A refrigeration system for supermarkets using natural refrigerant CO 2

Transcription

1 A refrigeration system for supermarkets using natural refrigerant CO 2 A. Campbell 1,2, G. G. Maidment 2 and J. F. Missenden 2 1 Tesco Stores Ltd, Shire Park, Welwyn Garden City 2 London South Bank University, 103 Borough Road, London, SE1 OAA maidmegg@lsbu.ac.uk Abstract This paper describes a refrigeration system using a natural refrigerant that has been developed to reduce significantly both direct and indirect greenhouse gas emissions. The new system uses both R-404A and CO 2 (R-744) as refrigerants. The new system has very low global warming potential compared with conventional HFC systems which reduces significantly direct emissions. Indirect emissions are also much reduced as the system efficiency is much greater than achieved using conventional HFC refrigeration. The paper firstly describes an experimental system that has been used to demonstrate the new technology. It then compares the relative efficiency, operating cost and greenhouse gas emissions from the new system with that from a conventional system. It can be seen that emissions are approximately 66% of those from a conventional installation. Introduction The average global temperature is predicted to rise by between 1.5 and 4.5 K in the next 100 years. The principal cause of global warming is the emission of greenhouse gases into the Earth s atmosphere. The effects of refrigerants on the environment are well documented. Supermarkets contribute both directly and indirectly to global warming. Directly, greenhouse gas emissions occur through the leakage of HFC refrigerants used in refrigeration systems for display and storage of food. These refrigerants have very high global warming potential (GWP) in excess of However, supermarkets are also indirectly responsible for producing CO 2 as they are large consumers of electricity, consuming as much as GWh ( TJ) per annum in the UK alone and approximately 50% of this is consumed by refrigeration equipment. Over the last 20 years, legislation has prohibited the use of ozone-depleting CFC refrigerants, however; the use of the HFCs is still legal and commonplace. In recent years natural refrigerants have been proposed as an environmentally friendly solution for the refrigeration industry, these refrigerants which include ammonia, hydrocarbons and carbon dioxide, do not contribute to ozone depletion and have low global warming potentials. Carbon dioxide offers a long-term solution suitable for many applications in refrigeration and heating, from domestic applications utilizing heat pumps to provide hot water and heating to commercial applications for supermarket refrigeration. The technology is future proof requiring no further retro-fitting to the next refrigerant solution promoted commercially. To investigate the performance of R-744 refrigeration, a prototype system was constructed and tested to demonstrate proof of principle. This paper reports on a

2 66 A. Campbell et al. practical investigation of an R-744 system in a commercial application and describes an investigation of its design, commissioning and energy performance. Also discussed are the practical experiences gained from building an R-744 system and the benefits of using such a natural refrigerant. R-744 (CO 2 ) as a refrigerant R-744 has been used in refrigeration for many years. Figure 1 shows a timeline detailing its proposal and discovery, decline in the 1930s, to its rediscovery in the 1990s. R-744 has ten noteworthy characteristics: non-toxic non-flammable environmentally benign low triple point low critical point high pressure high refrigeration volumetric capacity high heat transfer characteristics inexpensive readily available These characteristics are discussed as follows, in relation to its use as a refrigerant. Like most natural refrigerants, R-744 is regarded as being environmentally benign. It has an Ozone Depletion Potential (ODP) of zero and has a very low global warming potential (GWP = 1). The main benefits of R-744 compared to other natural refrigerants are that it is non-toxic and non-flammable, which often limits the application of other refrigerants. The main characteristics of R-744 as a refrigerant are the critical and triple points. The critical point is a relatively low temperature at 31 C although at a high pressure of 73 bar. The triple point occurs at 56.6 C with a pressure of 5.2 bar which is the only common refrigerant to have a triple point above atmospheric pressure. Proposal to use CO 2 as a refrigerant (Alexander Twining, British patent) The peak of utilizing CO 2 as refrigerant Renewed interest in CO 2 refrigeration technology. New system demonstrated in Kilmarnock Scotland Figure 1. The history of R-744 as a Refrigerant [1] [2].

3 A refrigeration system for supermarkets using natural refrigerant CO ,6 +31 Deg.C Supercritical Critical point bar 5,2 Solid Solid - Liquid Solid - Vapour - 78,4 Deg.C Liquid -10 o C 26 Bar -28 o C 11Bar Liquid - vapour - 56,6 Deg.C Enthalpy (kj/kg) Figure 2. Simplified P-H Diagram [3]. Triple point Vapour In order to remain below the critical temperature of 31 C it is necessary to use a cascade system to provide a condensing temperature below 0 C. The system restrictions in relation to temperature and pressure can be seen in the simplified P-H diagram (Figure 2). R-744 operates at a far higher pressure than standard refrigerants though this is not excessively high compared to similar engineering applications. Figure 3 demonstrates the differences in operating pressures of standard refrigerants and R-744. It can be seen from the graph that while R-744 operates at higher pressures than conventional refrigerants, the pressures relative to those say of automobile power steering systems (115 bar), are low. R-744 systems have a high volumetric refrigeration capacity, as a result of their very high vapour density when compared to other refrigerants. As a consequence, refrigeration compressors using R-744 are 6 8 times smaller than those of R22 systems. This can be seen in Figure 4 where the same compressor at the same conditions has a far greater capacity when using R-744. Further benefits of high volumetric capacity can be seen when applied to pipework. Figure 5 shows the reduction in pipe sizes when using R-744. This will result in a significant reduction in suction line valving costs. The improved heat transfer properties of R-744 allow the evaporator to be operated at a higher temperature than competitive R-404A evaporators. The literature [4]

4 68 A. Campbell et al. Temp-Pressure Comparison Power Steering 115 Bar R744 R404A R410A 1000 kw C 25 C 20 C 15 C 10 C 5 C 0 C Temperature Figure 3. Figure Bar (g) Pressure Temperature Relationship for Various Refrigerants. Compressor CMO 28 at 1470 rpm Compressor CMO 28 at 1470 rpm Evaporating temperature ( C) CO 2 R-404A Condensing temperature at 10 C Comparison of Volumetric Capacities for the same Compressor. suggests evaporators may work at 2 K higher than conventional R-404A evaporators. The improved heat transfer properties thus result in increased capacity in heat exchangers. The comparisons made in Figure 6 demonstrate a 10% increase in the capacity for the same coil geometry.

5 A refrigeration system for supermarkets using natural refrigerant CO 2 69 Dry Suction Line Refrigerant R404A R717 R744 CO 2 Capacity 150 kw 150 kw 150 kw Circuit Penalty 1.4K 1.5K 0.8K Velocity 11.3 m/s 25.6 m/s 7.7 m/s Diameter mm 72.6 mm 50.8 mm Area mm mm mm 2 LIquid Line Velocity 0.6m/s 0.3m/s 1.1m/s Diameter 38.1 mm 25.4 mm 25.4 mm Area mm mm mm 2 Figure 5. Comparison of required pipe sizes at 30 C saturated suction temp and 10 C saturated condensing temp [3] kw 15.0 kw 10.0 kw 5.0 kw 0.0 kw 2 C 16 C 28 C EVAPORATING TEMPERATURE Figure 6. Heat Transfer Comparison (same coil) [5]. R744 R404A R-744 systems The excellent safety and environmental properties associated with R-744 enable its systems to offer real advantages to the retail food industry. Its non-toxic and nonflammable nature are important because large refrigerant charges are used in supermarkets and much of the refrigerant runs in pipes through the customer area. Low GWP is important because of the high refrigerant leakage rates that are a characteristic of the complexity and size of the system. A typical vapour compression system used in supermarkets comprises of medium- and high-temperature sections serving chilled and frozen food chillers and freezers respectively.

6 70 A. Campbell et al. Figure 7. Danish R-744 supermarket pack [6]. R-744 systems have been proposed and installed in supermarkets in Denmark. However, as can be seen in Figures 7 and 8, these systems are industrial in nature. As these would be expensive to install in the UK, an alternative arrangement has been used that is more economic, but retains safety and integrity features. This system is shown in Figure 10. The system was designed as a dual temperature cascade arrangement with refrigerant R-404A as the heat rejecting, condensing medium for the R-744 two-stage cycle. This arrangement enables the R-404A system to be a small compact unit, with minimal charge and leakage risk. It also enables the overall system to be constructed in a more commercial format than the previous industrial systems. The R-404A cycle evaporates at 15 C and condenses at 36 C using two scroll compressors. The evaporator is a plate heat exchanger, which acts as the condenser for the R-744 cycle condensing the discharge gas from the R-744 condenser and the flash gas from the wet suction on the high temperature R-744 evaporator at 10 C. The R-404A evaporator was sized to remove the heat from both R-744 cooling systems. The R-744 is condensed and held in the pulse vessel, which acts as liquid receiver and separation vessel. The high temperature R-744 evaporator is fed from the pulse vessel via a refrigerant pump, which feeds the refrigerant at a 4 :1 circulation ratio. The R-744 at 10 C acts as a volatile secondary refrigerant in respect to sensible and latent cooling making the evaporator more efficient. The refrigerant returns to the pulse vessel from the evaporator as a saturated liquid via the wet suction where it is recondensed, the flash gas leaves the vessel and enters the condenser to liquefy and

7 A refrigeration system for supermarkets using natural refrigerant CO 2 71 Figure 8. Danish Supermarket Pack Installed at ISO Roskilde Denmark. then return to the vessel at 10 C. The low temperature liquid R-744 also leaves the pulse vessel at 10 C and feeds its evaporator where it boils at 28 C, returning via the dry suction to the R-744 compressor, thus completing the cycle. A schematic view of the simplified system is shown in Figure 9. The installed test system can be seen in Figure 10. Commissioning of test rig Charging the R-744 system If liquid R-744 is charged straight into a system below 5.2 bar it will freeze directly, forming dry ice. Having pressurized the system above 6.5 bar, liquid charging was still difficult to achieve. The plan was to close the valve on the Pulse vessel thus trapping any condensed liquid inside. This was also to allow the suction pressure to be maintained at a sufficiently low pressure but above 5.2 bar, for the liquid to be injected into the system at the inlet of the expansion valve of the low temperature evaporator. As it turned out the compressor would reduce the system pressure sufficient to cause a safety trip (low pressure). The pressure then dropped below 5.2 bar and the regulator froze. As the R-744 compressor was the only one in the UK, extra care was needed not to burn the motor out.

8 72 A. Campbell et al. R404a COMPRESSOR R744 COMPRESSOR R404A CYCLE Expansion Device PHE R744 L.T. CYCLE Control of plate heat exchanger A particular problem that had not been foreseen was the flooding of the plate heat exchanger acting as the R-404A evaporator and R-744 condenser (Cascade condenser). The R-404A system is controlled by the R-744, pressure transducer. The compressors are controlled by a Danfoss EKC 513A controller, which takes its run signal from the vessel pressure transducer. The purpose of this is to maintain the vessel pressure by means of condensing discharge and flash gas from both R-744 cycles. In order to accomplish this refrigeration effect, the electronic expansion valve must respond to the load requirements of the plate heat exchanger. The TX valve is controlled by a separate Danfoss EKC 315A controller. The system was thus working with an open loop control strategy with neither EKC controller having interlocking action. To overcome the slow response of the expansion valve and subsequent flood back to the compressor the super-heat had to be set high at 15 K. Performance calculations Table 1 shows the calculated results based on the manufacturers performance and the results gained from the readings on the test rig. The COP is the standard form, where COP = Q P cooling input Figure 9. PULSE VESSEL R744 PUMP PUMPED R C Simplified Schematic of the Cycle. Expansion Device Expansion Device

9 A refrigeration system for supermarkets using natural refrigerant CO 2 73 Figure 10. Picture Showing Pack, H.T. Case and L.T. Room. The COP of the R-404A/CO 2 and R-404A systems becomes [7]: and COP COP total total QeM.T. + QeL.T. = P + P + P + P 404A cond fans pump CO2 QeM.T. + QeL.T. = P + P + P + P 404A M.T. cond fans 404A L.T. cond fansl.t. where: COP total = the total system coefficient of performance, Q e = food cooling capacity kw MT = medium temperature section, LT = low temperature section P 404A = power input R-404A compressor kw, P CO2 = power input CO 2 compressor kw P pump = power input refrigerant pump kw, P cond fans = power input condenser fans kw The COP of the test rig was better than expected by about 0.3, although this COP could be improved with a correctly sized pump and a more balanced system. These crude

10 74 A. Campbell et al. Table 1. Test Rig Performance Actual High Side Expected High Side Copeland ZB45KCE-TFD 2 Copeland ZB45KCE-TFD 2 T c 36 C T c 36 C T e 15 C T e 15 C Q e 17.8kW Q e 15.0kW P 404A 7.0kW P 404A 8.0kW P cond fans 0.5kW P cond fans 1.7kW THR 24.8 kw THR 23.0 kw COP 404A 2.39 COP 404A 1.5 M.T. M.T. T e 10 C T e 10 C Q e 4.7kW Q e 4.7kW P pump 0.8kW P pump 1.0kW COP CO2 MT 5.7 COP CO2 MT 4.7 L.T. L.T. Bitzer 2KC-3.2K Bitzer 2KC-3.2K Tc 10 C Tc 10 C T e 28 C T e 28 C Q e 10.0kW Q e 10.0kW P CO2 1.8kW P CO2 1.5kW THR 11.8 THR 11.5 COP CO2LT 5.6 COP CO2 LT 6.7 System System P total 10.1kW P total 12.2kW COP Total 1.5 COP Total 1.2 calculations are not satisfactory to evaluate the system exhaustively and further work is required to ascertain comprehensive energy balances within the system. Comparison of technologies In order to evaluate the technology it was compared to existing systems. Standard supermarket refrigeration systems comprise of separate medium- and low-temperature sections with separate condensers. In the comparison, the pump power required has been calculated rather than using the pump power from the manufacturer s data due to the over sizing of the pump as previously mentioned. The pump power detailed in Table 1 is based on the actual pump s rating of 1 kw, whereas the calculated power input was 0.2 kw. Compressor power input was measured with power meters. As can been seen in Table 2, the R-744 system is more efficient in all aspects of energy usage because of improved refrigeration and ancillaries energy use. This may be examined further in terms of running cost based on a 61% running time for

11 A refrigeration system for supermarkets using natural refrigerant CO 2 75 Figure 11. Cycle Prediction for Low Temperature System from Cool Pack Software [8]. each month. The traditional R-404A system costs and the R-744 system costs at a 0.04/kWh rate. This is a running cost saving of 21% over traditional systems. With traditional supermarket refrigeration packs costing so much to operate each year the pay back period of such refrigeration systems can be significantly reduced. TEWI calculations Total equivalent warming impact (TEWI) is a calculation developed by the British Refrigeration Association to compute the total impact the refrigeration system has on the environment over its life time. The calculation is based on the direct effect of the refrigerant release over the system s service life and the indirect effect of the energy consumed by the system; these effects are combined and expressed as an equivalent mass of CO 2. TEWI = (GWP L N) + (GWP m (1 a recovery ) + (N E annual b) Leakage Recovery losses Energy consumption Where GWP = global warming potential relative to CO2 L = leakage rate per year N = system operating time M = refrigerant charge kg a recovery = recycling factor (assumes some lost in recycling i.e. 0.75) E annual = recycling (energy consumption per annum kwh) b = CO2 emissions per kwh of consumption

12 76 A. Campbell et al. Table 2. R-744 Pack versus R-404A Pack R404a/CO 2 Cascade High Side STANDARD System M.T. Copeland ZB45KCE-TFD 2 Copeland ZB26KCE-TFD 1 T c 36 C T e 10 C T e 15 C Q e 5.3kW Q e 17.8kW P 404A MT 2.9kW P 404A 8.0 kw THR 8.2 kw P cond fans 1.7kW P cond fans 1 THR 25.8 kw COP R404A MT 1.33 COP 404A 1.83 M.T. L.T. T e 10 C Copeland ZF48K4E-TFD 1 Q e 4.7kW T c 36 C P pump 0.2kW T e 28 C COP CO2 MT Q e 10.1kW L.T. P 404A LT 9.0kW Bitzer 2KC-3.2 K THR 21.4kW Tc 10 C P cond fans 1.5kW T e 28 C COP R404A LT 0.96 Q e 10.0kW P CO2 1.5kW THR 11.5kw COP CO2 LT 6.67 System System P total 11.4kW P total 14.4kW COP total 1.29 COP total 1.06 Tables 3 and 4 show the relative TEWI of traditional and R-744 systems. From this it can be seen that a large reduction in CO2 is projected with the R-744 system. These savings are mainly as a result of the much reduced leakage associated with the R-744 system. Conclusion The investigation has clearly demonstrated there are considerable energy savings to be made by using an R-404A/R-744 cascade system. the calculations demonstrate that the R-744 system reduces substantially CO 2 emissions when compared with traditional systems. A reduction in emissions of 1274 tons CO 2 for a 25 kw pack over 10 years. The practical experience gained from building and commissioning an R-744 system has shown that there are no insurmountable problems with using R-744 as

13 A refrigeration system for supermarkets using natural refrigerant CO 2 77 Table 3. TEWI Calculation for a Traditional R-404A System Standard Method of Calculation Input Data Application Sector Commercial Refrigerant Fluid R404A Refrigerant Charge (M) 70 Annual Energy Consumption Power Input 107,844 Ancillary 0.00 Sectoral Factors System Operational Life Time (N) 10 Refrigerant GWP 3750 L1 5 S1 25 L2 0 S2 0 Recovery Efficiency (α) 0.30 CO2 Emission Factor ß 0.43 Refrigerant Loss Operational 210 Refrigerant Loss Retirement 70 Total Lifetime Refrigerant Loss (L) 280 CO 2 Equivalent Indirect Effect TEWI a refrigerant. The problems associated with the procurement of equipment will diminish with growth in the use of R-744. The benefits are: smaller compressors smaller pipe work and valves smaller heat exchangers more energy efficiency environmentally more friendly reduction in greenhouse gas emissions cheaper refrigerant With the imminent arrival of the F-gas regulations, the running costs of systems can be further reduced by the avoidance of inspections, with the use of R-744. There is no reason to delay the design of new plants using R-744 as the components and technology are now available. The only hindrance to wider use is unfamiliarity of designers with these types of systems. Acknowledgements The authors would like to thank the engineers who worked on the CO 2 program and the directors of Space Cooling Systems for their permission to publish this paper.

14 78 A. Campbell et al. Table 4. TEWI Calculations for the R-744 Rig Standard Method of Calculation Input Data Application Sector Commercial Refrigerant Fluid R404A Refrigerant Charge (M) 10.0kg Annual Energy Consumption Power Input kWh Ancillary 0.0 kwh Sectoral Factors System Operational Life Time (N) Refrigerant GWP L1 5 S L2 0 S Recovery Efficiency (a) 0.3 CO2 Emission Factor ß 0.43 Input Data Application Sector Commercial Refrigerant Fluid R744 (CO 2 ) Refrigerant Charge (M) 40.0kg Annual Energy Consumption Power Input 725.3kWh Ancillary 0.0 kwh Sectoral Factors System Operational Life Time (N) Refrigerant GWP 1.00 L1 2 S L2 0.5 S Recovery Efficiency (α) 1 CO2 Emission Factor ß 0.43 R404A CALCULATIONS Refrigerant Loss Operational (L) Refrigerant Loss Retirement 7.00 Total Lifetime Refrigerant Loss CO 2 Equivalent Indirect Effect TEWI R404A R744 CALCULATIONS Refrigerant Loss Operational Refrigerant Loss Retirement 0.00 Total Lifetime Refrigerant Loss CO 2 Equivalent Indirect Effect TEWI R TEWI TOTAL

15 A refrigeration system for supermarkets using natural refrigerant CO 2 79 References [1] Danfoss, CO 2 Phase Changes Danfoss (2002), [CD Rom]. [2] S. F. Pearson, Development of improved secondary refrigerants, Proc. Institute of refrigeration, 89 (1993), [3] Danfoss, CO 2 Refrigerant for industrial Refrigeration, Danfoss (2002), Arhus. [4] K. Visser, Carbon Dioxide for the food processing and cold storage industries, AIRAH Natural Refrigerants Conference, Melbourne, [5] Palladio software Ver Alfa Laval. [6] Sabroe Refrigeration Ltd, Why is there renewed interest in old refrigerants, (2005), Sabroe X s Vej Christian, Hojberg, Denmark. [7] G. VanRiessen, NH 3 /CO 2 Supermarket refrigeration systems with CO 2 in the cooling and freezing section, 6 th IIR Gustav Lorentzen Natural Working Fluids Conference, Glasgow, [8] Cool pack software, Ver. 1.46, (2000) Department of energy engineering Technical University of Denmark. [9] A. Pearson, Carbon Dioxide Short Course, 6 th IIR Gustav Lorentzen Natural Working Fluids Conference, Glasgow, 2004.