Evaluation of natural working fluids for the development of high temperature heat pumps.

Abstract

For a sustainable future in energy, the use of natural fluids in heat pump systems can provide an efficient solution to combat the ozone depletion and the global warming effects of synthetic working fluids. These natural fluids, though environmentally friendly, must be able to operate at high temperature in heat pumps in an efficient and cost effective way to be viable for the industry heat demand. High temperature heat pumps with heat source and heat sink temperatures ranging from 40 oC to 200 oC have unique challenges with natural working fluids. This paper evaluates the performance and use of natural working fluids for high temperature heat pumps. The analysis is based on simple thermodynamic cycles. A simulation tool coupled with a thermodynamic properties database is used to evaluate the operating limitations of the natural fluids. Natural fluids such as ammonia, hydrocarbon and hydrocarbon mixtures are considered. The hydrocarbon mixtures, benefitting from both adjusted thermodynamic properties and a higher evaporating temperature due to better fit to heat source temperature, shows greatest potential for the development of high temperature heat pumps. The mixtures R290+R600 and R600+R601 were able to expand the working limits of their pure fluids while improving COP by up to 13.5 %.