The amount of solar energy that the sun provides in an hour can power the entire Earth for a year. Known formally as solar photovoltaic (PV) energy, solar power has become the most popular form of clean energy production, and is still growing at an incredible rate. Harnessing that energy, however, is difficult. While solar panels provide clean energy, they’re slower, take up more space, and are expensive to invest in when compared to nonrenewable energy production such as burning fossil fuels. Spearheading this solar power transformation is China, whose efforts to popularize clean energy have resulted in a total of over 43 gigawatts of solar capacity in 2016 and a projected 400 gigawatts by 2030. The country’s dedication to solar power is due largely to its role in the Paris Climate Agreement, in which China agreed to peak its carbon dioxide emissions and to produce 20% of its total energy production from renewable energy sources by 2030.

While solar energy is sustainable, it does have its drawbacks.

While solar energy is sustainable, it does have its drawbacks. The expense and uptake of space of solar panels compared to their relatively low energy output, when pitted against the cheap, high energy output of fossil fuels serve as deterrents to homeowners, large companies, and government officials when weighing the benefits and costs of investing in solar energy. Furthermore, while solar panels may have a certain potential output, they almost never hit that mark since they are dependent on weather conditions. The fact that the sun does not shine at night, clouds, and a sheer lack of sunlight on any particular day can significantly reduce the efficiency of PV cells. In China, where solar panels are starting to make a significant dent in the use of nonrenewable energy sources, strategic utilization of such PV resources can mean a net gain of several gigawatts of energy, enough to power millions of homes for a year. Solar panels need to be installed in such a way as to maximize their solar energy output, and Professor of Environmental Engineering and International Affairs Denise Mauzerall and her research team have quantitatively discovered another factor that can reduce its PV production: air pollution.

On average, photovoltaic generation is cut by more than 20% in China due to air pollution.

Though China has started the transition to clean energy sources, its dependence on fossil fuels and other nonrenewable energy is still high. This contributes to the already dangerously high level of air pollution, especially in its northern and eastern provinces that are the most polluted. For reference, in 2015 the level of PM2.5, an air particle known to cause cancer and heart disease, in the Chinese city of Shenyang had reached an alarming 1,400 micrograms per cubic meter; 25 micrograms per cubic meter is the threshold for air to be considered safe, according to the World Health Organization. These aerosols do more than just health-related harm, however: according to Mauzerall and her team, the air pollution in China, particularly in the polluted north-east, reduces the efficiency of PV electricity generation. By utilizing satellite measurements of the aerosol optical depth (AOD) of pollutants in China’s atmosphere at different locations and gauging the average point-of-array irradiance (POAI), which is a measure of the amount of sunlight hitting a surface area, between 2003 and 2014, Mauzerall and her team were able to determine the effects of smog on solar panel efficiency. They found that, on average, PV generation is cut by more than 20% in China due to air pollution, with the most polluted areas such as provinces in northern and eastern China experiencing up to 35% reduction.


A single-axis horizontal-tracking solar panel. Panels like these can maximize sun exposure and potentially minimize the effects of China's heavy pollution.

Photo by: Instructables

The findings have some important implications when it comes to strategically placing solar panels and which types of solar panels to use. For instance, in the areas of China least affected by air pollution, such as the western and very northern parts, it is advantageous to invest in one-axis horizontal tracking (One-T) solar panels, which follow the sun’s movement from day to night to maximize PV generation. Two-axis tracking panels, which allow movement in two directions to track the sun diurnally and seasonally, improves PV generation even more, by up to 40%, when compared to solar panels that are fixed. In the more polluted southern and eastern parts of China, however, the improvement in PV generation by investing in One-T and Two-T panels is less than 10%, since tracking panels rely more on perpendicular solar radiation, called direct radiation, than fixed panels do. Aerosols significantly reduce the amount of direct radiation by deflecting sunlight, and thus reduce the efficiency of tracking panels more than they do fixed panels.

When it comes to renewable energy, solar power is dominating the sector. Its dependence solely on our sun, a literal ball of energy that provides all the heat and light necessary to fuel life, makes it convenient and accessible almost anywhere on the planet. PV generation, however, is heavily dependent on factors like weather and possible obstructions such as aerosols. However, as Mauzerall points out, “There is the potential for a virtuous cycle. Improving air quality in China will increase the efficiency of solar PV generation. This will result in a positive feedback with increased PV efficiency and PV deployment leading to further reductions in air pollutant emissions as solar electricity replaces more polluting energy sources.” Mauzerall and her research team are leading the scientific community in the strategic planning of where to site PV, which could potentially improve the efficiency and appeal of solar panels and generate enough extra clean energy to power millions of homes.

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Victor Hua