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Energies 2019, 12, 457 26 of 35 radiation. The results showed that changing the operating temperature from 40 ◦C to 48 ◦C resulted in about a 20% increase in the compressor pressure ratio, which caused an increase of the compressor power consumption from 4.5 to 5.3 kW. Additionally, with the rise in daily solar radiation from 1.0 to 20 MJ/m2, the heat in the collector increased by around 50%. It was suggested to consider the operational characteristics and design parameters while designing a CO2 GSAHP to increase system reliability and reduce energy consumption. 5. CO2 Heat Pump Applications CO2 HPs are environmentally-friendly and energy-efficient, which makes them popular for various applications such as hot water production, space heating, and dehumidifying (drying). Moreover, a single CO2 HP unit can be utilized to serve for multiple purposes such as simultaneous heating and cooling. In this section, a comprehensive overview is presented on recent developments in CO2 HPs for different applications. 5.1. Water Heating Roughly one-half of the households in the USA use natural gas and one-third use electricity for space heating, which accounts for two-thirds of the utility bill in a typical US house [130]. Water heating accounts for roughly one-fifth of the energy consumption in a typical residential building in the USA [130]. Hence, water heating is the main purpose of a CO2 HP for providing both service hot water and space heating. Based on the heat transfer performance of each component in a CO2 HP water heater, Yamaguchi et al. [21] carried out a numerical simulation showing that the maximum attainable COP was 3.6 for a given water inlet temperature of 8 ◦C and an outdoor air temperature of 44 ◦C. The effects of the inlet water temperature and outside air temperature on the system characteristics were analyzed in the study. The study concluded that the CO2 HP water heater instead of an electric or gas water heater can lead to a significant reduction in the primary energy consumption. In addition, a CO2 HP water heater (HPWH) prototype [135] is being developed in Oak Ridge National Laboratory (ORNL) [136], which meets ENERGY STAR standards [137] for HPWHs at a low cost. The main design feature is that the prototype uses an optimized wrap-around gas cooler (roll flexible HX) instead of an external HX. Unlike the external HX, with a wrap around HX, no fouling can occur and there is no need for a pump to drive the flow. ORNL estimated that the CO2 HPWH can reduce energy use by 0.8 quads a year, whereas the annual energy consumed by electric water heaters are approximately 1.38 quads. Tammaro et al. [138] compared two air-to-water natural refrigerant HPs including one that utilized CO2 and the other used propane as the refrigerant. The results suggested that propane offers better energy savings when used in large HP systems for sanitary hot water production in warm and average climates. However, under cold climates, both delivered a similar energy performance. Similarly, Saikawa et al. [139] carried out a thermodynamic analysis of an HP using the fluorocarbons and natural refrigerants. The theoretical study illustrated that, among all the refrigerants tested, the CO2 HP had the highest COP and the theoretical limit was 6.0 from a technical viewpoint. They developed a prototype for residential hot water production, which could attain a COP of 3.0, and emphasized on an energy recovery in the expansion process and the efficiency improvement of the compressor and HXs to further improve the prototype’s COP. 5.2. Drying In a CO2 HP, cooling capacity can be utilized to dehumidify the moistened air, while the heating capacity heats the cold air. Klocker et al. [140] experimentally studied a CO2 HP dryer and compared its performance with a conventional electric resistance dryer. The results showed that the CO2 HP dryer could reach a temperature of about 60 ◦C whereas the typical electric resistance dryer could reach up to 130 ◦C. One advantage of reaching a low temperature is that the temperature sensitive merchandises’ safety can be preserved. Moreover, for the compressor with a larger heat capacity, the drying timePDF Image | Recent Advances in Transcritical CO2 (R744) Heat Pump System
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