1、456 2009 ASHRAEABSTRACTCarbon Dioxide (CO2), which is generally associated with green house effect and damage to the environment, can be used in the fight against ozone depletion. CO2 (R744) is a prac-tical replacement to existing fluoro-carbon based refrigerants. Although R744 has a global warming
2、potential (GWP) of 1, it is about 1000-3000 times lower than the GWP of most other commonly used refrigerants. This paper follows a theoretic approach to compare the performance of R744with other natu-ral refrigerants, employing the first law of thermodynamics, i.e., using coefficient of performance
3、 (COP). It shows the opti-mized pressure plots of a R744compressor and sketches the effect of compressor performance on the COP of the refriger-ation system.INTRODUCTIONCarbon Dioxide (R744) was a well known and widely accepted refrigerant in the early 1900s, but its popularity reduced with the intr
4、oduction of fluorocarbons. The revival of R744 as a refrigerant started over a decade ago in Europe with the work of Dr. Gusav Lorentzen and Dr. Jostein Petterson1. This sudden rediscovery was invoked by growing environ-mental concerns of global warming and ozone depletion. R744has some very attract
5、ive properties, which makes it destined to be used as a working fluid. It is non-flammable, non-ozone depleting, has good heat transfer properties, a high volumetric capacity, it is easily available and economic. However its critical temperature is 31.1oC, which is generally lower than the heat reje
6、ction temperature of a typical refrig-eration and air conditioning system. Thus, wherever the heat rejection temperature is greater than the critical temperature, R744must operate in a transcritical cycle, i.e, with a sub crit-ical low-side pressure and a supercritical high side pressure.The work of
7、 Dr. Peter Neksa has already proved the advantages of using R744 for water and space heating appli-cations. In the field of automobile cooling systems, R744 also has proved advantageous over the conventional system in terms of better cooling performance, improved fuel consump-tion and zero ozone dep
8、letion rate2. There is a drive to move R744towards space cooling and it is being developed and tested across Europe.TRANSCRITICAL CYCLEThe critical temperature is the temperature above which there is no clear distinction between liquid and gaseous phase. As the critical temperature is approached, th
9、e properties of the gas and liquid phases become the same. Above the critical temperature, there is only one phase (supercritical fluid) that is characterized by density and no latent heat effects. The crit-ical pressure is the vapor pressure at the critical temperature. R744has a critical pressure
10、of 7.38 MPa at the critical temper-ature of 31.1C.In a normal refrigeration cycle, the gas from the compres-sor outlet is condensed in the condenser, by removing latent heat of condensation. But in a transcritical CO2cycle the discharge pressure of the compressor is above the critical point, where h
11、eat transfer cannot take place by phase change 1.Man-Hoe Kim, Jostein Pettersen, Clark W. Bullard, 2003, Funda-mental process and system design issues in CO2 vapour compres-sion system.2.Peter Neksa, Jostein Petterson and Geir Skaugen, 2006, CO2 Refrigeration, Air Conditioning and Heat Pump technolo
12、gy.A Closer Look at CO2 as a RefrigerantNorbert Mller, PhD Jijo Oommen JosephMember ASHRAENorbert Mller is an assistant professor and Jijo Oommen Joseph is a masters student in the Department of Mechanical Engineering at Mich-igan State University, East Lansing, MI.LO-09-042 2009, American Society o
13、f Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2009, vol. 115, part 2. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permis
14、sion.ASHRAE Transactions 457(condensation). In such a cycle, the gas from the compressor is cooled in a gas cooler, causing the density of the gas to increase, while temperature decreases. The supercritical temperature and pressure are not coupled, so they can be opti-mized, giving us an additional
15、degree of freedom.COMPARISON OF NATURAL REFRIGERANTSIn this section we compare the COP and volume flow rates of ammonia (R717), carbon dioxide (R744) and water (R718). It has already been derived that the thermodynamic and the transport properties of R744 are comparable with other refrig-erants. 1In
16、 Figure 1, the x-axis represents the evaporator temper-ature and the y-axis, the temperature lift. Temperature lift is defined as the temperature difference between the evaporator and the condenser/gas cooler of a refrigeration system. The assumptions made for the comparative study are as follows:a.
17、 Single stage compressor with polytropic efficiency of 100% was assumed in each case.b. There is no superheating of the refrigerant.c. There is no energy exchanged with the environment.d. There is no subcooling of the refrigerant.e. There is no pressure loss in the piping or the heat exchangers.f. O
18、ptimized gas cooler pressure for R744Figure 1 shows an operating range for the evaporator temperature from -55C to 25C for R744, as compared to -1C to 265C for R718, and -40C to 105C for R717. From the figure it is also possible to obtain the performance of the shown refrigerants for a particular co
19、ndenser and evaporator temper-ature. As an example the performance of the various cycles for a condenser temperature of 35 C versus varying evaporator temperature has been plotted using the dash-dot line.Compared to R717 and R718, R744 always shows the best COP in the temperature range -55oC to -35o
20、C, for the temperature lift of up to around 55 K. With decreasing temper-ature lift down to 15K, this range extends to an evaporator temperature of about 15C. Therefore R744 can certainly be preferred for food freezing and cryogenic process industry and air-conditioning (AC) in moderate and northern
21、 regions.While this study was conducted for the use of an expan-sion valve at the inlet to the evaporator, exergy studies performed for transcritical systems have shown that the COP of the cycle can be enhanced 33% by using an expander in place of the conventional throttling valve. 3The volume flow
22、rate of R744is considerably lower than that of other refrigerants (Figure 2), which makes it ideal for miniaturization. For an evaporator temperature of 20 C, R744has a volume flow rate approximately 10 times lower than the nearest competitor (R-22). Figure 1 Iso-COP curves of R744, R717 and R718 in
23、 an ideal cycle.3.Jun Lan Yang, Yi Tai Ma, Min Xia Li and Hai Qing Guan, 2004, Exergy analysis of transcritical carbon dioxide refrigeration cycle with expander.458 ASHRAE TransactionsStudies comparing R744with other refrigerants have shown that the risk of the high pressure in a R744system is negat
24、ed by the fact that the energy contained in thesystem is relatively lower than that in a R-22 system owing to the lower volume and refrigerant charge.1The high-pressure low-flow rate also allows for the design of small diameter tubing or even micro-channel cooling. This property can well be used for
25、 micro applications like cooling of electronics and lasers systems. Due to the high pressures involved, the use of conventional material like silicon for the microchannels can be challenging, diamond microchannels can be an alternative. Diamond has good thermal conductivity and is already being used
26、 as a heat sink in micro chips. Using diamond, it is possible to attain high fin efficiency for heat sinks.4 However, pressure tests are yet to be done on diamond micro-channels for pressure ranges encountered in the R744 system. Research done on stainless steel and aluminum micro-channels has shown
27、 that due to the reduced diameter they are able to withstand the high pressure.4COEFFICIENT OF PERFORMANCER744 systems operates in two cycles, in a reversed Rankine cycle below the critical point and in a transcritical cycle, if the state of the discharge gas of the compressor is 4.Kenneth Goodson,
28、Katsuo Kurabayashi and R, Fabian W. Pease, 1997,Improved heat sinking for laser-diode arrays using micro-channels in CVD diamond.Figure 2 Volume flow rate comparison for a refrigeration capacity of 3.517 kW (1 ton) per hour.Figure 3 Iso-COP lines for R744.ASHRAE Transactions 459above the critical pr
29、essure. The enlarged iso-COP lines for R744 only, assuming the same condition as above are shown in Figure 3. All curves were obtained for optimized pressure ratio as discussed in the following section. Once the optimum pressure ratio is determined, the isentropic specific work done by the compresso
30、r can be calculated bywhere= the specific gas constant for CO2= the ratio of specific heats= the discharge pressure of the compressor= the suction pressure of the compressor= the suction temperature of the compressorThe specific evaporator heat can be calculated from the specific enthalpy difference
31、 between the fluid entering and leaving the evaporator. The ratio of the specific evaporator heat to the specific compressor work gives the COP.PRESSURE RATIOIn the reversed Rankine cycle the compressor outlet pres-sure is dictated by the condensation pressure corresponding to the condenser temperat
32、ure. In the transcritical cycle, the compressor outlet pressure can be varied to any value above the critical pressure. However, for a maximum COP there exists an optimum compressor discharge pressure correspond-ing to the gas cooler outlet temperature (Figure 4).Since the cycle has no superheating,
33、 the evaporator temper-ature dictates the compressor suction pressure (Figure 5).wisRCO2T1 1-p2p1- 1-1=RCO2p2p1T1Figure 4 For maximum COP optimized gas cooler pressure vs. gas cooler temperature.Figure 5 Evaporator pressure vs. evaporator temperature.460 ASHRAE TransactionsCombining the optimum pres
34、sure corresponding to the gas cooler temperature and the evaporator pressure, the compressor pressure ratio can be calculated (Figure 6). These results are consistent with those presented by Dr. Liao, Zhao and Kakobsen, which were limited to a very small temperature range. 5It is seen that although
35、R744 operating pressure is much higher than that of other refrigerants, the pressure ratio of the compressor is comparatively small. Thus the R744 compres-sor can operate with considerably higher isentropic efficiency if designed and produced with same quality (polytropic effi-ciency). However, some
36、 studies conducted for R744 compres-sors have shown that the advantage of higher efficiency is often reduced by an larger entropy production in the compres-sor. 6For a better understanding of the effect of the pressure ratio on the COP, both iso-COP lines and iso-pressure lines have been plotted aga
37、inst the same axis, i.e, evaporator temperature vs. temperature lift in Figure 7.COMPRESSOR EFFICIENCYIn this section the effect of polytropic efficiency ( ) of the compressor on the cycle performance is investigated. The polytropic efficiency can be related to the COP via the 5.Liao S.M, Zhao T.S.
38、and Jakobsen A., 2000 A correlation of opti-mal heat rejection pressures in transcritical carbon dioxide cycles.6.J.S. Brown, Y. Kim and P.A. Domanski, 2002, Evaluation of carbon dioxide as R-22 substitute for residential air-conditioning.Figure 6 Iso-COP lines showing pressure ratio at various evap
39、orator temperature.Figure 7 Iso-COP lines and constant pressure ratio lines.polyASHRAE Transactions 461isentropic efficiency ( ) of the compressor. For this, the polytropic exponent ( ) can be calculated with the isentropic exponent ( ) for various polytropic efficiencies:Once we have the polytropic
40、 exponent, we can calculate the polytropic work done by the compressor:We can now calculate the isentropic efficiency of the compressor corresponding to the polytropic efficiency. The variation in isentropic efficiency is a function of the pressure ratio.Using the definition of COP, we can calculate
41、 the COP incorporating the isentropic efficiency of the compressor. Figures 8 and 9 show the iso-COP lines for a fixed polytropic efficiency, indicating the variation of the isentropic efficiency with evaporator temperature as a function of the pressure ratio.isnn 1n- 1-1poly-=wpolyRCO2T1nn 1-p2p1-n
42、 1n-1=wisis-wpolypoly-=Figure 8 Isentropic efficiency vs. evaporator temperature for a polytropic efficiency of 70%.Figure 9 Isentropic efficiency vs. evaporator temperature for a polytropic efficiency of 40%.462 ASHRAE TransactionsFigure 10 shows the iso-COP lines, the iso-pressure ratio lines and
43、the isentropic efficiency lines corresponding for evaporator temperature vs. temperature lift considering 70% polytropic efficiency of the compressor. CONCLUSIONThe study presents COP maps for conventional and tran-scritical R744 (CO2) systems. It also provides the optimum pressure ratios for a wide
44、 range of gas cooler or condensation temperatures and evaporator temperatures as they have been used in this work. For certain compressor quality (polytropic efficiency) maps are generated that summarize the compressor requirements in terms of pressure ratio and isentropic effi-ciency, along with th
45、e obtainable COP for a wide range of combinations of evaporator temperature and temperature lift. It can be concluded that R744 is a viable natural refriger-ant, especially for lower temperatures or moderate tempera-ture lifts. The volume flow rates of R744 are very small as are also the pressure ra
46、tios. This combination renders R744 also as an ideal refrigerant for micro-scale cooling systems.DISCUSSIONPhillip Johnson, Director of Engineering, McQuay Inter-national, Staunton, VA: More than 100 years of industrial refrigeration industry practice agree with this papers conclusion that carbon di
47、oxide (R-744) is an excellent refrigerant for low-temperature refrigeration applications with moderate lift. The empirical evidence is the common use of carbon dioxide in the low-temperature side of cascade-type refrigeration systems,* which tend to operate in the region of Figure 1 identified as “R
48、744 best.” While the title and focus of this paper is on carbon dioxide, an inter-esting observation is the good performance of water (R-718) at higher-temperature applications, as shown in Figure 1. Contrary to the case of carbon dioxide, the air-conditioning and refrigeration industry has not hist
49、orically adopted water as a refrigerant. It would be interesting to see an analysis similar to Figure 1, comparing water to the refrigerants most commonly used by the industry. *See the following refer-ences in the 2006 ASHRAE HandbookRefrigeration: Chapter 3, pp. 3.263.27; Chapter 7, pp. 7.237.24; and Chapter 16, p. 16.7.Figure 10 Iso-COP lines, constant pressure ratio lines and isentropic efficiency fo
