Evaluating heat pump system design methods towards a sustainable heat supply in residential buildings

Aiming at an emission-free building energy supply, air-source heat pump systems (HPSs) are promising to replace conventional technologies such as gas boilers. Currently, the overall costs of HPSs are higher than the costs for conventional systems, inhibiting HPS market penetration in existing buildings. Increasing the profitability of HPSs by reducing overall costs simultaneously decreases buildings' emissions. Total costs consist of investments (depending on the design) and operating costs (depending on the operation). While design and operation are mutually dependent, it is necessary to consider them at once in the design stage to exploit the full potential of HPSs. However, in the literature, there is no general framework to consider operation already within an overall design process. To close this gap, we develop a framework called X-HD enabling the comparison of three different design methods: a heuristic method based on the normative standard DIN EN 15450 (1: HSM), a mixed-integer linear programming optimization design method (2: OBM), and a simulation-based design method (3: SBM). All three methods design reliable HPSs consisting of a heat pump, an auxiliary heater, and two thermal energy storages: one for space heating and one for domestic hot water. A single-family house serves as a case study representing a part of the German building stock, and the annuity method is applied to evaluate the economic efficiency. While the design according to OBM (2) and SBM (3) outperform HSM design (1) by 3 % in annuity and 14 % in operating costs, HSM design (1) offers easy and fast handling. The results highlight the necessity of considering HPSs' operation already in the design process. Although the OBM design (2) reveals the highest cost-saving potential, we recommend the SBM design (3) due to the consideration of nonlinear interdependencies, which occur during operation. We suggest coupling simulation-based design methods with an optimization method to determine optimal solutions while still integrating nonlinear system behavior for future work. Doing so, we pave the way towards an emission-free building sector based on HPSs.