Atmospheric Vapor Can Cool Solar Panels

Cooling Photovoltaics Inexpensively by Sucking Water from Air

Solar cell farm in power station for alternative energy from the sun. | Credit: Courtesy

Solar panels, typically made up of silicon cells, convert only about 20 percent of the sun’s energy that falls on them into electric current. Much of the rest turns into heat. It is this heat that, in part, limits the efficiency of the cells and degrades their effectiveness over time. This unwelcome heat can warm panels by as much as 40 degrees centigrade, causing a drop in output for every degree of temperature increase. For years, engineers have struggled to boost the conversion factor, with every 0.1 percent of improvement being significant.

Because of the damage heat does to cell efficiency, the cooling of panels has long been a goal. Raising panels 3-4 inches off the roof to create an air flow underneath has become the most prevalent strategy to dissipate heat. The main reason integrating solar cells into roofing products has not gone mainstream is due to the heat buildup and reduced efficiency by not having this surrounding air flow. For decades, researchers have tried to find cost-effective ways to cool panels with water, only to run into the challenges of cost, water availability, storage tanks, pipes, and pumps.

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In recent years, researchers have developed materials capable of absorbing water vapor from the air and condensing it into drinkable water. One of the best of these is a gel — a mix of carbon nanotubes and water-attracting calcium chloride salt — which causes the vapor to condense into droplets that the gel holds. It absorbs best at night when humidity is high and air is cool. During the day when the sun comes out and the heat rises, the gel releases the water vapor. If covered by a clean plastic dome, the released vapor condenses on the plastic and can be drained or channeled into storage containers.

This technology soon found another application, that of cooling solar panels. A sheet of gel less than ½ inch thick placed on the underside of a standard silicon cell panel pulls heat from the panel during the day by evaporating the water absorbed from the atmosphere the previous night. The evaporating water cools the panel just as evaporating sweat from our skin cools us down.

The results indicate that the temperature of a water-cooled panel falls around 10 degrees centigrade (18 degrees Fahrenheit), increasing the electricity output by between 15-20 percent. This translates into an overall increase in efficiency of 3-4 percent, a sizeable amount. Another variation being developed is to trap and re-condense the vapor after it cools the panel and to use it to clean accumulated dust from the surface of panels, thereby solving a second power-eroding problem. Solar electric generation is exploding and is expected to increase fivefold over the next decade. Integrating this inexpensive, atmospheric water-cooling component into panel design could further increase the speed and effectiveness of the expansion of this renewable power.


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