Ten things we know about telescopic heliostats

They're necessary. Ground loss—the quantity of sunlight falling between heliostats—is atrocious in current generation CSP's. Obvious in a satellite view is the fact that far more sunlight is reaching the ground than the mirrors. Land requirements are being tripled! The Second Law of Thermodynamics decrees that a heliostat without ground loss must act like a telescope, i.e., it must increase the divergence of the reflected beam while maintaining collimation (parallel rays map to parallel rays.)

Telescopic heliostats must have two mirrors. No fewer than two lenses (objective and eyepiece) compose a telescope.

Optimum power is about 6X. Magnifying the sun's disk, which is 0.5° in diameter, six times makes an intensified, but still collimated beam (or highbeam) that can be aimed at an elevation angle as small as its own diameter, 3.0°. That condition minimizes the height of the central, beam-down optics or lamp.

Central, beam-down optics are necessary. At 3.0° divergence, the highbeams are simply too spread to form a high quality focus without another optical stage.

The lamp (central beam-down optics)—shaped something like an overturned apple—will be only 60% as tall as a power tower on the same field. Larger heliostat fields are thus made practical.

The objective needs to move, the eyepiece doesn't. The objective (primary) mirror can redirect sunlight vertically to a fixed focus: the much smaller eyepiece (secondary) mirror gets to sit right there.

The objective needs to have its optical profile continuously fine-tuned to the sun's changing zenith distance. To first order, the adaptation needed is simply a thin-shell bending of the mirror.

Telescopic heliostats move in concert. The only real difference between two telescopic heliostats in a field is the azimuth aiming of their eyepieces. All the objectives could be mechanically ganged.

At at 0.70 mirror/land ratio, telescopic heliostats can be sited with negligible blocking. Surprisingly, the presence of the eyepieces adds no complication at all to heliostat siting when a phyllotaxis-based algorithm is used. The packing density achieved is more than three times that of a conventional heliostat field.

These improvements in field size and field packing make it practical to design a replicable 1 GW, 75% capacity-factor, standard solar plant covering one quarter-township (9 square miles) in the southwest U.S. The central lamp, about 190 m high, would be only 20% taller than current-generation power towers. Goodbye coal!