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Stirling engine
Stirling engine was invented by Robert Stirling in 1816. It works on the principle of the Stirling thermodynamic cycle. It became popular because of its ability to use air as a working fluid. With the advancement of power engine technology and the invention of the Otto cycle, it lost its significance. Stirling engine is a closed cylindric piston system which uses an inert working fluid – mostly helium or hydrogen. It operates on the closed thermodynamic cycle principle – temperature difference is converted into mechanical and/or electric energy.
Advantage of Stirling engine is the possibility to use all fuels including fossil fuels, biomass, solar, geothermal and nuclear energy. When fossil fuels and biomass are used, the heater with constant combustion avoids temperature jumps which results in very low controllable emissions. Stirling engine uses heat in a process similar to steam turbine. Due to its small capacity ranging from 1 to 25 kW, it is used in households as microgeneration. Although it is of very small dimensions, electric efficiency can exceed 30%. Stirling engine with a good heat utilization technology as cogeneration can achieve 98% efficiency because it uses total thermal energy (water or space heating). In the UK it has been put in commercial use only recently while the USA uses it in the space industry and the navy.
Its advantages are low levels of emissions, noise and vibration, high reliability and easy maintenance, relatively small number of simple movable parts, possibility of using different fuels and a long working life. Disadvantages include a high price (they are still rarely used) and low efficiency.
Reciprocating Engines
Gas reciprocating, or internal combustion, engines are nowadays the most sold and the cheapest distributed electricity generation technology. Although they are most widely used in car industry, they are also used as a drive unit for air compressors, pumps and electric generators (as generating units in facilities).
Reciprocating engines require fuel, air, compression and a combustion source in order to function properly. Depending on ignition source, they are divided into two groups: spark ignition, the Otto cycle (petrol or natural gas used as fuel) and compression ignition, the Diesel cycle (diesel petrol used as fuel).
Commercially used reciprocating engines which consume natural gas range from 0.5 kW to 10 MW with 37 to 40% efficiency. If properly used, these robust engines can use methane or biofuel. With a heat exchanger which gathers and releases exhaust heat, reciprocating engines can also be used as cogeneration in order to achieve 80% energy efficiency.
Reciprocating engines have numerous advantages in comparison with other low power technologies, which include a possibility of reliable and not too expensive auxiliary supply source, electricity supply of remote locations (as a distributed source), electricity generation during peak hours when the price of electricity is the highest and cogeneration for electricity supply of industrial facilities and housing units. If biogas is used, advantages include a low level of greenhouse gases emissions. Otherwise their level of nitrogen oxide is sky high. Disadvantages include high maintenance costs due to numerous movable parts, and loud operation especially if cooled by ventilators.
Reciprocating engines with used thermal energy represent the most widely spread cogeneration in the small power unit market.
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