15
C and+15
C(Fig. 1). One can also notice the large variation of hourlyCOPsys values in terms of the outdoor air temperature: it varies be-tween 0.8 and 2.5 at +15
C, and between 1.0 and 1.8 at
20
C.The annual average exergy efficiency is 2.9–3.8% at the systemlevel, and 2.15–2.82% at the power plant level (Table 2). The differ-ence between the hourly exergy efficiencies of both systems is al-most constant when the outdoor air temperature varies between
20
C and about +15
C(Fig. 2). The authors did not findTable 2Monthly and annual exergy efficiency and COP of VAV systemsMonth Exergy efficiency (%) Coefficient of performance (COP)System level Power plantlevelSystem level Power plantlevelVAV1 VAV2 VAV1 VAV2 VAV1 VAV2 VAV1 VAV2January 4.5 5.6 3.32 4.21 1.05 1.34 0.76 0.97February 5.4 6.5 4.07 4.87 1.16 1.38 0.84 1.00March 3.9 5.2 2.88 3.88 1.08 1.38 0.78 1.00April 3.1 4.7 2.33 3.47 1.03 1.43 0.75 1.04May 2.1 2.6 1.59 2.00 1.03 1.22 0.75 0.89June 1.6 1.9 1.18 1.37 0.93 1.04 0.67 0.75July 1.8 2.0 1.34 1.44 0.99 1.03 0.72 0.75August 1.6 1.8 1.18 1.33 0.97 1.02 0.70 0.74September 1.9 2.5 1.38 1.86 0.88 1.13 0.64 0.82October 2.5 3.7 1.83 2.77 0.89 1.28 0.64 0.93November 2.6 3.8 1.87 2.78 0.87 1.27 0.63 0.92December 3.8 5.2 2.83 3.86 0.98 1.33 0.71 0.96Average 2.9 3.8 2.15 2.82 0.99 1.24 0.72 0.90 published data about the exergy efficiency of VAV systemssupplied by the same electricity mix. The results presented in thispaper are, however, in the same range with average published val-ues of exergy efficiency of commercial and residential buildings.Alpuche et al. [24] calculated the exergy efficiency of packagedair-conditioning units at 2.5–6.3%, by using the hourly outdoorair temperature and humidity as the reference state. Utlu and Hep-basli [25] compiled results from other studies that estimate the na-tional exergy efficiency in the residential-commercial sector at8.7–9.7 (Saudi Arabia), 3% (Japan), 2% (Italy), 8% (Finland), 10%(Sweden), 14% (USA) and 15% (Canada). Saidur et al. [26] estimatedthe exergy efficiency of air-conditioning systems in Malaysia at2.3% by using statistical national data and information from similarstudies; the reference temperature was set at 25
C.Table 3 presents the exergy efficiency of major components ofthe VAV system no. 2, the most energy efficient system. The elec-tric boiler has the lowest annual exergy efficiency of 10.8% fol-lowed by the vapor-compression chiller with 22.8%. The use ofelectricity as an energy source for heating or cooling in a HVAC sys-tem is not a thermodynamically sound choice, and it should be re-placed by other sources such as waste heat, solar or geothermalenergy, in order to match the quality of energy source with thedemand.The annual GHG emissions are estimated at 47.9 tons of eq-CO2/yr or about 9.6 kg/m2of conditioned floor area or 240 kg/person(the case of VAV system no. 1), and 37.4 eq-CO2/yr or about 7.5kg/m2or 190 kg/person (in the case of second VAV system). This study found that the operation of the VAV system no. 1 generatesan amount of annual equivalent-CO2 emissions that is equivalentto about 6.3 Volkswagen Jetta car of 2.5 l, model 2006, while thesecond VAV system generates annual emissions equal to about fivecars [27].5. ConclusionsIn this study, energy and exergy analyses are utilized for evalu-ating two types of VAV systems operating in a large office buildingin Montreal. Although the COP of the system already shows a verylow performance according to the 1st law, the exergy efficiencyshows an alarming low value: only 2–3% of potential work thatcan be developed by using those energy sources is supplied for sat-isfying the environmental thermal conditions for human occu-pancy and indoor air quality in this case study office building.The remaining 97–98% is wasted. The results are even more sur-prising in the context of electricity mix of Quebec, where the hy-dro-power accounts for 95.4% of energy sources used in thegeneration of electricity.The VAV system is considered to be one of the most energy effi-cient systems used today in office buildings [28] because it elimi-nates or significantly reduces the simultaneous cooling andheating of the supply air, and also reduces the energy use for theair transportation at part load conditions. However, the resultsshow that either there is a large potential for improvement of thissystem that is currently not exploited, or other design and opera-tion approaches must be adopted. For instance, less use of air-con-ditioning systems and more use of the natural or hybrid ventilationfor cooling purposes in some climatic zones may avoid the waste ofenergy resources.The largest improvement of exergy efficiency of the system canbe obtained by changing the heating source, from electricity torenewable energy sources such as solar or geothermal. The use ofmechanical cooling in cold climate should be more questioned,and if it cannot be prevented then other means should be usedsuch as absorption chillers with waste heat or solar energy. In addi-tion, the recovery of heat rejected by the condenser should becomemore common in large HVAC systems.Exergy analysis should be combined with the energy analysis toevaluate the performance of buildings and HVAC systems, andeventually should be integrated in most building energy analysisprograms such as EnergyPlus and TRNSYS. AcknowledgementThe authors acknowledge the support received from NaturalSciences and Engineering Research Council of Canada and fromFaculty of Engineering and Computer Science of ConcordiaUniversity.References[1] Wepfer WJ, Gaggioli RA, Obert EF. Proper evaluation of available energy forHVAC. ASHRAE Trans 1979;85(1):214–30.[2] Tsaros TL, Gaggioli RA, Domanski PA. Exergy aalysis of heat pumps. ASHRAETrans 1987;93(2):1781–97.[3] Franconi EM, Brandemuehl MJ. Second law study of HVAC distribution systemperformance. ASHRAE Trans 1999;105(1):1237–46.[4] Ren CQ, Li NP, Tang GF. Principles of exergy analysis in HVAC and evaluation ofevaporative cooling schemes. Build Environ 2002;37:1045–55.[5] Asada H, Takeda H. 2002. Thermal environment and exergy analysis of a ceilingradiant cooling system. Sustainable Building Conference; 2002. slo.[6] Hepbasli A, Akdemir O. 办公楼变风量空调系统英文文献和中文翻译(5):http://www.youerw.com/fanyi/lunwen_39732.html