Wood-fired Stirling engine
The new system set up in Germany by a Danish firm consists of four cascading Stirling engines that provide both power and heat, making it the largest such cogen unit in the world.
The four engines collectively provide some 4,000 megawatt-hours of heat and 1,000 of power annually. Stirling DK, the supplier, says the system is the largest cogeneration unit ever built as Stirling engines running on biomass – in this case, wood chips. The plant serves a nearby spa in Tabarz, Germany, owned by the town.
The technology was first developed by Scotsman Robert Stirling in 1816. The misery he saw around him led him to do some tinkering. What he came up with was a reaction to the high-pressure steam engines that were increasingly becoming common at the time – and frequently exploding lethally because of technical flaws.
In a Stirling engine, a heated gas expands to drive a piston. It does not matter what the heat source is; timber can also be used. When the gas loses its energy, it decompresses, and the process can be repeated. In this cogeneration system, the Stirling engine drives a generator.
The project is also interesting because of the hopes for systems with small to midsize electric outputs – from less than 10 kilowatts to hundreds of kilowatts – fired with wood. But there is hardly any long-term experience with cogeneration units fired with wood with wood gasification on such a small scale. The key to success is sufficient use of heat, says wood-based cogeneration expert Holger Roswandowicz.
Stirling DK says it is a global leader as a developer and provider of Stirling engines for biomass. Roswandowicz agrees based on his experience with manufacturers. That helps explain why German energy giant RWE's has a holding in Stirling DK. (Dittmar Koop / Craig Morris)