Solar Thermal Power Plants - Renewable Energy: Technology, Economics and Environment - page 181

heliostats with surfaces ranging between 100 and 200 m 2 and possibly beyond.
However, there are also approaches to manufacture smaller heliostats to reduce
costs by efficient mass-production.
Heliostats are usually centrally controlled and centrally supplied with electrical
energy. As an alternative, autonomous heliostats have been developed which are
controlled locally. There, the energy required for the control processor and the
drives is provided by photovoltaic cells mounted parallel to the reflector surface.
The heliostats are individually controlled in order to control the radiation flux
density on the receiver. For this reason not all of the heliostats are focussed on the
same point of the receiver; their control rather ensures a smooth flux distribution
over the entire receiver surface.
Based on the developments of the last few years, faceted glass/metal heliostats
and membrane heliostats are distinguished (Fig. 5.6). These types are described
below.
10
5
0
0 5 10 0 5 10
Width in m Width in m
Fig. 5.6 Faceted glass/metal heliostat (left) and metal membrane heliostat (right) /5-3/
Faceted heliostats. Usually, faceted heliostats consist of a certain number of re-
flecting facets mounted on lattice work which in turn is positioned on a mounting
tube. The facets are commonly designed as individual mirrors of sizes between 2
and 4 m 2 . The orientation of the individual mirrors on top of the mounting struc-
ture (referred to as "canting") is different for every heliostat within the heliostat
field, and thus results very expensive. The heliostats are usually tracking in two
axes positioned vertically to each other (commonly mounting tube and vertical
main axis), according to the desired azimuth and elevation angle. Mostly to reduce
canting efforts and the number of individual drives, currently wide facetted helio-
stats are proposed. The glass/metal heliostat illustrated in Fig. 5.6 (left) as an ex-
ample has a concentrator width of 12.8 m and a height of 8.94 m. The size of the
individual facets is 3 by 1.1 m each. The total weight without foundation amounts
to scarcely 5.1 t /5-3/.