Drop-in replacement of R22 in heat pumps used for district heating — influence of equipment and property limitations

In Swedish district heating systems several large (25 MW) turbo-compressor driven heat pumps using R22 are installed. The only commercially available alternative is R134a, but its use could decrease the heating capacity by 35%. In this paper a method for finding the best working fluid for a specific heat pump plant is presented, and applied to a district heating plant. First, a screening is made among almost 2000 mixtures, using criteria such as condenser pressure, Mach number and temperature glide. Simulations of the plant are then made to investigate the change in heating capacity and COP when using a mixture instead of R134a. The results show that there are mixtures that can offer a substantially higher heating capacity than R134a, but there is a decrease in COP. The importance of considering the limiting parameters of the heat pump, such as maximum volume flow to each compressor stage and minimum evaporator pressure, is shown.