August 03, 2010
Solar Process Harnesses Light And Heat
Stanford researchers have developed a way to harness both light and high temperatures to generate electricity at a much higher level of efficiency.
A new process that simultaneously combines the light and heat of solar radiation to generate electricity could offer more than double the efficiency of existing solar cell technology, say the Stanford engineers who discovered it and proved that it works. The process, called "photon enhanced thermionic emission," or PETE, could reduce the costs of solar energy production enough for it to compete with oil as an energy source.
The key here is the ability to achieve the very high temperatures needed to use heat to generate electricity while still retaining the ability to convert a portion of the light directly into electricity.
If this wasted heat energy could somehow be harvested, solar cells could be much more efficient. The problem has been that high temperatures are necessary to power heat-based conversion systems, yet solar cell efficiency rapidly decreases at higher temperatures.
Until now, no one had come up with a way to wed thermal and solar cell conversion technologies.
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.
"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."
Because the process works best at 200 C and above it is more suited to solar concentrators. I've been wondering whether concentrating solar or PV would win in the long run. Sounds like maybe a hybrid of the two could become the biggest winner.
Efficiency of 55-60 percent might be possible.
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 percent – almost triple the efficiency of existing systems.
Since this design could work with existing parabolic mirror solar concentrators and requires a fairly small amount of material the incremental capital cost for boosting conversion efficiency is fairly low. Since a large amount concentrated light hits a small target a boost to the target's conversion efficiency lowers of of electric power generation.
Remember that high concentration solar collectors can only use direct sunlight, not diffuse sunlight scattered off/through clouds. They are best suited to desert areas or possibly space.
50% efficiency is astonishing. Could this replace heat engines? The ICE in the Volt is perhaps 25% efficient: it could get 100MPG with 50% efficient conversion.
EREVs with smaller batteries (perhaps 4kWh) would become instantly the optimal vehicle design, eclipsing hybrids like the Prius instantly. EREV designs would also take over everywhere else: trucking, shipping...
Paul, why would concentrated solar collectors not be able to utilize diffuse radiation? If solar thermal collectors can operate on a cloudy day, how would this be different? Obviously lower radiation intensity would affect the rate of generation but I don't see why the collector would seize operating. Particularly vacuum solar collectors could be of interest since they would be insensitive to ambient temperature and possible cooling effect of wind.
Concentrated solar collectors rely on focusing light from large mirrors to small collectors. Diffuse light won't work.
On the other hand, there are many large locations with the proper light.
Thanks for the reminder. So solar concentrators might out-compete conventional PV in, say, Arizona or SoCal while PV might do better in the US southeast.
Ever come across a study that measured average level of sunlight diffusion by region?
The diffuse light hits the concentrators at wrong angles, therefore reflects off at wrong angles, and so less of the light gets focused at the target.
I can't tell from the article whether this process converts only photons, or whether it converts heat directly to electricity. Any thoughts?
Randall, you asked Ever come across a study that measured average level of sunlight diffusion by region?
This site gives all the data you need - it doesn't provide a specific analysis of diffusion, but it shows insolation for concentrators and flat plate solar collectors, so you can make a comparison:
Nick and Paul,
There are ways to intensify diffuse light - obviously though, if the incoming intensity is very low, resulting intensity will also be limited. Go Google "fluorescent solar collectors".
Paul, why would concentrated solar collectors not be able to utilize diffuse radiation?
The larger the solid angle the incoming radiation occupies, the lower the maximum achievable concentration ratio. This follows directly from the 2nd law of thermodynamics. If one could violate the constraint (properly stated) one could build a perpetual motion machine of the second kind.
(The maximum ratio depends on the refractive index of the material in which the absorbing surface is encapsulated, which one reason I wrote "properly stated".)
Hello every body,
I post this comment because i don't really understand why the diffuse radiation are not concentrated?
i need more explanation. please.
"The larger the solid angle the incoming radiation occupies, the lower the maximum achievable concentration ratio", can you please explain more.