In an Organic Rankine Cycle process (ORC process), thermal energy is converted into mechanical energy in the same way as in a steam power plant. By using fluids other than water in the cycle, the process can be operated at lower temperatures and thus be used for heat recovery from low-temperature waste heat (from 80°C). The aim of the joint research project was to develop a small-scale ORC system as a retrofit module for biogas plants. This means that biogas plants previously operated without a heat utilisation concept can also meet the requirements of the EEG 2012, which stipulates the utilisation of waste heat by ORC plants as a prerequisite for further eligibility for funding. The prototype of a pressurised gas membrane engine was developed for the small ORC plant, which can be used as an expander machine. Its lower costs make the overall system more favourable and shorten the return on investment.

The main tasks in the joint project were the numerical simulation of the membrane engine, the design, adaptation and optimisation of the ORC process, the scientific monitoring of the project partners' results and a final evaluation of the ORC plant that had been built.

A 2-way fluid-structure interaction (FSI) and a pure flow simulation (CFD) were used in the numerical simulation. In the FSI simulation, the movements of the solid components such as the shaft, connecting rod and diaphragm as well as the fluid flow and their interaction were calculated. A pure CFD simulation, which was able to deliver adequate results more quickly, proved to be sufficient for the investigation of different design variants. The tests on a functional model in the laboratory and the evaluation of the measurement results using the simulations were used to determine the key design parameters, which were then adopted by the project partners.

The projected ORC plant was constructed by the project partners on the basis of the institute's process design. In contrast to the originally planned installation site at a biogas CHP plant, the ORC plant was installed at Nordhausen University of Applied Sciences, as better monitoring of the test runs is possible here. For this purpose, an additional electrical thermal oil system was set up at Nordhausen University of Applied Sciences. Building on the existing test rig, the membrane motor is to be replaced by a plastic scrolllexpander in a follow-up project. The project partners were the companies Galek & Kowald (Mühlhausen), Hörisch-Präzision (Apolda) and the Gesellschaft für Fertigungstechnik und Entwicklung (Schmalkalden)