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BASIC KNOWLEDGE – PHOTOVOLTAICS Everything you need to know about photovoltaics

From Luke James

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When people talk about “solar”, they’re usually referring to photovoltaics, the solar panels that you have probably seen sitting on several rooftops. But have you ever thought about how these actually work to generate clean electricity? This article takes a look at what a photovoltaic cell is, what it’s made from, the technology behind it, how it works, and more.

As for what photovoltaics is, it’s the direct conversion of light into electricity as the result of a reaction that takes place at the atomic level.
As for what photovoltaics is, it’s the direct conversion of light into electricity as the result of a reaction that takes place at the atomic level.
(Source: Zhou, Yilu - AdobeStock)

There are several ways of using solar energy to generate electrical power. This article focuses on the most popular method - the photovoltaic technology.

What is photovoltaics?

The term “photovoltaic” comes from the Greek word “phos”, meaning “light”, and from “volt”, the unit of electromotive force, the volt. Voltaic is also a word that relates to electricity produced by chemical action in a battery.
Photovoltaic definition: As for what photovoltaics is, it’s the direct conversion of light into electricity as the result of a reaction that takes place at the atomic level. By leveraging materials that exhibit the photoelectric effect, it’s possible to create PV solar cells and deploy them on a large scale, i.e., on the roofs of residential housing or in industrial solar cell farms to generate clean, renewable electricity.

The photoelectric effect & history of photovoltaics

The photoelectric effect is the emission of electrons in a material when it is exposed to light. It’s both a physical and chemical phenomenon with origins dating back almost two centuries. The photovoltaic effect (the generation of voltage and electric current in a material upon light exposure) is closely related to, but different from, the photoelectric effect.

The photovoltaic effect was first discovered by Edmund Bequerel, a French physicist, in 1839. Bequerel found that certain materials could produce small amounts of electric current when they were exposed to light.
Then in 1095, Albert Einstein’s Nobel Prize-winning research described the nature of light and the photoelectric effect on which photovoltaic technology is based.
It wasn’t until 1954 when the first photovoltaic module was built by Bell Laboratories as a “solar battery”, however. This is because it was far too expensive for the module to gain traction and be used in widespread applications.
Then in the 1960s, the space industry began to make use of the first serious photovoltaic technology for providing power to spacecraft. It’s during this period when the technology really took off (no pun intended!) and advanced. It became more reliable, the cost began to fall, and it started being used in more and more applications, especially during the energy crisis of the 1970s when photovoltaic technology gained widespread recognition as a source of power.

Solar photovoltaics

Solar photovoltaics (often referred to as “solar cells” or “solar panels”) is an electric power system which converts solar radiation from the sun (i.e., the sun’s light energy) into direct current (DC) electricity. A typical solar PV system will feature solar panels which absorb this sunlight and convert it into electricity, thus supplying clean and renewable energy, even when the sun isn’t shining bright.

Since the turn of the century, solar PV has been recognized as a promising renewable energy, and developments of all kind (scientific, technological, industrial, and logistical) have been on the rise, with production doubling every two years or so. This makes it one of the world’s fastest-growing renewable energy technologies.

And with more and more government-backed incentives being handed down to the owners of solar PV systems (e.g., tax breaks, payment for energy supplied to the grid, and feed-in tariffs), this trend is likely to continue.

How does photovoltaic technology work

In short, solar cells are thin wafers of crystalline silicon, the same element that’s used in virtually every electronic device in existence today. While these wafers were relatively big when PV solar cells were first developed, they’re now so small that they’re barely as thick as a human hair.

When these PV solar cells are exposed to light photons, they hit the negatively charged electrons inside the silicon atoms and knock electrons and knock them loose. When this happens, it leaves behind an empty, positively charged “hole” where the negative charge used to be.

In untreated silicon, electrons would just recombine with these holes to produce waste heat; no electricity would be generated. To get around this and make a working solar cell, the crystalline silicon wafers are treated (doped) with two other elements: boron and phosphorus.

When the boron and phosphorus meet, they interact with the silicon to create an electrostatic field just under the front surface of the cell. This field remains in the crystal structure permanently. Not, when sunlight photons hit the crystal, the negative electrons and positive “holes” are kept separate by this electrostatic field. This causes electrons to flow to the front of the cell while the holes flow to the back, creating a current.

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The electrons that flow to the front of the cell are collected by grid lines printed onto its surface. They flow into what are known as “busbars”, which are basically metallic strips used for power distribution. The current then flows into a circuit where its voltage potential is given up as electrical energy while the electrons flow back into the back end of the cell where they recombine with the empty positive “holes” that were left behind.

As long as there is sunlight, no matter how weak, a PV solar cell will never “run out” of electrons; they’ll always be buzzing around the circuit, completing it over and over and over again.

Types of photovoltaic technology

The solar PV panel is the main building block of a PV system. While these systems all tend to look very similar, the PV technology at the heart of these panels can vary. These include:

  • Monocrystalline silicon photovoltaic panels: Monocrystalline panels are made by using cells taken from a single cylindrical crystal of silicon. This is currently the most efficient type of mature PV technology (we’re not counting PV technologies still under research, such as organic PV) and typically converts around 15 % of the sun’s energy into electricity. However, the manufacturing process needed to produce monocrystalline silicon PV cells and panels is quite complex, thus resulting in a slightly higher cost.
  • Polycrystalline silicon photovoltaic panels: Polycrystalline silicon PV panels, also known as multi-crystalline cells, are made up of cells cut from an ingot of melted and recrystallized silicon. These ingots are then cut into extremely thin wafers and built into complete cells. Polycrystalline cells are usually cheaper to produce than monocrystalline cells due to the much simpler manufacturing process. However, this comes at the cost of efficiency which sits at around the 12 % mark.
  • Amorphous (“thin-film”) silicon photovoltaic panels: Amorphous silicon cells are made when silicon is deposited in a thin homogenous layer onto a substrate. Because this type of silicon absorbs light more effectively than crystalline silicon, the cells can be made much thinner. Amorphous silicon can be deposited onto both rigid and flexible substrates, making it ideal for curved surfaces or bonding onto roof materials directly. Although it absorbs light more efficiently, the actual efficiency of the cell is much lower than crystalline silicon, typically sitting at around the six percent mark. However, amorphous PV cells tend to be easier and cheaper to make.

Applications of photovoltaic technology

Today, energy generated by solar PV cells serves people living in some of the world’s most isolated places, as well as those living in big cities, to pump water, keep the lights on, charge batteries, supply the grid with electricity, and more. It doesn’t matter who you are, where you are, or what you do, PV technology will have already touched your life in ways that you might not know. These are some of the most important applications of PV technology:

  • Stand-alone power applications: In urban areas, PV technology can be used to power everything from standalone devices and tools to entire homes and communities, including infrastructures like traffic lights, radio transmitters, and water pumps. For the most remote and rural locations, running power line extensions is not always convenient or cost-effective. And in some cases, it’s simply not possible. Here, PV technology is the solution.
  • Power in outer space: From the very beginning, high-efficiency PV technology has been the primary power source for space applications like the International Space Station, Earth-orbiting satellites, and surface rovers such as those on Mars and the Moon.
  • Transportation: PV technology can be used to provide auxiliary power for electrified vehicles such as cars, boats, and even aircraft. Some automobiles even come with PV cells fitted to the sunroof to power so that the vehicle is provided with a source of power while on the move, i.e., by trickle-charging batteries. Many electric vehicle charging points are also powered by solar, either entirely or partly.
  • Solar farms: When deployed at scale over several acres, PV panels can provide utility-scale amounts of power, producing amounts well into the gigawatts. These large-scale systems tend to use fixed or sun-tracking panels which follow the sun as it moves across the sky, feeding power directly into municipalities and regional grids.

Advantages of photovoltaic technology

Especially with regard to the environment and availability, PV technology offers numerous advantages:

A source of clean energy

By far the biggest advantage of PV technology is that it enables the production of clean energy, and in a world plagued with climate problems and lofty carbon neutrality targets, this advantage is king. Unlike traditional sources of energy, PV solar panels don’t emit harmful greenhouse gases and other pollutants when they create electricity. They also don’t deplete natural resources. This helps to protect the environment and help the Earth’s natural resources last longer.

Quiet and unobtrusive

PV technology and the solar cells they power produce zero noise while they’re generating electricity. This is a key distinction from other energy generation methods, i.e., backyard generators which produce lots of noise. Not only is PV technology quiet, but it’s also unobtrusive. As PV technology has become more advanced, solar panels have become smaller, flexible, and more discreet.

Available anywhere

There’s no place on Earth where the sun doesn’t shine; solar radiation is available anywhere. So, regardless of location, the sun’s light can be used to generate electricity anywhere in a decentralized way. And since the sun produces huge amounts of energy, there are zero scarcity concerns like those that exist with other energy sources like fossil fuels and wind.

Disadvantages of photovoltaic technology

In spite of the convincing advantages, there are also some disadvantages of PV technology that need to be considered:

It can be location-dependent

The term “location, location, location” doesn’t just apply to houses, it’s important for solar panels, too. The availability of solar radiation can vary dramatically depending on where in the world you are. A solar panel in Scotland, for example, is going to have much less exposure to strong sunlight than those based in California.

Solar energy is more expensive

The energy generated by solar PV panels is somewhat more expensive to produce than conventional sources of energy like fossil fuels. This is mostly due to the cost of manufacturing photovoltaic cells and the conversion efficiencies in the systems themselves, which can vary depending on the PV technology being used. As conversion efficiencies continue to increase and manufacturing costs fall with further research, however, PV technology is coming much more cost-competitive when compared with conventional energy sources.

Photovoltaic technology is at the mercy of the sun

All renewable energy sources—solar, wind, tidal—are variable, and energy production is entirely dependent on weather conditions. This means that photovoltaic cells might have days where almost nothing is produced, and this could lead to an energy shortage if too much of a region’s power is reliant on renewables.

The future of photovoltaic technology

Despite the challenges facing PV technologies, it’s clear that it has the potential to be a gamechanger when it comes to meeting our carbon-zero goals. According to recent studies, accelerated solar PV deployment could deliver 21 % of the CO2 emission reductions (nearly 4.9 gigatonnes annually) by 2050, and solar PV could meet a quarter of the world’s electricity needs by 2050, becoming the second biggest generation source after wind. However, global capacity must reach almost 20 times current levels (more than 8,000 gigawatts) to achieve this.

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