Understanding the Efficiency of Solar PV
Why aren’t solar PV cells more efficient? Here’s an answer from a physicist who looks at the problem from the perspective of the Carnot Limit, i.e., the theoretical limit to the efficiency of a heat engine, which is a function of the difference between the absolute temperature of two bodies (in this case, the sun and the Earth).
Where this obviously applies to solar thermal (aka concentrated solar power, or CSP), I don’t understand the relevance to PV. The way I’ve always explained this is as follows: the sun bombards us with photons of varying energies (which are proportional to the frequencies of the photons). The goal of PV is to see how large a percentage can be converted to kicking an electron out of a piece of semiconductor. But a) some have energy too low (and that energy is therefore wasted), b) some get the job done, but with energy that is far more than sufficient (and that extra energy is wasted), and c) some miss the target altogether; they either pass through the PV or are reflected off it (and thus their energy is entirely wasted).
The goal of modern PV R&D is to use quantum physics to eliminate as much of this as possible, e.g., getting one photon to kick out more than one electron, something that can be attained via the Mossbauer Effect, where the energy of a single photon is thought to be absorbed into a matrix, rather than actually hitting an electron as if these particles were billiard balls, which was believed to be the case under classical physics.
I’d appreciate help on this from a real-live physicist.
Until such help comes along, here’s a suggestion I can make even with my less-than-perfect command of the subject: tell all those scantily dressed girls in the pic above to get off the array and stop blocking the sun. Oh, wait a sec…maybe they’re there for a reason….
Your understanding is spot on. far as I can tell.
The only other way to boost efficiency is to use both electricity and heat from the same system. There are a number of solar panels designed to capture electricity whilst at the same time heating water. That way, you are likely to achieve around 75 to 80% sunshine to useful energy with today’s technology – very similar to a fuel fired power station in CHP mode.
Gary that is a great real world example of making efficiency gains in existing pv panels. There are a few companies marketing these and I saw the data collected from some installs in Arizona and NM and West Texas. They reach around 70 to 80 % on a btu produced basis.
The water or coolant circulating on the back side of the pv panels actually cools off the pv panels and lowers the heat degradation energy losses common to most pv in the hot desert climates. The thermal liquids can produce hot water , space heating and the Germans have some thermal Adsorption or Absorption I( my memory slips me) Air Conditioning units. So they can provide year round energy and load reductions for users. Has excellent Peak demand reduction benefits for utilities and end users.
Due to weight issues the best installations tend to be ground mounted. Lots of institutional end users have the combination of both electric and annual thermal loads to utilize these systems. Economics are greater than standard pv systems.
Once Natural gas prices go up this sector will get deserved attention.
In respect to traditional pv there are several well funded research efforts ongoing that are reaching 24 % to 30 % conversion efficiency’s for third generation pv. They should hit the market in the next 3 years. Progress is still on the Horizons
Here is an alternative view Craig. We have commercial single layer PV cells that can achieve an efficiency of around 23% to a theoretical limit around 32%. Not too shabby. We have triple layer cells that capture a broader selection of wave-lenghts with an efficiency of about 43%. Depending upon the value of the application we can choose some reasonable alternatives (space applications and solar cars: high end, house installations lower end.)
When we attempt to capture not the energy in light but the heat energy in solar radiation, efficiencies up to 80% are possible with solar thermal panels (for hot water and absorbtion air conditioning. Additional equipment (changing from heat energy to electrical energy) lowers the efficiency for CSP electrical power plants.
The most efficient method of converting solar radiation to commercial electrical energy is using a solar dish (a method of CSP) powering a stirling engine which powers a generator for AC electricity where efficiencies of around 35% have been achieved. Unfortunately efficiency does not directly lead to cost effectiveness. Noise and mechanical maintenance are less preferred when compared to the simpler methods.
Now compare that to what nature can do and how we start the process for some bio-fuels. Photosynthesis is approximately 1% efficient at converting solar radiation to stored chemical energy. We are not doing too badly.
Nature gives us abundant energy but we also want to convert it to electricity. The solar PV panel does both. Naturally it is going to be less efficient than other methods of collecting solar energy and using it as heat. Maybe we should be considering more methods to use solar heat energy than how to convert solar energy to electricity.
(We also do the same thing with wind energy which is more efficiently used as mechanical energy than to convert it to electricity.)
Hi Craig;
Have you seen what Rice University has been doing? While their device is mainly aimed at producing hydrogen directly from sunlight, their discussion has some additional insight as to what happens to the high-energy electrons in a solar cell.
Their device is aimed at capturing the extra energy of the hotter electrons. The device uses a nanothin layer of nickel oxide on an aluminum substrate, over which gold nanoparticles are placed. Rather than being pushed over an energy barrier, the electrons stay on the gold layer, producing plasmons until the energy is exhausted.
What is interesting is that if you place water over the cells they will produce hydrogen gas.
Unlike most conventional electrolyzers, the water doesn’t have to be pure; rain water or waste water from water treatment plants can be used.
Not a product yet, but looks promising for the future.
I see it as a matter of perspective. My PV operates at 100% efficiency! Everything it produces is free! And I get paid to produce it. The return has been much better than stock market investment for me. Wall Street knows that too… That’s why they are scrambling to push and garner all the corporate investment in PV they can before the GP wakes up to this also. And lobbyists want incentive programs for corporate benefit, while private encouragement is all but completely lost. I really hope the people wake up and toss the govt bums out of office. Unfortunately, that means most every one of them! Voting Green is the only answer that works.!
what any come out in the next ten years might be very amassing
Solar thermal collectors can make a much bigger contribution to the power grid than solar PV panels and the energy they supply is more useful because it is so much cheaper to store. The power grid demand peaks are caused by the thermal demands for heating and cooling buildings. By using thermal storage to handle both those heating and cooling needs the power demand peaks are reduced (achieving the same end objective as a system that uses electric batteries for storage) and the photon to heat conversion efficiency is so much higher that the panels can be much smaller.
What you say here is correct, but that doesn’t seem to be turning the tide in the direction of CSP at this point.
CSP (concentrated solar power, sometimes used with molten salt storage) is largely limited to countries that have long sunny days. It is not popular in northern N. America or Europe. What I was referring to are ordinary solar thermal panels combined with thermal storage in the ground. Such panels are widely used in N. America and Europe but in general they do not make use of ground thermal storage, which is the key to making such panels really useful because they can then supply most of the energy needs for our homes (heating, cooling and hot water), Ground thermal stores can also utilize ground heat (and cold), heat extracted from the summer air, and heat from house cooling systems and they can utilize cheap nighttime electricity to boost the exergy of the heat collected from the low temperature sources. See the following article for an explanation:
http://www.mdpi.com/2071-1050/6/11/8297/pdf
Did I find myself back in the 1950’s? What’s up with the pictures of the women on solar modules? It made me want to puke.
“They’re there for a reason”???!!!!
“They’re there for a reason” was my way of remarking on the sexism the photo invoked. Sorry that wasn’t more clear.