Stirling engines for biomass – what is the problem?

The External combustion of the Stirling engine makes it very attractive for small-scale Combined Heat and Power (CHP) plants using bio-fuels. Especially wood chips are an attractive fuel because of the high melting point and the low content of ash. Unfortunately, it is more complicated than expected to use bio-fuels for a Stirling engine. The high temperature in the hot heat exchanger transferring heat from the combustion to the Stirling engine combined with the low heating value of the fuel reduce the obtainable efficiency of the plant. The limitations of the combustion temperature in order to avoid melted ash in the combustion chamber decrease the obtainable efficiency even further. If a Stirling engine, which has an efficiency of 28,5% using natural gas, is converted into utilization of bio-fuel, the efficiency will decrease to 17,5%. Another problem for utilization of bio-fuels in Stirling engines is, that the combustion of bio-fuels and transfer of the heat from the combustion gases to the Stirling engine need much more space than for natural gas as fuel. Because of the large differences in specific heat transfer on the inside and the outside of the heater tubes, the specific power of the Stirling engine is restricted, and it is difficult to design a Stirling engine for direct combustion of bio-fuels, which has a mean pressure above 60 bar. This is shown by a simplified calculation of the specifications and the performance for typical Stirling engines compared to the conditions on the outside of the hot heat exchanger tubes including heat transfer from the combustion gases and calculation of fin efficiency etc.