A Study on the Reduction of Throttling Losses in Automotive Air Conditioning Systems Through Expansion Work Recovery

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1 Purdue Univerity Purdue e-pub International Refrigeration and Air Conditioning Conference School of Mechanical Engineering 2008 A Study on the Reduction of Throttling Loe in Automotive Air Conditioning Sytem Through Expanion Work Recovery Vitor A. Gonzalve Pontific Catholic Univerity of Rio de Janeiro Joe A. R. Parie Pontific Catholic Univerity of Rio de Janeiro Follow thi and additional work at: Gonzalve, Vitor A. and Parie, Joe A. R., "A Study on the Reduction of Throttling Loe in Automotive Air Conditioning Sytem Through Expanion Work Recovery" (2008). International Refrigeration and Air Conditioning Conference. Paper Thi document ha been made available through Purdue e-pub, a ervice of the Purdue Univerity Librarie. Pleae contact epub@purdue.edu for additional information. Complete proceeding may be acquired in print and on CD-ROM directly from the Ray W. Herrick Laboratorie at Herrick/Event/orderlit.html

2 2416, Page 1 A tudy on the reduction of throttling loe in automotive air conditioning ytem through expanion work recovery Vitor de Araujo GONÇALVES 1, Joé Alberto Rei PARISE 2 * 1 Pontifical Catholic Univerity of Rio de Janeiro, Department of Mechanical Engineering, Rio de Janeiro, RJ, Brazil Phone: (+5521) , vitor.a.goncalve@gmail.com 2 Pontifical Catholic Univerity of Rio de Janeiro, Department of Mechanical Engineering, Rio de Janeiro, RJ, Brazil Phone: (+5521) , Fax: (+5521) , parie@puc-rio.br ABSTRACT Refrigerant flow control of the vapor compreion refrigeration cycle ha been largely relied upon throttling device, uch a capillary tube, hort tube orifice and expanion valve. The expanion proce through thee device repreent an obviou thermodynamic lo that can be mitigated by the application of different olution uch a intracycle evaporative cooling, economizer and ejector cycle or work producing expanion device. The preent paper concentrate on tudying the latter, pecifically applied for automotive air conditioning ytem. A baic thermodynamic model wa developed to imulate a vapor compreion cycle for automotive air conditioning purpoe employing expander. Data and cycle conception from device that were uccefully teted and reported in the literature are briefly commented. A preliminary tudy wa carried out taking into account ome of the everal characteritic that are inherent to mobile air conditioning ytem, including high rejection temperature. Thi preliminary tudy concentrated in the ubcritical vapor compreion R134a cycle with an expander and a thermotatic expanion valve placed downtream the expander. 1. INTRODUCTION Due to an entropy generation during ienthalpic expanion that i 3 to 4 time higher than in ubcritical vapor compreion refrigeration ytem, trancritical CO 2 cycle are mot uceptible to COP improvement with reduction of loe aociated with the expanion proce (Huff et al., 2003). An early tudy on the efficiencie of trancritical CO 2 cycle with and without expanion work recovery wa preented by Robinon and Groll (1998). A thermodynamic model for the analyi of both cycle wa developed. An empirical equation for the compreor efficiency wa employed and expanion turbine efficiency wa aumed to be either 0.3 or 0.6. Expander and compreor mechanical efficiencie were precribed at 0.9. The effectivene of the internal heat exchanger wa taken into account in the model. An intereting finding from Robinon and Groll (1998) i that the combined ue of an internal heat exchanger and the expanion turbine lead to a lower COP (a reduction to the order of 8%) if compared to thoe hould no internal heat exchanger be ued. Depending on the operating condition and modeling aumption, an ideal expander could improve the ytem COP by 45% to 75% (Huff et al., 2003) and capacity by 5% to 15% (Huff et al., 2002). Thi etablihe an attractive paradigm, in pite of the inevitable inefficiencie that are inherent to uch machine. Zha et al. (2003) ummarize the poibilitie for expander, which can be claified into two main categorie: poitive diplacement expander or turbine. Poitive diplacement device can be either of the reciprocating, rotary or orbital type. Rotary expander, like compreor, can make ue of the liding-vane, rolling piton or crew technologie, and orbital expander, of the croll type. Zha et al. (2005) lit the pro and con of the ue of each technology in regard to different application (e.g., mall, medium or large capacity ytem, commercial refrigerator, mobile air conditioner, heat pump, and reidential air conditioning ytem). For automotive air conditioning ytem, Zha et al. (2005) recommend the rolling piton expander, for it compactne, low cot, wide

3 2416, Page 2 application and reaonable efficiency. All other poitive diplacement poibilitie were not dicarded, though, in particular, the reciprocating expander, ince, according to Zha et al. (2005), thi i the type of compreor preently in ue for CO 2 ytem. Huff et al. (2005) alert for the additional expene for the required ue of active valve (i.e., valve, like in internal combution engine, that require a mechanical or electronic mechanim for it operation) which lead to lower operation of the expander (and, conequently, to a larger diplacement volume), and, due to the preence of nearly incompreible fluid, high force peak and accelerated wear and damage. In particular, for CO 2 expander, Zha et al. (2003) lit the main barrier to the development thi technology, namely: ealing, due to high preure difference, cavitation and liquid lugging, due to vapor liquid flow, lubrication, and vibration and noie control. Yet, the decription of a number of expander prototype can be found in the literature. The majority of them refer to the CO 2 trancritical cycle. Quack and co-worker (Heyl and Quack, 1999; Heyl and Quack, 2000; Nickl et al., 2003; Nickl et al., 2005) report a development program of a free-piton expander compreor for the CO 2 vapor compreion refrigeration cycle. A total of 13 cycle, ranging from the conventional throttling device-compreor cycle to expander-compreor cycle, with one or two tage, with or without expanion work recovery for compreion, with combination of throttling device and expander, have been propoed (Heyl and Quack, 1999). Their compreor-expander ytem i baed on free-piton engine technology, with double acting piton working with a common piton rod. The firt generation expander operated in the o-called full-preure mode, with the expander going through the ame preure difference a the compreor and with expander and compreor piton moving with exactly identical troke. Subequent prototype relied upon partial decoupling between compreor and expander, with the expander accounting for only part of the whole compreor preure difference, in fact, only the econd compreion tage (Nickl et al., 2003). Reported improvement of the COP over the throttle cycle wa of the order of 30% (Nickl et al., 2003). According to (Nickl et al., 2005), vapor compreion trancritical refrigeration cycle, running on CO 2, face a relatively low COP in view of (i) a high compreion dicharge temperature, which can be mitigated with the ue of multi-tage compreion with intercooling, and (ii) large exergy loe with the throttling. The ue of a three-tage expander cycle, where a vapor-liquid eparator, intalled between the econd and the third expanion tage, upplie vapor to the third expanion tage and liquid to the electronic or thermotatic expanion valve, addree effect (i) and (ii). In Germany, Heidelck and Krue (2000) propoed a combined compreor-expander reciprocating machine, compriing a rotating lotted dik to determine opening and cloing poition. An experimental apparatu to tet the device wa preented. A piton-cylinder type work output expanion device wa alo developed and teted by Baek et al. (2005a). The prototype wa baed on a commercially available mall diplacement two-cylinder four-troke park ignition engine. The engine wa highly modified to accommodate it new function. Valve opening-cloing control and phae angle between piton were two iue that led to the firt ubtantial modification to the original engine. Deign conideration revealed that the geometry of the expander (maximum to minimum volume ratio) et the expanion ratio of the device, and the rotational peed, the ma flow rate. Should full preure expanion work recovery, directly to the compreor, be ued, the latter concluion impoe a new link between compreor and expander, namely, rotational peed (with or without peed reduction), beide refrigerant ma flow rate. Thi require a delicate balance between compreor and expander performance curve. Tet were performed with the prototype of the piton-cylinder type expander (Baek et al., 2005a) and, in pite of expected low ientropic expanion efficiency, approximately 11%, an increment of 10.5% in the COP wa found. A imulation of the device wa alo carried out (Baek et al., 2005b). More recently, two prototype of free piton expander, ingle and double acting type, to operate in a CO 2 trancritical cycle, were developed uccefully by Liu et al. (2007). For the double acting prototype, the meaured (via indicated diagram) ientropic efficiency wa 62%, operating between preure of 0.78 MPa and 0.33 MPa. Poitive diplacement rotary and orbital device have alo been adapted to form work producing expander. Zha et al. (2003), for example, provide guideline for the deign of a CO 2 rolling piton expander. Yang et al. (2007c) propoe a combined CO 2 hermetic compreor-expander device, with both compreion and expanion procee taking place with rolling piton device and with two-tage. The paper report on experimental data of the compreor, a a firt tep of a development program. An inight on the experimental P-V diagram of a CO 2 rolling piton expander i provided by Zeng et al. (2007). Indicated diagram were produced and analyzed for three different operating condition. Expander efficiency varied from, approximately, 0.3 to 0.5, within a rotational peed between 650 rpm and 1800 rpm. Under and over-expanion, typical phenomena of poitive diplacement machine operating with no valve, have been oberved for peed below and above the deign peed, repectively.

4 2416, Page 3 Fukuta et al. (2003) preent preliminary reult of the prototype of a vane expander, adapted from a vane-type oil pump. Experimental condition were uction and dicharge preure of 4.1 and 9.1 MPA, repectively, and inlet temperature of 40 ºC. A maximum volumetric efficiency of 0.64 and a maximum total efficiency of 0.43, were reported, with thee value reducing to approximately 0.2, when rotational peed decreae from 2000 rpm to 500 rpm. Prediction of COP improvement, a a function of compreion and expanion efficiency, were carried out for a CO 2 cycle with 10 MPa, 3.48 MPa and 40 ºC a operating condition, for high and evaporating preure and expander inlet temperature, repectively. For example, with a compreor efficiency of 0.7 and an expander efficiency of 0.6, Fukuta et al. (2003) pointed to a COP improvement of 30%. Preliminary data on a vane-type prototype i reported by Yang et al. (2007a). Theoretical model for the trancritical to two-phae expanion proce of vane-type expander were alo developed (Fukuta et al., 2003; Yang et al., 2007a). An et al. (2007) addre the problem of lubrication CO 2 rolling piton expander. Huff et al. (2003) developed two prototype of croll expander adapted from two exiting automotive R134a compreor. The two prototype differed in the converion meaure from the original compreor, which affected the reulting expander diplaced volume and, a a conequence, the leakage path length to pocket volume ratio. Ientropic and volumetric efficiencie were meaured and plotted againt expander rotational peed. The econd expander prototype operated in a peed range between 1400 rpm and 220 rpm. Volumetric efficiency remained within 50% and 68%, and the ientropic efficiency reached a maximum of 42%, at approximately 1800 rpm. A imulation model, including the effect of internal leakage, heat tranfer and valve loe, wa alo developed to predict the overall performance of vapor compreion cycle with integrated compreor-expander device (Huff et al., 2002). A numerical imulation of a two-tage compreion CO 2 heat pump water heater, with a combined croll expander-compreor unit, i alo reported by Kim et al. (2007). Braz et al. (2000) report on the development program of a device, named the expreor (Braz, 2001), which combine a twin crew compreor to a twin crew expander, forming an independent free running device. Tet reult, with R113, indicated an overall expanion-compreion efficiency of the order of 55%, which correponded roughly to 70% expanion efficiency and 80% compreor efficiency. The device, later teted with R134a (Braz, 2003), wa regarded a a highly cot effective mean of power recovery from the expanion proce in refrigeration cycle, due to it characteritic which include a high efficiency ealed unit with only one pair of rotor, low vibration, adaptability to two-phae flow, and no need for any eal or valve and timing gear (Braz et al., 2000; Zha et al., 2003). Stoic et al. (2002) propoe the twin crew compreor and expander technology for CO 2 refrigeration ytem, preenting olution to overcome the high bearing force likely to be aociated with the preure difference that are inherent to thi refrigerant. Finally, turbo expander are alo cited in the literature (Braz, 1995a; 1995b). The ue of a Pelton-type expander, with a backward-curved-blade impeller, ha been recently reported in the literature (He et al., 2007). Thi expander wa teted in a R410A refrigeration ytem and iolated performance data (rotational peed and pecific recovery power) were obtained a a function of condening and evaporating temperature. 2. VAPOR COMPRESSION CYCLES WITH EXPANSION WORK RECOVERY A review on the available literature reveal a reaonable number of propoed option for thermodynamic cycle to make the bet ue of the expanion work recovery from vapor compreion refrigeration cycle. Cycle can be claified under many different categorie, following pecific criteria. One major criterion i if the heat rejection temperature i above the refrigerant critical temperature. For reaon explained by many author (e.g., Huff et al., 2003), the CO 2 trancritical cycle i the mot uceptible to COP improvement with the recovery of the expanion work. On the other hand, the high preure difference encountered in the CO 2 trancritical cycle poe new technological challenge. Therefore, performance and technological outcome are expected to differ ignificantly between trancritical and traditional vapor compreion cycle with expander, even if the ame thermodynamic cycle i ued (a far a component dipoition in the flowchart i concerned). The exiting propoed cycle are briefly outlined next. Heyl and Quack (1999) lit 14 of thee cycle, tarting with the baic vapor compreion refrigeration cycle (compreor, condener or ga cooler, throttle device and evaporator). The throttle device i then ubtituted by the

5 2416, Page 4 expander which can either upply work directly to the compreor (direct drive type Hiwata et al., 2003) or to another power ink (generator type Hiwata et al., 2003). The firt option reult in an obviou reduction in power conumption (typically, electricity or haft power from an internal combution engine) but inevitably tie expander and compreor peed. Even with a peed reduction between them, compreor and expander may find themelve operating away from their maximum efficiencie over the operational range of the ytem. Thi iue wa alo addreed by Huff et al. (2002) and Hiwata et al. (2003). The ingle tage direct drive cycle wa the option propoed by Braz et al. (2000), for the twin crew compreor-expander combination, and by Yang et al. (2007b), for a theoretical optimal heat rejection preure tudy. Both recovery cheme (expanion power recovery to compreor or to a eparate power haft) can alo be applied to the cycle with internal heat exchanger (liquid line-uction line heat exchanger in ub-critical vapor compreion cycle). Becaue of refrigerant thermodynamic characteritic, the internal heat exchanger i an almot mandatory preence in the traditional CO 2 trancritical cycle without work recovery. However, Robinon and Groll (1998), in a eminal paper, pointed out that the ue thi exchanger in conjunction with the expander would, in fact, degrade the performance of the work recovery cycle. Huff et al. (2002) included two other variation of the expanion work recovery trancritical cycle, by fitting an expanion valve (ienthalpic expanion) either in parallel or downtream the expander. Thi wa done to provide room for adjutment of the optimum heat rejection preure, in view of non-adjutable expander peed and volume ratio. Finally, Hiwata et al. (2003) propoed another variation for ingle-tage combined expander and expanion valve, with the latter placed uptream the expander, and a by-pa valve intalled in parallel to the expanion valve/expander line. With the adoption of multi-tage compreion, the poibilitie of expanion work recovery increae, a reported by everal author (Heyl and Quack, 2000; Nickl et al., 2003; Hiwata et al., 2003; Nickl et al., 2005; Kim et al. 2007; Yang et al., 2007c; Yang et al., 2007d). For the purpoe of the preent tudy, although it i recognized that work recovery with multi-tage compreion may lead to an even greater COP (Nickl et al., 2005), effort were concentrated in the mot probable outcome from an adaptation of current automotive air conditioning ytem, namely, the ingle-tage compreion cycle. The ingle-tage vapor compreion refrigeration cycle with expanion work direct recovery to compreor, with an expanion valve placed downtream the expander, a propoed by Huff et al (2002), i here analyzed. Figure 1 depict the chematic of the cycle. Heat rejection i via a condener (CD) or a ga cooler (GC), depending on whether ub-critical or trancritical cycle i employed. All recovered power from the expander (EX) goe to the compreor (CP). The expanion valve (XV) i located downtream the expander to control either the heat rejection preure (trancritical cycle) or the evaporator outlet uperheat (ub-critical cycle). 3 CD or GC 2 1 EX CP XV 4 5 EV Figure 1: Single-tage compreion cycle with expanion work direct recovery to compreor and downtream thermotatic expanion valve. For thi preliminary tudy, the ub-critical cycle, running with refrigerant R134a, wa choen. In pite of the uperior improvement in COP one find when expanion work recovery i applied to trancritical CO 2 cycle, the choice for the preent analyi wa baed on the evidence that current automotive air conditioning ytem till run on the ubcritical vapor compreion cycle. Thi tudy aim at preenting the potential for COP improvement with expanion work recovery applied to a cycle, Figure 1, that apparently require the leat of modification to exiting ytem.

6 2416, Page 5 3. THERMODYNAMIC MODEL A baic thermodynamic model wa developed to evaluate the performance of the cycle under different operating condition. Fundamental conervation ma and energy conervation equation were applied to each component of the cycle. Steady-tate regime wa aumed. Heat loe and preure drop were neglected. w w cp, The work of compreion, cp, i given in term of the ientropic work of compreion, : wcp, h2 h1 wcp (1) cp, cp, The volumetric and ientropic compreor efficiencie were taken from Brown et al. (2002). 1 vcp, cp 1 (2), cp cp (3) where cp i the compreor preure ratio. P cp (4) Pev and, the pecific heat ratio, taken at uction condition. According to Brown et al. (2002), equation (2) wa obtained by curve-fitting data from different author for CO 2 and R134a compreor. Equation (3) wa ued by Brown et al. (2002) for both CO 2 and R134a compreor, for the purpoe of their comparion analyi. A precribed degree of ubcooling i aumed for the condener: T T T (5) c 3 The work of expanion, w ex, i : ex ex, ex, 3 4 ex, w w h h (6) The literature i carce on information about the efficiencie of poitive diplacement expander for ubcritical vapor compreion cycle, in particular, their dependence on operational condition, uch a preure ratio or rotational peed. For trancritical cycle one could mention experimental data from: (i) Fukuta et al. (2003) and (ii) Huff et al. (2005), with volumetric and ientropic efficiencie of CO 2 vane and croll expander, repectively, a a function of rotational peed; from (iii) Baek et al. (2005), with ientropic efficiency and ma flow rate at three different operating condition of a CO 2 piton-cylinder expander; (iv) from Zeng et al. (2007), who preent data for a CO 2 rolling piton, with three ditinct working condition, from which the ientropic efficiency can be derived, and (v) from Braz et al. (2000), with a combined efficiency for the compreor expander device running on R113, a a function of haft peed and compreor ma flow rate. Adiabatic expanion i aumed, o that: wex h3 h 4 (7) Refrigerant expanion with work recovery i from to P : P 4 P ex (8) P4

7 2416, Page 6 The thermotatic expanion valve wa imulated from it function, i.e., to provide a precribed degree of uperheat at the compreor inlet. And the expanion proce through the valve i uppoed to be adiabatic and, conequently, ienthalpic. T1 Tev Th (9) The refrigerating effect,, i, of coure: q ev h4 h 5 (10) qev h1 h 5 (11) and the refrigerating coefficient of performance, taking into account the expanion work recovery and both compreor and expander mechanical loe, i: qev COP wcp w cp, m ex ex, m (12) With no expanion work recovery, the coefficient of performance i: COP h h 1 3 n (13) 1 wcp cp, m 4. RESULTS A imulation program, uing on the EES (Engineering Equation Solver) platform, wa developed to olve the ytem of equation above. Figure 2 how the predicted reult for the improvement in the COP, COP COP COP n, a compared to the ratio between the compreion preure ratio and the expanion ratio, ex, cp ex cp. The following value were ued in the imulation: T o C, T 0 o ev C, T 5 o c C, T 15 o h C, ex, m cp, m 0.9. The expander preure ratio wa made to vary from the compreor preure ratio, full preure cycle, with no expanion valve, to a minimum of 20% of that value. The expander ientropic efficiency aumed value from 0.2 to 0.8. It can be oberved, from Figure 2, that, even with the ub-critical vapor compreion cycle, gain on the COP are reaonable, at leat for large expander efficiencie. However, with operation of more realitic value for ex,, theoretical COP gain lie within 0-5%, approximately. Thi improvement will alo depend on the expanion ratio, which i a function of expander characteritic and of the coupling between compreor and expander, not covered in the preent analyi. The COP improvement wa more prominent for a higher condening temperature. A expected, the oberved variation on the refrigerating effect wa marginal. 5. CONCLUSIONS A theoretical analyi wa carried out on the potential for COP improvement with the partial recovery of the expanion work for a vapor compreion cycle running with R134a. In pite of the attention that ha been primarily given for thi application to trancritical CO 2 cycle, it ha been proven that gain in cycle efficiency may till be relevant to traditional automotive air conditioner cycle. The preent analyi alo etablihe condition for future tudie of uch cycle running on new LGWP ynthetic fluid.

8 2416, Page 7 1,1 T =40 [C] 1,1 T =50 [C] ex, 0.8 COP 1,05 ex, COP 1, ,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1 ex;cp Figure 2 Improvement on cycle COP a a function of expander preure ratio and expander ientropic efficiency, for condening temperature of 40 o C and 50 o C (R-134a). ex;cp NOMENCLATURE COP coefficient of performance (-) Greek Symbol h pecific enthalpy (kj/kg) pecific heat ratio (-) P preure (kpa) Tc degree of ubcooling ( o C) q ev refrigerating effect (kj/kg) Th degree of uperheating ( o C) T temperature ( o C) COP COP improvement ratio (-) T condening temperature ( o C) m mechanical efficiency (-) T evaporating temperature ( o C) ientropic efficiency (-) ev w work of compreion (kj/kg) v volumetric efficiency (-) cp w cp, ientropic work of compreion (kj/kg) preure ratio (-) w ex work of expanion (kj/kg) ex, cp ex to cp ratio (-) w ex, ientropic work of expanion (kj/kg) Subcript 1 compreor inlet 5 evaporator inlet 2 compreor dicharge condener 2 dicharge at ientropic compreion cp compreor 3 condener outlet ev evaporator 4 expander dicharge ex expander 4 dicharge at ientropic expanion xv expanion valve REFERENCES An, Q., Ma, Y., Tian, H., 2007, The Function of Lubrication in CO 2 Heat Pump with Expander, Proceeding of the 22 nd International Congre of Refrigeration, IIR/IIF, paper ICR07-B2-340, Beijing, China. Baek, J.S., Groll, E.A. Lawle, P.B., 2005a, Piton-cylinder Work Producing Expanion Device in a Trancritical Carbon Dioxide Cycle. Part I: Experimental Invetigation, Int. Journal of Refrigeration, vol. 28, pp Baek, J.S., Groll, E.A. Lawle, P.B., 2005b, Piton-cylinder Work Producing Expanion Device in a Trancritical Carbon Dioxide Cycle. Part I: Theoretical Model, International Journal of Refrigeration, vol. 28, pp Braz, J.J., 1995a, Two-Phae Flow Turbine, United State Patent, number 005,467,613. Braz, J.J., 1995b, Improving the Refrigeration Cycle with Turbo-Expander, 19 th International Congre of Refrigeration, IIR/IIF, vol. IIIa, pp , The Hague, The Netherland. Braz, J.J., 2001, Single Rotor Expreor a Two-Phae Flow Throttle Valve Replacement, US Patent, n

9 2416, Page 8 Braz, J.J., 2003, Screw-Expreor Teting on an R134-a Chiller Efficiency, Liquid Carry-over, and Chiller Benefit, Int Conference on Compreor and Their Sytem, paper C615/051/2003, pp , London, UK. Braz, J.J., Smith, I.K., Stoic, N., 2000, Development of a Twin Screw Expreor a a Throttle Valve Replacement for Water-Cooled Chiller, Fifteenth International Compreor Engineering Conference at Purdue Univerity, Wet Lafayette, IN, USA, pp , July Brown, J.S., Yana-Motta, S.F., Domanki, P.A., 2002, Comparative Analyi of Automotive Air Conditioning Sytem Operating with CO 2 and R134a, International Journal of Refrigeration, vol 25, pp Fukuta, M., Yanagiawa, T., Radermarcher, R., 2003, Performance Prediction of the Vane Type Expander for CO 2 Cycle, International Congre of Refrigeration, IIR/IIF, paper ICR0251, Wahington, D.C., USA. He, T., Li, L., Xia, C., Shu, P., 2007, Application of Pelton-type Expander to Replace Throttling Valve in Refrigeration Sytem, 22 nd Int Congre of Refrigeration, IIR/IIF, paper ICR07-E2-958, Beijing, China. Heidelck, R., Krue, H., 2000, Expanion Machine for Carbon Dioxide Baed on Modified Reciprocating Machine, Final Proceeding of the 4 th IIR-Gutav Lorentzen Conference on Natural Working Fluid at Purdue, pp , Wet Lafayette, USA. Heyl, P. Quack, H., 1999, Free Piton expander-compreor for CO 2 Deign,, Application and Reult, 20 th International Congre of Refrigeration, IIR/IIF, Sydney, Autralia, vol. III, paper 516, pp Heyl, P., Quack, H., 2000, Trancritical CO2-Cycle with Expander-Compreor, Final Proceeding of the 4 th IIR- Gutav Lorentzen Conference on Natural Working Fluid at Purdue, pp , Wet Lafayette, USA. Hiwata, A., Lida, N., Sawai, K., 2003, A Study of Cycle Performance Improvement with Expander-Compreor in Air-Conditioning Sytem, International Conference on Compreor and Their Sytem, paper C615/028/2003, pp , London, UK. Huff, H., Linday, D., Radermarcher, R., 2002, Poitive Diplacement Compreor and Expander Simulation, International Compreor Engineering Conference at Purdue, paper C9-2, Wet Lafayette, USA. Huff, H., Radermarcher, R., Preiner, M., 2003, Experimental Invetigation of a Scroll Expander in a Carbon Dioxide Air-Conditioning Sytem, Int Cong Refrigeration, IIR/IIF, paper ICR0485, Wahington D.C., USA. Kim, H.J., Ahn, J.M., Cho, S.O., Cho, K.R., 2007, Numerical Simulation on Scroll Expander-Compreor Unit for CO2 Tran-critical Cycle, Applied Thermal Engineering, doi: /j.applthermaleng Liu, B., Zhang, B., Peng, X., Shu, P., 2007, Reearch Development of Free Piton Expander for Trancritical CO 2 Refrigeration Cycle, e 22 nd International Congre of Refrigeration, paper ICR07-B2-920, Beijing, China. Nickl, J., Will, G., Krau, W.E., Quack, H., 2003, Third Generation CO 2 Expander, International Congre of Refrigeration, IIR/IIF, paper ICR0571, Wahington, D.C., USA. Nickl, J., Will, G., Quack, H., Krau, W.E., 2005, Integration of a Three-Stage Expander into a CO 2 Refrigeration Sytem, International Journal of Refrigeration, vol. 28, pp Robinon, D.M., Groll, E.A., 1998, Efficiencie of Trancritical CO 2 Cycle with and without an Expanion Turbine, International Journal of Refrigeration, vol. 21, no. 7, pp Stoic, N., Smith, I.K., Kovacevic, A., 2002, A Twin Screw Combined Compreor and Expander for CO2 Refrigeration Sytem, Sixteenth Int Compreor Engineering Conference at Purdue, Wet Lafayette, USA. Yang, B., Zeng, H., Guo, B., Peng, X., Xing, Z., 2007a, Mathematical Modeling and Experimental Invetigation of the Vane Motion of a Rotary Vane Expander in CO 2 Heat Pump Sytem, Proceeding of the 22 nd International Congre of Refrigeration, IIR/IIF, paper ICR07-E2-1475, Beijing, China. Yang, J., Ma, Y., Li, M., Zeng, X., 2007b, Invetigation on the Optimal Heat Rejection Preure of CO 2 Trancritical Expander Cycle, 22 nd Int Congre of Refrigeration, IIR/IIF, paper ICR07-B1-254, Beijing, China. Yang, J., Zhang, L., Cheng, J., Lu, P., 2007c, Development of a Combined Compreor-Expander for Trancritical CO 2 Sytem, 22 nd International Congre of Refrigeration, IIR/IIF, paper ICR07-B2-1177, Beijing, China. Yang, J.L., Ma, Y.T., Liu, S.C., 2007d, Performance Invetigation of Trancritical Carbon Dioxide Two-Stage Compreion Cycle with Expander, Energy, vol. 32, pp Zeng, X., Ma, Y., Liu, S., Wang, H., 2007, Teting and Analyzing on P-V Diagram of CO2 Rolling Piton Expander, 22 nd International Congre of Refrigeration, IIR/IIF, paper ICR07-B2-431, Beijing, China. Zha, S., Ma, Y., Sun, X., 2003, The Development of CO 2 Expander in CO 2 Trancritical Cycle, International Congre of Refrigeration, IIR/IIF, paper ICR0089, Wahington, D.C., USA. ACKNOWLEDGEMENT Thank are due to CNPq and FAPERJ, Brazilian reearch funding agencie, for the financial upport provided.