RENEWABLE ENERGY

Solar power has lots of headroom to make it stronger, cheaper and global

First of a two-part series. Read part two here.

On paper, the amount of sunlight hitting the Earth in one hour could fulfill humanity's energy needs for a year, an equation that's hard to ignore.

However, harnessing more of this vast potential remains a major challenge, since the sun has to compete with coal, oil and natural gas -- fuels that are abundant and cheap and retain substantial political support in many parts of the world, including the United States.

Over the past few years, solar energy technologies, particularly photovoltaics, have rapidly closed the cost gap with fossil fuels and are being rushed into deployment. Yet solar panels still need to do much better if they're going to take a meaningful bite out of greenhouse gas emissions.

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Total solar capacity in the world is on track to hit 200 gigawatts this year. But that's just over 1 percent of global electricity production, a drop in a massive bucket.

Though policy and market forces drive the spread of all forms of energy, for solar power, improving the technology remains an open frontier. To this end, researchers are looking at where they can improve the devices that turns the sun's rays into electricity in hopes of staving off diminishing returns in performance.

Sarah Kurtz, the photovoltaic program manager at the National Renewable Energy Laboratory, explained that a conventional solar panel consists of three main components: a coating (usually glass), a semiconductor (usually silicon) and a backing layer.

When a unit of sunlight, a photon, strikes the semiconductor, it excites an electron in the material, harnessing the photoelectric effect, a phenomenon whose discovery led to Albert Einstein's Nobel Prize in 1922.

The excited electron generates a current that can then power a light bulb, run a refrigerator or charge a battery. However, not every part of the light spectrum goes to work to excite electrons, and out of the excited electrons, not all have enough energy to generate a current.

This leads to an inherent limitation in a single-junction solar cell's efficiency of about 33 percent, a value known as the Shockley-Queisser limit.

Most of the solar panels on the market still fall far short of this barrier, so there is room for improvement in efficiency. Higher efficiency means a panel of a given size can produce more electricity for the same space.

Besides bringing efficiency up, the other main research thrust is drawing prices down. The biggest driver for solar energy deployment is cost, according to many analysts, and that's what drove the sector's blossoming over the past few years.

"The industry has achieved a hundredfold decrease in prices," Kurtz said. "There is opportunity to bring those prices down even more."

The race to lighter, thinner and more concentrated

Much of this price drop has come from economies of scale and a drop in prices of semiconductors. "In general, the semiconductor is the most expensive part of the module," she said.

The majority of the world's solar panels use polycrystalline silicon as their semiconductor. For a long time, solar panels using this material were costly, until Chinese manufacturers flooded the market around 2010.

This led to a precipitous price drop, undercutting other approaches to solar but also creating a worldwide solar energy boom.

Earlier this year, researchers at the Massachusetts Institute of Technology published a 322-page study titled "The Future of Solar Energy." Among its key findings, authors reported that silicon photovoltaics are a mature technology but they have intrinsic performance limits, so public research dollars should go toward breakthrough technologies instead of incremental gains.

One strategy is to use less of the semiconductor, applying it instead as a thin film that ranges from microns to nanometers in thickness. Materials include cadmium telluride and copper indium gallium diselenide.

A thinner semiconductor also means a thinner backing layer since it has to support less weight. Less weight in a cell means lighter panels, reducing installation costs and mounting hardware requirements.

The tradeoff with this approach is that thin film cells tended to be less efficient than their crystalline silicon brethren, though they are closing the gap, with laboratory tests showing thin film cells with efficiencies approaching 20 percent.

Another approach is to use a concentrator, a lens or mirror that focuses sunlight onto the cell. More intense sunlight means more electricity from a solar generator. With focused light, you can shrink the size of the cell, thereby reducing materials costs.

If you concentrate light enough, more efficient and more expensive semiconductors can become viable. Multijunction cells, for example, stack several cells together, each tuned to a different part of the solar spectrum.

This circumvents the Shockley-Queisser limit, joining the suite of technologies that comprise third-generation photovoltaics. Commercial multijunction cells can reach efficiencies around 35 percent, while labs have set laboratory efficiency records topping 50 percent (ClimateWire, Oct. 20, 2014).

The drawback is that the panels have to track the sun as it moves through the sky during the day to keep light concentrated on the cells. Trackers add cost and complexity to the system, but researchers are working on ways around this (ClimateWire, Feb. 12).

A market that can handle more strategies

"This technology is a little bit easier to plan on," Kurtz said. "It's more of an engineering project than a fundamental chemistry project."

Scientists are also experimenting with a mineral structure called perovskite. The substance is cheap and absorbs light in the ultraviolet part of the spectrum, so it pairs nicely with silicon (ClimateWire, March 25). However, perovskites have issues with durability, which is a big concern for solar panels that have to remain exposed to the elements for decades.

Probing the fundamental physics behind photovoltaics may also yield a better way to produce an electric current. Experiments show that tweaking the structure of a semiconductor at nanometer scales can produce a material that can move more than one electron for a given photon (ClimateWire, April 19, 2013). Solar panels that exploit this effect are at least a decade away, scientists say, but this approach could lead to higher efficiencies for similar costs in materials.

However, these technologies would struggle to dethrone the reigning king, silicon. Kurtz explained that the global supply chain around silicon photovoltaics gives it a huge edge compared with upstart chemistries. And it's still getting better.

"I think that silicon still has a lot of headroom," she said.

Still, panel technology can only go so far. Solar energy, like wind power, suffers from intermittency as clouds obscure the sun. These problems will only get more severe as solar makes up a greater share of the generation mix, requiring ancillary systems like gas turbines or batteries to fill the void.

Meanwhile, as technology costs decline, installation, regulations and financing costs are making up a greater share of the price tag for solar energy.

"Efficiency is helpful because any gain in efficiency helps reduce all area-related costs (glass, racking, roof/land space, installation labor, etc.) but it is not the only way of addressing costs," said Justin Baca, senior director of research for the Solar Energy Industries Association, in an email. "The efficiency question is a science question, but most solutions to solar challenges right now are engineering and regulatory solutions."

In the competition for the next generation of clean energy, Kurtz said it's not a winner-take-all race. The market may still have room for all of these strategies. "These different photovoltaic products also have different characteristics," she said. "I think it's a mistake to focus on the winners and losers."

Tomorrow: What the solar market wants.

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