Solar Energy Perspectives - IEA

Chapter 7: Solar heat
Figure 7.8 Scheffler dish for community kitchen
Source: Wolfgang Scheffler, www.Solare-bruecke.org.
Key point
Using a simple clock mechanism, Scheffler dishes concentrate sunrays on a fixed focus.
Solar towers
Solar towers, or central receiver systems, are made of a field of heliostats surrounding
a central receiver atop a built structure (Figure 7.9). Heliostats reflect the sunlight onto the
receivers. Alt-azimuth mounting of heliostats is almost universal in towers. The simplicity of
equatorial mount for dishes is lost with heliostats, because their surface area is not
perpendicular to the pointing direction, but to the bisect of the angle formed by the direction
of the sun and that of the tower.
Heliostats can vary greatly in size, from about 1 m 2 to 160 m 2 . The small ones can be flat
and offer little surface to winds. The larger ones need to be curved to send a focused image
of the sun to the central receiver, and need strong support structures and motors to resist
winds. For similar collected energy ranges, however, small heliostats need to be grouped by
the thousand, multiplying the number of motors and connections, and their orientation
requires much more computing power. As the cost of computing power is rapidly declining,
the trend towards more and smaller heliostats is likely to persist. Heliostats need to be
distanced from each other to reduce shading but also blocking, which takes place when
a heliostat intercepts part of the flux reflected by another (Figure 7.10).
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© OECD/IEA, 2011