Development of a hot heat exchanger and a cleaning system for a 35 kW hermetic four cylinder Stirling engine for solid biomass fuels

Over the past few years, a small-scale CHP technology based on hermetic Stirling engines has been developed and two prototype plants with a 35 kWel four cylinder and a 70 kWel eight cylinder Stirling engine have been erected in Austria. The prototype plant with a 35 kWel Stirling engine has already been operated for more than 9,000 hours.

Operating experiences gained from these plants formed the basis for the further development of this technology. The experiences showed that the efficiency of the Stirling hot gas heat exchanger and of the hot gas heat exchanger cleaning system have to be further optimised. Within the scope of a RD&D project, a new hot gas heat exchanger and a new cleaning system have been developed and optimised in cooperation of the AUSTRIAN BIOENERGY CENTRE GmbH, the Technical University of Denmark, MAWERA Holzfeuerungsanlagen GmbH, Austria, and BIOS BIOENERGIESYSTEME GmbH, Austria.

The new design of the Stirling hot gas heat exchanger has been developed in order to optimise the performance of the engine and simplify the geometry. In this respect, an equal distribution of the heat transfer across each tube in the hot gas heat exchanger, the reduction of the internal Helium volume, a functional design of the flue gas duct and the geometrical possibilities to implement an efficient automatic cleaning system to reduce ash depositions have been the most relevant objectives.

Previous investigations showed that it is not possible to reduce the ash depositions on the surfaces of the hot gas heat exchanger by primary measures efficiently. Consequently, a new pneumatic cleaning system has been developed, in order to increase the intervals between manual cleaning and thus the availability of the Stirling engine.

The system is integrated into the furnace and works fully automatically. Based on the new technology developed, a pilot plant was designed and erected. The nominal electric power output of the plant amounts to 35 kWel and the nominal thermal output to approx. 250 kWth. The plant is already in operation since the end of May 2005.

First operating experiences proved that the pilot CHP plant can be operated fully automatically and reaches the design target concerning electric efficiencies and electric power output. Furthermore, the new hot gas heat exchanger cleaning system seems to work as expected. Within the scope of detailed test runs planned for autumn 2005 it is foreseen to further optimise the new CHP technology developed.