This paper deals with a heat pump system with two heat sources, a medium-term ground collector heat storage and a façade-integrated massive solar-thermal collector. Aiming at affordable system cost, a medium-term heat storage is applied instead of a seasonal heat storage. During milder phases of the heating period, the heat storage is regenerated by the solar absorber to be used as ambient heat source under frosty weather conditions. Preparing for the design of heat pump systems with multiple

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... at sources, the energy balance of a medium-term underground heat storage has been investigated, based on physical component models. Yearly and seasonal analyses for different absorber sizes from 7.5 to 37.8 m² lead to the following statements. In general, even for small absorber sizes solar yields during winter are sufficient for continuous operation of the heating system, with increasing average temperature of the storage for larger sizes of the solar system. Higher ground temperatures result in higher seasonal coefficients of performance of the heat pump, and in higher annual heat losses of the storage. The medium-term character of the storage is shown by the in-season heat gain contributed by the solar absorber. Finally, to assure frost-free operation of the storage the absorber size must be at least 15% of the ground collector size.