Air-source ammonia heat pump for district heating: a field modeling approach with focus on frosting-defrosting cycles

• Evaporator field optimization for district heating heat pumps. • Dedicated model for frost formation in fin-and-tube evaporators. • Experimental validation with transient tests in frosting and defrosting. • Numerous field configurations and defrosting strategies considered. • Best design options largely different when including or excluding frost formation. Frost formation on air-source evaporators in cold and humid climates significantly increases energy consumption due to airflow obstruction and thermal insulation. While this issue is well recognized, effective control strategies for multi-evaporator fields remain underexplored. This paper presents a novel evaporator model that combines experimental validation under transient frosting conditions with a new algorithm for solving frost accumulation and defrosting process across an entire evaporator field coupled to the heat pump cycle. The model is applied to simulate a large-scale ammonia heat pump serving a Danish district heating network over an annual period. Results show that neglecting frosting in design analyses can lead to strongly overestimated performance, with total costs increasing by up to 70 % once frost and defrosting cycles are considered. The study also identifies an optimal field configuration of 80 evaporators with a fin pitch of 9 mm and an airflow velocity of 3.5 m/s, and shows that defrosting 20 % of the field simultaneously is more effective than defrosting 10 %. Overall, the outcomes emphasize the critical importance of accounting for frost and defrosting cycles in thermo-economic optimization, showing that the optimal design depends on local climatic conditions and should be supported by dedicated modelling tools.