Solar energy is crucial to many futures. On the micro level, there's a booming solar industry in America and across the globe. Since Congress passed a tax credit in 2006, the Solar Energy Industry Association (SEIA) says that the industry has been averaging an annual growth rate of 50 percent in the last decade. In most fields, that would be macro news. But solar energy has a mission beyond making money-it's supposed to save the planet.
There's no plan to prevent man-made global warming from permanently warping the Earth's climate without solar panels and the energy they can convert. "The role of renewable energy solutions in mitigating climate change is proven," says the United Nations Development Program. Some in the industry think that solar will grow 6,500 percent as an industry by 2050 in order to mitigate that need.
But for all their importance, solar panels still feel mysterious. Stiff and slightly menacing black rectangles, they have neither the look or the feel of a savior. Majestic waterfalls and dams look heroic, but solar panels do not. So...how do they work anyway?
A Brief History
Work in solar energy started in 1839, when a young French physicist named Edmond Becquerel discovered what is now known as the photovoltaic effect. Becquerel was working in the family business-his father, Antoine Becquerel, was a well-known French scientist who was increasingly interested in electricity. Edmond was interested in how light functioned, and when he was just 19 their two interests met-he discovered that electricity could be produced through sunlight.
The years went on and the technology made small but steady steps. During the 1940s, scientists like Maria Telkes experimented with using sodium sulphates to store energy from the sun to create the Dover Sun House. When investigating semiconductors, the engineer Russell Shoemaker Ochs examined a cracked silicon sample and noticed that it was conducting electricity despite the crack.
But the biggest leap came on April 25, 1954, when chemist Calvin Fuller, physicist Gerald Pearson, and engineer Daryl Chapin revealed that they had built the first practical silicon solar cell.
Like Ochs, the trio worked for Bell Labs and had taken on the challenge of creating that balance before. Chapin had been trying to create power sources for remote telephones in deserts, were regular batteries would dry up. Pearson and Fuller were working on controlling the properties of semiconductors, which would later be used to power computers. Aware of each others work, the three decided to collaborate.
These earliest solar cells were "basically hand-assembled devices," says Robert Margolis, senior energy analyst at the National Renewable Energy Laboratory (NREL).
How Do Solar Panels Work?
To understand how silicon solar panels make electricity requires shrinking down to the atomic level. Silicon has an atomic number of 14, which means that it has 14 protons in its center and 14 electrons circling that center. Using the classic imagery of atomic circles, there are three circles moving around the center. The innermost circle is full with two electrons, and the middle circle is full with eight. However, the outermost circle, which holds four electrons, is half-full. That means it will always look to fill itself up with help from nearby atoms. When they connect, they form what is called a crystalline structure.
With all those electrons reaching out and connecting to each other, there isn't much room for an electric current to move. That's why the silicon found in solar panels is impure, mixed in with another element, like phosphorous. The outermost circle of phosphorous has five electrons. That fifth electron becomes what is known as a "free carrier," able to carry an electrical current without much prodding. Scientists boost the number of free carriers by adding impurities in a process called doping. The result is what's known as N-type silicon.
N-type silicon is what's on the surface of a solar panel. Below that resides its mirror opposite-P-type silicon. Whereas N-type silicon has one extra electron, P-type uses impurities from elements like gallium or boron, that have one less electron. That creates another imbalance, and when sunlight hits the P-type, the electrons starts to move to fill the voids in each other. A balancing act that repeats itself over and over again, generating electricity.
What Makes Up a Solar Panel?
Solar cells are made out of silicon wafers. These are made out of the element silicon, a hard and brittle crystalline solid that is the second most abundant element in the Earth's crust after oxygen. If you're at the beach and see shiny black specks in the sand, that's silicon. As Ochs discovered, it naturally converts sunlight into electricity.
Like other crystals, silicon can be grown. Scientists, like the ones at Bell Labs, grow silicon in a tube as a single, uniform crystal, unrolling the tube, and cutting the resulting sheet up into what are known as wafers.
"Visualize a round stick," says Vikram Aggarwal, the founder and CEO of EnergySage, a comparison-shopping marketplace for solar panels. That stick is cut like a "pepperoni, a roll of salami cut thin for sandwiches-they shave them very thinly," he says. That’s where it has historically been very difficult-either too thick, a waste, or too thin, making them not precise and prone to cracking."
They try to make these wafers as skinny as possible, to get as much value out of their crystal as possible. This type of solar cell is made out of mono-crystalline silicon.
While the first solar cells resemble today's cells in terms of look, there are a number of differences. Back at Bell Labs, the initial hopes was that solar cells would be good for the coming space race, Margolis says, so there was a premium on keeping weight down. The photovoltaic cells, as they came to be known, were put into a lightweight encapsulate.
And it worked. Just four years after the first working solar cell was developed, on March 17 1958, the Naval Research Laboratory built and launched Vanguard 1, the world's first solar-powered satellite.
Solar Panels Today
Nowadays, photovoltaic cells are mass-produced and cut by lasers with greater accuracy than any scientist at Bell Labs could have imagined. While they're used in space, they've found far more purpose and value on Earth. So instead of putting an emphasis on weight, solar manufacturers now put an emphasis on strength and durability. Goodbye lightweight encapsulate, hello glass that can withstand the weather.
One of the main focuses on any solar manufacturer is efficiency-how much of the sunlight that falls on every square meter of the solar panel can be converted into electricity. It's "a basic math problem" that lies at the center of all solar production, says Aggarwal. Here, efficiency means how much of the sunlight can be properly converted through P and N-type silicon.
"Lets say you have 100 square feet available on your roof," he says in a hypothetical. "In this limited space, if panels are 10 percent efficient, its less than 20 percent. Efficiency means how many electrons they can produce per square inch of silicon wafers. The more efficient they are, the more economics they can deliver."
Around a decade ago, Margolis says, solar efficiency was hovering around 13 percent. In 2019, solar efficiency has risen to 20 percent. There's a clear upward trend, but one that says Margolis has a limit with silicon.
Due to the nature of silicon as an element, solar panels have an upper limit of 29 percent. So...where do we go from here?
The Future of Solar
Some scientists are working on using new materials. There's a mineral known as perovskite that Aggarwal describes as "very exciting." First discovered in the Ural Mountains in western Russia, perovskite has raised eyebrows in testing-from 10 percent efficiency in 2012 to 20 percent in 2014. It can be made artificially with common industrial metals, making it easier to find, and it uses a simpler process than the balancing dance of P and N type silicon to conduct electricity.
But both Aggarwal and Margolis caution that it the technology is still in its earliest phases. "Efficiency in the lab has gone up rapidly, but there’s a difference between the lab and the real world," Margolis says. While perovskite has shown great progress in clean environments, it has shown rapid declines when introduced to elements like water, which it could encounter in daily use.
Rather than new materials, Margolis and his team are working on a concept he calls "solar plus." As solar energy use increases, there's a potential to improve how "solar interacts with other buildings as a whole," he says.
So imagine it's a brutally hot summer in the city. You go to an office for work, and then back home at night. It's hot and humid, so you turn on the air conditioner-and so does every other person in the city. The electrical grid becomes strained.
But Margolis imagines it could be possibly to store and utilize solar energy to lessen the strain. "Two hours before you come home, when the sun is still running, the AC could pre-run and get your house cool beforehand." The same applies during a cold winter, risking frozen pipes. "You can super heat your water during the heat day, and still use that hot water to clean your dishes or take a shower the next morning...we’re just at the beginning of thinking of how to integrate solar into our system."
Despite struggles facing solar domination like competition from natural gas and a political climate favors fossil fuels, Margolis is optimistic.
"We’re at this point where the utilities and the engineers are understanding that solar is getting big enough that we have to deal with it. They’re fun challenges."