It is important to realize that heliostats and sun-trackers are different types of machine. A sun-tracker always aims at the sun. A heliostat always reflects light in a constant direction. The mirror of a heliostat therefore aims along the bisector of the angle between the directions of the sun and the receiver.
Automatic sun-trackers are fairly easy to design and build. The sun appears to revolve around the earth at a speed of 15 degrees per hour. The tracking machine must therefore turn at the same speed to keep aiming at the sun. This is simple to achieve.
Automatic heliostats are more difficult. Most of the ones in practical use have computers that figure out how the mirror should be aimed. Purely mechanical designs, and ones that use light sensors to determine where the sun is in the sky, are possible, but are considerably more complicated than simple sun-trackers.
In a third-world situation, the only type of heliostat that is reasonably practicable is operated manually. This was done in ancient Egypt, where servants or slaves turned mirrors so as to keep reflecting sunlight into the interiors of buildings. It is still done in a few places in Egypt, for the benefit of tourists. Sun-trackers, of course, can also be operated manually. In places where manpower is cheap and everything else is expensive, manual operation is best.
David Williams
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