-Solar Point Focus

Concentrating solar power is good idea. But concentrating sunlight can be dangerous and is not as easy as it looks. Idealab and eSolar show a solar array and an energy receiver in Figure 1. The receiver collects the reflected sun’s rays. Notice that the mirrors are rectangular. And notice that the receiver cavity has an opening that is also rectangular. It matches the shape of the modular mirrors. In many respects it’s a good design. However, the reflected image of the sun is not rectangular. The reflected image of the sun is round with some asymmetry. And it also gets larger the farther away the image is from the reflected surface. See demonstration video below.

To concentrate the sun to the theoretical limit requires near perfect optics, astronomical optics in fact. It is impossible to concentrate sunlight beyond certain levels no matter how perfect the optics because the reflected image always has a certain minimum size depending on the distance from the point of reflection. The image subtends an arc. The cost of astronomical quality optics is prohibitive because the slightest imperfection in the surface means the rays will not reflect at a perfect angle and may not reach the intended receiver.

At the distances shown in the figure the reflected image of the sun takes on the shape of the sun, not the shape of each facet. The size of the image grows as well. The cavity needs to be large enough to collect all the rays. But the larger cavity opening means greater heat loss from the receiver because the cavity radiates heat. So there needs to be a balance between the quality of the reflectors and the size of the round receiver.