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Name: Hilda
Status: student
Grade: 9-12
Location: Outside U.S.
Country: USA
Date: Summer 2012


Question:
I saw a science fair project on how when a photovoltaic cell is heated and cooled again, its efficiency drops. For example, the power of the photovoltaic cell at 30 degrees Celsius is 10 Watt, then it is heated to 40 degrees Celsius and cooled back to 30, and the power measured drops to 5 Watt. If the photovoltaic cell at 30 degrees Celsius is heated to 35 degrees Celsius and cooled back to 30, the power measured will be less than 10 but more than 5 Watts. What is the physics principle behind this?



Replies:
Hi Hilda,

I will come right out with it: The results being claimed by the science fair exhibit are clearly false! The claim that photovoltaic (PV) cells suffer a non-reversible, and very large permanent reduction of efficiency when heated to 30 or 40 degrees Celsius, then returned to room temperature, is completely false. If this were true, most PV arrays on rooftops and even those massive hectare-sized power-producing arrays in the US south, would be essentially useless.

A question arises, namely, exactly what type of photovoltaics are in question here? There are many types (single crystal, polycrystalline, thin film, bifacial, and so on), and almost a dozen different chemistries. All these have different characteristics and all behave differently, yet none suffer the type of extreme loss of efficiency you refer to.

To be sure, higher temperature somewhat reduces the power output of most types of PV cells. But this only starts to become significant at PV cell temperatures above 40 degrees Celsius, and can typically result in the output power falling off by as much as 40% at an operating temperature of 75 degrees Celsius, in the case of polycrystalline silicon PV cells (the type most affected by temperature). Of course, if the PV cell is exposed to extreme temperatures (typically above 120 degrees), irreversible chemical changes can begin. But as long as the PV cell is not overheated, it must be emphasized that once it cools, its original efficiency returns.

There is an important lesson that can be taken from the claimed results of this science fair exhibit. The question that one must always ask when presented with scientific claims is "Do these results seem to fit reality?". If they do not, then closer scrutiny would seem to be needed before accepting the results as fact.

In this case, the extreme and non-reversible reduction of PV cell efficiency, when exposed to temperatures commonly found in moderate climates (or even when simply exposed to the heating effect of the sun), would clearly mean that PV cells would be completely impractical as a useful source of power. Yet we know that in reality, large PV cell "farms" in the US southwest, and other hot climates around the world, generate huge amounts of power! Similarly, any PV cell exposed to the sun (and heated by it) in North America, will easily reach temperatures that, according to the claimed results of this science fair project, would result in an impractically large loss of efficiency. Yet we also know these PV cell installations work very well. So in general, when experimental results do not fit reality, you must always be suspicious!

Regards, Bob Wilson



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