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Novel CO 2 heat pump systems for domestic hot water supply are modelled using an object-oriented thermodynamic model library written in Modelica. Due to the variety of mathematical structures of thermodynamic models and the possibilities of objectoriented modelling languages, powerful libraries are often very complex and hard to understand. The scope of the presented library lies on good readability and usability for both users and model designers. The basic structure and design concepts of the library are outlined and also the most important model concepts are explained. The heat pump system is modelled and simulation results are compared with test stand measurements. Simulation results are used to analyse the design of the system and to get information about possible further improvements. IntroductionCompression heat pump systems have the potential to decrease the primary energy consumption of domestic heating and hot water supply. Different studies [1] have shown that heat pump systems using the natural, environmentally friendly refrigerant CO 2 are able to achieve higher efficiencies than systems using conventional refrigerants when warming up heating water or potable hot water at a high temperature level (55-60 C). Thereby, new concepts of heat supply systems for low-energy buildings have resulted. Low-energy buildings are characterized by a high level of heat insulation, which leads to a reduced energy demand for heating water. However, the amount of heat required to warm up potable hot water rises compared with the overall energy demand of the building. The heating system as a whole has to supply less heat, but considered relatively, it has to deliver heat more frequently at higher temperature demands. To come close to the theoretical maximal efficiency in real systems, an accurate design is mandatory. To really lower the primary energy consumption, it is important to not only look at certain operating points but on the overall performance over long-term periods. For both aspects, simulation of heat pump systems is very useful and can support the design of heat pump systems significantly. Due to high variability of different concepts, a library with ready-to-use component models is a great advantage.There have been many research activities about simulation of thermodynamic systems during the last decades. A variety of different programming and modelling languages are used. Richter [2] gives an overview of what is state of the art in this area. The latest activities