# CPC and Fresnel Lenses for Solar Cell Power

1. Oct 2, 2011

### Yachtsman

I want to maximize solar cell efficiency regardless of the sun's angle and so I've been looking at placing Compound Parabolic Concentrator (CPC) over the top of a high efficiency round solar cell. If I come up with a good design then I would like to try and manufacture some of them to see if I could start a small business for myself. I have 3 questions that I am working on, I will appreciate any advice.

1. Will it be most efficient to use CPCs placed over the top of highly-efficient round solar cells? Or will it add efficiency to place a Fresnel lens over the top of each CPC? or perhaps using a "DTIRC" (Dielectric Total Internal Reflection Concentrator) rather than a CPC?

I could design the Fresnel lens to be a square that will refract all light into the round CPC directly below it so that I gain extra light, but I don't know if I will lose efficiency in other ways; e.g. it captures less sun angle than the CPC alone.

2. If I use a Fresnel lens can I design the prisms to filter out the IR frequency to reduce the heat of the solar cell? I've read that the cells become less efficient with heat.

3. What are the most efficient solar cells right now, especially the round shapped cells?

A fourth question is regarding books on this subject; can you tell me any recommended books that would help me with the mathematics of the CPC and Fresnel designs; and also to learn about high efficiency designs in general. Thanks for your help.

Last edited: Oct 2, 2011
2. Oct 4, 2011

### Born2bwire

There is a lot of research in this area at the moment. There are a number of factors that need to be considered. For example, sending the light in at normal incidence to the solar cell may not be very efficient. This is because the path length of the transmitted light is small since it will pass in and out via the shortest distance. It is more desirable to trap the light as a travelling wave inside the cell so that the light wave travels large distances, allowing a long absorption. This is more easily obtained by sending the light in at an angle. Another problem is that the index of refraction of the solar cell is different from the air which causes some of the light to be reflected. Allowing an impedance transformer between the air/cell interface helps increase the transmission into the cell (at the same time it unfortunately helps facillitate the transmission out of the cell).

So an example of some of the ideas is to use a gathering lens in the front with a Bragg diffractor on the cell backing. The Bragg diffractor will scatter the light inside the cell at large angles so that when it reflects it does so in a travelling (or more a leaky) wave mode. Light gathering elements on the front of the cell can be far more sophisticated than a simple external lens. There are ideas of having a spherical lens on the surface and there are a lot of research in using nanoparticles as well. I would suggest doing an extensive literature research into what is currently being done on the subject.

3. Oct 5, 2011

### Yachtsman

Given what you have said the first thing that comes to mind is to stack the solar cells at an angle, providing enough gap in between each cell to insert an impedence transformer to refract the light onto them at an optimal angle, or better yet, by stacking them vertically in sandwiched pairs with a shared aluminum backing, aligning them into parallel banks and providing an optimal light gap between each of the banks to facilitate the impedence transformer.

This kind of impedence transformer design allows light to bounce back and forth between the Left facing half of bank 1 with the Right facing half of Bank 2. Perhaps it may be possible to maximize absorption and minimize leakage by using the "Illumination Problem" proposed by Ernst Straus in the 1950's (see http://mathworld.wolfram.com/IlluminationProblem.html and http://numb3rs.wolfram.com/401/).

Here the impedance transformer now becomes a refraction chamber designed with either right triangles or ellipticals designed to maximize refraction and absorption, while minimizing leakage.

Please let me know if my comments spark any further ideas. Thanks.

Last edited by a moderator: Apr 26, 2017
4. Oct 9, 2011

### Isaacsname

Are you thinking of using something along the lines of what Solyndra manufactures, but with Fresnel lenses integrated into the system ? Just out of curiousity, are you familiar with electrowetting ?

5. Oct 9, 2011

### Yachtsman

Solyndra is very interesting, but I can't tell if how well they are truly bouncing light around in their glass tube to harvest all of the available light; they are very expensive--I'm hoping to provide a more reasonably priced solution.

I would rather use cheaper refraction material to focus a larger concentration of light onto the solar cells; and I'd like to use designs that maximize on harvesting the full spectrum of light.

I am also curious if work has been done to capture frequencies at ultra violet and greater. At what frequency does glass or other typical refraction materials stop refracting?

Thanks for asking about electrowetting; i will not rule it out, but I do think that concentrating large amounts of light is key. It's the cheapest way to turbo charge the solar cell I think.

6. Oct 9, 2011

### Bobbywhy

This material was studied and written up about forty years ago. It may applicable to your search:

Nonimaging optics (also called anidolic optics) is the branch of optics concerned with the optimal transfer of light radiation between a source and a target. Unlike traditional imaging optics, the techniques involved do not attempt to form an image of the source; instead an optimized optical system for optical radiative transfer from a source to a target is desired.

See: http://en.wikipedia.org/wiki/Nonimaging_optics

The use of a compound parabolic concentrator as field collector, in conjunction with a primary focusing concentrator for photovoltaic applications is studied.

See: http://www.archive.org/details/nasa_techdoc_19760005394