Hundy, Guy

Winandy, Eric

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Purdue UniversityPurdue e-PubsInternational Refrigeration and Air ConditioningConference School of Mechanical Engineering2008Refrigerant and Scroll Compressor Options forBest Performance of Various European Heat PumpConfigurationsEric WinandyEmerson Climate TechnologiesGuy HundyEmerson Climate TechnologiesFollow this and additional works at: http://docs.lib.purdue.edu/iraccThis document has been made available through Purdue e-Pubs, a service of the Purdue University Libraries. Please contact epubs@purdue.edu foradditional information.Complete proceedings may be acquired in print and on CD-ROM directly from the Ray W. Herrick Laboratories at https://engineering.purdue.edu/Herrick/Events/orderlit.htmlWinandy, Eric and Hundy, Guy, "Refrigerant and Scroll Compressor Options for Best Performance of Various European Heat PumpConfigurations" (2008). International Refrigeration and Air Conditioning Conference. Paper 895.http://docs.lib.purdue.edu/iracc/8952397, Page 1International Refrigeration and Air Conditioning Conference at Purdue, July 14-17, 2008Refrigerant and Scroll Compressor Options for Best Performance of various European HeatPump ConfigurationsEric Winandy and Guy HundyEmerson Climate Technologies Europe, Welkenraedt, BelgiumABSTRACTA variety of refrigerants is used for today’s heat pump systems in Europe. In a theoretical analysis the impact ofchosen refrigerant upon performance is discussed from a theoretical and from a compressor viewpoint.The requirements for the heating system, as well as from the weather conditions are not uniform throughout Europe.Therefore compressor versions have been developed to achieve the required COPs as well as the necessary operatingrange in different types of applications. Analysis of seasonal performance using statistical weather data for differentlocations shows how various scroll compressor types can be matched to types of heat pumps to give bestperformance, considering cost.In considering heat pump types the requirements for air source can differ from those for ground source, and the heatdistribution system also needs to be taken into account. This may be underfloor heating operating at relatively lowtemperature or radiator systems requiring higher temperature. In both cases, the domestic hot water option may beadded.1. INTRODUCTIONThe European emphasis on the reduction of CO2 emissions stimulates an increased use of heat pumps replacing thetraditional heating systems for residential houses presently equipped with gas or oil burners. The specifics of thisapplication require some adaptations to the compressors to make the heat pump system competitive. The primeaspects are operating range of the compressor and the compressor coefficient of performance, as this has a directimpact upon the economics. The choice of the refrigerant is also a matter of great importance to the manufacturer.This paper considers the most commonly used refrigerants in Europe in their impact on the performance of bothground source and air source heat pumps (GSHPs and ASHPs) Seasonal COPs, referred to as SCOP or SPF(seasonal performance factor) calculated from compressor selection data, have been used to compare differentR407C compressor types.2. REFRIGERANT CHOICE2.1 RefrigerantTable 1 presents the refrigerants in heat pumps mostly discussed nowadays, either in industry or in investigationcentres, together with their main characteristics. There are 3 groups:? Hydrofluorocarbons HFC: mainly R407C R134a, R410A, and R404A,? Hydrocarbons HC: propane 290 and iso-butane R600a,? Mineral Substances: Ammonia R717 and Carbon dioxide R744.2397, Page 2International Refrigeration and Air Conditioning Conference at Purdue, July 14-17, 2008Table 1. Refrigerant properties1: Global Warning Potential (100 years)2: at 1 bar absoluteHydrocarbons and Minerals have a negligible GWP, which makes them attractive for environment reasons. On theother hand, HC are flammable and then present a risk that is not currently accepted for HP applications.Ammonia R717 is toxic and is prohibited in residential and commercial applications. CO2 is non-flammable, non-toxic and cheap. On the other hand, its physical properties (low critical temperature and high density) make itdifficult to apply with existing technologies. Moreover there are serious doubts on its efficiency potential for thisapplication. Keep in mind that COP is mainly proportional to critical temperature (Calm and Domanski 2004).Based on this first analysis, only HFC will be considered in the discussion that follows.R134a presents the lowest GWP and the highest critical temperature. On the other hand it presents a low refrigeranteffect and a high boiling temperature, not ideal for component cost and low temperature applications like air-to-water heat pumps.R410A has the highest refrigerant effect but the lowest critical temperature, it is currently the most expensive and itshigher working pressure requires different component and system design. Its thermodynamic properties limit its useat high-pressure ratios due to high discharge temperatures. On the other hand, it has excellent heat transfer propertiesand pressure drop effects are less. Both these factors assist in reducing system cost.R404A is used in refrigeration applications in Europe due to its high density, low boiling point and good behaviourat high-pressure ratio (low discharge temperatures). On the other hand, it has the highest GWP of all HFCconsidered here and its low critical temperature makes it less attractive for Heat pumps.Finally R407C is a good compromise. Its glide can be beneficial in counter flow plate heat exchangers resulting inlower heat exchanger temperature differences.Figures 1 and 2 show a comparison of refrigeration COPs for the HFCs considered here. The heating COP, definedwith reference to the condenser enthalpy difference is equal to refrigeration COP + 1. Figure 1 shows the influenceof the condensing temperature on the COP at constant evaporating temperature (-5°C). This represents typicallyBrine to Water heat pumps. Figure 2 is aimed at showing what would happen in the case of an Air-to-Water heatpump at variable evaporating temperature.The conclusions are about the same for both conditions, Compared to R407C at up to 50°C condensing temperature,R134a is up to 5% higher efficiency while R410A is a few percent lower and R404A close to 10% lower.R407C R134a R410A R404A R290 R600a R717 R744Mixture/SingleSubstance Mixture Single Mixture Mixture Single Single Single SingleGWP(100y) 1 1520 1300 1725 3260 3 3 ~0 1Toxicity No No No No No No Yes NoFlammable No No No No Yes Yes Slight NoBoiling Temp, °C 2 -44.1 -26.1 -51.8 -47 -41.2 -12.2 -33.6 /Critical Temp, °C 87 101 72 73 97 135 133 31Condensing Temp@ 26 bar abs, °C 58 80 43 55 70 114 60 -11Refrigeration Effect@ 0°C, MJ/m3 4.2 2.9 6.9 5.1 3.9 1.5 4.3 232397, Page 3International Refrigeration and Air Conditioning Conference at Purdue, July 14-17, 2008Figure 1. Refrigeration COP for different condensing temperaturesFigure 2. Refrigeration COP at different evaporating temperatures25 30 35 40 45 50 55 60 650.80.850.90.9511.051.1tcd [°C]COP/COP R407C@ tev = -5°C, ?s =1R407CR404AR134aR410A-25 -20 -15 -10 -5 0 5 10 150.80.850.90.9511.051.1tev [°C]COP/COP R407C@ tc=50°C, ? s =1R407CR404AR134aR410A2397, Page 4International Refrigeration and Air Conditioning Conference at Purdue, July 14-17, 2008Figure 3 shows the isentropic discharge temperature comparison. This does not give the good estimate of the realdischarge temperature but gives the trends among the refrigerant options. It can be seen that R404A and R134a are“low discharge temperature” refrigerants, as it is well known, on the other hand R410A gives the highest dischargetemperature of all. High discharge temperature means a restricted operation envelope, which can imply higherrunning costs due to the need of direct heating.Figure 3. Isentropic compressor discharge temperature2.2 CompressorsThe compressor isentropic efficiency depends on the electromechanical losses (motor and friction) and thermo-fluidlosses (mainly throttling and superheating). A compressor is in general optimised for one given refrigerant so thisnot completely fair to make a general statement on what happens with other refrigerants. Nevertheless the way thedifferent losses vary with different refrigerants are known and some general trends can be given on what wouldhappen in a specifically optimised compressor. Among all the losses mentioned above, the mechanical frictionlosses are probably the ones mostly variable with the refrigerant choice since its thermodynamic properties willdictate the needed swept volume and hence friction surface and friction pressures. So what can be expected is alower isentropic efficiency for R134a compared to R407C and a slightly higher for R404A and even more withR410A.The vapour injected R407C compressor is now starting to become available for R407C. The economiser cycle offersbetter theoretical COP at all conditions compared to the standard vapour compression cycle. Vapour injection alsohas the effect of cooling the compressor, thus extending the range of operation. Increased compressor capacity canoffset the additional cost of the heat exchanger required for subcooling. Vapour injection is a very attractive optionfor ASHPs, delivering more capacity at extreme conditions, and ability to operate at lower ambient temperaturesthan the standard scroll.2.3 SystemFinally, the heat transfer properties can vary quite significantly with the refrigerant type. Higher-pressurerefrigerants give better heat transfer properties. Typically R134a will need additional heat transfer surface to keepthe same DT compared to R407C while the use of R410A can reduce the DT in the heat exchanger of a few K’s.-25 -20 -15 -10 -5 0 5 10 15556065707580859095tev [°C]t exs,cp[°C]@ tc=50°C, ? s =1R407CR404AR134aR410A2397, Page 5International Refrigeration and Air Conditioning Conference at Purdue, July 14-17, 2008This last refrigerant is becoming widely used in air-conditioning applications despite a lower refrigerant COP butdue to the compressor and systems benefits mentioned above (Hundy and Pham 2001).Impact on costs: according to the above analysis, the use of R134a will dictate a larger compressor swept volume(more than 40%), which means an extra cost of about 25% compared to R407C. Also the heat exchangers will needmore surface which typically increase its cost of about 10%.R410A is more expensive, and components such as the compressor, the heat-exchanger and the controls designedfor higher pressure and are at present more expensive.From this discussion, R407C and R410A become the two short listed candidates based on performance,compactness and price. For special heat pump applications requiring high condensing temperatures if disregardingthe price, R134a can be the only remaining choice.3. SEASONAL PERFORMANCE3.1 MethodCompressor selection software has been used to derive comparative seasonal performance data as described byWinandy and Hundy, 2007, 2008. This method uses the pre-configured heat pump characteristics of evaporator andcondenser temperature differences, refrigerant superheat and sub-cooling. The fan/pump power is added and there isalso an allowance for defrost de-rating in the software. Ambient temperature profiles from Meteonorm are used todefine the heat load and the evaporating temperature in the case of air source. In this way comparative seasonal COPfor various locations, refrigerants and compressor types can be readily investigated without the need for extensivetests. A comparison of performance characteristics with actual rating point data was made in the referenced paper(2008) with good results.The model can be configured for constant heated water temperatures or for compensated control. For the purpose ofthis example, an R407C ASHP has been chosen and a comparison is made between a standard heating scroll and avapour injected equivalent. The heating scroll has a discharge valve that enhances high pressure ratio efficiency, andthe vapour injected scroll additionally has an intermediate pressure injection port. The heat pump temperaturedifferences are taken as 7 – 10K (evaporator), 4K (condenser) with zero subcooling and 4K superheat. A 6% defrostde-rating is applied at evaporating temperatures below zero when the air temperature is above zero. Hot waterdelivery temperature is 55°C.For compensated control the conditions are the same as above – the only change made is to set the watertemperature at 55°C for ambient –10°C and it is then reduced by 0.7K for each degree rise in ambient temperature,to compensate for lower building heat loss.3.2 ResultsThe results are summarised in Table 2. The improved SCOP with compensated control results from lowercondensing temperatures during most of the season and correspondingly lower compressor run hours.Table 2 SCOP comparison for heating only, location – FrankfurtHeat pumpcompressorSCOPConstant water temperature55°CSCOPCompensated Control with watertemperature 55°C at –10°C ambientHeating Scroll 2.06 2.85Vapour InjectedScroll2.69 3.182397, Page 6International Refrigeration and Air Conditioning Conference at Purdue, July 14-17, 2008The heat pump is sized for 20% excess capacity at +2°C air temp. Below 0°C, capacity becomes inadequate andback up heating required. This is shown as dark shaded in Figure 4.Figure 4 Heat load in each ambient bin. All heat in lowest bin supplied by back up heaterCompensated control has been set to give a maximum water temperature of 55°C at –10°C air temperature. If the airtemperature falls below this value the water temperature is correspondingly increased, and there are just 4 hours inthe –15°C bin. This demands a condensing temperature of 62.5°C which is outside the compressor operatingenvelope The calculation has been made with this heat supplied 100% by the back up heater. Other options arepossible but the overall result changes very little due to the small number of hours involved.The envelope of the standard scroll restricts its use at low ambient temperatures in this application. At airtemperatures below –5°C water temperature of 55°C cannot be delivered and in this situation the SCOP is againcalculated using 100% back up heat for these low ambient bins.3.3 Addition of Domestic Hot Water (DHW)DHW can be added in the model as a percentage of the heating load. In the following calculations the total heatingload at 2°C is the same as before, except that 20% is taken for DHW supplied at a constant 58°C. The HP isassumed to cycle between the DHW load and the heating load as required, with the condensing temperature adjustedaccording to the compensated control requirement. This is set up in the same way as before.The calculation for SCOP now includes the bins for summer period where the load is DHW only. This is shown inFigures 5 and 6. When the air temperature is 25°C and above the operating conditions are outside the envelope andthe calculation is made on the assumption of back up heating for DHW at these conditions. Alternative options arepressure reducing valve to enable the compressor operation to continue or solar heat, and both of these optionswould benefit the SCOP. When the air temperature is –5°C and below back up heating is required for DHW (Thecompressor hours are prioritised for heating). Back up heating in these bins could be reduced with a largercompressor.2397, Page 7International Refrigeration and Air Conditioning Conference at Purdue, July 14-17, 2008Figure 5 SCOP calculation – Location: Frankfurt, compensated control, with DHW at 58°C, vapour injected scrollA comparison between three scrolls has been made for these conditions and loads, each HP having approximatelythe same capacity at 2/55°. The standard air conditioning compressor, and the standard heating compressor arecompared with the vapour injected equivalent. These results are summarised in Figure 6.Figure 6 SCOP results comparing standard scroll types and vapour injection type2397, Page 8International Refrigeration and Air Conditioning Conference at Purdue, July 14-17, 20084. CONCLUSIONIn summary R407C has advantages as a refrigerant for heat pump applications, but the climate at the locationconsidered and the choice of the compressor model are influential factors. Among the refrigerants considered thedifferences are not dramatic and this may explain why essentially all refrigerants are found in existing European heatpumps today and the choice made by each supplier may very well be influenced by considerations other than SPF.Whilst the air conditioning scroll exhibits high COPs at moderate conditions, its operating range limits itsapplication in air source heat pumps. This is also partially true with heating scroll. The vapour injected scrollpresents the best overall performance, and also delivers more capacity at low ambient conditions, reducing the needfor back up heating.One should keep in mind that this SPF calculation was performed for a specific heating system (air-to-water withwall-hung radiators) and other systems are important as well like ground-source to water and low temperatureunder-floor heating .It becomes clear that there is no easy and straight-forward approach to the choice of refrigerant.The fact that R407C is the predominantly used refrigerant in European heat pumps may be attributed to the fact thatthis refrigerant has advantages in some heat pump applications but suppliers also make the decisions for theirrefrigerant of choice base on cost, market region, product proliferation and equipment availability.5. REFERENCESCalm J.and Domanski P.. “R22 Replacement status,” ASHRAE Journal, August 2004.Hundy G. and Pham H. 2001. “Effect of refrigerant choice”, IOR conference, London, 2001.Kämmer N., 2004. “Scroll compressors for heat pumps - Application to space heating in new and existing residentialbuildings”, Paper No IMECE 2004-62546, Anaheim.Liègeois O. and Winandy E., Scroll compressors for dedicated Heat Pumps: Development and performancecomparison, Paper 293, Purdue Compressor Conference, 2008Meteonorm 5.0, Meteorological Database for Solar Energy and Applied Meteorology by Meteotest, Switzerland.PrEN14335 2003. “Method for calculation of system energy requirements and system efficiencies(Part 4: Heat pump systems”).SELECT 6, Version 6.6, Copeland Compressor Selection Software, 2007.Winandy E and Hundy G, “Investigation Of The Benefits of the Vapour Injection Scroll Compressor in Heat PumpsUsing a Seasonal Efficiency Model”, IEA heat pump conference, Zurich 2008Winandy E, Hundy G, Hewitt N, “Modelling Air Source Heat Pump Seasonal Efficiency”, International Congress ofRefrigeration, Beijing 2007

Refrigerant and Scroll Compressor Options for Best Performance of Various European Heat Pump Configurations

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Refrigerant and Scroll Compressor Options for Best Performance of Various European Heat Pump Configurations

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