Solar Cells — CPVs, Fresnel Lenses, and Efficiency

5 min readDec 28, 2019

So you want to learn about fresnel lenses?
Only a nerd would read about that! So…
Leave your world behind and take a trip
Across a foreign territory of photovoltaics and plastic.
Ready yourself for an article of the ages!!

Want to watch the video based on the article? Same content and same order, just geared for visual and audio learners! Check it out here:

https://www.youtube.com/watch?v=MPApbmHCffg&t=1s

Now back to the article…

CPVs

Before we get into fresnel lenses, let’s take a look at their mother category, CPVs. CPV is an abbreviation for concentrated photovoltaics. Stripped down to its most bare meaning, a CPV is a solar panel with magnification. However, the ‘style’ of magnification varies between CPVs.

Magnification with Certain CPVs

— Parabolic Concentrators (the top)
This type of concentrator is a u-shaped mirror (hence, parabola) and the point where these mirrors direct the most light is a receiver tube. This converts the thermal energy it receives into electricity.

— Quantum Dot Concentrators (the middle)
This concentrator is basically a sheet of QD doped plastic surrounded by mirrors (on three sides) and a solar cell on another side. When the light hits the QDs, it slows down and the mirrors can guide the light to the cell.

— RXI Concentrators (the bottom)
This system utilizes refracting (R), reflecting (X), and internal reflecting (I)to get the most light to the cell. It takes all incoming light and does each of these actions once to direct a great amount of light to the cell.

FROM: “A Review of Solar Photovoltaic Concentrators”

Fresnel Lenses

By far, the coolest CPV is a fresnel lens concentrator. It takes advantage of a regular solar cell by minimizing its size and concentrating as many of the sun’s rays toward it.

The fresnel lens (on the left) is a flat surface on top which absorbs sunlight with concentric grooves on the bottom which move the light (the lines) to one specific point where a typical solar cell is.

Capitalizing on the Lens

This lens works because it puts as much light as possible onto the solar cell, giving it the most chances possible to convert the photons into electricity.

But different circumstances make the lens more (or less) efficient, like the distance between the solar cell and lens, the direction the system is facing and the angle the system is at, and the factor of imaging.

1. Distance

A fresnel lens concentrates the rays of the sun to a certain point. There is a singularly focused point as well as the areas above and below where the rays are less focused (orange in the diagram).

In one study, the lens was put at different distances away from the solar cell, putting the cell at different points between the rays (a, b, c for example). It was found that at 0.8 feet away from the cell, the fresnel lens could help create the most output. This was most likely because there was still a highly concentrated amount of light hitting the panel, but it was far enough from the focal point as to not overheat the cell.

The solar cell with this fresnel lens produced 17.77% (cloudy) and 15.49% (sunny) more output than a solar panel without a lens in the same conditions. Now, what does this 15% percent mean? In your house, 15% of the electricity you use in a day is for your AC system. One house could use an entire solar system but a fresnel lens system could provide electricity for a house and its AC.

FROM: “Impact of the Location of a Solar Cell in Relationship to the Focal Length of a Fresnel Lens on Power Production”

2. Orientation and Angle

A separate study looked into what angle the fresnel solar panel was at and the cardinal direction the system was facing.

After a year of research, it was found that the system at 45 degrees and facing South was the most efficient. These rates of output were 12–30% and 1–10% higher than other degrees and directions respectively.

By simply changing where a panel is facing you can optimize a fresnel solar panel. What a panel facing the north can power, a panel facing the south can power a third extra.

FROM: “The Use of Fresnel Lenses to Improve the Efficiency of Photovoltaic Modules for Building-integrated Concentrating Photovoltaic Systems”

3. Imaging

Another factor of fresnel lenses is imaging. This is the fancy way of saying that the lens is flat on top and groovy on the bottom. This type of lens creates paraxial rays; this is when the rays of sunlight hitting the top are bent and move inward toward the center axis of the lens (the line that leads up to the focal point).

The biggest problem with imaging fresnel lenses was a tracking issue, where the system would have to follow the sun to be more efficient. Equipment like this is expensive to manufacture and therefore causes a huge downside for imaging fresnel lenses.

FROM: “Concentrated solar energy applications using Fresnel lenses: A review”

Implications

Alright, here’s a quick recap before I put in my own two cents!

CPVs — fancy magnifying glasses to make solar panels better
Fresnel lenses — a piece of glass or plastic that’s flat on top and has circles etched on the bottom
Distance, position, and imaging — ways to change a fresnel that sometimes help and hurt

Now, what do those factors have to do with helping a solar panel? Well, the more efficient a solar panel is the better it will be at converting electricity in a timely and cost-effective manner. If these two barriers can be crossed, solar energy systems could skyrocket in commercial and industrial uses.

Furthermore, something as simple as adding a piece of material with grooves in it, changing the way a cell is facing, or how close or far a solar panel is could drastically improve solar systems. An average solar panel has the efficiency of about 20%, and the theoretical limit is still as low as 34%. By adding a fresnel lens at 0.8 feet away and making it face the south at a 45-degree angle already puts one at 40% efficiency. THAT’S DOUBLE THE ACTUAL AVERAGE AND ABOVE THE THEORETICAL AVERAGE.

Small tweaks and changes are and will be the key to expanding solar energy to a larger audience. Little things that can grow efficiency and diminish costs are essential to expansion.