Energy Conversion and Management 76 (2013) 332–341

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Energy Conversion and Management

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Assessing energy and thermal comfort of different low-energy coolingconcepts for non-residential buildings

0196-8904/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.enconman.2013.07.064

⇑ Corresponding author. Tel.: +39 3486937648.E-mail address: graziano.salvalai@polimi.it (G. Salvalai).URL: http://www.polimi.it (G. Salvalai).

Graziano Salvalai a,⇑, Jens Pfafferott b, Marta Maria Sesana a

a Department of Architecture, Built Environment and Construction Engineering, Politecnico di Milano, 20133 Milan, Italyb Offenburg University of Applied Sciences, 77652 Offenburg, Germany

a r t i c l e i n f o a b s t r a c t

Article history:Received 30 January 2013Accepted 26 July 2013

Keywords:Office buildingsThermal comfortLow-energy cooling technologiesDynamic building simulations

Energy consumption for cooling is growing dramatically. In the last years, electricity peak consumptiongrew significantly, switching from winter to summer in many EU countries. This is endangering the sta-bility of electricity grids. This article outlines a comprehensive analysis of an office building perfor-mances in terms of energy consumption and thermal comfort (in accordance with static – ISO7730:2005 – and adaptive thermal comfort criteria – EN 15251:2007 –) related to different coolingconcepts in six different European climate zones. The work is based on a series of dynamic simulationscarried out in the Trnsys 17 environment for a typical office building. The simulation study was accom-plished for five cooling technologies: natural ventilation (NV), mechanical night ventilation (MV), fan-coils (FC), suspended ceiling panels (SCP), and concrete core conditioning (CCC) applied in Stockholm,Hamburg, Stuttgart, Milan, Rome, and Palermo. Under this premise, the authors propose a methodologyfor the evaluation of the cooling concepts taking into account both, thermal comfort and energyconsumption.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

During the last few decades, a trend towards mechanical cool-ing in non-residential buildings has been observed in Europe, evenin moderate and cold climates such as Central and Northern Eur-ope. This is due to high internal gains, especially in modern build-ings and increased comfort expectations [1]. The seriouspenetration of air conditioning has an important impact on theabsolute energy consumption of buildings. Studies have shownthat refrigeration and air conditioning are responsible for about15% of the total electricity consumption in the world [2], while inEurope air conditioning increases in average the total energy con-sumption of commercial buildings to about 40 kW h/m2 year [3,4].To avoid excessive reliance on energy for cooling, there is a need toexamine alternative cooling concepts coupled with passive coolingstrategies for natural cooling of buildings during summer [5]. Aframework that is widely accepted for passive cooling is underthe frame of three steps: prevention of heat gains (solar shadingdevices [6]), modulation of heat gains (thermal mass) and heat dis-sipation (natural ventilation) [7–9]. Existing experience has shownthat passive cooling provides excellent thermal comfort and indoorair quality, together with very low energy consumption [10,11].

New materials, systems and techniques have been developed, ap-plied and now also commercially available [12]. Heat dissipationtechniques deal with the disposal of the excess heat of a buildingto a sink characterized by lower temperature. Effective dissipationof the excess heat depends on two main pre-conditions: (a) theavailability of a proper environmental heat sink with sufficienttemperature difference for the transfer of heat and (b) the efficientthermal coupling between the building and the sink. As also dem-onstrated in this paper, in some European climates the effect ofpassive cooling strategies are low, it is thus necessary to studysolutions that integrate both, passive cooling strategies and lowenergy technologies like hybrid ventilation and radiant coolingsystems coupled with heat pumps.

In this field different researches are carried out in order to ana-lyse the potential of passive and low energy cooling strategies.

Kalz et al. [13] demonstrated that the ground, groundwater,rainwater and the ambient air constitute efficient heat sources/sinks, but the choice of suitable plant components, the accurate de-sign of the hydraulic system and the correct dimension of the envi-ronmental heat source/sink play a central role in achieving higherefficiencies.

Birtles et al. [14] and Kolokotroni et al. [15] showed that usingthe night time ventilation to pre-cool exposed mass inside thebuilding and simple controls based on external temperature, it ispossible to improve the building summer comfort. Schulz andEicker [16] studied the effect of the natural ventilation of buildings

Nomenclature

FC fan coilsNV natural ventilationMV mechanical night ventilationSCP suspended ceiling panelsCCC concrete core conditioningTABS thermo active building componentsPPD predicted percentage of dissatisfiedACH air change hourORT operative room temperatureV(t) volumetric air flow rate m3/h

q air density kg/m3

c specific heat of air J/kg kTroom room air temperature (�C)Tamb ambient air temperature (�C)Tsupply supply water temperature (�C)Pelectric,fan electrical power of fan WOF opening windows factorm2

net net floor area

G. Salvalai et al. / Energy Conversion and Management 76 (2013) 332–341 333

in different climate and found out a cooling energy saving potentialbetween 13 and 44 kW h/m2 year and about 4 kW h/m2 year forelectrical energy for fan ventilation.

Artmann et al. [17] showed a high cooling potential for night-time ventilation over the whole Northern Europe and a significantpotential in Central, Eastern and even some regions of SouthernEurope [18].

Night ventilation reduced the mean room temperature by 1.2 Kduring working hours for a building in Freiburg/Germany [19] andbetween 1.5 and 2 K in la Rochelle/France compared to a referenceroom [20].

Pfafferott [21] and Turrent with Barlex [22] showed a high po-tential on reducing energy use and costs, by up to 80% of the airconditioned solutions, in some energy efficient buildings withlow energy cooling strategies coupled with natural ventilation.As high temperature level technology, radiant cooling systemshas received increasing attention in recent years, especially inNorthern Europe.

Henze et al. [23] and Kalz et al. [24] demonstrated that build-ings with TABS (Thermo Active Building Systems) showed higheroccupant satisfaction with low energy consumption, about 20%less, compared to traditional cooling system solutions.

Miriel et al. [25] and Imanari et al. [26] investigated water ceil-ing panels in terms of comfort and energy consumption and dem-onstrated that they are suitable technology for office buildingswith low thermal load.

Jeong and Mumma [27] observed that the cooling panel capac-ity is enhanced by the mechanical ventilation system installed in aspace within the range of 10–39% when the diffuser discharge airvelocity and the inlet fluid temperature vary within their typicalranges.

Feng et al. [28] demonstrated throughout simulation case stud-ies the high efficiency of the radiant cooling technologies compar-ing to an air system for equivalent comfort conditions (operativetemperature). Zone level 24-h total cooling energy removed bythe RCP system hydronic loop was 5–13% higher, 6–15% higherfor the ESCS, and 6–15% higher for the TABS, due to lower surfacetemperatures at the inside surfaces of the building envelope.

A promising solution to reduce the building energy consump-tion is to design low-energy buildings with synergy within archi-tectural aspects, passive cooling technologies and low energycooling systems. Under this premise, the authors propose an holis-tic approach to evaluate the performances of different cooling sce-narios through primary energy consumption and thermal comfortanalysis all over Europe. These solutions include the integration ofpassive or low-energy strategies: natural and mechanical ventila-tion coupled with concrete core conditioning systems or sus-pended ceiling panels [29–31]. The work presented in this paperis part of the ThermCo project [32] founded by the Intelligent En-ergy Europe in order to evaluate low-energy cooling concepts all-over Europe.

2. Building signature for cooling

The authors analyse the building performances introducing abuilding signatures correlating primary energy consumption[kW hprim/(m2

net year)] and thermal comfort according to the EN15251:2007 (adaptive approach) and to the ISO 7730:2005 (staticapproach). The scales are chosen individually for each criterion:

� cooling energy should not exceed a target of 50 [kW hprim/(m2

net year)];� thermal comfort classes are assigned using EN 15251:2007 and

ISO 7730:2005 considering the comfort Class B (90% of user sat-isfied) since this class resembles good thermal comfort and highuser satisfaction.

3. Building simulation model and performance evaluation

3.1. Methodology and modelling approach

A series of energy simulations to investigate the impacts of thecooling technologies in different European climate has been con-ducted with Trnsys 17 [33] dynamic simulation environment.The authors adopted the following methodology:

� A building model characterized by a sufficient thermalmass, appropriate windows fraction, external shading, win-dows opening and typical internal heat loads (Table 1 – basecase) were studied. Because the construction of each radiantsystem type is different and is highly influential on overallbuilding response, the comparison of NV, MV and FC sce-nario were configured to match the construction of the radi-ant system (CCC and SCP).

� The simulations were carried out for the summer period(from 1st May to 30th September) and their respectiveresults are presented in this paper as a function of the totalworking days (109) and of the working hours (1090 h from8:00 to 18:00). The comfort was evaluated by analysing theoperative room temperature (ORT) of the office space.

� Since no monitoring data were available, the performanceevaluation of each scenario were analysed following thecomparative method.

� The building energy consumption includes the energy forventilation and cooling and were compared consideringthe primary energy related to the net floor area.

� The energy consumption and the thermal comfort studywere carried out for both the East–West and South–Northorientation (Fig. 1). The simulation results presented inthe following study are only related to the East–West orien-tation, chosen by the authors being critical for the SouthEuropean Climate.

Table 1Building physic characteristics of the simulation models.

Case study Building components Building plant Thickness(mm)

Area(m2)

U-value(W/(m2 K))

Heat capacity(kJ/m2 K)

Solar absorptance(–)

Convective heat transfer coefficient(W/(m2 K))

External wall 260 11.70 0.29 260 0.8 18.0 ext – 5.0 intFloor/ceiling 320 20.28 0.36 280 0.6 5.0Windows 4.68 1.4

Base case Shading devices According to the incident solar radiation on surface: the external blinds are rolled down reducing the incomingradiation of 80% when the solar radiation exceeds 200 W/m2

Solar heat gain coefficient 0.59Internal wall 125 15.6 0.45 28 0.6 5.0

Case 1 Floor/ceiling (concrete) FC and NV 320 20.28 0.36 280 0.6 5.0Case 2 Floor/ceiling (concrete) CCC 320 20.28 0.36 280 0.6 5.0Case 3 Floor/ceiling (concrete) SCP 550 11.70 0.50 280 0.6 5.0

(a) Stockholm, office facing east-west for NV scenario [°C] (b) Stockholm, office facing south-north for NV scenario [°C]

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ORT_west ORT_east

EN 15251

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ORT_north ORT_south

EN 15251

Ope

rativ

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ratu

re [°

C]

Ope

rativ

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mpe

ratu

re [°

C]

Running mean ambient temperaure [°C] Running mean ambient temperaure [°C]

Fig. 1. Comparison between simulation results of south–north and east–west building orientation.

Table 2Description of the five cooling conditions under which the simulation has been carried out. All the scenarios are used for all the climate conditions.

Scenario Ventilation strategy Cooling systems

Natural ventilation (NV) Natural night ventilation AbsentThe windows are automatically open if between 18:00and 6:00 Troom > 21 �C and Troom–Tamb > 2 K

Mechanical ventilation (MV) 4.0 ACH during night (from 19:00 to 5:00) 1.3 ACHduring day (from 8:00 to 18:00)

Absent

No heat recovery systems is implemented in the buildingmodel. The MV is activated if the following conditionsare met: Troom > 23 �C and Troom–Tamb > 2 K

Fan-coil (FC) 0.3 ACH during night Fan coil with inlet water temperature of 9 �C during workinghours

1.3 ACH during day (from 8:00 to 18:00) Strategy control: ON if Troom > 24 �CSuspended ceiling panels (SCPs) 0.3 ACH during night Suspended ceiling chilled panels (SCP) with inlet temperature

according to the ambient air temperature [9]Tsupply = 0.35 � (18�Tamb) + 18 (�C) Eq. (1)

1.3 ACH during day (from 8:00 to 18:00) The area of ceiling panel is equal to 19 m2, the mass flow rateis constant and equal to 10 kg/(h m2). Reversible heat pumpcoupled with borehole heat exchangers as cool generation.Strategy control: ON if Troom > 25 �C Time operation: from 8:00to 18:00 during workdays, while in weekend the systems wasconsidered off

Concrete core conditioning (CCC) 0.3 ACH during night Concrete core conditioning (CCC) with inlet temperatureaccording to the ambient air temperatureTsupply = 0.35 � (18�Tamb) + 18 (�C) Eq. (1)

1.3 ACH during day (from 8:00 to 18:00) The area of TABS (Thermo Active Building Systems) is equal to19 m2, the mass flow rate is constant and equal to 10 kg/(h m2) reversible heat pump coupled with borehole heatexchangers for cooling. Strategy control: ON if Troom > 25 �CTime operation: from 22:00 to 6:00 during workdays, whileduring the weekend the systems was considered off

334 G. Salvalai et al. / Energy Conversion and Management 76 (2013) 332–341

From the modelling point of view, the radiant technologies, SCPand CCC, were modelled with the active layer model which usesthe RC-representation implemented by Koschenz and Lehman

[34]. The Trnflow Plug-in [35], that integrates the multi-zone airflow model COMIS [36] into the Type 56, was set-up for the perfor-mance evaluation for both, natural and mechanical ventilation

Fig. 2. Geometric vertical section of the office building model.

Fig. 3. Internal gains of the building model for equipment, lights and occupants.

G. Salvalai et al. / Energy Conversion and Management 76 (2013) 332–341 335

strategies. The windows opening for ventilation and the sun shad-ing device (venetian blind) was set and moved according to inter-nal and external climate conditions analysis (Tables 1 and 2).TheFC system was modelled coupling the Type 52 into the multi-zonebuilding (Type 56) with a vapour compression chiller.

The water to water heat pump was implemented using the Type927 coupled with a vertical borehole heat exchangers (Type 557a).The supply water temperature for CCC and SCP scenarios was cal-culated according to Eq. (1) (see Table 2). The heat pump model isbased on:

I. user-supplied data files containing catalogue data for thenormalized capacity and power draw,

II. entering load and source temperatures,III. normalized source and load flow rates.

(a) Operative room temperature [°C]

18.0

18.5

19.0

19.5

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20.5

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22.0

6/7 10/7 14/7 18/7 22/7 26/7 30/7 3/8

Time

ORT_TRNSYS ORT_IDA_ICE

Ope

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ratu

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C]

Fig. 4. Building model validation through comparative method.

3.2. Model specifications

From the architectural point of view, the building simulationmodel is a simple office space, typical of current design practices,characterised by good values of thermal mass (external wall andfloor), external thermal insulation, external shading devices andHVAC systems to provide ventilation and cooling (the model is de-fined by the authors as building with potential). The building ischaracterized by two zones (length of 5.20 m and width of3.90 m) separated by a corridor (length of 2.60 and width of3.90 m); the height of the space is 3.0 m (Fig. 2). Each zone has awindow of 4 m2 (windows size is about 30% of the wall area) withU-value (window and frame) of 1.4 W/m2 K, and g-value of 59%.The U-values of the building components are reported in Table 1.

The windows have mechanically activated shading devices. Thesolar control is considered as a fraction of the direct radiation onthe vertical surface: the external blinds are rolled down, reducingthe incoming radiation of 80%, when the direct solar radiation onthe windows exceeds 200 W/m2. The users’ zone occupancy isfixed at 10 h, starting at 8:00 until 18:00 during workdays (fromMonday till Friday). Hypothetically the simulations are carriedout considering the thermal internal loads due to occupants, tech-nical equipment, and lights as represented in Fig. 3. During theweekend (unoccupied time) the internal loads are consideredequal to 1 W/m2. The solar gains are not represented varying be-tween the different climate.

3.3. Building model validation

The authors investigated the radiant system (CCC) model accu-racy and its calibration using the comparative method [37,38]. Inparticular, the Trnsys simulation results was compared with theresults from IDA-ICE (Indoor Climate and Energy) tool [39–41],considering the building model as described in the Section 3.2and under the five cooling scenarios. The IDA-ICE building modelsimulation results are also validated by the author comparingmonitoring data campaign [42].

Fig. 4a and b shows the results of the simulations in particulartwo different parameters are analyzed:

– the operative room temperature (ORT) [�C];– the ceiling surface temperature [�C].

The graphics shows a similar trend between the two simulationmodels. In both cases, the heat flux and the interaction of thebuilding components are well modelled. Since Trnsys integrateTrnflow plug-in, well known for its capability to assess natural

(b) Ceiling surface temperature [°C]

18.0

18.5

19.0

19.5

20.0

20.5

21.0

21.5

22.0

6/7 10/7 14/7 18/7 22/7 26/7 30/7 3/8

Time

Tceiling_Trnsys Tceiling_IDA

Cei

ling

Tem

pera

ture

[°C

]

The Trnsys results are compared with IDA-ice output data.

Fig. 5. Schematic diagram of floor with suspended ceiling panels (SCPs) on the right and concrete core conditioning (CCC) on the left.

336 G. Salvalai et al. / Energy Conversion and Management 76 (2013) 332–341

ventilation [43], the authors decide to adopted this building simu-lation environment for the proposed studies.

3.4. Cooling strategies description

The cooling scenarios were represented by different plant sys-tems technology, ventilation and control strategies as describedin Table 2.

The first scenario describes the impact of the natural night ven-tilation (NV) on the building performances. With this aim the win-dows opening (described by the opening factor: OF) was set equalto 0.12 for each climate zone with a correspondent ventilation ratebetween 1.5 and 2.5 ACH. The second scenario evaluated the cool-ing potential of the mechanical ventilation systems, consideringthe impact of the night ventilation on comfort results. The thirdscenario investigated the suitability of the fan coil scenario and isused as reference case, as an example of full air conditioning sys-tem. The fourth scenario analysed the performances of the ceilingpanels systems as an alternative system to classic air conditioningequipment. The ceiling panels were incorporated into the false-ceiling for 70% (14 m2) of the available surface area with an airgap in between equal to 170 mm (Fig. 5a).

In order to allow a privileged downward transfer, a thick insu-lation equal to 20 mm was considered on the back side of the ceil-ing water panels. The water capacity of these panels was equal to2 l/m2 with a mass flow rate of about 15 kg/h m2. The floor is com-posed by the following layers: 10 cm of light concrete, 6 cm ofinsulation and 16 cm of concrete (Fig. 5b). In the fifth scenario,the thermo active building systems (TABS) was implemented inthe model and represented specifically by the concrete core tem-perature control (CCC) system [44,45] with pipes directly embed-ded in the center of the concrete slab.

3.5. Occupant thermal comfort and energy evaluation

According to EN 15251:2007 [46], except for the NV scenario,the building models investigated belong to the category of

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EN 15251

CLA

SS A

CLA

SS B

CLA

SS C

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Running mean ambient temperaure [°C]

Fig. 6. Graphical indication of comfort class A, B and C according to EN 15251.

‘‘mechanically cooled buildings’’. As a result, the thermal comfortshould be evaluated according to EN ISO 7730:2005 [47], with anoperative room temperature of 24.5 �C for the summer period, withthe tolerance range depending on the predicted percentage of dis-satisfied (PPD) occupants: ±1.0 �C, ±1.5 �C and ±2.5 �C correspond-ing to:

I. class A, PPD < 6%;II. class B, PPD < 10%;

III. class C, PPD < 15%.

It is believed that this very inflexible classification into only twobuildings types does not accommodate the necessary requirementson buildings in terms of energy efficiency and sustainability. Thebuilding model investigated constitute a category that allows, onthe one hand, the occupants to influence their surroundings byoperating windows and sun shading devices. On the other hand,the occupants are not required to meet a dress code, so they canadapt their clothing to the indoor conditions. In addition, the build-ings’ energy concepts employ environmental heat sinks, which, infact, prove to be energy-efficient, but, obviously, cannot guaranteethat very stringent specifications on thermal comfort will be met atall times. Deviating from the requirements of EN 15251:2007, thethermal comfort of the buildings is also deliberately evaluated bythe authors according to the adaptive comfort model. Again, thetemperature range defining thermal comfort correlates with usersatisfaction: ±2.0 �C, ±3.0 �C and ±4.0 �C (class A: PPD < 6%; classB: PPD < 10% and class C: PPD < 15%) as represented in Fig. 6.

Whereas the EN ISO 7730:2005 defines temperature settingsindependently of the meteorological conditions, the adaptive ap-proach by EN 15251:2007 evaluates the operative room tempera-ture in relation to the running mean of the ambient airtemperature, Eq. (2.4). The upper comfort boundaries are as fol-lows: boundary ha (Eq. (2.1)), boundary hb (Eq. (2.2)) and boundaryhc (Eq. (2.3)). The running mean ambient air temperature hrm isgiven by Eq. (2.4) as function of the running mean ambient air

Table 3Definition of method for the evaluation of thermal comfort ratings in summer.

Scenario Standard Method

NV–MV–CCC–SCP

EN 15251:2007 Running mean ambient airtemperature greater than15 �C

FC ISO EN 7730:2005 Daily mean ambient airtemperature greater than15 �C

Evaluationperiod

Summer season

Tolerancerange

5% of time of occupancy duringsummer days

Evaluationmethod

Method A (Annex F – EN 15251)exceedance of upper boundariesclass B

Table 4Results from the simulation study in terms of energy consumption [kW hprim/(m2

net year)] and thermal comfort (violation of comfort limit class B) of the office facing west.

Scenario 1 Scenario 2 Scenario 3 Scenario 4 Scenario 5Ventilationstrategy

Natural Night ventilation(NV)

Mechanical ventilation (MV) Mechanical ventilation Mechanical ventilation Mechanical ventilation

Coolingstrategy

– – Fan coil (FC) Ceiling panels (SCP) Concrete core conditioning(CCC)

Energycooling + vent.[kW hprim/m2 y]

Exceedingcomfortlimit class BEN15251 (%)

Energycooling + vent.[kW hprim/m2 y]

Exceedingcomfortlimit class BEN15251 (%)

Energycooling + vent.[kW hprim/m2 y]

Exceedingcomfortlimit class BISO7730 (%)

Energycooling + vent.(kW hprim/m2 y)

Exceedingcomfortlimit class BEN15251 (%)

Energycooling + vent.(kW hprim/m2 y)

Exceedingcomfortlimit class BEN15251 (%)

Stockholm 0 2.1 7.8 0 15.0 0 8.9 0 9.1 0Hamburg 0 1.2 10.6 0 24.7 0 11.6 0 12.2 0Stuttgart 0 12.1 11.9 0 27.3 1.9 13.2 0 13.5 3.2Milan 0 44.1 12.7 30.1 43.1 3.5 14.7 2.2 15.2 8.0Rome 0 49.2 13.2 37.2 52.6 4.3 34.5 8.1 36.2 8.5Palermo 0 75.8 14.1 62.0 69.7 8.2 45.9 8.9 47.5 9.9

G. Salvalai et al. / Energy Conversion and Management 76 (2013) 332–341 337

temperature of the previous days (hrm�1) and the daily mean ambi-ent air temperature of the previous days (hed�1) with a = 0.8.

ha = 0.33�hrm + 18.8 + 2 (2.1)hb = 0.33�hrm + 18.8 + 3 (2.2)hc = 0.33�hrm + 18.8 + 4 (2.3)hrm = (1�a) hed�1 + a hrm�1 (2.4)In this study the authors calculated the exceedance of upper

comfort limits during the summer defined by the with a runningmean ambient air temperature above 15 �C [48]. The authors eval-uated the comfort according to the method A (Annex F – EN15251:2007) calculating the percentage of occupied hours (those

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CLASS B

Ope

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Running mean ambient temperaure [°C]

Fig. 7. Natural ventilation (NV) scenario. Thermal comfort analysis according to EN15251 for Hamburg climate.

(a) Rome, natural night ventilation east-west orientation (NV)

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ORT_west ORT_east

EN 15251

Fig. 9. Natural ventilation (NV) scenario. Thermal comfort analysis accordi

during which the building is occupied) when the operative temper-ature is outside a specified range. The comfort ratings consider thetolerance range of 5%.

In conclusion, the thermal comfort were evaluated as following(Table 3):

� NV, MV, SCP and CCC scenarios – Adaptive approach.� FC scenario EN ISO 7730:2005 – Static approach.

The seasonal energy performances of the technologies werecompared in terms of primary energy [kW hprim/(m2

net year)]

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EN 15251

CLASS A

Fig. 8. Mechanical night ventilation (MV) scenario. Thermal comfort analysisaccording to EN 15251 for Stuttgart climate.

(b) Palermo, natural night ventilation east-west orientation (NV)

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EN 15251

ng to EN 15251 for two climate conditions: Rome (a) and Palermo (b).

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5 10 15 20 25 30Running mean ambient temperaure [°C]

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EN 15251

CLASS C

Fig. 10. Mechanical night ventilation (MV) scenario. Thermal comfort analysisaccording to EN 15251 for Rome climate.

338 G. Salvalai et al. / Energy Conversion and Management 76 (2013) 332–341

considering the delivered energy simulated and the primary en-ergy conversion factors of the European country as defined bythe international standards [49–51].

4. Building signature for cooling technologies: results

Following the approach discussed in Section 1, the authors illus-trated the performance of the buildings strategies in an holistic ap-proach, which correlates the cooling energy consumptions[kW hprim/(m2

net year)], with the thermal comfort performance.The numerical results are summarized in Table 4 (the values ofexceeding hours and energy consumption reported in this sectionare related to the room orientation facing west).

The findings were derived separately for the Northern (Stock-holm, Hamburg), Central (Stuttgart) and Southern Europe (Milan,Rome and Palermo).

(a) Palermo, concrete core conditioning (CCC)

(c) Rome, concrete core

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CLASS B

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EN 15251

CLASS B

Fig. 11. Thermal comfort according to EN 15251 for two different cooling concept in Pale(a and b); concrete core conditioning in Rome (c).

� In the North of Europe (Hamburg, Stockholm), the natural ven-tilation strategy (NV) ensure high comfort levels, the class B isviolated only for 2.1% and 1.2% respectively (Fig. 7).

The MV scenario allows reaching the comfort class A with a en-ergy consumption equal to: 7.8 kW hprim/(m2

net year) in Stockholmand 11.4 kW hprim/(m2

net year) in Hamburg (the values were calcu-lated considering a typical energy demand for fans equal to0.4 W/(m3 h)). In case of the radiant cooling by means SCP, the val-ues of energy consumption calculated were equal to 8.9 kW hprim/(m2

net year) in Stockholm and 11.6 kW hprim/(m2net year) in Ham-

burg. Considering the CCC systems, the energy consumption wasequal to 9.1 kW hprim/(m2

net year) in Stockholm and 12.2 kW hprim/(m2

net year) in Hamburg. The FC scenario allows reaching the max-imum thermal comfort, but involving much more energy:15 kW hprim/(m2

net year) for Stockholm and 24.7 kW hprim/(m2

net year) for Hamburg climate.

� In Central Europe, represented by Stuttgart climate, the NV sce-nario shows a good cooling potential. In particular, the comfortlimit is violated only with a magnitude of 12.1%. These valuescan be easily reduced by increasing the air change rate or usingMV (Fig. 8).

The specific values of energy consumption were estimated asfollowing; considering the radiant cooling strategies (SCP orCCC), the value of energy consumption is lower than thatcalculated for the FC for about 51%. This is due to the relative highwater temperature levels, close to the room air temperature, thatallows the direct use of the ground as heat sink (with a correspon-dent high performance factor). The primary energy consumptionfor these strategies were ranging from 11 to 14 kW hprim/(m2

net year). Moreover, it is notice that the energy needed for the

(b) Palermo, suspended ceiling panels (SCP)

conditioning (CCC)

18

20

22

24

26

28

30

32

5 10 15 20 25 30

Running mean ambient temperaure [°C]

Ope

rativ

e ro

om te

mpe

ratu

re [°

C]

ORT_west ORT_eas

EN 15251

CLASS A

20 25 30

nt air temperaure [°C]

ORT_west ORT_east

rmo and Rome: concrete core conditioning and suspended ceiling panels in Palermo

MN

V

0%

10%

20%

30%

40%

50%

60%

70%

80%

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90

Exce

edin

g ho

urs

[%]

Primary Energy [kWh/m2y]

PALERMO

FC

NV

CC

C

SCP

Comfort evaluation according to ISO 7730

5%

0%

10%

20%

30%

40%

50%

60%

70%

80%

0 5 10 15 20 25 30 35 40 45 50 55 60 65

Exce

edin

g ho

urs

[%]

Primary Energy [kWh/m2y]

ROME

NV

MN

V

CC

C

SCP

FC

Comfort evaluation according to ISO 7730

5%

0%

10%

20%

30%

40%

50%

60%

70%

80%

0 5 10 15 20 25 30 35 40 45 50 55 60

Exce

edin

g ho

urs

[%]

Primary Energy [kWh/m2y]

MILAN

SCP

NV

CC

C

MN

V

FC

Comfort evaluated according to ISO 7730

5%

0%

2%

4%

6%

8%

10%

12%

14%

16%

0 5 10 15 20 25 30 35 40

Exce

edin

g ho

urs

[%]

Primary Energy [kWh/m2y]

STUTTGART

NV

CC

C

MN

V

SCP

FC

Comfort evaluated according to ISO 7730

5%

0%

2%

4%

6%

8%

10%

12%

14%

16%

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

Exce

edin

g ho

urs

[%]

Primary Energy [kWh/m2y]

HAMBURG

NV

MN

V

CC

CSC

P

FC

Comfort evaluated according to ISO 7730

5%

0%

2%

4%

6%

8%

10%

12%

14%

16%

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

Exce

edin

g ho

urs

[%]

Primary Energy [kWh/m2y]

STOCKHOLMM

NVNV

CC

CSC

P

FC

Comfort evaluated according to ISO 7730

5%

Fig. 12. Analysis of different cooling strategies in different climatic zone. Comfort levels and primary energy consumptions of the different cooling technologies for thestudied cities.

G. Salvalai et al. / Energy Conversion and Management 76 (2013) 332–341 339

CCC configuration is higher than the energy required by the build-ing equipped with ceiling panels (about 3%). This is mainly due tothe thermal inertia of the concrete slab.

Using the MV, the energy consumption is equal to11.9 kW hprim/(m2

net year), comparable with the energy absorbedby the CCC or SCP strategies.

� In the South of Europe (Milan, Rome and Palermo), the NV strat-egies have a low cooling potential due to high mean ambient airtemperature, respectively equal to 21.1, 23.1 and 24.6 �C. More-over, the temperature difference between night and day is lowwith a consequent cooling effect reduction. In particular, theexceeding of comfort level is equal about to 75.8% in Palermo,49.2% in Rome and 44.1% in Milan (Fig. 9a and b).

The mechanical night ventilation provides an high discomfortfor the three analysed climate: in Milan the exceeding hours ofcomfort limit is higher than 30.1%, in Rome and in Palermo thecooling efficiency is even lower with respectively 37.2% and62.0% (Fig. 10).

However, the exceeding hours was reduced by 18.3% in Palermoand 24.3% in Rome respect to the NV scenario through an increaseof ventilation air volume (4 ACH during night) and an optimizedcontrol strategy that integrates internal and external air tempera-ture monitoring.

In Palermo and Rome, the CCC scenario allows reaching a com-fort class B, while the class A is reached by using SCP scenario inPalermo (Fig. 11a–c).

Considering the FC scenario, the thermal comfort is violated inPalermo and Rome for 8.2% and 4.3% respectively. The related en-ergy consumption is equal to 43.1, 52.6 and 69.7 kW hprim/(m2

net year)] for Milan, Rome and Palermo respectively.In case of the radiant cooling strategies (SCP or CCC), the value

of energy consumption is lower to the disadvantage of the thermalcomfort expectation (reduced cooling capacity). However, the rel-ative high water temperature levels, close to the room air temper-ature, allows utilizing directly the ground as heat sink.Consequently, the energy consumption for these systems wasequal to 14.7 kW hprim/(m2

net year) for Milan, 34.5 for Rome and45.9 for Palermo for the SCP scenario and 15.2, 36.2 and

R† = 0.9596

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

12 14 16 18 20 22 24 26

Exce

edin

g co

mfo

rt lim

it C

lass

B [%

]

Ambient temperature level [ C]

Hamburg Stockholm

Stuttgart

Milan Rome

Palermo

Fig. 13. Correlation between the frequency distribution of the ambient airtemperature and the exceeding of comfort limit considering the NV scenario underdifferent climate condition.

0

10

20

30

40

50

60

70

80

Stockholm Hamburg Stuttgart Milan Rome Palermo

Prim

ary

ener

gy [k

Wh p

rim/m

2 ] MNV FC SCP CCC

High potential of NV High potential of water based technologiesSCP CCC

Good potential of MNV

Fig. 14. Primary energy consumption for different cooling strategies.

340 G. Salvalai et al. / Energy Conversion and Management 76 (2013) 332–341

47.5 kW hprim/(m2net year) for CCC scenario, respectively. Using the

MV, the energy consumption is equal to 12.7, 13.2 and14.1 kW hprim/(m2

net year).

5. Summary and conclusion

A comprehensive model of a typical architectural office coupledwith five cooling strategies was developed in order to characterizethe most suitable solutions in the Mediterranean area.

The main results are summarized in Fig. 12 in which the x-axisrepresents the primary energy consumptions, while the y-axis rep-resents the comfort level. The combination of these two parame-ters, represented by grey dots, permits to identify graphically thesuitability and the potential of each technology for different cli-mate conditions.The main conclusions, which derives from thebuilding model simulations, are following described.

If the buildings and control strategies are carefully and ade-quately designed, the passive cooling by ventilation strategiescan be successfully employed in Northern and Central Europe. Inparticularly the simulations highlight high efficiency in climateswith an ambient air temperature frequency distribution belowthe threshold of 18 �C for almost 70% of the overall hours (mayto September). Fig. 13 shows the almost linear correlation betweenthe frequency distribution of the ambient air temperature and thecomfort level obtained by the simulations.

The climate of Stuttgart, with 30% of the ambient air tempera-ture above 19 �C, represents the boundary limit in which the NVscenario cannot reach the comfort class B. In this case, it is neces-sary to increase the night ventilation rate (equal to at least 4 ach)

by means of mechanical ventilation. In climate such that of Milan,Rome and Palermo (30% of the hours with temperature level over23.2 �C), the ventilation strategies show low cooling capacity. Theuse of the right windows openings control strategy is crucial in or-der to increase the thermal comfort. The on–off ventilation control,based only on the recorded internal temperature is not sufficientand the control of the external temperature should be included,in order to avoid ventilation with hot ambient air.

To improve the cooling effect, the systems must be activatedonly during night (or when the condition Troom–Tamb > 2 K is met)in order to avoid inappropriate diurnal heat gains. Using the MVstrategy, it is also observed an high impact of electricity consump-tion on building energy balance. This is particularly observed inStuttgart and Milan with an increasing of the energy consumptionequal to 8.6% and 3.4% respectively, comparing to CCC and equal to10.8% and 6.8% respectively, comparing to SCP scenario. In North-ern climate, the use of radiant cooling can reduce the energy con-sumption by approximately 5–8% respect to FC scenario (Fig. 14).

In Milan, both the water based technologies allow reaching thecomfort class A with an exceeding percentage of 2.2 and 8.0 forrespectively SCP and CCC scenarios. The water based radiant sys-tems, coupled with ground heat pump source, represent a goodcompromise between comfort and energy consumption even inhot climates like Rome and Palermo reducing the energy consump-tion by approximately 30–35% in comparison with FC scenario.

The economic feasibility of the cooling technologies analysedare not discussed in this paper, being the main purpose of the pre-sented work is the energy and thermal comfort performance anal-yses. The economic analysis being a fundamental aspect, will beanalysed in a further research paper.

Acknowledgements

The work described in this paper was co-founded by the Euro-pean Commission within the ThermCo project Thermal comfort inbuildings with low-energy cooling – establishing an annex forEPBD-related CEN-standards for building with high energy effi-ciency and good indoor environment under the reference EIE/07/026/SI2.466692.

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