Performance of a transcritical carbon dioxide heat pump for simultaneous refrigeration and water heating : a thesis presented in partial fulfilment of the requirements for the degree of Master of Technology at Massey University
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Date
1998
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Massey University
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Abstract
Many industrial processes require both refrigeration to less than 0°C and water heating to greater than 60°C. Traditional independent refrigeration and boiler systems have relatively poor energy efficiency, whilst conventional heat pumps can provide both cooling and heating but are limited in terms of the temperature lift that can be achieved. A novel heat pump using CO
2
as the refrigerant in a transcritical cycle has been proposed as a new technology that can overcome these disadvantages. The use of CO
2
as a refrigerant has many advantages. It is environmentally benign, safe, and has good thermodynamic properties, especially compared with fluorocarbons. The transcritical cycle involves evaporation of CO
2
at constant temperature and pressure below the critical point to provide refrigeration, while cooling of the CO
2
occurs at temperatures and pressures above the critical point to provide heating of water. The objective of this project was to design and construct a prototype transcritical CO
2
heat pump to simultaneously provide refrigeration and water heating, and to test its performance over a wide range of operating conditions. The prototype CO
2
heat pump had a nominal cooling capacity of 90 kW at -6°C and nominal water heating capacity of 127 kW from 10°C to 90°C. The prototype was designed to operate with a suction pressure of 30 bar and discharge pressure of 130 bar. The major components were a gas cooler, recuperator, flooded evaporator, low pressure separator/receiver, compressor, expansion valve, connecting piping and a control system. All components were standard high pressure equipment used by the natural gas processing industry. The gas cooler had a reasonably unique design to ensure close to pure counter-current heat exchange between the cooling CO
2
gas and the water being heated, both of which had relatively low flowrates. The compressor used was an open crankcase, reciprocating type with special gas seals on the piston rod to prevent CO
2
leakage. Refrigeration capacity (suction pressure) was controlled by varying the compressor speed. Water heating capacity was controlled by both using the expansion valve to control the CO
2
discharge pressure and varying the water flowrate through the gas cooler. The main problem encountered during commissioning of the prototype was CO
2
leakage through the compressor piston rod seals. Alternative sealing systems were tried, but the leakage remained an on-going problem that prevented prolonged operation of the prototype, such as would be necessary in industrial applications. Performance of the prototype was determined by energy balances based on measurements of CO
2
and water flowrate and temperature when it operated at steady-state. The energy balances generally agreed to within 6%. Trials were performed with suction pressures from 29.6 to 35.5 bar, discharge pressures from 80 to 130 bar, with hot water outlet temperatures from 65°C to 90°C, and evaporator water inlet temperatures from 11°C to 21°C. When heating water to 90°C and providing refrigeration at 1°C (35.5 bar suction pressure), the maximum overall Coefficient of Performance (COP) achieved was 5.4 at a discharge pressure of 114 bar. Below this optimum discharge pressure, the COP declined due to gas cooler heat transfer limitations (lower compressor discharge temperature led to lower temperature difference in the gas cooler and high CO
2
outlet temperature). Above the optimum, the decline in thermodynamic and compressor efficiency as pressure ratio increased caused the COP to decrease. The maximum heating and cooling capacities were about 13% less than the design values. This was attributed to the lower than expected volumetric efficiency of the compressor. The performance of the heat exchangers were generally close to the design values when allowances for lower than design water flowrates were taken into account. As expected, when suction pressure was reduced to 29.6 bar (-6°C), there was up to a 10% decrease in optimum COP as well as reduced heating and cooling capacity. When heating water to 65°C rather than 90°C, the optimum COP was about 20% higher. When suction pressure or hot water outlet temperature was decreased, the optimum discharge pressure became slightly lower due to the gas cooler heat transfer being less of a limitation on overall system performance. Addition of oil to the CO
2
did not reduce the CO
2
leakage sufficiently to allow long-term operation without recharging, and had minimum impact on the performance of the gas cooler, recuperator and compressor. However, oil fouling caused a significant drop in heat transfer performance of the evaporator. The measured prototype performance agreed well with process simulations of the equipment and with results for similar laboratory scale equipment reported in the literature. Therefore, simulations could be used to optimise component and system design with a reasonable level of confidence. It was shown that the biggest increase in COP could be achieved by improving compressor isentropic efficiency rather than increased heat exchanger size. Overall, the concept of the transcritical CO
2
heat pump for simultaneous refrigeration and water heating was proven and the required energy efficiency was sufficiently high that the heat pump is likely to be economically competitive with traditional heating and cooling systems. Further work should concentrate on improving compressor design to eliminate CO
2
leakage and to improve both isentropic and volumetric efficiency.
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Keywords
Heat pumps, Refrigeration and refrigerating machinery