Take the 2-minute tour ×
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free, no registration required.

I was just wondering why Fresnel Lenses are not widely used in the production of solar electricity. Their use there would mean that you could produce heat within a fraction of a second, up to a few minutes and run a turbine to produce electricity.

Though it is used in welding, I am not sure what are the problems in producing electricity, as stated by this Wikipedia article:

New applications have appeared in solar energy, where Fresnel lenses can concentrate sunlight (with a ratio of almost 500:1) onto solar cells.

share|improve this question

4 Answers 4

up vote 16 down vote accepted

This is mainly an engineering & economics question; and we can deal with those aspects of it over on the Sustainability Stack Exchange, if you want.

And there is one conceptual physics aspect too.

No, fresnel lenses are not widely used for solar power. Occasionally, but rarely.

Concentrated solar power (CSP), including concentrated photovoltiacs (CPV) depend on direct rays. Ordinary photovoltaics do not; they generate electricity from light however it comes in; reflected off snow, or scattered by the atmosphere and by clouds. And you can find light like that all around the world. Whereas if you need high-intensity direct rays for a lot of the year, you're pretty much confined to the tropics and near the tropics. You can go further away (and there are indeed concentrating solar power stations further from the equator), but then you've got an economic problem. That's not something we can deal with here, so I'll leave that for now

Here's a real-world CPV using a linear fresnel lens:

enter image description here source

So, the first problem with CPV is that you need a lot of direct light, not just ambient light. Ordinary PV can make do with either. The second problem, is that no PV cell is anywhere near 100% efficient, and although CPV cells have got up to (roughly) 30-45% efficient, that still means that up to 70% of the energy could end as heat. Some will get reflected, but there'll still be 30-50% of the direct light energy going into concentrated heat. And that's all heat on the expensive CPV cell, that you've got to dissipate. The more successful the fresnel lens is in concentrating the light, the bigger your problem in preventing heat build-up, and the higher the equilibrium temperature on your CPV cell.

share|improve this answer
    
Should work well for spacecraft then? –  gerrit Sep 28 '13 at 11:44
2  
@gerrit Dumping heat is one of the hardest things we ask our spacecraft to do, so it seems there would be better ways, but the technicalities would be the basis for a whole other question. –  Chris White Sep 28 '13 at 13:02
    
@ChrisWhite Right. Space Exploration may know and there even is a CSE-question there already. –  gerrit Sep 28 '13 at 13:08
1  
@EnergyNumbers Thanks,nice answer. –  Abhishek Oct 1 '13 at 15:31

There are several reasons why Fresnel Lenses are not frequently used for solar energy collection; although they can be for small niche applications. Fresnel lenses are inherently single surface lenses; all of the optical power is on the serrated surface. The grooved structure of this surface is easily contaminated , needing frequent cleaning. They are expensive to make in glass, and most moldable optical plastics are UV degradable. The surface contamination problem can be overcome by putting the grooved side on the inside, towards the focus. This is the worst possible way to use a single surface refracting lens; the lens aberration are extremely large, whereas they are not too bad, with the grooved side out.

The axial edges of the grooves scatter a lot of light, greatly reducing the transmission efficiency. You already have an 8-10% Fresnel reflection loss from the surfaces anyway, so Fresnel lenses are inefficient compared to reflector mirrors, and even with plastic molded lenses the costs are high. Numbers like 500 x concentration are not possible with linear (one directional trough) Fresnels, and even with rotationally symmetrical lenses, achieving 500 x is difficult because of the scattering from the non active edges of the grooves.

share|improve this answer

Fresnel lenses are not used, but fresnel reflectors may be soon. There are two main ways in which electricity is generated from sunlight:

  1. Photovoltaics: These (what are commonly thought of as "solar panels") are generally used without optics. The reason is that they will accept light from nearly any direction, and the power generated is directly proportional to both the surface area exposed, and the intensity of the light. One could use optics to focus light from a wider area onto a smaller amount of photovoltaic material, and there have been some trials of using arrays of small lenses to do this, but in general it would produce no more power than just covering the wider area with photovoltaic material. Whether this is more economical comes down to engineering optimisation (e.g. perhaps it would enable a smaller amount of a more expensive photovoltaic material to be used), but to date most photovoltaics do not use optics.

  2. Concentrating Solar Power (CSP): A heat transfer fluid is heated by concentrated sunlight and this heat is used to drive a turbine. At present there are a number of approaches to this. The most prevalent is to use linear parabolic troughs with a heat transfer fluid flowing through a pipe at the focal point. It has been suggested that linear Fresnel reflectors would be more economical, being cheaper to construct for the same receiving area (see http://social.csptoday.com/technology/hovering-wings-linear-fresnel-technology). The second approach is the "solar tower" method, where an array of reflectors is used to focus sunlight from a wide area onto a single tower (example). Normally, tracking mirrors are used to follow the sun across the sky. In some respects one could think of this mirror array as being similar to a Fresnel reflector - the array, seen as a whole, is a relatively flat structure that mimics the optical effect of a curved lens. I seem to recall that there have been some studies on sculpting desert sand into the shape of a fixed Fresnel reflector and painting it white, but I cannot find any links for this, and it may be an idea that rapidly faded!

share|improve this answer

Though it is not common to use Fresnel lenses for electricity generation but 100 MW power plant is nearing completion in Rajasthan state of India using linear Fresnel lens technology. So to say that this technology is not feasible for large scale use is not correct and time may come if that above mentioned power generation goes smoothly, the scene may change in Fresnel lens technology's favor.

share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.