Environmental Engineering Reference
In-Depth Information
Membrane heliostats. To avoid or reduce manufacturing and assembly efforts
associated with individual facets and at the same time obtain high optical quality,
so-called "stretched membrane" heliostats have been developed. The reflecting
surface consists of a "drum", which in turn is composed of a metallic pressure ring
with stressed membranes attached to the front and rear side. For this purpose plas-
tic foils or metal membranes are used. In case of metal membranes, characterised
by a considerably longer technical lifetime, the front side membrane is covered
with thin glass mirrors to achieve the desired reflectivity. Inside the concentrator,
a slight vacuum (only a few millibars) is created either by a vacuum blower or a
vacuum pump. By this measure the membrane shape is altered so that the even
mirror is transformed into a concentrator. Other designs use a central mechanical
or hydraulic stamp to deform the membrane. Both configurations are advanta-
geous since the focal length can easily be set and may even be altered during op-
eration. Disadvantageous is the impact of wind on the optical quality of the helio-
stat and, in the case of using a vacuum blower, the energy consumption of the
blower.
Fig. 5.6 shows the example of such a metal membrane heliostat equipped with
a simple tubular steel space framework moving with six wheels on a ring founda-
tion for vertical rotation. Two bearings form the horizontal axis. For this type of
tracking, forces are introduced into the stable pressure ring far from the rotation
axis (approximately 7 m for the illustrated heliostat). The reduced drive torque
keeps the gear units small and inexpensive. The concentrator diameter of the
heliostat shown (ASM 150) with a mirror surface of 150 m 2 amounts to 14 m. The
concentrator thickness is 750 mm and its weight excluding the foundation is ap-
proximately 7.5 t.
Heliostat fields and tower. The layout of a heliostat field is determined by tech-
nical and economic optimisation. The heliostats located closest to the tower pre-
sent the lowest shading, while the heliostats placed north on the northern hemi-
sphere (or south on the southern hemisphere) show the lowest cosine losses.
Heliostats placed far off the tower, by contrast, require highly precise tracking
and, depending on the geographic location, have to be placed farer from the
neighbouring heliostats. The cost of the land, the tracking and the orientation pre-
cision thus determine the economic size of the field.
The height of the tower, on which the receiver is mounted, is also determined
by technical and economic optimisation. Higher towers are generally more fa-
vourable, since bigger and denser heliostat fields presenting lower shading losses
may be applied. However, this advantage is counteracted by the high requirements
in terms of tracking precision placed on the individual heliostats, tower and piping
costs as well as pumping and heat losses. Common towers have a height of 80 to
100 m. Lattice as well as concrete towers are applied.
The costs for piping or the technical challenge of a thermal engine mounted on
top of the tower can be avoided by a secondary reflector installed on the tower
top, which directs incident radiation to a receiver located at the bottom (beam-
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