How Solar Energy is Generated
Among all the renewable forms of energy, solar energy generation has always been the most challenging.
All the other forms involve easy concepts like burning off some fuel or rotation of some kind of wheel.
Pretty easy concepts.
Whereas light falls on a plate and generates electricity–that’s amazing, but how does that happen?
Let’s try to understand how solar panels generate electricity from sunlight in the most straightforward concept possible.
From Sunlight to Electric Current
Well, we all know the fuel for solar energy is sunlight, but what some of us don’t know is that sunlight travels in the form of photons of energy.
Starting its journey from the sun and then traveling a distance of 9.3 million miles in about 8.5 minutes until it finally reaches our beautiful planet, “Earth.”
The magnitude of these photons reaching our planet in one hour can generate enough energy to fulfill the energy needs of our entire planet for a year.
When they reach the earth and collide with the photovoltaic solar panel, each photon loses an electron.
There are two reasons why this happens:
(1) The electromagnetic nature of light and (2) the design of the photovoltaic panel.
The Electromagnetic Nature of Light
Let’s first discuss the phenomenon of the electromagnetic spectrum of light.
The light that we see is only a part of the electromagnetic spectrum.
There are other energies/radiations that we can’t see.
They have their wavelengths and, therefore, their energy levels.
It’s these variations in wavelengths that create different colors of light, and this can be witnessed clearly in a rainbow.
While a part of the light falling on our solar panel is playing a useful role, the other part of it isn’t because either they have too less energy than required to knock off an electron-hole pair or they have too high energy.
The ones with too low energy would pass through the cell as if it were transparent, whereas those with extra energy will use the required energy to knock off an electron, and the excess energy will be lost.
These losses account for about 70% of the energy lost.
Only the energy defined by our cell material is required to knock an electron lose and that for semiconductors made from silicon is approximately 1.1 eV.
This is called the bandgap energy.
One might suggest that we choose a material that has a very low bandgap so that we could utilize the maximum number of photons.
The problem with choosing a very low bandgap is that it wouldn’t generate enough current.
Therefore, a very low bandgap would create a small voltage, and since voltage times current is power, the power generated wouldn’t be enough for any use.
An ideal bandgap that balances these two factors is around 1.4 eV for a cell made from a single material.
Design of the Solar Panel
Now we discuss the design of the solar panel.
As the name suggests, “photo” meaning light and “Voltic” meaning electricity, thus photovoltaic means electricity made from light energy.
A photovoltaic cell is made up of a semiconductor or combination of semiconductors, in such a way that one side of the material has a positive charge (electron-deficient) and the other hand has negative charge (electron abundance).
Therefore, this creates a space of high potential to a low potential, which causes a flow of electron or in other words, current.
So, when photons of light strike a semiconductor, their energy is absorbed by the semiconductor, which in return sets electrons free, which then starts moving along the direction of the electric field, generating an electric current.
This is the functioning of one cell, but in one solar panel, there can be up to hundreds of such cells.
The more the cells, the more the energy that the panel will generate, and the more the panels, the more energy the whole system will make.
This shows that semiconductors are at the heart of solar energy systems.
Still, semiconductors are made in a variety of combinations according to the availability of the material and the efficiency.
Before we go into specific properties of semiconductors made from different materials, let’s look into the electrical properties of a semiconductor and their different types based on their design.
Usually, in conductors, like copper, electrons are the charge carriers, but in semiconductors, it’s the combination of electrons and electron-holes that cause the flow of current.
Therefore, mixing of two or more than two types of elements that would create an electron abundance at one end and electron scarcity at the other end, would cause a flow of current.
To accomplish this, we need to add impurities in the semiconductors, and the process of doing so is called doping.
Different proportions of impurities will give different properties to the semiconductor.
The type of doping where electrons are in abundance is called the N-type doping, whereas P-type doing is when electrons are in scarcity.
Silicon is one of the most widely used semiconductors, probably due to its durability and overall efficiency.
But how much is this efficiency, one might ask?
Well, not much.
It’s around 12 to 18 % for residential units, and the best ones are approximately 23 %, but these are improving every year.
And even a slight improvement of 1% is enough to make a big difference.
To know more about the improvements in efficiency over the past century, click here.
Now back to our semiconductor design.
The problem with silicon is that it happens to have a gleaming surface, on which light bounces back without doing their job.
Thus, an antireflective coating is applied to minimize the reflection.
Another protection is added between the cells and the elements, usually glass cover plate.
Once the semiconductor starts separating the electrons and then making them flow in a direction, this generates electricity.
Still, what we’re getting is direct current (DC), while we need to convert it into Alternative Current (AC) to power our home appliances or any commercial or industrial machines for that matter.
For this, we use an inverter, which takes the direct current (DC) from the solar panel and converts into Alternating current (AC).
Along with converting DC into useful AC, Inverters also provide ground fault protection and system stats, including voltage and current on AC and DC circuits, energy production, and maximum power point tracking.
Classification of Inverters
Now inverters in itself have two different classifications: one classification is based on where the inverter is located, and the other is based on its on-grid or off-grid network.
a. Location-Based Classification
First, we discuss the classification done based on location.
It has two types one is the central inverters and micro-inverters.
Central inverters were traditionally used, but their drawback being; their interdependence on all solar panels at a time.
That means if one solar panel isn’t performing well, either due to blockage due to shade or dirt, while others are working fine, the whole performance of the system is brought down if a central inverter is connected.
Microinverters are a recent invention and have overcome the barrier that central inverters faced.
Their invention was a significant technological advancement, and now one solar cell would not affect the performance of other cells.
b. Network-Based Classification
Now the other classification is based on the off-grid/grid-tied network.
Off-grid inverters, in other words, standalone inverters, are disconnected from the local area grid.
They store their surplus energies in batteries and use later on.
However, in case the panel isn’t working or requires maintenance, or if the winter is prolonged, the homeowner will not have an alternate source of electricity.
Grid-tie inverters connect to local area grids, and that’s where their excess energy goes.
The plus point about grid-tie inverters is that its owner gets compensation for the energy that they sent to the grid, either if it’s a homeowner, investor, or organization.
The concept of net metering is introduced due to grid-tie inverters and is being considered as a good option for homeowners and offices to reduce their electricity bills.
The problem with this type of inverter is that during power outages, the owner of the panel might not receive the energy that their panel has previously given to the grid.
However, a third type has been recently introduced, which is a hybrid combining the best of both worlds.
It stores backup energy in the batteries and sends the rest to the grid but becomes a bit costlier than the other two.
From Inverters to Appliances
Now that the inverter has thankfully provided us with the alternating current that we need to run our appliances, we need to look further into its journey until it powers our machines.
It sends the AC to the distribution board (DB), where it’s divided into separate circuits to propagate it into different corners of the building for various appliances such as lights, fans, refrigerators, ACs and heating systems, etc.
Circuit breakers and fuses are fixed at different points to avoid an overload of electricity, which could burn some appliances.
Now we’re free to use this electricity to power our appliances like phones, refrigerators, toasters, and whatnot.
Other Types of Semiconductors
Until now, we have been discussing semiconductors made from silicon because they’re the most popular and best known, but other materials with their unique properties have also been tested and used.
Such as germanium, gallium arsenide, silicon carbide, and the most recent and interesting one among these are organic photovoltaics (OPV).
Each one of them has its advantages as well as disadvantages, as some cost more than their performance, whereas some cannot withstand extreme temperatures.
Some have low efficiency, while others respond too slowly to signals.
Organic Photovoltaics (OPV)
Let’s focus on organic photovoltaics (OPV) in specific.
They have attracted much attention from the industry due to their promising potential of being much cheaper than silicon.
For large-scale manufacturing, this could be very profitable, and that might ultimately lower the prices of the solar panels for the consumers.
The core principle in OPVs is their carbon-based structures that can be manufactured in many compositions and with different techniques that would give them unique properties.
Another one of their great plus-points is their flexibility and adaptability.
They can be printed on thin rolls of plastic; they can bend or curve around structures or even incorporated them into clothing.
And they can be modified to respond to different frequency bands of sunlight.
Previously it was their low efficiency, which was half that of silicon-based semiconductors, that was stopping them from becoming the leader in solar panels.
Recent advancement has shown their efficiency has reached 17% and may reach 25% very soon.
This efficiency is astonishing given the estimates that with a 15% and a 20-year lifetime, OPVs will be producing electricity at a cost less than 7 cents per kilowatt-hour.
Whereas data from the US Energy Information Administration show, the average cost of electricity is 10.5 cents per kilowatt-hour.
How Far Have We Advanced in Solar Technology?
If we compare the solar panels we have today to where we started, we can see the stark advancements.
We started with 1% efficiency in the 19th century, which was not even enough to be considered a useful energy source.
The significant improvement was made by Bell Labs when they invented the first useful silicon solar panel with 6% efficiency.
Over the last decade, growth has been exponential.
The future is very bright for all renewables as they’re expected to increase 50% over the next five years, and solar energy is expected to account for 60% of this growth, according to a recent report from International Energy Agency (IEA).
Apart from better efficiency, reduction in cost price and government tax credit are the major factors for this increase.
In this article, we covered everything about solar energy, starting from the light emitted by the sun until the alternating current powering our appliances.
Along the way, we learned about semiconductors and their different types, as well as inverters and their different types.
We also learned how technology has improved over time and is now serving us better and much more affordable.