Large eight.cylinder Stirling engine for biofuels

A large Stirling engine with an electric power output of 70 kW has been developed for small-scale CHP using wood chips and other sorts of biomass as fuel. The development of the engine is based on the results from the development of a four-cylinder Stirling engine with a power output of 35 kW, which has been reported before.

The new 70 kW engine has eight cylinders, which are connected in a 2 x 4 double acting configuration. Like the four-cylinder engine, the eight-cylinder engine is designed as a hermetically sealed unit. A 75 kW asynchronous generator, which is incorporated in the pressurised crankcase, has 6 poles corresponding to approximately 1000 rpm.

Working gas is helium at a mean pressure of 45 bars. The eight heater panels, which form two separate square sections, are exposed directly to radiation from the combustion chamber. Each heater panel is divided in a section for heat transfer by radiation and heat transfer by convection. The convection part the heater has been optimised in order to obtain an equal distribution of heat transfer on each tube and at the same time maximise the heat transfer from the combustion products to the engine.

In a double acting Stirling engine, the cold volume below the piston in one cylinder is connected to the hot volume above the piston in the cylinder next to it. Normally this connection is made between the compression space and the cold end of the regenerator. In the four- and eight-cylinder engines for biomass the connection is made in the hot end connecting the expansion space with the hot end of the regenerator through the heater panel.

However, this has resulted in comparably large dead volumes and flow losses in the connections between the heater and the regenerator/expansion volume. For the new eight-cylinder engine the design of the connections between the heater and the regenerator/expansion volume have been improved considerably, reducing the flow losses and internal dead volume.

Results from simulations indicate an improvement of power output and efficiency of about 10%. A four cylinder double acting Stirling engine is basically balanced concerning first order mass forces but not first order torques. An analysis of balancing mass forces and torques show, that the eight cylinder double acting engine can be balanced concerning both first order mass forces and torques, resulting in less vibrations transferred from the engine to the surroundings.

Preliminary results from tests are compared with results from simulations.