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Active Solar Energy: How It Works

There are two primary kinds of direct solar power: passive and active solar power. Passive solar energy involves the capture of the sun’s thermal energy. Everyone gets that pretty instinctively, since simply standing outside on a sunny day demonstrates the process.

But when it comes to active solar energy, how it works is murkier. It’s also fascinating, and as solar power will continue to become a more significant part of our energy footprint, it makes sense to have at least a rudimentary grasp on how it actually works. At the same time, most of us aren’t interested in becoming theoretical physicists, so I won’t make this wildly technical.

We’ll start with a smidge of talk about the structure of atoms. Most people know that atoms consist of positively charged particles (protons) and negatively charged particles (electrons). It’s also pretty common knowledge that the protons are tucked away in the center of an atom, while the electrons orbit the middle; the image commonly used by way of analogy is our solar system. The sun stands for the nucleus made of protons in the middle of the arrangement, while planets illustrate electrons circling in orbits.

While helpful in getting a handle on atomic structure, this image is incomplete. Instead of single electrons in a given orbit (and all spinning on a flat plane) like the planets do, electrons orbit the atomic nucleus in a structure more reminiscent of concentric shells – a little like those Russian nesting dolls – and each shell can have multiple electrons flying around within it.

While each of the concentric shells or layers surrounding the nucleus of can have more than one electron occupying space, there is an absolute upper limit to the number of electrons which can be in each layer (before a new layer can be added, the other shells must all be full). But, if the outside layer (called the valence shell) is not filled with its maximum number of electrons, the atom is said to have one or more holes. It is these holes which allow for atoms to connect with one another, by sharing electrons.

Let’s use an example that is specific to solar energy and how solar cells do their thing: the element silicon. The valence shell of silicon has a capacity for 8 electrons. The silicon atom actually only has 4 electrons in the valence shell, leaving 4 holes. The atom actually wants to fill those holes, and the easy way to do so is to share up to 4 electrons with another atom (or atoms). When you have a large group of silicon atoms lined up properly, each of them can share electrons with up to four other silicon atoms, rendering a very neat and stable lattice structure that has all the holes filled and no extra electrons wondering what to do with themselves.

That last observation is key – pure silicon is horrible conductor of electricity, because there are no electrons in the mix which aren’t spoken for. A good conductor will have at least one electron in the molecular mix which isn’t involved in binding one atom to another.

So to get our solar energy, how do we get some of those ‘free’ electrons? The answer is called ‘doping’. In the process of doping, we add a small amount of another element to our pure silicon. This impurity will either have five electrons in its valence shell (providing the so-called ‘free’ electron) or three electrons in the valence shell (making a hole).

Here is where things get really cool: when you have a layer of silicon with some free electrons (called the n-layer) laid against a layer with some holes (the p-layer), the difference in electrical charges (negative n-layer and positive p-layer) creates an electrical field. That field gives electrons a preference for the direction, all other things being equal, they want to move. All that’s needed is some impetus to start the electrons flowing. One way of doing that is to apply another electrical field (for instance, hooking up the layered silicon to a circuit with a battery). Another way, and the one we are interested in here, is to juice the free electrons with light photons.

When the light from the sun is absorbed by the n-layer of a properly prepared silicon wafer one of two things happen: the photonic energy increases the temperature of the wafer, or the photon knocks one of the free electrons loose. When this happens that electron skitters about, looking for a home. And because of the field created by the interface between the n- and p-layers, it knows which way to go. Remember, we are talking about a large number of electrons moving around here, so as they continue to trend from the n-layer to the p-layer we have the makings of an electrical motivating force. If you connect this arrangement to a circuit, the electrons will begin moving through it, and you end up with a full blown electrical current.

Once that circuit is in place, you have free electrical energy. Electricity to run anything from a small calculator or watch, to satisfying the demand of entire communities (given enough solar panels). Electricity without any accompanying air or water pollution. Electricity without handing money over to government who don’t really like us. It’s a great approach to powering our world.

If you’d like to learn more about solar energy (both active and passive), stop by and sign up for my free solar power mini-course. In that course, I cover many more elements of what solar power is, how it has been used historically, and how people like you and I can get on board with it right now.

Until then,

Sullivan Kincaid

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