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Energies 2019, 12, 457 11 of 35 and fitted inside a larger tube. The theoretical and experimental results showed that a greater number of inner tubes increases the outlet water temperature, but, at the same time, the pressure drop increases sharply. Kim et al. [32] developed a multi-tube counter-flow gas cooler consisting of parallel smaller tubes bundled together inside a larger tube for a geothermal HP. In this configuration, the CO2 and water flow in the opposite direction to ensure maximum heat transfer. Compactness of the gas cooler is another key focus of recent research, which can be achieved by reducing refrigerant charge and tube size, varying the air volume flow rate, and considering fin types. Marcinichen et al. [51] numerically studied these factors to reduce the size of an existing gas cooler used in a beverage vending machine. They found that the existing gas cooler was oversized by a factor of two, and a more compact design could be realized by increasing the air volumetric flow rate and replacing a plain fin with a wavy fin. In case of an undersized gas cooler, a ‘pinch point’ might occur due to the strong nonlinear variation of the specific heat capacity of CO2. A high value of pinch point indicates high thermal losses in the HX due to irreversibility. Chen et al. [52] analyzed the pinch point occurrence in a water-cooled CO2 HP using the log mean temperature difference (LMTD) method and found that the gas cooler was undersized by 30% to 40%. Likewise, Sánchez et al. [53] developed an accurate finite element model for properly sizing a CO2 gas cooler. Yin et al. [54] developed and studied a compact gas cooler model using a finite element approach where the micro-channel gas cooler consisted of three passes of 13, 11, and 10 tubes. They found an increasing number of passes in the gas cooler might improve the performance, but the three-pass design is the best since there is not much difference between a five-pass and a three-pass gas cooler concerning the exit CO2 enthalpy. Additionally, the simulations showed that using a multi-slab rather than a multi-pass could further reduce the gas cooler size due to improved performance of the gas cooler. Similarly, Tsamos et al. [55] carried out an experimental study on two different configurations of fin-tube gas cooler (two-row and three-row) considering both the supercritical and subcritical pressure at the HX. They found a linear relationship between the air temperature at the inlet and the pressure while transitioning from the subcritical to the supercritical stages. A simulation of the same fin-tube gas cooler showed that 90% of the pressure drop occurred in the first 17% of the HX length from the inlet [56], and the HX size could be reduced if the air flow rate is increased. If the refrigerant mass flow rate is lower than the optimum amount, the degree of superheat (DSH) at the evaporator outlet increases, which causes an excessive pressure-drop, and, as a result, COP decreases. Conversely, if the refrigerant mass flow rate is excessive, the specific volume of the refrigerant at the compressor inlet increases, which causes a high pressure-drop at the evaporator and a low specific enthalpy change at the gas cooler. Therefore, for the same compressor power consumption, the specific enthalpy exchange becomes less, and the system COP decreases. As such, Liu et al. [57] developed a genetic algorithm for maximizing CO2 HP COP based on gas cooler pressure and the water flow rate. Similarly, to reduce the power consumption in real-time, Hu et al. [58] adopted an extremum seeking control (ESC) strategy considering the gas cooler pressure and outlet water temperature. A similar control strategy was developed by Peñarrocha et al. [59] to reduce the consumption by regulating the high-side pressure. by To optimize the performance of a CO2 refrigeration system, Kim et al. [60] suggested a new control strategy. They utilized the difference between the required total HTC in the gas cooler for optimum COP and total HTC at any concurrent gas cooler pressure. When the difference between the total HTCs is zero, the maximum COP is realized. However, this control strategy requires an exact value of total HTC at the gas cooler. Other than ESC, Ge et al. [61] proposed a heat recovery optimization system for a supermarket CO2 refrigeration system. The developed system used an opmimum gas cooler pressure to maximize heat recovery. 3.2. Evaporator The CO2 HP evaporator functions like conventional CFC or HFC HP evaporators except that it experiences a much higher pressure (2–7 MPa) in the subcritical region. The CO2 evaporator operates at a reduced pressure higher than 0.36 at a saturation temperature of −10 ◦C whereas an R134aPDF Image | Recent Advances in Transcritical CO2 (R744) Heat Pump System
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