An air source heat pump model for operation in cold humid environments

There is considerable interest in the use of heat pumps as a potential low-carbon alternative to fossil fuel-based domestic space heating and hot water systems. In many cases, heat pumps are combined with other energy sources such as solar thermal and/or electric resistive heating, to ensure that building thermal loads can be met, and in order to minimise carbon emissions from such integrated systems. Whilst meeting the comfort demands in the occupied space, relatively complex control strategies are required in comparison to simple thermostatic control typically implemented to control gas fired heating systems in domestic buildings. Well characterised models of the principal components of these systems are required to explore and identify the most appropriate strategies in simulation. However, models of air source heat pumps (ASHPs) operating in humid climates, such as the UK, are limited. This paper presents an experimental setup designed to capture the operation of the ASHP in conditions similar to those found throughout a typical heating season in the UK. Results from a number of tests on a 10kW ASHP are presented in terms of the coefficient of performance (COP) and the steady-state operation are used to develop a model using empirical curve fitting. The overall maximum time constant is also established. The resulting model calculates COP as a function of air humidity, air temperature on the evaporator side and water temperature on the condenser side.