The Stirling engine is an external combustion heat 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 is based on the expansion and contraction of a gas that can be helium, hydrogen, nitrogen or air. This gas is forced to move cyclically from a cold source where it contracts to a hot source where it expands.
This type of machine is considered as a heat engine. It is a thermodynamic motor due to the presence of a temperature gradient between the two thermal sources.
Development is still under investigation. The fact that you only need an external heat source gives it great versatility since this fact gives you the possibility of using many energy sources for its operation. Examples of energy sources used in this technology are solar thermal energy, fossil fuels such as coal or oil, biomass, geothermal energy, and others.
How does a Stirling engine work?
A Stirling motor is an alternative motor that operates in a thermodynamically closed regenerative cycle. This cycle is the so-called Stirling cycle. The main characteristic of this cycle is that it works with cyclical 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 levels of temperature are used that cause a net conversion of thermal energy into mechanical energy (mechanical work).
The compressed gas puts pressure on a piston. The piston is connected to a crankshaft that allows the linear movement of the piston inside the cylinder to be converted into a rotational 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 done through the engine wall. On the other hand, in an internal combustion engine the heat input is made by the combustion of a fuel inside the body of the working fluid.
As is typical for heat engines, the general cycle consists of:
- Compress the gas that is at a lower temperature.
- Heat the gas in a hot spot.
- Expand the hot gas generating mechanical work.
- Cool the gas in a cold spot to be able to repeat the Stirling cycle.
What is the remorse of a Stirling engine?
The efficiency of the process is highly restricted by the efficiency of the Carnot cycle, which depends on the temperature difference between the hot zone and the cold zone. In any case, the Stirling engine is the only one capable of approaching the theoretical maximum performance known as Carnot performance.
In thermodynamics, the Carnot cycle or the Carnot process is an ideal circular process. This cycle consists of two adiabatic processes and two isothermal processes. In the Carnot process, the thermodynamic system performs mechanical work by exchanging heat with two heat tanks. The two heat deposits have constant but different temperatures.
In Carnot cycles there are two sources of heat:
- The heater, which tank with higher temperature
- The refrigerator, which is a tank with a lower temperature.
What is the difference 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. On the other hand, 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 greater ease of using different heat sources.
This compatibility with alternative and renewable energy sources has become increasingly important as the cost of conventional fossil fuels has risen, and social sensitivity to climate change. The Stirling engine is currently of interest as the core of microcombine units for heat and energy, that is to say cogeneration, in which it is safer and more efficient than steam energy.
Applications and uses of this thermal machine
This technology is considered to be of great application for regions where there are a large number of dispersed settlers, which would be very expensive to reach with an electrical network.
It is expected that manufacturers of Stirling engines will build small units of the same type on a large scale, (with a solar disk), such as with the capacity to produce around 200 to 400 kWh per month (equipment of approximately 1 to 2 kW of power) ; especially for countries located between the tropics, because in these areas the amount of solar radiation is large throughout the year and in turn is the region where there is the most dispersed population.