It seems strange to many people that you can take a very thin sheet of material covered with extraordinarily thin crystals, and just by laying it in the Sun, and attaching it to a couple of non-descript boxes, you suddenly get electricity. It is very counterintuitive—no windmills or turbines are turning; there are no magnetic fields; it’s absolutely silent—and yet there is power.
Just 100,000 years ago, it could be a hundred or even a thousand years between new inventions. Human creativity was very limited and stayed that way for the next 70,000 years. A usefully shaped rock would be passed down for generations from parent to child.
Neanderthals and Homo sapiens lived together until 30,000 years ago. Suddenly brains and vocal cords developed (without explanation—we call it the Great Awakening), so everyone suddenly became smarter, and began to develop speech. Neanderthals, despite their bigger brains, had hands that couldn’t manipulate fine tools for carving or painting, or for any delicate work, so modern humans (you and I) took over the hike down evolution’s path.
M ore recently, in the last 20,000 years, when something new was invented, such as weaving, people wanted to know how and why it worked. Bone sewing needles were invented. People were getting curious and seeking more pragmatic explanations. Real skills started to be passed along and dispersed to others.
T ime moved on and glass was invented. Greek and Roman soldiers soon used to hang around early outdoor glassworks looking for blobs of glass that landed on flat stones having a curved top. These were treasured tools used to focus the Sun’s rays and quickly start fires while their army camped. By our modern standards, these little blobs of glass were of terrible quality, full of impurities, and very low power—but they worked!
We had stopped looking for magic to explain things and science was becoming dominant. Solving the mysteries of the world ruled for thousands of years. Now, in the 21st century, technology is advancing so rapidly and inventions are coming so fast and furious that people are being overwhelmed by the complexity of how something works.
When inquiring minds (like yours) show up to find out the “how” of something, it’s a sign that there are still intelligent, thinking people in the world and we are so glad to see you here! Curious people are the inventors—the ones that push the frontiers of knowledge forward—and knowing how something works is essential if you’re going to make it better!
How Do Solar Panels Work?
At the very simplest level, solar panels work by possessing a surface that is constructed of atoms with a very loose attachment to their electrons. When a single photon of light arrives and crashes into one of those electrons, it adds enough energy for it to break free of its weak association with its atom and that electron begins to flow. And what do we call flowing electrons? Electricity, of course!
Now, of course, there is a great deal more to it than that, but that gives you a good starting point. Understanding photons becomes important here because the photon is the carrier of the electromagnetic force in all of its various forms.
A photon is a quantum (plural: quanta) of energy, the smallest possible unit in our current understanding of physics. In quantum mechanics we define it as an elementary particle with an electrical charge of zero, a rest mass of zero, and a spin of +1. Luckily you don’t need to understand anything more than that about quantum mechanics for this discussion!
How It’s Made
The construction of a solar panel is vital to how well it functions. Basically it is composed of three parts. There is the structural layer, the functional layer, and the protective encapsulation. The top surface is often protected with a sheet of glass for earthly-use, and plastic for satellite-use (to save launch-weight).
The structural layer holds everything in place. The functional layer does the work of generating electricity. The protective encapsulation is often ethylene vinyl acetate (EVA) which keeps the functional layer clean, dry, and uncorrupted. It all goes into a lightweight (often aluminum) framework to make handling and installation easy.
An Electronic Sandwich
The largest component of a solar cell is one of the most plentiful substances on Earth…silicon. It also happens to be the largest component of glass, which as you probably know, is an excellent insulator. It resists the movement of electrons.
We want the electrons to move, of course, so we deliberately contaminate the pure silicone with other substances that have desirable electrical characteristics. The technical name of this precisely controlled “pollution” is doping. Doped silicon is therefore known as a semiconductor since it can conduct under the right circumstances, but normally does not…
That functional layer itself is actually two discrete layers. By virtue of them being connected, and because of their opposite properties, they manifest a third layer in between which we call a P-N junction.
The “P”, or positive side, of the silicon layer has “holes” that can accept electrons. The “N”, or negative side, has excess electrons that it can share. Where the two sides meet, the electrons and holes will slip across the border and balance each other out, and in so doing create a depletion zone, the P-N junction. This stops any further movement of electrons.
There are many substances that could fulfil the doping roles. Two of the most popular are boron and phosphorous.
Let’s Talk About Atoms
Boron fits into the silicon’s molecular matrix by joining to three silicon atoms. That leaves it with the ability to add one more electron, giving it a “hole” that can be filled. This gives that side an overall “positive” charge, so it is called the P side.
Conversely, phosphorous has five electrons in its outer shell. It connects to the silicon matrix in four places, but that leaves it with one extra electron. That allows for the possibility for that electron to move about freely.
Photons For Everybody!
Sunlight drives just about every process on Earth, if you analyze it closely enough. Yes, all radioactive substances came from other stars, and are an exception, but all the remaining energy is what the Earth collects from our centrally located nuclear fusion reactor! It feeds all our plants; it drives our water cycle so rivers can power electric generators; it even fed plants and dinosaurs that were buried millennia ago and eventually became fossil fuels.
Photons of light are small and there is a constant barrage of them arriving from all directions. If you cupped your hands in front of you, you would receive quadrillions of photons per second. The Sun actually gives us 2 ⨯ 1018 photons per square inch of area (3.1 ⨯ 1017 per cm2) by the time it reaches the Earth’s surface. On average, that’s 164 watts per square meter, per day.
How Do They Do What They Do?
Photons strike the P-N junction, and the electrons and holes are forced apart because the electrons acquire more energy. This allows the attraction between the positive and negative side to “feel” each other. When you connect a load (such as a motor, light, or some batteries) across the two opposite sides, the electrons start to circulate moving from where there are an abundance, through the load (doing some work along the way), and then escaping back to the side where there are fewer electrons.
Sadly, as Admiral Akbar knows, it’s a trap. The photons knock the electrons loose once again and they have to make the journey over and over. As long as there is light, they have to keep circulating, pushed by the repulsion of too many electrons on one side, and the paucity of electrons on the other…
How Much Power Can We Get?
You’ll often see it stated that the Sun delivers 1000 watts per square meter, but that is on a square meter at the equator, at high noon, that is completely perpendicular to the incoming sunlight. This is an ideal situation that rarely actually exists.
In our real world a standard one square meter solar panel cannot deliver more than 337 watts in full sunlight. The reason for this is that there are only so many electrons that can flow, and at a limited rate in silicon. We can get past this, with additional technology, which we’ll discuss shortly.
This limitation was first described in the Shockley-Queisser Limit, calculated in the early 1960s, which puts the theoretical maximum at 30% based on the number of electrons that can flow across a single silicon P-N junction in a normal (full) solar spectrum. This is boosted to 33.7% with a mirror surface behind the cells to maximise that available photon absorption.
This fundamental rule guides solar cell design. In fact, it has taken since the 1800s when solar cells were first invented up until the present time to increase efficiency from 1% to about 23%. That is very slow progress.
Which Solar Cells are Best?
That question is dependent on your needs. For example, it you have no particular limitation on space, polycrystalline cells are less costly, and are rated mid-efficiency. Having a large roof area can provide significant savings.
Monocrystalline cells, on the other hand, are more expensive to produce, but because they are so uniform, the electrons flow more easily, making it several percent more efficient. They are smaller for the same amount of power generation so you can get more in the same space.
Lastly, in big commercial arrays, with unlimited space, say the top of a factory, there is flexible thin-film solar cells. These are by far the least efficient, but if you have an acre or two of roof top, who cares? They are also useful for conforming to surfaces, such as car bodies, RVs, boats, or anything with a peculiar shape.
A vailable power is usually described as a range (Polycrystalline 12-18%, or 15% average; Monocrystalline 15-23%, or 19%; and Flexible Film 9-14%, or 11.5%). As you can see, the best PolyC cells can outperform the worst MonoC cells, and the best FlexFilm could slip by the worst PolyC.
A new crystal technology, Perovskite solar cells, have been in development since about 2009, but just this year (2020), they have reached 25.5% efficiency in single-junction architectures! How can we improve on this? More layers!
By layering this cell with a silicon cell, more of the energy can be harvested providing up to 29.1%—better than any silicon cell, ever! And it will get better as we progress. Multi-layer films will enable us to harvest the (for example) red part of the spectrum with the most efficient technology for that color; another layer will let us collect the green part; yet another could let us collect blue light; and, finally, we could harvest in the ultraviolet, too.
Could We Be More Efficient?
Certainly we can do better, and with different technologies it will improve. But, what if we were to harvest a different kind of photon? Infrared photons may not be very useful for generating electricity, but they are good at transferring heat.
A popular type of solar cell, especially with off-grid RV-users, is the solar air-cell. These are essentially boxes with a couple of square meters of surface area that are colored to collect the maximum amount of solar radiation. Some have very intricate designs to present as much surface area to the moving Sun as possible, while others are simply a flat-black colored box.
They contain a large volume of air and can reach temperatures in excess of 76º Celsius (~170º Fahrenheit), keeping a coach/camper pleasantly warm inside with no energy expenditure such as propane, battery, or electricity.
In either case, these boxes are equipped with their own solar power cell to run the electronics and circulating fans which take internal air, heat it, and return it to the interior. These are capable of heat generation even in mid-winter and can run unattended, if desired. If you’re particularly clever, you could set it up with smartphone controls so it could preheat your cottage before you arrive.
If You Can Heat Air…
Water can be heated the same way—certainly enough for a shower or daily use, with a solar water panel. If nothing else, it can serve as a preheater for your regular water tank to decrease propane/electric use. Naturally, these are going to be heavier, so they need to be mounted properly on an RV. On a fixed object, such as house or cottage, the mounting can be more robust.
There are models designed for areas where freezing occurs, which are more costly. That is definitely a consideration if you want one for an RV where you may visit Montana in the winter!
Are Solar Panels the Best Solution?
Perhaps not! Forest camping may make it hard to collect solar energy, so a gasoline, diesel, or propane electric generator may be useful. In broad, open locales with a good steady wind, a wind turbine could be a really good choice, too. They can provide lots of watts, and 120 volts with an inverter.
And, if you are backpacking, setting up a portable water turbine generator that weighs less than a kilogram (~2 pounds) in a stream or river could be sufficient to run LED lighting and charge all your portable devices. There are dozens of choices from tiny (that you can tow behind your kayak as you paddle), to large stream diverters that can provide electrical power year-round.
We all love “free” stuff and, after investing a bit in equipment to get started, having free power-on-demand is quite a good freebie! Solar Cells may seem “magical” in a way, but in truth, it’s just good old physics at work because a smart person took the time to be curious and investigate.
Curiosity may be a cat-killer, but it is the key to how we obtained all of the technology that we take for granted every day.