Solar Energy Perspectives - IEA

Chapter 7: Solar heat
Figure 7.10 Blocking, shading and cosine losses in heliostat fields
Incident
solar flux
Blocking loss
Heliostat
Cosine loss
Shadowing loss
Source: Stine and Geyer, 2011.
Key point
Heliostats must be distanced from each other to minimise blocking and shading.
Field design optimisation also depends on the desired power. Typical heliostat fields in low
latitudes tend to be circular and surround the central receiver, while in higher latitudes they
tend to be semi-circular, to reduce cosine losses. Small fields will be more concentrated to
the polar side of the tower, larger ones more circular, as atmospheric attenuation reduces the
efficiency of far-field heliostats.
While linear systems require flat land areas, central receiver systems may accommodate
some slope, or even benefit from it as it could reduce blocking and shadowing, and allow
increasing heliostat density.
There are two basic receiver designs: external and cavity. External receivers offer vertical pipes
to the concentrated solar flux from the heliostats, in which a heat transfer or working fluid
circulates. In case of direct steam generation, different heliostats might be pointed to two or
three different stages where the water is pre-heated, then vaporised, and if required the steam
superheated. In the cavity design, the solar flux enters the cavity, possibly closed by a window.
“Beam-down” designs use a secondary reflector on top of the tower, of hyperboloid shape,
which redirects the concentrated solar flux to ground level, where it might be refocused by
a secondary compound parabolic concentrator before it falls on the receiver (Photo 7.8). This
design, conceived and first assembled at the Weizmann Institute in Israel, reduces optical
efficiency but conveniently allows keeping the receiver at ground level.
Heat transfer or working fluids, or reactants, are specific to the applications and are described
in Chapters 5, 8 and 9.
Recently, scientists at the Massachusetts Institute of Technology have suggested a simpler
concept called CSPond – provided hilly landscape. Light from a hillside of mirrors goes
through a small open window in a small insulated building and is volumetrically absorbed
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© OECD/IEA, 2011