CO 2 AS REFRIGERANT FOR SYSTEMS IN TRANSCRITICAL OPERATION PRINCIPLES AND TECHNOLOGY

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1 This is the second in a two-part examination of carbon dioxide as a refrigerant in transcritical applications. The full paper was originally presented at AIRAH s 2004 natural refrigerants conference, held at the Museum of Sydney on July 28. For copies of the conference proceedings, contact the AIRAH office on AS REFRIGERANT FOR SYSTEMS IN TRANSCRITICAL OPERATION PRINCIPLES AND TECHNOLOGY STATUS P.Nekså Dr.ing, Senior Research Scientist SINTEF Energy Research, NO-7465, Trondheim, NORWAY Introduction This month EcoLibrium presents the second part of Petter Neksa s examination of carbon dioxide as a refrigerant in transcritical applications. The first part of this paper was published in the September 2004 issue of EcoLibrium and the full paper was presented at the AIRAH natural refrigerants conference, held at the Museum of Sydney on July 28, For copies of the full conference proceedings, contact the AIRAH office on Keywords: Carbon dioxide,, natural refrigerants, refrigeration 6. COMMERCIAL REFRIGERATION Commercial refrigeration systems for shops, supermarkets, larger kitchens etc. have large refrigerant emissions, and the energy use is often high. Palandre et al [37] reports that commercial refrigeration has the largest emissions by sector within refrigeration, 37% of the total emissions. On average the annual leakage rates is reported to be about 30% of the system charge per year. A recent study of 220 supermarkets in Norway showed a leakage rate of 14% (not including standalone equipment) [38]. Taking into consideration the high focus leakage reductions has had in Norway, also using taxation as an instrument, and the fact that this is a supermarket chain with a great focus on the issue, it is expected that this number is in the lower range of the numbers representative for the sector. Thus, there is a need for efficient, safe and environmentally friendly refrigeration systems. New concepts based on have been demonstrated for centralized systems using as a volatile heat transfer fluid, in a low-temperature cascade stage or as the only refrigerant by utilising a trancritical cycle. Decentralised concepts with heat recovery have also been investigated. Some of these developments are outlined in the following text. Theoretical and experimental studies have been carried out for centralised all- systems, showing very promising results [35]. The company Costan has installed centralized all- systems in medium sized supermarkets in Italy, Denmark and Sweden, after a field-testing period of several smaller systems [36]. Whilst energy efficiency was still somewhat lower for medium temperature unit, methods have been studied to reduce energy consumption, and it is expected that an all- system can be made with a seasonal efficiency as a good direct expansion R404A system in the near future. It was also believed that a system like the one installed was better in terms of cost and efficiency than that of a MT secondary / LT cascade system, which was, until then, the best option available to reduce HFC charges. Compared to a direct expansion HFC- 404A system, the cost indicated was already at that stage only 10% higher for the system. This gap should be eliminated when components for are mass-produced. Life-cycle cost of an all- system, including cost of installation and refrigerant should already be competitive. Figure 11 cost comparison of direct expansion HFC-404A system and state of the art all- system [36] Eggen and Aflekt reviewed the possibilities for : i) as secondary refrigerant, ii) as a primary refrigerant in a low temperature stage in a cascade system, and iii) in all- centralised systems [2]. They also presented a prototype /NH 3 cascade system built in Norway. During the last years a considerable number of cascade systems has been installed in Europe, using in the low stage and different refrigerants in the high stage of the cascade. Advantages of cascade systems include the greatly reduced low-temperature compressor sizes, the absence of a liquid pump, and fewer stages of heat transfer. Several secondary fluid systems are also already operating in the Nordic countries, using as a volatile heat transfer fluid. The safety aspects and good thermophysical properties of, leading to small pipe dimensions and good heat transfer, make it a preferable heat transfer fluid in indirect systems. The decentralised distributed supermarket system described by Nekså, Girotto et al. uses as the only refrigerant. Self-contained display cabinets, each with refrigeration units, are connected to a hydronic heat recovery circuit that heats service water and buildings, see Figure 12 [14]. By utilising the transcritical process, it is possible to have a large temperature glide in the hydronic circuit, typically 26 EcoLibrium October 2004

2 50-60 K, and a correspondingly low volume flow rate and small pipe dimensions. Waste heat with high temperature (70-75ºC) is available for tap water and/or space heating. Excess heat is rejected to the ambient air by direct heat exchange. The system offers a very easy installation and gives the owner of the store a great flexibility in arranging and rearranging the cabinets. System simulations for a medium size supermarket have been carried out. Optimum hydronic supply and return temperatures to the cooling and freezing cabinets were identified. A comparison of the system and a conventional R-22 system with respect to the overall energy consumption of the supermarket for one year of operation in a southern European climate was carried out. The system was found to reduce the energy consumption by 32% compared to the R-22 system. 7. HEAT PUMPS FOR SPACE CONDITIONING Until now, most of the focus for heat pumps has been on tap water heating. The market for heat pumps would of course be extended significantly if the demand for space heating could be covered efficiently in addition to the demand for tap water heating. Schiefloe and Nekså investigated a system design as shown in Figure 13 [39, 40]. In order to achieve a lowest possible return temperature from the heating system, radiator and air heating are connected in series. Tap water is pre-heated in parallel with the space heating and heat exchange against hot discharge gas is used to achieve the required hot water temperature. In order to simplify the system design, the tap water heating part could also be implemented as a separate system or covered when space heating is not required. Figure 13 system design for a combined space and water heating system. The process is also illustrated in the T-s diagram Figure 12 decentralised supermarket refrigeration system with central heat recovery/heat rejection Each unit can also be equipped with a condensing unit in order to reject heat directly to the shopping area when space heating is required. In the warm season with a heat surplus, the waste heat recovery circuit removes the heat. This concept reduces the power demand for the refrigeration units to the same level as for the baseline R-22 system, and the resulting overall energy consumption of the supermarket will then be further reduced. Also in light commercial equipment, may become an alternative to HFC refrigerants. The Coca Cola Company has announced during the one-day conference Refrigerants, Naturally, -based refrigeration is currently the best option for the global needs of Coca-Cola s sales and marketing equipment [51]. SINTEF has participated in a group that has assisted TCCC in evaluating refrigeration systems based on the Shecco Technology. During Refrigerants, Naturally McDonalds also announced that they will concentrate, but not limit, research and development on the exploration of alternatives [52]. A comparison with a system using HFC-134a as working fluid showed favourable seasonal performance for the system when more than 30% of the power demand for space heating was demanded by the air heating system. The rest is then a demand of the radiator system. A 70/50ºC radiator system and heat recovery efficiency of the balanced ventilation system of 60% was assumed. In larger buildings in Norway typically more than 50% of the heating demand is air heating and this percentage is increasing due to better insulation and increased air quality requirements. This indicates that may be a promising candidate for this application. Residential heat pumps for heating only are quite common in Europe, especially in the North, and in certain other areas in the world. Stene studied both experimentally and theoretically a residential brine to water heat pump for combined space heating and hot water heating [41]. He found that the seasonal performance factor of integrated brine to water heat pump system was competitive to state of the art systems if the tap water heating demand constituted minimum 25% of the total annual heating demand and if the return temperature for the space heating system was sufficiently low (30ºC). It was also crucial to avoid heat transfer from hot to cold water in the storage tank. EcoLibrium October

3 Rieberer, Halozan et. al. made detailed theoretical studies of controlled ventilation air heating systems with an integrated heat pump. The results looked very promising [42]. The overall system seasonal performance factor for a Graz, Austria climate was calculated to be in the range 6.15 to 6.5. This corresponds to a seasonal performance factor of the heat pump of above 4 (author s remark). Ground source heat is important as heat source for heat pumps in Europe. The brine systems frequently used may cause problems regarding polluting ground water if leakages occur. Both Univ. of Graz [43] and FKW in Hannover have developed earth probe systems with. The probes are filled with and collecting heat from the ground by evaporation and rejecting heat in a cold head as heat source for a heat pump. The principle can be compared to that of a heat pipe. Very promising results have been reported, and the product is already commercialised. evaporating temperature. A description and analysis of the pre-series one- and two-stage compressors can be found in Nekså, Dorin et al. [16,17]. Figure 14 shows a picture of the compressor, and measured overall isentropic and volumetric efficiency figures for medium sized compressors at the current stage of development. The single-stage machine reaches quite high efficiencies, especially at lower pressure ratios, representative for medium temperature refrigeration and upward. Two-stage figures are representative for the compressor operating in a system without throttling to medium pressure, indicating how it would perform in a single stage process compared to the single stage compressor. As the efficiency figures indicate, two-stage compression will give an advantage regarding energy efficiency in low temperature applications, typically for pressure ratios in the range of 6 to 9 when using. Several studies have investigated air-to-air reversible heat pumps with as refrigerant. This has been well summarised in [45]. In general it was found that the heat pumps compete with the HFC systems in heat pump mode, while air conditioning mode at high ambient temperatures lacked in efficiency. Even though the seasonal performance in cold climates may be superior for the systems already, these systems have not found the way to the market place yet. SINTEF is doing development work on an Asian/European type of split system, using a state of the art HFC-410A system as baseline. So far, the system performs better than the baseline system in heating mode, while lacking somewhat in efficiency in cooling mode. Heat exchangers better adapted to reversed cycle operation are expected to improve the system performance. 8. HEAT PUMP DRYERS Another interesting application is heat pump dryers. Based on theoretical considerations, Steimle reported that energy saving is possible due to better temperature adaptation in the heat exchangers, compared to subcritical processes [25]. It is also possible to achieve higher air temperatures without loss in efficiency, thus increasing the moisture extraction rate. Experimental results from Schmidt, Klöcker et al. report hp-cops in the range 5.5 and 55% reduction in the energy consumption, including fan power, compared to a traditional electrically heated clothes dryer [24]. The results were achieved after a first optimisation of the prototype system, and it is hoped that further essential improvements still can be reached. Eikevik investigated heat pump dryers for food drying applications [46]. Very favourable efficiency figures for the system were reported. Also very important was the high degree of flexibility the system could offer, with a temperature range for the inlet of the drying chamber in the range of 30ºC to +110ºC. This is due to very favourable thermophysical properties of as working fluid in such systems. 9. COMPRESSORS The company Dorin, Italy, developed the first high-pressure semi-hermetic compressor series in the range of m 3 /h swept volume. The series comprises single- and two-stage compressors with two cylinders, running at nominal speeds of 1450 and 2900 rpm (50Hz). This corresponds to cooling capacities in the range of 3-25 kw at 10ºC Figure 14 compressor design and measured volumetric and isentropic efficiency for a single-stage and a two-stage pre-series compressor with a swept volume of 2.7m 3 /h, as function of the pressure ratio, for high-pressures of 80, 95 and 110 bar. A constant suction gas superheat of 10ºC was applied. For the two-stage compressor the intermediate pressure gas was cooled to 20ºC. The German company Bock developed a high-pressure, opentype compressor for transport applications. This development is described by Kaiser [9]. A semi-hermetic version of this compressor was introduced in Mycom also announced in 2003 that they were developing a semihermetic compressor for high-pressure compression. Several compressors are developed or under development from a variety of companies. Omitting all the efficient compressors developed for mobile applications, it is important to mention the variety of hermetic compressors developed in Japan, primarily to serve for the heat pump water heater systems launched in the market. From an European perspective it may be important to mention development of small hermetic compressors from the company Danfoss, see figure 15, and from the company Embraco. The latter is a development SINTEF has been participating in. 28 EcoLibrium October 2004

4 Figure 15 Danfoss hermetic compressor for maximum operating pressure 130 bar By introducing an expander, the thermodynamic losses in the cycle can be greatly reduced, thus improving the competitiveness even more, but also open application areas for which it so far has shown difficult to meet the efficiency requirements. Several research groups are working on expander concepts for systems, including the free-piston expander by Heyl [8] and the axial-piston machine by Heidelck [7]. 10. HEAT EXCHANGERS AND HEAT TRANSFER Owing to the high operating pressure, heat exchangers generally use small-diameter tubing. Studies on compact air based heat exchangers for mobile and unitary applications have demonstrated the potential for compact and lightweight Figure 16 principles of heat exchanger geometry using multi-port extruded tubes with microchannels, folded fins, and a compact double barrel manifold. The heat exchanger is assembled by brazing in a furnace. From Pettersen et al [18]. designs with high performance, especially when using extruded microchannel tubing, Figure 16 (Pettersen et al.) [18]. Extensive studies have been conducted on heat transfer and pressure drop in microchannels, both covering supercritical-pressure cooled flow (Pettersen et al.) [20], and flow vaporization (Pettersen) [21]. Supercritical-pressure microchannel heat transfer is correlated well with well-known single-phase correlations, while flow vaporization is greatly influenced by nucleate boiling, dryout and post-dryout heat

5 transfer and thus need more advanced correlations. Important contributions to understand these phenomena better can be found in [47], where also flow visualisation of vaporizing pure in a 1 mm heated glass tube are reported. Flow visualisation results of vaporizing with lubricant in the same test tube are reported in [48]. Two-phase flow distribution in heat exchanger manifolds is an important issue in microchannel heat exchangers. The potential drop in heat exchanger capacity could be more than 30% due to maldistribution of two-phase flow in the inlet manifold. This phenomenon was investigated in [49]. Figure 17 shows the flow maldistribution visualized in a glass manifold. and HFC-134a were used as refrigerants and a total of eleven manifold geometries were tested. The experimental data was compared to existing semi empirical models for phase split in T-junctions and a correlation for two-phase manifold flow split was developed. Figure 17 flow visualisation of two-phase distribution in a glass manifold [49] Water/liquid based heat exchangers has also had a certain focus. Most attention has been paid to gascoolers for heat pump water heaters where counter-current flow conditions are required due to the large temperature glide, with emphasis on double-tube concepts. 11. CONCLUSION The revival of as a refrigerant started in Europe more than 15 years ago, and there has been a strong development of new technology worldwide using this refrigerant in several application areas since then. Developments which initially were driven primarily by environmental concerns have often resulted in disclosing additional advantages by using, such as higher COP, higher cooling and heating capacity, better comfort, and added possibilities of heat recovery. Cost- and energy efficient systems have been developed and commercialised for some applications and more seem to come in the near future. With increasing focus on climate gas emission reductions, strict regulations on the use of HFC chemicals may be expected, possibly followed by phase-out targets and dates as announced by some European countries. These trends will clearly drive the interest in the direction of natural refrigerants in general and in particular. REFERENCES [2] Eggen, G. and Aflekt, K., Commercial Refirgeration with Ammonia and as Working Fluids, Natural Working Fluids 98, IIR - Gustav Lorentzen Conference on Natural Working Fluids, Oslo, Norway, IIR, June 2-5, [7] Heidelck, R., Expansionsmaschinen auf der basis modifizierter Hubkolbenmaschinen. Ki Luft- und Kältetechnik, Vol 37, No 3, pp , [8] Heyl, P., Untersuchungen transkritischer -Prozesse mit arbeitsleistender Entspannung - Prozeßberechnungen, Auslegung und Test einer Expansions- Kompressions-Maschine. Dissertation, Institute of Refrigeration and Cryogenics, TU Dresden, [9] Kaiser, H., Verdichter fur naturliche Kaltemittel in Nutzfahrzeugen und Omnibussen. Ki Luft- und Kaltetechnik, No 8, [14] Nekså, P., Girotto, S. and Schiefloe, P. A., Commercial Refrigeration Using as Refrigerant - System Design and Experimental Results. IIR - Gustav Lorentzen Conference on Natural Working Fluids, Oslo, Norway, IIR, June 2-5, [16] Nekså, P., Dorin, F., Rekstad, H., Bredesen, A.M., Serbisse, A., Development of Semi-Hermetic - Compressors, 20th International Congress of Refrigeration, IIR/IIF, Sydney, [17] Nekså, P., Dorin, F., Rekstad, H. and Bredesen, A.M., Development of two-stage semi-hermetic - compressors, 4th IIR-Gustav Lorentzen Conference on Natural Working Fluids, Purdue, USA, July 25-28, [18] Parsch, W., Status of Compressor Development for R-744 Systems, VDA Alternate Refrigerant Wintermeeting, Saalfelden, Austria, January 30-31, [18] Pettersen, J., Hafner, A., Skaugen, G. and Rekstad, H, Development of Compact Heat Exchangers for Air Conditioning Systems, International Journal of Refrigeration, Vol 21, No 3, pp , 1998 [20] Pettersen, J., Rieberer, R., Munkejord, S.T., Heat Transfer and Pressure Drop for flow of Supercritical and Subcritical in Microchannel Tubes. Final Technical Report. US Army, European Research Office, Contract N M Report issued by SINTEF Energy Research and Norwegian University of Science and Technology, February [21] Pettersen, J., Flow Vaporization in Microchannel Tubes, Dr.techn. thesis, Faculty of Engineering Science and Technology, NTNU, February [24] Schmidt, E. L., Klöcker, K. and Flacke, N., Heat Pumps for Dehumidification and Drying Processes in Residential and Commercial Applications. - Hot Air Drying Heat Pump using a Transcritical Process - Technology in Refrigeration, Heat Pump and Air Conditioning Systems, Mainz, Germany, [25] Steimle, F., -Drying Heat Pumps. Technology in Refrigeration, Heat Pump & Air Conditioning Systems, Trondheim, Norway, IEA Heat Pump Program, May 13-14, [35] Nekså, P. and Girotto, S.: as Refrigerant within Commercial Refrigeration, Theoretical Considerations and Experimental results, 5th IIR - Gustav Lorentzen Conference on Natural Working Fluids, September 17-20, Guangzhou, China, 2002 [36] Girotto, S., Minetto, S. and Nekså, P.: Commercial Refrigeration with as Refrigerant, Experimental Results, XXI IIR Int. Congress of Refrigeration, Washington, August 17-22, EcoLibrium October 2004

6 [37] Palandre, L., Clodic, D. and Kuijpers, L.: HCFCs and HFCs emissions from the refrigerating systems for the period , Earth Technologies Forum, Washington D.C., 2004 [38] Veiby O.J.: Internal records/documentation in the ICA supermarket chain in Norway, Oslo, Norway, 2003 [39] Schifloe, P.A. and Nekså, P.: varmepumpe for bygningsoppvarming, forprosjekt (in Norwegian), SINTEF Report TR F4875, Trondheim, Norway, 1999 [40] Nekså, P. (2002): Heat Pumps, International Journal of Refrigeration, Vol 25, Issue 4, June 2002, p [41] Stene J.: Residential brine to water heat pump for combined space heating and hot water heating, Dr.ing (PhD) thesis, Norwegian University of Science and Technology, 2004 [42] Rieberer, R. and Halozan, H.: heat pumps in controlled ventilation systems, Proceedings of the third IIR- Gustav Lorentzen conference on Natural Working Fluids, Oslo, Norway, 1998, p [43] Halozan, H., Rieberer, R. et al.: Direct expansion ground coupled heat pumps, Proceedings of the 21st IIR Int. Congress of Refrigeration, Washington, 2003 [45] Kim, M.H., Pettersen, J. and Bullard, C.: Fundamental process and system design issues in vapor compressin systems, Progr. in Energy and Combustion Science, in press, 2004 [46] Eikevik, T.M., Alves-Filho, O. and Strømmen, I.: Carbon Dioxide Heat Pump an Measurement on Coefficient of Performance and Specific Moisture Extraction Ratio, 2nd Nordic Drying Conference, Copenhagen, June 25-27, 2003 [47] Pettersen, J.: Flow vaporization of in microchannel tubes, Dr. technicae thesis KKT-rapport 2002:1, Norwegian University of Science and Technology, 2002 [48] Argento, A.: Influence of lubricant in flow vaporization of in microchannel tubes, Diploma work no 03/65, FH Kalshruhe in cooperation with SINTEF, Trondheim, December, 2003 [49] Vist, S.: Two-phase flow distribution in heat exchanger manifolds, Dr.ing (PhD) thesis to be published, Norwegian University of Science and Technology, 2004 [51] Cola%20Company%20backgrounder.pdf [52] McDonalds%20backgrounder.pdf About the author Petter Nekså is group manager, senior research scientist for SINTEF Energy Research, Norway. He has written more than 90 international and national reports/publications within the area of refrigeration engineering including system design for refrigeration, air conditioning and heat pumps, trans-critical vapour compression cycles, technology, low temperature refrigeration systems, compressor and heat exchanger technology and working fluids with emphasis on natural working fluids.