hybridventilation - divakth.diva-portal.org/smash/get/diva2:725266/fulltext02.pdf · 2014-06-17 ·...

Hybrid Ventilation

Simulation of Natural Airflow in a Hybrid Ventilation System

DAÐI SNÆR PÁLSSON

Master’s Thesis at KTH School of Architecture and the Built EnvironmentDivision of Building Service and Energy Systems

Supervisor: Viktor SjöbergExaminer: Ivo Martinac

TRITA IES 2014-05

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Abstract

This thesis investigates the possibilities of using hybrid ventilation in anoffice building in Stockholm. The focus is on simulating the natural airflowto find out for which conditions it is sufficient. The thesis is done at WhiteArkitekter AB in cooperation and under the supervision of environmental spe-cialists working there. A literature study is carried out to study what has beendone before in Sweden as well as in other countries. Computer simulationsare used to simulate the airflow to examine the conditions and architecture.A synthetic computer model representing a realistic office building is built upas a starting point. The ventilation method for the natural ventilation partis to take air in through the façade and use the stack effects in an atrium fornatural ventilation. By altering the architecture and the sizes of the openingsaccording to the results from the simulations the building is dimensioned andformed to cope with the rules and requirements about the indoor air quality inworkplaces. The simulations are done with a multi zone energy performancesimulation tool that can simulate airflows and indoor air climate conditions inthe zones as well as the energy consumption. Computational fluid dynamicscalculations are then used to more closely simulate the conditions within thezones.

The results from those simulations suggest that the natural ventilationas a part of a hybrid ventilation works for all the floors of the building forup to 10 ◦C. The computational fluid dynamics simulations showed that thethermal comfort of all the occupants is fulfilled for these conditions but thereis a risk of occupants experiencing draught because of to high velocities inthe air especially for the colder outdoor temperatures. For the higher outdoortemperatures the airflow needs to be enforced to ensure sufficient conditions forthe occupants and for the colder temperatures mechanical ventilation is neededto decrease heat losses and avoid the risk of draught.

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AcknowledgementsI would like to acknowledge and thank Prof. Ivo Martinac who gave valuable inputand advice during the process and who established contact with White ArkitekterAB that gave me the opportunity to work on this project.

I would like to thank the people at White for the giving me the opportunity towork on this project and providing equipment and facilities to carry out the work.I would also like to thank them for all the workshops, meetings, lectures and otheroccasions that I have been able to attend and take part in and expand my knowledgeabout various subjects and get insight into their valuable work.

At last I would like to thank my supervisor at White Viktor Sjöberg for allhis help and I would like to thank all the others involved in the hybrid ventilationproject at White as well for the cooperation, support and help.

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Nomenclature

ASHRAE American Society of Heating, Refrigerating and Air Conditioning EngineersCFD Computational Fluid DynamicsBBR Boverkets ByggreglerIAQ Indoor Air QualityHVAC Heating, Cooling and Air ConditioningIFC Industry Foundation ClassesMET Metabolic EquivalentNPL Neutral PlanePPM Parts Per Million of volume

Programs used in this study:

• Autodesk Revit

• Autodesk Simulation CFD

• IDA ICE

• Inventor Fusion

v

List of symbols

A Area of opening [m2]C Flow coefficient [kg/s · Pan]Cd Discharge coefficient of opening [ ]Cp Pressure coefficient [ ]dp0 Limit pressure difference for linearisation [N/m2]g Acceleration of gravity [m/s2]gx x-component of the gravity acceleration vector [m/s2]gy y-component of the gravity acceleration vector [m/s2]gz z-component of the gravity acceleration vector [m/s2]Hd Height difference between the midpoint of openings [m]n Flow exponent [ ]m Mass [kg]µ Dynamic viscosity [N · s/m2]P Pressure [N/m2]ρ Density of fluid [kg/m3]SDR Moment from resistance [N/m3]Sω Moment from rotation [N/m3]P Pressure [N/m2]Q Volume flow [m3/s]Ti Average indoor temperature [K]To Outdoor temperature [K]t Time [s]U0 Reference velocity away from building [m/s]u x-component of the velocity vector [m/s]v y-component of the velocity vector [m/s]w z-component of the velocity vector [m/s]

Contents

Contents vi

List of Tables viii

List of Figures ix

1 Introduction 11.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41.3 Literature Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.3.1 History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51.3.2 Natural Ventilation . . . . . . . . . . . . . . . . . . . . . . . . 61.3.3 Hybrid Ventilation . . . . . . . . . . . . . . . . . . . . . . . . 101.3.4 Openings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121.3.5 Risks and Concerns . . . . . . . . . . . . . . . . . . . . . . . 121.3.6 Benefits of Hybrid Ventilation . . . . . . . . . . . . . . . . . . 131.3.7 Other Sustainable Design Methods . . . . . . . . . . . . . . . 13

1.4 Objectives and Expected Outcomes . . . . . . . . . . . . . . . . . . . 161.5 Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

1.5.1 Validation of IDA ICE . . . . . . . . . . . . . . . . . . . . . . 191.5.2 Validation of Autodesk Simulation CFD . . . . . . . . . . . . 19

2 Design and Simulations 212.1 Building Regulations . . . . . . . . . . . . . . . . . . . . . . . . . . . 212.2 Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

2.2.1 Natural Ventilation . . . . . . . . . . . . . . . . . . . . . . . . 232.2.2 IDA ICE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242.2.3 Autodesk Simmulation CFD . . . . . . . . . . . . . . . . . . 26

2.3 Modelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 282.3.1 Architectural Models . . . . . . . . . . . . . . . . . . . . . . . 282.3.2 IDA ICE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 292.3.3 Autodesk Simulation CFD . . . . . . . . . . . . . . . . . . . . 32

2.4 Risks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

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CONTENTS vii

3 Results and Discussions 393.1 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

3.1.1 Combining the Models . . . . . . . . . . . . . . . . . . . . . . 393.1.2 IDA ICE Results . . . . . . . . . . . . . . . . . . . . . . . . . 403.1.3 Autodesk Simulation CFD Results . . . . . . . . . . . . . . . 65

3.2 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 723.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 753.4 Suggestions for Future Work . . . . . . . . . . . . . . . . . . . . . . 76

Bibliography 77

A Input values for IDA ICE 81A.1 Default Values in IDA ICE . . . . . . . . . . . . . . . . . . . . . . . 81A.2 Heating Design Default Values . . . . . . . . . . . . . . . . . . . . . 84A.3 Materials Defined in IDA ICE . . . . . . . . . . . . . . . . . . . . . 85A.4 IDA Openings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87A.5 Openings for 2014.03.21 . . . . . . . . . . . . . . . . . . . . . . . . . 89A.6 Openings for 2014.05.14 . . . . . . . . . . . . . . . . . . . . . . . . . 90A.7 Outdoor Temperatures . . . . . . . . . . . . . . . . . . . . . . . . . 91

B Operative Temperatures 93

C Input Values for Autodesk Simulation CFD 97C.1 Material Properties Autodesk . . . . . . . . . . . . . . . . . . . . . . 97C.2 Boundary Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . 101C.3 Meshing in Autodesk Simulation CFD . . . . . . . . . . . . . . . . . 104

List of Tables

2.1 The relevant regulations . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

3.1 Temperature and CO2 concentration for Variation 0 . . . . . . . . . . . 403.2 Airflow and power input of the office zones on every floor for Variation 0 413.3 Temperature and CO2 concentration for variation 1 with larger openings

in atrium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 413.4 Airflow and power input of the office zones on every floor for variation

1 that uses larger openings in atrium . . . . . . . . . . . . . . . . . . . . 413.5 Temperature and CO2 concentration for variation 2 with larger openings

on floors 3 to 6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 423.6 Airflow and power input of the office zones for variation 2 on every floor

with larger openings on floors 3 to 6 . . . . . . . . . . . . . . . . . . . . 423.7 Temperature and CO2 concentration for variation 3 with elevated atrium 433.8 Airflow and power input of the office zones on every floor for variation

3 with elevated atrium . . . . . . . . . . . . . . . . . . . . . . . . . . . . 443.9 Temperature and CO2 concentration for variation 4 with elevated atrium

and variable openings . . . . . . . . . . . . . . . . . . . . . . . . . . . . 443.10 Airflow and power input of the office zones on every floor for variation

4 with elevated atrium and variable openings . . . . . . . . . . . . . . . 443.11 Temperature and CO2 concentration for variation 5 with elevated atrium

and variable openings on every floor . . . . . . . . . . . . . . . . . . . . 453.12 Airflow and power input of the office zones on every floor for variation

5 with elevated atrium and variable openings on every floor . . . . . . . 453.13 Temperature and CO2 concentration for variation 6 with elevated atrium

and variable openings on every floor . . . . . . . . . . . . . . . . . . . . 463.14 Airflow and power input of the office zones on every floor for variation

6 with elevated atrium and variable openings on every floor . . . . . . . 46

viii

List of Figures

1.1 Simplified section that shows how stack effects in atrium can be used fornatural ventilation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.2 Culvert system that has been used in Sweden driven by natural forcesby keeping equal pressure difference on all floors A+C=B+D. . . . . . . 11

2.1 Pressure difference between openings at different heights. . . . . . . . . 242.2 A floor plan of the office set-up. . . . . . . . . . . . . . . . . . . . . . . . 282.3 IFC model of the office building. . . . . . . . . . . . . . . . . . . . . . . 292.4 Floor plan and zones used in IDA ICE. . . . . . . . . . . . . . . . . . . 302.5 Backflow of air in CFD simulations. . . . . . . . . . . . . . . . . . . . . 332.6 Rising of air into atrium in the CFD simulations. . . . . . . . . . . . . . 332.7 Materials defined in Autodesk Simulation CFD. . . . . . . . . . . . . . . 342.8 Boundary conditions defined in Autodesk Simulation CFD. . . . . . . . 352.9 Mesh as defined in Autodesk Simulation CFD. . . . . . . . . . . . . . . 36

3.1 IFC model of the modified office building. . . . . . . . . . . . . . . . . . 433.2 The temperature development throughout the day for office zone on the

first floor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 473.3 The temperature development throughout the day for office zone on the

second floor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 473.4 The temperature development throughout the day for office zone on the

third floor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 483.5 The temperature development throughout the day for office zone on the

fourth floor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 483.6 The temperature development throughout the day for office zone on the

fifth floor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 493.7 The temperature development throughout the day for office zone on the

sixth floor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 493.8 The airflow development throughout the day for office zone on the first

floor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 503.9 The airflow development throughout the day for office zone on the second

floor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

ix

x List of Figures

3.10 The airflow development throughout the day for office zone on the thirdfloor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

3.11 The airflow development throughout the day for office zone on the fourthfloor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

3.12 The airflow development throughout the day for office zone on the fifthfloor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

3.13 The airflow development throughout the day for office zone on the sixthfloor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

3.14 The indoor air quality development throughout the day for office zoneon the first floor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

3.15 The indoor air quality development throughout the day for office zoneon the second floor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

3.16 The indoor air quality development throughout the day for office zoneon the third floor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

3.17 The indoor air quality development throughout the day for office zoneon the fourth floor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

3.18 The indoor air quality development throughout the day for office zoneon the fifth floor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

3.19 The indoor air quality development throughout the day for office zoneon the sixth floor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

3.20 The temperature development throughout the day for office zone on thefirst floor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

3.21 The temperature development throughout the day for office zone on thesecond floor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

3.22 The temperature development throughout the day for office zone on thethird floor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

3.23 The temperature development throughout the day for office zone on thefourth floor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

3.24 The temperature development throughout the day for office zone on thefifth floor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

3.25 The temperature development throughout the day for office zone on thesixth floor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

3.26 The airflow development throughout the day for office zone on the firstfloor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

3.27 The airflow development throughout the day for office zone on the secondfloor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

3.28 The airflow development throughout the day for office zone on the thirdfloor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

3.29 The airflow development throughout the day for office zone on the fourthfloor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

3.30 The airflow development throughout the day for office zone on the fifthfloor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

3.31 The airflow development throughout the day for office zone on the sixthfloor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

List of Figures xi

3.32 The indoor air quality development throughout the day for office zoneon the first floor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

3.33 The indoor air quality development throughout the day for office zoneon the second floor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

3.34 The indoor air quality development throughout the day for office zoneon the third floor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

3.35 The indoor air quality development throughout the day for office zoneon the fourth floor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

3.36 The indoor air quality development throughout the day for office zoneon the fifth floor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

3.37 The indoor air quality development throughout the day for office zoneon the sixth floor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

3.38 The velocities according to the simulation seen in a section between theoccupants. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

3.39 The velocities according to the simulation seen in a section through someof the occupants. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

3.40 The temperature distribution through the room according to the simu-lation seen in a section. . . . . . . . . . . . . . . . . . . . . . . . . . . . 65














xii List of Figures





3.58 The temperature distribution through the room according to the simula-tion seen in a section, temperature range on legend up to 30 ◦C, becausetemperature rises above 25 ◦C in some areas. . . . . . . . . . . . . . . . 71

Chapter 1

Introduction

1.1 Introduction

Global warming and climate change continue to scope the life on the planet andinfluence our decisions. Global warming will soon have caused more increase in theearth’s temperature than experts believed possible to cope with and is growing ata faster rate than any time in the previous 20 million years[16]. A lot of actionis needed to slow down this process and a lot is being done. The cause is carbondioxide that is released from human activities and combustion of fossil fuels forenergy usage, it is altering the balance in the atmosphere.

Energy usage of buildings accounts for about 40% of the world’s total energyend use [21]. Therefore minimizing the energy consumption of buildings has thepotential to reduce the energy consumption in the world substantially. Decreasedenergy consumption of buildings should be of economical interest for the buildingowners, tenants and the authorities. The natural resources available and renewableenergy should utilized when possible to create the best sustainable design solutionsfor buildings. The natural interaction between the indoors and outdoors should beused as much as possible to provide ventilation and thermal comfort inside. Asthe modern HVAC technology evolves and with the strict demands on the indoorclimate the knowledge about naturally ventilated spaces appears to have decreasedin Sweden [6]. The implementation of natural ventilation in new buildings hasdecreased over the last century [23]. Types of natural ventilation are usually cate-gorized by the forces that drive the airflow. The main categories are the wind drivenventilation and buoyancy driven ventilation [27]. These solutions can also be usedin combination with each other or in combination with mechanical ventilation tomake a hybrid solution. With the general public’s increased awareness of the effectsof global warming and it’s causes there seems to be some interest in returning tothe traditional methods for ventilating buildings. Natural and hybrid ventilationtherefore make an interesting and important area of research.

Ventilation is the process of exchanging air in a space moving out humidity andpollution and bringing in fresh air. This is a crucial aspect to consider when de-

1

2 CHAPTER 1. INTRODUCTION

signing buildings, for the well being of the occupants and their thermal comfort.The ventilation of different types of buildings are subjected to regulations and re-quirements to ensure that. It is not simple dealing with peoples experience andcomfort. Even though all regulations and requirements are met the occupants maystill not be comfortable and may not like the solution and the indoor climate. Thereis therefore a human aspect to the ventilation design that needs to be consideredand studied.

Office buildings are an interesting subject for implementation of natural andhybrid ventilation solutions because of the strict regulations on the working climateand the regular occupancy. The size of office buildings often provide conditions forlarge thermal and pressure differences that can be utilized. Fulfilling the require-ments and ensuring a healthy and comfortable indoor environment influences theefficiency and performance of the workers and should be of great interest to em-ployers. The economical aspect and energy consumption of the different solutionsaffect the life cycle cost of the building and are important for the building owners.In office buildings in the United Kingdom for example about 60% of the energy goesto heating and about 10% to cooling and ventilation [27].

Buildings that only depend on natural ventilation can be a risky design strategywhere the environmental conditions are subjected to frequent change. For examplein climates with cold winters and small temperature differences between the indoorsand outdoors in the summertime. The uncertainties of the natural forces can makeit difficult to fulfil the strict demands on indoor climate at all times. The imple-mentation of hybrid ventilation solutions is therefore a very interesting topic andperhaps a more applicable to the Scandinavian climate. Hybrid solutions is a com-bination of mechanical and natural ventilation used on the same building. A hybridsolutions can make the most of the advantages of both systems to optimize the per-formance considering, thermal comfort, costs and energy consumption. They havebeen shown to be an economically, environmentally and a socially good solution fora Danish climate [15].

To make the most of the natural ventilation concept in the hybrid ventilationsystem the orientation and the architectural forming of the building are importantto consider in the early design phases. By smart design the time of the year that thebuilding is dependent on the mechanical ventilation can be minimized. Then againmore energy might be needed for heating the building if it is ventilated with coldoutdoor air and if air exhaust air is not used for heat recovery before exhausted.The energy consumed by buildings during their lifetime includes the embodied en-ergy of all materials and installations as well as the buildings specific energy duringthe operational time. To achieve low energy consumption the choices of materialsneed to be smart and the whole design of the building needs to aim at energy sav-ings. This needs to be considered in the design process at an early stage for mosteffective outcome.

Natural and hybrid ventilation solutions are therefore important to considerwhen planning to build energy efficient houses. The more natural indoor environ-ment can also feel more comfortable for the occupants than mechanical air that

1.1. INTRODUCTION 3

stays the same all the time. People are more satisfied with the indoor environmentif they feel like they have control over it themselves [7]. Research also implies thatpeople that work in naturally ventilated spaces have more adaptability for changesin the indoor climate than people that have gotten used to a stable air-conditionedenvironment [28].

The possibilities of using hybrid ventilation systems instead of mechanical venti-lation in Sweden will be studied in this thesis with a literature study and computersimulations.


1.2 BackgroundThis research came about because of the increased interest for passive ventilationsolutions from environmental specialists and architects at White Arkitekter AB.The thesis is done at White, in cooperation with and under the supervision of theenvironmental specialists that work in the company’s Sustainability Group. Thegroup works with environmental aspects of the buildings that the architects design,such as daylight and energy questions as well as environmental certifications.

The book Som man bygger får man ventilera has a good amount of informationabout history of ventilation in Sweden including natural and hybrid methods[19].There was interest within the company to start a project to restore the knowledgeabout natural and hybrid ventilation in Sweden and to update the information inthe book based on research of what has been done around Sweden in the recentyears and with computer simulations of natural airflow within buildings. Since it isan architectural firm the interest lies in how it would influence the architecture anddimensioning of buildings. There is an ongoing project within the company andin cooperation with external specialist and other companies to revoke knowledgeon the matter in Sweden. The first phase of that project includes research on thethings being done in Sweden and elsewhere, looking into energy and economicalquestions and the relevant building regulations as well as coming up with a way tosimulate the airflow in buildings that can be used to examine architectural aspectsand compare different solutions. This thesis is a part of that work and focuses onthe simulations of natural airflow, comparing different dimensions, the regulationsthat need to be fulfilled and a brief literature review. In the later phases the focuswill be on the social aspect, to further develop simulation methods and increase theknowledge amongst architects and other people within the sector.

1.3. LITERATURE REVIEW 5

1.3 Literature Review

Hybrid ventilation is a fairly new concept, natural ventilation methods howeverhave been used and developed for centuries. When mechanical ventilation cameto the table the natural and mechanical ventilation methods were developed sepa-rately and independently of each other for the most part[6]. In recent years withthe increased environmental awareness interest has grown in using the two methodstogether to achieve better indoor climate and energy efficiency [18]. Natural venti-lation is a complicated concept and there are many different principles and methodsthat can be used and have been used throughout the years. It requires intricatearchitectural design that considers the climate and surroundings. If natural venti-lation is integrated for all year round usage in the Swedish climate then there needto be solutions for how to heat the incoming air in wintertime and how to ensuresufficient airflow during the summer when there are little temperature differencesbetween the indoors and outdoors. A study from Lunds Tekniska Högskola observ-ing school buildings in the southern part of Sweden ventilated with enforced naturalventilation, suggests that fans are needed to enforce the airflow when the outsidetemperature is more than 10 ◦C [24]. Therefore it is likely that hybrid ventilationis a more feasible solution for Swedish climate conditions than completely naturalventilation. Hybrid ventilation makes the most of the two concepts, the mode thatis used to ventilate at each time is dependent on the ambient and environmentalconditions. This makes hybrid ventilation a very feasible solution for passive designssince it is likely to make the most of the environmental conditions each time. Ithas the potential to save energy but at the same time having the mechanical optionso indoor air quality is ensured in all weathers and conditions. There are somedifferent ways to combine these systems. They can be completely separate, havingthe house formed to utilize the natural forces from wind and buoyancy and havinga complete mechanical ventilation system separate[18]. There is a possibility of anintelligent system that switches on the mechanical ventilation automatically whenneeded and controls the openings of the natural ventilation system, or it can all bedone manually. They can also be combined in a way to reinforce each other, like fanreinforced natural ventilation systems and a wind or buoyancy enforced mechanicalventilation [17].

1.3.1 History

Ventilation has been an important concern when constructing buildings for cen-turies. It is the most important part in creating and maintaining a good and healthyindoor environment. Ventilation is especially important in warmer countries andhas in large parts been developed there. Architecture in the Middle East has for longbeen influenced with and taken advantage of the combination of shadows, daylight,evaporation and air movements to make the indoors environment comfortable [9].That is really important in areas where the temperature outdoors can be unbear-able. These systems often work in reverse during night and colder periods, utilizing


the thermal mass of the earth to either heat or cool the air. Sometimes the houseshave different zones as well that are used during different times of the year andthe day [9]. Before mechanical ventilation was introduced all ventilation problemsneeded to be solved using the natural forces and the architecture of the buildings.

1.3.2 Natural Ventilation

Natural ventilation is a wide concept and it can be as simple as only using windowsor other openings in the façade to let air in and out. There are also more complicatedsystems possible, such as culvert systems and complex architectural formations thatconvey the air through a building. Various ways have been used to preconditionthe incoming air like culvert systems that use the thermal mass of the ground forheat exchange. The ground can heat or cool the incoming air depending on theseason using the thermal mass [3]. This is a more complex system than taking inair through the façade and can be more difficult for the users to understand andcontrol. A system where the airflow needs to be enforced with fans is a hybridventilation system.

Draught is a common complaint where air for ventilation is taken in through thefaçade [17]. When the outside temperature is lower than the indoor temperatureand the air does not reach a comfortable temperature before reaching the occupantsor it has too high velocity, draught can be experienced. There is less risk of draughtwhen the air is taken in via ducts and is preheated before entering the room. CFDanalysis is therefore very useful when looking at natural ventilation and when theair enters through the façade to estimate the air velocities and study if the thermalcomfort of all the occupants is sufficient.

The surroundings play a very important role in the relationship between theoutside air and the indoor climate. For a successful integration of a hybrid ventila-tions and making the most of the natural airflows to save energy the surroundinglandscape and neighbouring buildings need to be considered. The wind patterns,sunlight and shading resulting from the surroundings all influence the airflows intothe building [9] [19].

1.3.2.1 Buoyancy Driven Ventilation

Stack effect arise in shafts and atrium in taller buildings from the pressure differencescreated by the different pressure drop inside and outside of atmospheric pressure,shown in Figure 2.1. It is caused by density and temperature differences betweenthe indoor and the outdoors [9]. The airflow created by these effects can be used forand has been used to help ventilation in buildings. It can have similar ventilationcapacity as cross ventilation, driven by wind but can sometimes be a better option ifthe architecture and purpose of the building provides opportunities to utilize theseeffects [18]. The advantage is that it is independent of the wind and orientation,meaning they do not need the wind but the wind can help the airflow if present.Many buildings are not constantly exposed to wind, for example buildings that are


shielded by other buildings in urban areas, or where the winds are weaker duringcertain parts of the day or year so it may be hard to rely on. Stack effects can beenhanced by the height difference and temperature differences. Many methods areknown and have been used throughout the years to enforce these effects, for exampleusing atrium, shafts or chimneys to increase height differences between openings andvarious methods usually using the sun to increase the temperature differences. Inthe more northern hemisphere there is a big difference in the conditions throughoutthe year. There is a need for cooling of buildings during the hottest months andheating during a large part of the year. It is likely that natural ventilation ismost suitable during the periods between the hottest and coldest parts [24]. Largetemperature differences during the colder times will cause big airflows, there is a riskof draught and a lot of energy might be needed to maintain comfortable temperatureindoors. During the hottest months of the summer there are very small temperaturedifferences between the indoors and outdoors and that can cause low and unstableairflows.

Shafts Shafts in multi storey buildings are necessary for mechanical ventilationsystems to channel air between floors. Shafts can also be useful in a house that usespassive solutions and relies on natural forces to drive the ventilation. For buoyancydriven ventilation some height difference is needed so a shaft stretching betweenthe floors is often the ideal place to create airflows with buoyancy forces. They arethus often the central part of different systems that rely on the buoyancy to driveventilation [19].

Atrium Stack effect in atrium in large buildings can be used for ventilation ofthe building. Figure 1.1 shows a simplified case explaining the idea. The pressuredifference created with the height difference can be used to drive air through thebuilding. Atrium and a set of shafts or chimneys that have the same height differencecan have essentially the same effect, but atrium is more useful. Atrium can be usedto get daylight into the building and it can be used for staircases and as a placefor social activities. Glazing of atrium can enhance the stack effects and increasethe effectiveness [17]. In naturally ventilated spaces that use atrium it is a generalrecommendation to have the ventilated space not wider than five times the ceilingheight from the façade to the shaft. Since 3 m is a common ceiling height in officespaces 15 m width is often recommended both for the ventilation capacity andconsidering the daylight in the space [17]. A common practise in Denmark is totake the air in through openings in the façade and out through an atrium. It isrecommended to take the air in through openings located high in the room to haveit further away from the occupied zone to avoid draught [17].

Solar Chimneys Solar chimneys can be used to enhance the stack effects in abuilding. By elevating the exhaust openings high above the building with a chimneystack effects are created. If the chimney is from a heat absorbing material it warms


Figure 1.1: Simplified section that shows how stack effects in atrium can be usedfor natural ventilation.

up the air inside and the it rises faster and the effects are increased. Heating up theair can also help airflow in atrium buildings, increasing the stack effect with largewindows for example. It can be challenging to rely on since there also needs to bea bearable temperature there for people that need to stay in or pass through theatrium [11] [17].

Double Skin Façades Double glazed façades use the buoyancy created by thepressure and temperature differences in the space between the glazing in the façade.The stack effects are increased with the sun warming up the air and creating moretemperature differences between the indoors and outdoors [6] [9].

1.3.2.2 Wind Driven Ventilation

There is a wide variety of different ventilation solutions that have been used through-out the years that rely on the pressure differences created by the wind. There area lot of methods that aim at maximizing the wind effects. By planning the orien-tation of the building, the building form and placement of openings with respect tothe surroundings and the governing wind directions. The pressure difference cre-ated between the inlet and outlet of the building enforce the airflow through thebuilding.

Cross Ventilation A common ventilation concept that relies on the wind is crossventilation. Open floor plans, permeable buildings and usually the governing winddirection are combined to create airflow through the building for ventilation. Usuallythe inlet openings are facing the prevailing wind direction and the outlets on theleeward side, making use of the pressure differences created between the openings.It is a useful method in hotter areas where there is a need for cooling, because it


can both remove heat from the building like any other ventilation method and itincreases peoples rate of evaporation and thus the sensation of cooling [9]. It is notconsidered as efficient as night time cooling method. The cross ventilation methodlimits the size of the building. It can usually only be one room in thickness andneeds to be very wide to maximise the exposure to the wind. If the building is tohave corridors along its width and more than one room in thickness cleaver designis needed to channel the air through all the spaces, often making use of stack effectsto enforce the flow [9]. The wind can be used in combination with stack effects. Anexample of that is when cross ventilation is used in combination with double glazedfaçades. That is a common way of ventilating large office buildings in Germany [6].

Single Sided Ventilation Single sided ventilation is similar to cross ventilationin a way that it takes the air in through the faa̧ade and relies on the wind. Insteadof having the air travel through the building or the ventilated space the inflow andoutflow happen on the same side of the room. This method can also utilize somestack effects within the space by placing the inlet and outlet at different heights.This method is not as effective as cross ventilation and does not efficiently ventilateas wide rooms as cross ventilation is capable of. A general recommendation is thatit can ventilate spaces that are twice as wide as the floor to ceiling height [17].

Wind Driven Fan The stack effect can be reinforced by wind blowing by theopening of chimneys or shafts and thus creating under pressure. The under pressurehelps to increase the airflow. This effect can be enforced even more by placing awind driven wheel or fan there that can create even stronger under pressure. Thishas been used in smaller houses in Sweden [19].

Wind Catchers and Wind Towers Wind catchers and wind towers and havebeen used for many centuries in the Middle East for ventilation. The towers aredesigned to capture the wind and use the air for ventilation. Openings that facethe wind direction capture the wind, the pressure pushes the air down into thebuilding. This has been used in dense cities and in other structures where there islittle exposure to wind in warmer countries. Different types are designed to capturewind from a certain direction. Some designs can catch wind from all directions sothe ventilation of the building is independent of the wind direction [9]. This designis probably not suitable for the northern hemisphere where the air is colder and thelarge openings make the indoor climate vulnerable to damages because of rain orstorms, but is more suitable for desert climate with little precipitation.

Surroundings and Environment Where the wind has been used throughoutthe years to drive ventilation people have developed tricks and techniques to max-imize the effects of the wind and to lead it to the right direction. When crossventilation is used the façade where air is taken in should face the prevailing winddirection. Trees and vegetation can also be used in a smart way to control the


wind pressure on different surfaces [9]. That is perhaps more applicable for singlefamily homes or buildings with fewer floors. The geographical location of coursealso provides the basis for which methods are possible and feasible to use and whatnatural forces are available.

In spite of all of these techniques that have been developed the wind still re-mains an unpredictable source. Both the direction of the wind and the wind velocitychange from time to time and can be hard to evaluate and simulate accurately [27].

1.3.3 Hybrid VentilationThe hybrid ventilation concept is quite new and utilizes the best of mechanicaland natural ventilation in combination to create good indoor environment withminimum environmental impact and energy consumption.

Enforced Exhaust A type of hybrid ventilation is when the exhaust of a natu-ral ventilation system is enforced mechanically to prevent recirculation and ensurestability of the system. The airflow through the exhaust can be enforced in twoways. A natural ventilation exhaust can be enforced with a simple fan or with acompletely mechanical exhaust system.

Fans that are used for enforcing exhaust only operate when the natural forcesdo not provide sufficient airflows and also to prevent air from flowing in the wrongdirection.

In mechanical exhaust systems fans are used to pump indoor air out, creatinga constant under pressure in the building that helps pulling outside air in throughopenings. Unlike in an enforced natural ventilation the exhaust system is runningconstantly. In larger spaces it is very hard to have a sufficient amount of airflowinto room without a high risk of draught. With completely natural airflow throughexhaust windows and openings heat is lost because the warm air flows out. Witha mechanical exhaust system it is possible to use heat recovery at the fans to saveenergy. The operating costs for this kind of system are still relatively low [19].

When the outside air is warmer than inside are probably rare in Sweden [24].When that is the case natural ventilation airflow might be reversed. It will onlyremove pollution and not provide any cooling. Mechanical ventilation is capable ofcooling down buildings in the hottest periods by cooling the air before it is pumpedinto the rooms. Sustainable designs should minimize the solar gains during hotperiods to keep down the energy used for cooling.

Culvert System A number of schools were built in the 90’s in Sweden usingnatural ventilation with with exhaust fans. The air is taken in outside the buildingand conveyed through a culvert in the ground to the house and from there toall the spaces almost like a regular mechanical ventilation system. When the airgoes through the duct under ground to the house it can be heated during the


winter and cooled during the summer, utilizing the thermal mass effects of theground. Interviews with the users in those schools imply that most of them aresatisfied with the indoor conditions [6]. The thermal mass of the ground causes thetemperature fluctuations in the ground to be much lower and about 2 m down inthe ground the temperature has small fluctuations throughout the year[3]. Equalpressure differences can be created in these systems for every level if the heightdifferences of the intake and out take openings combined are equal for all floors theconcept is explained in Figure 1.2 [26][19].

Figure 1.2: Culvert system that has been used in Sweden driven by natural forcesby keeping equal pressure difference on all floors A+C=B+D.

This method is energy efficient since it uses the ground to heat or cool the air.Less energy is needed to heat the spaces compared to the case where the air wouldhave been taken through the façade. The culvert also gives the opportunity to filterparticles from the incoming air by only using gravity. Large diameter of the groundculvert cause low velocities of the air and particles can fall to the bottom. In thesesystems fans are often used to reinforce the airflow [24]. They can be used to enforcethe airflow but also to ensure that the air is going in the right direction and air isnot going from the classrooms into the hallways or other classrooms. The wrongdirection of airflow can happen when the temperature differences are unfavourableor it can be caused by the wind conditions. The exhaust air exits either out throughthe windows or through an exhaust duct, utilizing the stack effects. If the air needsto exit out through windows it can cause problems in certain wind conditions if forexample the windows are all facing in the same direction and the wind is blowingair into these openings [24]. The location of the inlet and outlet openings in relationto the interior architecture and the furniture needs to be considered to ensure thatthe airflow is not blocked. The occupants need to be aware of this, for example inschools where the set-up is flexible and the furniture can be moved around [24].


Wind or Buoyancy Enforced Mechanical Ventilation The natural forcescan also be used to help mechanical ventilation to decrease the operational energy[18]. This method can be defined as a mechanical ventilation system with smallpressure losses where the natural forces can provide most of the pressure differencesneeded [18].

Mechanical and Natural Ventilation Combined The combination of sepa-rate systems, natural and mechanical needs to be formed to utilize the natural forcesas well as being equipped with a full mechanical ventilation. Strategy on how toswitch between those systems is needed, that is also relevant when the systems en-force each other. Some methods that have been used in previous hybrid ventilationdesigns are: Based on censors that measure the IAQ parameters, timers, motiondetectors or done manually by the occupants[18]. The advantages of the differentmethods depend on the occupancy level and frequency.

1.3.4 Openings

Many different ways are available to take in air. A common way is through thefaçade, the most common being simply to open a window. There are ways to heator cool the incoming air and control the volume flow to prevent draught. A cleaversolution is to use the heat that is escaping a room to heat the incoming air or, viceversa, using cold air that is escaping to cool down the incoming air [9]. Alvar Aaltoused this strategy in the Tuberculosis Sanatorium in Paimio Finland where the heatthat escapes through windows is used to heat the incoming air. That is done bymulti layer windows that brings air in through the layers gradually getting closerto the indoor temperature [9]. Other methods that have been used throughout theyears include using radiators to heat the incoming air, for example the air can beconveyed through a radiator so it reaches comfortable temperatures before enteringthe room [19].

1.3.5 Risks and Concerns

The natural air flow through a building is a result of pressure differences and the airtravels through openings and also with infiltration. The strict energy and thermaldemands on the modern buildings make it harder to utilize the natural flow of airsince they are more tightly built. The difference in costs and energy of using thenatural ventilation and mechanical ventilation needs to be studied to make a gooddecision about for which temperatures to use natural ventilation or mechanical.When relying on the natural airflow the building becomes more susceptible to riskssuch as easier spread of pollution, noise, and in emergencies the spread of fire andsmoke. The air pollution in Stockholm is quite low for a big city [10]. Airflow that isdependent on the natural forces is more unreliable. Draught can be more difficult tocontrol. The path of the air can be obstructed by the interior architecture, privacyor shading devices. The whole design therefore needs to be included when planning


natural ventilation and it needs to be considered early on in the architectural designprocess. It can not be dimensioned afterwards like mechanical solutions. The designof the openings needs to take into consideration the security of the building and forexample risk of burglary [18].

1.3.6 Benefits of Hybrid Ventilation

Studies from Denmark of the implementation of hybrid ventilation in actual projectsfrom around the world have shown some of the benefits of the concept [18]. Thepossible economical benefits of hybrid ventilation need to be compared to otherprojects on a life cycle cost basis. That is because the implementation of naturaland hybrid ventilation concepts is included in the design phase and dimensioningof the building and is hard to compare to the implementation costs of a mechanicalventilation solutions. The same goes as for the embodied energy and CO2 emissionsthat need to be compared on a life cycle basis. Research done by the FraunhoferInstitute in Stuttgart, Germany and research from the University of Aalborg inDenmark suggests that regarding energy consumption and CO2 emissions on a lifecycle basis, the hybrid ventilation performs best of the three options, natural venti-lation, hybrid ventilation and natural ventilation [18] [15]. But regarding the totallife cycle costs the natural ventilation appears to be the most economical option[15]. Studies have shown that occupants are generally happy with the indoor cli-mate when natural ventilation solutions are used, they are more tolerant for changesand feel better if they have some control over the environment [6] [28] [7].

1.3.7 Other Sustainable Design Methods

Many concepts that are used in passive designs of buildings are useful and essentialto help the airflow in natural ventilation. During the design phase all of the methodsthat are going to be used need to be considered as they need to work together forthe best results.

Architectural Forms When designing a sustainable building, the architecturaldesign is the most important. It will not work well to implement a lot of sustainabletechniques and methods afterwards, they need to be implemented in the design andneed to be a part of the designs from the beginning. The architect needs to considerthe climate conditions, location and the surroundings and form the building in away that minimizes its energy need, by for example maximizing solar gains whenneeded and using shadows and passive cooling methods for energy efficient coolingduring warmer periods [22] [9].

Evaporative Cooling Evaporation of water requires energy and thus takes awayheat from the surroundings. This has been used for centuries to cool air, and haseven been implemented into some modern sustainable designs. It can be imple-mented with wind driven natural ventilation, for example in wind towers to cool


the incoming air by spraying drops to the incoming air[22]. It has also been used inrecent designs to cool down the air in glass buildings and spaces with large windowson sunny days.

Night Cooling Cooling of buildings during the night when the outdoor temper-atures are lower than during the day can decrease the cooling needed the day after.The temperature differences provide better conditions for buoyancy driven airflowthan during the day. During nights when the temperature indoors is higher thanthe outside temperature, the building radiates heat to the night sky. If this processis used to cool down buildings during the night it can be enhanced by sprayingwater on the warm roof and the evaporation helps to cool down the building evenmore [12]. The temperature therefore remains low and less cooling and ventilationmay be needed during the following day. This is more applicable for lower buildingssince cooling through the roof is ineffective in taller buildings [9] [12].

Shading Use of shadows and shading of windows are important in the design ofbuildings. Shading of windows is used to minimize solar gains and thus energyfor cooling needed in the warmer parts of the year decreases. Shading panels foreach window can have a smart control system, only being drawn down when thesun is shining directly on it, to maximize the use of daylight and minimize solargains. Shadows outside from cleverly designed buildings have created conditionsfor staying outdoors in warm weather. This increases the quality of living becausepeople can choose to stay outside in warm sunny weather. This can be used incombination with natural ventilation so the air is cooled to some extent before it isused to ventilate indoors [9].

Thermal Mass Thermal storage in walls and ground are important componentsthat can be used to minimize energy needed for heating and cooling. To use thermalstorage of the walls and ground to balance temperature differences has been usedfor ages in warmer climates [9]. The abilities of thermal mass and storage have beenused in designing natural ventilations systems for example when air is channelledthrough the ground for heating or cooling and the thermal mass in walls are used tobalance the temperature fluctuations of the indoor environment. The airflow thusremoves heat from the mass when it is heated and brings heat when cooled. Thishas been used in Sweden in naturally ventilated school buildings [24].

Vegetation Vegetation can be used in many ways to form the surroundings andinfluence the airflow and temperature of buildings. Trees can be used in formingthe wind patterns surrounding a building to increase or eliminate effects from wind.The trees can be used to shelter houses or to direct the airflow so it makes desirablepressure distributions on the buildings that help the natural ventilation [17]. Thisis most applicable for lower buildings that are relying on cross ventilation. Plantsand vegetation can also be used indoors to influence the indoor climate. Plants have


positive effect on the experience of humans indoors, they have positive influence onthe CO2 concentration and the humidity [26].


1.4 Objectives and Expected Outcomes

The objectives of the thesis is to look at the possibility of using hybrid ventilation inoffice buildings in Stockholm by looking at what has been done in the past and withcomputer simulations. A literature study will examine the many methods availablefor enhancing natural airflow in other climates and also what methods have beenused in Sweden and the nearby countries.

Today it is very important to be able to simulate different performance aspectsof buildings during the design phase to see if the building will perform sufficientlyfor example regarding the indoor climate and energy consumption. Therefore thegoal is to be able to simulate natural airflow in a synthetic office building locatedin Stockholm that uses natural ventilation as a part of a hybrid ventilation solu-tion. The simulations can be used to determine if the airflows are sufficient and forwhat conditions the natural airflow will work and when it needs to be enforced ormechanical ventilation should take over. The relevant building regulations will beinvestigated and used to evaluate the performance of the designs.

The starting point will be a model of a typical office building with an atrium.The focus will be on the simple case where air is taken in through the façade andusing stack effects in an atrium to channel the air out of the building, i.e. utilizingan already very common design for such office buildings. Atrium in large buildingscan serve as a provider for natural light for parts of the building and as a lobby,social area and can have space for elevators and staircases. The model will be builtup with dimensions and interior design features and floor plans inspired by existingand planned real buildings to have a realistic starting point. The model will bebuilt up in Autodesk Revit. The whole building model will be simulated in IDAICE for different temperatures to get an idea of the airflows available for each caseat every floor. The model can then be changed and altered to examine variationsthat can create better conditions without compromising for temperatures, draughtor energy consumption. The airflow, the indoor temperature and the energy neededfor heating will then be used to make more accurate CFD simulations of how thetemperature is distributed in the office spaces and to check if the velocities arewithin the building regulation limits using Autodesk Simulation CFD. The resultsfrom the simulations can be used to examine if the conditions are sufficient for allthe occupants in the space. The office modules that are used in the CFD simulationsare built up using Autodesk Revit and then some adjustments are done in Inventorfusion to make the model usable for CFD calculations, like creation of fluid volumes,extension of openings and the approximation of the atrium effects.

The stack effects are a well known phenomena and widely used for naturalventilation, the expected outcome from the airflow simulations is therefore thatthey show the stack effect and the models can then be altered to examine how tobest make use of the natural forces for ventilation. The expected outcomes fromthe CFD calculations are that they show somewhat the same temperatures in thespace for the given boundary conditions from the IDA ICE model and can thus beused to further evaluate the design.

1.4. OBJECTIVES AND EXPECTED OUTCOMES 17

It is expected that with these simulations the feasibility and conditions for thenatural part of a hybrid ventilation can be examined.

This study will only focus on simulations of the actual airflows. Comparisonof different ventilation options considering the environmental impact, energy con-sumption as well as the costs on a life cycle basis are left for future studies.


1.5 Limitations

Natural ventilation in large buildings is a complicated concept and is influenced bythe natural forces, the formation, the location, orientation of the building and itssurroundings. Simulations that take all of these factors into account are compli-cated and computationally heavy. Simplifications and assumptions are done in themodelling process to simplify the calculations. The scope of this thesis will only fo-cus on simulating natural airflow in an office building in the Stockholm climate thatuses the stack effects in atrium and shafts using a multi zone energy performancemodel IDA ICE and Autodesk Simulation CFD for CFD simulations.

As with any computer modelling there is a risk for errors and uncertainties whenmaking assumptions and simplifications. One program will be used to simulate theeffect of the natural forces on the whole building and another one to simulate theconditions in every room. Using two different modelling methods, the multi zonemodel and the CFD model and using them together and to compliment each otherrequires that the simulations agree and give results that can be compared.

The limiting factor in IDA ICE is that the program gives average values for thezones and the distribution of values over the zone is unknown. But that is also thestrength of the program because by doing so it can simulate with sufficient accuracythe energy usage, airflows and indoor air quality parameters for larger structures.The CFD calculations are therefore used to complement these simulations for moreaccurate analysis of the conditions in the zones. CFD calculations take a lot of timeand computing power. The models tested in with CFD calculations therefore needto be simplified. It would be ideal to simulate the whole building with this techniquebut that is unreasonable considering the size of the models and the computationneeded to solve the calculations. By only looking at smaller zones from the wholebuilding is limiting since natural ventilation is very much dependent on the interac-tion of all the zones and the atmosphere outside. The driving forces therefore needto be simulated accurately with the IDA ICE multi zone model in order to get theright input data to use as boundary conditions for the CFD model.

From IDA ICE the results are used in Autodesk Simulation CFD as boundaryconditions to a standardized module of the office zone to simulate the conditionswithin the zone.

The cooperation of the IDA ICE program and Autodesk Revit was a limitingfactor in this study. Some data is lost when exporting models from Revit to IFC touse in IDA ICE. The zones are exported but openings, windows and doors need tobe modelled manually every time. That limits the amount of variations that can besimulated in IDA, since changing of parameters is fairly easy in Revit, but buildingup the module every time in IDA is time consuming. Perhaps some other programthat can easily change parameters in IFC models could have been more useful forthis study.

There are no field measurements available for this case to validate the resultsso the validation of the programs needs to suffice. The guidelines for the Autodesksoftware do not show how to use the program both to simulate the temperature

1.5. LIMITATIONS 19

and air movements as well as the CO2 concentrations for an indoor environment inan architectural engineering project in the same model. That has limited this CFDstudy to only look at the distribution of the temperature and the velocities in thezones. That is definitely a limitation of this study since it is a useful indicator ofhow the ventilation performs.

The openings and the variation of openings is a factor that could cause inac-curacies. The percentage of the total opening available in IDA is varied betweenfloors to dimension the flow for different pressures. In Autodesk Simulation CFDthe volume flow boundary is assigned to the whole surface of the assumed opening.If the actual decreased area would be used it would require more intricate modelling,changing the opening for every case and unnecessary complexity in the model.

The study will be limited to simulations of the airflows and not the energyconsumption or costs of different solutions. To compare the feasibility of differentoptions, the energy consumption and costs of those options for the whole life cycleof a building need to be considered.

1.5.1 Validation of IDA ICEIDA ICE is used around the world for simulating energy performance of buildings.IDA ICE has been validated with tests and comparison with measured data andother software. It has been validated with respect to some of the commonly usedenergy and environmental standards used, including ASHRAE 140, CEN StandardEN 15255 and 15265 and International Energy Agency SHC Task 34 [13].

1.5.2 Validation of Autodesk Simulation CFDOn the Autodesk homepage there are references to simulations that simulate realcases that are compared to the correct values. The natural convection of air insteady state simulation with buoyancy driven, incompressible flows have been com-pared to test values with sufficient outcome [4].

Chapter 2

Design and Simulations

2.1 Building Regulations

The indoor working environment in Sweden is subject to general regulations fromBoverket and Arbetsmiljöverket. The requirements from those two sources will beused as references to evaluate the performances of the designs using simulations.The occupied zone of a room is specified in BBR as the volume between 0.1 m and2.0 m above the floor and 0.6 m from walls and 1.0 m from doors or windows. TheCFD analysis will be used to check that the conditions within this zone fulfil therules and regulations on the indoor environment from BBR and Arbetsmiljöverket[8].

Temperature in the occupied zone can not be lower than 18 ◦C according to BBRbut there is no upper limit [8]. According to Arbetsmiljöverket the temperaturesshould be between 20-24 ◦C during the winter and between 20-26 ◦C during thesummer [2].

The air velocity can not exceed 0.15 m/s during colder seasons and 0.25 m/sduring other periods [8]. According to Arbetsmiljöverket air velocities of 0.15 - 0.2m/s are experienced by most people as draught free depending on the season, thisneeds only to be considered in the occupied zone of the workspace [2].

Working spaces should have a ventilation system that provides air changes thatremoves pollution from the room and prevents it from further spreading. TheCO2 concentration in air can be used as an indicator for how well the space isventilated. Preferably the CO2 should not rise above 1000 ppm. The danger level ofCO2 concentration is significantly higher, but it indicates poor ventilation and thatlevels of other pollutants might be too high as well as odours start to be noticed toostrongly [2]. For office workspaces or other places where people work sitting downand the people themselves are the main source of pollution it is recommended bythe Arbetsmiljöverket to have a flow of outdoor air of 7 l/s per person and 0.35 l/sper m2 [2].

The required volume airflow is compared to the calculated values from IDA ICE.The openings are dimensioned so that the ventilation fulfils the requirements. The

21

22 CHAPTER 2. DESIGN AND SIMULATIONS

airflow calculated by IDA ICE is then used as an input for the CFD modelling andthe calculated temperature and velocities are then compared to the requirementsto indicate the quality of the indoor air and performance of the ventilation.

When people have control over the ventilation and can influence the indoorclimate themselves there is evidence that they will be more satisfied than in the sameconditions created with automatically controlled mechanical ventilation. There ismore tolerance when there is understanding of how things work [7].

Table 2.1: The relevant regulations

Condition LimitsTemperature winter 20− 24 ◦CTemperature winter 20− 26 ◦C

Velocity winter ≤ 0.15 m/sVelocity summer ≤ 0.25 m/s

CO2 concentration ≤ 1000 ppmAirflow per person 7 l/sAirflow per m2 0.35 l/s

2.2. METHODOLOGY 23

2.2 Methodology

For this study IDA ICE and Autodesk Simulation CFD were used to examine theindoor air quality of the models. The driving forces for a whole building model aremodelled with IDA ICE, a multi zone model that calculates the temperatures inthe zones, airflows and infiltration rates that can be used as boundary conditionsfor the Autodesk Simulation CFD modelling of smaller sections of the model.

2.2.1 Natural Ventilation

Equation 2.1 estimates airflow in buoyancy driven ventilation between two openings:

Q = CdA

√2∆Pρ

= CdA

√2gHd

Ti − To

Ti(2.1)

[1]This only assumes the flow between two openings and neglecting the friction withobstacles inside. When the air is taken in trough many openings on different floorsthat are connected directly or indirectly, the calculations become more complex [27].In such calculations the size of the inlets and outlets, floor height, heat source andtemperature differences all need to be considered. Therefore computer simulationsare needed to calculate this with some accuracy. The pressure difference betweentwo openings relates to the density of the air and the temperature inside and outsideas can be seen in Figure 2.1. As the figure shows, when the outside pressure dropsfaster than the indoor pressure conditions for buoyancy driven natural ventilationare created. The pressure differences drive the airflow. The neutral pressure planeis where the inside and outside atmospheric pressures are the same. In buildingsthat have stack effects in shafts or aria, the zones below the neutral plane will haveunder pressure, meaning that the air from outside will flow in. The zones that lieabove the neutral plane will have overpressure making the air flowing from shaftsor atria to the zones and out through openings. This needs to be observed at in thesimulations.

The airflow caused by pressure differences created by the wind can be estimatedwith a similar simplified formula:

Q = CdAU0

√∆Cp

2 (2.2)

[1]Equation 2.2 only considers a simple flow between two openings with different pres-sure coefficients Cp neglecting the internal obstacles.


Figure 2.1: Pressure difference between openings at different heights.

2.2.2 IDA ICE

IDA ICE is a multi zone modelling calculation method. IDA ICE can simulate theinternal heat gains of equipment, lights and the occupants. With schedules it ispossible to accurately simulate these loads and how they differ over the day. Thecontrol of heating, cooling and ventilation can be according to schedules or can becontrolled by set points, decided by the user. This is very useful since it allows fora model that can calculate the airflows and energy usage for the given conditions.

It is possible to either, simulate the conditions for a synthetic weather conditionsor actual days of the year for the given location. The weather data for Stockholmis from the year 1977 and taken at the Bromma airport, according to the climatefile in the program. It is also possible to simulate for a given fixed temperature.

The hypothetical building that is studied does not have an exact geometricallocation or neighbouring landscape so the automatic values for the pressure co-efficients provided in IDA are used (Appendix A). These pressure coefficients arehandbook data values, from the Air Infiltration and Ventilation Centre [14]. Theprogram uses the temperatures and pressure differences to calculate the airflows.The automatic values can be considered the critical condition with the wind condi-tion set to default urban urban conditions.

2.2. METHODOLOGY 25

2.2.2.1 Calculations

The calculation are done in iterative periods. The simulations are done on a 24hour basis even though the ambient conditions are kept constant to get steady stateconditions with realistic values for the internal gains, i.e. the heat and the CO2emissions. The periods are therefore 24 hour intervals that IDA simulates a certainnumber of times according to the number of periods chosen. The conditions aresimulated in 3 to 4 periods. IDA calculates the energy consumption, temperatures,airflows and indoor air quality parameters.

Airflows IDA differs from many other multi zone models because it can simulatenatural ventilation by taking wind pressure and stack effects into account [14]. IDAcalculates the airflows based on the pressure differences that can be calculated fromthe density of the air which is correlated to the temperatures. The model is made upof nodal points where the openings and cracks are represented as flow resistances.The flow is then calculated with this empirical power law equation:

δmδt

= C ·∆Pn (2.3)

And where there is a 5 Pa or less pressure difference over the opening, which is thedefault limit value

δmδt

= C0 ·∆Pn, |∆P | < dp0 (2.4)

Where:

C0 = C · dpn−10 (2.5)

C is a flow coefficient and has units (kg/s · Pan) and n is a unitless flow exponent.The constant n varies with the Reynolds number of the crack and C is related to thesize of the opening. Both the constants are independent of the pressure differenceand flow rate and this method can be used for multi zone models and has beenstudied and validated [20].

Carbon Dioxide Concentration Calculations IDA calculates the CO2 con-centration for the whole room at each time step of the simulation. It is done bycalculating the balance between the emissions and the airflows through the zone.First the emissions per time step are calculated from occupants and/or equipmentif that is applicable. The mass fraction of CO2 in the air is then calculated fromthe balance between the emissions, the current amount in the air and the airflowsthrough the air terminals, to or from the zone divided by the total mass of air that issimply calculated from the volume of the zone. The mass fraction is then convertedto volume fraction to get the PPM value of CO2 at all times. This only gives onevalue for the whole zone.


Heat Balance Heat balance takes into account the temperature of the incomingairflows, heat and radiation from room units and internal gains, solar radiation fromother zones or from the outside and temperatures of the surfaces. The model thencombines these calculations for all the zones to find the temperature developmentin the whole structure giving the mean calculated value at all times during thesimulation for both mean air temperature and operative temperature.

2.2.3 Autodesk Simmulation CFD

Autodesk Simulation CFD is a commercial program that uses CFD methodologyto calculate the movements and conditions in a fluid at every point in a volume.This means that the conditions are calculated at every point on the mesh that hasbeen defined beforehand. The mesh consists of elements and nodes. The flow ofthe fluid and the heat transfer are governed by partial differential equations. Thoseinclude the Navier-Stokes equations and the energy equation. These equations arecoupled and non linear, meaning that there is no analytical solution available exceptin some rare simple cases. Numerical methods are therefore needed to approximatethe solution.

2.2.3.1 Governing Equations

Continuity Equation:∂ρ

∂t+ ∂ρu

∂x+ ∂ρv

∂y+ ∂ρw

∂z= 0 (2.6)

The Navier-Stokes or the momentum equations in three directions:The moment equilibrium equation in the x - direction

ρ∂u

∂t+ ρu

∂u

∂x+ ρv

∂u

∂y+ ρw

∂u

∂z

= ρgx −∂p

∂x+ ∂

∂x

[2µ∂u∂x

]+ ∂

∂y

[µ

(∂u

∂y+ ∂v

∂x

)]+ ∂

∂z

[µ

(∂u

∂z+ ∂w

∂x

)]+Sω + SDR

(2.7)

The moment equilibrium equation in the y - direction

ρ∂v

∂t+ ρu

∂v

∂x+ ρv

∂v

∂y+ ρw

∂v

∂z

= ρgy −∂p

∂y+ ∂

∂x

[µ

(∂u

∂y+ ∂v

∂x

)]+ ∂

∂y

[2µ∂v∂y

]+ ∂

∂z

[µ

(∂v

∂z+ ∂w

∂y

)]+Sω + SDR

(2.8)

2.2. METHODOLOGY 27

The moment equilibrium equation in the z - direction

ρ∂w

∂t+ ρu

∂w

∂x+ ρv

∂w

∂y+ ρw

∂w

∂z

= ρgx −∂p

∂z+ ∂

∂x

[µ

(∂u

∂z+ ∂w

∂x

)]+ ∂

∂y

[µ

(∂v

∂z+ ∂w

∂y

)]+ ∂

∂z

[2µ∂w

∂z

]+Sω + SDR

(2.9)

Where Sω and SDR are source terms for the rotating flow and distributed resis-tances [5].

2.2.3.2 Discretization

Autodesk simulation CFD uses the finite element method to approximate the solu-tion. The dependent variables are represented by polynomial shape functions overthe elements, that lie between the nodes. The weighted integral over the elementfor the dependent variables are calculated with the shape functions as the weightingfunctions, resulting in equations for the dependent variables at the nodes.

Advection terms are treated with upwind methods as well as the weighted in-tegral method. There are five different advection methods available in AutodeskSimulation CFD. The one used for these simulations was Advection method 2 fromAutodesk Simulation CFD, that uses the Petrov-Galerkin discretization method,recommended for buoyancy driven flows [5].

2.2.3.3 Convergence

The CFD solution is built with an iteration process. The maximum iterations canbe set by the user. Before that iteration is reached the solution should alreadyhave converged. The solution has converged when the components being tested donot change or the change is within a previously decided convergence criteria. Therecommendations from the program are to use 750 iterations and the convergencecriteria set to “Resonable” that means that variations of 1% in the components isconsidered converged [5].


2.3 Modelling

2.3.1 Architectural Models

The objective of the thesis is to look at the conditions for hybrid ventilation inoffice buildings in Stockholm. The model building that is used is an ideal syntheticoffice building in Stockholm built using Autodesk Revit. The parameters used arebased on advice from architects at White Arkitekter AB Stockholm’s office. Thebuilding is made up of two buildings standing parallel to each other and connectedat the ends creating an atrium in the center. That is quite a common design inoffice buildings that allows daylight to enter the center of the building. It can bea place for socializing, can have room for stairs and elevators and it can be usedto create temperature and pressure differences to get the air moving through thestructure. The most important parameters are for example the width of the twomain buildings and the façade elements. The width of office buildings is typically17 - 18 m, because they are usually built on top of a basement for car parking,so the dimensions are depended upon the standard dimensions and combination ofparking spots. The total distance between floors is 3.75 m, that is also a dimensionthat is common in office building projects to allow for installations but also thehigh ceiling height is recommended in designs for naturally ventilated houses [18].The façade is made of elements that are 4.8 m wide, this allows for standardizedindoor dimensions and flexibility for changes in the function of the space. It is veryimportant in the lifetime of an office building to be flexible to facilitate any changesneeded, for example for new tenants or organisational changes. The exact depth ofthe offices in the model are 15.77 m, the width of the two buildings separated byan atrium is 18 m and the balconies are 2.33 m wide. The recommendations fornaturally ventilated spaces that use atria is for the depth to be within five timesthe ceiling height which is satisfied with this design [18].

Figure 2.2: A floor plan of the office set-up.

2.3. MODELLING 29

The closed office spaces, the staircases and the installation rooms and the combi-nations of the furniture inspired by existing designs used to approximate the amountof occupants for the indoor climate evaluations. Shafts are strategically placed ata few points to provide the same effect as the atrium to the spaces that do not lienear the atrium for example the zones at the corners.

2.3.2 IDA ICE

Figure 2.3: IFC model of the office building.

The model used for the IDA ICE simulation represents the same building as theoriginal architectural model but substantial simplifications have been made. Onlythe dimensions and details that are important for the simulation are left in themodel, all unnecessary detailing is therefore removed. The floor plan is dividedup into several zones to represent the office areas in between the closed spaces.The whole area around the atrium could not be a single zone since zones cannotbe enclosed within another zone and the atrium needed to be in the middle. Theenclosed spaces like meeting rooms, toilets and other spaces are not considered inthis model since they most likely will need a separate ventilation solution and cannot use the natural ventilation. The focus in this study will be on the zones inthe middle that can have a direct airflow from the windows through the space andout into the atrium. The number of people in one such zone is assumed to be 36.The density of the people working is determined by considering an interior layoutin existing buildings designed by architects at White.

2.3.2.1 Materials

The materials in the IDA model are representing the material choices in the origi-nal model. The exterior walls are from concrete. The walls and glazing are chosenfrom the available materials in IDA. To simplify the calculations all other compli-cations are omitted. The model is realistic enough to get reliable numbers from thesimulations, and it is used to compare architectural alterations in the model not


Figure 2.4: Floor plan and zones used in IDA ICE.

to compare to real values for now. The definition of the materials can be seen inAppendix A.3.

2.3.2.2 Initial and Boundary Conditions

The synthetic building is located in Stockholm so the location and climate settingsin IDA are set to match that location. The program then uses weather informa-tion from 1977 taken at the Bromma airport [14]. Since the exact surroundingsand interaction with the neighbouring buildings are unknown for the building. Thepressure coefficient settings are set to Auto fill and Semi-Exposed(Appendix A.1).This is very important to the dynamics of the airflow and natural ventilation so thissetting is kept constant in all the simulations for comparison. When simulationsare done on a real buildings the interactions with the surrounding buildings andorientation could be used to optimize the performance even further. The standard-ized pressure coefficients can be used to compare the performance of the differentvariations in the design. The wind pattern used is a default setting for urban envi-ronment available in the program. The weather conditions from the climate files inthe program are used when real dates are simulated. Heating load calculations tocalculate the power needed to maintain the minimum indoor temperature of 21 ◦Cfor open windows for different outdoor temperatures are possible(Appendix A.2).The flow numbers from one office per floor will be used as input into the CFD cal-culations and will be used to check the feasibility of natural ventilation i.e. if theairflow is sufficient, too much or non-existent. The solar radiation clearness number

2.3. MODELLING 31

that is taken into account in the calculations is set to zero, that is there will be nosolar radiation included.

2.3.2.3 Internal Gains

The office zones that are being studied at have 36 occupants. They are consideredto be present between 7 and 17 on weekdays. Lights and equipment are workingon schedule between 7 and 17 on weekdays. The load from lighting and equipmentis according to the automatic setting in IDA, 7.5 W/m2 from equipment and 10W/m2 from lights. The people are considered to be working and sitting down, themetabolic rate 1 MET or 58 W/m2 of the exterior surface of the person, the valueapplied is 108 W per person. The ideal heater, uses district heating and has nogeometric location in the IDA model. It is given an unrealistic maximum power sothat will not be the limiting factor and the room temperature will be kept at 21 ◦C,the minimum set point when the windows are open. If very much power is neededfrom the radiator the natural ventilation option can be considered infeasible for thattemperature. No cooling systems are included in the model, to check the efficiencyof the natural ventilation. Only openings in the façade let in air to ventilate thespaces and provide cooling when the outdoor air temperature is lower than theindoor temperature.

2.3.2.4 Openings

The natural airflow is simulated with openings in the façade and openings fromthe zones into the atrium and shafts. In each zone there is an array of windowsthat if modelled individually would make the modelling heavy and demanding. Thewindows are therefore simulated as two windows with the same total glazing areaas the windows they represent. One of them is closed and the other one can beopened according to a schedule, that is considered to represent all the openings onthe façade. The airflow that IDA simulates into the zone is then considered to enterthrough all the openings, evenly distributed across all all of them. The airflow thatis used as input for the CFD analysis will be divided on some of the windows tosee if the outdoor air spreads to all the occupants. The office on each floor has anavailable opening of 3.636 m2 the airflows can than be adjusted by controlling thepercent to which the opening is opened. The openings can be dimensioned withregards to the BBR regulations and the CFD calculations. The openings used inthe simulations are defined in Appendix A.4.

The program also takes into account the leakage through cracks and or otherholes in the walls. The leakage is defined according to the BBR and ASHRAEstandards for new buildings. That is 0.5 ACH at pressure difference 50 Pa.

2.3.2.5 Ventilation Method

The natural ventilation method is therefore a constant flow of air through the façadefor the conditions available at every time. It could also be possible to have periodical


airing and then let more air in per second. That requires more computationaltime than a model with constant conditions. Periodical openings cannot fulfil theregulations since they specify airflow per second whenever the occupants are present.Shading of windows according to schedule or set-points could be integrated and theindoor temperatures could be kept comfortable for higher outdoors temperatures.

2.3.2.6 Modelling Process

The office model needs to be dimensioned and varied to maximize the capabilitiesof the natural ventilation and for as large part of the year as possible. The originalmodel is run for the temperatures from -5 ◦C to 25 ◦C on a 5 ◦C interval to see theperformance at each temperature. The goal is to find a design that has acceptableindoor air quality and airflows on all the floors.

• Variation 0: That is the original model and it’s input values can be foundin Appendix A

• Variation 1: Increased number of openings in the atrium, see Appendix A.4

• Variation 2: Larger openings on floors 3-6 see Appendix A.4

• Variation 3: Elevated atrium openings the same as in Variation 3

• Variation 4: Elevated atrium and openings varied between floors, openingsdefined in Appendix A.4

• Variation 5: Elevated atrium and openings varied between floors, openingsdefined in A.4

• Variation 6: Elevated atrium, varied openings between floors and windowopenings added in atrium, the top floor has openings to the ceiling instead ofinto atrium. The openings are defined in Appendix A.4

To see the performance in real conditions, two days that have outdoor temper-atures on the range that appears to allow for natural ventilation from the heatingdesign simulations are simulated dynamically.

2.3.3 Autodesk Simulation CFDThe standard office space used to simulate the flow indoors represents a half of thezones used in IDA ICE. The occupants in each zone are 36 so in the CFD modelthere will be 18 people. The geometry is simplified, only the window openings, theoccupants, radiator and the doors or exhaust openings to the atrium are representedin the model. Columns are used to represent the human occupants. The windowopenings are represented by surfaces smaller than the window in the architecturalmodel. The initial location of the opening is high up, that has been shown tobe a good solution for naturally ventilated buildings that take in air through the

2.3. MODELLING 33

façade [6]. How the air enters and how much is dependent on the type of window,orientation and the wind conditions [27]. The airflow from the IDA model wasassumed to be evenly distributed among the openings in each zone along the façade.

The general recommendation when simulating natural airflows with AutodeskSimulation CFD is to have no pressure as a boundary for the exhaust or whereit is expected that the air will leave. The air volume is extended out from thezone and the boundary condition 0 Pa is applied at the end. During the modellingprocess it was noticed in the results that the air appeared to be flowing back intothe simulated space, shown in Figure 2.5. To fix this and represent the actual

Figure 2.5: Backflow of air in CFD simulations.

conditions even better a part of the atrium was included in the model. Adding airvolume with one inlet at the bottom and outlet at the top, to simulate the desiredupward flow of the air in the atrium, as can be seen in Figure 2.7. The air thenappears to be rising from the opening towards the outlet that emulates the stackeffects of the atrium, shown in Figure 2.6.

Figure 2.6: Rising of air into atrium in the CFD simulations.


2.3.3.1 Materials

The exterior wall and the ceiling are represented by solid concrete, the internalwalls by a gypsum board. The radiator is assumed to be from aluminium. A specialmaterial property that represents humans is available in Autodesk Simulation CFD.The floor is assigned the material properties of wood. Material properties are definedin C.1.

Figure 2.7: Materials defined in Autodesk Simulation CFD.

2.3.3.2 Boundary Conditions

The boundary conditions of the model are part constants and some are dependedon the outdoor conditions and are taken from the IDA ICE simulation. The flowinto the zone and the power from the heater, equipment and lights is output fromthe IDA ICE simulation. The heat dissipated by the humans is set as a boundarycondition on the volume representing the human. The assumed heat generation is108 W per person [14] and the heat generated by the equipment is divided amongthe humans. The outlet in the atrium has the boundary condition 0 Pa as wellas an inlet at the bottom and the air that will come in there has the temperature20 ◦C to represent the temperature in the atrium. The heat loss or gain through thefaçade or the external wall are represented with a film coefficient that has the value20 W/m2K recommended by the Autodesk Helpdesk for external layers where theair around it is moving [5]. The film coefficient boundary condition is also given areference temperature and that is the external temperature in each simulation[5].The wall between the office zone and the atrium is left adiabatic. The sides ofthe air volumes perpendicular to the external wall are assigned the slip/symmetry

2.3. MODELLING 35

boundary since they model is representing only a part of a bigger office space. Thedefinition of the boundary conditions can be seen in figure 2.8 and in more detailin Appendix C.2.

Figure 2.8: Boundary conditions defined in Autodesk Simulation CFD.

2.3.3.3 Openings

The air inlets in the model represent openings at the top of or above the windows.The air volume at the openings are extended out so that the airflow is representedmore correctly when it comes into the room [5]. Boundary conditions of volumeflow and temperature are assigned to the end of the extended openings to simulatethe airflow into the room.

2.3.3.4 Meshing

The fluid volume needs to be meshed. The program uses a finite element methodto calculate the conditions at every point of the mesh. The geometries representingthe humans, the radiator and the ceiling need to be meshed as well so the programis able to calculate the heat transfer from the workers, the radiator, lights andequipment. The automatic meshing command is used at first but then the mesh isrefined as needed every time. The mesh is shown in Figure 2.9, and further detailscan be found in Appendix C.3.


Figure 2.9: Mesh as defined in Autodesk Simulation CFD.

2.4 Risks

The most common risk involved with in the design of naturally ventilated buildingsis the risk of discomfort because of draught [24]. There are other risks that can notbe measured or simulated with the simulations but need to be considered in thedesigning of buildings that use natural or hybrid ventilation systems. As previouslymentioned, relying on the natural forces always involves uncertainties because ofchanges in the weather.

Location The location and orientation are very important aspects that need tobe considered when designing a building that is going to rely on natural or hybridventilation. The location and surroundings are the prerequisites that decide if nat-ural or hybrid ventilation are feasible options. The environment and surroundingsshould be used to the designs advantage to enforce or decrease the airflow whenneeded. Risks of air and noise pollution creating discomfort for the occupants aredirectly related to the location and the environment.

When the air is taken in through the façade it is hard to prevent noise from theoutside to be heard indoors. The indoor environment is also exposed to outdoor airpollution. In systems that convey outdoor air through duct systems can use gravityto remove particles and precondition the air. Cleaning incoming air is possible withfilters but that will affect the flow rates and the dynamics of the system significantly.

2.4. RISKS 37

Weather Natural ventilation uses the natural forces provided by the weather ateach time but the weather can also work to the disadvantage of the concept and dodamage for example in storms and high wind velocities. Relying on openings in thefaçade into the occupied zone makes the indoor climate vulnerable and exposed tofrequent changes.

Wind influences the pressure difference that drives the natural airflow and eventhough the building is dimensioned and formed considering the prevailing windconditions, there is always the possibility of other wind directions or extreme casesof very high wind velocities. This could result in too much airflow and risk ofdraught and discomfort as well as increased energy usage. Wind could also changethe dynamics of the indoor airflow preventing it from performing sufficiently.

In places where precipitation and wind coexist, there is risk of water enteringthrough the openings that can damage the interior of the building and the layers inthe building envelope.

Indoor Climate The well being of the occupants, in this case the office workersand their working environment is dependent on the performance of the ventilationconcept chosen. If the ventilation is not working properly it may lead to discomfort,decreased productivity and illnesses. When depending on stack effect through manyfloors there is a risks of the air flowing from the atrium into the office zones on thefloors higher up. The indoor air quality will not be sufficient and humidity andpollution from the lower floors can spread to the upper floors instead of going upand out. Infectious diseases can easily spread if some zones of the building arereceiving the exhaust air from another one.

Safety Designs of natural and hybrid ventilation need to take into considerationthe safety of the building and the occupants, with the regards of burglary risk anddangerous animals getting in. The dimensioning of the openings and placement alsoneeds to consider these risks.

Chapter 3

Results and Discussions

3.1 Results

3.1.1 Combining the Models

The first step after developing the models of the office building and the office zoneswas to make sure that the IDA ICE model and the CFD model agree and the resultscan be compared and evaluated. The values from IDA for the airflow, the powerneeded to maintain the minimum temperature and the outdoor temperatures shouldsimulate similar conditions indoors.

The airflow on the lower floors is very high for the lower temperatures and thepower needed to maintain the minimum temperature at 21 ◦C is high. The spaceshigher up seem to have a more unstable inflow of air because of the smaller heightdifference between the openings there and the outlets on the roof. The openingsare made smaller by setting the opening percentage lower in the whole building.By having a smaller percent of the total opening area the flow was dimensionedto be close to the minimum required flow but still fulfilling the indoor air qualityparameters.

The temperature was used as reference to determine if the CFD model was sim-ulating correctly. In the model that was built for the CFD calculations representinghalf of the office zone looked at in IDA ICE the overall temperatures appear to besimilar to the temperature calculated by IDA ICE. Some deviation can be seen inthe temperatures throughout the room The temperature shown in the CFD calcu-lations results is usually a little bit higher than the mean air temperature but veryclose to the operative temperatures from the IDA simulations. It has been shownin previous studies before that IDA ICE and CFD calculations can differ slightly onthe results for the same input values and same volumes [25]. The strength of IDAICE is that it can produce quite accurate temperature, energy and air flow numbersfor large and complicated models, so even though the CFD calculations do not givethe exact same results they can be used to complement each other.

39

40 CHAPTER 3. RESULTS AND DISCUSSIONS

3.1.2 IDA ICE ResultsThe simulation was run for the original model and recorded, the temperature, CO2airflows and the power of the heater. The empty space in the airflow column meansthat there was no airflow or it was negative, from the atrium and out. For thepower outlet it means that the outside air is warm enough or the internal gainsare sufficient to maintain the minimum temperature and no additional heating isneeded. These numbers need to be compared to the regulations set by BBR andArbetsmiljöverkets. The zone that is being studied is 266m2 and has seats for 36occupants. That means that:

• The air velocities should not exceed 0.15 m/s or 0.25 m/s depending on theseason.

• The required airflow in the zones is 345 l/s for this size of office for 36 people.

• The temperatures should be between 20 − 24 ◦C or 20 − 26 ◦C depending onthe time of the year.

• The CO2 concentrations should be below 1000 ppm.

• The air velocities should not exceed 0.15 m/s or 0.25 m/s depending on theseason.

Results from IDA: The tables show the mean air temperature to be able tocompare the results to the CFD calculations. The operative temperatures give bet-ter information of how the occupants experience the temperature because radiationis taken into account. The operative temperatures for the following simulations arefound in Appendix B.

Variation 0

Table 3.1: Temperature and CO2 concentration for Variation 0

Floor 1 Floor 2 Floor 3 Floor 4 Floor 5 Floor 6Temp. Temp. CO2 Temp. CO2 Temp. CO2 Temp. CO2 Temp. CO2 Temp. CO2[ ◦C] [ ◦C] [PPM] [ ◦C] [PPM] [ ◦C] [PPM] [ ◦C] [PPM] [ ◦C] [PPM] [ ◦C] [PPM]-5 21.0 639 21.0 744 21.0 888 23.6 1137 24.2 1035 23.4 9780 21.0 672 21.0 790.4 21.0 954 24.3 1160 24.9 1085 24.3 10365 21.0 712 21.0 846 22.3 1028 25.3 1211 25.9 1150 25.2 110610 21.6 775 23.3 917 25.0 1051 27.1 1277 27.5 1226 26.8 118815 24.9 817 26.7 978 28.1 1113 29.8 1358 29.9 1305 29.4 126620 28.0 882 29.8 1055 31.0 1199 31.9 1463 32.3 1405 31.6 136625 30.7 983.8 32.4 1167 33.4 1329 34.2 1608 34.2 1545 33.7 1508

3.1. RESULTS 41

Table 3.2: Airflow and power input of the office zones on every floor for Variation 0

Floor 1 Floor 2 Floor 3 Floor 4 Floor 5 Floor 6Temp. Air

flowPower Air

flowPower Air

flowPower Air

flowPower Air

flowPower Air

flowPower

[ ◦C] [l/s] [W] [l/s] [W] [l/s] [W] [l/s] [W] [l/s] [W] [l/s] [W]-5 650 24216 450 15216 310 7826 50 - - - - -0 570 14993 400 8017 290 2151 50 - - - - -5 510 7076 350 1315 250 - 40 - - - - -10 450 - 310 - 210 - 30 - - - - -15 410 - 390 - 200 - 10 - - - - -20 370 - 250 - 170 - 10 - - - - -25 320 - 220 - 140 - - - - - - -

Variation 1

For better flows the openings in the atrium were made larger, having 12 instead of4 0.8x0.8m2˙

Table 3.3: Temperature and CO2 concentration for variation 1 with larger openingsin atrium

Floor 1 Floor 2 Floor 3 Floor 4 Floor 5 Floor 6Temp. Temp. CO2 Temp. CO2 Temp. CO2 Temp. CO2 Temp. CO2 Temp. CO2[ ◦C] [ ◦C] [PPM] [ ◦C] [PPM] [ ◦C] [PPM] [ ◦C] [PPM] [ ◦C] [PPM] [ ◦C] [PPM]-5 21.0 607 21.0 681 21.0 746.2 21.0 900 23.7 1103 23.5 9900 21.0 635 21.0 720 21.0 794.1 21.0 971 24.2 1116 24.1 10295 21.0 674 21.0 771 21.0 858 22.3 1002 25 1154 24.8 108410 21.0 725.2 22.2 838 23.3 929 24.5 1039 26.3 1217 26.0 115915 23.8 780 25.2 911 26.1 997 27.0 1121 28.2 1317 27.7 125720 26.5 865 28.0 1014 28.7 1107 29.2 1255 29.8 1457 20.0 140025 28.9 1099 30.4 1234 30.9 1347 31.4 1564 31.5 1698 31.0 1660

Table 3.4: Airflow and power input of the office zones on every floor for variation 1that uses larger openings in atrium


flowPower Air

flowPower Air

flowPower Air

flowPower Air

flowPower Air

flowPower

[ ◦C] [l/s] [W] [l/s] [W] [l/s] [W] [l/s] [W] [l/s] [W] [l/s] [W]-5 740 29141 550 19746 440 14766 300 7813 40 - - 3920 660 18460 490 11330 400 7582 270 2238 30 - - -5 560 9248 430 4045 350 1186 240 - 20 - - -10 500 822 370 - 300 - 200 - 10 - - -15 450 - 320 - 260 - 160 - - - - -20 370 - 270 - 210 - 140 - - - - -25 270 - 200 - 160 - 70 - - - - -


Variation 2

The same model was than run again with larger openings on floor 3, 4, 5 and 6since there is less pressure differences between those floors and the atrium openingsand thus less airflow.

Table 3.5: Temperature and CO2 concentration for variation 2 with larger openingson floors 3 to 6

Floor 1 Floor 2 Floor 3 Floor 4 Floor 5 Floor 6Temp. Temp. CO2 Temp. CO2 Temp. CO2 Temp. CO2 Temp. CO2 Temp. CO2[ ◦C] [ ◦C] [PPM] [ ◦C] [PPM] [ ◦C] [PPM] [ ◦C] [PPM] [ ◦C] [PPM] [ ◦C] [PPM]-5 21.0 607 21.0 682 21.0 652 21.0 906 23.2 1041 23.1 8720 21.0 636 21.0 721 21.0 681.8 21.0 978.8 23.2 1054 23.1 807.65 21.0 675 21.0 773 21.0 717 22.3 999 24.6 1059 24.3 98710 21.0 727 21.8 726 21.0 734 24.5 1033 25.8 1164 25.5 106715 23.8 782 25.0 914.7 24.3 803 27.0 1114 27.9 1256 27.5 114120 26.5 866 28.0 1015 27.4 893 29.2 1245 29.6 1382 29.8 128225 28.9 1074 30.4 1227 30.3 1125 31.2 1544 31.3 1604 30.9 1513

Table 3.6: Airflow and power input of the office zones for variation 2 on every floorwith larger openings on floors 3 to 6


flowPower Air

flowPower Air

flowPower Air

flowPower Air

flowPower Air

flowPower

[ ◦C] [l/s] [W] [l/s] [W] [l/s] [W] [l/s] [W] [l/s] [W] [l/s] [W]-5 740 28958 550 18495 850 33554 300 7384 60 - - -0 730 18103 550 10934 850 20687 300 2068 60 - - -5 580 9291 430 3817 680 10157 230 - 40 - - -10 500 1078 360 - 570 622 200 - 20 - - -15 450 - 320 - 500 - 160 - - - - -20 370 - 270 - 420 - 130 - - - - -25 270 - 200 - 290 - 70 - - - - -

3.1. RESULTS 43

Variation 3

Since the neutral plane of the pressure needs to be lifted to get more sufficientairflows on the floors higher up. That is done by elevating the atrium and theopenings, creating more height and pressure differences, the changed model can beseen in Figure 3.1.

Figure 3.1: IFC model of the modified office building.

Table 3.7: Temperature and CO2 concentration for variation 3 with elevated atrium

Floor 1 Floor 2 Floor 3 Floor 4 Floor 5 Floor 6Temp. Temp. CO2 Temp. CO2 Temp. CO2 Temp. CO2 Temp. CO2 Temp. CO2[ ◦C] [ ◦C] [PPM] [ ◦C] [PPM] [ ◦C] [PPM] [ ◦C] [PPM] [ ◦C] [PPM] [ ◦C] [PPM]-5 21.0 600 21.0 669 21.0 723 21.0 831 21.1 1322 23.4 10190 21.0 628 21.0 706 21.0 767 21.0 891 22.5 1107 23.8 10515 21.0 667 21.0 758 21.0 830 21.7 960 23.9 1099 24.6 109410 21.0 717 22.0 823 23.0 900 24.1 990 25.7 1161 25.8 116615 23.7 772 25.0 894 25.9 967 26.7 1066 27.8 1264 27.6 126320 26.4 854 27.9 993 28.5 1074 29.1 1191 29.8 1425 29.5 140725 28.8 1070 30.4 1205 30.8 1300 31.2 1456 31.5 1694 31.1 1531


Table 3.8: Airflow and power input of the office zones on every floor for variation 3with elevated atrium


flowPower Air

flowPower Air

flowPower Air

flowPower Air

flowPower Air

flowPower

[ ◦C] [l/s] [W] [l/s] [W] [l/s] [W] [l/s] [W] [l/s] [W] [l/s] [W]-5 760 30036 560 20885 480 16183 350 10688 160 883 - 9700 690 19588 510 12028 430 8659 320 4155 140 - - -5 600 9470 450 4664 370 2021 280 - 110 - - -10 500 1100 380 - 310 - 240 - 80 - - -15 450 - 340 - 280 - 200 - 50 - - -20 370 - 280 - 230 - 160 - 30 - - -25 290 - 210 - 170 - 100 - - - - -

Variation 4

The openings were then varied and kept a little larger on the floors higher up definedin Appendix A.4.

Table 3.9: Temperature and CO2 concentration for variation 4 with elevated atriumand variable openings

Floor 1 Floor 2 Floor 3 Floor 4 Floor 5 Floor 6Temp. Temp. CO2 Temp. CO2 Temp. CO2 Temp. CO2 Temp. CO2 Temp. CO2[ ◦C] [ ◦C] [PPM] [ ◦C] [PPM] [ ◦C] [PPM] [ ◦C] [PPM] [ ◦C] [PPM] [ ◦C] [PPM]-5 21.0 602 21.0 671 21.0 637 21.0 647 21.0 956.4 23.1 8980 21.0 637 21.0 722 21.0 618.8 21.0 691 23.7 1000 23.6 8565 21.0 679 21.0 778 21.0 654 21.0 745 24.4 1017 24.2 90810 21.0 740 21.9 859 21.0 706 22.4 825 25.4 1065 25.0 97615 23.8 802 25.0 911 24.3 758 25.5 893 27.5 1141 27.0 105420 26.6 888 27.9 1080 27.5 1027 28.4 1304 29.6 1428 29.0 136325 29.0 1428 30.4 1363 30.3 1027 30.9 1304 31.3 1428 30.8 1363

Table 3.10: Airflow and power input of the office zones on every floor for variation4 with elevated atrium and variable openings


flowPower Air

flowPower Air

flowPower Air

flowPower Air

flowPower Air

flowPower

[ ◦C] [l/s] [W] [l/s] [W] [l/s] [W] [l/s] [W] [l/s] [W] [l/s] [W]-5 760 29766 560 19228 910 36095 690 26133 280 6135 - 9380 650 18315 490 10957 780 20850 540 12095 60 - - -5 570 8940 430 3660 680 10319 460 3423 50 - - -10 490 627 350 - 550 436 380 - 20 - - -15 410 395 300 - 480 - 300 - 5 - - -20 350 - 250 - 400 - 250 - - - - -25 270 - 200 - 290 - - - - - - -

3.1. RESULTS 45

Variation 5

The openings were then dimensioned further and made linearly larger on every floorand than completely open on the top floor, openings are defined in A.4.

Table 3.11: Temperature and CO2 concentration for variation 5 with elevated atriumand variable openings on every floor


Table 3.12: Airflow and power input of the office zones on every floor for variation5 with elevated atrium and variable openings on every floor


flowPower Air

flowPower Air

flowPower Air

flowPower Air

flowPower Air

flowPower

[ ◦C] [l/s] [W] [l/s] [W] [l/s] [W] [l/s] [W] [l/s] [W] [l/s] [W]-5 790 29705 880 33202 990 36936 910 35770 630 23315 - 11450 700 19225 790 20942 870 23625 800 22738 550 13470 - 6085 600 9630 680 10618 750 12462 700 11598 450 4436 - -10 500 1357 550 1824 600 2509 570 1300 310 - - -15 410 - 450 - 500 - 450 - 160 - - -20 350 - 380 - 420 - 360 - 80 - - -25 260 - 280 - 300 - 220 - - - -


Variation 6

Windows that can be opened are added to the atrium. Floor six that appearsto lie above the neutral pressure plane acted as an exhaust in the previous runs.That is air was flowing out through the zones on floor 6. Even though it performssufficiently regarding the temperature and CO2 concentration the incoming air needsto be from the outside [8]. Cross contamination of air between floors can spread,humidity, other pollution and bacteria. The openings were then dimensioned forevery floor for every temperature to fulfil the requirements for as many cases aspossible. The definition for all the openings can be seen in Appendix A.4.

Table 3.13: Temperature and CO2 concentration for variation 6 with elevated atriumand variable openings on every floor


Table 3.14: Airflow and power input of the office zones on every floor for variation6 with elevated atrium and variable openings on every floor


flowPower Air

flowPower Air

flowPower Air

flowPower Air

flowPower Air

flowPower

[ ◦C] [l/s] [W] [l/s] [W] [l/s] [W] [l/s] [W] [l/s] [W] [l/s] [W]-5 400 12249 470 14902 490 15501 550 19138 440 13545 350 101540 500 11688 450 9600 490 10714 470 10081 550 13702 460 12525 500 6595 550 7116 540 6697 460 5043 480 5590 420 580610 500 1203 460 - 450 - 400 - 450 - 390 -15 440 - 400 - 390 - 350 - 350 - 350 -20 360 - 380 - 350 - 390 - 200 - 300 -25 280 - 280 - 260 - 210 - 60 - 350 -

The two dates are tested on the latest design, Variation 6. The dates thatwere simulated for comparison were 21 st of March and the 14 th of May. Theoutdoor temperatures from the climate file in IDA ICE can be found in AppendixA.7 and the openings used can be seen in Appendices A.5 and A.6. The followinggraphs showing temperature, airflow and IAQ are produced by IDA ICE for everysimulation. The temperature graph shows both the mean air temperatures and theoperative temperatures. The operative temperatures take into account radiationand are calculated for an occupant at the center of the zone with a center of gravity0.6 m above the ground.

3.1. RESULTS 47

Day 1: 21 st of March

Figure 3.2: The temperature development throughout the day for office zone onthe first floor.

Figure 3.3: The temperature development throughout the day for office zone onthe second floor.


Figure 3.4: The temperature development throughout the day for office zone onthe third floor.

Figure 3.5: The temperature development throughout the day for office zone onthe fourth floor.

3.1. RESULTS 49

Figure 3.6: The temperature development throughout the day for office zone onthe fifth floor.

Figure 3.7: The temperature development throughout the day for office zone onthe sixth floor.


Figure 3.8: The airflow development throughout the day for office zone on thefirst floor.

Figure 3.9: The airflow development throughout the day for office zone on thesecond floor.

3.1. RESULTS 51

Figure 3.10: The airflow development throughout the day for office zone on thethird floor.

Figure 3.11: The airflow development throughout the day for office zone on thefourth floor.


Figure 3.12: The airflow development throughout the day for office zone on thefifth floor.

Figure 3.13: The airflow development throughout the day for office zone on thesixth floor.

3.1. RESULTS 53

Figure 3.14: The indoor air quality development throughout the day for officezone on the first floor.

Figure 3.15: The indoor air quality development throughout the day for officezone on the second floor.


Figure 3.16: The indoor air quality development throughout the day for officezone on the third floor.

Figure 3.17: The indoor air quality development throughout the day for officezone on the fourth floor.

3.1. RESULTS 55

Figure 3.18: The indoor air quality development throughout the day for officezone on the fifth floor.

Figure 3.19: The indoor air quality development throughout the day for officezone on the sixth floor.


Day 2: 14 th of May

Figure 3.20: The temperature development throughout the day for office zone onthe first floor.

Figure 3.21: The temperature development throughout the day for office zone onthe second floor.

3.1. RESULTS 57

Figure 3.22: The temperature development throughout the day for office zone onthe third floor.

Figure 3.23: The temperature development throughout the day for office zone onthe fourth floor.


Figure 3.24: The temperature development throughout the day for office zone onthe fifth floor.

Figure 3.25: The temperature development throughout the day for office zone onthe sixth floor.

3.1. RESULTS 59

Figure 3.26: The airflow development throughout the day for office zone on thefirst floor.

Figure 3.27: The airflow development throughout the day for office zone on thesecond floor.


Figure 3.28: The airflow development throughout the day for office zone on thethird floor.

Figure 3.29: The airflow development throughout the day for office zone on thefourth floor.

3.1. RESULTS 61

Figure 3.30: The airflow development throughout the day for office zone on thefifth floor.

Figure 3.31: The airflow development throughout the day for office zone on thesixth floor.


Figure 3.32: The indoor air quality development throughout the day for officezone on the first floor.

Figure 3.33: The indoor air quality development throughout the day for officezone on the second floor.

3.1. RESULTS 63

Figure 3.34: The indoor air quality development throughout the day for officezone on the third floor.

Figure 3.35: The indoor air quality development throughout the day for officezone on the fourth floor.


Figure 3.36: The indoor air quality development throughout the day for officezone on the fifth floor.

Figure 3.37: The indoor air quality development throughout the day for officezone on the sixth floor.

3.1. RESULTS 65

3.1.3 Autodesk Simulation CFD ResultsThe CFD calculations were done for a few of the cases believed to be most criticalin Tables 3.13 and Table 3.14. The office modules that were simulated are 15.77 mlong, 8.2 m wide and 3.5 m high. The scale on the results pictures can be seen fromthe x and y axes, where the numbers shown represent the distance from where theaxes cross in millimetres.

Floor 4 for -5 ◦C

Figure 3.38: The velocities according to the simulation seen in a section betweenthe occupants.

Figure 3.39: The velocities according to the simulation seen in a section throughsome of the occupants.

Figure 3.40: The temperature distribution through the room according to thesimulation seen in a section.


Floor 1 for 0 ◦C




3.1. RESULTS 67

Floor 5 for 0 ◦C





Floor 2 for 5 ◦C




3.1. RESULTS 69

Floor 1 for 10 ◦C





Floor 4 for 10 ◦C




3.1. RESULTS 71

Floor 4 for 15 ◦C



Figure 3.58: The temperature distribution through the room according to thesimulation seen in a section, temperature range on legend up to 30 ◦C, becausetemperature rises above 25 ◦C in some areas.


3.2 Discussion

The results from the original design, seen in Table 3.2 show the stack effects quiteclearly, there is air coming in on the lower floors and going out through the atriumand through the top two floors. That indicates that the program can simulate thephenomena. The performance of this original design regarding indoor air qualityand airflows was not sufficient in all of the zones. The openings were increased tocheck if more air would exit through the atrium instead of through the upper floors.That resulted in some airflows on the fifth floor in the right direction, but verysmall numbers, seen in Table 3.4. The openings were then made different betweenthe floors, making bigger openings where the airflow appears to be too little andsmaller where the airflow was unnecessarily large, the results can be seen in Table3.6. The airflows on the highest floors were still low or in the reversed direction.

The model was then modified to enhance the stack effects so the atrium andother shafts were elevated one storey above the rest of the building. The airflowsincreased everywhere. Tables 3.7 to 3.12 show the results for the elevated atriumfor differently varied openings that are defined in Appendix A.4.

In the last design the openings were altered more but the zones on floor 6 wereclosed off from the atrium and special openings made in the ceiling for each zone.Since the atrium is only one story higher than the last floor, the zones on thesixth floor appear to lie above the neutral point of the pressure difference for alltemperatures and the simulation always showed a flow from the atrium into thezones and out the windows. This might be possible to prevent with an even higheratrium, but for an office building of this type that seems not fitting but could besuitable in some other designs. Air moving up past the openings of the zones on thesixth floor might cause some negative pressure that could help the airflow out andup into the atrium from the highest floors. The calculations depend on pressuredifferences caused by temperature and density differences and not the movementof air. According to the simulation the stack effects cause the air to exit throughthe sixth floor if it is not ventilated separately. The offices on the sixth floor couldalso be single sidedly ventilated but since they are on the top floor the stack effectswithin themselves are utilized to be less dependent on wind.

The performance of the current design can be seen in Tables 3.13 and 3.14. TheIDA ICE simulations show that the required airflows and other indoor air qualityparameters can be fulfilled for outdoor temperatures from -5 to 15 ◦C on all thefloors of the office building. The indoor temperatures are quite high for the casewhere the outdoors temperature is 15 ◦C but still just within Arbetsmiljöverketslimits [2]. The design does not manage to keep the acceptable indoor air quality orthermal comfort for the higher outdoor temperatures. The indoor temperatures aretoo high and the CO2 concentrations rise above the limits on some floors. For thesetemperatures the flow needs to be reinforced with a fan or mechanical ventilationused on some of the floors.

For the dates that have been simulated the openings have been dimensionedaccording to the temperatures to try to keep the flow constant. The natural ven-

3.2. DISCUSSION 73

tilation is working for the date 21 st of March where the temperatures are between1 ◦C and 4 ◦C during the occupancy, see Appendix A.7. The other test simulationfor the 14 th of May where the temperatures are higher shows instabilities in the flowfor the two highest floors. The zones on the fourth floor also have a little bit toolow airflows towards the end of the workday and only the lowest floors have enoughpressure difference to fulfil the airflow requirements for the whole day. Even thoughthe heating simulations suggested that the design would work for up to 15 ◦C thedynamic simulation suggests that when the outdoor temperatures rise above 10 ◦Cthe flow becomes unstable on some of the floors and does not fulfil all the require-ments and needs to be reinforced by fans or taken over by a mechanical system.This dynamic simulation suggest that the airflows are unstable and depend on thethere is a fine balance that needs to be kept in the relationship between the indoorand the outdoor temperatures. When the outdoor temperature rises during the daythe natural airflow becomes more unreliable. In a dynamic situation the pressuredifferences and the flow on the fifth floor appear to be more unstable than in theconstant temperature simulations. Fans or mechanical ventilation are needed onthe upper floors to keep up the same indoor air quality like on the lower floors forthe higher temperatures.

The CFD simulations show that the thermal comfort of the occupants can bekept for the temperatures from -5 ◦C to 10 ◦C. For the case on floor 4 for 15 ◦Cthat the IDA ICE model suggests will work with the minimum airflow, the CFDcalculations show slightly higher values that exceed the limit of 26 ◦C suggested bythe Arbetsmiljöverket. It has been shown in studies before that IDA ICE and CFDcalculations can differ slightly on the results for the same input values and samevolumes [25]. The CFD calculations show that the flow available at 15 ◦C will resultin too high temperatures in the occupied zone on the fourth floor.

All of the velocity figures show that there is a risk of draught for these volumeflows. The highest velocities appear to be closest to the ceiling but there are veloc-ities in the occupied zone and near the simulated people up to 0.3 m/s. Figure 3.39clearly shows too high velocities near the occupant sitting closest to the window. Inthese cases where the velocities are above 0.25 m/s the temperatures are within thelimits of Arbetsmiljöverkets recommendations so there is never too cold air blowingat the occupants, but is can cause discomfort anyway and does not fulfil the require-ments. This is done in a steady state simulation, but as the dynamic simulationsfrom IDA show, the airflow changes throughout the day because of changes in thetemperature. The wind conditions can also influence the flows. All the simulationshave the standardized pressure coefficients and the default urban wind conditionsprovided in IDA ICE. This means that if there is a risk of draught somewhere inthe room it is subject to changes throughout the day.

The CFD simulations suggest that the thermal comfort of the occupants issufficient for the lowest outdoor temperature examined -5 ◦C, but there is high riskof draught in the space. When deciding on for how low outdoor temperatures thenatural ventilation can be used in the risk of draught end heating costs need to betaken into account. It needs to be studied when it is more economical to use the


energy for ventilation rather than heating up the space that is ventilated with thecold air.

Even though natural ventilation has been simulated for many different scenarios,there are always uncertainties when relying on the natural forces. The mean tem-peratures and the prevailing wind directions may be known but the system needsto work for all the other scenarios as well.

Satisfying conditions need to be created on all the floors. Having satisfying airquality on some floors but not others is unacceptable since that could affect thevalue of the property on different levels and the well being of the occupants.

Natural ventilation is always at work in most houses, since they are never com-pletely airtight. Air flows between spaces through openings or cracks and nonairtight parts because of pressure differences. Buildings have been made more andmore impenetrable in recent years to prevent energy losses, that affects the naturalflow of air and the indoor climate.

The simulations from IDA ICE and Autodesk Simulation CFD show that it ispossible to rely on the natural airflow for ventilation in certain conditions for thisdesign, with the risk of draught being present. From these simulations it can beassumed that most probably the natural ventilation needs to be a part of a hybridsolution. It needs to have either fans that can reinforce the flow when necessaryor perhaps a mechanical ventilation that can both provide sufficient airflow in thesummer and provide heated air during the coldest periods.

3.3. CONCLUSIONS 75

3.3 ConclusionsNatural ventilation can work certain parts of the year fulfilling the requirementson indoor air quality, temperatures and required airflows. The higher up in thebuilding the more unstable the airflows are in warmer weathers and they need tobe reinforced.

The CFD models show however that there is a substantial risk of people feelingdiscomfort because of draught, especially for the lowest outdoor temperatures. Thatis a known risk for this type of ventilation method. If people have control over thesituation and are used to the natural ventilation they are however more likely toaccept it.

Hybrid ventilation that can retain sufficient airflows for warmer weathers andprevent draught during the coldest times is therefore a good and environmentallyfriendly ventilation option.

Office buildings that are ventilated with natural or hybrid ventilation are ideallyplaced in quiet suburbia surroundings or have a spacious yard, rather than in areaswhere there is risk of noise and air pollution like in densely built central areas.

The simulation shows that natural ventilation works for this design in certaintemperatures and conditions.


3.4 Suggestions for Future WorkIn this thesis the focus was on if the concept could be modelled and then for whichconditions the IAQ was sufficient. It is important to compare more accurately, theenergy consumption for a naturally ventilated building and mechanical ventilation inthe colder outdoor temperatures. Simulations are needed that compare the energyusage on a yearly basis for natural ventilation, hybrid ventilation and mechanicalventilation to see the advantages and disadvantages of the methods from energyconsumption perspective for Swedish conditions.

More dynamic simulations would be interesting to carry out. That could bedone to find out more accurately when the airflow on the upper floors becomesunstable and if there are other factors than the temperature that influence that.

Energy is needed to heat up the incoming air and energy is lost from the build-ing if hot air exits the building envelope without any heat recovery. Advancedmethods of heating up the incoming air and heat recovery methods would be in-teresting to include in the simulations. More detailed simulations could look intoand simulate more complicated systems, for example including thermal masses andshading of windows and night ventilation. CFD wind analysis can be used to simu-late the relationship with the surroundings and to get accurate pressure coefficientsfor buildings where the surroundings are known.

The CFD calculations of the indoors environment should add CO2 concentrationcalculations to complement the values calculated by IDA ICE and see how it spreadsin the zone more accurately and measure the efficiency of the ventilation.

Technical aspects of the design of the natural part of a hybrid ventilation such asthe one studied in this thesis needs to be researched. Simulations and experimentsshould be done to find out what technical details are most fitting for Swedish climateand conditions. The control strategy of the openings and exact formations of theinlets and outlets need to be determined, they can differ depending on the purposeof the building and the cost for the owners.

Since there is a risk of draught in natural ventilation designs the location of theinlet and outlets relative to the interior layout of the zone is important. A studythat finds out the best location could be done with CFD calculations.

Research on the relation between the real estate space occupied or saved byusing one of the methods compared to the other, using hybrid ventilation insteadof only mechanical should be examined. That relates to the life cycle costs thatshould be done with simulations of the energy usage for the different methods andcompared to real reference cases. There are not many reference objects or researchgoing on in this are in Sweden as opposed to some of the neighbouring countries.

Some of the suggestions from this thesis will be looked at by White Arkitekterin the continuation of the hybrid ventilation project to restore the knowledge andunderstanding of natural and hybrid ventilation in Sweden.

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BIBLIOGRAPHY 79

[27] C. E. Walker. Methodology for the Evaluation of Natural Ventilation in Build-ings Using a Reduced-Scale Air Model. PhD thesis, MIT, MA, USA, 2006.

[28] J. Yu, Q. Ouyang, Y Zhu, H. Shen, G. Cao, and W. Cui. A comparison ofthe thermal adaptability of people accustomed to airconditioned environmentsand naturally ventilated environments. Technical report, Tsinghua University,2011.

Appendix A

Input values for IDA ICE

Input values for IDA simulation and the defined materials

A.1 Default Values in IDA ICEInput values for IDA simulation and the defined materials:

Figure A.1: The default materials and components defined in IDA ICE.

81

82 APPENDIX A. INPUT VALUES FOR IDA ICE

Figure A.2: The infiltration defined in IDA ICE.

A.1. DEFAULT VALUES IN IDA ICE 83

Figure A.3: The pressure coefficients used in IDA ICE.


A.2 Heating Design Default Values

Figure A.4: The set up for the heating design with 100% internal gains and noeffects from the solar radiation

A.3. MATERIALS DEFINED IN IDA ICE 85

A.3 Materials Defined in IDA ICE

Figure A.5: The external wall section in IDA

Figure A.6: The internal wall section in IDA.

Figure A.7: The internal floor section in IDA.

Figure A.8: The external floor section in IDA.

Figure A.9: The roof section in IDA.


Figure A.10: The glazing in IDA.

A.4. IDA OPENINGS 87

A.4 IDA OpeningsThe different openings for the different variations of the IDA ICE model, the windowarea available to open in the model was 3.636m2 , the opening areas are calculatedas a percentage of that.

Figure A.11: The window openings specifications for Variations 1 - 5

Variation 0 had only 3 openings in atrium of size 0.8× 0.8m2 . The other vari-ations have 12, they were assumed to be fully open.


Figure A.12: The openings specifications for Variation 6 different depending onthe ambient temperature in order to get sufficient airflow in as many cases as possibleand not unnecessarily much

The openings are specified as a percentage of the total opening available in theIDA model for every floor, these values can be seen in Figure A.12 below are thecorresponding opening areas in square centimetres.

A.5. OPENINGS FOR 2014.03.21 89

A.5 Openings for 2014.03.21

Figure A.13: The column to the right indicates percentage of the total openingarea used, the central column indicates the total opened area per window


A.6 Openings for 2014.05.14

Figure A.14: The column to the right indicates percentage of the total openingarea used, the central column indicates the total opened area per window

A.7. OUTDOOR TEMPERATURES 91

A.7 Outdoor TemperaturesThe outdoor temperatures for the simulated dates.

Figure A.15: The outdoor temperature from the climate file

Figure A.16: The outdoor temperature from the climate file

Appendix B

Operative Temperatures

Table B.1: Operative temperatures for Variation 0

Outd.Temp.

Floor1

Floor2

Floor3

Floor4

Floor5

Floor6

[ ◦C] [ ◦C] [ ◦C] [ ◦C] [ ◦C] [ ◦C] [ ◦C]-5 23.3 22.7 22.3 23.7 24.3 23.60 22.7 22.4 22.0 24.3 24.9 24.35 22.2 21.9 23.0 25.3 25.9 25.310 22.3 23.8 25.4 27.1 27.5 27.015 25.4 27.1 28.4 29.6 29.9 29.420 28.2 29.9 31.0 31.9 32.0 31.625 30.6 32.4 33.3 34.0 34.0 33.6


Outd.Temp.

Floor1

Floor2

Floor3

Floor4

Floor5

Floor6

[ ◦C] [ ◦C] [ ◦C] [ ◦C] [ ◦C] [ ◦C] [ ◦C]-5 23.5 23.0 22.6 22.2 23.8 23.50 22.9 22.5 22.3 21.9 24.2 24.15 22.3 22.0 21.9 22.9 25.0 24.810 21.8 22.8 23.8 24.9 26.3 26.015 24.2 25.6 26.4 27.1 28.1 27.720 26.6 28.1 28.7 29.2 29.8 29.325 28.6 30.2 30.7 31.0 31.2 30.8

93

94 APPENDIX B. OPERATIVE TEMPERATURES


Outd.Temp.

Floor1

Floor2

Floor3

Floor4

Floor5

Floor6

[ ◦C] [ ◦C] [ ◦C] [ ◦C] [ ◦C] [ ◦C] [ ◦C]-5 23.5 23.0 23.9 22.2 23.4 23.20 22.9 22.5 23.1 22.6 24.0 23.85 22.3 22.0 22.4 22.9 24.6 24.510 21.8 22.5 22.0 24.8 25.8 25.615 24.1 25.4 24.9 27.1 27.8 27.520 26.6 28.0 27.8 29.2 29.6 29.225 28.6 30.2 30.3 31.0 31.1 30.7


Outd.Temp.

Floor1

Floor2

Floor3

Floor4

Floor5

Floor6

[ ◦C] [ ◦C] [ ◦C] [ ◦C] [ ◦C] [ ◦C] [ ◦C]-5 23.6 23.1 22.8 22.4 21.8 23.30 22.9 22.5 22.3 22.0 22.9 23.85 22.3 22.1 21.9 22.4 24.1 24.610 21.8 22.6 23.5 24.5 25.7 25.815 24.2 25.5 26.2 26.9 27.8 27.620 26.5 28.0 28.6 29.1 29.6 29.325 28.5 30.2 30.7 31.0 31.2 30.8


Outd.Temp.

Floor1

Floor2

Floor3

Floor4

Floor5

Floor6

[ ◦C] [ ◦C] [ ◦C] [ ◦C] [ ◦C] [ ◦C] [ ◦C]-5 23.6 23.0 24.0 23.4 22.1 23.10 22.9 22.5 23.1 22.5 23.8 23.75 22.3 22.0 22.4 22.1 24.4 24.310 21.8 22.5 21.9 23.1 25.4 25.215 24.3 25.4 25.0 25.9 27.5 27.120 26.6 28.0 27.9 38.6 29.6 30.625 28.6 30.2 30.3 30.8 31.0 30.6


Outd.Temp.

Floor1

Floor2

Floor3

Floor4

Floor5

Floor6

[ ◦C] [ ◦C] [ ◦C] [ ◦C] [ ◦C] [ ◦C] [ ◦C]-5 23.6 23.9 24.1 24.1 23.2 22.20 22.9 23.1 23.3 23.3 22.6 22.45 22.3 22.5 22.8 22.5 22.1 22.610 21.8 21.9 22.0 21.9 22.9 23.115 24.1 24.4 24.5 24.9 26.0 25.220 26.5 27.4 27.6 27.9 28.8 27.825 28.6 29.9 30.2 30.5 30.9 29.9

95


Outd.Temp.

Floor1

Floor2

Floor3

Floor4

Floor5

Floor6

[ ◦C] [ ◦C] [ ◦C] [ ◦C] [ ◦C] [ ◦C] [ ◦C]-5 22.5 22.6 22.8 23.0 22.6 22.30 22.5 22.4 22.5 22.4 22.6 22.45 22.2 22.2 22.2 22.1 22.1 22.010 21.8 21.8 22.0 22.3 22.0 22.015 24.2 24.9 25.2 25.4 25.4 25.120 26.5 27.5 27.8 28.1 28.4 27.725 28.6 29.9 30.3 30.6 30.7 29.8

Appendix C

Input Values for Autodesk SimulationCFD

Input values for Autodesk Simulation CFD

C.1 Material Properties AutodeskMaterial properties in Autodesk Simulation CFD

Figure C.1: Material properties of air in Autodesk Simulation CFD

97

98 APPENDIX C. INPUT VALUES FOR AUTODESK SIMULATION CFD

Figure C.2: Material properties of aluminium in Autodesk Simulation CFD

Figure C.3: Material properties of concrete in Autodesk Simulation CFD

C.1. MATERIAL PROPERTIES AUTODESK 99

Figure C.4: Material properties of gypsum in Autodesk Simulation CFD

Figure C.5: Material properties of human in Autodesk Simulation CFD


Figure C.6: Material properties of wood in Autodesk Simulation CFD

C.2. BOUNDARY CONDITIONS 101

C.2 Boundary ConditionsInput values for the boundary conditions in Autodesk Simulation CFD

Figure C.7: Boundary conditions defined in Autodesk Simulation CFD.

Figure C.8: Boundary conditions defined in Autodesk Simulation CFD. Here thefluid volume of the air in the atrium and inside the office ate hidden to show theinteriors.


The boundary conditions listed for all the cases, that were simulated

Figure C.9: Boundary conditions defined in Autodesk Simulation CFD for Floor4 and -5 ◦C outdoor temperature.

Figure C.10: Boundary conditions defined in Autodesk Simulation CFD for Floor1 and 0 ◦C outdoor temperature.


C.2. BOUNDARY CONDITIONS 103






C.3 Meshing in Autodesk Simulation CFDThe meshing was kept constant for all the different simulations. Information aboutthe mesh from the message window from Autodesk Simulation CFD:

** FINITE ELEMENT SUMMARY FOLLOWS...166442 Total Nodes, 158997 Fluid Nodes , 7445 Solid Nodes637046 Total Elements, 579287 Fluid Elements , 57759 Solid Elements

Figure C.16: The mesh defined in Autodesk Simulation CFD, the grey volumes aremeshed, they are fluid volumes or volumes that represent heat generating objects,the other volumes shown blue here are suppressed.

Figure C.17: The mesh defined in Autodesk Simulation CFD, the grey volumes aremeshed, they are fluid volumes or volumes that represent heat generating objects,the other volumes shown blue here are suppressed. The volume representing the airinside the office is hidden in this figure to show the interiors.

hybridventilation - divakth.diva-portal.org/smash/get/diva2:725266/fulltext02.pdf · 2014-06-17 ·...

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