# Helping Solar Cells Keep Their Cool

A new transparent coating helps solar panels stay cool, increasing their production.

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The great paradox of photovoltaic solar power is the fact that PV panels work best in direct sunlight, but the solar heat actually reduces their efficiency. PV thermal (PVT) systems provide a nice method of capturing heat energy and cooling panels, but PVT requires plumbing and a water pump, so it’s better suited for rooftop PV rather than utility-scale PV farms.

Engineers at Stanford University recently developed a transparent silica  film which, when applied to the front of a PV panel, absorbs and radiates heat while passing all visible light to the PV cell. The result is a temperature reduction of up to 13oC.

Left to right: Linxiao Zhu, Shanhui Fan, and Aaswath Raman. Image courtesy of Stanford University

How does the temperature reduction translate into efficiency? A PV panel has a negative temperature coefficient with respect to its voltage. A typical value is -0.08V/oC. A PV panel that’s 13oC cooler will produce about 1V more than it would have without the cooling. Since power is the product of voltage and current, and current is minimally affected by changes in temperature, we can say that the power output is proportional to the voltage.

If we start with a 75W panel whose original efficiency is 20%, the coated panel will generate about 80W under the same lighting conditions. That brings its efficiency up to 21%. Going from 20% to 21% efficiency may seem insignificant at face value, but … you knew this was coming … let’s do the math.

A March 2015 study by the International Energy Agency says that there is 177 GW of PV installed worldwide. For the sake of argument, let’s say those panels are 20% efficient – that’s our baseline. That means that 885 GW of solar irradiance reaches those panels. (20% of 885 = 177.) If the panels were 21% efficient, they would generate almost 186 GW. That’s an increase of 9 GW.

At the time of this writing, the largest PV farm in the world is the Solar Star in California, with an installed capacity of almost 600 MW, or 0.6 GW. If all existing PV panels had the Stanford coating, it would be equivalent to adding fifteen Solar Star PV farms. Obviously we wouldn’t add the coating to existing panels, but since PV capacity is increasing rapidly, future PV farms could be much more productive if the coating were used. Moreover, a reduction in cell temperature increases the cell’s longevity, giving each solar panel a longer productive life.

Stanford researchers believe that a manufacturing method such as nanoprint lithography could be used to apply the coating to new PV panels.