Solar Cells with 60% Efficiency?

Jan 09, 2008 by Lisa Zyga weblog
Lonnie Johnson
Lonnie Johnson with his invention of the Super Soaker, the top-selling toy in the US in the early ´90s.

Nuclear Engineer Lonnie Johnson, best known for his invention of the super soaker squirt gun, has recently designed a new type of solar energy technology that he says can achieve a conversion efficiency rate of more than 60 percent. Considering that the best solar energy systems today have an efficiency of 30-40 percent, Johnson´s method could cut the cost of solar energy nearly in half.

A recent article in Popular Mechanics describes how Johnson´s system would work. Rather than use photovoltaic cells, where silicon converts light into electricity, the new system works like a heat engine. But instead of using heat to turn an axle, it uses heat to force hydrogen ions through a membrane-electrode, and create electricity.

The system, called the Johnson Thermoelectric Energy Converting System (JTEC) consists of two stacks of electrodes - a high-temperature stack heated by the sun (and by concentrated mirrors) and a low-temperature stack.

An electrical jolt triggers a voltage across the electrode stacks, with the low-temperature stack pumping out hydrogen from low to high pressure in order to maintain the pressure differential. As the hydrogen passes through the high-temperature stack of electrodes, it generates power. In a sense, the system works similar to a fuel cell.

Johnson plans to build a system whose high temperature reaches 600 degrees centrigrade, within the current solar concentration ability of parabolic mirrors, which can produce temperatures of more than 800 degrees centigrade. At 600 degrees, the system would have an efficiency of close to 60 percent. At higher temperatures, the efficiency would increase even more.

The system should be able to produce several megawatts of power, according to Johnson. It could also harvest waste heat from internal combustion engines, turbines, and even the human body.

Johnson, a former NASA employee, funds his work with the millions of dollars he made from inventing the super soaker.

via: Popular Mechanics

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User comments : 19

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nilbud
1.7 / 5 (3) Jan 09, 2008
A super soaker that fires a beam of superheated steam, excellent.
Nikola
3.4 / 5 (5) Jan 09, 2008
He should call the system "The Solar Johnson".

Rametarin
3.3 / 5 (3) Jan 09, 2008
Awesome!
Not only is it very efficient, it's an alternative to the traditional way. I saw on the Science Channel that they're introducing a moderately efficient solar cell that's as cheap to produce as paper. Combining these two ways of harvesting energy from the sun will be spectacular.
gopher65
3.8 / 5 (4) Jan 09, 2008
This sounds neat, if it:
1) Works as advertised,
2) Is cheap to manufacture,
3) Has a long lifespan before needing to be replaced,
4) Isn't made from something nasty like lead or mercury, like many other products which claim to be "green". Sure, save me from AGW and kill me with mercury. Works for me!
MattJ
3 / 5 (4) Jan 09, 2008
Am I the only one to notice? According to this article, it exceeds Carnot efficiency. Clearly this is not possible.

Recall that the Carnot efficiency is the efficiency of an ideal heat engine, more efficient than any realizable engine, and it equal to (Thigh - Tlow)/Thigh. In this case, that would be (600-273)/600 = 327/600 = 0.455.

Also, recall that an electrical generator _is_ (from the viewpoint of classical thermodynamics) a heat engine, since electrical energy is a form of work.
Quantum_Conundrum
3.7 / 5 (3) Jan 09, 2008
mattj:

Your math is wrong.

327/600 = 0.545

Which is still smaller than the claims in the article.
Graeme
1 / 5 (1) Jan 09, 2008
Tlow must be smaller than 273 then!
gopher65
3 / 5 (2) Jan 09, 2008
They do say "close to 60%". But rounding up 54.5 to 60 is one heck of a round up.

I wonder if this is another case of someone doing the math on paper, going "Holy $%@((! Look at the theoretical efficiency!" and then claiming they made a breakthrough without doing any actual experimentation. That happens way more often than it should:(. Just look at EEStor.
MattJ
5 / 5 (3) Jan 09, 2008
Twice I got "327/600=.455" from the Windows calculator, not sure how the digits could have got transposed.

Anyway, thanks for the correction. But since writing that comment, I realized that there is one other way he could do better than 54.5%: if the generator is not relying solely on the _heat_ energy in light, but relying on another physical process to directly convert visible light photons into electrical energy. But the article did not make it clear whether this is what Johnson's invention does. Indeed: it makes it sound like it really _is_ relying solely on heat energy with the comment about "a high-temperature stack heated by the sun". Likewise with the comment about "heat engine".

The comment comparing to a "fuel cell" was not very informative. Fuel cells are not that efficient, either. They tend to reach only about 50% efficiency.

gopher65
3.7 / 5 (3) Jan 10, 2008
You typed in "273/600" instead of "327/600". Easy mistype to make.
out7x
1.6 / 5 (5) Jan 10, 2008
pseudo science at its best. What is an "electrical jolt"????
TomJames
5 / 5 (4) Jan 10, 2008
OK. First of all Carnot efficiency is measured using absolute temperatures. The quoted temperatures are in degrees centigrade. So (assuming the cold sink is at 25 degrees C):

efficiency = 1 - ((T COLD)/(T HOT))

efficiency = 1 - ((273 plus 25)/(273 plus 600)) = 0.66

Giving a theoretical efficiency of 66%.

However the endoreversible process is a slightly more accurate method of measuring the efficiency of a heat engine (at least according to the Wikipedia article), which is given by:

efficiency = 1 - (T COLD/T HOT)^0.5.

So:

efficiency = 1 - ((273 plus 25)/(600 plus 273))^0.5 = 0.42

Giving a more realistic theoretical efficiency of 42%.

Of course there will probably be other factors to consider that affect efficiency.

I agree that the article seems poorly written and does not give a concrete explanation.
gopher65
1 / 5 (1) Jan 10, 2008
Ah TomJames... I can't believe I didn't see that. I feel stupid heh. *waps head with palm*
poiuyt
5 / 5 (2) Jan 11, 2008
This may be a dumb question but where does the hydrogen come from?
norbozo
5 / 5 (1) Jan 15, 2008
Another article explained that this is like a Stirling Engine, the "fluid" is a closed cycle. So you put the H2 in this closed system, and the heat drives the H2 across the voltage generating membrane, and presumably, the low temperature mebrane somehow pumps the H2 back towards the high temperature membrane? Sort of like the old joke, a proof on the blackboard, lots of equations, step 5 says "Then a miracle occurs"...
Professor says "You need to be a little more rigorous in step 5".
JohnMudd
5 / 5 (1) Jan 19, 2008
"The JTEC is an all solid-state engine that operates on the Ericsson cycle. Equivalent to Carnot, the Ericsson cycle offers the maximum theoretical efficiency available from an engine operating between two temperatures."

From: http://www.johnso...tec.html

Run calculations again?
jabe
not rated yet Jan 20, 2008
Math aside how practical is needing a startup temp of over 1100 deg F to achieve this theoretical efficiency?
Not very, I'd say.
CTYankee
not rated yet Jan 22, 2008
Thank you TomJames,

The article is as confusing as the claim. I can't tell if the H2 is running up the pressure gradient or down? Where does the pressure come from? What is a "jolt"? I thought "jerk" was the derivative of acceleration?

The only thing I see forcing the ions across the gap is the force of will, which we all know causes thrown dice to come up 7 on the first roll every time!
Ulg
not rated yet Jun 24, 2009
The jolt is a electrical charge that acts as a catalyst for the membrane hydrogen exchange to start. And 1100 F is not hard at all for a solar concentrator- even a plastic fresnel lens for a overhead projector can focus heats up to 500 F on a partially cloudy day.



The pressure is generated from heating a gas, many types of stirling engines use hydrogen this just happens to be solid state (the hydrogen moves in a loop but no mechanical parts move)



In a fuel cell the hydrogen is attracted to oxygen which provides the energy to pass the hydrogen proton through- the electron is free and that giving each side of the membrane varied charges.



This requires no oxygen, and does not create water as no chemical energy is required to push the hydrogen proton through the membrane and also makes it a closed system. But hydrogen is extremely difficult to contain forever.