Though solar energy provides a sliver of the world’s electricity now, it is on a trajectory to expand rapidly. Solar power installations are surging globally and in the U.S. as this method to generate renewable electricity becomes cost competitive. Meanwhile, to solve the sustainability problems of oil- and gas-derived fuels, researchers are inventing methods to make liquid fuels from sunlight, water, and waste gases like carbon dioxide.
Solar power capacity and cost
Globally, solar power capacity reached 623 gigawatts in 2019, more than one-sixth of which was installed that year. That capacity represents the solar installations' power rating under ideal conditions. The amount of power actually delivered is much lower, as the amount of electricity solar panels produce depends on the length of the day and its cloudiness, the angle at which light hits the panels, their temperature, and how much snow or dirt has accumulated on them.
Solar and wind projects now account for the majority of electrical capacity growth. China leads in solar power capacity and expansion, with its photovoltaic installations generating 3.9 percent of the country's energy for electricity. Australia, Germany, and Japan have the most solar power capacity per capita.
The U.S. has a solar capacity of about 97 gigawatts, which is equivalent to the average power usage of 17 million American homes. The U.S. saw a million solar PV installations in 2016 and 2 million in 2019. That year, solar technologies produced about 2.6 percent of electricity generated in the U.S., up from a fraction of a percent five years earlier.
Solar cells connected together in photovoltaic modules (or solar panels) are the main mode of producing power with sunlight. In each cell, a material that generates an electric charge when hit by sunlight, typically silicon, is sandwiched inside weatherproof layers. Electronics connect cells and panels together into arrays and route the electricity to an inverter, which changes it from DC to AC so that it is ready to use.
Solar photovoltaics now perform well in a variety of materials and forms, thanks in part to Caltech researchers. Photovoltaics may be thick and stiff or thin and pliable, single-sided or double-sided. Arrays may be raised on metal supports or configured as solar shingles and used as roofing material.
In a less-common method called concentrated solar power or solar thermal, mirrors focus sunlight on a heat-absorbing target that typically contains a fluid. The mirrors can be trough- or dish-shaped or flat, and some track the sun. The heated fluid is used to generate electricity via a heat exchanger and a steam-powered turbine. While solar panels stop producing power at sunset, some of these systems stay online longer. That is because the liquid retains heat and can also be heated by natural gas. In fact, plants also often start heating the water for the steam cycle with natural gas before sunrise so that the system is fully operational at dawn.
Who produces solar power?
In the U.S., utility companies account for more than half of the solar power capacity and growth. Solar installations on homes are expected to increase as costs fall, new policies take effect, and people invest in combined solar and storage systems to defend against outages and shut-offs during extreme weather. People who cannot or would rather not install their own solar arrays have bought or leased panels in 1,200 community solar power projects in 40 states, receiving financial credit for the electricity their panels generate.
Challenges and anticipated solutions
Photovoltaic panels stop generating electricity when the sun is down, so utility companies typically use fossil-fuel-derived power to meet consumer demand. Renewable power sources can also complement solar. And both utilities and rooftop-solar buyers increasingly purchase battery storage that extends the time when solar energy can be used, an active area of Caltech research. Another ambitious option that Caltech is exploring is to avoid sunsets entirely by generating solar power in space and beaming it to antennas on Earth.
Non-hardware costs for solar rooftop installations are decreasing more slowly than desired. Buyers may be put off by unnecessarily costly, complex, and variable permitting processes, connections to grids, and utility pricing. Promising options for streamlining include apps and better federal guidance and policies.
Finally, while solar panels last for decades, they will eventually need to be replaced. In anticipation, researchers are exploring methods to recycle, reuse, and resell them.
Perhaps the greatest challenge in solar energy is what photovoltaics cannot do. They cannot make fuel. The amount of sunlight that beams to Earth could more than supply all of humanity's energy if we knew how to convert the energy in sunlight into liquid fuels, like gas for cars. Plants, plankton, and algae can do it; they produce the fuel they need to grow from sunlight, water, and carbon dioxide.
Chemists, engineers, physicists, and materials scientists have collaborated for years to develop artificial photosynthesis, a process that would harness the energy of sunlight to make liquid fuels from water and waste gases. To make it a reality, these researchers are developing new catalysts, which speed up chemical reactions; new materials to carry electrical charges during those reactions; and new strategies to streamline the conversion process. Caltech has had a leading role in this research, first through its Joint Center for Artificial Photosynthesis and now through its successor, the Liquid Sunlight Alliance.
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