Today we are going to talk about a different type of engine than the conventional internal combustion engine. Vehicles generally use engines powered by the Fossil fuels whose efficiency is usually not very high. In this case, we present you the Stirling engineThis type of engine offers significantly greater efficiency than gasoline or diesel engines and is also environmentally friendly.
In this article, we will explore the main features of the Stirling engine, how it works, and what its main advantages and disadvantages are. We will also delve into some of the most common uses of this engine, which can play a crucial role in the energy transition.
The Stirling engine
The Stirling engine is a type of external combustion engine that uses the pressure generated by a gas that is heated and cooled, rather than internal combustion where fuels are burned. It is an invention that dates back to 1816, when it was designed and patented by the Scottish reverend Robert StirlingIntended as a safer alternative to the steam engine that dominated at the time, the Stirling offered greater efficiency and safety thanks to its simple construction.
Although it had difficulties in establishing itself in large-scale applications due to technical and economic limitations, it remains very relevant in certain sectors, mainly due to its quiet operation and its ability to generate energy from a variety of sources, including renewable energy.
Currently, its use is focused on submarines and the electricity generation, especially in solar plants, where Stirling engines harness the sun’s heat to efficiently generate power. In the next sections, we will explore more details about their operation and applications.
Stirling engine operation
The Stirling engine follows a thermodynamic cycle known as Stirling cycle, which consists of four basic phases: heating, expansion, cooling and compression. Unlike internal combustion engines, in the Stirling engine, the gas remains sealed within the system, meaning that no polluting emissions, such as carbon dioxide or toxic gases, are released. This makes it an attractive option from an ecological point of view.
Key principles of operation: The Stirling cycle is based on two fundamental principles:
- The pressure inside a gas increases when its temperature rises in a closed volume.
- The compression of a gas in a constant volume also raises its temperature.
These principles are applied by means of two chambers, one hot and one cold, which contain the working gas (which can be helium, hydrogen, nitrogen or even air). The gas moves between the two chambers, causing variations in the pressure that drive the pistons.
The process starts by heating the gas in a hot cylinder. As it heats up, the gas expands and pushes the piston down. The hot gas is then transferred to a cold chamber, where it cools and its pressure decreases, allowing it to be compressed again. This creates the cyclical motion of the pistons and converts thermal energy into useful mechanical energy to generate electricity or power a vehicle.
Stirling engine parts
The Stirling engine is made up of several key parts that work together to perform the energy conversion cycle:
- The displacement cylinder: Responsible for moving the gas between the hot and cold source.
- The power piston: Converter of thermal energy of gas into mechanical work that can be used to move a machine.
- The regenerator: A heat exchanger that absorbs heat from the gas when it is in the cooling phase and returns it when the gas is heated again. This helps to improve the efficiency of the cycle.
- The burner or heat source: Provides the thermal energy needed to heat the gas.
- Flywheel: It acts as a stabilizer that maintains a uniform rotary motion, essential for the continuous operation of the engine.
- Crankshaft: Converter of linear motion of the piston into a rotary motion.
In the next section, we will detail each phase of the engine operating cycle to understand how these parts work together to generate mechanical work.
Stages of the Stirling engine cycle
The Stirling engine operating cycle consists of the following four phases:
- Hot gas expansion: Heat is applied to a portion of the gas in the hot cylinder, causing it to expand. The hot gas pushes the piston downward, doing mechanical work.
- Transfer to cold cylinder: The hot gas is transferred to the cold cylinder, where it is rapidly cooled.
- Cold gas compression: The cold gas is compressed as the piston rises, removing some of the heat it has gained during the expansion phase.
- Return to the hot cylinder: The compressed gas returns to the hot cylinder where the process is repeated.
Advantages of the Stirling engine
The Stirling engine has numerous advantages over internal combustion engines:
- Quiet operation: Since there is no internal combustion, the Stirling engine operates extremely quietly, making it ideal for noise-sensitive applications such as submarines or electric generators in urban areas.
- High efficiency: Its ability to harness multiple external heat sources and its efficient design allow it to achieve performances close to the Carnot cycle. In applications such as cogeneration, this efficiency is very valuable.
- Flexibility in heat sources: The Stirling engine can run on a variety of heat sources, from fossil fuels to completely clean energy sources such as solar energy.
- Low environmental impact: The fact that the gas is sealed means that it does not emit polluting gases, making it a very environmentally friendly option.
- Reduced maintenance: Thanks to its simple design and the absence of internal explosions, the Stirling engine requires relatively little maintenance compared to traditional internal combustion technologies.
- Long useful life: The robust design and simplicity of the system allow Stirling engines to have a longer service life, giving them great value in long-term applications.
- Versatility: From submarines to solar power generators to cogeneration systems, the Stirling engine has a wide range of applications, making it a useful tool for multiple industries.
Disadvantages of the Stirling engine
Despite its many advantages, the Stirling engine also has some disadvantages that have held back its mass adoption:
- High initial cost: Building a Stirling engine, with its heat exchangers and specific materials to withstand pressure and temperature, is expensive, which limits its competitiveness compared to other technologies.
- Lack of popularity: Although its features are impressive, the lack of general knowledge about the Stirling engine has been an obstacle to its large-scale adoption.
- Sealing problems: Preserving working gas can be complicated, especially in engines that operate at high pressures, which affects their performance and durability.
- Size and weight: Stirling engines are typically larger than internal combustion engines of similar power due to the need for larger heat exchangers.
- Limited response time: Although efficient for constant power applications, the Stirling engine is not suitable for systems requiring rapid changes in power, such as vehicles.
Stirling Engine Applications
The Stirling engine has found applications in several key areas. Among the most notable are:
- Solar power generation: In sunny areas, the Stirling engine can use concentrated solar energy to generate electricity with high levels of efficiency. Experimental plants have shown that this technology can be highly competitive compared to other renewable energy sources.
- Underwater propulsion: Because of its quiet operation and the absence of air for combustion, the Stirling engine has been used in submarines for long underwater missions.
- Water pumping: In rural areas, where lack of electricity can be a problem, the Stirling engine has been used for water pumping thanks to its ability to run on biomass or crop residues as a heat source.
- Industrial applications: The Stirling engine is also being tested in industrial applications as auxiliary power generators in industrial plants that can take advantage of waste heat from industrial processes.
- Refrigeration: By reversing the thermodynamic cycle, Stirling engines can be used for cryogenic refrigeration, reaching very low temperatures.
The Stirling engine stands out for its versatility and high efficiency in specialized applications that require constant operation and low emissions. Despite its limitations, it is a technology with great potential for the future, especially with regard to reducing the carbon footprint and integrating with renewable energy sources.