## 1 Introduction  
Today's commercially available air source heat pumps (HPs) work highly efficient and make a valuable contribution to achieve climatic goals such as the reduction in the amount of CO\({}_{2}\) in our atmosphere. In the United States, for example, the share of HP sales for newly constructed buildings exceeds 40% for single-family dwellings, and is nearly 50% for new multi-family buildings. In addition, the EU market is expanding quickly, with 12% annual average growth since 2015. France, Italy and Spainare responsible for half of all sales in the European Union, while Sweden, Estonia, Finland and Norway have the highest penetration rates, with more than 25 HPs sold per 1000 households each year [1]. Different concepts using latent heat thermal energy storage technologies together with HPs were proposed in the past to further increase the efficiency of the state-of-the-art HPs. Usually, the main aim of such storage technologies is to decrease the size of buffer tanks and domestic hot water (DHW) storage devices, to avoid the oversizing of HPs by reducing thermal peak loads and ON/OFF-cycles by shifting heating and cooling demands in time, to optimize the integration of solar thermal collectors, to enhance the defrosting performance of air source HPs or to reduce the size of a ground-source heat exchanger by the use of PCMs in boreholes [2; 3; 4; 5; 6; 7]. This wide range of applications with PCMs in combination with HPs was made possible by the availability of PCM materials over a wide temperature spectrum [8; 9; 10]. In most of the reported studies, HP compressors with fixed speed were investigated. The performance of these systems was mainly increased by reducing the ON/OFF cycles with the aid of the latent storages. Applications with variable speed compressors designed for heating and DHW generation are usually not the focus for combined systems with PCMs [2; 3]. In the latter, the benefits of using PCM storage, besides a reduced size, usually result from an energy shift from times with high to times with low electricity prices and from the possibility to scale down the HP capacities, and therefore from reducing investment costs. Applications with PCMs used to store the sensible heat of the hot gas in the superheated section of a HP after the compressor are not known to the authors. However, the concept of using this heat directly to improve the overall performance of DHW heating has been studied for many years [11; 12]. The sensible energy which is available at a higher temperature level as the condensing temperature is described as a desuperheater, which is usually an additional small heat exchanger that is installed directly after the compressor and before the actual condenser of the HP. The desuperheater can either be integrated directly into a stratified storage, as in [13], or connected via a secondary water cycle to a DHW storage, as in [14]. The present work employs an integrated three-media refrigerant/phase change material (PCM)/water heat exchanger (RPW-HEX) as a kind of desuperheater-storage combined with a HEX in the hot gas section after the compressor of an air source HP. For this purpose, the water side of the RPW-HEX is connected to decentralized DHW storage devices located in individual apartments of a multi-family house. The original concept was first presented in [15] and results were presented for the annual performance of a low energy building located in an average European climate (Strasbourg, France). The results were computed adopting a quasi-static approach similar to the one used in the EN14825 standard [16] for air source HPs. A payback time of 18 years and annual energy savings of 200 kW\({}_{\text{el}}\) were found in this study.