Whereas fuel combustion takes place within the cylinders of an internal combustion engine, the heat energy used to drive a Stirling engine is
applied outside the cylinders which are completely sealed. The engine was designed by a Scottish Presbyterian minister, Robert Stirling, who received his first patent in 1816. The original engines used air within the cylinders and were called air engines but modern Stirling engines often use helium or hydrogen.
A normal Stirling engine has two cylinders, an expansion cylinder and a compression cylinder. The two are linked and heat is applied to the expansion cylinder while the compression cylinder is cooled. Careful balancing of the system allows the heat energy to be converted into rotational motion as in an internal combustion engine.
The great advantage of the Stirling engine is that the heat energy is applied externally. Thus the energy can, in theory, be derived from any heat source. Stirling engines have been used to exploit solar energy and for biomass applications. However their use is not widespread. Typical engines' sizes in use and development range from 1 to 150 kW.
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The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.