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Energiza Magazine

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     Stirling dish technology in solar thermal power plants


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1. INTRODUCTION

Dish-Stirling systems correspond to one of the principal means for decentralized energy production of electricity from solar energy. Consist of a highly reflective solar concentrator, a solar receiver cavity and a Stirling engine.


Stirling engine

Thermodynamic engines run on heat cycles. They are divided into endothermic (internal combustion) and exothermic (external combustion). Among external combustion engines, Stirling engine highlights.


Stirling engine was invented by Scottish Robert Stirling in 1816, believing that steam engine was too complicated and dangerous. This engine runs on a gas expansion and contraction, usually helium, hydrogen, nitrogen or air. This gas is transported through a cooling cycle in a cold focus, it is contracted, and then, in a hot focus, it is expanded. Thus a thermodynamic cycle takes place.


Within the cycle, it highlights the importance of a regenerator, which is based on absorbing and transferring heat at constant volume changes of the cycle. The engine can work without the regenerator, but it allows higher efficiencies.


Figure. 1: “Stirling model” Source:

http://www.ecotec2000.de/espanol/stir2.htm#

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Dish-Stirling System

It has been demonstrated thermal power plants efficiency with Dish-Stirling systems because these installations are able to reach a maximum of 30% and up to 25% of daily average in power of 7 to 25 kW.
One of the main elements of the system is the concentrator that can reach values over 3,000, due to its parabolic curvature and the low ratio focal length / diameter (f / D = 0.6,). Thus achieving very high operating temperatures between 650 and 800 º C. Concentrator’s concave surface consists of glass mirrors as a second surface, with its corresponding parabolic shape. Also, as a first surface, it consists of thin mirrors supported on a fiberglass or stretched membrane structure.

Another element is the receiver, which consists of a cavity with a small opening and an insulation system.

For heat transference from solar radiation to the working gas, two methods are used.

In the first, is to reflect and to concentrate on the absorbent tubes the direct solar radiation that it incident on the area. Inside the tubes circulates the gas (helium, hydrogen or air).

In the second, a liquid metal is vaporized (usually sodium), which then condenses on the surface of the tubes, through them the working gas flows, and flows back to the absorber.


Figure. 2: Dish Stirling operation diagram.

Source: www.abengoasolar.com

Evolution

In U.S.A and Europe, solar thermal power plants with dish-Stirling systems have progressed considerably.

Dishes first generation consisted of glass configurations that could reach high concentrations (C = 3000), but them supposed high cost in addition to heavy structures.

Germany achieved great progress with companies such as Steinmüller and Schlaich, Bergermann und Partner (SBP), and SOLO company as a supplier of solar Stirling engine. In Spain we should mention the Solar Platform in Almeria with more than 30,000 hours of operation and an availability of 90%. Its engine runs on helium at 630 º C and provides an efficiency of 20%.

The European project EURODISH built solar thermal plants tested in Vellore (India), Milan (Italy) and in the solar platform of Almeria (Spain). During 2002-2005, German project ENIVIRODISH began its market activity.

Currently, in Europe, the largest investments are carrying out in Spain and France.

Spain has begun with the second generation of dishes. The first plant will be located in Villarobledo, in Albacete. It will have a capacity of 71 MW.

NASA uses internal combustion engines for years as a system of power generation and refrigeration.



Figure. 3: Eurodish Project picture, high concentration collector with Stirling engine.

Source: CIEMAT- Solar platform in Almeria

There are several advantages of using a Stirling engine: it does not need water to produce electricity, as well as being non-friction engine it requires no lubrication, it is quiet and can be connected to a stages power supply.

A 50 MW plant can be built in less than a year. Like all systems of power generation from renewable energy, initial investment cost is high, but operating costs are very low.




Figure. 4 Cost comparison on electricity generation

Source: http://www.sbp.de/es/html/solar/dish-stirling.html







Bibliography

http://es.wikipedia.org/wiki/Motor_Stirling

Jesús Andrés Álvarez Flores; Ismael Callejón Agramunt; Sergi Forns Farrús; others.

“Motor thermal machines-1” First edition; upc 2002 editions; Polytechnic university of Catalonia; pages 391-436.

http://www.sbp.de/es/html/solar/dish-stirling.html

Article “SOLAR THERMAL ENERGY” Dr. Manuel Romero Álvarez Solar Platform in Almeria-CIEMAT. Section 22; 04200 TABERNAS (Almeria)




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