In the process of doing research for an article I did on solar green products, I came across the Stirling air engine. The engine was once called a hot air engine because it requires no internal diesel or fuel to run. Let’s take a deeper look at this unique engine that requires no fuel and has recently been adapted for use on solar collector dishes.

The Stirling engine was originally invented and patented as an air engine by Robert Stirling in 1816. It may have been designed as a replacement for the steam engine, because the steam engines used in factories would explode frequently, causing many injuries and fatalities. These days, because of the cost of fuel and the environmental problems with coal, many new designs are incorporating the Stirling engine in solar power systems.

The Stirling engine has many different designs, some more efficient than others. Some designs use the two cylinder Alpha design, some the Beta single cylinder design and some the Gamma design (a variation of the beta for use in multiple cylinder applications). The beta Sterling design has a single chamber with two pistons and uses a highly efficient regenerator gas as the thermal conductor.

The heat source needed to start and maintain the Stirling cycle depends on the temperature difference between the hot and cold chambers. The greater the differential between chambers, the faster the engine will operate. In some cases the difference that occurs when the cold chamber is cooled by wetting it is enough differential to start the engine.

The Stirling cycle is pretty simple. When a heat source is applied to the large end, it causes the air to expand which moves a piston. A flywheel is used to maintain movement between cycles and add a convenient place to tap the power from the engine. What is so amazing about this design is that the design is very scalable. Engines have been produced that are as small as your hand to a large as a car but the concepts remain the same.

The system is ideal when a concentrated heat source is applied, in this case sunlight. The sunlight is collected with parabolic mirrors and focused on to the surface of the hot chamber, causing the chamber to be heated and the engine to start. This temperatures can be very high. You know this from, when as a child, you took a magnifying glass and focused it on a piece of paper. When done correctly the paper would get dark and then a flame would appear.

With better-designed engines and new types of material, a movable (tracking) parabolic mirror, and a single Stirling engine you can produce 25 kilowatts of electricity.

In a future article about green products I will list the different solar technologies and how they compare to the competition.

Michael