The Energy in Light and How Solar Panels Work

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Solar Panels or Solar Modules

When light (particularly, sunlight) strikes a silicon semiconductor, it causes electrons to flow, creating electricity. This is the fundamental principle behind solar panels (also called solar modules).

^{_{Pixabay Creative Commons: A Roof Mounted Solar Panel}}

Solar panels are used on power generating systems which utilise the property of certain types of semiconductors to convert sunlight into electric energy. The energy so captured is in the form of direct current and we shall examine direct current and the reason for it before the end of this post. Most home appliances use alternating current so an inverter is used to convert the electric energy to alternating current that can be used at home and offices. This setup is called a Solar Power Generating System

Two types of solar power generating systems exist:

• stand-alone solar power generating systems and;
• grid-connected systems.

Stand-alone systems are systems that operate in isolation such that the power they generate is fed to the facility (home or office) where the power is needed. They are used in a variety of applications, like backup power supply as a contingency for the failure of the primary power system. They are also used in remote areas where centrally generated power is not available. In the tropics, where the sun shines most parts of the year, the solar power system can act as a primary power supply for homes all through the year. In my part of the world, this is the predominant installation of solar power generating systems. One of the reasons for this is perhaps because we do not have a very smart grid.

A favourite type of solar power system installation in locations where electric utility companies operate more robust systems is the grid-connected systems. Grid-connected systems are used for homes, public facilities and offices. The system is connected to the grid so that during the day, when the sun shines, some of the generated power is used and the rest is sold to the utility. At night or rainy and cloudy days, the power company would supply to the user. At the end of the month, the costs are checked and balanced with the help of a monitor. In areas where electricity is expensive, this could help a user to reduce electric utility bills in the long run.

The science behind the nature of light and the energy stored in it, how a semiconductor converts this energy to electric energy, how this turns to usable electricity in homes and everything else in between, are the objectives of this post So we have a long way to go.

The Nature of Light

Light is produced through two means: luminescence and incandescence. Luminescence in when light is released as excited electrons fall to their ground state as in the case of the firefly discussed here. Incandescence is when the light is produced when an object is heated to very high temperatures, (usually up to 800 Kelvin).

This is the kind of light produced by the sun. The surface of the sun is at a temperature of about a temperature of about 6000 Kelvin due to nuclear fusion of hydrogen to form helium, releasing incredible amounts of energy in the form of heat and photons (light).^[2]. Light is an electromagnetic wave, and electromagnetic waves carry energy. Great.

Solar (Photovoltaic) Cells - The Semiconductor Connection

The older I get, the more I'm conscious of ways very small things can make a change in the world. Tiny little things, but the world is made up of tiny matters, isn't it? ~ Sandra Cisneros^[6]

The above quote is very accurate in physics as in life. Solar panels are made up of solar cells that have sizes no more than the palm of a hand. They are fused together to form solar modules and solar panels. In Quantum Mechanics, every particle or quantic entity is known to have dual behaviour. This concept is explained by the wave-particle duality.

Albert Einstein realised this when he noticed that the subsisting wave laws failed to explain all the behaviours of waves and particle laws failed to described some of the action of particles. However, when you consider a wave such as light as having dual properties, then this explains all the behaviours of light. So Einstein postulated the existence of photons which he called quanta of energy. It is this energy which would strike a solar cell made with a semiconductor, knocking electrons (called photoelectrons) out of it which flows to generate electricity.

But What's in a Solar Cell?

Solar cells are made of semiconductors. In my previous post transistors were discussed extensively. They are small switches made from semiconductors like silicon. Ordinarily, silicon does not allow much current to flow, but when we manipulate it by adding some impurities, we can improve the ability of silicon to conduct electricity. This is called doping. If you are interested in reading more about doping, you may read it here. Depending on how the silicon is doped, we may have a p-type semiconductor or an n-type semiconductor.

In a solar cell, these two types of semiconductors are laid one on top of the other. The p-type has fewer electrons and is placed under the n-type. A junction is formed where these two types of semiconductors meet and no electron can flow past this junction. But when a beam of light (or photons) is shined on the cell, the light transfers its energy to the atoms of the p-type semiconductor, knocking of electrons which jump over to the n-type thereby causing current to flow. The higher the intensity of the light, the more electrons that would be knocked out and the greater the current and electricity generated.

Unfortunately, solar cells are not as efficient as we would like them to be with the best ones having an efficiency of about 30% (known as Shockley-Queisser's limit). The reason for this is that not all the photons have the required strength to knock off electrons from the silicon and not all the photons have the appropriate frequency. So this leaves our solar cell inefficient, but it is not all that bad as there is progress in recent years through which panels with up to 46% efficiency have been made. The solar panels we see on roofs of houses are mostly first-generation solar panels with an efficiency of 30% or less. And because of other environmental factors their efficiency may drop to as low as 15%. However, they are still some of the most efficient panels in the market. There are also second and third generation solar panels which differ from the first-generation mainly due to the kind of materials they are made from such as amorphous Silicon, a-Si.

What Can Solar Panels Power?

If you have ever told someone to try solar generating systems, the question they would ask you, especially in my part of the world is: will it power my air conditioning?

So how much power can we get from solar panels?

As already mentioned, solar cells are marred by inefficiency. However, if we consider the energy from the sun shining perpendicular to the earth, the energy within a square meter of ground receiving insolation is potentially up to 1000 watts._[5]. After adjusting for the efficiency, the angle of incidence, and location, solar panels covering one square meter may be able to generate only about 160 watts. Therefore one needs to cover a large space to generate substantial energy from sunlight, but it is still worth the trouble because the sun is free.

As mentioned at the beginning, solar panels generate direct current, and most of our appliances use alternating current. To use the power from solar panels in the home or other facilities, an inverter is required perform a conversion.

An Inverter

Inverters are electrical devices that are capable converting direct current (DC) to alternating current (AC). Most of the things discussed in basic electricity classes are DC. AC is different because, unlike DC where we describe the electrons are just flowing in one direction, AC flows more like in a cycle that reverses about 50 - 60 times a second. Electronic devices build to utilise DC often come with built-in devices made with semiconductor devices called diodes. These diodes work as rectifiers which converts AC to DC.

Inverters are and do the reverse of rectifiers by converting DC to AC for appliances that are powered by AC. The process through which inverters achieve this conversion and the different types of inverters shall be the subject of another post sometime soon.

I hope this has been informative for you and I'd like to thank you for making it to the end. Please leave a comment if you have more information on this topic or you just feel there are areas I can improve.

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References

1. Dictionary | Incandescence
2. Space | What Causes the Sun to Give Off Heat
3. Physics.Org | What are Photovoltaic Cells
4. Kyocera | Solar Power
5. Explain That Stuff | Solar Cells
6. Brainy Quotes
7. Wikipedia | Quantum Mechanics
8. Wave-Particle Duality

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