If you grew up in the 80s or 90s, there’s a good chance you used a solar-powered calculator. They were ubiquitous, though solar was used for little else at the time. A lot of kinks needed to be worked out before solar power could be effectively used for more than basic arithmetic. Only now is the technology catching up to sunlight’s vast potential to power our world.

Before we dig into the story of solar panels, let’s cover some basics: solar cells — also called photovoltaic cells — consist of a semiconductor material (usually silicon) topped with a grid of fine metal lines and sealed with glass. The semiconductor in a solar cell is mixed with minerals that give its top layer a positive charge. When sunlight hits the cell, negatively charged electrons are knocked loose and attracted to the positively charged top layer, where they encounter the metal conductor strips. The electrons move through the conductor strips as an electrical current that can be transported along wires or stored in batteries.

Regular silicon solar cells are known as “single-junction cells.” Researchers are currently exploring ways to layer multiple semiconductor materials in what are known as “multi-junction cells,” which increase efficiency by capturing different parts of the light spectrum. Another emerging technology is thin-film solar, in which a coating of photovoltaic semiconductor is deposited on regular glass, no silicon required.

Scientists started trying to harness the sun’s energy nearly 150 years ago.

Your solar-powered calculator was likely powered by three photovoltaic cells. Cells like these (but in a larger, standard size of 15.6 centimetres squared) are assembled to create a solar panel or “module,” which can power more than a calculator. A “solar array” — sometimes confusingly called a “solar system,” like the one we share with Mars — is made of many linked solar panels.

Those large-scale arrays were a long time coming. Scientists started trying to harness the sun’s energy nearly 150 years ago. American inventor Charles Fritts made the first solar cells in 1883, but they were only able to convert less than 1% of the light that hit them into electricity. It took until 1954 for scientists at Bell Labs to create the first commercially viable solar panel. It was 6% efficient.

A question of efficiency

If 6% efficiency doesn’t sound like much, that’s because it isn’t — which is the main reason solar panels weren’t immediately adopted as a major energy source.

It took decades for scientists to build the efficiency of solar cells up to where it currently sits, at around 20%. Silicon has a natural efficiency ceiling — known as the Shockley-Queisser limit — of around 34%. This means it can only absorb that percentage of the sunlight that hits it, with the rest either reflecting out or passing through the cells. This ceiling is why the development of multi-junction technology is so important.

A 100-kilowatt system installed under the Carter administration was a big deal at the time. Now you’re talking about hundreds of thousands of kilowatts.

John Rogers, a senior energy analyst with the Union of Concerned Scientists (UCS) — a charity dedicated to solving the planet’s pressing problems with science — illustrates how the tech has improved over the decades. “There’s a 100-kilowatt solar system that was installed under the Carter administration, that was a big deal at the time,” he says. “Now you’re talking about projects of hundreds of megawatts, so hundreds of thousands of kilowatts.”

And the rate of improvement has been speeding up. In 2010, the Sarnia Photovoltaic Power Plant in Ontario was said to be the largest solar farm in the world, with a capacity of 97 megawatts. Just over 10 years later, the solar farm with the largest capacity is likely in India’s Bhadla Solar Park which has a capacity of 2,250 megawatts.

It takes energy to save energy

Solar panels’ environmental shine comes from their potential to reduce our dependence on greenhouse-gas-emitting energy sources. But before they can make green power, they need to be made themselves. And that process has environmental impacts that can’t be ignored.

Photovoltaic technology is evolving quickly, and solar cells of all kinds are complex, so we can’t delve deeply into all possible manufacturing impacts here. (A 2020 literature review of solar panel life cycle analyses runs to 39 pages.) But we can hit the highlights.

By far the largest impact comes from the energy-intensive manufacture of silicon. Depending on where the process is taking place — and most solar cells are currently made in China — silicon production can have a large carbon footprint. Life cycle analyses of solar cells often report on “energy payback time” (EPBT), the time it takes for a cell to generate the quantity of energy used to manufacture and ultimately dispose of it. The review mentioned above found that silicon-based photovoltaics can have EPBTs ranging from 10 months up to 6 years. Factors affecting EPBT include not only the efficiency of the solar unit, but also the amount of sunlight it has the opportunity to absorb. Lower insolation leads to longer payback times.

A 2011 spill at a Chinese solar factory killed dozens of pigs that were cleaned with contaminated river water.

Newer solar technologies are less energy-intensive than silicon-based ones, but silicon is still the dominant player (making up 95% of the market by some estimates). Newer technologies come with other challenges. The major thin-film options use cadmium, which is a carcinogen and can cause heritable mutations. Workers making these cells are at risk of exposure, and toxic leaching is a risk when thin-films are disposed of.

Silicon-based cells also pose a toxicity risk, as their production requires hazardous chemicals. A 2011 hydrofluoric acid spill at a Chinese solar factory killed dozens of pigs that were cleaned with contaminated river water. It’s easy to imagine much worse outcomes. Mitigation of these toxic risks is possible, but requires proper environmental regulation and oversight, which will be particularly important as the solar energy market grows.

Newer solar technologies’ reliance on precious metals like gold, silver, and platinum is also a concern the literature review describes as “non-negligible.” The authors state that it’s “important to consider the real sustainability of a scenario of large-scale diffusion of these devices.” The impacts of mining, and/or potential scarcity of raw materials, make recycling at end-of-life critical to keeping solar technology as sustainable as possible.

Recycling matters

Disposal is a major pain point with solar panels, at least in North America. Most panels have a lifespan of around 20–30 years, which means that the first wave of widely available solar panels is at or nearing the end of their use.

The main components of solar panels — metal and glass — are eminently recyclable. It’s the other materials you need to watch out for, the UCS’ Rogers says. Most silicon cells contain lead; cadmium, as mentioned above, is carcinogenic. And separating the elements in thin-film CIGS cells (made by combining copper and selenide with the rare elements indium and gallium) is a complex undertaking.

Solar can fit in the cracks. Whether on a landfill, or the edge of farmland, or other industrial sites, there are opportunities.

Putting the onus on companies to provide recycling options for the products they sell isn’t something we’re used to doing in North America, but Rogers hopes recycling initiatives will develop as more people adopt solar power. In contrast, the EU created a law mandating all electric and electronic equipment manufacturers take responsibility for recycling their own products, including solar panels. The result is a not-for-profit association, PV Cycle, dedicated to doing just that in countries across the EU. Australia, Japan, and India are working on similar initiatives.

Currently, Canada has no facilities capable of recycling solar modules; any companies that do offer solar-panel recycling ship the modules elsewhere, primarily the US or Malaysia. As a country, Canada’s energy use is only 1% solar power, which works out to about 700 metric tonnes of photovoltaic cells in need of recycling. A 2016 study by the International Renewable Energy Agency projected that North America could create up to 1,110,000 metric tonnes of photovoltaic cell waste by 2030, and potentially over 12 million tonnes by 2050. Clearly, photovoltaics recycling will need to be ramped up before then.

Facing the future

The International Energy Agency estimates that the amount of solar power generated globally almost quadrupled between 2014 (190 terawatt-hours) and 2019 (720 TWh), in no small part due to the technology’s plummeting cost. Rogers is excited about the possibilities.

“Solar is modular, so it can sort of fit in the cracks, in between other stuff,” he says. “Whether it’s on a landfill in my community, or on the edge of farmland, or low quality land, or in other industrial sites, there are opportunities.”

In addition, he mentions the concept of “floatovoltaics,” or floating solar panels, as being of particular interest. Such arrays already exist throughout Japan and China, on inland lakes and particularly on reservoirs. The water has a cooling effect on the panels that can enhance efficiency in hot climates, and the shade they create can reduce evaporation and reduce destructive algae blooms in the water below.

With the US, Canada, and the EU all committing to pandemic recovery plans that emphasize climate action, it’s clear much remains to be told in the story of solar panels.