PHOTOVOLTAIC TECH

We need to increase our production of renewable energy fast, very fast in the teeth of the climate breakdown that threatens us (though that does not mean plating hundreds of hectares of our rural green spaces with solar cells – see last week). Although the government has removed the original subsidies (hopefully not for long), people are fitting solar panels to their roofs in large numbers and are to be congratulated for that, but few know how they work. This short article is to try to explain how they function without too much tech speak and quantum mechanics.

Below is a public domain diagram from Wikipedia uploaded by user Rfassbind. Individual solar cells are fitted to solar modules which are then made into solar panels to fit on roofs etc., or made into larger solar arrays for larger surfaces. Sometimes a tracking system is incorporated to follow the sun.

That’s the easy bit to understand, but how do the cells actually work to deliver energy as electricity in a form that can be delivered to the national grid or to a home or other receptor? The sun radiates to Earth as much as 1,000 Wm^-2 (watts per square metre). Remember that a watt is a measure of power flow – the voltage (the speed of electrons passing along an electrical circuit) multiplied by the current in amperes, or amps for short (the volume of electrons in the current). Electrons are subatomic particle components of atoms; electrons have a negative electrical charge.

The Solar cells are a sandwich of two layers of a crystalline semiconductor, usually silicon. A semiconductor conducts electricity but only under certain conditions. In a solar cell, the intensity of the sun’s radiation varies the conductance of the semiconductor of which the specific properties can be controlled by the addition of impurities. These added impurities are known as dopants and solar cells using silicon as the semiconductor layers are doped with phosphorus and boron. The phosphorus-doped silicon layer on top of the cell is known as the N-type semiconductor and the lower boron-doped layer as the P-type semiconductor. Between the two is what is known as a depletion region. Note that newer and more efficient solar cells are now based on perovskites as the energy harvesting semiconductor. For more on that please see https://bit.ly/pskite, but let’s stick with the silicon ones for the purposes of this article.

I have to assume here that you know a little about atoms and electrons, but if not, Wikipedia or the Encyclopedia Britannica https://www.britannica.com are good sources of help. When photons[1] from the sun strike a solar photovoltaic cell, they dislodge electrons, which have a negative charge, from their atoms and the semiconductor forces them into a directional flow which creates an electric current. It is due to the arrangement of the semiconductors creating an electric field that the electrons are driven in one direction to create the flow of electricity. The solar cells are fitted, connected together, to the solar panels, usually under an anti-reflective coating and a tempered glass cover with a metal back. The current generated flows along conductors but then has to be converted from direct (DC) to alternating (AC) current by an inverter before it is ready for connection to the grid or a local user. There is a another helpful diagram by National Geographic here.

Even the new perovskite photovoltaic cells cannot match what a leaf does, though, and there are now “artificial “leaf” designs beginning to be developed. For more, see here.

At Betts Ecology we keep informed of this developing field and hope that renewable energy technology will soon halt climate breakdown, but, as I said last week, this must be by installations on roofs, car parks and unvegetated spaces, not at the expense of our precious green spaces and the natural world.

 © Betts Ecology

[1] For non-scientists, think of photons as tiny particles of light, in other words a discrete amount or quantum of electromagnetic radiation carrying energy. At a diameter of something like 10^-5 ångström, this is much smaller than the diameter of an atom. An ångström (Å) is the SI unit of wavelength and is one ten-billionth of a metre or a tenth of a nanometre. The photon is a wave-particle duality, stemming from the ideas of the French physicist Louis de Broglie in 1924 who extended this concept to cover quantum particles generally as having “une dualité onde-particule, comme pour la lumière”. This was shown to be the case by Young’s famous double slit experiments so that today we think of multiple photons and other quantum particles as adopting the behaviour of a wave even if individually they are particles but that’s a whole new area for you to explore and follow the fascinating science as it evolves, which it is doing all the time.