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Energies 2019, 12, 457 17 of 35 Among many types of expansion devices, the electronic expansion valve (EEV) and capillary tubes have been studied for CO2 HPs. Zhang et al. [102] experimentally studied the effect of the refrigerant charge amount and EEV opening on the performance of a CO2 HP water heater. The EEV opening of 40% was found to be optimal for their system. Increasing the EEV opening from its optimal value to 60% decreased the heating capacity up to 30% due to an increase in the refrigerant charge and supercritical pressure, but the undercharged condition had a more severe consequence on the performance than an overcharged condition. Baek et al. [9] investigated the control methods of the gas cooler pressure in a CO2 HP using an EEV. The EEV integrated CO2 HP showed enhanced COP due to optimized pressure in the gas cooler. Besides EEV, capillary tubes are preferred as an expansion device particularly in small vapor compression refrigeration and air conditioning systems due to their simplicity, low initial cost, and low starting torque of compressor. However, flow characteristics inside the capillary tube under adiabatic condition are complex. Song et al. [103] found that the CO2 HP using a capillary tube is promising with its COP close to (above 80% of) that of a system using an EEV. In addition, in another study of CO2 HP, Madsen et al. [104] found that the use of an adiabatic capillary tube was better than a fixed high-pressure expansion valve but inferior to an adjustable expansion valve. The study recommended the use of a capillary tube in transcritical CO2 HP when the system is relatively small. Other than capillary tube, Hu et al. [105] studied and developed an improved design of a two-rolling piston expander for CO2 systems using sealing techniques to minimize the leakage. 3.5. An Auxiliary Component: Internal Heat Exchanger As discussed in Section 3.3, studies have confirmed that single-stage compression is less efficient than the two-stage system. More commonly, an IHX has been installed to ensure better and efficient operation of the compression system [106]. Use of an IHX reduces the possibility of damaging the compressor when the liquid refrigerant exits from the evaporator and promotes the superheating of the vapor entering the compressor. Xian et al. [29] modeled a high-performance two-stage CO2 HP using two ejectors and incorporated an IHX to the model. The researchers found that it could attain a 30% increase in the heating COP than a baseline two-stage HP at a standard operating temperature of −15 ◦C. Likewise, Kim et al. [32] developed a steady-state model to analyze the thermodynamic performance of an IHX in a geothermal CO2 HP, where a counter-flow multi-tube HX with several tubes encompassed in a larger tube was used as the IHX. They found that the system with IHX could achieve up to 6% increase in COP compared to the system without IHX at the 20% EEV opening. The proposed simulation program can serve as a useful tool for a thermodynamic performance analysis when optimizing complex system variables and establishing efficient operating conditions in CO2 geothermal HP systems. Similarly, Yamaguchi et al. [21] formulated a static mathematical model to study a CO2 HP with a twin-tube type HX as the IHX. They found that the addition of the IHX could enhance the COP up to 3.5 and had a positive influence on the heat transfer in both the gas cooler and evaporator. In a comparative study, Jiang et al. [25] utilized a double pipe copper HX as the IHX in which the high-pressure CO2 flew in the inner pipe and the low-pressure CO2 flew in between the inner and outer pipe. The experimental results demonstrated that, for a gas cooler temperature of 15 ◦C, the system with IHX had 6.5% to 11.5% higher COP than that of without IHX.PDF Image | Recent Advances in Transcritical CO2 (R744) Heat Pump System
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