Solar Energy Perspectives - IEA

Solar Energy Perspectives: Solar heat
Parabolic dishes
Parabolic dishes concentrate the sun’s rays at a focal point above the centre of the dish. The
entire apparatus tracks the sun, with the dish and receiver moving in tandem. Parabolic
dishes offer the best optical efficiency, as they entail no cosine losses.
Different systems can be used to track the sun on two axes. One is the equatorial mount, with
which an axis is set parallel to that of the earth: a continuous rotation at a constant rate during
the day compensates for the earth’s rotation, while discrete adjustments follow the changes
in the sun’s elevation over the seasons. In small devices, the adjustment can be made
manually, so operation can be quite simple.
A more common double-axis system is the alt-azimuth mount, which is based on a horizontal
rotation and direct command of the elevation, with the two axes perpendicular to each other.
Alt-azimuth mounts are usually preferred in large installations with many dishes, for their
precision, mechanical simplicity and robustness. They do, however, require variable speed
motions on both axes to track the sun.
A few large experimental parabolic dishes have been built in Australia. But usually individual
dishes are relatively small and assembled in large numbers, so the heat received at each focus
point can be collected and gathered. Although this scheme has been used in one industrial
installation in the United States, it has now been abandoned, as the limitations in piping
material, transport fluids, and joint technology seem to preclude transfer of heat at the high
temperatures that such concentration levels provide. Today, almost all parabolic dishes are
designed as independent electricity generators.
Scheffler dishes
Scheffler dishes are made of a light flexible steel frame with many small pieces of mirror.
They are formed into a flexible parabola focusing the sun’s rays on a fixed receiver. They
rotate on an axis parallel to that of the earth, making an angle with the horizontal equal
to the latitude. This allows tracking the sun during one day with a single rotational
mechanism. A simple clock mechanism can be used, and the drive requires minimal
power. Adjustment of the direction of the parabola, to follow the sun’s height across the
seasons, is made manually every few days. A continuous deformation of the surface of
the parabola over the year allows the sun’s rays to be effectively concentrated on the fixed
target. The surface deformation takes place automatically as the direction of the parabola
is adjusted at sunset to direct the concentrated reflective beams on the receiver
(Figure 7.8).
Output obviously depends on the solar resource. Under Indian skies, where most Scheffler
dishes have been locally built and installed at a cost of USD 1 450 to USD 2 900 (EUR 1 000
to EUR 2 000), a ten-square metre dish provides about 22 kWh per day.
A further development combines two Scheffler dishes, one “standing” and the other
“sleeping” to target one receiver. This allows keeping the total effective aperture constant
throughout the year. This approach is most often used to produce steam collected by
pipes (as on Photo 7.7) and it could also be used for solar ovens with secondary reflectors.
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