There are two main types of solar panels: mono- and polycrystalline. Monocrystalline ones work a little better than polycrystalline ones due to higher efficiency, but there are panel sellers who say that they have “special” solar panels designed for northern latitudes and such panels do not need direct solar radiation, because they run on ultraviolet (UV) light. They work even in cloudy weather, when the sky is lead-colored, supposedly because UV is not trapped by clouds, but passes freely through them.
Let’s figure out whether this really is and how efficiently solar panels work from ultraviolet radiation.
First, a little about the Sun
Our Sun is a giant natural thermonuclear reactor in the sky that continuously releases massive amounts of energy. If we compare the Sun with other “celestial” thermonuclear reactors, then it eclipses 85% of the stars in our galaxy.
How powerful is it?
For example, if you take:
- all the energy that humanity produces by burning coal, oil and natural gas
- all energy from uranium fission in nuclear reactors at nuclear power plants
- all wind energy
and let’s summarize all this for a year, this obtained value is approximately equal to the energy that the Earth receives from the Sun in 7 seconds! At the same time, it must be said that only 0.00000005% of the energy generated by the Sun falls on the Earth.
This energy reaches the Earth in the form of photons and these photons have different wavelengths, the shorter the wavelength, the more energy it carries. So, a “violet” photon (wavelength 360nm, where nm is a nanometer – 10-9m) carries 2 times more energy than a “red” photon (wavelength 720nm). If we delve a little into physics, then to the Planck formula, the photon energy is equal to E = hν = hc / λ, where h is Planck’s constant, ν is the frequency, and λ is the wavelength.
Our eyes are only able to see photons from the visible range, with wavelengths of 360 – 720nm. All that we see with our eyes is visible light, if the photons do not have enough energy, then these are infrared photons and our eyes are not able to see, if there is too much energy, then these are ultraviolet photons, our eyes also cannot see them.
What from the Sun reaches the Earth’s surface
If you look at the composition of sunlight reaching the Earth, then 4% of it is ultraviolet, 43% is visible light and 53% is from the infrared range. Most solar panels work in the visible range, also capture about half of the infrared range and only the smallest part of the ultraviolet range.
Why are UV solar cells a hoax?
Because ultraviolet radiation is a small percentage of solar energy, so if someone tries to sell you a solar panel that uses UV light and UV light is all that it can “recycle”, then this is outright nonsense (to put it mildly) in comparison A “regular” panel. If it somehow works like a conventional solar panel and also uses ultraviolet light, then the increase in generating capacity will not be so great and will be ~ 5%. As a result, a solar panel with an efficiency of 20% will become just a solar panel with an efficiency of 21%.
Since in reality there is no solar panel that can make good use of ultraviolet light, even such a modest improvement would be unrealistic. Although you can find solar panels that are more or less efficient at “recycling” ultraviolet radiation in space, solar cells are not used in rooftop panels.
Sunlight in space
As you already know, the Sun is a gigantic, uncontrolled nuclear reactor and you might think that it creates a huge amount of dangerous radiation. And you will be damn right. There is only one BUT. Nuclear reactions take place deep in the core of the Sun and because of its gigantic size, radiation simply cannot escape.
Light on its own can have a hard time getting out of the solar core. For example, it may take a photon 100,000 years to travel from the core to the surface of the Sun. From there, it takes a photon 8 minutes and 20 seconds to meet someone’s solar panel.
Compared to the total radiated energy, the Sun produces only a small amount of high-energy radiation, such as X-rays or gamma rays. But for fragile organic beings (that is, people), even a small amount of such radiation can become significant.
Sunlight on the surface of the earth
By the time the solar radiation reaches the upper layer of the earth’s atmosphere, its intensity will be approximately 1366W / m² (data link, satellite). After passing through the atmosphere, the radiation intensity will decrease by 18% and amount to 1120W / m². You just need to keep in mind that this intensity will be only at noon, only at the equator and only on a clear day.
Since conditions are rarely ideal, the Standard Test Conditions (STC) for solar panels is a radiation intensity of 1000W / m². This means that if you have a solar panel with a rated power of 300W, then it will give out this number of watts at a solar radiation intensity of 1000W per square meter.
But do not worry, nothing will happen to your solar power plant, nothing will burn or explode in it, even if the intensity of sunlight will exceed 1000W / m². Equipment manufacturers and solar planners take this into account. They also take into account that the intensity of sunlight will be even higher if the light shines through a hole in the clouds, and solar panels are simultaneously exposed to both direct sunlight and non-direct rays scattered by the surrounding clouds.
The diagram below is taken from Wikipedia. It shows how much solar radiation reaches the Earth’s surface. The yellow area of the diagram shows the amount of sunlight entering the upper boundary of the atmosphere, and the red area shows how much reaches the earth’s surface.
At noon, in the equatorial region, we trap ~ 18% of the solar energy passing through it. However, the graph above is not a snapshot taken at the equator at noon in ideal weather conditions, but a representative snapshot of solar radiation generally falling on Earth. Therefore, it can be seen from the graph that the atmosphere absorbs more than only 18% of the transmitted light. In the morning and evening, the sun’s rays must pass through a thicker layer of the atmosphere before reaching the ground. rays fall tangentially to the earth. Also higher latitude coordinates have a similar effect.
From the UV region of the graph, it can be seen that the atmosphere absorbs more than half of the UV light, mainly due to the thin ozone layer (O3 in the lower left corner of the graph). If we move to the right along the graph, then in the visible region of the spectrum the atmosphere retains more than a quarter of the sunlight, moving further along the graph we will see that the atmosphere “takes” several large pieces of radiation from the infrared region. Such large chunks, missing in the IR region, are a result of the gases in the atmosphere absorbing specific bands of sunlight energy.
If we separately consider only the visible region of the solar spectrum, we will find that this region consists of a beautiful rainbow of colors, as seen in the image below.
Ultraviolet Solar Panels
We see that visible light consists of 7 primary colors, we move from right to left along the spectrum: red, orange, yellow, green, blue, violet. But these colors can be divided into a huge number of shades and call them whatever you want.
Many of you probably know from the kindergarten the mnemonic rule for memorizing the colors of the rainbow: Every Hunter Wants to Know Where the Pheasant Sits.
The spectrum of sunlight absorbed by a solar panel
Below is a spectrum that we kindly took from the University of NSW website, this spectrum is similar to the spectrum of solar radiation reaching the earth’s surface, the only difference is that the dirty green color indicates part of the spectrum that can absorb a silicon solar cell and convert it into electricity.
This graph has a slight inaccuracy, which is that according to it ~ 49% of the absorbed sunlight is converted to electricity. To date, the maximum efficiency of silicon solar cells is 23%, which is more than 2 times less than from the graph. Therefore, a slightly augmented graph is shown below, in which the absorption corresponding to the efficiency of modern solar panels is marked in purple. (Note: the horizontal part of the spectrum in the range of 500-1100nm is an exclusively assumed form of the spectrum).
Absorption spectrum of a commercial solar panel As you can see from the left area of the graph, solar panels can absorb and convert some of the UV rays and this part becomes slightly larger as it moves into the visible area. The graph also clearly shows that solar panels generate a significant portion of their electricity from photons in the visible solar spectrum.
In contrast to the UV region, in the infrared part of the spectrum we see a vertical dip in absorption, or we can say a cutoff at a wavelength of 1100nm. This cutoff in absorption is due to the fact that the wavelength of light becomes larger than the size of a silicon atom and the waves simply pass through. That is, silicon becomes transparent for wavelengths of 1100nm and above.
Multi-junction solar cells
Multi-junction or solar cells from several p-n-junctions are essentially several solar cells combined into one, each part of which is focused on absorbing a certain part of the solar spectrum. The graph below (right) shows the absorption spectrum of such a solar cell, with different colors showing the absorption region for which different p-n junctions of the solar cell are responsible. Shown on the left is the structure of a multi-junction solar cell.