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¼Ptimeð _UrecÞPtimeð _Uinp;BÞð17Þwhere _Urec ¼ _ mHW [1] DwHW; _ mHW is the mass water flow rate of hotwater; _Uinp;B is the electric input to the boiler; and DwHW is thedifference in specific exergy between the inlet and outlet of theboiler.DwHW ¼ hHW;outlet
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hHW;inlet
T0 [1]ðsHW;outlet
sHW;inlet Þð18Þ3.7. Estimation of GHG emissionsThe annual equivalent-CO2 emissions, EMISCO2ðkg=yrÞ that aredue to the generation of electricity used by VAV system are esti-mated by using the specific equivalent-CO2 emissions from differ-ent power generation plants in Quebec [23] and the contribution ofdifferent primary energy sources to the electricity mix in Quebec:EMISCO2¼ ðb1Ehydro þ b2Enuclear þ b3Egas þ b4EoilÞ1000ð19Þwhere Ehydro,Enuclear,Egas, and Eoil are the annual off-site primaryenergy input (kWh/yr) to different types of power plants;b1 = 15kton/TWh is the specific equivalent-CO2 emissions (kton/TWh) from hydro power plants with reservoirs, b2 = 15kton/TWhis for nuclear power plans; b3 = 443kton/TWh is for gas-fired powerplants; and b4 = 778kton/TWh is for heavy oil-fired power plants.4. Results and discussionThe simulated annual on-site energy use of the VAV system no.1 is 132.5 kWh/m2, and for the system no. 2 is 103.4 kWh/m2. Thisannual energy performance for heating and cooling is in the samerange with average values of office buildings in Montreal area.While the annual average COP of chillers is about 4.0, the annualCOP of system no. 1 is only 0.99, and it is 1.24 for the system no.2(Table 2). The difference between the COP of chillers and COPsysis due to the parasitic losses (e.g., fans, pumps, and the mixing ofair and water streams) plus the operation of boiler. The annualCOP of VAV systems is of the same order of magnitude as resultspresented by Dun et al. [8]. If the off-site generation of electricityis considered, the annual COP of the whole system drops furtherto 0.72 (VAV no. 1) and 0.90 (VAV no. 2).The annual energy performance of the VAV system no. 2 is bet-ter, by about 26%, than that of system no. 1, due to the use of dis-criminator control that changes the setpoint value of supply airtemperature to satisfy the thermal zone with the highest coolingload. The difference is more apparent in winter and swing months,when the space cooling loads are generally much smaller. TheCOPsys of system no. 2 is higher by about 44% in October andNovember (e.g., 1.28 for VAV no. 2 versus 0.89 for VAV no. 1), whilein July and August the difference is only of about 4%. The differencebetween the two VAV systems, in terms of the COPsys is more sig-nificant when the outdoor air temperature is between