Co2 As Refrigerant for Systems in Transcritical Operation Principles and Technology Status

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This is the first in a multi-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 03 8623 3000.

CO2 AS REFRIGERANT FOR SYSTEMS IN TRANSCRITICAL OPERATION PRINCIPLES AND TECHNOLOGY STATUS

P.Nekså Dr.ing, Senior Research Scientist SINTEF Energy Research, Trondheim, NORWAY

Abstract Petter Nekså

The present paper outlines key development areas and results for CO2 systems, mainly focusing on projects where SINTEF/NTNU

has been or are involved, but also referring to development of CO2 technology in general. Emphasis is also given to systems

operating or able to operate in a transcritical cycle. Initially, a brief introduction to the peculiarities of CO2 as a refrigerant is given, before the status and trends within selected areas of the technology are discussed, including heat pump water heaters, mobile air conditioning and heat pumps, commercial refrigeration, heat pumps for space conditioning and heat pump dryers.

It is shown that CO2 is a viable alternative refrigerant for all these application areas. Some comments are also given regarding component development, mainly focusing on compressors and heat exchangers. Finally, some concluding remarks are given on general trends and outlook for the next years.

Keywords: Carbon dioxide, CO2 , natural refrigerants, refrigeration

1. INTRODUCTION This paper mainly concentrates on systems featuring transcritical operation. There has also been an extensive development during Although CO (R-744) was widely used as refrigerant in the early 2 the last 10 years in using CO as a volatile heat transfer fluid for 20th century, its use disappeared from around 1940 with the 2 indirect systems or in a low temperature stage of cascade systems, advent of the fluorocarbon chemicals. Thus, when professor especially within industrial and commercial applications. This is Gustav Lorentzen at NTNU/SINTEF in the late 1980s proposed only sparsely covered here. to reconsider the use of CO2, it had been absent for almost half a century. Increasing focus on environmental issues of fluorocarbon 2. CARBON DIOXIDE AS WORKING FLUID chemicals created a strong interest in systems using natural Compared to conventional refrigerants, the most remarkable refrigerants in general, and CO in particular due to its non- 2 property of CO is the low critical temperature of 31.1ºC. Vapour flammability and non-toxicity [11]. 2 compression systems with CO2 operating at normal ambient New concepts of high-side pressure control in what came to temperatures thus work close to and even above the critical

be called a “transcritical” cycle were devised in early patent pressure of 73.8 bar. This leads to three distinct features of CO2 applications by Gustav Lorentzen and his co-workers. The systems: industrial group Norsk Hydro acquired all commercial rights to this • Heat is rejected at supercritical pressure in many situations. technology in 1990, and through a joint R&D program at SINTEF/ The system will then use a transcritical cycle that operates partly NTNU in the early 1990s, the feasibility and competitiveness of the below and partly above the critical pressure. High-side pressure in technology were demonstrated. Norsk Hydro offers licenses and a transcritical system is determined by refrigerant charge and not technology development under the trademark Shecco Technology by saturation pressure. The system design thus has to consider (www.shecco.com). the need for controlling high-side pressure to ensure sufficient COP A lot of research programs have been ongoing within the industry, and capacity. An example of the measured effect of varying high- in research organisations and at universities around the world. side pressure (compressor discharge) on heating capacity and COP in a heat pump water heater system is shown in Figure 1. CO2 systems have proven to be viable alternatives in several applications. • The pressure level in the system will be quite high (around 30-100 bar). Components therefore have to be redesigned to We have seen the first commercial use of transcritical CO2

systems, in hot water heat pumps starting from 1999, in fuel fit the properties of CO2. Due to smaller volumes of piping and

cell electric vehicles starting from 2003 and lately in commercial components, the stored explosion energy in a CO2 system is refrigeration systems. not much different from a conventional system (Pettersen) [19]. A benefit of high pressure is the 80-90% smaller compressor

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displacement needed for a given capacity. Compressor conditions, which typically are at an extreme ambient temperature. pressure ratios are low, thus giving favourable conditions for high A more sensible basis for comparison is to use mean/average compressor efficiency. conditions, or to apply a seasonal analysis based on climatic variation. • Large refrigerant temperature glide during heat rejection. At supercritical or near-critical pressure, all or most of the heat 3. EFFICIENT HEAT PUMP WATER HEATERS transfer from the refrigerant takes place by cooling the compressed The first application of CO systems on the market is heat pump gas. The heat rejecting heat exchanger is then called gascooler 2 water heaters, where the thermodynamic properties are very instead of condenser. Gliding temperature can be useful in heat favourable. Figure 3 shows, in a temperature-entropy diagram, pumps for heating water or air. With proper heat exchanger design how the temperature characteristics of the transcritical cycle the refrigerant can be cooled to a few degrees above the entering matches the temperature profiles of the heat source and heat sink, coolant (air, water) temperature, and this contributes to high COP giving small heat transfer losses and high efficiency. of the system.

Figure 3 – T-s diagram showing the transcritical CO cycle used for water heating. Figure 1 – variation of heating capacity, heating-COP and 2 compressor shaft power with the discharge pressure for a CO2 heat pump water heater

Figure 4 – 50 kW prototype heat pump water heater in SINTEF/NTNU laboratory

Figure 2 – principal COP behaviour of CO2 system and conventional (baseline) system at varying ambient temperature.

Experience from testing and modelling of CO2 refrigeration and air conditioning systems shows that cooling COP is more sensitive to ambient temperature variation than with conventional refrigerants. This typically leads to the situation shown in Figure 2, where the

CO2 system is superior at moderate and low ambient temperature, and slightly inferior at very high temperature. In this situation, it would be misleading to base the comparison on design-point Figure 5 – measured heating COP of lab prototype system, at water inlet temp 10ºC

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Studies on CO2 heat pump water heaters were initiated at SINTEF/ The EU-commission is now working on a legislation to phase out NTNU from the late eighties, and a full-scale laboratory prototype HFC-134a in mobile air conditioning systems (Callaghan) [1]. The system of 50 kW heating capacity was completed in 1996, current proposal is to ban HFC-134a in all new vehicle types from Figure 4. Results from extensive measurements on this prototype 2011. Alternatives having a GWP value less than 50, alternatively showed that a COP above 4 was achievable even for a hot water 150, are accepted in the current proposal. This means that the

temperature of 60ºC, Figure 5 (Nekså, Rekstad et al.) [15]. options looked at is covered by CO2 and the flammable alternatives The high process efficiency is partly due to good adaptation of the HFC-152a and hydrocarbons. The legislation proposal is currently process to the application, but also due to efficient compression due to a public inquiry, and the start of the phase-out may be

and the good heat transfer characteristics for CO2. A CO2 heat altered. The legislation proposal also introduces measures in order pump water heater may produce hot water with temperatures up to reduce HFC emissions in other applications, but so far phase- to 90ºC without operational problems, and with only a small loss in out is not proposed for these areas. efficiency. Increasing the required hot water temperature from 60ºC Lorentzen and Pettersen published the first experimental data on to 80ºC reduces the heating COP only slightly (from 4.3 to 3.6 at an CO in a mobile air conditioning laboratory prototype system in evaporating temperature of 0ºC). Thus one of the big advantages 2 1992, demonstrating COP data that were competitive to baseline of this technology is the ability to supply water at high temperature CFC-12 system performance [11]. Based on these positive test with good COP. Important application areas for commercial-size results, the automobile industry initiated several development systems are in hotels, apartment houses, hospitals and food projects and further studies on CO systems. The European RACE industries. 2 project from 1994 to 1997 included development and testing of The above heat pump water heater system was a part of the car-installed prototype systems, with results confirming the potential

European Union (EU) cooperative project “COHEPS” from 1996 to for CO2-based car air conditioning (Gentner) [3]. Members in the 1998. Here research groups in Germany (University of Hannover, RACE project included car manufacturers (BMW, Daimler-Benz, Essen University), Belgium (Catholic University of Leuven), Austria Rover, Volvo, Volkswagen), system suppliers (Behr, Valeo), and a (University of Graz) and Norway (SINTEF/NTNU) together with compressor manufacturer (Danfoss). their industrial partners studied various aspects of heat pumping

applications for CO2, including commercial-scale heat pumps, residential heat pumps, systems for hydronic heating circuits, and heat pump dryers.

A 25 kW pilot plant was installed in a food-processing factory in Larvik, Norway in 1999, using waste heat from an industrial NH3 refrigerating system as a heat source. Performance has exceeded the initial expectations, and the system has proven to be a very profitable investment for the company [27].

Several Japanese companies have introduced heat pump water Figure 6 – equivalent emissions (in million tonnes CO2-equivalents) heaters for residential applications into the market during the last from mobile AC systems in Germany, using a business-as-usual years [28]. Currently field-testing of commercial sized systems in scenario (BAU) and a reduction scenario with phase-in of CO2- Europe has also been reported [29], with a possible market launch based AC systems from 2007. From (Scwartz) [22]. in 2004. A presentation given by a group of 17 members from car Concepts combining tap water heating with cooling purposes is manufactures and companies involved in manufacturing of MAC also a very interesting option, especially for hot climates zones. systems worldwide, considered CO2 systems as one of the most Adriansyah investigated this option in detail with very interesting promising alternatives, and estimated that the remaining technical results [26]. Different concepts for heat rejection, using multiple issues can be solved within 2 to 3 years (Mager) [13]. gascoolers, were among the topics under investigation. Further development of such systems is ongoing [30]. Over the last years, the German Motor Vehicle Industry Association

(VDA) has coordinated development and testing of CO2 systems, 4. MOBILE AIR CONDITIONING and several car manufacturers have had test vehicles on the road Mobile air conditioning systems have been and still are a since the late nineties. Presentations made by BMW, Audi and dominating source of refrigerant emissions to the atmosphere, and DaimlerChrysler at an industry meeting in 2002 (Mager et al.) [12], the growing production volume of HFC-134a for this purpose is a showed the following consistent results from independent studies raising concern. As a result, government bodies and environmental by the three companies: organisations are focusing on the need for reducing the emissions. • higher performance in cool-down mode for R-744 (CO2) A study made for the German environmental agency shows that a than for R-134a full replacement of HFC-134a by CO in mobile AC systems from 2 • lower compartment temperature and faster temperature 2007 would cut the greenhouse gas emissions of Germany by 1 pull-down with R-744 million tonnes CO -equivalents in 2010 and completely eliminate 2 • reduced fuel consumption for R-744 system the emissions by 2021, Figure 6 (Schwartz) [22]. A comprehensive

study using statistical data from German automobile workshops The technology for CO2-based mobile air conditioning systems showed that the average annual emission rates relative to the has reached a very advanced level after years of development. system charge from HFC-134a mobile AC systems was 10.2% One example is a compressor model shown by Parsch, where the

(Schwartz) [23]. potential for a compact design with CO2 has been exploited, Figure 7 [18].

30 EcoLibrium™ September 2004 R134a compressor R744 compressor Figure 7 – mobile AC compressors with variable displacement. State-of-the-art R-

134a design (left) and a design for CO2 by LuK-Sanden (right). From Parsch [18]. Detailed experimental investigations to obtain comparable results for enhanced

HFC-134a system and enhanced CO2 system performance and efficiency data have been conducted according to conditions given by SAE Alternative Refrigerant Program. Hrnjak presented these results in detail [31]. Based on the results, a Life Cycle Climate Performance (LCCP) study was conducted by Pettersen et al [32]

and Hafner et al [50], showing 20-40% reduced LCCP values for the CO2 (R-744) system compared to the enhanced HFC-134a system, the lower value representative for one of the hottest climate zones, see figure 8. It was also shown that the fuel consumption related to the air conditioning system would be significantly less for the

CO2 system, even in the warm climates.

Figure 8 – LCCP comparison between enhanced HFC-134a and enhanced CO2 (R-744) systems for warm climates (R-134a leakage: 80 g/year). Contribution due to fuel consumption for running the ac-system (indirect), fuel consumption due to the ac-system mass (mass) and direct contribution due to leakage (direct) are shown. From [32].

City bus air conditioning systems with CO2 have also been developed, and the results from two years (1800 hours) of road testing are very positive. (Köhler) [10].

CO2 systems are also under consideration as a possible natural working fluid in air conditioning systems backing up the traditional air cycle in air crafts [33]. SINTEF is a sub-contractor to the Univ. of Padova, which is partly responsible for this study.

For these applications, the non-flammability and non-toxicity of CO2 gives special advantages, since the charge size of flammable refrigerants would require measures influencing cost, system weight and energy efficiency, due to safety issues. 5. HEAT PUMPS IN AUTOMOBILES

Modern cars with fuel-injection engines often have insufficient waste heat for heating of the passenger compartment in the winter season. The long heating-up period and slow defroster action is unacceptable both in terms of safety and comfort. Supplementary heating is therefore necessary, and one attractive solution may be to operate the air conditioning system as a heat pump. Carbon dioxide systems have special benefits in heat pump mode, since high capacity and COP can be achieved also at low ambient temperature and with high air supply temperature to the passenger compartment. forum

Hafner et al. proposed an advanced circuit for reversible cooling The more rapid heating up with heat pump is clear, with almost and heating [4], but work is also progressing on simplified system 50% reduction in the heating-up time from –20 to +20ºC. Since concepts for internal-combustion engine cars and electric/hybrid the heat pump used engine coolant as heat source, the possible vehicles. The heat pump feature may turn out to be an important risk of extended heating-up time for the engine was of some

factor for the introduction of CO2 systems in motor vehicles. One concern. Measurements showed that owing to the added load of the key questions is the choice of heat source. The simplest on the engine by the heat pump compressor, the heating-up time solution is of course to use ambient air, but this may give problems was in fact slightly reduced even when heat was absorbed from the related to frosting and defrosting. Other solutions being studied coolant circuit. use engine coolant or exhaust as heat source. Hammer and Wertenbach showed test data for an Audi A4 car with 1.6 liter

gasoline engine, comparing a standard heater and a CO2 heat pump system based on engine coolant as heat source [6]. Figure 9 shows measured air temperatures at foot outlet nozzles and passenger compartment temperatures using standard heater core (“production”), and a heat pump system (without heater core).

Figure 10 – process data for air-to-air mobile heat pump operated at +5ºC interior and exterior temperature, using components designed for AC operation. From Hafner [5].

Systems using air as heat source will be simpler and less costly, and there is quite some interest in clarifying the practical possibilities and limits of reversible air-to-air systems. Frost build Figure 9 – measured air temperatures in during start-up of an Audi up may in many situations be slow enough to allow heat pump

A4 test vehicle (production) and same car with CO2 heat pump operation until the heating system can take over, and solutions may (“heat pump”). From Hammer and Wertenbach [6]. be developed that control and delay frost build up.

Figure 10 shows experimental data from a test on a “reversed” AC system operated as a heat pump, with interior/exterior air temperature 5ºC [5]. As may be observed, the heat pump delivers an air temperature of more than 60ºC, i.e. a temperature rise of almost 60 K. Further results has been reported by Hafner [34]. ��

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Norway, June, 1998 [13] Mager, R et al, “New Technology: CO2 (R-744) as an Alternative Refrigerant”, MAC Summit 2003, Brussels, 10/11 02. [4] Hafner A. Pettersen J. Skaugen G. Nekså P., An 2003 Automobile HVAC System with CO2 as the Refrigerantî. IIR – Gustav Lorentzen Conference on Natural Working Fluids, Oslo, [15] Nekså, P., Rekstad, H., Zakeri, G. R. and Schiefloe, P.

Norway, June 2-5, 1998. A., “CO2-Heat Pump Water Heater: Characteristics, System Design and Experimental Results”, Int. Journal of Refrigeration 21: [5] Hafner, A., “Experimental Study on Heat Pump Operation 172-179, 1998 of Prototype CO2 Mobile Air Conditioning System”, 4th IIR-Gustav Lorentzen Conference on Natural Working Fluids, Purdue, USA, [18] Parsch, W., “Status of Compressor Development �� July 25-28, 2000. for R-744 Systems”, VDA Alternate Refrigerant Wintermeeting, Saalfelden, Austria, January 30-31, 2002. [6] Hammer, H., and Wertenbach, J., “Carbon dioxide (R- 744) as supplementary heating device”. 2000 SAE Automotive [19] Pettersen, J., “Comparison of explosion energies in

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[22] Schwartz, “Forecasting R-134a emissions from car air [32] Pettersen, J. and Nekså, P.: Consequences of the Newest conditioning systems until 2020 in Germany”, translation of lecture Improvements in R-744 Systems, SAE Automotive Alternate at DKV Deutsche Kaelte-Klima-Tagung, Bremen, 22.-24. November Refrigerant Systems Symposium, Scottsdale AZ, July, 2003 2000, http://www.oekorecherche.de/english/ac-2000.html [33] www.poa-project.com [23] Schwartz, W, “R-134a Emissions from Passenger Car Air [34] Hafner, A.: Compact interior heat exchangers for CO2 mobile Conditioning Systems”, VDA Alternate Refrigerant Wintermeeting, heat pumping system, Dr.ing (PhD) thesis no. 2003:05, Norwegian Saalfelden, Austria, January 30-31, 2002. University of Science and Technology, 2003 [26] Adriansyah, W., “Combined Air-conditioning and Tap Water [50] Hafner, A., Nekså, P. and Pettersen, P.: Life Cycle Climate Heating Plant, Using CO as Refrigerant for Indonesian Climate 2 Performance (LCCP) of Mobile Air-Conditioning Systems with HFC- Condition”, Dr.ing (PhD) thesis no. 2001:54, Norwegian University 134a, HFC-152a and R-744, Mobile Air Conditioning Summit 2004, of Science and Technology, 2001 Washington, 2004 [27] Zakeri, G.R., Nekså, P., Rekstad, H., Lang-Ree, K and Olsen,

T.: Results and experiences with the first commercial pilot plant CO2 heat pump water heater, 4th IIR-Gustav Lorentzen Conference on About the author Natural Working Fluids, Purdue, USA, July 25-28, 2000 Petter Nekså is group manager, senior research scientist for SINTEF Energy Research, Norway. He has written more than 90 international and [28] Nekså, P.: CO2 Heat Pumps for the Building Sector, Cold Climate HVAC 2003, Trondheim, June 15-18, 2003 national reports/publications within the area of refrigeration engineering including system design for refrigeration, air conditioning and heat pumps, [29] UTC (2003): http://www.utc.com/investors/2003-09-24_ trans-critical vapour compression cycles, CO technology, low temperature techday/heat_pump.pdf 2 refrigeration systems, compressor and heat exchanger technology and [30] Adriansyah, W.: Development of combined air conditioning working fluids with emphasis on natural working fluids.

and tap water heating CO2 test rig in Indonesia-Preliminary results, International Conference on Fluid and Thermal Energy Conversion Petter is commission vice president of the International Institute of 2003, FTEC 2003,Bali, Indonesia, December 7 – 11, 2003 Refrigeration (IIR) and commission E2, Heat pumps and energy recovery. He is also a board member of the Norwegian Society of Refrigeration, was [31] Hrnjak, P.: Design and performance of improved R744 system a consulting and reporting member of UNEP TEAP on refrigeration, air based on 2002 technology, , SAE Automotive Alternate Refrigerant conditioning and heat pumps and lead author of IPCC/TEAP Special Report Systems Symposium, Scottsdale AZ, July, 2003 on ëSafeguarding the ozone layer and the global climate system.

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