The Stirling engine is an external combustion thermal engine. Originally it was conceived as an industrial main engine to compete with the steam engine, but in practice, for more than a century it was only used for domestic applications and for low power engines.
The Stirling engine was invented in 1816 by Robert Stirling, a Scottish priest. One of the concerns of the time was the safety of steam engines. Stirling's goal was to get a less dangerous engine than the steam engine.
The operation of the Stirling engine is based on the expansion and contraction of a gas that can be helium, hydrogen, nitrogen or air. This gas is forced to cycle cyclically from a cold source where it contracts to a hot source where it expands. It is considered as a thermal engine. It is a thermodynamic engine due to the presence of a temperature gradient between the two thermal sources.
Currently, development is still being investigated. The fact that you only need an external heat source gives you great versatility since this fact gives you the possibility of being able to use many sources of energy for its operation. Examples of energy sources used in a Stirling engine are solar thermal energy, fossil fuels such as coal or oil, biomass, geothermal energy and others.
Operation of a Stirling engine
A Stirling engine is an alternative engine that works in a thermodynamically closed regenerative cycle. The Stirling engine works with cyclic compression and cyclical expansion of the working fluid at different temperature levels.
The Stirling engine is a thermal engine that works through a compression and expansion cycle of a gas. Two temperature levels are used that make a net conversion of thermal energy into mechanical energy (mechanical work). The compressed gas exerts a pressure on a piston. The piston is connected to a crankshaft that allows to convert the linear movement of the piston inside the cylinder in a rotation movement of an axis.
Within the group of thermal engines, like the steam engine, the Stirling engine is classified as an external combustion engine; all heat transfers with the working gas are made through the motor wall. In contrast, in an internal combustion engine the heat input is made by the combustion of a fuel inside the body of the working fluid.
Performance of the Stirling engine
As is typical of thermal engines, the general cycle of the Stirling engine consists of:
- Compress the cold gas
- Heat the gas
- Expand the hot gas
- Cool the gas before repeating the cycle.
The efficiency of the process is very restricted by the efficiency of the Carnot cycle, which depends on the temperature difference between the hot and cold tanks. Anyway, the Stirling engine is the only one capable of approaching the theoretical maximum performance known as Carnot performance.
Differences between the Stirling engine and the steam engine
Unlike a steam engine, the Stirling engine closes a fixed amount of fluid in a permanently gaseous state such as air. In contrast, in the steam engine the working fluid undergoes a phase change from liquid to gas.
The Stirling engine is characterized by its high efficiency compared to steam engines, quieter operation, and easier to use different heat sources.
This compatibility with alternative and renewable energy sources has been increasingly important as the cost of conventional fossil fuels went up, and social sensitivity with climate change. The Stirling engine is currently of interest as the core of the micro-combustion units of heat and energy, that is to say of cogeneration, in which it is safer and more efficient than a steam energy.
Stirling engine applications
This technology is considered to be of great application for regions where there are large numbers of dispersed settlers, which would be very expensive to reach with an electric network.
It is to be expected that the Stirling engine manufacturers will build small units of the same type on a large scale, (with solar disk) as for example with capacity to produce about 200 to 400 kWh per month (equipment of 1 to 2 kW of power approximately ); especially for countries located between the tropics, because in these areas the amount of solar radiation is great throughout the year and in turn is the region where there is more dispersed population.
Last review: December 13, 2017