Modeling air source heat pumps under severe frosting conditions and evaluating performance during demand response events

Managing the electric demand of heat pumps on the coldest days is an important aspect of congested low voltage grids. Specifically, severe frosting of the evaporator deteriorates system performance. Contemporary models are limited in representing severe frosting conditions. To understand the potential impacts of these conditions on the grid, a new model for the initiation and operation of defrost cycles that incorporates relative humidity, outdoor temperature, and heat pump loading was developed. To investigate regular and flexible operation of the system, various heating system design parameters were simulated. The simulations show that the heat pump was able to coordinate all tasks without additional electric resistive heating. Furthermore, the impact of a flexibility request on the amount of defrost cycles was found to be negligible. Finally, the results highlight the need to consider both efficiency and flexibility during the design process.