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abstractThis paper presents the exergy analysis of variable air volume (VAV) systems used in office buildings forair-conditioning. The mathematical models are developed using the engineering equations solver (EES)environment. The following indicators are presented: energy efficiency, exergy efficiency, and equiva-lent-CO2 emissions due to the generation of electricity used by the VAV system.[1] 2008 Elsevier Ltd. All rights reserved. 1. IntroductionThe exergy analysis of thermal systems is a powerful tool forassessing the performance and locate irreversibilities. The use ofthis tool may become more important when the concepts of exergyefficiency is related to the sustainable development of buildings.Although today there is no standard method of calculating the in-dex of sustainability of buildings, the use of exergy analysis mayreveal some information about the inefficient use of natural re-sources and where, in a thermal system, the change may havemaximum effect. Only a few publications, however, discussed theapplications of exergy analysis to heating, ventilating and air-con-ditioning (HVAC) systems used in commercial buildings. Energyanalysis programs such as EnergyPlus, TRNSYS and DOE-2 arebased only on the first law analysis. Therefore, this paper addressesthe building simulation using another dimension, which is cur-rently neglected.Some papers presented the exergy analysis of HVAC systems,mostly for small residential buildings. For instance, Wepfer et al.[1] calculated the exergy of different fluids commonly encountedin HVAC applications and presented the exergy analysis of someselected psychrometric processes such as adiabatic mixing anddehumidification. They used the outdoor environment state asthe dead or reference state. Tsaros et al. 39435
[2] presented the exergy flow diagram of a heat pump. Franconi and Brandemuehl [3] com-pared the exergy performance of two types of air distribution sys-tems, the constant air volume and variable air volume (VAV)systems. They concluded that negative exergy load might be calcu-lated for zones that require cooling and where the indoor air tem-perature is higher than the instant outdoor temperature, or forzones that require heating and where the indoor air temperatureis lower than the instant outdoor temperature. Ren et al. [4] eval-uated the performance of evaporative cooling by choosing the deadstate as the saturated state at ambient temperature and pressure.Asada and Takeda [5] used exergy analysis and experimental datato evaluate the radiant ceiling cooling system using well water.Hepbasli and Akdemir [6] calculated the exergy efficiency of aground source heat pump (GSHP) system to be about 3%, whilethe corresponding coefficient of performance (COP) is around 1.7.Ozgener et al. [7] estimated the exergy efficiency of a geothermaldistrict heating system at about 46% and the energy efficiency at42%.Only a few published papers have discussed the energy effi-ciency of the whole HVAC system. For instance, Dunn et al. [8]compared the rated coefficient of performance (COP) of chillerswith the measured daily average COP of four HVAC systems usedin office buildings.
For instance, the daily average COP of a fan coilsystem with a screw chiller of 1275 kW was evaluated at 0.8–1.6,while the rated efficiency of the chiller was 4.46. They also foundby computer simulation, using the DOE-2 program that the sea-sonal COP of a VAV system is 1.19. The literature review did not reveal publications about theapplication of exergy analysis to HVAC systems in commercialbuildings, by considering the whole system that includes heatingand cooling plants, power plant and delivery systems such astransmission lines for electricity.This paper presents the exergy analysis of two VAV systems fora large all-electric office building. Since no building energy analysisprograms can perform this type of analysis, next section presentsthe mathematical models developed for this purpose. The perfor-mance of a VAV system is evaluated by using the following indica-tors: the COP and exergy efficiency of the whole system, the exergyefficiency of each component of the system, and the equivalent-CO2 emissions due to the off-site generation of electricity usedby the VAV system.2. Description of the systemThe description of the original existing office building, which isused in this study, is presented in [9]. The sample building has fiveidentical floors of about 1000 m2each. For simulation purposes,each floor was pided in five thermal zones: one core zone andfour perimeter zones. The VAV system is composed of a centralair-handling unit (AHU), ducts and VAV boxes with hot-water re-heat coils. The AHU contains supply and return fans, a centralhot-water heating coil, a central cooling coil, a mixing chamber,an outdoor air intake and an indoor air exhaust. The return andoutdoor air flows are mixed and treated in the central air-handlingunit, and then supplied to each zone through VAV boxes. Free cool-ing is available by using the air-side economizer control, which re-duces the need for mechanical cooling. The central heating andcooling coils operate to maintain the setpoint temperature of airleaving the AHU. The VAV system controls the zone air tempera-ture by varying the airflow rate rather than varying the zone sup-ply air temperature. At design conditions, the dampers of the VAVboxes are fully opened. As the zone cooling load decreases, thedamper of VAV box modulates until the airflow rate reaches theminimum value that satisfies the ventilation requirements. If thecooling load of a zone continues to diminish, the reheating coil ofcorresponding VAV box modulates the supply air temperature tomaintain the room setpoint temperature. The hot-water loop thatsupplies the heating and reheating coils includes an electric boilerand a constant-speed circulating pump. The chilled water loop thatsupplies the central cooling coil includes a vapor-compression chil-ler, and a constant-speed circulating pump. The condenser waterloop rejects the heat to a cooling tower using a constant-speed cir-culating pump.Design parameters are presented in Table 1 [10]. The nominalcapacity of equipment is selected as follows: the chiller capacity is about 330 kW, the capacity of cooling tower is 420 kW, the heat-ing capacity of the boiler is 250 kW, and the supply and return fanscirculate about 17 m3/s of air each.In this study, two types of VAV systems are separately simu-lated. The system no. 1 uses the conventional constant supply airtemperature, while the system no. 2 uses the discriminatory con-trol where the supply air temperature is controlled in terms ofthe highest cooling load of all spaces.3. Mathematical modelsMathematical models are written for each component as well asfor the whole system. The equations are written in the English-likelanguage of the engineering equation solver (EES) [11] that usesNewton’s methods for solving systems of non-linear algebraicequations. The hourly weather conditions and space thermal loads,which are generated by the DOE-2 program, are input to the EESprogram.3.1. Energy performance of the whole HVAC systemThe coefficient of performance (COPsys) of the HVAC system isdefined as the ratio of the systemthermal loads for heating, coolingand ventilation, Q