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Energies 2019, 12, 457 18 of 35 Shariatzadeh et al. [107] analyzed four different transcritical refrigeration cycles with and without IHX along with an expander and throttling device. They found the COP of the expander cycle is always higher than the throttling cycle due to the optimized gas cooler pressure and reduced exergy loss in the expander cycle. However, when the system worked in the expander cycle, use of an IHX reduces the average COP from 2.8 to 2.5 due exergy efficiency loss in the gas cooler, but, in case of the throttling cycle, integration of IHX reduces exergy loss and, as a result, increases the COP from 1.9 to 2.0. Zhang et al. [108] carried out a similar study and found that, when IHX is used in an expansion valve cycle, the system performance could increase by 17%. On the contrary, if IHX is used in an ejector cycle, the system performance degrades by 16%. This is attributed to a decrease in ejector isentropic efficiency with the integration of IHX. In addition, the effects of IHX on system parameters rather than efficiency were studied. From an experimental investigation, Sánchez et al. [109] reported that the integration of IHX increases the compressor suction temperature, which reduces mass flow rate, and, as a result, specific compression work increases. However, this increase is very insignificant. Moreover, superheating induced by IHX increases the discharge temperature, and this effect is high at a low evaporating temperature. Pérez-García et al. [110] found the effectiveness of the IHX largely depends on the gas cooler outlet CO2 temperature, and the increased energy efficiency with the IHX is due to the subcooling of CO2. However, the mechanical subcooling system without an IHX can also improve the energy efficiency [111]. Furthermore, Ituna-Yudonago et al. [112] analyzed the effect of transient thermal effectiveness of an IHX on a CO2 system, and they reported that increasing the CO2 temperature by 10 ◦C at the IHX inlet of the gas cooler side could decrease the COP of about 3% due to reduced HTC and IHX effectiveness. 4. CO2 Heat Pump Systems The resurrection of CO2 as the working fluid in vapor compression cycles started in the early 1990s due to phasing out of ozone-depleting refrigerants. In 1993, Lorentzen et al. [113] developed and tested one of the first prototypes of the CO2 automotive air conditioning system. In 2015, soft drinks manufacturing giant Coca-Cola announced to utilize CO2 as the refrigerant in all their new vending machines as part of the global effort to phase out fluorinated refrigerants [114]. The commercialization of the CO2 HP system started a decade ago in Japan and sales went past 2 million units in October 2009 [115]. Research and development on CO2 air conditioning, refrigeration, and HP systems are continuing and still has scope for performance improvement. According to the available heat sources at the evaporator side, HPs can be classified as the water source, the air source, and the ground source HP, while a hybrid HP is defined as an HP using more than one heat source. In hybrid systems, most commonly solar and geothermal energy are incorporated with the conventional heat sources to improve the system performance. In the following subsections, each type of system is discussed and detailed. Table 3 summarizes the salient features of different types of CO2 HP systems found in the literature.PDF Image | Recent Advances in Transcritical CO2 (R744) Heat Pump System
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