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Stirling engine
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== Comparison with internal combustion engines == {{procon|date=May 2021}} In contrast to internal combustion engines, Stirling engines have the potential to use [[renewable heat]] sources more easily, and to be quieter and more reliable with lower maintenance. They are preferred for applications that value these unique advantages, particularly if the cost per unit energy generated is more important than the capital cost per unit power. On this basis, Stirling engines are cost-competitive up to about 100 kW.<ref name="WADE" /> Compared to an [[internal combustion engine]] of the same power rating, Stirling engines currently have a higher [[capital cost]] and are usually larger and heavier. However, they are more efficient than most internal combustion engines.<ref name="Krupp-57" /> Their lower maintenance requirements make the overall ''energy'' cost comparable. The [[thermal efficiency]] is also comparable (for small engines), ranging from 15% to 30%.<ref name="WADE" /> For applications such as [[micro-CHP]], a Stirling engine is often preferable to an internal combustion engine. Other applications include [[water pump]]ing, [[astronautics]], and electrical generation from plentiful energy sources that are incompatible with the internal combustion engine, such as solar energy, and [[biomass]] such as [[Zero waste agriculture|agricultural waste]] and other [[waste]] such as domestic refuse. However, Stirling engines are generally not price-competitive as an automobile engine, because of high cost per unit power, & low [[power density]].{{citation needed|date=July 2020}} Basic analysis is based on the closed-form Schmidt analysis.<ref name="Herzog-2008" /><ref name="Hirata-1997" /> Advantages of Stirling engines compared to internal combustion engines include: * Stirling engines can run directly on any available heat source, not just one produced by combustion, so they can run on heat from solar, geothermal, biological, nuclear sources or waste heat from industrial processes. * If combustion is used to supply heat, it can be a continuous process, so those emissions associated with the intermittent combustion processes of a reciprocating internal combustion engine can be reduced. * Bearings and seals can be on the cool side of the engine, where they require less lubricant and last longer than equivalents on other reciprocating engine types. * The engine mechanisms are in some ways simpler than other reciprocating engine types. No valves are needed, and the burner system (if any) can be relatively simple. Crude Stirling engines can be made using common household materials.<ref name="Make-2006" /> * A Stirling engine uses a single-phase working fluid that maintains an internal pressure close to the design pressure, and thus for a properly designed system the risk of explosion is low. In comparison, a steam engine uses a two-phase gas/liquid working fluid, so a faulty overpressure relief valve can cause an explosion. * Low operating pressure can be used, allowing the use of lightweight cylinders. * They can be built to run quietly and without an air supply, for [[air-independent propulsion]] use in submarines. * They start easily (albeit slowly, after warmup) and run more efficiently in cold weather, in contrast to the internal combustion, which starts quickly in warm weather, but not in cold weather. * A Stirling engine used for pumping water can be configured so that the water cools the compression space. This increases efficiency when pumping cold water. * They are extremely flexible. They can be used as CHP ([[combined heat and power]]) in the winter and as coolers in summer. * Waste heat is easily harvested (compared to waste heat from an internal combustion engine), making Stirling engines useful for dual-output heat and power systems. * In 1986 NASA built a Stirling automotive engine and installed it in a [[Chevrolet Celebrity]]. Fuel economy was improved 45% and emissions were greatly reduced. Acceleration (power response) was equivalent to the standard internal combustion engine. This engine, designated the Mod II, also nullifies arguments that Stirling engines are heavy, expensive, unreliable, and demonstrate poor performance.<ref name="NASA, Automotive Stirling Engine" /> A catalytic converter, muffler and frequent oil changes are not required.<ref name="NASA, Automotive Stirling Engine" /> Disadvantages of Stirling engines compared to internal combustion engines include: * Stirling engine designs require [[heat exchanger]]s for heat input and for heat output, and these must contain the pressure of the working fluid, where the pressure is proportional to the engine power output. In addition, the expansion-side heat exchanger is often at very high temperature, so the materials must resist the corrosive effects of the heat source, and have low [[creep (deformation)|creep]]. Typically these material requirements substantially increase the cost of the engine. The materials and assembly costs for a high-temperature heat exchanger typically accounts for 40% of the total engine cost.<ref name=Hargreaves /> * All thermodynamic cycles require large temperature differentials for efficient operation. In an external combustion engine, the heater temperature always equals or exceeds the expansion temperature. This means that the metallurgical requirements for the heater material are very demanding. This is similar to a [[Gas turbine]], but is in contrast to an [[Otto engine]] or [[Diesel engine]], where the expansion temperature can far exceed the metallurgical limit of the engine materials, because the input heat source is not conducted through the engine, so engine materials operate closer to the average temperature of the working gas. * The Stirling cycle is not actually achievable; the real cycle in Stirling machines is less efficient than the theoretical Stirling cycle. The efficiency of the Stirling cycle is lower where the ambient temperatures are mild, while it would give its best results in a cool environment, such as northern countries' winters. * Dissipation of waste heat is especially complicated because the coolant temperature is kept as low as possible to maximize thermal efficiency. This increases the size of the radiators, which can make packaging difficult. Along with materials cost, this has been one of the factors limiting the adoption of Stirling engines as automotive prime movers. For other applications such as [[Ship#Propulsion|ship propulsion]] and stationary [[microgeneration]] systems using [[Cogeneration|combined heat and power]] (CHP) high [[power density]] is not required.<ref name="BBC_CHP" />
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