The global energy economy is changing gear. The cost of solar energy is dropping in price so dramatically that in much of the world solar energy is now cheaper than that produced from fossil fuels.
According to a recent report from Energy Innovation, wind and solar energy are now cheaper than around three-quarters of US coal-fired energy production. In May 2019, the International Renewable Energy Agency (IRENA) reported that new photovoltaic projects in some countries such as Chile, Mexico, Peru, Saudi Arabia and the UAE have enabled a levelised cost of electricity of as low as $0.03 /kWh, compared to new fossil fuel plants, which globally range from $0.05 /kWh to over $0.15 /kWh. What’s more, IRENA predicts this trend to continue over at least the next ten years.
It undeniable that solar energy has and will continue to become a core player in the future of the energy economy. Businesses and households will all benefit from these falling renewable energy prices. Understanding solar energy and how to take advantage of it will be essential for both in the coming years.
Here we are going to answer the key questions about solar:
- What are solar panels
- How do solar panels work?
- How do solar panels produce electricity?
- And what is the installation process?
What are solar panels?
So let’s start with the basics. If you haven’t seen them on rooftops or in fields you’ve seen them in the media. Big blue panels that somehow silently generate clean electricity. But what materials are they composed of? Why do the individual cells have such a specific shape? Why do they take up so much area? And why are they blue?
Solar panels, sometimes called solar modules, are made up of individual solar cells. These cells convert the sun’s light energy into electrical energy. This can then either be stored or used directly. Each cell can generate in the range of 5 watts, about enough to power a smartphone charger. To power anything more (i.e. most things) the cells have to be joined together to form a solar panel.
The cells are most often made from silicon, many in the form of a single crystal. These monocrystalline silicon cells are sliced from a large cylindrical crystal of silicon. So in order to maximise the size of solar cell that can be cut from a round slice like this, the edges are sacrificed.
What about solar panel size? Typical solar panels have around 60 to 72 cells and a power rating of around 320 watts. The daily energy use of an average house in the US is around 32,000 watt hours (32 kWh), in the UK around 12.7 kWh and in India around 2.5 kWh, according to 2010 data. Crunching a few numbers, that means the average US house would need a 30-panel array to generate all of its energy from solar.
What’s the physical principle behind solar energy? It turns out there’s more than one way to catch the sun’s rays. There are a few different solar energy technologies, including solar thermal, concentrated solar and solar photovoltaic, or PV. The former two convert solar energy into thermal energy, while the latter converts solar energy into electrical energy.
PV is the most common and what most people think of when they think of solar panels. These PV panels are made from silicon, as described above, with the two most common types being monocrystalline and polycrystalline silicon. Polycrystalline solar panels are generally not made up of individual cells, but one larger piece of silicon. Other PV technologies include thin-film PV cells and concentrated photovoltaic cells.
All PV technology works on much the same principle, so to keep things manageable let’s focus on silicon cells and how they work.
How do solar panels work?
Silicon solar panels, and therefore solar cells, have a very basic working principle. Sunlight hits the cell and causes electrons to depart from their otherwise stable position. These leave the silicon and move through an electrical circuit, powering our devices.
Solar cells are made up of a number of layers:
1. Transparent protective layer
This first layer is usually made of glass or another transparent material which allows light to pass through to the silicon beneath while protecting the silicon and metal layers from damage from the environment.
2. Antireflection coating
This layer is quite impressive. The silicon layer itself has high reflectivity and would result in around 30 % of the light incident on it to be reflected. The antireflection coating vastly reduces the amount of reflected light by permitting light to pass through to the silicon layer but not to escape again. It does this by being exactly the correct thickness.
The light, in fact, does reflect off both the top and the bottom of the antireflection layer, yet the thickness of the layer is perfectly chosen so that the light waves reflected from either surface exactly cancel each other out (called destructive interference). As a result, only the light which is transmitted through the antireflection layer remains.
The catch is, however, that this light cancellation is dependent on the wavelength of the light. As sunlight is a mixture of many wavelengths (the colours of the rainbow), the antireflection cannot work perfectly, but it can be optimised for certain wavelengths. It turns out that the reason solar cells are blue is that the antireflection layer is most effective when reducing reflection from the green – through yellow – to red region of the spectrum, leaving blue to be reflected slightly more than the other colours.
The P-type and N-type silicon are where the magic happens. Here the all-important light splits electrons and generates a current. This is achieved through the use of two slightly different types of silicon in contact with one another.
One has a positive charge (P-type) and the other has a negative charge (N-type). This results in a voltage across both types. The light (via photons) imparts energy to electrons in these two types of silicon, which allows them to pass through an electrical circuit with the help of the aforementioned voltage. This will be explained more clearly below!
4. Metal contacts
After the electrons have been mobilised in the silicon, these need to be extracted via metal conductors. The bottom contact is a simple metal plate covering the whole area of the solar cell. The front contact of the solar cell is a grid. This is to maximise the amount of light reaching the silicon while minimising the length that electrons must travel through the silicon before reaching the metal.
If the metal contact covered the whole surface, no light could pass through. If the metal only ran around the outer edge of the solar cell, most electrons would never reach the contact at all.
How do solar panels produce electricity?
So now we know the main parts of a solar cell. But what really happens inside the silicon? To answer this question, firstly, we have to understand that silicon is not a highly conductive material, like metal, nor is it entirely terrible at conducting either, as an insulator such as rubber. Instead, it falls somewhere between.
Silicon is a semiconductor, meaning that under normal conditions it has a low conductivity and jumps to a much higher conductivity under certain other conditions, such as under an applied voltage. This is why it can be used as a switch in integrated circuits.
Crystalline silicon uses its four available electrons to bond with four surrounding silicon atoms. It, therefore, has no electrons remaining for conducting electricity. If we add a small amount of phosphorus, which has five available electrons, to the silicon we are effectively adding an extra electron for each extra phosphorus atom. This is called doping. This area of silicon is now negatively doped and so we call it an N-type semiconductor.
We can do the same with boron, which has one electron less than silicon. This removes an electron from the silicon structure. The resulting absence of an electron acts like an effective positive charge, which we call a hole. This area of the silicon is called P-type. We can now create a silicon wafer with the top of N-type silicon and the bottom P-type.
When the N-type and P-type silicon is in contact, the extra electrons in the N-type rush to combine with the holes of the P-type, creating a zone with no free electrons or holes, called the depletion layer. Now, because the phosphorus atoms have lost their fifth electron they now have a net positive charge. And the boron atoms, now with an extra electron, have a net negative charge. The result is an electric field between the two areas, or more importantly, a voltage (voltage is the electric field divided by charge).
So now we have a silicon semiconductor with a voltage. But all of the electrons in the depletion layer are unable to move. If we can give them the energy to move, the voltage will tell them where to go. That energy comes from sunlight.
Light can be thought of as particle-like packets of energy called photons. When a photon of the correct energy collides with an electron in the depletion layer, it knocks the electron out of its position, (re)creating an electron-hole pair. Under the influence of the voltage, the electron migrates towards one side of the silicon and the hole to the other.
Now when we connect an electrical conductor (a wire) from one side of the wafer to the other, the electrons can flow through that conductor.
Now hopefully you know the basics of what solar panels and solar cells are and how they work. To find out how to go about installing solar panels for your home o business, read solar panels: what’s the installation process? (Coming soon…)
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