Environmental Engineering Reference
In-Depth Information
Heat
engine
Direct normal
solar irradiance
Parabolic
mirror
Effective
area
Figure 1.12
Principle of a Dish-Stirling System
Another system using mirrors is the solar furnace. The solar furnace in
Odeillo (France) consists of various heliostat mirrors that have been set up on
sloping ground. These heliostats reflect the sunlight onto a concave mirror
with a diameter of 54 m. At the focus of this mirror, temperatures up to
4000°C can be reached and used for scientific experiments or, in a commercial
product, for industrial processes. Further solar furnaces exist in Almería
(Spain) and Cologne (Germany).
So-called Dish-Stirling systems can be used to generate electricity in the
kilowatt range. A parabolic concave mirror (the dish) concentrates sunlight. A
two-axis tracked mirror tracks the sun with the required high degree of
accuracy. This is necessary in order to achieve high efficiencies. The receiver at
the focus is heated to 650°C. The heat absorbed drives a Stirling motor, which
converts the thermal energy into mechanical energy that is used to drive a
generator producing electricity. If sufficient sunlight is not available,
combustion heat from either fossil fuels or bio-fuels can also drive the Stirling
engine and generate electricity. The system efficiency of Dish-Stirling systems
can reach 20 per cent or more. Some Dish-Stirling system prototypes have
been tested successfully in a number of countries; however, the cost of
electricity generation using these systems is much higher than that of trough or
tower power plants. Large-scale production might achieve significant cost
reductions for Dish-Stirling systems. Figure 1.12 shows the principle of a
Dish-Stirling system.
A
solar chimney power plant
has a high chimney (tower), with a height of
up to 1000 metres. This is surrounded by a large collector roof, up to 5000
metres in diameter, that consists of glass or clear plastic supported on a
framework. Towards its centre, the roof curves upwards to join the chimney,
creating a funnel. The sun heats up the ground and the air under the collector
roof, and the hot air follows the upward slope of the roof until it reaches the
chimney. There, it flows at high speed through the chimney and drives wind
generators at the bottom. The ground under the collector roof acts as thermal
storage and can even heat up the air for a significant time after sunset. The
