Stanford’s Nick Melosh, Assistant Professor of Materials Science and Engineering, seems to have cracked the problem of getting the heat of sunlight with the photons working to produce electricity in a solar cell.  That would not be just a photovoltaic; it would be infraredvoltaic plus photovoltaic.  The efficiency gets past 55% and near 60% in some configurations.  This is news.

PETE Solar Cell Test. Click image for more info

Melosh says in the Stanford press release, “This is really a conceptual breakthrough, a new energy conversion process, not just a new material or a slightly different tweak. It is actually something fundamentally different about how you can harvest energy.”

“Just demonstrating that the process worked was a big deal,” Melosh said. “And we showed this physical mechanism does exist; it works as advertised.”

Melosh’s group figured out that by coating a piece of semiconducting material with a thin layer of the metal cesium, it made the material able to use both light and heat to generate electricity.  Most silicon solar cells have been rendered inert by the time the temperature reaches 100º C; the “photon enhanced thermionic emission” device doesn’t hit peak efficiency until it is well over 200º C.  “What we’ve demonstrated is a new physical process that is not based on standard photovoltaic mechanisms, but can give you a photovoltaic-like response at very high temperatures,” Melosh said. “In fact, it works better at higher temperatures. The higher the better.”

And the materials needed to build a device to make the process work are cheap and easily available, meaning the power that comes from it will be affordable.

The process is called “photon enhanced thermionic emission,” or PETE and could reduce the costs of solar energy production enough for it to compete with oil as an energy source.  Because PETE performs best at temperatures well in excess of what a rooftop solar panel would reach, the devices will work best in solar concentrators such as parabolic dishes, which can get as hot as 800º C. Dishes are used in large solar farms similar to those proposed for the Mojave Desert in Southern California and usually include a thermal conversion mechanism as part of their design, which offers another opportunity for PETE to help generate electricity as well as minimize costs by meshing with existing technology.  There go all those plans for the huge arrays.  But its OK, getting to 60% efficiency changes the plans in a massive way as well.

Melosh explains how a PETE works, “The light would come in and hit our PETE device first, where we would take advantage of both the incident light and the heat that it produces, and then we would dump the waste heat to their existing thermal conversion systems. So the PETE process has two really big benefits in energy production over normal technology.”

Getting the efficiency up: Photovoltaic systems never get hot enough for their waste heat to be useful in thermal energy conversion, but the high temperatures at which PETE performs are perfect for generating usable high-temperature waste heat. Melosh calculates the PETE process can get to 50 percent efficiency or more under solar concentration, but if combined with a thermal conversion cycle, could reach 55 or even 60% – almost triple the efficiency of existing systems.  These technologies are already understood.

Its still way early – perhaps the paper published online Aug. 1 in Nature Materials is a competitive effort to get on record first.  Using a gallium nitride semiconductor in the “proof of concept” tests, the efficiency achieved in their testing was well below what they have calculated PETE’s potential efficiency to be – which they had anticipated. But the Melosh team used gallium nitride because it was the only material that had shown indications of being able to withstand the high temperature range they were interested in and still have the PETE process occur.  With the right material – most likely a semiconductor such as gallium arsenide, which is used in a host of common household electronics – the actual efficiency of the process could reach up to the 50 or 60 percent the researchers have calculated. They are already exploring other materials that might work.

Another advantage of the PETE system is that by using it in solar concentrators, the amount of semiconductor material needed for a device is quite small.  Melosh explains, “For each device, we are figuring something like a 6-inch wafer of actual material is all that is needed. So the material cost in this is not really an issue for us, unlike the way it is for large solar panels of silicon.”

That answers the questions where the cost of materials has been one of the limiting factors in the development of the solar power industry, so reducing the amount of investment capital needed to build a solar farm is a big advance.

Melosh closes with an academic’s understatement, “The PETE process could really give the feasibility of solar power a big boost. Even if we don’t achieve perfect efficiency, let’s say we give a 10 percent boost to the efficiency of solar conversion, going from 20 percent efficiency to 30 percent, that is still a 50 percent increase overall.”

The Stanford writer hits the competition with oil drum with “And that is still a big enough increase that it could make solar energy competitive with oil” while what they really need to beat is coal.  There is little oil used for power generation while coal is a major fuel source.  Getting competitive with coal would be great, burning coal is still a massive nasty effluent producer, and daylight high output solar would answer peak needs.

Taking the concept further, the PETE idea would be quite a basis to recalculate for orbital solar too.

Melosh and the Stanford team deserve some thanks for the breakout and some encouragement to keep at it.  The working prototype needs built, a bit of demonstration – seeing solar at anything in the 50% plus range is quite a feat, it’s a welcome crack in the efficiency war!


2 Comments so far

  1. Asif on August 9, 2010 12:09 PM

    Thanks a lot for your article. As an engineer I agree with you and you know there is no way without solar energy for future demands, the solar energy advantages
    are so many.

  2. administrative assistant on November 8, 2010 9:15 AM

    Great site. A lot of useful information here. I’m sending it to some friends!

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