District heating networks, heat recovery, apartment buildings, industry

The plan was to develop a heat pump (Flex-WP) that could be used flexibly for a variety of applications, based on natural zeotropic refrigerant mixtures with a medium to high temperature glide on both the cold and hot sides, with a capacity range of approximately 50 kW.

The project is divided into two main areas of work. In the first area, the concept and a functional prototype of the Flex heat pump were developed. Initial theoretical work was carried out to select a mixture of natural refrigerants and to perform a preliminary design of the functional prototype. Subsequently, the interaction between the compressor, oil, and the mixture was investigated. This was followed by the design and construction of the functional prototype. The creation of a safety concept for the flammable mixture was also part of the project. Finally, the functional prototype was successfully commissioned and measured with various modifications. A second area of work was necessary to provide a test facility for the functional prototype at PEWO. For this purpose, a suitable test bench was designed, built, and commissioned.

The following findings were obtained:

• For large temperature differentials between heat sources and sinks, a mixture of propane and butane is optimal in terms of efficiency and system compactness.
• For large differentials, the mixture demonstrated a significant improvement in COP (+0.5) with a slight reduction in heating capacity.
• The mixture also enabled temperature levels (90°C) to be reached that would not be possible with propane alone and would result in significantly lower heating capacity and efficiency with butane alone in the same system.
• For the mixture to be efficient, it is extremely important that a suitable temperature spread is present.