Solar Thermal Power Plants - Renewable Energy: Technology, Economics and Environment - page 180

the efficiency of a turbine; current steam turbines reach efficiencies above 40 %,
while those of gas turbines even exceed 55 %.

5.2 Solar tower power stations

Within solar tower power plants (also called "central receiver systems") mirrors
tracking the course of the sun in two axes, so-called heliostats (Greek term for
"immobile sun"), reflect the direct solar radiation onto a receiver, centrally posi-
tioned on a tower. There, radiation energy is converted into heat and transferred to
a heat transfer medium (e.g. air, liquid salt, water/steam). This heat drives a con-
ventional thermal engine. To ensure constant parameters and a constant flow of
the working medium also at times of varying solar radiation, either a heat storage
can be incorporated into the system or additional firing using e.g. fossil fuels (like
natural gas) or renewable energy (like biofuels) can be used. Such systems are
described in detail below.

5.2.1 Technical description

In the following the technology of solar tower power plants including all related
components are described.

5.2.1.1 System components

Heliostats. Heliostats are reflecting surfaces provided with a two-axis tracking
system which ensures that the incident sunlight is reflected towards a certain tar-
get point throughout the day. In addition, heliostats commonly concentrate
sunlight by means of a curved surface or an appropriate orientation of partial ar-
eas, so that radiation flux density is increased.
Heliostats consist of the reflector surface (e.g. mirrors, mirror facets, other
sunlight-reflecting surfaces), a sun-tracking system provided with drive motors,
foundations and control electronics. The individual heliostat's orientation is com-
monly calculated on the basis of the current position of the sun, the spatial posi-
tion of the heliostats and the target point. The target value is communicated elec-
tronically to the respective drive motors via a communication line. This informa-
tion is updated every few seconds. The concentrator surface size of currently
available heliostats varies between 20 and 150 m 2 ; to date, the largest heliostat
surface amounts to 200 m².
The heliostat field accounts for about half the cost of the solar components of
such a power plant. This is why tremendous efforts have been made to develop
heliostats of good optical quality, high reliability, long technical life and low spe-
cific costs. Due to economic considerations there is a tendency to manufacture