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134 M.B. Blarke et al. / Energy and Buildings 50 (2012) 128–138 Table 3 Key results for simulated cycles for Danfoss and Sanyo CO2 heat pumps. Result Carnot efficiency Electricity consumption Heat production Cooling production COP cooling COP heating COP total Unit Danfoss Sanyo – 0.19 0.23 kW 0.61 0.49 kW 2.17 1.83 kW 1.52 1.50 – 2.49 3.07 – 3.54 3.73 – 6.02 6.81 Fig. 5. Heat pump cycles in PV diagrams for R744. Danfoss (top) and Sanyo (bottom). operation according to the weekly report from the US Energy Infor- mation Administration [15]. According to the Californian Energy Commission, the average thermal efficiency of CCGT plants in oper- ation is 53.7% measured for the lower heating value of natural gas. Furthermore, based on international figures [16], we assume a transmission and handling cost for natural gas of 2.21 USD per MWh, and a variable cost of CCGT operation of 3.73 USD per MWh. Thus, CCGT’s short-term marginal cost of operation is found to be 36.95 USD per MWh. According to the marginal system methodology described ear- lier, this implies that for hours where the spot market price is below 36.95USD per MWh, electricity consumption is technically CO2 neutral as illustrated in Fig. 3. 4.2. PG&E’ssystem:tariffs For the analysis using current consumer costs according to PG&E’s tariffs, the standard option applicable to residential single- family dwellings is the E1 tariff schedule. The E1 baseline electricity price for San Jose is USD 0.12233 per kWh composed of generation cost, distribution cost, and additional transmission and utility costs as specified in Table 2. For the analysis using optional Smart Grid tariffs based on PG&E’s day-ahead spot market, the fixed generation cost compo- nent of E1 in Table 2 is replaced by the hourly day-ahead spot market prices illustrated in Fig. 3. The unweighted average elec- tricity price under this real-time pricing schedule is USD 0.11779 per kWh, which is less than 4% lower than the E1 tariff. This sug- gests that the chosen day exhibits an approximately average market performance making it useful for analysis. 4.3. Referencebuildingheatingandcoolingtechnologyand demand In 2003, the “California Statewide Residential Appliance Satura- tion Study” surveyed 21,920 residential customers for information about appliances and end-use among households [17]. For single- family residential buildings in PG&E’s service area, the study finds that 9% use conventional electric resistance heaters for water heat- ing and 39% use conventional central A/C units for space cooling. For the purpose of this study, we assume that a conventional elec- tric resistance heater has a conversion efficiency of 100%, while a conventional central A/C system has an effective COP of 3.0, roughly assuming a CEER 13 standard. For water heating, the survey finds that electricity consump- tion for electrical water heating ranges from 1567 kWh per year in multi-family units in large buildings to 3079 kWh per year in single family houses with an average of 2585 kWh per year, corre- sponding to an average daily electricity consumption of 7.08 kWh. An empirically based hourly distribution of hot water consump- tion is not available for California specifically, but the distribution may be assumed to be rather similar in all developed countries. In this analysis, the hourly demand for hot water is found by heating supply, nor is existing space heating demand considered for replacement. Two cost models are assessed and compared: Current consumer costs according to PG&E’s tariff schedule, and optional Smart Grid costs according to an economic real-time pricing schedule based on PG&E’s day-ahead spot market. 4.1. PG&E’ssystem:loads,intermittentrenewables,marginal producers For the day of July 19, 2011 in PG&E’s system, Fig. 2 plots the hourly electricity loads and the hourly production from intermittent renewables, while Fig. 3 plots the hourly day-ahead spot market prices for electricity. In 2009, PG&E’s generation mix was 35% natural gas (mainly combined-cycle gas turbines (CCGT)), 47% CO2 neutral supply (nuclear, hydro, renewables), 1% coal, and 16% un-specified. We assume that natural gas fired CCGT is the dispatchable marginal producer when the day-ahead spot market price for electricity is above CCGT’s short-term marginal costs, while CO2 neutral producers are marginally dispatched whenever electricity prices are below this threshold. Identifying this threshold, we find that the Californian aver- age natural gas spot price was 15.63 USD per MWh in the week of Table 2 PG&E’s E1 electricity tariff by component. Tariff component Generation (under baseline) Distribution (under baseline) Transmission and other utility costs Total Unit Value USD/kWh 0.03552 USD/kWh 0.03603 USD/kWh 0.05078 USD/kWh 0.12333PDF Image | Thermal battery with CO2 compression heat pump
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