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Physics, Sustainability

Floating photovoltaics: Do they reduce evaporation?

I saw this interesting article about installing floating photovoltaics on Lake Mead and Lake Powell in the U.S. as a means of generating power using renewables and reducing some of the significant evaporation from the two lakes.

It makes sense that the solar panels will generate electricity, but I’m curious about two of the claims: 1) how much will evaporation be affected by the panels, and 2) how will being on the water improve efficiency of the solar panels.

1) Evaporation.

Evaporation happens when water (or other liquids) is excited to the point that some of the molecules of water become gas or vapour. The vapour, being lighter, then travels upward where it then exists in the atmosphere. Higher temperatures result in higher rates of evaporation because the molecules have more energy. Wind also increases evaporation because it blows the water vapour away which makes it more likely for other molecules to change to gas.

So, the floating panels can reduce the amount of direct solar radiation contacting the water molecules. They also provide a surface for water that does evaporate to condense on allowing it to fall back into the water body rather than be taken into the atmosphere. The panels can also decrease the wind effects. This keeps the humidity directly above the water which reduces the amount of evaporation.

2) Increased productivity

The water will keep the solar panels cooler, this in turn will increase productivity compared to panels on land. But how does this work?

Solar panels use semiconductors. In a conductor resistance increases as temperature increases. This is because the electrons are more active and more likely to bump into each other (think about the difference between walking through a school hallway when it is empty, compared to walking through during a class change when there are lots of people in the hallway. But in a semiconductor which has greater resistance and lower conductivity most of the time actually decreases resistance as the temperature increases. This is because there are a few more electrons available to create the current. The current then increases but as a result the voltage decreases. The voltage is what is needed to create the usable electricity from the solar panels. So higher temperatures means decreased voltage and decreased power output. 

This is why floating solar panels are more efficient than the ones based on land. Water changes temperature more slowly which moderates the temperature of the solar panels, keeping them cooler and more efficient.

http://energy.gov/eere/energybasics/articles/solar-performance-and-efficiency

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About Peyto

I am passionate about making science, sustainability, and sport accessible through engaging information and activities. Peyto is a reference to Bill Peyto who was an outfitter, trapper, and eventually a park warden in Banff National Park. Peyto Lake and Peyto Glacier are both named after him. He is also a distant relative of mine.

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