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A comparative analysis on experimental performance of CO 2 trans-critical cycle

A performance comparison of experimental results for CO 2 trans-critical cycle is presented for an overview of the current level of technology. The published performance data were collected as research objects through comprehensive literature review
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  HVAC&R Research  (2014)  20 , 1–13Copyright  C  2014 ASHRAE.ISSN: 1078-9669 print / 1938-5587 onlineDOI: 10.1080/10789669.2014.913959 A comparative analysis on experimentalperformance of CO 2  trans-critical cycle SHUAI DENG 1,2, ∗ , RUZHU WANG 1 ,  and  YANJUN DAI 11 Institute of Refrigeration & Cryogenics, Shanghai Jiao Tong University, Shanghai, China 2 Key Laboratory of Efficient Utilization of Low and Medium Grade Energy, Tianjin University, Ministry of Education of China,Tianjin, China A performance comparison of experimental results for CO 2  trans-critical cycle is presented for an overview of the current levelof technology. The published performance data were collected as research objects through comprehensive literature review onexperimental research. The methods for data processing, error analysis, and performance evaluation are introduced in the researchmethodologysection.Throughtheproposedresearchmethod,28groupsofperformanceresultsfromdevelopedprototypesortestrigsarecomparedandanalyzedusingthecoefficientofperformanceandthesecondlawefficiencyofthermodynamics.AdiscussionoftheperformancecomparisonbetweendevelopedCO 2  devicesandcommercialproductsofsyntheticworkingfluidisalsopresentedbasedon China’s national standards (General Administration of Quality Supervision, Insection and Quarantine of the People’s Republicof China 2001, 2003). Based on the comparison results, the state-of-art and possible research directions for CO 2  trans-critical cycletechnology are summarized and presented. Introduction Studies undertaken by various scientists during the 1970s re-vealed that chlorofluorocarbon (CFC) released into the at-mosphere accumulates in the stratosphere, where they had adeleterious effect on the ozone layer (Molina and Rowland1974, Ramanathan 1975). Thus, the use of CFC has beenheavilyregulatedsincethe1970s(Morrisette1989).Withsucha pressing need to replace CFCs, a revival of natural workingfluids emerged since the 1990s (Lorentzen 1994). During thepast two decades, technologies related to trans-critical CO 2 cycle (TCC) in cooling and heating applications have been ex-tensively exploited through the persistent efforts of numerousresearch groups all over the world.Several research groups have published some review oranalysispaperstoshowthedevelopmentlevelofTCCtechnol-ogy. The status of CO 2  systems until the middle of the 1990swas presented by Pettersen et al. (1995). In 2004, Bullard dis-cussed the development of CO 2  technology by reviewing pre-vious developments and exploring likely applications. Kimet al. (2007) published a detailed review article regarding fun-damentalprocessandsystemdesignissuesinCO 2  vaporcom-pression systems. Kim et al.’s (2007) article includes sectionsfor component design and application areas, which coveredthe representative research results up to 2003. In addition, Received October 7, 2013; accepted March 6, 2014 Shuai Deng, PhD, isaLecturer. Ruzhu Wang, PhD, isaProfessor. Yanjun Dai, PhD,  is a Professor. ∗ Corresponding author e-mail: edward versions of one or more of the figures in the article can befound online at tworeviewarticlesonheattransferofpoolboiling,two-phaseflow, and boiling flow in macro- and micro-channels for CO 2 were published in 2005 (Gorenflo and Kotthoff 2005; Thomeand Ribatski 2005). Next, Groll and Kim (2007) reviewed re-searchactivitiestowardTCCtechnologyandadvancesinthe-oryandexperimentaldata.Areviewfocusedonmodificationsandefficiencyimprovementsfordifferentcycleconfigurationswas presented by Sarkar (2010). Recent research on CO 2  heatpumps up to 2010 were summarized by Austin and Sumathy(2010).Thecontentofthatarticlecoveredsystemcomponents,configurationsandmodifications,andhowthesefactorsaffectthe overall system performance.Existing review work, however, does not provide sufficientcomparisons of research on device performance or efficiency,so that an overview about the technology development levelof TCC is not presented in a direct way. This article attemptstocarryoutacomprehensivecomparisonontheperformanceof the prototype, test rig, or experimental system, based onthe literature research, data acquisition, and calculation. Inthis way, state-of-art TCC technology and future trends canbe provided through a comparison among published perfor-mance data and a comparison between performance rangesofexperimentaldevicesandrequestedperformanceofmatureproducts in China’s national standard (General Administra-tion of Quality Supervision, Insection and Quarantine of thePeople’s Republic of China 2001, 2003). Methodology The methodology used in the article is presented in terms of research procedure, data source, performance indicator, mod-eling and assumption, and tools.  2  HVAC&R Research Research procedure The research procedure used in this article contained threesteps: Step 1: Data collection Experimental results and test conditions from previous re-searches were collected as data sources for comparison. Be-cause of limitations in the scholar database, some publishedpapers that contain valuable experimental results may not beincludedorhavenotbeenyetindexed.Thus,someexperimentsmay not be covered in this article. The search conditions of the literature research, such as time range and keywords, aredescribed in the Appendix. Step 2: Data arrangement Because experimental results in collected papers are com-monly presented in different formats or measured under dif-ferent test conditions, it is nearly possible to compare themdirectly without any appropriate conversions. Thus, a simpli-fied cycle model and several assumptions were developed forthe comparative analysis. In this case, based on the experi-mental data, performance parameter can be calculated andconverted to a result in the classical thermodynamic indica-tors. Then the performance comparison can be carried out ona uniform platform. Step 3: Data analysis Through calculations and comparisons, an overview of thecurrenttechnologicallevelcanbeobtained.Performancecom-parisons in heating and cooling for different applications re-veal the development potential and possible development di-rectionsofCO 2 trans-criticalcycletechnology.Someproblemsregarding the development barrier are discussed, as well. Data source Performance data of existing experiments on trans-criticalCO 2  cycle (TCC) were collected from indexed papers fromthe Science Citation Index, from the date range January 1998to May 2013. As showed in Figure 1, the amount of pub-lished research on TCC and related applications has greatlyincreased during the past 15 years. The sudden rise (and fall)in specific years, such as 1998, 2005, and 2011, was caused bythe particular emphasis on TCC by academic organizationsor journals, such as special issue 28 of   International Journal of Refrigeration  in 2005, issue 15 of   HVAC&R Research  in 2009.By classifying collected papers, a distribution about re-searchdirectionscanbeobtained,asshowninFigure2.Asidefromreviewandtheoreticalresearch,experimentalstudytakesabout 78% of the total amount of collected papers. The pub-lished data on ejector, expander, and cascade system of TCCare not contained in this research, as our work mainly fo-cuses on the classical TCC structure with compressor, gascooler,evaporator,andthrottle.Intheexperimentalinvestiga-tionsectioninFigure2,researchoncoolingperformance,suchas refrigeration and air conditioning (AC) account for 31% of the total amount of collected papers, while the percentage of research on heating performance, such as heat pump or heat Fig. 1.  Paper amount per year related to TCC (SCI indexed:1998.01-2013.05). pump water heater, is about 28% of the total amount of col-lected papers.As previously mentioned, a cycle model was proposed forthecomparisonofcoefficientofperformance(COP)andther-modynamic second law efficiency. Experimental data foundin references were collected to form a closed cycle in theory.Not all collected papers provided enough experimental datafor comparative research. For example, some papers that fo-cused on the performance of components in the cycle ratherthan the performance of the entire system were not includedin the database. Through a literature screening, 28 groups of experimental data were available for analysis. Brief informa-tion for the experimental system or prototypes is provided inTables 1 and 2 for cooling and heating, respectively. The re-ported experimental data in the papers listed in Tables 1 and2 were chosen as the data source for the comparative analysis,not only becausethey canbeaccessed through theonlineaca-demicdatabase, butalso becausetheyhavepassed theprocessof peer review with the reliability. Fig. 2.  Distribution for various research directions regardingTCC.  Volume 20, Number 5, July 2014  3 Table 1.  Information summary of cooling experiments.Year Author(s) Application Cycle Source and sink Q cooling , kW Compressor1993 Lorentzen and Pettersen AAC IHX A → A 4.5 Recip1999 Hwang and Radermacher R Baseline W → W 10.8 Recip, hermetic1999 Beaver et al. RAC IHX A → A 10.5 Recip, open2000 Culter et al. ECU Baseline A → A 11.7 Recip, semi-hermetic2001 Preissner AAC IHX A → A 3.5 Recip, open2002 Giannavola and Hrnjak AAC IHX A → A 6.3 Recip2004 Girotto et al. R IHX, TST A → A — —2005 Rigola et al. R Baseline W → W 0.8 Recip, open2005 Cavallini et al. R TST A → A — Recip, semi-hermetic2008 Cabello et al. R Baseline W → W 5.3 Recip, semi-hermetic2009 Cho et al. R Baseline A → A — Scoll, variable speed2009 Xie, Liu et al. R IHX W → W 9.3 —2010 Cecchinato et al. R IHX W → A 94.0 Recip, semi-hermetic2012 Deng et al. R Baseline W → A 6.0 Rotary Note: A = air source or air sink; AAC = automotive air conditioning; ECU = environment control unit; IHX = internal heat exchanger; R = refrigeration;RAC = residential air conditioning; W = water source or water sink; Recip = reciprocate; Scoll = scroll compressor; TST = two stage. →= heat transferdirection. Performance indicator Thevaporcompressioncycle,regardlessifitisasub-criticalortrans-criticalcycle,isaclosed,steadyflowcycleinwhichwork-ingfluidschangetheirstatesastheycirculatesthroughvariouspieces of equipment. Thus, the analysis method for the theo-retical thermodynamic cycle can be used in the performanceanalysisonrealexperimentalsystemsofrefrigerationandheatpump.Thecommonevaluatedindicatorforrefrigerationandheatpump (R&HP) cycle is COP. However, COP values in pa-pers are commonly obtained under different test conditions,e.g. different temperatures of heat source and sink, so theperformance comparison only in COP with different externalconditions is unreasonable and commonly causes confusion.Moreover, COP analysis cannot assign a “quality” to thermalenergytransferredinthecycle.Therefore,thesecondlawanal-ysis was comprehensively applied in studies on R&HP cycleto provide an evaluation of the performance perfectness rel-ative to reverse Carnot cycle. The corresponding indicator isthe second law efficiency of thermodynamics ( η ex ). Its calcu-lation equation is shown as Equation 1 with two simplifiedexpressions on cooling and heating (Equations 2 and 3): η ex  = COP   p COP  C  (1) η ex c  = COP   p T  l  / ( T  h − T  l  ) (2) η ex h  = COP   p T  h / ( T  h − T  l  ) (3) Table 2.  Information summary of heating experiments.Year Author(s) Application Cycle Source and sink Q heating , kW Compressor1998 Neksa et al. HPWH IHX W → W 50.0 Recip, open2000 Cutler et al. ECU Baseline A → A 6.0–10.0 Recip, semi-hermetic2001 Wang et al. HP IHX W → W — Recip, semi-hermetic2001 Richter et al. HP IHX A → A 9.8 Recip, semi-hermetic2002 Giannavola andHrnjakHP IHX A → A 4.1 Recip2002 White et al. HPWH IHX W → W 115.0 Recip, open2005 Stene HPandHPWH IHX W → W 6.5 Rolling piston, hermetic2008 Sun et al. HPWH IHX W → W — Recip, hermietic2008 Liu et al. HPWH IHX A → A — Recip, hermietic2009 Xie, Sun et al. HP IHX W → W — Piston2009 Cho et al HP Baseline A → A 2.7 Scroll2010 Fernandez et al. HPWH Baseline A → W 4.5 Rotary piston2011 Baek et al. HPWH IHX A → W 5.2 Single-rotary2011 Lin et al. GHP Baseline G → W 12.7 Recip Note: A  =  air source or air sink; AAC  =  automotive air conditioning; ECU  =  environment control unit; GHP  =  ground source heat pump; HPWH  = heat pump water heater; IHX = internal heat exchanger; R = refrigeration; RAC = residential air conditioning; W = water source or water sink; Recip = reciprocate; Scroll = scroll compressor; →= heat transfer direction.  4  HVAC&R Research where  COP  P   is the COP of the practical experimental deviceor system from references,  COP  c  is the COP of an ideal cycle. T  l   is the temperature of low temperature side in the cycle.  T  h is the temperature of high temperature side in the cycle.Particularly for TCC, the Lorenz cycle is the theoreticalcycle used in  η ex  calculation (Stene 2001). Thus,  η ex  for TCCis commonly defined as follow: η ex  = COP   p COP  LZ  (4)Because the isothermal process in the reverse CarnotR&HP cycle is replaced by a polytropic process in Lorenzcycle, the mean temperature of secondary fluid in the temper-ature gliding process is defined as: T  m  = T  2 − T  3 ln  T  2 T  3   (5)where  T  2  is temperature of secondary fluid (e.g., air or wa-ter) at a start position of the heat exchange process and  T  3  istemperature of secondary fluid at an end position of the heatexchange process. Provided that a real process in the evapora-tor is also nonreversible, the mean temperature of secondaryfluid can be obtained through a similar equation to Equation5.In this case, the identification of test conditions in the ref-erences provides two mean temperatures for the calculationof   η ex  and, therefore, the equation for cooling (or heating) isupdated as follows: η ex c  = COP   p T  ml  / ( T  mh − T  ml  ) (6)for which  COP   p  was obtained from published data, as previ-ously mentioned. Modeling and assumptions The simplified cycle model for the comparative analysis wasbased on a basic TCC with or without an internal heat ex-changer (IHX), as shown in Figure 3. The processes in themodel include compression (from point 1 to point 2), heat re- jection(frompoint2topoint3),expansion(3to4),andevap-oration (from point 4 to point 1). The heat rejection processabove the critical point (point C in Figure 3b), which occursfrom point 2 to point 3, is the primary difference from con-ventional subcritical cycles. The corresponding Temperature-Enthalpy (TH) diagram is shown in Figure 3b. Without con-sideration on the detailed realization of compression, heatabsorbing (aka: heat addition), heat rejection, and expansion,only four temperatures of secondary fluids are necessary forCOP calculation of the ideal cycle. These four temperaturesare shown in Figures 3a and 3b as starting and ending pointsof heat rejection and absorbing processes of secondary fluids.Based on these four temperature values, calculations can becarried out for  η ex . It is also implied that the major difficulty Fig. 3.  a. Simplified cycle model for performance comparison. b.TH diagram for simplified model. for the calculation is the clarification and determination of temperatures for required state points.Notallcollectedpaperscanprovidethenecessarytempera-tures for the required state points due to limited experimentalconditions.Forinstance,asshowninFigure4a,someauthorsrecordedandreportedtheinletandoutlettemperaturesoftherefrigerant for the heat rejection process and, thus, only testresults of CO 2  were presented in the paper without any infor-mation regarding heat source. Another type of experimentalresult that was common in the references chose the inlet tem-perature of secondary fluid and the inlet temperature of CO 2 on the both sides of heat-exchange in a gas cooler (Figure 4b)toshowamatchperformancebetweensecondaryfluidandre-frigerant during the heat rejection process. In order to realizea comparison for these cases, two assumptions for calculationwere established as follows. Temperature difference (  T) assumption As shown in Table 3, different temperature differences areassumed for cooling and heating, respectively. In this case, T  mh  (or  T  ml  ) can be calculated based on Equation 7 and the  Volume 20, Number 5, July 2014  5 Fig. 4.  a. Cycle diagram: temperature difference assumption. b.Cycle diagram: approach temperature assumption. meantemperatureofrefrigerantcanbeconvertedtothemeantemperature of secondary fluid: T  m  = T  2  sf   − T  3  sf  ln  T  2  sf  T  3  sf    +  t  (7) Approach temperature (AT) assumption Approach temperature (AT) is a temperature difference be-tween the outlet temperature of the refrigerant and inlet tem-perature of secondary fluid. Research on automotive air con-ditioning (AAC) shows that AT in CO 2  TCC is smaller thanthat in hydrofluorocarbon (HFC) subcritical cycle because of a great temperature glide of refrigerant (Pettersen et al. 1998).Representative test results of AT are recorded and comparedin the reference (Pettersen et al. 1998). Based on the results,the assumption conditions of AT are shown in Table 4. Asshown in Figure 4b, the temperature of state 3 can be ob-tained based on the AT assumption. The mean temperatureof CO 2  is calculated out based on Equation 5 and a knowntemperaturevalueofstate2.ThenEquation7and   T   canbeused to calculate the mean temperature of the secondary fluidand compare it with known temperature conditions of heatsink, for a checking of the relative error (see the Case studysectioninthisarticle).Iftherelativeerrorisbiggerthan5%,aniterative trial and error comparison process is needed throughan adjustment of estimated values in the assumed condition.Finally, a relative accurate result is obtained based on theso-called estimate calculation (EC), as introduced earlier.Itshouldbenotedthatvaluesof   T   andATinassumptionsare not only specified based on research results of referencesin Tables 2 and 3, but also based on the testing experience of theauthors’researchonTCCsystem.Furthermore,ourworkfocuses on a performance comparison of ratio values ( η ex ),rather than 100% accurate data recurrence of collecting ex-perimentalresearchinreferences.Thus,weattempttoprovidethe calculated results based on limited data and assumptions,and compare the relative values of various research results.That is the principal limitation of this research and it can beovercome as more researchers focus on thermodynamic sec-ond law efficiency analysis on TCC and publish their owncalculated results. Table 3.  Temperature difference (   T  )assumption.Temperature difference,  ◦ CCooling HeatingSecondary fluid Sink side (heat rejection) Source side (heat absorb) Sink side (heat rejection) Source side (heat absorb)Water 30 . 0 ± 3 . 0 a 5.0 23 . 5 ± 2 . 5 b 5.0Air 23 . 5 ± 1 . 5 c 10 ± 1 . 5 d  22 . 5 ± 2 . 5 e 7 ± 2 . 0  f  Note: The values of temperature difference in assumptions are based on the experiment results of references. a Xie et al. 2009. b Wang et al. 2001, Sun et al. 2008. c Giannavola and Hrnjak 2002, Beaver et al. 1999. d  Giannavola and Hrnjak 2002, Beaver et al. 1999. e Richter et al 2000, Giannavola and Hrnjak 2002.  f  Richter et al 2000, Giannavola and Hrnjak 2002.
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