The Stirling Engine: Betamax of the 19th Century


While following the links provided alongside a very interesting article on how to reduce household energy consumption in A-to-B magazine, I came across some information about highly efficient, power generating domestic boilers which contained Stirling engines, and I wondered again why we don't see Stirling engines everywhere. Why did VHS (steam power and then the infernal internal combustion engine) win?

I first learnt about Stirling engines several years ago on a TV programme (who says TV isn't educational?) and marvelled at their elegant design. Invented by Rev. Robert Stirling in 1816, the Stirling engine was intended as a safer, quieter and cleaner alternative to steam power. But modern Stirling engines using modern materials can even be more efficient than many petrol or diesel engines.

The principle is fairly simple; the cylinders are sealed and contain a fluid (usually a low molecular weight gas like hydrogen or helium), and contain a piston. One end of the cylinder chamber is heated externally, expanding the gas. This causes the piston to rise up, but as the opposite end of the cylinder is cooled, the gas compresses again, returning the piston to its starting position. So all you really need is to create a temperature gradient from one end of the cylinder to the other. The engine is quiet because there are no explosions going on inside the cylinders, and the components can be much lighter and simpler because no combustion happens inside the cylinder and it doesn't need an air intake or exhaust or complicated fuel valves. The process is extremely efficient, and because it just needs a source of heat (or coolness!) rather than something to combust, you can run a Stirling engine on gas, oil, solar energy, geothermal energy, waste methane from landfill sites or even the heat from a cup of coffee.

Unfortunately, Stirling engines aren't really suited to road vehicles (though there is a lot of interest in using them in aircraft) because of the size of the heat exchangers needed, and because they take a little while to get going. However, they do make excellent domestic boilers. New micro-CHP boilers use Stirling engines running on gas or oil to generate hot water for radiators and taps, and also cleverly use the moving piston to generate a small amount of electricity, which can either be used in the home or sold back to the grid. It's true that it's not renewable energy, but the micro-CHP boilers are supposed to reduce emissions by about 25% over a conventional combi-boiler. That could make a huge difference if all new homes were fitted with them, and people were encouraged by subsidies to replace old boilers with micro-CHP units.

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