It’s been said that if we covered just one percent of the Sahara Desert with solar panels, we could generate enough electricity to power the whole world. It’s clear, then, that solar energy is powerful stuff with huge potential, but how exactly is it produced?
Photovoltaic - turning light into electricity
All light contains energy. When light hits an object, this energy turns into heat. However, when light hits certain materials its energy is turned into an electrical current that can be used to power all sorts of things. This is called the photovoltaic effect.
Photovoltaic cells, more commonly known as PV solar panels, have been used to produce power from light energy since the 1950s. The name says it all: photo meaning light in Greek and voltaic being a reference to the Italian electricity pioneer Alessandro Volta, inventor of the battery.
As far back as 1958, PV cells were being used in space to power satellites’ electrical systems. Today, they can be found on calculators, emergency road signs and buoys out at sea, but they’re most often associated with rooftop power production.
Each PV panel contains several PV cells, and the common rooftop solar panels you see are usually clustered together in solar arrays. There are now enough solar arrays on residential and public rooftops and on solar farms to supply electricity to 1.5m UK homes, but what is going on inside the PV cell itself?
Inside the cell - A ‘silicon sandwich’ of photoelectric effect
In order for an electrical current to be produced, electrons must be able to move freely within matter. This is where Einstein’s photoelectric effect comes in; he observed the ability of matter to emit electrons when a light is shone through it. In the case of solar power, the matter is the ‘sandwich’ of two layers of thin silicon crystal inside each PV cell, and it is the movement of the electrons emitted by the silicon that creates the electrical current to power your dishwasher and kettle.
Solar energy is the key here. The electrons within the silicon are unable to move anywhere without the energy that’s carried by photons in the sun’s rays.
When these photons hit the atoms in the silicon, they give their energy to the electrons which have been stuck to these atoms; meaning the electrons can now move freely.
Now, in order for these freed electrons to generate an electrical current, they have to be directed to move in a certain way. PV panels generate electricity by stimulating the movement of electrons within this ‘silicon sandwich’ and this requires an electrical imbalance between the two different layers of silicon, just like the opposing ends of a magnet.
Composing the ‘silicon sandwich’
The two layers of silicon within the PV cell both contain atoms that have been arranged into a bound structure but each has been modified by the addition of other elements to create two different types of silicon:
The ‘silicon sandwich’ is created when a layer of n-type silicon is placed on top of a layer of p-type with a barrier between the two that makes it impossible for the electrons to cross without energy from the light.
When light is shone on this ‘silicon sandwich’ a current will flow because the surplus electrons from the n-type silicon jump down to fill the holes in the electron-poor p-type silicon.
The movement of electrons in the same direction produces electricity and leaves the n-type silicon layer positively charged and the p-type layer negatively charged. An electric field is created across the cell and the addition of two metal contacts on either side of the ‘silicon sandwich’ creates a circuit with electricity flowing through it. The electrical current is then conducted out of the sandwich and into a breaker box.
The more light that shines on to the ‘silicon sandwich’ the more electrons move down on to the lower layer of p-type silicon and the more current flows. This is why solar panels are most productive in the sun.
The two-storey house
It may help to visualise the PV cell as a two-storey house, full of people rather than electrons, with a metal roof and basement. The movement of people in this house creates the electrical current.
The top floor of the house (the n-type layer of silicon) has been filled to capacity with people and a certain number of these people need to move to the emptier lower floor (the p-type layer of silicon) where there’s more space.
However, the people on the top floor have no energy of their own to move downstairs and this is where sunlight is important. When the sun shines on the top floor, the people receive the energy they need to move downstairs from the photons in the sun’s rays. In a PV cell, it’s this movement ‘downstairs’ that creates the electrical current which circulates the ‘house’, passing through the metal roof and basement, before being conducted out of the cell.
Where to next?
Before this electricity can be used to power your household appliances, it must be converted from a DC (direct current) to an AC (alternating current) by passing through an inverter. The AC power then travels to a breaker box, which distributes the power to any electrical items being used in the home.
The power produced in the PV cell is measured in kilowatt peak (kWp); this is the rate at which the cell generates energy at peak performance in full direct sunlight during the summer. The UK’s Energy Saving Trust estimates that a 4kWp system installed on a south-facing roof can generate around 3,800 kilowatt hours of electricity a year in the South of England, which is roughly the equivalent of an average household’s electricity needs.
How does solar energy work with an immerSUN?
The solar energy produced by the photovoltaic effect within PV cells is used to power household appliances but when you aren’t at home to use this power it gets exported straight back to the grid.
By installing an immerSUN you can divert surplus energy generated by your solar panels into element-driven devices in your home such as a hot water systems and storage heaters. No more waiting for the water to heat up for your evening bath and you could save more than £250 per year on your energy bills! Click here to see the immerSUN in action.