# Solar energy questions

What is the maximum efficiency for a solar cell? I’ve read a few things, for example, like on this site http://secondsilicon.com/what-happens-when-solar-cells-reach-maximum-efficiency/
But can someone explain this in plain English?

Also, how many square feet (or meters) of today’s solar panels would it take to supply enough energy to meet today’s demand? Thansk

I’m in a rush so let me just give a partial answer. There are many things that limit the efficiency of a solar cell, some more fundamental than others. The 30% limit is perhaps the most important practical limit, but not fundamental. It has been surpassed many times, but never in a device as cheap as simpler device structures, and price per Watt is king.

This is the Shockley-Queisser limit, caused by the fact that the sun does not emit light at just one color. If you use a semiconductor diode to do the conversion, you can get close to 100% efficiency for photons whose energy is just slightly above the “band gap” energy of the material from which the diode is made. Less energetic photons aren’t converted at all and higher energy photons are converted, but their extra energy is wasted. For our sun’s spectrum, this limit is about 33% with the optimal band gap. Silicon’s band gap is close to optimal, giving approximately 30% maximum efficiency.

86% is the absolute, hard limit for solar power operating on Earth. That comes straight from the laws of thermodynamics, so you can be confident that there’s no way, nohow, of getting around that limit, no matter what kind of technology you come up with. Any given technology will have its own limits, however, and in practice you’ll never even reach those limits (though you might come very close). For the current technology used in solar cells, there’s a limit of about 30%. Some people have ideas for other sorts of technology that they think might reach 50%. And someone might, in principle, come up with a technology that could get close to that 86% number (in fact, at least one way of doing it is well-known), but for now, it’s impractical.

http://www.richhesslersolar.com/solar-articles/solar-panels-power-world/

I figure that if every place with a parking lot would put a strip of panels over the rows where the cars actually park you would not only get valuable shade over the cars but generate electricity. The downside would be installing the panels so that random idiots hitting poles wouldn’t destroy the system. Probably something like these that South Carolina is proposing to install but with curbs to keep the cars from hitting the poles.

Hell, if I had land in California’s Central Valley in say Kerman or Madera, I would consider tossing up solar panels covering areas that I was parking or keeping poultry in to provide shade and electricity. I could see popping up an acre and selling juice back to PG&E - at least having enough toavoid the rolling brownouts that California gets occasionally, or even rolling blackouts. We currently have petroleum based small generators for the house - winter in Connecticut is not really great for trying to power up with solar [though we do have a couple small solar toys to charge up stuff like phones and tablets] I wouldn’t mind getting something like a yeti for taking to pennsic and occasionaly blackout duty here. \$2k is a bit much out of the budget with only one of us working right now. Though I wonder how long it would take me if I did 10 hours a day on Amazon Mechanical Turk … :eek:

I like how they compare laying solar panels to paving roads as if there are no additional complications like materials, manufacture, maintenance, wiring and lifetime. These sorts of over-simplistic comparisons are why I have difficulty taking alternative energy seriously. We should definitely invest in it heavily, but the idea that it is ready to go now is not realistic.

The total world energy consumption in 2008 was 143.851 petawatt-hours (per year) (from here).

The average insolation in the deserts of the Southwest US is approximately 6.5 kWh/sq. meter/day (from here).

So, at 100% efficiency, a solar array would need to be 1.43851 x 10^17 / 6.5 * 10^3 / 365 = 6.06 * 10^10sq. meters in area, or 246 kilometers on a side. That’s a pretty big chunk of Arizona, and that size needs to be increased by a factor of 3 or more to allow for efficiency considerations.

I like how you like that fact.

And nobody is saying you have to grab a chunk of desert and do all the generation from one point. As I pointed out, you could place the solar units in non-agricultural areas like parking lots, which admittedly there are a fuckload more than the above stated amount of turf between the states of Az, NM, CA, FLA, LA, MI and Texas alone. [going for higher insolation areas than say Maine and Seattle WA.] And if every householder kicks in with a few panels, then they are adding to the collection amount. As WW2 wartime pundits claimed, every little bit helps.

Plus, of course, it’s unrealistic to expect that solar would ever be our sole source of energy. Even if we run out of all fossil fuels entirely, we’ll still be getting some from hydro, and wind, and geothermal, and possibly tides and nuclear (fission or fusion). Even being able to get 10% or so from solar would be a really big deal.

Put it in lots of different places doesn’t change any of the problems solar power has. It isn’t just a matter of every household buying a few panels. All of those panels have to be built, the raw materials need to be mined, the raw materials need to be refined, the manufacturing facilities need to be built, the panels need to maintained. It doesn’t matter if all of the panels are in one location or in every parking lot individually. When the solar power proponents start dealing with those problems realistically, I’ll take them seriously. Until then, every year there is a little meeting in Boston for the Materials Research Society, and every year about 25% of the lectures are on how to make solar power work scalably, because right now, as far as the materials science community is concerned, it doesn’t work.

Fossil has Nuclear energy has distinct advantages over Solar and Wind. For big cities, Solar cannot provide the power density needed - unless you install it somewhere else and build a huge grid network to bring it back in.

For overcast days and nights (obviously) you need a backup generator for Solar. Wind typically is strongest when power is needed the least (the night).

If you look at the pure economics and take out government subsidies or political will, fossil and nuclear always win. The three numbers that are most important for power production (as an economic venture not a social cause) are : /MW (capital investment), /MW (Operating cost) and Hrs/year (reliability and availability). (Meeting the environmental and safety laws is assumed).

I don’t know, there were 31,000 MW of solar installed in 2012, and it increases all the time, from here. Compare that to an estimated 86 GW of new nuclear capacity projected to be installed by 2030, from here. It depends what market share target you’re shooting for I suppose, but I’d say the solar industry is not waiting for you to take them seriously.

It is getting better. Don’t misunderstand. I think solar energy has a key role in our energy future. I want research funded. I want installation funded. But I also want realistic projections based on solid economic and manufacturing principles. The, “look at all the roads we paved.”, stuff reenforces the conspiracy mind. It builds distrust in science on the psychology that it can all be bought off.

I suppose the roads argument isn’t very good, but that still leaves plenty of potential for as much solar capacity as we want. Solar parking canopies are already here. And in a country where nearly every state gets more sunshine than Germany, the world’s current solar generation leader, rooftop or installation systems have tremendous potential. The question is how long will it take to build all this infrastructure, and how much will storage systems cost.

As I aid, it doesn’t matter if you put it on parking lots, rooftops or in one big spot in the desert. The material all needs to be made. It isn’t just about building infrastructure, it’s about the cost of raw materials. When everybody want’s lots of tellurium, indium and cadmium the price will skyrocket.

Germany is an interesting example. They are making a good show for there efforts. I am curious as to how much power they are importing. I’d be all for that sort of effort in the US, but this effort is also heavily subsidized. It isn’t really solar power working because it is cost effective. It is solar power working because it is subsidized by the government. Good luck getting that here.

Don’t solar panels that use rare metals like tellurium and such find uses in satellites and other applications where absolute maximum efficiency is economically advantageous? I though that solar panels that are installed on rooftops use silicon which is abundantly found on earths crust. I believe that the race now is on to lower manufacturing costs, and not necessarily increase efficiency since land area cost for installation is essentially free.

Is there a reason why conversion to heat comes into play that is easily explainable? Is this due to infrared to electrical energy?

Fine, so lets all just sit on our asses waiting for the perfect solar panel to be invented … instead of putting up what we can as we get it, and upgrading when it becomes available.:rolleyes:

I don’t know about you, but I plan on adding solar as I can. It may not save oodles of resources, but every bit I can take away from burning coal and gas I will. Maybe if everybody did, we would save more than you consider possible.

It’s not a matter of conversion to heat-- It’s already heat. There’s nothing special about infrared. A hot object can give off light of any wavelength, and light of any object, when absorbed by an object, will increase its temperature. The only reason we think of infrared specifically as heat is that that’s the dominant “color” emitted by objects at temperatures we’re familiar with, like warm-blooded animals or campfires. But for an object at about 6000 degrees, like the Sun, the light emitted (through exactly the same process) is mostly in the visible light range.

So, ultimately, any sort of solar power scheme is a heat engine, with the Sun (at ~6000 degrees) as the hot reservoir, and the ambient environment on Earth (~300 degrees) as the cold reservoir. And thus, the maximum efficiency of any solar power scheme is the Carnot efficiency for those two temperatures.