PUBLICATION DATE: Q1 2009 REF NUMBER: HVAC SWGII 006 03 HVAC SPECIAL WORKING GROUP Analysis of HVAC Control Strategies 06
Table of Contents 1 Introduction 2 2 Scope 3 3 Analysis 4 3.1 Strategy 1 4 3.2 Strategy 2 5 3.3 Strategy 3 6 3.4 Strategy 4 7 3.5 Strategy 5 8 3.6 Strategy 6 9 4 Comparison 10 5 Conclusion 12 1
1 Introduction Heating, ventilation and air conditioning (HVAC) systems control the temperature, humidity and quality of air in buildings to a set of chosen conditions. This is achieved by transferring heat and moisture into and out of the air. Heating systems increase the temperature in a space. Ventilation systems supply air to the space and extract 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 increasing costs of maintaining the correct conditions in production and cleanroom areas, while providing the correct conditions for employees in other areas of the site. Fuel efficiency and sustainability are now as important as maintaining comfort in buildings. As part of SEI s Heating Ventilation and Air Conditioning Working Group, it was proposed that an analysis 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 to investigate their impact on energy consumption and running costs. It meant breaking down the energy required by each section of the unit including the heating coils, cooling coils and humidifiers based on a year s simulation data generated by Integrated Environmental Solution s Virtual Environment, IES VE. The aim of this study is to illustrate the impact various types of control strategies 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 coil and a humidifier in series, with later additions such as return ductwork, a mixing box and modulating dampers as different strategies are investigated. Each strategy provides a constant volume of supply air. 2
2 Scope Six different control strategies were examined as part of the study. Typically supply air is delivered into zones to maintain the temperature and humidity at setpoints which are critical to the production process being undertaken in an area. From experience the strategies that were modelled contain the most common configurations of components and operating setpoints that are used in installations throughout the country. Below is a list of the options analysed through modelling and simulation, to give general guidelines for energy savings that may be obtained by implementing different control and operational strategies in AHUs. All the options are based on delivering 1m 3 /sec of air at the specified setpoints. Strategy 1: The supply-air temperature is fixed at 21 C with a zone relative humidity requirement of 45%. There is no dead-band on these setpoints and the AHU operates on full fresh air. Strategy 2: The supply-air temperature is 21 C ± 1 C with a zone relative humidity requirement of 45% ± 15%. The AHU operates on full fresh air. Strategy 3: The supply-air temperature is fixed at 21 C with a zone relative humidity requirement of 45%. 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 21 C ± 1 C with a zone relative humidity requirement of 45% ± 15%. The AHU operates on 15% fresh air and 85% return air. Strategy 5: The supply-air temperature is fixed at 21 C with a zone relative humidity requirement of 45%. There is no dead-band on these setpoints and the AHU has a modulating fresh-air intake. Strategy 6: The supply-air temperature is 21 C ± 1 C with a zone relative humidity requirement of 45% ± 15%. The AHU has a modulating fresh-air intake. 3
3 Analysis 3.1 Strategy 1 The first option has very tight control over the supply-air state. It has a fixed temperature of 21 C and a relative humidity of 45%. There is no dead-band on these setpoints, and the incoming air for the AHU is 100% fresh air at a rate of 1m 3 /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 figures against which the other operational strategies will be compared. The figures are illustrated in the following flow diagram. The energy consumption and cost lines are weighted to represent the actual usage 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/kWh for thermal energy were used. 4
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 21 C, with a tolerance of +/- 1 C. A relative humidity of 45%, with a tolerance of +/- 15%, is used as the setpoint. The incoming air for the AHU is still 100% fresh air at a rate of 1m 3 /sec. In this mode of operation, the AHU consumed 170,856 kwh of thermal energy at a cost of 8,543 and 7,355 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 unit Figure 2: Strategy 2 component energy consumption and associated costs 5
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 a fixed temperature of 21 C 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 and 12,855 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 unit The minor 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 of the supply air. Therefore, a larger cooling demand would be required when the moisture content of the air needs to be reduced. This is as a result of the mixing with the return air. Figure 3: Strategy 3 component energy consumption and associated costs 6
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 21 C with a tolerance of +/- 1 C, 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 and 8,815 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 unit Figure 4: Strategy 4 component energy consumption and associated costs 7
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 enthalpy control on the dampers. The supply air exiting the AHU has a fixed temperature of 21 C and a relative humidity 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 and 13,725 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 unit The 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 of the supply air. Therefore, a larger cooling demand would be required when the moisture content of the air needs to be reduced. This is as a result of the mixing with the return air. Figure 5: Strategy 5 component energy consumption and associated costs 8
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 21 C with a tolerance of +/- 1 C, while a relative 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, and 8,111 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 unit Figure 6: Strategy 6 component energy consumption and associated costs 9
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 AHU Figure 7: Comparison of energy consumption, from Option 1 to Option 6 Energy Consumption Comparison 250,000 200,000 Electrical Energy Thermal Energy Energy Consumption (kwh) 150,000 100,000 50,000-1 2 3 4 5 6 10
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 of conditioned air from 12,161 for the annual operation of Strategy 1 to 3,573 for the annual operation of Strategy 6. This is as a result of including re-circulating ductwork and modulating dampers, as well as more flexibility in supply-air setpoints. Figure 8: Comparison of the annual costs of Options 1 to 6 Cost Comparison 14,000 12,000 Electrical Energy Thermal Energy 10,000 Cost ( ) 8,000 6,000 4,000 2,000 0 1 2 3 4 5 6 11
5 Conclusion The aim of this study is to illustrate the impact various types of control strategies have on the energy consumption and operational costs of air handling units. As can be seen from the simulation results employing a modulating damper mixing section in conjunction with setpoint deadbands results in the lowest energy consumption and operational costs of the six strategies investigated. There is a 71% reduction the operational cost from 12,161 to 3,573 per metre cubed of air delivered when Strategy 1 (full fresh air, with no setpoint deadbands) is compared to Strategy 6 (modulating damper control, with temperature and relative humidity deadbands). The use of recirculation ductwork, modulating dampers and a mixing section in an AHU has significant benefits. This operational strategy provides free heating and cooling by obtaining the optimum mixture of fresh outdoor air and re-circulated return air to achieve the most advantageous air condition exiting the mixing section of the unit. This minimises the load on the heating and cooling coils of the AHU. For example when you compare Strategy 2 (full fresh air, with temperature and relative humidity deadbands) to Strategy 6 (modulating damper control, with temperature and relative humidity deadbands) the benefit of employing modulating damper control to reduce the energy demanded by the AHU is apparent. There is a 62% reduction the operational cost from 9,325 to 3,573 per metre cubed of air delivered. The study also illustrates the increased operational costs that are incurred as a result of employing a close control strategy. The strategies with a deadband on the control setpoints show significantly less energy consumption and lower operational costs when compared to the same system with no setpoint deadband. This is illustrated in Error! Reference source not found.. Table 1: Operational cost reductions utilising deadbands. Full Fresh Air (Strategies 1 and 2) 15% Fresh Air (Strategies 3 and 4) Modulating Dampers (Strategies 5 and 6) No Deadband Deadband Percentage Reduction 12,161 9,352 23.1% 9,352 5,442 41.8% 5,060 3,573 29.4% 12
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