Techno-Economic Assessment of a Condenser-Side Phase Change Material Buffer Integrated with an Air-to-Water Heat Pump

Heat pumps are increasingly used for low-temperature heating applications. But their operation can create electricity demand peaks during periods of high thermal load. This paper presents a concept and calculation study for integrating phase change materials (PCMs) with air-to-water heat pump (AWHP) systems as a condenser-side thermal buffer for residential buildings. The analyzed retrofit-oriented storage unit is a 200 L tank containing 30% PCM by volume, corresponding to 60 L of paraffin PCM and 140 L of water. The sizing calculation gives 3.08 kWh of PCM thermal storage, 0.81 kWh of sensible storage in the remaining water over the 55–60 °C interval, and almost 3.89 kWh of total short-term thermal reserve for the PCM-plus-water tank. A 200 L water tank over the same 5 °C interval would store approximately 1.16 kWh. At the nominal 14 kW heat pump output, the PCM buffer corresponds to about 13.2 min of operation. At representative part load it corresponds to about 53 min at 25% load and 26 min at 50% load. The PCM sizing model predicts storage capacity, buffer time and possible cycling relevance, but it does not by itself predict a seasonal COP increase. Therefore, annual electricity and economic effects are evaluated only as COP-uplift sensitivity cases, including 0%, 3%, 4%, 5%, 8%, 15% and 28.6%. Given the assumptions in this study, the PCM add-on reaches positive net present value at approximately 11% COP uplift when the initial marginal investment and discounted year 10 replacement cost are considered. Approximately 14.1% is reached if the replacement cost is added upfront as a simplified lifecycle basis. The results indicate that the condenser-side PCM buffer may be technically useful for short-term thermal buffering and possible on-off cycle reduction, but its annual energy and economic benefit requires dynamic simulation or experimental validation. Moreover, peak-demand reduction, demand shifting and renewable electricity utilization are discussed as potential applications of the buffer concept.