Increasing Efficiency of Stirling Engine

Following my readings on thermodynamics in the Stirling Engine I read about ways to increase the efficiency of the engine. There a number of ways to increase the efficiency of the engine. To have 100% efficiency, all of the thermal energy must be converted to mechanical energy. This is theoretically impossible; however the Stirling Engine is able to come close to the theoretical limit. This is done through increasing the temperature differences on the hot and cold sides of the engine and a matrix of wires, known as a regenerator.

As explained in a previous post thermal energy is transmitted through collisions of particles. An average of faster particles equates to more thermal energy, otherwise known as an increase in temperature. A larger difference in speed between the particles indicates a larger increase in speed for the slower particle. Likewise, the faster-moving particle will lose more energy. This means that the heating and cooling of particles can create enough of a difference to move the working piston faster, and results in a slight increase in efficiency.

100% efficiency is when all of the thermal energy is converted to mechanical energy. Thusly, if the thermal energy lost during the cooling phase was able to be reused in another phase of the engine, efficiency would be increased because more thermal energy would be converted to mechanical energy. The regenerator is responsible for this action. Shown to the right, the regenerator is a matrix of wire along the edges of the cylinder. When the displacer is moved gas is pushed, resulting in the gas rushing around the edges of the displacer to the opposite end of the cylinder. When the regenerator is added the gas is forced to flow through the regenerator. This results in an increase in efficiency.

Efficiency is increased when this occurs because the regenerator pre-cools and pre-heats the gas. For example when the gas rushes from the hot end of the cylinder to the cold end of the cylinder the hot gas flows through the regenerator. When the gas flows through the regenerator the gas particles collide with the slower particles of the regenerator. This heats the regenerator and effectively pre-cools the gas. Likewise, when the gas returns to the hot end of the cylinder it must flow through the regenerator. The gas is now cooler than the regenerator, thusly when the particles collide they heat the gas. This pre-heats the gas. By pre-heating and pre-cooling the gas less energy has to be put into the system each cycle as some of the energy already put into the system is re-used each cycle. This results in an increase in efficiency.

For the regenerator to be effective a few criteria must be met. First the gas must flow through the regenerator when changing locations within the cylinder. Secondly, the material of the regenerator must be able to increase and decrease in temperature easily. Most regenerator’s use steel, which would indicate that steel, is able to receive and expel thermal energy easily. Finally, the surface area of the regenerator must be carefully considered. If the regenerator is too large, each particle of the regenerator may not be able to gain enough thermal energy to see an increase in efficiency.  Similarly, if the regenerator is too small the gas may not be able to effectively cool and heat itself resulting in no increase in efficiency.

During these readings I had little trouble understanding the general ideas of the regenerator and ways to increase efficiency. I did however struggle to understand why a larger difference in temperature results in more efficiency. This is because my intuition tells me that even if there was a larger difference in temperature, proportionately the same amount of thermal energy is being converted to mechanical energy. However, the textbooks and internet readings I looked at all specifically stated that a larger difference in temperature increases efficiency. I suppose this makes some sense, because if the temperature can be increased in fewer collisions than the efficiency must be increased. Beyond this struggle, I was able to understand how and why the regenerator works and the importance of including one in the engine.

Sources:

Hooper, C & Reader, T. G. (1983). STIRLING ENGINES. New York, NY: E. & F. N. Spon.

Stirling Engine. (n.d.). The operating principles of Stirling engine. Retrieved June 3, 2014 from “Robert Stirling Engine”:www.robertstirlingengine.com