This document describes the details of the above self-starting solar-powered Stirling engine prototype. The white box at the lower right houses the controller which automatically starts this engine at a temperature ratio of 1.0675 which is about 35degF temperature difference at typical operating temperatures. Typical operation is between 100 and 200 rpm depending on the temperature ratio. The controller requires an external power source for monitoring temperatures, starting the engine, and logging and displaying run data.
Running on such a low temperature differential this engine has minimal power output and probably never exceeds 10mW. I’ve never measured it. This engine has hundreds of hours operating outside under solar power. It has been very reliable.
I also include recommendations for improvements that I plan to use in future versions of this engine. Unfortunately this engine evolved as a prototype and as such I have no complete plans to offer for this engine. The key dimensions are provided below and should help anyone with average machining skills and access to a lathe and drill press to design and build one. I have not withheld any design or constructions secrets that I have learned through many hours of experimenting with these types of engines. I think that information is more useful than plans, especially if you are interested in designing your own engine.
Key dimensions from current engine:
cylinder ID: 3.25 inches
cylinder OD: 3.5 inches
cylinder length: 2.75 inches
Piston diameter: 3.10 inches
Piston length: 2.0 inches
Stroke: 5/8 inch
Power piston and cylinder:
Bore: .625 inches
Stroke: .875 inches
Piston length 1.0 inch
Heat collector mirror
During the summer the engine will run for 6 hours with a flat piece of glass or acrylic cover over the heat collector. The cover is important to reduce convection cooling of the heat collector plate. During the winter the sun is at such a low angle that the heat collector mirror shown above is useful to improve performance. It should work fine in the summer too although I haven’t actually tested it during the summer.
The cover above, using aluminum foil as the mirror surface, gives good performance for sun elevation angles down to 24 degrees, allowing the engine to run for more than 4 hours even on the shortest day of the year at 37 degrees north latitude (assuming a clear sky). The heat collector mirror is oriented so that the open side faces true south (in the northern hemisphere) and need not be moved to obtain the 4+ hours of operation. The prototype was not designed to accommodate rain so it is taken indoors when rain threatens.
After removing the top cover and the top insulating foamcore square, the black round disk visible on top is the heat collector plate. It is 5.25 inches in diameter. The heat collector plate was made from 0.063 inch aluminum plate. This is the minimum thickness that should be used for this plate to avoid flexing. It also helps with heat conduction because the collector is larger in diameter than the displacer. When the top cover and insulating foamcore square are on top, a 5.00 inch diameter section of the heat collector plate is exposed to the sunlight.
You can also see the temperature sensor mounted to the edge of the heat collector plate.
Displacer cylinder gaskets
The bottom of the heat collector plate is forced against the top of the displacer cylinder by the four nuts with washers visible on the top cover. A 1/16 inch thick neoprene gasket or other material able to withstand 200 degF should be used to seal the connection between the heat collector plate and the clear polycarbonate displacer cylinder. The gasket is visible on the bottom of the heat collector plate. The gasket used on the prototype is not neoprene and has taken a compression set.
An identical neoprene gasket should be used to seal the connection between the bottom of the displacer cylinder and the cold plate. The cold plate is .125 inches thick.