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Energies 2019, 12, 457 27 of 35 was shorter, and the system COP was higher due to the lower discharge pressure of the compressor. This HP dryer was found to be 65% more energy-efficient as compared to an electric resistance dryer. Schmidt et al. [141] thermodynamically compared a transcritical CO2 dehumidification cycle with a subcritical R134a cycle for drying purposes, and found that the two cycles could be energetically equivalent if a better isentropic efficiency of compression can be achieved in the CO2 cycle. Henceforth, the CO2 is the effective candidate for its environmental concern. 5.3. Cold Climate Heating HPs’ heating performance diminishes with the ambient temperature decrease, and specific Energies 2019, 12, x FOR PEER REVIEW 25 of 33 modifications are required for energy efficient performance in cold climates. The ASHP loses efficiency in a low outdoor temperature due to frost formation on the evaporator coil, while GSHP COP reduces HPs’ heating performance diminishes with the ambient temperature decrease, and specific in cold climates due to a soil thermal imbalance in long-term operation. The HP compressor suffers modifications are required for energy efficient performance in cold climates. The ASHP loses premature wear and tear in cold climates due to load imbalance at a low ambient temperature. efficiency in a low outdoor temperature due to frost formation on the evaporator coil, while GSHP MoCreOoPverre,dthuecerseqinuicroeldhcilgimhainteistiadluceostot liamsiotsilththeerumsealofimHbPaslaincceoilnd lcolinmg-atteersm. operation. The HP coMmpanreysssotrusduifefesrshparvemcaotunrteriwbueaterdantdoteanrhiannccoilndgclHimPatepsedrfuoermtoalonacde iimnbalacnocledactlaimloawtea.mAbisenstuch, Watnegmepteralt.u[r7e2. ]Meoxrpeoevriemr,etnhetarlelyqusitruediheidghainCitOial cHosPt lfiomritcsotlhde culsime oafteHPospienractoilodnclaimndatefos.und that the ◦ achieve a COP of 3.1 at the outdoor temperature of –20 °C (Figure 8). However, integration of a COP by 19% compared to a CO2 HP without a secondary loop. Hakkaki-Fard et al. [142] found that the 2 Many studies have contributed to enhancing HP performance in a cold climate. As such, Wang HP could achieve a COP of 3.1 at the outdoor temperature of −20 C (Figure 8). However, integration et al. [72] experimentally studied a CO2 HP for cold climate operation and found that the HP could of a secondary loop cycle in the system increased the gas cooler outlet temperature and reduced the secondary loop cycle in the system increased the gas cooler outlet temperature and reduced the COP 80/20 R32-CO2 zeotropic mixture could enhance the heating performance of an ASHP in cold climates by 19% compared to a CO2 HP without a secondary loop. Hakkaki-Fard et al. [142] found that the by 30% compared to an R410a system. The zeotropic mixture could also reduce the flammability of R32 80/20 R32-CO2 zeotropic mixture could enhance the heating performance of an ASHP in cold climates and the CO2 pressure in the operation. They reported R32-CO2 is the best-suited refrigerant mixture by 30% compared to an R410a system. The zeotropic mixture could also reduce the flammability of for the cold climate among all the mixtures tested. In another study, Hakkaki-Fard et al. [143] found R32 and the CO2 pressure in the operation. They reported R32-CO2 is the best-suited refrigerant a variable mixture of the R32-CO2 refrigerant, which could save up to 23% energy in a cold climate mixture for the cold climate among all the mixtures tested. In another study, Hakkaki-Fard et al. [143] while reducing GWP by 16%. Other than research studies, several split type systems are commercially found a variable mixture of the R32-CO2 refrigerant, which could save up to 23% energy in a cold developed to provide smooth operation in severe climate conditions. The split-system components climate while reducing GWP by 16%. Other than research studies, several split type systems are arecroombmusetrcainaldlycdaenveolpopereadtetoinpraonviydegisvmeonocthlimopaetrea.tiSouncinh saevCeOre cslipmliatteHcPoncdaintiolenas.dTthoeasnplietf-fisycsiteenmcy of 2 upctom4p0o0n%en[t1s4a4r]e, raonbdusitanlsdoccaannopgernaeteraitneahnoytgwivaetnercluimpatoe.8S0uchCawChOe2resapslititHsPefcfaenctilveaedotpoearnation ◦ temepffeicriaetnucryeoaftutphetoe4v0a0p%or[1a4to4]r, caannd ibtealasso lcoawn gaesne−ra3t0e hCot. wAaltehroupghtoth80er°eCawrehesrevaesriatsl ecfofmectmivercial operation temperature at the evaporator can be as low as –30 °C. Although there are several split-systems available in the Asian and European market (e.g., Sanyo [145]), only the Sanden’s commercial split-systems available in the Asian and European market (e.g., Sanyo [145]), only the SANCO2 split-system HPWH [146] is available in North America. Mitsubishi is also developing such Sanden’s SANCO2 split-system HPWH [146] is available in North America. Mitsubishi is also a CO2 HP system for residential applications [144]. developing such a CO2 HP system for residential applications [144]. ◦ Figure 8. Effects of indoor and outdoor air inlet temperatures on COP. Reproduced with permission from [72]. Elsevier, 2016. Figure 8. Effects of indoor and outdoor air inlet temperatures on COP. Reproduced with permission from [72]. Elsevier, 2016. 5.4. Food Processing The food processing industry consumes a major portion of energy, and the consumptions are growing rapidly. Brush et al. [147] reported that about $1.5 billion per year is spent only by the dairyPDF Image | Recent Advances in Transcritical CO2 (R744) Heat Pump System
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