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Professor Clark Bullard co-founded the Air Conditioning and Refrigeration Center (ACRC) in 1989 at the University of Illinois at Urbana-Champaign, and served as Director until 2002. Now retired from teaching, he remains an active researcher in the Center. He is the author of more than 100 technical publications covering a wide range of technologies, many focusing on how market forces interact with public policies to influence the path of technological innovation. His experience with technology policy dates back to the 1970s when he served as Director of the Conservation and Advanced Systems Policy Office at the US Department of Energy, and as senior analyst at the US Congress Office of Technology Assessment. His current research is focused on development of new components required to maximize efficiency and reduce charge in the next generation of a/c and refrigeration technologies.

An active member of IIR, he currently serves as President of Commission B2. He was the 2004 recipient of the Anderson Medal, the highest technical award given by the American Society of Heating, Refrigerating and Air-conditioning Engineers (ASHRAE). Full details of his activities and those of the ACRC can be found at http://acrc.mie.uiuc.edu/

Transcritical CO2 systems – recent progress and new challenges by Clark Bullard Air Conditioning and Refrigeration Center University of Illinois at Urbana-Champaign, USA e-mail: [email protected]

INTRODUCTION substantial – less This article was prepared in response to a than their ozone-depleting predecessors, but request by IIR to address the recent still more than 1000 times the potential of “increasing interest in CO2 technology”, by . The early focus on auto air reviewing the state of the art and exploring its conditioning is a consequence of that sector’s likely applications. Although the benefits and historically high leakage rate, compared with disadvantages of carbon dioxide refrigeration other applications. systems have been the subject of lively debate in the scientific community for 15 years, it has Carbon dioxide re-emerged as a possible only recently been brought to the general replacement for HFC in 1989, and public’s attention by the European Parliament’s the first major results were published by 1 March 2004 vote to out the Lorentzen and Pettersen at a 1992 IIR universally used for auto air conditioning. This symposium in Norway. Subsequently, major marks the first international action aimed at research efforts were initiated at universities in banning the use of R-134a, one of the “new” North America and Asia. By 1999, the Society refrigerants developed in response to the of Automotive Engineers (SAE) began Montreal Protocol (1987), which phased out organizing annual symposia, where the world’s chlorine-containing refrigerants to protect the automotive industry meets to exchange Earth’s ozone layer. technical information and compare vehicle performance in the extremely hot climate of In 1987 policymakers and the public were Phoenix USA.2 dealing with the urgent problem of protecting the ozone layer, and few foresaw the Unlike most refrigerants that can reject to possibility of phasing out those replacement the air via condensation at typical outdoor refrigerants in the foreseeable future. Today’s temperatures, carbon dioxide operates on a interest in carbon dioxide is the result of transcritical cycle, evaporating in the concern about greenhouse emissions and subcritical region and rejecting heat at global warming, and the European Parliament’s temperatures above the critical point in a gas commitment to compliance with the Kyoto cooler instead of a condenser. It is this cycle Protocol. The new generation of that is being considered for automotive a/c, hydrofluorocarbon (HFC) refrigerants have domestic space and water heating and other applications. Research on conventional

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Figure 1. Vapour curve for refrigerants Courbe de pression de vapeur des frigorigènes

transfer with only a small increase in subcritical CO2 applications is also underway in . Heat transfer in the gas the industrial refrigeration sector. cooler is enhanced by another thermodynamic property, as the isobaric specific heat is The state of the art for the transcritical CO2 infinite at the critical point and relatively high cycle technology in various refrigeration, air- at the typical operating slightly conditioning and heat applications has above it. The unique thermodynamic properties recently been critically reviewed in great 3 of the transcritical cycle produce nearly flat detail. That article covered the history and re- isotherms just above the critical point, as emergence of the natural refrigerant CO2, its shown on the P-h diagram in Figure 2 from [3]. thermodynamic and transport properties, basic In a/c applications the gas cooler exit CO2 transcritical cycles and some proposed temperatures are typically above the critical cycle modifications, heat transfer and pressure point, so the gas cooler exit can be drop characteristics in CO2 systems, and decreased dramatically by using the expansion design issues related to high operating valve to raise the pressure by a small amount pressure. It also explored component design above the critical point. Further increases in challenges and barriers to be overcome before pressure yield diminishing returns, so the cycle commercialization. Unless otherwise cited efficiency can be maximized by adjusting the herein, the reader can find support and high-side pressure. HFCs, hydrocarbons, elaboration for the following summary report ammonia and other refrigerants operate on a by consulting that extensive review of the subcritical cycle where a local maximum does technical literature. not exist, so the expansion valve is used to

PROPERTIES OF CARBON DIOXIDE control the outlet. A low-side receiver is generally used in the transcritical The vapour pressure curve for CO2 is 4 to 10 cycle to ensure that the CO2 exits the times steeper than that of other, less dense, evaporator in the saturated vapour state, and refrigerants, as shown in Figure 1 from [3]. to provide sufficient refrigerant charge for Therefore and internal (suction high-side pressure control. line) heat exchangers can be designed to operate at high mass flux to achieve high heat

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Offsetting the inherently lower thermodynamic diameter pressure-tolerant microchannel ports efficiency of the transcritical cycle are several greatly increase the surface area, while transport properties of CO2 itself: higher pressure drop can be reduced by simply adding thermal conductivities of and vapour, more parallel ports.5 and lower liquid viscosity. Its lower surface tension enhances evaporative heat transfer by promoting nucleate boiling, but capturing its full benefit may require cycle modifications (e.g. flash gas bypass) or control strategies that maintain a two-phase exit state in the evaporator.4

FLOW MODULATION Figure 3. Cross-section of an internal heat In one respect the transcritical cycle is no exchanger different from its competitors – its efficiency Section transversale d'un échangeur de chaleur can be increased by modulating the refrigerant interne flow to eliminate cycling losses and to utilize To reduce the losses associated with gas the heat exchangers continuously, instead of cooler heat rejection across high ∆Ts intermittently via on/off cycling. Over the last (temperature changes), the compressor decade, the prospect of global warming has discharge temperature can be reduced by signaled the potential for higher energy costs multi-stage compression with intercooling. and controls on refrigerants. In Shown schematically in Figure 4, intercooling parts of the world where energy prices are reduces the of gas to be handled in the already high, the simple vapour-compression second stage, thus reducing the cycle efficiency is being increased by adding required.6 By reducing the internal pressure sensors, actuators and controls to modulate differentials, multistage compression also refrigerant flow. Unfortunately the maximum enables the use of rotary , which efficiency attainable through this approach would otherwise be subject to excessive soon reaches a refrigerant-specific limit: the internal leakage across the line contacts along COP of that particular refrigerant on the the piston.7 Analyses by Huff et al. suggest standard vapour-compression cycle operating that various combinations of internal heat between the specified heat source and sink exchange and two-stage compression could temperatures. Therefore, in the case of CO2 as increase cycle efficiency by as much as 38- well as conventional refrigerants, today’s 63% over the standard cycle.8 research is focusing on ways to modify the standard -compression cycle, as is now Internal heat exchange is not the only way to done routinely in industrial-scale systems reduce the large expansion losses inherent in where energy costs dominate. the transcritical cycle. The potential work recoverable from the large changes in pressure CYCLE MODIFICATIONS and volume during isenthalpic expansion There exist many opportunities for improving suggest that some kind of expander could the of any refrigerant by generate enough work to operate a pump or bringing it closer to the Carnot ideal – a blower, or possibly the second stage of the rectangle on the T-s diagram. In the case of compressor.9 The disadvantage of this CO2, its largest exergy losses (irreversibilities) approach is that it competes for the same are associated with the expansion process and exergy losses as the internal , its high temperature of heat rejection. The which is a much simpler and less costly simplest way to reduce expansion losses is to device. However, the potential payoff is larger cool the refrigerant upstream of the valve by if the isentropic efficiency exceeds 0.5, so transferring heat internally to the cold suction research continues on mechanisms to recover gas. The same approach has been applied for and transmit this “lost work”. decades with many conventional refrigerants, but the high-pressure requirements of CO2 COMPONENT TECHNOLOGIES have led to development of a unique Aside from the cycle-modifying components “microchannel sandwich” design shown in described above, the high operating pressures Figure 3, that outperforms the simple tube-in- and unique properties of CO2 also place tube designs employed in the past. The small- different constraints on the design of

F:\Reda\rev2004-5.doc REVIEW ARTICLE conventional components, such as the heat aluminium heat exchangers had been in exchangers and compressor. For example, the widespread use for more than a decade as R- high pressures dictate the need for small- 134a condensers in automotive a/c systems. diameter tubes to minimize stresses and However, the need to employ the same material requirements. Despite early concern technology in CO2 evaporators is focusing that the high pressures would present attention on the problem of distributing 2- insurmountable safety hazards, the phase refrigerant droplets uniformly among development of ultra-compact microchannel hundreds of parallel channels.11 This problem heat exchangers has reduced that risk of refrigerant distribution, and the need to considerably, by confining the refrigerant develop microchannel heat exchangers that are inside hundreds of parallel sub-millimetre frost-tolerant, pose serious obstacles to the channels. For example, the lower refrigerant- development of CO2 evaporators. Meanwhile, side volume nearly offsets the higher pressure in residential air conditioners and heat to produce an explosion energy that is where frosting is an issue and volume comparable to that of R-22, and the reduced constraints are not as severe, small-diameter charge mitigates the inhalation risk associated circular tubes (5-7 mm) with enhanced flat fins with prolonged or sudden release into an are being explored at the prototype stage, to occupied space.10 handle the distribution problem by minimizing the number of circuits so conventional Compressor compactness is another distributors can be used. consequence of the high pressure/low volume characteristics of CO2; displacement volumes Transcritical CO2 system performance is very are 6-8 times smaller than those of R-134a sensitive to gas cooler refrigerant outlet and sensitivity to valve losses is reduced by temperature, where a small temperature the high vapour . In automotive change causes a large enthalpy change due to compressors where compactness is valued the high isobaric specific heat. Figure 5 shows highly, smaller displacements may enable how cooling the refrigerant by an additional reductions in component volume. 2ºC can increase COP 11% while reducing the Nevertheless, some early prototypes were COP-optimizing discharge pressure in an heavier because of the increased strength automotive a/c system’s gas cooler.12 This required to contain the crankcase pressure in observation led to the design of a multi-slab open-shaft automotive compressors or the cross-counterflow gas cooler shown in Figure motor cavity in hermetic compressors, but it is 6, the invention of a fabrication method by a not yet clear whether commercial versions will major manufacturer, and subsequent prototype incur substantial weight penalties. The low validation in the lab and in vehicle tests.13 pressure ratios have led to higher efficiencies, and the high pressure differences experienced FORCES DRIVING INNOVATION inside the compressor have resulted in most Three major factors are influencing the early prototypes being of reciprocating or 2-stage stages of commercialization of CO2 rotary design. The assessment of lubricants for technology. First are the regulatory actions the transcritical cycle is still underway; with related to the Kyoto Protocol, which are polyalkylene glycol (PAG) favoured for its driving current generation of mobile a/c lubricity, aromatic ester derivatives for their prototype development. Over the long term, tolerance of high temperatures and however, advances in vehicle energy efficiency polyolesters (POE) for their miscibility will create shortages of the “waste” heat throughout the transcritical cycle. The poor traditionally used for space heating in winter, miscibility of synthetic mineral oils is offset by and are stimulating the development of their superior flow properties at low alternative technologies. A number of temperature, so the ultimate choice may technologies now in use (e.g. electric involve blending two or more lubricants. resistance heaters, friction heaters) have a

COP=1, while an air-source transcritical CO2 The need for light weight and compactness in could have delivered high- transport applications has led to the use of flat temperature heat at COP>3 instantly upon multiport (microchannel) tubes having vehicle startup.14 diameters less than 1 mm. Such brazed

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4.0 o 28 C – refrigerant temperature at 30 the gas cooler exit 3.5 32

3.0

34 2.5

COP 36

38 2.0 42 40 45 + A 50 + B 1.5 7.5 8.5 9.5 10.5 11.5 Discharge pressure [MPa]

Figure 5. Effect of gas cooler exit temperature on transcritical cycle COP Effet de la température de sortie d'un refroidisseur de gaz sur le COP du cycle transcritique

Refrigerant inlet Refrigerant inlet

First pass, 14 tubes

Air Second pass, 13 tubes

Third pass, 11 tubes Refrigerant exit

Refrigerant exit Figure 6. Crossflow and cross-counterflow gas cooler designs

Companies’ desire to appeal to “green several large food processing companies are consumers” is the second driving force. designing two-stage cascade (CO2 and Commitments by a major soft drink company ammonia) refrigeration systems to use natural to switch to natural refrigerants in its beverage refrigerants as chlorine-containing conventional coolers prior to the 2004 Olympics have refrigerants are phased out. spurred competition among developers of CO2 and hydrocarbon vapour-compression systems, Conventional market forces are the third as well as Stirling cycles. For similar reasons driver, and are the most pervasive and durable.

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At present they are strongest in the domestic closer to its refrigerant-specific thermodynamic hot water heating sector in Japan where ideal “standard vapour-compression cycle”.17 demand for bathing is greatest in the evenings, Most of the refrigerant-specific research on and where low off-peak electric rates CO2 has focused on learning how to exploit its encourage economical recharging of water unique thermodynamic and transport storage at night. For the commercial sector, properties to reduce losses in the compressor multinational companies are developing larger and the heat exchangers. The logical outcome (~20 kW) water heaters for cafeteria of such a research strategy is to magnify the dishwashing, and using them to importance of refrigerant choice, favouring the simultaneously produce chilled water as a by- most efficient fluids whose standard vapour- product, for space cooling or other uses.15 compression cycles most closely resemble a rectangle on the T-s diagram. Other applications are still in the research stage. Clothes drying is one example where This traditional research strategy is obviously the transcritical cycle provides the required not sufficient for CO2 because of the high-temperature heat without loss of inherently lower standard-cycle efficiency efficiency, and the cold side is available to associated with transcritical operation. It has, condense water vapour for disposal in liquid however, opened up niche markets in several form. In residential space heating, preliminary applications where the most prominent non- research has confirmed that transcritical CO2 is rectangular part of the transcritical cycle (its less efficient and has greater total equivalent high temperature of heat rejection) has value warming impact (TEWI) in a/c mode than R- that offsets its thermodynamic loss. Those 410A, the current HFC refrigerant of choice. applications are water heating and mobile air However, its heating seasonal efficiency was conditioning, as detailed in the following found to be potentially higher, because the paragraphs. CO2 system’s higher capacity at low temperatures reduces the need for Heat pumping applications supplemental electric resistance heat. The Demand for domestic hot water occurs during results suggested that overall annual energy heating and cooling seasons alike, and desired use could match that of HFC systems in mild delivery temperatures (60-90ºC, depending on winter maritime climates like Seattle, and it the cost of storage volume) match the could be more efficient in cold continental supercritical temperature glide in the CO2 gas climates like Chicago, provided that the heat cooler. Therefore, it can be provided at near- pumps were sized for heating rather than zero energy cost on very hot days, and at cooling capacity.16 small incremental cost when cooling loads are low, simply by using the expansion valve to OUTLOOK FOR THE FUTURE raise the high-side pressure enough to reject The global warming potential of HFCs initially the desired amount of heat at the desired spurred the search for refrigerants that are temperature. Similarly, during the heating naturally degradable in the atmosphere, in season, the compressor can switch between contrast to newer man-made substances (air or hydronic) space and domestic water whose atmospheric chemistry is harmful or heating as needed, again using the expansion uncertain. In the case of CO2, the primary valve to provide each at the appropriate enabling technology has been the development temperature. During all seasons, there is an of technologies for mass-producing lightweight opportunity to provide domestic water heating heat exchangers that can contain high- at efficiencies far exceeding that of electric pressure refrigerant in small-diameter channels. resistance heat.

Research conducted since the renewal of The same is true for the demand for hot water interest in transcritical CO2 systems a decade in the commercial sector, as in the cafeteria ago has been very conventional in several example cited above. Many commercial respects. Primarily, it has mimicked, at an buildings have a modest year-round demand accelerated pace, the types of research done for chilled water, so hot water can be supplied on subcritical systems during the last century. as a co-product by the transcritical cycle at At the same time, improvements on the air or little incremental cost. However, the economic water side of CO2 heat exchangers have advantage of transcritical CO2 is limited to helped all refrigerants equally by bringing applications requiring delivery at temperatures systems’ real thermodynamic performance exceeding ~40ºC, otherwise the water could

F:\Reda\rev2004-5.doc REVIEW ARTICLE be preheated using heat rejected from a components so that the real cycle approaches subcritical system and merely “topped off” the simple cycle. For cooling applications the using more costly electric resistance heat. cycle itself must be altered to bring it closer to Since modern natural gas combined-cycle ideal efficiency. The most cost-effective step, plants can generate electricity at >50% internal heat exchange, has already been taken efficiency, heat pumps can already meet space in automotive and water heating applications, and water heating demands more efficiently but future research must focus on reducing the than direct . However, the future air-refrigerant temperature difference at the economic viability of widespread heat pumping gas cooler, achieving a temperature glide that will be determined by the relative prices of matches that of the heat sink. The next logical capital and fuel. Only after those conditions steps will require additional components and are met will the question of heat pumping via more complex controls, including such features subcritical HFC vs transcritical CO2 systems as multistage compression with intercooling, arise. The answer will be influenced to some further reduction of expansion losses, etc. extent by the temperature of heat required, Unless the transcritical cycle can be cost- and to a greater extent by the need to reduce effectively modified to the point where its emissions directly and energy penalty is smaller than the direct global indirectly, as measured by indicators such as warming impact of subcritical HFCs, then it TEWI. will be unlikely to prevail in most cooling applications. Auto a/c applications In mobile a/c where there is a need for light- It is therefore instructive to look to the long- weight and ultra-compact systems, the high term future and imagine what kinds of future heat rejection temperature in the CO2 gas technological developments might be required cooler favours small heat exchangers. The for transcritical CO2 systems to see negligible direct global warming impact of CO2 widespread use in cooling applications. One is a major advantage that tends to offset its potentially plausible scenario would be loss of efficiency on extremely hot days, and characterized by continued reductions in the its higher heat rejection temperature enables cost of sensors, actuators and microprocessor the heat exchangers to be downsized to controls, and embedding them in mass- increase fuel savings via more aerodynamic produced items. Such R&D progress would streamlining of the vehicle. The car’s diminish the cost advantage now enjoyed by aerodynamic drag coefficient is very sensitive refrigerants having favourable standard-cycle to the shape and slope of the front of the thermodynamic properties, and enable vehicle where the condenser or gas cooler has refrigerants to be selected for their traditionally been located to minimize environmental safety and compatibility with recirculation of hot air off the while various cycle-improving component designs. idling, and to receive ram air while moving. Just as modern methods for refrigerant-side Control systems are being developed to exploit area enhancement have decreased the the extra degree of freedom associated with importance of refrigerant transport properties, the transcritical cycle, enabling efficiency to be microprocessor control and component- traded for additional capacity when needed for integration technologies may also diminish the faster pull-down of cabin temperature at importance of refrigerants’ thermodynamic vehicle startup. The ability to control high-side properties. pressure in this manner also provides an opportunity for further downsizing of the gas Research on alternative refrigerants is cooler at the high-temperature design continuing on many parallel paths, including condition. explorations of the use of hydrocarbons or ammonia by minimizing charge or using Refrigeration and a/c applications secondary refrigerant loops. Each of these To envision truly widespread use of CO2 over paths efforts is yielding surprising results, as the long term, one must look beyond niche illustrated by the case of carbon dioxide. What markets where the high temperature heat these efforts have in common is that they are rejection has economic value (e.g. for heating, bringing fundamental research to bear on a or for compactness as in auto a/c). The technology whose development has standard-cycle efficiency of CO2 is so low that traditionally proceeded incrementally in the it is not sufficient to simply tailor the interest of reliability. Since the Montreal

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