Beam-down optics for telescopic heliostat arrays

The focal zone of an array of telescopic heliostats scaled to 140 m height—yielding about 750 MWe.

Save for the intervention of the beam-down optics, light from a field of telescopic heliostats would converge onto a focal zone shaped something like an ice cream cone (the orange region in the figure above.)

Taking parameters from an earlier post:

Height to top of the beam-down optics: 140 m

Field radius: 1085 x 1.59 = 1725 m

Beam divergence: 3°

Angular elevation of beam center = 1.6° + 1.5° = 3.1°

[For comparison: the height of the Washington Monument is 169 m (152 m to the base of the pyramidal cap); the distance between the monument and the U.S. Capitol is 1800 m. The Capitol's dome is 88 m in height and 29 m in diameter.]

If it received only light from the outermost ring of heliostats, the focal zone would be a sphere centered at 1725 m x tan(3.1°) = 93 m high, with a radius of 1725 m x tan(3°) / 2 = 45 m. That is the "ice cream" at the top of the cone; superimposing many smaller and lower spheres for each ring of heliostats yields the "cone" itself. 

The beam divergence of a 6x telescopic heliostat (the pink spot) drawn as an area on the globe.

The beam divergence found near the center of an array of telescopic heliostats. Divergence has increased in longitude, but not in latitude.

We will describe the directions light propagates in by reference to an earth globe oriented with north pointing up, and with the prime meridian facing the heliostats we are concerned with. So, for example, close to the heliostat, the light lies within a circle 3 degrees in diameter that is centered on 0 degrees longitude and 3.1 degrees S latitude (the top picture above.) Closer to the focal zone, the light will have the same latitudinal extent, but its longitudinal extent will have increased because the light of many heliostats is being concentrated along that dimension (lower picture above.) The exact longitudinal extent can be calculated by taking a look in the opposite direction and calculating the angular subtense of the cone-shaped focal zone.

At times we need to know the divergence of light where the beam-down optics are located. We can get the same answer by reversing the light rays and considering the focal zone—rather than the heliostats—as the source of light.

Some considerations in designing beam-down optics:

1. Placing beam-down optics too close to the focal cone will make the incident light too widely spread out in the longitudinal direction to be handled efficiently by the optics.
2. We want the surface normal of the beam-down structure to make roughly equal angles with the incoming and outgoing light. This will avoid the same thermodynamic "gotcha" that afflicts conventional heliostat fields that direct high sun to a low target. 
3. We want the incident light to cover the same area on the globe (i.e., subtend the same solid angle) as the opening of the furnace, because that is what we can do most efficiently, reshape, not resize, divergence.