maggiekb at May 28th, 2014 11:38 — #1
jim_r at May 28th, 2014 12:37 — #2
I remember reading an article a while back, going into some detail about the thermodynamics of the silicon PV mechanism. I can't recall the details just now, but the bottom line was that 15% efficiency is about all you can expect from ordinary sunlight on ordinary panels.
By concentrating the sunlight with mirrors, and building multiple layers, and using exotic semiconductors (like the CIGS or perovskite in your article), the conversion efficiency could be raised a bit. But still, TANSTAAFL applies to this as it does to everything else. The thermodynamic limit is probably about 40%. And to keep that advantage up, you have to polish mirrors, run cooling water, refine exotic elements, create nanostructures, and so forth. You end up with fragile high-maintenance panels.
The advantage of silicon is that it is abundant here on the earth, and PV cells made from it are very close to optimal for passively harvesting sunlight. For everything else there is photosynthesis, which is about 0.8% efficient or so. But photosynthesis has the advantage of making a lot of useful things like wood and fiber and food.
jandrese at May 28th, 2014 14:16 — #3
I think that's why most of the research into solar panels has been focused on making them cheaper, not increasing their efficiency.
And it's working. Per-watt solar prices have been declining steadily, but there is still a ways to go until it is truly affordable. I double checked what it would cost to do my house with a 50% power offset in my area, and it is still luxury car prices.
anthonyc at May 28th, 2014 15:11 — #4
This long timeline is also a big part of why so little money gets put into materials research, relative to IT. What VC (other than a CVC with a strategic interest) wants a 25 yr design cycle?
The first limit on efficiency is the Shockley-Queissir limit. For a normal PV cell the bandgap of your semiconductor (1.1 eV for silicon) draws a line through the solar spectrum. Anything below that can't be absorbed/used. Anything above that generates one electron-hole pair. But if your incoming photon had more energy than 1.1 eV (ultraviolet, for example) then the electron and hole will be moving faster, but that excess is lost as heat. So too small a bandgap --> very little usable energy per pair generated, too high bandgap--> too few pairs generated. This gives a limit of ~34% at ~1.3 eV, or 29% for Si at 1.1 eV.
Other effects - like incomplete absorption of light or electrons and wholes recombining in the device - lower the efficiency in practice.
Multi-junction cells try to use different bandgap materials together so as the extract more energy from high energy photons without losing the low-energy ones. Today people have made 2, 3, and 4 junction cells with demonstrated efficiency in the 44% range at least. But these are really expensive to manufacture so they use concentrating mirrors and active cooling so you only need a PV cell hundreds of times smaller than the area of light you're capturing.
There are also multi-exciton multi-photon, and solar antenna designs (in the lab anyway) that can raise the limit in other ways.
Note: not disagreeing with anything you wrote, just clarifying the physics
anthonyc at May 28th, 2014 15:25 — #5
As always, PV should be low on your priority list. I'm guessing you've already done this, but for all of us:
Make all of your appliances, heating, cooling, lighting systems, insulation etc. as efficient as possible.
If by chance you're building new, design for efficiency and for your local climate - if you're not familiar with passivhaus and other such standards, look into them and determine whether a 5%-10% higher construction cost is worth it for 60%-70% lower energy use and 90% lower heating/cooling energy use for the life of the house. But if you go this route use a contractor familiar with these standards or you could end up with a house that is prone to mold and poor air quality, among other things. Ditto for trying to make existing buildings really airtight and insulated.
Take advantage of energy audits your utility can provide to identify missed opportunities.
If you live in PA or other places with electric choice, choose a supplier with cost-effective renewable power. You can often find wind and solar electricity within 1 cent/kWh of the cost other suppliers charge.
Look at solar leasing companies like SolarCity, which will install panels on your roof for you, maintain them, and charge you per kWh but 80% less than the rate your normal utility charges.
Home solar installations are expensive because doing anything at small scale is more expensive per unit, but in this case there are (not yet available everywhere, but still) workarounds to that problem. This should be one of the very last items on anyone's personal energy savings and greenhouse gas reduction todo list.
jandrese at May 28th, 2014 15:36 — #6
Yeah, in my case I did the math more for academic purposes. I didn't call any of the contractors to see if they could do better than web quotes or anything, although I did apply all of the tax breaks I could find.
The situation has definitely improved. They're down from supercar prices to mere mid-tier luxury car prices.
pieromatarasso at May 28th, 2014 17:14 — #7
In 1974 I was in charge (with others) of the definition of priorities for our newly born solar research program at CNRS (french national center for scientific research) Obviously, PV research was on the top of the list. But very rapidly we decided to begin a serious investigation on CSP (concentrating solar power). Five years later our solar plant prototype THEMIS was delivering few megawatt of electricity. At the time and for thirty years we have been heavily criticised by poeple saying that PV cost would drop quickly...that solar "paint" will be created soon...that our "Jules Verne" device was using only direct sunshine.....etc. We always replied that we would be delighted if the cost of pv was in the situation to drop..... but for the time being there was a possibility that 30 years in the future (ie 2004) the manufacture of PV would still rely on silicon crystall pulling, sawing slices of these crystalls, depositing electrodes, wiring all that...etc. Therefore from our opinion even if the crystalls could have a bigger surface, the saw being better, the slices thinner etc....the cost would be linked to the production cost measured by square meters. On that level the comparison with CSP was relevant....as long as you are in a sunny climate. Besides....with CSP you might have a thermal storage.... and you gather already concentrated energy ( in the case of PV plants....batteries are expensive...and also the copper to connect the pannels). I still hope very much that someday..someone will discover a "solar paint" but meanwhile...CSP plant can be a great way to provide electricity in many places of the world....and even with the french, american, spanish prototypes....and the actual building of a plant at Ivampah...USA...the potential and funding of CSP plants is underestimated
jomchenry at May 29th, 2014 02:33 — #8
What's wrong with 15% efficiency. It's sunlight, it's not going to run out any time soon. It heats my water and provides my power and I can't wait to get one of the new dual/solar powered cars that just came on the market.
The article above is way behind on both research and practice. Get with it - not with the oil, coal and nuclear corporations who seem to have written most of the comments here.
Let's see if the corporations do their usual thing and try to prevent development of new solar technology with this one - http://www.echo.net.au/2014/04/renewables-changing-nature-power-manufacturing/
Here's an amazing new use for solar. https://www.youtube.com/watch?v=qlTA3rnpgzU#t=17
jim_r at May 30th, 2014 00:09 — #9
That was the point I was sort of trying to make. You can sputter a thin layer of unobtainium on a sheet of glass and get a novel cell with slightly more efficiency (so long as no dust falls on the shiny surface) ... or just refine, pull, saw, grind, polish, dope, and wire for an easy 15%. And there are some interesting shortcuts to those steps for making polycrystalline cells a little cheaper. Furthermore, you don't need the nine-nines silicon used to make microprocessors, a mere five-nines will do nicely for PV.
For capturing energy, it's way better than photosynthesis.
The bad news is, it doesn't have the concentrated embedded energy of coal, so it's harder to run a steel mill on it, but the good news is that every house could be net-zero or net-positive, and a reasonable strategy for keeping civilization going.
pieromatarasso at May 31st, 2014 05:33 — #10
To be a little more precise.....than in my previous contribution...... although PV is very convenient in many situations...athough...it will be more and more convenient with its cost getting lower its efficiency getting higher.....and above all the efficiency of electricity use growing very quickly ( with LEDS replacing incandescent bulbs...w
We do NOT have discovered.....Unobtainium..... or Kryptonite for mass production of PV
But we need TODAY solutions to fight Climate Change.....And CSP (solar power tower) IS such a contemporary SOLUTION with a tremendous potential of lowering cost! Just for fun...... an examples of simple solutions......which emerged...... very strangely late: the rolling suitcase ( industrally developped in the 70's)..... The air balloon of the Montgolfier brothers.....in 1783......but Archimedes could have made it...) ( this remark for Google fellows.....You can make it today....with very contemporary solutions....an much more cheap than present instalations like Solar Two....Ivanpah...etc...)
We designed and builded the first modern CSP Plant THEMIS at CNRS before the 80'.....and the first realy commercial CSP plant are developped today at Ivanpah.....FOURTY YEARS later!
To those interesred in the matter...I recommend the lecture of a patent ( now in public domain) for very cheap heliostats....you will find it in Google Scholar under the names Deflandre, Matarasso. Traisnel.....I will gladly help any volunteer to adapt this to rough soil. to the concept of family heliostat for a single house...to air cooling devices for hot climate....etc
maggiekb at June 2nd, 2014 11:38 — #11
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