The necessity of telescopic heliostats

A telescopic heliostat converts sunlight into an intense—but slightly divergent—beam of light.

A drawback of solar power plants that use mirrors (heliostats) to beam sunlight onto an elevated target is that one heliostat might block a neighboring heliostat's view of the target. Trying to reduce this blocking causes the land utilization of an optimized plant of this type to be low. Since the blocking problem is the worst at the far edges of the heliostat field, for a given target height, the blocking effect also limits the useful radius of the heliostat field—and thus limits the power rating of the entire plant.

The heliostat blocking problem is actually a consequence of the Second Law of Thermodynamics: thermal radiation cannot be concentrated without increasing its divergence. When we try to redirect sunlight from a high-elevation sun toward a low-elevation target using an array of parallel mirrors, we are attempting exactly such an evasion of the Second Law.

Redirecting sunlight toward a low elevation target without a compensating increase in beam divergence is an attempt to evade the Second Law of Thermodynamics—the heliostat blocking effect rises up to trump our attempt.

A telescopic heliostat, by dealing realistically with the need for increased beam divergence can largely eliminate the heliostat blocking problem, and thereby increase by an order of magnitude the power rating of a solar plant having a given target height. About half of this prospective improvement comes from increased utilization of land within the heliostat field, and half from increasing the useable radius of the field. 

Concentrating a sunbeam causes its divergence to increase: that fact is inescapable. Nonetheless, there is room to increase the divergence of natural sunlight. For example, a very narrow-beam flashlight may have a beam divergence (the full angular spread between its half-intensity shoulders) of 5 degrees. That is about ten times the divergence of natural sunlight. So a sunbeam can be concentrated about 100 times without increasing its divergence beyond that of a very narrow-beam flashlight beam.

This narrow-beam flashlight has about ten times the divergence of natural sunlight.  Concentrating a sunbeam to this level of divergence allows a hundred-fold increase in intensity.