Stirling Engines                                                                                                                                                                       

Stirling engines are mechanically simple but complex in design. I have known about their existence for a long time but had not really “gotten it” until after some additional reading this week. In a Beta Stirling engine, a gas is cyclically heated and cooled by moving the gas in and out of regions containing solids that are already hot or cold. This movement of gas takes little energy because it is accomplished with a light-weight displacer in a closed vessel. The displacer does not compress the gas and except for some gas rushing around in there, no work is done. Warming gas expands and cooling gas contracts. The energy of the changing pressure in the closed vessel is captured by a piston and, when the piston moves, is referred to as the power stroke. The work done by the power stroke is transferred to a flywheel,  which in turn powers the displacer. Because gas is difficult to heat and to cool rapidly, most of the heat transfer happens in a regenerator which is filled with a high surface area material like steel wool. The regenerator temporarily stores the heat (or lack of heat) of the alternating cycles and improves the efficiency of the engine. 



In an Alpha Stirling engine, there is no displacer. The gas is pushed back and forth through the regenerator by two  pistons. The power stroke force comes from the increase in combined volumes from both pistons.

Wikipedia has some great animations of Stirling engines.

http://en.wikipedia.org/wiki/Stirling_engine

Stirling engines that use air as the working gas are said to be inefficient in extracting work from the heat energy supplied. These weak engines have more power if the heat difference is higher. The spa is 20C when cold and 40C when hot. I am thinking of circulating vegetable oil in a parabolic solar array for attaining input temperatures of greater than 100C. The engine’s low extraction of the heat from the solar array fluid may actually help keep the input temperature nice and high.

I am very impressed with the SunPulse engine (beta type I think) in the Youtube video (prior post). Their solar fed engine produces 1.5 KWatts of electricity. I am thinking of an alpha type engine about half that size to power two cheap sump pumps. I have some ideas on how to improve the efficiency of the alpha engine. 



My Working Concepts for Building a Stirling Engine.

1. Use a stationary regenerator with two identical but opposing pistons to move the gas back and forth through the regenerator; an alpha type engine. 

2. Heat exchangers are incorporated into the regenerator so that the air exiting to a piston is heated (or cooled) as it passes. Air coming into the regenerator takes a different path.

3. Use rolled cloth piston seals for a completely closed system. It could be possible to use helium as the fluid gas.

4. Power stroke (sum of both piston volumes) is variable. May change from zero to high compression ratios during engine operation.

5. Phase angle between pistons is changeable during engine operation. 

6. Pragmatically, find optimal engine parameters by studying the effects of temperature, flow-rate, phase angle, compression and running speed on an operating engine.

7. Make  a ¼ or ½ scale model of the engine to study efficiency and power. Because power is proportional to the volume of the working gas, doubling the size of an engine should increase the power produced by a factor of eight and quadrupling by a factor of 64. There is no need to make an engine that requires more heat energy than the solar collectors can produce.

8. Use scrounged, recycled or hardware-store-off-the-shelf materials. Use simple bench-top tools for fabrication. Budgeting $100 for the first model. That does not include anything solar at this time.