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Stanford researchers develop self-cooling solar cells

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July 25, 2014

The Stanford University system uses a glass layer patterned with micro-pyramids and cones ...

The Stanford University system uses a glass layer patterned with micro-pyramids and cones to shed heat

Photovoltaic cells are one of the more promising alternative energy sources. Mechanically they are very simple, with no moving parts, and are clean and emission-free. Unfortunately they are also inefficient. One of the reasons for this is that they overheat, a problem that a Stanford University team under electrical engineering professor Shanhui Fan is addressing with the development of a thin glass layer that makes solar cells self-cooling.

Despite many advances in recent decades, solar cells suffer from efficiency problems. Only a small amount of the energy from sunlight that falls on solar cells is converted to electricity, peaking at below 20 percent for most cells on the market today. Overheating is a constant problem because the sunlight used to generate electricity routinely heats up the panels to 130⁰ F (55⁰ C) or higher.

This heating causes all sorts of problems – not the least of which is a dramatic drop in efficiency. According to the Stanford team, each degree Celsius (1.8⁰ F) heating results in an efficiency drop of 0.5 percent. Equally unpleasant, with each increase in temperature of 10⁰ C (18⁰ F) the deterioration rate of the cells doubles.

Cooling is an obvious step, but the question is, how to do it? Active systems, such as coolant pumps or ventilation, consume power, and passive systems aren't very effective and can interfere with the the panel’s operation. The solution developed by Shanhui Fan, who has previously applied similar principles to passive cooling for buildings, is a system where ordinary solar cells are given an extremely thin layer of specially patterned silica glass that is designed to draw heat away from the cells in a manner that requires no energy and exploits the atmosphere’s infrared window to shed the heat.

The pattern consists of micro-pyramids and cones measuring only microns in thickness, which are sized and shaped to draw away heat in the form of infrared radiation to the top of the layer, where it radiates and disperses into the atmosphere. It’s based on the fact that different wavelengths of light have different properties and refract differently. The Stanford team tailored the silica glass layer to allow visible light in and heat in the form of infrared light out.

“Silica is transparent to visible light, but it is also possible to fine-tune how it bends and refracts light of very specific wavelengths,” says Fan. “A carefully designed layer of silica would not degrade the performance of the solar cell but it would enhance radiation at the predetermined thermal wavelengths to send the solar cell’s heat away more effectively.”

The Stanford team is testing and fine tuning the cooling layer with the aim of lowering the solar panel’s operating temperature in order to make the more efficient and with a longer operating life.

The team’s results were published in Optica.

Source: The Optical Society

About the Author
David Szondy David Szondy is a freelance writer based in Monroe, Washington. An award-winning playwright, he has contributed to Charged and iQ magazine and is the author of the website Tales of Future Past.   All articles by David Szondy
9 Comments

for distributed power solar pv obvious will benefit from applications like this.

however, for power stations, pretty much everything explained in this article as well as numerous other factors involving , wind , weight, dirt, etc...

the reality is that advanced thermal management systems are at their most efficient when the energy load scales upwards to a centralized thermal system ( in our current circumstances we're talking about turbines, but the future of solar energy extraction might be marked by futuristic thermal management technologies ) .

ultimately a whole bunch of ultra refective light,infrared,uV frequency mirrors will simply redirect massive quantities of heat energy to be managed at a central location. solar thermal is the present and forseeable future of power plant scale solar electricity production...

zevulon
25th July, 2014 @ 10:03 pm PDT

Wouldn't it make more sense to cool the solar cells by harvesting the heat for say preheating water for the water heater, or driving an absorption refrigeration system that can also drive a "steam engine". http://www.aqpl43.dsl.pipex.com/MUSEUM/POWER/ammonia/ammonia.htm

Slowburn
26th July, 2014 @ 01:09 am PDT

Interesting part about the deterioration rate of solar cells when it gets hotter. Similarly, to computer electronics. Clearly, there is a lot of potential and incentive to cool the panels down in order to maximize the benefits of your investment on the long term. Although this is of more immediate concern for places where there is a lot of direct sunlight vs. places with a lot of diffuse light.

Fretting Freddy the Ferret pressing the Fret
26th July, 2014 @ 03:02 am PDT

Slowburn:

Right as usual, Slowburn. See http://www.solimpeks.com/pv-t-hybrid-collectors/ for details of the Turkish company selling combined PV hot Water panels worldwide.

Stanford's heat shedding glass layer could be very useful, however in situations where there is no hot water requirement. Powering road signs, charging off grid batteries etc.

Dirk Scott
27th July, 2014 @ 05:27 pm PDT

@ Dirk Scott

Did you look at the ammonia engine? Inherent energy storage and refrigeration.

Slowburn
27th July, 2014 @ 10:14 pm PDT

"Equally unpleasant, with each increase in temperature of 10⁰ C (18⁰ F) the deterioration rate of the cells doubles."

Are you sure this is true? I think not

Why not put an IR reflective coating on the protective glass?

katgod
27th July, 2014 @ 11:50 pm PDT

Yea - that would not hold dust/dirt at all.....which would have an even more negative effect on performance. Maybe a good idea in a clean lab - BAD idea in the real world.

(I do love the fact that his name is Fan, though....)

bullrun
28th July, 2014 @ 10:18 am PDT

mass produce, awesome, lisc with estd solar cell makers alone.

Stephen N Russell
28th July, 2014 @ 03:45 pm PDT

I agree that the real world of dust and dirt would have a pretty severe impact on the performance of this. Not sure why the fancy pyramids are needed - it looks like what they are doing is tailoring the emissivity and absorbtivity of the surface for the wavelengths at which they are receiving radiation and emitting it. I'm not an optics guy, so maybe that is a lot easier said than done. Also, as katgod mentioned, the 10C increase leading to a 50% reduction in reliability is a rule of thumb that will draw an arc with many people in the electronics reliability area. As I recall, it is based on the Arrhenius equation and assumes things are already operating pretty close to their maximum temperature (which is well above the 55C mentioned in article) before applying the 10C increase. I have seen papers submitted for thermal conferences be thrown out for no other reason than the fact that the author mentioned this rule of thumb and thereby trashed his or her credibility and it was assumed that the rest of the paper was useless.

r_wil
5th August, 2014 @ 11:22 am PDT
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