Stirling Heat Engine & the Peltier Device

Stirling heat Engine

A Stirling engine is a heat engine that operates by cyclic compression and expansion of air or other gases at varying temperatures. Every stirling engine has a sealed cylinder with one part hot and the other cold. The working gas inside the engine (usually air, helium, or hydrogen) is moved by a mechanism from the hot side to the cold side. When the gas is on the hot side, it expands and pushes up on a piston. When it moves back to the cold side it contracts. Properly designed Stirling engines have two power pulses per revolution, which allows smooth running. Two of the more common types are two piston Stirling engines and displacer-type Stirling engines.

A displacer type engine has one piston and a displacer. The displacer controls when the gas chamber is heated and when it is cooled. This type of Stirling engine is sometimes used in classroom demonstrations. Below is a diagram of the displace type engine.

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In order to run, the engine requires a temperature difference between the top and bottom of the large cylinder. Above, you can see a smaller piston at the top of the engine. This is a tightly-sealed piston that moves up as the gas inside the engine expands. The displacer is the large piston in the diagram above. This piston is very loose in its cylinder, so air can move easily between the heated and cooled sections of the engine as the piston moves up and down. The engine repeatedly heats and cools the gas, extracting energy from the gas’s expansion and contraction.

Below is a detailed diagram of this Displacer model and how it works.

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Below is a short clip of a common class demonstration of the MM-1 Coffee Cup Stirling engine.

http://www.stirlingengine.com/graphics/videos/mm-1_hot.MPG

 In the Two-piston Stirling engine, the heated cylinder is heated by an external flame. The cooled cylinder is air-cooled, and has find on it to aid the cooling process. A rod stemming from each piston is connected to a small disc, which is connected to a larger flywheel. This keeps the pistons moving when no power is being generated by the engine. Below is a diagram of this two piston engine.

 

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The Peltier Device

Thermoelectric modules are solid-state heat pumps that operate on the “Peltier effect” (the presence of heating or cooling at an electrified junction of two different conductors). A thermoelectric module consists of an array of “p” and “n” type semiconductor elements that have many electrical carriers. The elements are arranged into an array that is electrically connected in series, but thermally connected in parallel. This array is then affixed to two ceramic substrates.

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The “p” type semiconductor is doped with certain atoms that have fewer electrons than necessary to complete the atomic bonds within the crystal lattice. When a voltage is applied, there’s a tendency for conduction electrons to complete the atomic bonds. When conduction electrons do this, they leave “holes” which are atoms with the crystal lattice that now have positive charges. Electrons are then continually dropping in and being bumped out of the hole and moving on to the next available hole. In effect, it’s the holes that are acting as the electrical carriers. Now, electrons move much easier in the copper conductors but not easily in the semiconductors. When electrons leave the p type and enter into the copper on the cold side, holes are created in the p type as the electrons jump out to a higher energy level to match the energy level of the electrons already moving in the copper. The extra energy to create these holes comes by absorbing heat. The newly created holes travel downwards to the copper on the hot side. Electrons from the hot side copper now move into the p type and drop into the holes, releasing the excess energy in the form of heat. The n-type semiconductor does essentially the same thing.

Below is a diagram of this device.

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Thermoelectric modules have no moving parts and do not require the use of chloroflurocarbons. Therefore they are safe for the environment, reliable, and practically maintenance free. They can be operated in any orientation and are ideal for cooling devices that might be sensitive to mechanical vibration. Their compact size also makes them ideal for applications that are size or weight limited. Their ability to heat and cool by a simple reversal of current flow is useful for applications where both heating and cooling is necessary or where precise temperature control is critical.

Thermoelectric coolers are used for some of the most demanding industries such as medical, laboratory, aerospace, industrial, and consumer. Uses for these coolers could be as simple as a food or beverage cooler or for more sophisticated temperature control systems in missiles and space vehicles. A thermoelectric cooler permits lowering the temperature of an object below ambient as well as stabilizing the temperature of objects above ambient temperatures.

Resources:

Home

http://en.wikipedia.org/wiki/Thermoelectric_effect

FAQ

http://auto.howstuffworks.com/stirling-engine3.htm

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