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Energies 2019, 12, 457 24 of 35 4.2.1. Hybrid Solar Use of low-grade heat sources such as solar energy as an auxiliary energy source integrated with a HP to obtain net zero energy balanced (nZEB) has been considered and explored by many researchers. Energies 2019, 12, x FOR PEER REVIEW 22 of 33 A solar assisted CO2 HP is typically more energy efficient than baseline HPs. Chen et al. [132] experimentally demonstrated that a solar assisted CO2 HP for space heating could attain an attractive attractive COP of 13.5 and reduce the electricity consumption by 53.6% when compared to a baseline COP of 13.5 and reduce the electricity consumption by 53.6% when compared to a baseline CO2 HP CO2 HP under typical test conditions. In this study, a numerical model was formulated to analyze under typical test conditions. In this study, a numerical model was formulated to analyze the effects the effects of HP components on the performance, while a multi-parameter optimization was of HP components on the performance, while a multi-parameter optimization was performed to performed to minimize the electrical consumption considering the influence of storage and operation minimize the electrical consumption considering the influence of storage and operation tanks. tanks. Faria et al. [122] developed a dynamic mathematical model to investigate a CO2 HP (Figure 7) Faria et al. [122] developed a dynamic mathematical model to investigate a CO2 HP (Figure 7) integrated with a flat plate collector (solar evaporator) and expansion valve assembly under both integrated with a flat plate collector (solar evaporator) and expansion valve assembly under both the the steady state and transient conditions. The model is a useful tool particularly when HP operates steady state and transient conditions. The model is a useful tool particularly when HP operates under under various operating conditions that include solar radiation, ambient temperature, and wind speed. various operating conditions that include solar radiation, ambient temperature, and wind speed. They reported that adjusting the expansion valve opening had a noticeable effect on the system’s They reported that adjusting the expansion valve opening had a noticeable effect on the system’s performance. If the CO2 is heated to a high DSH, HP’s performance drastically decreased because the performance. If the CO2 is heated to a high DSH, HP’s performance drastically decreased because the DSH reduced the boiling region, which consequently increased the dry area. This resulted in a low DSH reduced the boiling region, which consequently increased the dry area. This resulted in a low convective coefficient value. They recommended that, for a CO2 driven HP with a solar evaporator, convective coefficient value. They recommended that, for a CO2 driven HP with a solar evaporator, the use of an EEV is essential to continuously adjust the CO2 flow to the evaporator in order to the use of an EEV is essential to continuously adjust the CO2 flow to the evaporator in order to maximize the system’s performance. Unlike Faria et al.’s design [122], a U-pipe solar collector was maximize the system’s performance. Unlike Faria et al.’s design [122], a U-pipe solar collector was utilized as the evaporator in Islam et al.’s work [133], which showed that the system COP could utilized as the evaporator in Islam et al.’s work [133], which showed that the system COP could be be improved by an average of 57% with a compressor speed decrease from 1500 to 900 rpm. They improved by an average of 57% with a compressor speed decrease from 1500 to 900 rpm. They found found that the collector efficiency was in the range of 50% to 55%. Though the initial cost of the that the collector efficiency was in the range of 50% to 55%. Though the initial cost of the U-pipe solar U-pipe solar collector HP systems is relatively higher when compared to the non-focusing collectors, collector HP systems is relatively higher when compared to the non-focusing collectors, the system the system can be a better prospect in the future for thermal applications since such a system is can be a better prospect in the future for thermal applications since such a system is environmentally environmentally benign. benign. Figure 7. Schematic representation of the experimental device. Reproduced with permission from [122]. Figure 7. Schematic representation of the experimental device. Reproduced with permission from Elsevier, 2016. [122]. Elsevier, 2016. Deng et al. [134] carried out a comparative study of a novel air-cooled solar assisted CO2 HP and Deng et al. [134] carried out a comparative study of a novel air-cooled solar assisted CO2 HP and an ideal vapor-compression CO2 HP. The hybrid HP integrated a conventional CO2 HP with a solar an ideal vapor-compression CO2 HP. The hybrid HP integrated a conventional CO2 HP with a solar thermal driven absorption chiller, which was used to super-cool CO2 in a vapor-compression loop thermal driven absorption chiller, which was used to super-cool CO2 in a vapor-compression loop when working fluid leaves the gas cooler. Simulation results showed that the cooling performance when working fluid leaves the gas cooler. Simulation results showed that the cooling performance could improve with an increased super-cooling temperature difference. The cooling COP reached could improve with an increased super-cooling temperature difference. The cooling COP reached 3.8 3.8 at the super-cooling temperature difference of 7.7 ◦C. Additionally, the hybrid system was found at the super-cooling temperature difference of 7.7 °C. Additionally, the hybrid system was found to to reduce the power consumption at about 19% compared to a conventional CO2 HP without the reduce the power consumption at about 19% compared to a conventional CO2 HP without the absorption chiller. Further experimental work [134] showed that, using such a hybrid system, 58% absorption chiller. Further experimental work [134] showed that, using such a hybrid system, 58% 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 forPDF Image | Recent Advances in Transcritical CO2 (R744) Heat Pump System
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