Secondary Mirror in Solar Concentrator Dish Designs

[vladpaln]

I have been looking at various solar power dish designs. They have one thing in common, they all have their power generators located at the focal point.

Why don't any of the designs use a secondary mirror located at the focal point and the generator located center of dish and right behind it. The design using secondary mirrors is used in all the large telescopes you find in Chile and Hawaii and seems to work fine.

I see several advantages using the secondary mirror design including full dish utilization, less material required, lighter design, and easier maintenance. The only con I can think of is the secondary mirror will have to be of high quality with >99% reflective surface possibly with some kind of cooling (active/passive).

What am I missing??

Standard Design

Dish with Secondary Mirror

 

[mfb]

Probably more complicated to build - needs a better alignment of everything, needs an additional element that is cooled. Support for the secondary mirror might be a bit lighter, okay.

Large telescopes need an excellent resolution, something you don't get with a single mirror, but that is not a concern for these concentrators.

 

[tech99]

It looks as if the tilt mechanism penetrates the dish and creates a strip of shadow. The collector is located in this shadow and there would be no advantage in the sub reflector design in this case.

 

[Rive]

full dish utilization

The shadow of the secondary mirror is there, just as the opening on the main mirror.

easier maintenance.

Actually, the design on the picture is quite clever. The 'rest' position of the mirror is not only cuts off the heat but the crew will get the generator at convenient height and position for repair/maintenance. Also I have a bet that the support point goes through the center of the structure, so not much power is needed for tracking or positioning. The generator is the counterweight for the mirror.

The only con I can think of is the secondary mirror will have to be of high quality with >99% reflective surface possibly with some kind of cooling (active/passive).

That is also true. Just keep in mind, that the goal is not about the maximal efficiency, but about cheap power.

 

[256bits]

The design using secondary mirrors is used in all the large telescopes

That cuts down the total length of the optical imaging telescope - which means it can be housed in a smaller building as one benefit.

You may be interested in the optics of solar collectors.
They do not need to focus the light to form an image, but only to direct the light to a target of fixed size ( too small a target and it might melt ).
https://en.wikipedia.org/wiki/Nonimaging_optics

The mirrors can be as simple as polished flat sheets or panels of metal aimed at the target, and since being out in the elements, they will get dusty and dirty so a reflectivity something less than 100% is taken into account in the design for the amount of power one can collect. Need more power - just add a few more panels - and it could be cheaper to do it that way than to form perfect parabolic curves for the individual mirrors as seen in the picture. The dish is parabolic, but each panel mirror does not have to be. Cost goes up with each fabrication step from flat panel to 1-d curve to a 2-d curve, but that is something the designers would calculate and make decisions on.

 

[johnbbahm]

The only limitation I can think of would be a concern for air ionization.
As the energy density increases in the ever tightening focused beam, you
might pass the point where the air ionized and becomes less transparent.
This can be observed with lasers or just a few watts, the power observed at just a meter or two
appears to cycle up and down.

 

[mfb]

Lasers can be focused extremely well. You won't have that problem with solar concentrator designs.