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design operating conditions. The Sanyo compressor is a hermetic 2-stage rolling compressor with a volume ratio of 1.55, an isen- tropic efficiency of 0.698, and a discharge pressure of 100 bars. The Danfoss compressor is a semi-hermetic reciprocating piston compressor with an isentropic efficiency of 0.58 and a discharge pressure of 90 bars. The characteristics of these compressors are applied to the slightly larger capacities required in the case study TB. This is a reasonable assumption at this point, as the TB may in fact utilize several smaller compressors in parallel to allow for better partial load characteristics, or even opt for a large compressor with slightly better characteristics. The heat pump cycles with each of the two compressors have been modeled using Coolpack/EES software [21] for a steady- state mode that simultaneously produces useful hot water at 70 ◦ C and useful cold water at 10◦C. Fig. 5 illustrates the design cycle including steady-state parameters, while Table 3 summarizes key cycle results. The Sanyo compressor provides a heating capacity of 1.87 kW and a cooling capacity of 1.50 kW using 0.49 kW of electric- ity. This corresponds to a heating COP of 3.7, and a cooling COP of 3.0. For simultaneous supply of useful heat and cooling, the Sanyo heat pump reaches a total thermal COP of 6.7, which is the high- est COP of the two compressors. While the choice of compressor for the TB will depend upon various end-use aspects, particularly the ratio between heating and cooling demands in order to min- imize the dumping of thermal production, the higher COP of the Sanyo compressor is here decisive for the subsequent use in the techno-economic case-study. With no storage, the reference system is necessarily operated continuously within each hour. However, the TB is assumed to operate only at full capacity within each hour as devised in the mathematical unit dispatch model above. This reduces the number of on-off switches, thus, allowing for a realistically simulated oper- ation according to steady-state parameters, while also increasing the life time of the compressor. Today, thermal storage tanks are often left without insulation; however, to reduce thermal losses, both thermal storage tanks are insulated with 10 mm BASF water tank foam having a thermal con- ductivity of 0.035 W/(mK). While the material is among the best available options in terms of thermal conductivity, similar low ther- mal conduction losses may be obtained with other materials at greater thickness. Both storages are located indoors at a constant ambient temperature of 20 ◦ C. The hot thermal storage supplies heat to cold potable water at 70 ◦ C with a return temperature from the heat exchanger of 20 ◦ C. The potable water increases in temperature from 8 ◦ C to 60 ◦ C. The cold thermal storage supplies cooling to a chilled water system at 5 ◦ C with a return temperature from the heat exchanger of 15 ◦ C. Thermal losses for empty storage states (20 ◦ C for hot thermal stor- age, 15 ◦ C for cold thermal storage) are insignificant and are ignored for both storages. Key parameters are summarized in Table 4. 5. Techno-economicresults Key techno-economic results are summarized in Table 5 5.1. Thermalstoragesizing Fig. 6 illustrates the relationship between the size of each stor- age and the intermittency-friendliness coefficient Rc as a result of the identified least-cost operational strategy for each option with variable storage capacities. The highest value for Rc is found for a hot storage volume of 200 L and a cold storage volume of 1000 L, which are the volumes used subsequently. 5.2. Electricity consumption profiles and Rc Fig. 7 illustrates the net electricity consumption profiles and accumulated storage content as a result of the identified least-cost operational strategy for each option. It is observed that both TB options dispatch operation to only a few hours, operating at full capacity within these hours and utilizing the storages effectively. For the TB with real-time tariffs, the auxiliary electric resistance heater is utilized during the first few hours of the days; awaiting the low-price hours that allow the TB to operate at full capacity during those hours. On the basis of the electricity consumption profiles, the Rc for the existing option is found to be negative, −0.11, increasing to 0.36 for the TB with E1 tariffs, and increasing further to 0.46 for the TB with real-time tariffs. Thus, the TB offers a significant Rc improvement over the existing system, and the TB with real-time tariffs provides the highest Rc. 5.3. Electricity consumption and system-wide CO2 emissions Due to the preference that the TB with real-time tariffs will have for low price hours, it is likely to exhibit higher thermal losses than the TB with E1 tariffs. In fact, it is found that the electricity con- sumption of the TB with real-time tariffs is 3.42 kWh/day, which is slightly higher than the electricity consumption of the TB with E1 tariffs of 3.33 kWh/day. Still, both TB options offer a reduction in electricity consumption of 63–64% compared to the 9.15 kWh/day consumed by the reference option. However, higher electricity consumption is not necessarily a problem for the TB with real-time tariffs. The ability to support intermittent renewables and reduce emissions are critical crite- ria rather than the electricity consumption. The preference for low price hours correlates with lower CO2 emissions, and the TB with real-time tariffs will always offer similar or lower CO2 emissions than the TB with E1 tariffs. Nevertheless, in this case, both TB options result in zero CO2 emissions as both options only consume electricity during hours in which the day-ahead spot market price for electricity is lower than the short-term marginal cost of CCGT. The existing option emits 2kg of CO2 per day. 5.4. Economic cost of operation The operational cost for the existing option is found to be 1.12 USD per day; for the TB with E1 tariffs, 0.41 USD per day, and for the TB with real-time tariffs, 0.31 USD per day. Thus, both TB options offer significant reductions in operational costs. The TB with real- time tariffs results in a 72% cost reduction compared to the existing option, and a 25% cost reduction compared to the TB with E1 tariffs. 6. Conclusion A Smart Grid enabling concept for providing heating and cooling to buildings in a configuration that supports intermittent renew- ables in the energy system, while minimizing operational costs and CO2 emissions, is introduced and investigated. The so-called thermal battery (TB) converts electricity simul- taneously to hot and cold reservoirs at useful temperature levels using a high-pressure CO2 compression heat pump. An optimized TB design is offered for a proof-of-concept case study in which the TB replaces an existing electrical hot water heater and a central A/C unit, and the techno-economic consequences are evaluated. It is concluded that the TB with real-time tariffs allows for signif- icant improvements in the intermittency-friendliness of operation M.B. Blarke et al. / Energy and Buildings 50 (2012) 128–138 137PDF Image | Thermal battery with CO2 compression heat pump
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