14 May 2015 13:55:31 IST

How sunlight is converted into electricity

A primer on solar thermal, or heat-induced, and solar PV, or light-induced

The Sun is a free and abundant source of energy for humankind: and we make good use of it in everything we do, especially in the food chain — right from growing our plants and crops [plants convert the sun’s light energy to chemical energy in the form of bonds of sugar for later use, a process known to us as photosynthesis] to drying and storing the produce of the land and the seas.

But we need energy in a lot more forms than just food. We mainly need it in the form of heat or electricity, the two easiest forms of energy, in our daily lives. Again, between the two, electricity would be a sort of first-among-equals.

How do you convert sunlight into electricity? There are two ways of doing it, but before I proceed to describe them, we need to keep this in mind: both methods are still very energy inefficient. They convert only small quantities of the sun’s energy into electricity and it costs a lot even to do that. Solar companies are scrambling to make the process more efficient, and any firm that can come up with a cost-effective solution has a ready market on hand.

Let’s look at the two ways of converting the sun’s energy into electricity. The first is to trap the heat and use the heat to convert water into steam, then hose the steam into turbines. When the turbines turn, they produce electricity. (see Quick Guide below)

Don’t confuse this ‘heat’ method — let’s call it ‘solar thermal’ hereafter — with devices that straightaway use solar heat. Solar water heaters and solar cookers, for instance, use the sun’s heat but they do not convert it into electricity. In fact, some industries trap the solar heat to produce, for instance, hot air, which could be used to, say, dry recently-painted surfaces. So, there are many, many devices that make use of the heat of the sun’s rays. We are not talking about that here. We are talking about making use of the heat to produce electricity.

Photo voltaic method

‘Solar thermal’ plants typically have mirrors, mirrors, and, well, more mirrors. Only, these mirrors are curved so that any light falling on them gets focussed on a particular point. There are people experimenting with lenses instead of mirrors, but at the moment, solar thermal is mostly mirrors. Concentration of sunlight produces heat. They use the heat to make a specially prepared liquid very hot and use the hot liquid — called ‘heat transfer fluid’— to convert water into steam. The hotter the steam is, the faster it flows and the more the mobile energy (called kinetic energy) it possesses and you can use this energy to turn the turbine blades, and out flows the electricity.

The second way is more common and it is what you see everywhere. Let’s call it the ‘photo voltaic method’, or simply ‘solar PV’. Solar PV uses the sun’s light instead of heat. So, the two methods of making electricity from sun’s energy are (a) solar thermal, or the ‘heat method’, and (b) solar PV, or the ‘light method.’

Solar PV works with material called semiconductors. Now, if you are not a science student, you might find the word ‘semiconductor’ intimidating. But it’s actually quite simple: To put it very simply, semiconductors have loose electrons that can be ripped away from their parent atoms, and made to flow along a wire. Just in case you don’t remember your high school lessons about atoms, here is a brief recap. Everything in the world is made of atoms, which basically have a nucleus with particles called protons and neutrons and have another type of particles called electrons going round and round the nucleus. Therefore, their paths are ‘rings’ around the nucleus and an atom could have several rings around the nucleus, with a specified number of electrons on each ‘ring’. Imagine an atom as the solar system — the sun is the nucleus and the planets are electrons.

Now, the outermost ‘ring’ could have electrons that are itching to fly off. Only, they need to be given a push. Sunlight provides this push. Light (any light) is made of particles called ‘photons’ and absolutely nothing in this universe can run faster than these photons. They are the Usain Bolts of the Universe. That’s why scientists say ‘nothing can travel faster than light’. Since these photons move very, very fast, they have a lot of energy, and therefore, when the photons fall on the loose electrons in a semi-conducting material, they give the electrons the ‘push’ to fly off.

Then, it is simple. You channel the ‘foot-loose’ electrons into wires and you have a river of electrons: so Electricity is nothing but a river of electrons.

So, all you need for solar PV is to have a surface of semi-conducting material facing the sun and have a wire sticking into it, to siphon off the electrons. Of course, I’m over simplifying, but that is basically what it is.

A little more about PV, before we conclude this article. If you happen to overhear two solar guys talking, you’ll probably hear the words ‘crystalline silicon’ and ‘thin film’. They just mean two different ways of preparing the semi-conducting surfaces to face the sun.

You can make blocks of silicon-based semiconductors, like loaves of bread, and slice them into very, very thin wafers and run wires into them to make ‘cells’, then stitch cells into ‘modules’— well, those are the blue-coloured panels you see on rooftops and on street lights. This is ‘crystalline silicon’.

The other way is to simply paint semi-conducting material on glass. These would have to be very special semiconductors, as opposed to the commonly-used silicon-based stuff and it is a very hi-tech business to give glass a coat of these. These are ‘thin film’.

You can generally tell ‘crystalline silicon’ and ‘thin film’ apart by looking at the panels. Crystalline silicon is blue and panels have small squares. Thin film is brownish-black and would look like some sort of suiting material.

Quick Guide

How is electricity is produced from turbine-generators? You know that when you take a pin sufficiently close to a magnet, the magnet attracts the pin. The magnet has its ‘sphere of influence’ around it – the magnetic field. When the pin enters the magnetic field, the magnet pulls it.

One of the basic phenomena of the universe is, if you take a wire inside a magnetic field and shake the wire, electricity will flow through the wire. Don’t take a magnet and a wire and try it, because the amount of electricity will be very, very small for you to feel it.

But if you arrange powerful magnets in a circle and shake a wire in the middle of them, a pulse of current will flow through the wire. A generator is such an arrangement. Shaking the wire disturbs the magnetic field, or to put it a little more technically, cuts the magnetic flux lines. When that happens, electricity is generated.

But how does that happen? Well, no one really knows. There are four ‘fundamental forces’ in nature and the electro-magnetic force is one of them. It just exists. The generator converts the ‘magnetism’ into ‘electricity’ — ‘electro’ and ‘magnet’ are interchangeable. (The other three forces are: gravity, strong and weak nuclear forces, which are two forces inside the nucleus of an atom.)

Therefore, a generator has an arrangement of magnets so that a powerful magnetic field is created and a ‘coil’ is inserted inside the field and is made to move. But to make the coil move in order to produce electricity in such huge quantities of electricity as we need, you need a correspondingly large force. This force is provided by the turbines. The turbine blades spin when steam, (or water, in hydro electric plants, and wind, in windmills) hits them. This spinning movement is used to move the coil inside the generator. Turbine moves, coil moves, magnetic lines are cut, electricity is produced.

The reverse can also happen. If you run electricity through the coil, the arrangement of magnets will spin. This is what we call a ‘motor’.