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Energies 2019, 12, 457 25 of 35 of the total energy consumed per year in nZEB apartments could meet the demand for HVAC and domestic hot water (DHW). Given that the thermophysical properties of a refrigerant play an important role in dictating the HP performance, CO2 has been compared with other conventional refrigerants in hybrid solar HPs. Li et al. [123] presented a simulation on a hybrid solar assisted HP using R134a, R744, and R22 for water heating in a residential apartment. The results showed that the given ambient temperature ranged from −5 to 13 ◦C while R744 attained a higher COP compared to R134a and R22. However, R134a attained the maximum COP when the ambient temperature was above 13 ◦C, while CO2 HP attained a low cooling efficiency under all operating conditions. Cho et al. [124] presented a similar study on a hybrid HP system using R744 and R22, and found that, during sunny days, COPs for both R744 and R22 were higher by 20% compared to cloudy days. However, concerning the second law efficiency, the R22 system had a 6.2% higher efficiency compared to the R744 due to the higher irreversibility and performance sensitivity of the R744 system. Additionally, Chaichana [125] assessed the use of natural working fluids-Carbon dioxide (R744) and Ammonia (R717) in solar boosted HPs and compared them with the HP using Chlorodifluoromethane (R22). The study concluded that choosing R744 for HP applications would not be an appropriate choice due to its low critical temperature and high working pressures, which leads to a low COP. Instead of R744, R717 was suggested as a potential natural working fluid for solar boosted HP operation due to its similar performance to R22. 4.2.2. Hybrid Geothermal To further improve the performance of CO2 HP and increase the energy saving potential, hybrid geothermal HP systems with a secondary heat source/sink such as solar energy, ambient air, or water has been studied. Jin et al. [126] theoretically analyzed a hybrid GCHP system, which utilized a combination of ambient air and ground boreholes as the heat sink for cooling mode, while only boreholes were engaged as the heating source during the heating mode. The system used two separate gas coolers known as the air-cooled gas cooler and the water-cooled gas cooler for the heat rejection process to take advantage of the large temperature glide. An optimal control strategy was developed for the gas cooler to minimize the annual ground thermal imbalance depending on the desired cooling COP and the heat rejected to the ambient air. The results showed that both heating and cooling performance of the CO2 hybrid GCHP improved when compared to its baseline ASHP. Depending on the indoor temperature, the time-dependent COPcool varied from 2.2 to 4.1, while the COPheat varied from 2.53 to 3.15. Jin et al. [127] further extended this study using a reverse CO2 transcritical cycle for a CO2 hybrid HCHP system. The system was able to operate under space cooling or heating mode with a simultaneous service water heating. For instance, while operating in the cooling mode, this system could simultaneously provide both space cooling and hot water supply. Such an arrangement gives the possibility to eliminate the underground heat accumulation in a warm climate and, thereby, paves a way to enhance the system performance in the long-term operation. The results showed that the combined COP for space conditioning and hot water varied from 3.0 to 5.5 with 65 ◦C of the service hot water supply. During the last decade, the hybrid geothermal-solar assisted HPs (GSAHPs) have gained researchers’ attention, potentially for both space heating and hot water supply. GSAHP technology has proven to be efficient not only in the warm climate but also in the cold climatic regions. In a cold climate, the HP works mainly in a heating mode, and the ground temperature gradually decreases due to the continuous heat extraction from the ground, which deteriorates the HP performance. Emmi et al. [131] simulated a GSAHP and evaluated its performance in cold climates where the heating and cooling load profiles of buildings were not balanced. The integration of solar collector with the HP was found to enhance the efficiency by 30% with the COP ranging from 3.59 to 4.70 for different seasonal conditions. In a real application of CO2 HP, the actual system performance and reliability depend on operating characteristics rather than only on the design conditions. Kim et al. [128] investigated the performance of a CO2 hybrid GSAHP under different operating conditions such as operating temperature and solarPDF Image | Recent Advances in Transcritical CO2 (R744) Heat Pump System
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