Solar Heat Engines

Maker Faire 2008

From 2008_05_05 Ma…

I want to thank all the people who stopped by to see the solar power Stirling engine in operation. I appreciate the questions, comments and interest in the engine. I regret that I was unable to offer a better response to those that asked about the efficiency. I simply cannot make a meaningful measurement of efficiency on this engine because it has several thermal shorts that are part of the engine. I’ll post more details on efficiency and the low-temperature engine in a future post.

One of my goals in attending the Maker Faire was to get a feel for potential interest in the engine and see if there might be applications that could make use of the engine. My impression is that there is a lot of interest—I just need to deliver the power. I was showing a development engine with an output of about 1/10 watt. I think interest would be better for at least 100 watts, approximately the power a person puts out exercising, or preferably 1kwatt.

One application that sounded particularly promising is the possibility of using the engine to make use of the excess solar-heated water available in buildings that use solar hot water for space heating. Space heating requires considerable power, the cause of you high energy bills in the winter. In the summer these systems are essentially unused. I’ve heard from users that they’ve seen steam coming out of these systems as the temperatures soar in the summer. A hot water engine need not be highly efficient as long as it can generate power to pay for itself plus some extra.

I’m always interested in hearing about potential application where a low-temperature low-power engine might be useful. There are potential applications such a powering fountains or operating kinetic sculpture that might be able to use power in the 10 watt range. One benefit of the type of Stirling engines that I am designing is that they produce torque at low RPM so that they tend to be more useful for mechanical operations. High RPM power often requires a gear train to reduce RPM and increase torque.

As I mentioned at the Maker Faire, I’ll be putting out some free plans on this website for making a simple, small, Stirling engine. I’m still getting a few bugs out (mostly reducing friction) so that it can operate from sunlight. My goal is to have it done by May 18.

After that I’ll put out a simple simulator for Excel that will help you estimate the power output of a Stirling engine design. Essentially it will give you a maximum possible work per revolution and information on pressures during the cycle. It won’t tell you things like what RPM the engine will turn—that depends on the design of your engine and how efficiently you transfer heat and how low the friction is in your engine.

You can see a video of the Maker Faire 2008 Configuration