The Solar Dish Stirling engine: function and basic ...



The Solar Dish Stirling engine: function and basic thermodynamics of the cycle

Introduction:

The Stirling engine was invented by Robert Stirling in 1817. The principle behind the Stirling engine is that it holds a fixed volume of gas which is sealed inside, where pressure changes are caused by the series of actions, results it to do the work.

The Stirling engine’s potential for high efficiency and ability to operate on number of fuel sources has attracted the attention of engine makers for over the years. In 1986, according to a NASA report,[1] they have made an engine according to Stirling cycle principles using pressured Hydrogen gas as the working gas, could reach to a thermal efficiency of 38.5% which is higher that the spark ignition of internal combustion engine. Also it burned fuel with cleaner emissions and produced less noise during operation.

How ever there were some technical problems as it requires bulky components and specialized seals to maintain high pressure requirements of some components.

Unfortunately the automotive industry did not attract to this invention as Stirling engine has few drawbacks compare to ICE. Slow responsiveness is one major disadvantage where Striling engines has slow responsiveness to changes in power requirements and also take longer time to warm up before it reaches its full power.

Although in that time automobile market had little concerns over the harmful emissions and use of fossil fuel, currently world is more concerned over the conservation and alternative sources of energy.

So the Stirling engine can be an option for us to move away from fossil fuels as this engine can use any source of heat to run even bio mass and Solar.

What is a Stirling Engine?

The Striling engine is a heat engine, where it works on the foundation of externally applied temperature difference. The Engine can run and produce mechanical power, by maintaining hot and cold temperature difference in the engine. It is a closed cycle, which means that the working gas is sealed inside the engine. Not like ICE’s the stirling engines does not need engine valves and timing mechanisms as it does not have process of drawing gas from out side or gas expelled as exhaust. Further, the Stirling engine doesn’t depend on the type of fuel used. Anything which can use as a heat source (e.g.: Solar Energy, Bio mass burning) can be used.

The Stirling cycle

The Stirling cycle is the thermodynamic cycle that Stirling engine operates. The working gas that Stirling engine uses is hydrogen or helium. The all the processes of Striling cycle are assumed as reversible. Therefore when gas heated the engine produce work and then when work is supplied to the cycle it works as the refrigerator.

The Processes of Ideal Stirling cycle:

The ideal Stirling cycle comprises of two iso thermal processes and two constant volume processes. Here following p-v diagram describes the processes of Ideal Stirling cycle

(Graphic source: ohio.edu)

1) Isothermal expansion process 1-2: Piston moves upwards, pressure of the gas reduces and volume increases. Work produced by the cycle.

2) Isochoric heat removal process 2-3: heat within the gas is transferred to the generator Temperature of the gas reduces.

3) Iso thermal compression process 3-4 : piston moves down inside cylinder and compresses the gas inside at constant temperature

4) Isochoric heat addition process 4-1: compressed gas moves toward the generator and absorbs the heat

Likewise the gas expands again iso thermally producing work as in process 1-2 and in this way cycle repeats.

[pic]

(Graphic source: )

Solar dish Stirling engine:

Solar dish Striling engine transfer solar heat/ thermal energy in solar radiation to mechanical energy.

The figure 02 shows the components of a solar dish engine which uses to generate electricity:

The dish engine system uses a mirror array to reflect and concentrates sunlight to a receiver. The concentrated solar radiation is absorbed by the receiver and transferred to the engine

(Figure 02 /Graphic source: )

When Striling cycle, used in the solar dish engine, uses hydrogen or helium as working gas and temperature over 700˚C and pressure as high as 20MPa

The working gas is alternatively heated and cooled by iso thermal and iso choric processes.

The Solar Dish engine systems are known for high efficiency, modularity and autonomous operation , hybrid capability. These systems have the potential to be on of the least expensive sp

Dish/engine systems are characterized by high efficiency, modularity, autonomous operation, and an inherent hybrid capability (the ability to operate on either solar energy or a fossil fuel, or both). Of all solar technologies, dish/engine systems have demonstrated the highest solar-to-electric conversion efficiency (29.4%)[1], and therefore have the potential to become one of the least expensive sources of renewable energy. The modularity of dish/engine systems allows them to be deployed individually for remote applications, or grouped together for small-grid (village power) or end-of-line utility applications. Dish/engine systems can also be hybridized with a fossil fuel to provide dispatchable power. This technology is in the engineering development stage and technical challenges remain concerning the solar components and the commercial availability of a solarizable engine. The following describes the components of dish/engine systems, history, and current activities.

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