Sterling Engines

The Sterling engine uses a closed system and makes no use of steam, explosives or fire. The engine being compatible with renewable sources of energy, is invoking interest worldwide, amidst the fuel crisis faced today. The fundamental concept behind the working of the Sterling engine, is to convert heat energy into mechanical energy. The main components of a Sterling Engine are the regenerator, heat exchanger and the heat source.

• Regenerator: The regenerator is also known as the regenerative heat exchanger. It carries out internal heat exchange in the Sterling engine. In the process of heat exchange, the fluid alternately passes from the regenerator in opposite directions. The regenerator does the work of not allowing the heat to escape out of the system. This increases the thermal efficiency of the engine to a great extent; which in turn results in greater engine output as compared to the condition in which the heat is lost to the environment.
• Heat Exchanger: Its main function is to transfer heat from one medium to another. A bore tube can be used as a heat exchanger.
• Heat Source and Sink: The sources of alternative fuels used for a Sterling engine may range from solar energy, biological energy, geothermal energy, waste heat to nuclear energy. It also runs on heat produced by combustible fuels like siloxane and landfill gas. The sink or cold sink is to lower the temperature. Water at room temperature or flowing air is used for cooling.

Working of the Sterling Engine
The Sterling engine either uses helium, air or hydrogen as its 'working fluid'. The fluid is contained in the engine and is sealed. The cycle through which the sterling engine processes energy includes cooling, compression, heating and expansion. The hot and cold heat exchangers are in contact with a heat source and a cold sink respectively.

When the gas is heated, the expansion in its volume results in the rise in pressure inside the chamber. It pushes the piston, which results in a stroke. Further, when the gas cools down, the pressure in the chamber lowers. This requires the return stroke of the piston to do less work. This 'less work' translates into a net gain in power.

The Sterling engine thus, makes use of expansion and contraction of the enclosed gas to produce mechanical energy.

Advantages of Sterling Engines

• Since, a continuous process of fuel combustion is used in the Sterling engine, it produces very less emissions.
• The design and working of the Sterling is a simple one. Thus, there is no requirement of valves.
• These engines don't require any kind of air supply and also, they run without making any noise.

Disadvantages of Sterling Engine

• Heat exchangers are used in Sterling engines. These components need to bear the pressure generated by 'working fluids' and also the corrosion caused by heat. Thus, assembling efficient heat exchangers raises the cost of the sterling engine by 40%.
• The big size of radiators used in sterling engines makes it problematic in packaging and also increases the cost.

Applications
The Sterling engines are used in various applications enlisted below.

• Generating Power from Solar Energy: The Sterling engine is more efficient in producing power from solar energy than the photovoltaic cells.
• Submarines: The Sterling engine has been used in operating submarines. A German company, 'Kockums' based in Sweden, has built 8 submarines which run on Sterling engines.
• Nuclear Power Generation: Electricity can be generated in nuclear power plants by using Sterling engines. A Sterling engine is a better option than steam engines that are currently used in nuclear power stations.
• Aircraft: Sterling engines have the potential to be used in aircraft, provided that its 'power density' is enhanced and the cost is reduced.

Robert Sterling patented the Sterling engine in 1816. The intention behind creating this engine was to reduce fuel consumption. It was also safer than the steam engines used in those times. With the invention of electric motor in the 20th century, the popularity of the Sterling engines reduced. Its use was further limited only in making ventilators and toys. The Philips company took interest in the Sterling engine and worked on improving the engine. They created different types of engines using the fundamental principles of the Sterling engine and achieved commercial success by the production of the 'reverse Sterling engine cryocooler'.

The expansion work carried out by the Philips company in the area of Sterling engines led to many modern inventions. The simple yet neglected technology of the Sterling engines was, in a way, revived by Philips. The Sterling engine, a simple yet effective form of powering many different kinds of machines, has proved to be useful over the years.