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Energies 2019, 12, 457 14 of 35 In terms of system configuration, the hermetic and semi-hermetic types are the recent research focus. In a study of a semi-hermetic compressor integrated with a recuperative HX. Rozhentsev and Wang [26] reported that the HP performance was sensitive to even a marginal change in compressor efficiency and CDP (Figure 3). For example, at an optimum discharge pressure (85 bar), when the Energies 2019, 12, x FOR PEER REVIEW 12 of 33 compressor efficiency decreased from 1 to 0.75, the overall HP performance reduced by 30% from the maximumCOPoff44.5.5..Hooweevveer,r,ththeyeyfofuonudndthtehienitnertenranlasluspueprheerhateiantginhgadhadmainmiminaiml imalpimacpt aocnttohne tHhPe HCOPP.CCOaPv.alClinaviaeltlainl.i[7e8t]aolp.t[im78i]zeodpatimtwizoe-sdtaagettwraon-sctarigteicatlracyncslceriutiscianlgcayscelemiu-hsienrgmeaticsermeci-phreorcmateintigc rceocmipprroecsastoinr gcocuopmlepdrewssitohr acnouinptlerdcowolietrh. Tanheyinrteprcoroteledr.thTehsepylitr-ecpyocrletewd itheanspinlite-cgyracltedwsiuthctioan ilnintegHraXtecdousuldctinocnrelianseHthXecCoOulPduinpctroea2s5e%thceoCmOpParuepd to 2a5b%asceolminpeatrwedo-tsotagbeacsyeclilne.eZtwhaon-sgtaegteacl.y[c7l9e]. Zexhpaenrgimeteanlt.a[l7l9y]ceoxmpepraimreednthalelypceorfmorpmaraendctehoefpaerfoolrlimnganpciestofnahroerllminegtipcisctoonfihgeurrmateitoinc cionnsfinggulreatainodn itnwosi-nstgalgeeacnodmtpwroes-stoargse.Tchoemypfroeusnsodrtsh.aTthtehyefvoulunmdethtraictethffiecvieonlucymoeftrtihceesfifnicgilen-sctyagoefcthome spirnegslseo-rstwagaes choigmhperetshsaonrwthasthofigthweor-tshtagnetahtatcomfptwreos-sitoangerattiocsombeplorewss2io.8n,brauttiotshebetwloow-s2ta.8g,ebsuhtotwhedtwaoh-isgtahgeer vshooluwmedetarihciegfhfiecrievnocluymateatrcicoemffpicriesnscioynatratciomofp2re.8ssoironhirgahtieor.oTf 2h.e8 iosrehnitgrohperic. Tehffieciisenctyrohpaicdetfhfieciseanmcye htraedndthaes svaomluemteretrnidc eafsficvieonlucmy.etHriocweeffviceire,nthcye.DHSoHwehvaedr,athmeinDimSHal hefafdecat omninbiomthalveoflfuemcteotrnicbaontdh visoelnutmroeptirciceaffincdieinsecnietsrofopricbeoftfhiccieoncfiigesurfoartiobnosth. configurations. Figure 3. Effect of discharge pressure on the system COP. Reproduced with permission from [26]. Figure 3. Effect of discharge pressure on the system COP. Reproduced with permission from [26]. Elsevier, 2001. Elsevier, 2001. Recent studies have shown that, from a thermodynamics point of view, it is favorable to use a Recent studies have shown that, from a thermodynamics point of view, it is favorable to use a two-stage compressor in a transcritical CO2 HP due to a high working pressure. Cecchinato et al. [80] two-stage compressor in a transcritical CO2 HP due to a high working pressure. Cecchinato et al. [80] found a 70% increase in energy efficiency could be achieved using a two-stage double-throttling system found a 70% increase in energy efficiency could be achieved using a two-stage double-throttling compared to a simple single-stage single-throttling system for a severe evaporating temperature of system compared to a simple single-stage single-throttling system for a severe evaporating −30 ◦C. Moreover, the two-stage system with an injection throttling outperformed the system with an temperature of -30 °C. Moreover, the two-stage system with an injection throttling outperformed the IHX in term of COP and heating capacity. Similarly, for water heating applications, Pitarch et al. [81] system with an IHX in term of COP and heating capacity. Similarly, for water heating applications, reported that the COP of the two-stage system was 11% higher than the single stage system. Pitarch et al. [81] reported that the COP of the two-stage system was 11% higher than the single stage A commercially developed two-stage compressor for residential CO2 HP water heaters equipped with system. A commercially developed two-stage compressor for residential CO2 HP water heaters an intermediate gas-injection mechanism between the first and second stages helped to reduce the equipped with an intermediate gas-injection mechanism between the first and second stages helped vapor mixture losses [76]. It was found that such a two-stage configuration improved the compressor to reduce the vapor mixture losses [76]. It was found that such a two-stage configuration improved performance by 15% and 30% for compression ratios of 3 and 4, respectively. In another study, the compressor performance by 15% and 30% for compression ratios of 3 and 4, respectively. In Cho et al. [82] found that, in a two-stage refrigeration cycle, integration of gas injection could increase another study, Cho et al. [82] found that, in a two-stage refrigeration cycle, integration of gas injection the cooling COP by up to 17% compared to a non-injection cycle. Similar to the two-stage design, could increase the cooling COP by up to 17% compared to a non-injection cycle. Similar to the two- Xing et al. [29] combined a high-pressure compressor and a low-pressure compressor using an IHX, stage design, Xing et al. [29] combined a high-pressure compressor and a low-pressure compressor and the simulation showed that the new design could attain 10% to 30% higher COP compared to a using an IHX, and the simulation showed that the new design could attain 10% to 30% higher COP baseline system at a given evaporating temperature of −15 ◦C. Based on six different configurations of compared to a baseline system at a given evaporating temperature of –15 °C. Based on six different configurations of two-stage CO2 refrigeration systems, Liu et al. [83] found that the gas cooler outlet temperature and the low-stage compressor significantly affect the system performance. Recently, the BoostHEAT Company introduced a new and original concept of a thermal compressor for transcritical CO2 HP. Unlike the rotary or scroll compressors where the mechanical power generates the pressure increase of working fluid, in the new design, a heater provides thermalPDF Image | Recent Advances in Transcritical CO2 (R744) Heat Pump System
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