No, not sterling as in silver…although I am a fan of that too. This is in regards to the use of the Stirling Cycle engine to power a fan. So called “hot air engines” are an external combustion engine that was developed by Robert Stirling in 1816. The key part of his patent was the use of a heat exchanger which he called an “economiser” due to its enhancement of fuel efficiency. This is a key element in the closed cycle regenerative types that are seen today. While subject to some dispute, it is possible that the development of this engine in the early 1800’s was in part to provide an alternative to the steam engine at a time when boilers were not reliable and could fail with catastrophic results.
The Stirling cycle engine operates by simply moving air from one cylinder to another in the presence of a thermal differential. The resulting expansion and contraction of the air provides the mechanical force that is converted into rotary motion by the crankshaft. In the more simple forms, the pistons are set up to be 90 degrees out of phase. Heat applied to the hot side causes this air (or other gas if so desired) to expand which moves that piston. The continued expansion then pushes the cold side piston…as this happens, the hot side piston moves back which forces the majority of the gas to the cold side…cooling there causes this piston to move back which then forces the now denser gas back to the hot side to be heated and expanded again. There are of course variations on the design, including some with concentric pistons, a rhombic drive scheme, and even a free piston design….yet regardless of the variation that is basically how it derives its power.
The efficiency of a Stirling engine can actually be rather high, approaching 50% in some modern designs. And they can operate on any heat source…which makes them attractive for alternative energy solutions or capturing waste heat from other processes….some tiny examples will even function from the heat of a cup of coffee. The power density, however, is rather low and so any modern use usually needs to be of a stationary type where size and weight are not as much of a factor. Another important consideration is that unlike a steam engine, the torque output is also rather low–making them unsuitable for some applications.
Early engines of this type were used for pumping water. In the late 1800’s up to about 1920, very ornate iron and brass fans were made that would burn alcohol or kerosene to power them and help the owner keep cool on a hot day. The irony of lighting a fire to cool off. In more recent years, they were used for electric power generation on some submarines, owing to their quiet operational nature. During the 1940’s, Philips did a great amount of research in order to develop one that could power a small generator to run a radio. Only about 150 or 200 of these sets were produced by 1951/52 when the project was dropped due to the advent of practical low power consumption transistor sets that negated the need for the portable genset.
Even so, such a simple and quiet generator that could burn almost any fuel would be rather attractive to those who are in remote areas. I’d kinda like to find one at a garage sale! In spite of the marketing failure, the research was of such value that much of it is still quoted and relied upon today by those working on new designs…including some for deep space probes where sunlight is too weak and thermoelectric generation is not enough power to meet the needs…in such an application, nuclear decay will be the heat source but instead of thermoelectric junctions the heat will be applied to a variation of a Stirling Cycle engine to run a generator for powering the onboard systems.
Aside from the educational models and a few experimental uses being worked on, one of the only successfully marketed uses of the Stirling engine in very recent years has been for powering fans–this time, however, the hot end is designed into the base to be set upon a wood burning stove. The heat from the stove drives the fan, which in turn helps to circulate the hot air from around the stove and into the rest of the cabin or house. I have seen some smallish almost mass produced looking versions and also, very recently, a very unique looking example with a 14″ diameter from Canada that appeared to use built up wood for the fan hub and cold side connecting rod. It was too spendy for me to buy it today…..but sometime I may pick up an example to test and evaluate with real measurements. Granted, you can obtain the same result with a modern Peltier thermoelectric junction powering a small electric fan….but there is something both visually and aurally appealing about the lightly chugging mechanical approach from 200 years ago.
