A Cold Winter’s Light: Photovoltaic Power Solutions for Winter

Reference: Kahl, A., Dujardin, J., & Lehning, M. (2019). The bright side of PV production in snow-covered mountains. Proceedings of the National Academy of Sciences, 116(4), 1162-1167. www.pnas.org/cgi/doi/10.1073/pnas.1720808116

The sun is the most plentiful and cheapest source of heat and fuel on earth. It’s always in the sky. Even on cloudy days, its light illuminates the earth’s surface. Unlike most other fuel sources, it doesn’t run out. There’s never a need to mine or drill for more sunlight. Using sunlight for energy doesn’t affect the sun in any way. All that sunlight is essentially there for the taking, all over the planet. Acquiring and using sunlight for electricity requires almost no pollution whatsoever.

Photovoltaic cells in winter. Source: US Department of Energy.

For these reasons, solar or photovoltaic power has been a lucrative source of electrical energy. As the technology behind photovoltaic cells and related equipment has improved and gotten cheaper, more and more individuals, organizations, cities, and countries have embraced photovoltaic power.

A lingering issue with photovoltaic power has been inefficient power generation during the winter months. This has to do with the earth’s axis (the tilting of the planet).

The Earth’s axial tilt during different seasons in each hemisphere. Source: NASA.

During the Northern Hemisphere winter (left picture), the Northern Hemisphere (top half of the earth) is tilted away from the sun, while the Southern Hemisphere (bottom half of the earth) is tilted toward the sun.

During the Southern Hemisphere winter, the opposite is true. This is why winter in the Northern Hemisphere coincides with summer in the Southern Hemisphere, and vice versa.

Imagine standing directly outside your window and looking through it at a light inside. As you turn away from the window, you’ll see less and less of the light within – even though the actual amount or intensity of the light itself hasn’t changed. That’s similar to what happens during the winter months. During each hemisphere’s winter time period, its tilt away from the sun leads to less sunlight reaching that area.

Less sunlight results in less photovoltaic energy generation during the winter. This supply shortage is compounded by increased energy demand during the winter months – from heating devices being on for longer time periods and from increased electric light usage due to fewer daylight hours. So, even with today’s relatively cheap and advanced photovoltaic technology, generating sufficient amounts of electricity from the sun to satisfy winter demand is problematic.

A paper recently published in the Proceedings of the National Academy of Sciences sheds light on methods that could be used to address this issue. The researchers focused on Switzerland, much of which has long, snowy winters, and high, snowy mountains.

Figure 2 from Kahl et al 2019. The higher the elevation, the more snow-covered days (SCDs) shown in Figure B. Figure A above shows the map outline of Switzerland with average SCDs. Copyright (2019) National Academy of Sciences.

Interestingly, the relationship between elevation and irradiance (the amount of sunlight in a given area) follows similar trends, as shown below.

Figure 1 from Kahl et al 2019. Irradiance increases with elevation (Figure B). Map of Switzerland showing average irradiance (Figure A). Copyright (2019) National Academy of Sciences.

So, higher elevation is correlated with more snow and more light. This sense because first, higher elevations generally have less cloud cover – which lets more sunlight reach the ground. Secondly, anything white reflects light. So snow cover acts as a giant mirror, reflecting sunlight.

Thus, one of the key findings of this paper was that during the winter, photovoltaic panels should be placed in areas with minimal cloud cover or lots of snow cover, or both. This would allow for increased photovoltaic electricity generation. In Switzerland, that translates particularly well to high, mountainous areas which raise above the cloud-covered valleys and are snowy much of the year.

Another finding was that the angle or tilt of the photovoltaic panel can significantly increase electricity production in the winter. This is due to the tilted panels catching more of the winter sun’s rays (remember the axis figures at the beginning of this post), and also to less snow accumulation compared to less tilted (flatter) panel installations. Also, the more tilted the photovoltaic panel, the less light-blocking snow accumulation there is on it.

The researchers compared power produced by the more common flat, roof-top photovoltaic panel installations (“urban”) with a hypothetical tilted installations in the mountains with snow (“mountain”) and no snow (“mountain no snow”) throughout the year.

Figure 5 from Kahl et al 2019. Average photovoltaic electricity (power) generation in six years in urban, mountain, and mountain without snow environments. Copyright (2019) National Academy of Sciences.

The red lines (urban) show much higher power production in the summer months and drop drastically during the winter. The green and blue mountain lines show higher power production in the winter months, with snowy mountain installations showing the highest winter power production.

Much of the results of this paper are based on modeling and is at the moment theoretical. Nonetheless, the relatively simple tweaks suggested by the researchers seem to have the potential to greatly increase winter photovoltaic electricity production. This would directly address a current shortcoming of this promising method.

Sometimes technological advances don’t require new technology – but improved use of already-existing technology.

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Munim Deen

Munim is an epidemiology doctoral student. His primary interests are infectious disease outbreaks and their intersection with the environment, public policy, and society at large. A geographic information system (GIS) devotee, he incorporates mapping and spatial analysis into his work whenever possible. A former newspaper columnist, he holds a bachelor's degree in microbiology and a master's degree in epidemiology.

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