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#
apli
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)
