hvac appendix d

8/6/2019 HVAC Appendix D

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03PUBLICATION DATE: Q1 2009

REF NUMBER: HVAC SWGII 006

Analysis of HVAC Control StrategiesHVAC SPECIAL WORKING GROUP

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Table of Contents

1 Introduction 22 Scope 33 Analysis 4

3.1 Strategy 1 43.2 Strategy 2 53.3 Strategy 3 63.4 Strategy 4 73.5 Strategy 5 83.6 Strategy 6 9

4 Comparison 105 Conclusion 12

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1 Introduction

Heating, ventilation and air conditioning (HVAC) systems control the temperature, humidity andquality of air in buildings to a set of chosen conditions. This is achieved by transferring heat andmoisture into and out of the air.

Heating systems increase the temperature in a space. Ventilation systems supply air to the space andextract polluted air from it. Cooling is needed to bring the temperature down in spaces where people,equipment or the sun give rise to heat gains.

With ever-growing energy awareness and rising fuel prices, facilities must shoulder the increasingcosts of maintaining the correct conditions in production and cleanroom areas, while providing thecorrect conditions for employees in other areas of the site. Fuel efficiency and sustainability are nowas important as maintaining comfort in buildings.

As part of SEIs Heating Ventilation and Air Conditioning Working Group, it was proposed that ananalysis of different forms of control strategies used in operating Air Handling Units (AHU) be carried

out. This required modelling and simulating various control options and AHU configurations toinvestigate their impact on energy consumption and running costs. It meant breaking down theenergy required by each section of the unit including the heating coils, cooling coils andhumidifiers based on a years simulation data generated by Integrated Environmental SolutionsVirtual Environment, IES VE. The aim of this study is to illustrate the impact various types of controlstrategies have on the energy consumption and operational costs of air handling units.

A standard AHU system is used in this analysis. It consists of a frost coil, a cooling coil, a heating coiland a humidifier in series, with later additions such as return ductwork, a mixing box andmodulating dampers as different strategies are investigated. Each strategy provides a constantvolume of supply air.

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2 Scope

Six different control strategies were examined as part of the study. Typically supply air is deliveredinto zones to maintain the temperature and humidity at setpoints which are critical to the productionprocess being undertaken in an area. From experience the strategies that were modelled contain themost common configurations of components and operating setpoints that are used in installationsthroughout the country.

Below is a list of the options analysed through modelling and simulation, to give general guidelinesfor energy savings that may be obtained by implementing different control and operationalstrategies in AHUs.

All the options are based on delivering 1m3/sec of air at the specified setpoints.

Strategy 1:The supply-air temperature is fixed at 21C with a zone relative humidity requirement of45%. There is no dead-band on these setpoints and the AHU operates on full fresh air.

Strategy 2:The supply-air temperature is 21C 1C with a zone relative humidity requirement of45% 15%. The AHU operates on full fresh air.

Strategy 3:The supply-air temperature is fixed at 21C with a zone relative humidity requirement of45%. There is no dead-band on these setpoints and the AHU operates on 15% fresh air and 85%return air.

Strategy 4:The supply-air temperature is 21C 1C with a zone relative humidity requirement of45% 15%. The AHU operates on 15% fresh air and 85% return air.

Strategy 5:The supply-air temperature is fixed at 21C with a zone relative humidity requirement of45%. There is no dead-band on these setpoints and the AHU has a modulating fresh-air intake.

Strategy 6:The supply-air temperature is 21C 1C with a zone relative humidity requirement of45% 15%. The AHU has a modulating fresh-air intake.

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3 Analysis

3.1 Strategy 1

The first option has very tight control over the supply-air state. It has a fixed temperature of 21C anda relative humidity of 45%. There is no dead-band on these setpoints, and the incoming air for theAHU is 100% fresh air at a rate of 1m3/sec.

In this mode of operation, the AHU consumed 215,064 kWh of thermal energy at a cost of 10,753

and 12,794kWh of electrical energy at a cost of 1,407 per annum.1 These are the baseline figuresagainst which the other operational strategies will be compared. The figures are illustrated in thefollowing flow diagram. The energy consumption and cost lines are weighted to represent the actualusage of each component.

Figure 1: Strategy 1 component energy consumption and associated costs

1 For the purposes of this analysis, assumed costs of 0.11/kWh for electrical energy and 0.05/kWhfor thermal energy were used.

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3.2 Strategy 2

The second option is very similar to the first, except that the controls on the supply-air setpoints are

loosened. The supply-air temperature is controlled at 21C, with a tolerance of +/- 1C. A relativehumidity of 45%, with a tolerance of +/- 15%, is used as the setpoint. The incoming air for the AHU isstill 100% fresh air at a rate of 1m3/sec.

In this mode of operation, the AHU consumed 170,856 kWh of thermal energy at a cost of 8,543 and7,355 kWh of electrical energy at a cost of 809 per annum. Compared to Option 1, this led to:

a 21% reduction in the thermal energy consumption , a 43% reduction in electrical energy consumption a 23% overall improvement in the operational cost of the unitFigure 2: Strategy 2 component energy consumption and associated costs

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3.3 Strategy 3

The third option includes return ductwork and a mixing box. The supply air comprises 15% fresh air

and 85% re-circulated air. This is controlled in the mixing box. The supply air exiting the AHU has afixed temperature of 21C and a relative humidity of 45%. There is no dead-band on these setpoints.

In this mode of operation, the AHU consumed 80,553 kWh of thermal energy at a cost of 4,028 and12,855 kWh of electrical energy at a cost of 1,414 per annum. Compared to Option 1, this led to:

a 63% reduction in the thermal energy consumption a slight 0.5% increase in electrical energy consumption a 55% overall improvement in the operational cost of the unitThe minor increase in electrical energy consumption is due to (a) the air before the cooling coil beingat a higher average temperature than in Option 1 and (b) the tight restrictions on the relativehumidity of the supply air. Therefore, a larger cooling demand would be required when the moisturecontent of the air needs to be reduced. This is as a result of the mixing with the return air.


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3.4 Strategy 4

The fourth option includes the return ductwork and the mixing box along with the loosening of the

controls on the supply air setpoints. The temperature is controlled to 21C with a tolerance of +/- 1C,while a relative humidity of 45% with a tolerance of +/, while 15% is used as the setpoint.

In this mode of operation, the AHU consumed 62,388 kWh of thermal energy at a cost of 3,119 and8,815 kWh of electrical energy at a cost of 970 per annum. Compared to Option 1, this led to:

a 71% reduction the thermal energy consumption a 31% reduction in the electrical energy consumption a 66% overall improvement in the operational cost of the unitFigure 4: Strategy 4 component energy consumption and associated costs

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3.5 Strategy 5

The fifth option incorporates modulating dampers on the fresh-air intake and the return duct in order

to get the optimum condition of supply air exiting the mixing box. This is done by using enthalpycontrol on the dampers. The supply air exiting the AHU has a fixed temperature of 21C and a relativehumidity of 45%. There is no dead-band on these setpoints.

In this mode of operation, the AHU consumed 71,009 kWh of thermal energy at a cost of 3,550 and13,725 kWh of electrical energy at a cost of 1,510 per annum. Compared to Option 1, this led to:

a 67% reduction in the thermal energy consumption a 7% increase in the electrical energy consumption a 58% overall improvement in the operational cost of the unitThe increase in electrical energy consumption is due to (a) the air before the cooling coil being at a

higher average temperature than in Option 1 and (b) the tight restrictions on the relative humidity ofthe supply air. Therefore, a larger cooling demand would be required when the moisture content ofthe air needs to be reduced. This is as a result of the mixing with the return air.


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3.6 Strategy 6

The sixth option incorporates the modulating dampers along with the loosening of the controls on

the supply-air setpoints. The temperature is controlled at 21C with a tolerance of +/- 1C, while arelative humidity of 45% with a tolerance of +/- 15% is used as the setpoint.

In this mode of operation, the AHU consumed 53,611 kWh of thermal energy at a cost of 2,681, and8,111 kWh of electrical energy at a cost of 892 per annum. Compared to Option 1, this led to:

a 75% reduction in the thermal energy consumption a 37% reduction in the electrical energy consumption a 71% overall improvement in the operational cost of the unitFigure 6: Strategy 6 component energy consumption and associated costs

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4 Comparison

Figure 7 shows the energy consumption of the various strategies. Between Strategy 1 and Strategy 6,the difference is significant:

a 75% reduction in the thermal energy consumed by the AHU a 37% reduction in the electrical energy consumed by the AHUFigure 7: Comparison of energy consumption, from Option 1 to Option 6

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50,000

100,000

150,000

200,000

250,000

EnergyConsumption(kWh)

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Energy Consumption Comparison

Electrical Energy

Thermal Energy

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The reduction in energy consumption is also borne out in a cost comparison of the various strategies,as shown in Figure 8. This illustrates a 71% reduction in the energy cost of delivering a metre cubed ofconditioned air from 12,161 for the annual operation of Strategy 1 to 3,573 for the annualoperation of Strategy 6. This is as a result of including re-circulating ductwork and modulatingdampers, as well as more flexibility in supply-air setpoints.

Figure 8: Comparison of the annual costs of Options 1 to 6

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6,000

8,000

10,000

12,000

14,000

Cost()

1 2 3 4 5 6

Cost Comparison

Electrical Energy

Thermal Energy

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5 Conclusion

The aim of this study is to illustrate the impact various types of control strategies have on the energyconsumption and operational costs of air handling units. As can be seen from the simulation resultsemploying a modulating damper mixing section in conjunction with setpoint deadbands results inthe lowest energy consumption and operational costs of the six strategies investigated. There is a71% reduction the operational cost from 12,161 to 3,573 per metre cubed of air delivered whenStrategy 1 (full fresh air, with no setpoint deadbands) is compared to Strategy 6 (modulating dampercontrol, with temperature and relative humidity deadbands).

The use of recirculation ductwork, modulating dampers and a mixing section in an AHU hassignificant benefits. This operational strategy provides free heating and cooling by obtaining theoptimum mixture of fresh outdoor air and re-circulated return air to achieve the most advantageousair condition exiting the mixing section of the unit. This minimises the load on the heating andcooling coils of the AHU.

For example when you compare Strategy 2 (full fresh air, with temperature and relative humiditydeadbands) to Strategy 6 (modulating damper control, with temperature and relative humiditydeadbands) the benefit of employing modulating damper control to reduce the energy demandedby the AHU is apparent. There is a 62% reduction the operational cost from 9,325 to 3,573 permetre cubed of air delivered.

The study also illustrates the increased operational costs that are incurred as a result of employing aclose control strategy. The strategies with a deadband on the control setpoints show significantly lessenergy consumption and lower operational costs when compared to the same system with nosetpoint deadband. This is illustrated in Error! Reference source not found..

Table 1: Operational cost reductions utilising deadbands.

No Deadband Deadband Percentage ReductionFull Fresh Air(Strategies 1 and 2)

12,161 9,352 23.1%

15% Fresh Air(Strategies 3 and 4)

9,352 5,442 41.8%

Modulating Dampers(Strategies 5 and 6)

5,060 3,573 29.4%

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Sustainable Energy Ireland is funded by the Irish Governmentunder the National Development Plan with programmes

Sustainable Energy Ireland

Glasnevin, Dublin 9, Ireland

Glas Naon, Baile tha Cliath 9, ireann

T. +353 1 8082100 [email protected]

F. +353 1 8372848 www.sei. ie

hvac appendix d

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