Method of linear approximation of COP for heat pumps and chillers based on thermodynamic modelling and off-design operation

Often, simple estimates of the coefficient of performance (COP) for heat pumps (HPs) and chillers are used. Depending on the purpose, this may not be sufficient. There are more accurate methods for determining COP, but they may not be used due to complexity, nonlinearity, or limited application of advanced COP estimation methods. Here, we present a new COP approximation method suitable for HPs and chillers. It is based on linear relationships and was developed from a thermodynamic two-stage HP model for design and off-design operation using ammonia as a refrigerant. This approximation method was then applied to a case study investigating the potential of district heating supplied by HPs in Tallinn, Estonia. Groundwater, sewage water, seawater, river water, lake water, and a district cooling return line were explored as potential heat sources. The results show a deviation in COP of less than 1.5 % compared to the thermodynamic model. Annual calculations show the applicability of the COP approximation method for calculating hourly COPs at different heat source and heat sink temperatures, as well as changing heat loads, seasonal COP, heat demand ratio, and hourly Lorenz efficiency.