OOps, sorry Uncle........I missed that being the space constant....guess I should do my own home work.
Here's a reply from Joe Rudmin from the Physics dept at James Madison University:
"Fundamentally, solar power cannot compete with a good wind turbine. With solar power, you start with solar constant --1 kW per square meter. You lose some of that coming in through clear air. How much depends on how much of the spectrum you're using.
Next lose half of it to night time. Less than 500 W/m? left. Next multiplly by the cosine of your latitude--383 watts left in Virginia. Next multiply by the cosine of the longitude averaged over the day--you don't get much at sunrise and sunset --244 W/m? left. Next you lose the cloudy days--typically 10%--220 W/m? left. Next you have conversion efficiency. The best you can do with voltaics is 1/2, provided you use all of every photon (which you can't do) Basically photo-voltaics are the photo-electric effect. If the transition energy you're using is 2.5 eV, then you can't convert 2.2 eV photons, and you only get half of 0.5 eV photons. With expensive multi-layered systems, you'll get 40% at best -- less than 88 W/m?. The electrons which get ionized must have enough kinetic energy left to climb the voltage hill in the PN junction. Half of them are headed in the wrong direction, and some of the rest won't make it. With much less expensive single-layered semiconductors, you'll get about 25% at best, so now you're down to 55W/m?.
Sure you can concentrate the energy with mirrors, but even mirrors aren't cheap, must be cleaned, and must occasionally be resurfaced. For solar power, it's hard to beat a solar tower-- reflecting the sunlight from mirrors towards a black boiler on a tower. You can convert maybe 35% of the steam energy to electricity, and maybe do something useful with the waste heat.
35% of 220W/m? is 77 W/m? averaged over the year in a good sunny region. If you're competing with windmills, you need to produce this electricity for about 2.5 cents per kWhr, so you might earn $16.80 /m? per year.
If you want 7% return on you're investment, you need build and install your mirrors and motorized computer-controlled aiming systems for less than $240/m?. It better be quite a bit less, because you still need to pay for the tower, the steam handling system, the turbine, the generator (used less than half the time), buy the land, pay your staff, and pay your taxes and depreciation from that $240 too. A much safer and easier way to get 7% is to invest it in US Savings Bonds. Inflation-indexed treasuries are paying 7%. Photovoltaics starting with 55 W/m? are worse than the solar tower.
In contrast, with a windmill, you can get 500W /m? average power through the blades almost anywhere. With a big system--500 to 1000 feet up--you can get 1 to 2 kW/m?, depending on the quality of the site. The theoretical maximum conversion efficience is 56%, but almost any reasonable well-engineered system will get 40 to 45% of the windpower passing through the disk swept by the blades. AND THE BLADES THEMSELVES CAN BE A SMALL FRACTION OF THE SWEPT AREA. So you pick up a factor of, say, 10 and get 450% of that 1 to 2 kW PER SQUARE METER OF BLADE AREA. The blades don't need to be kept clean, and they can be made of inexpensive materials including aluminum, wood, plastics, or even canvas or nylon sails.
For centuries wind pushed sailboats around the ocean with sails on masts up to 180 feet high, and if you can put that mast on a boat, you can put it on axle. We can even do better with modern materials. All this without even trying to be clever about energy conversion and storage. If all the energy used in the contiuguous 48 states were produced with wind, we'd be harvesting about 2 or 3 per cent of the available wind energy.
The atmosphere is a huge convection system converting temperature differences to kinetic energy in the wind. Because the visconsity of air is quite low, a large fraction, probably most, of this energy, is dissipated through friction in a relatively thin area near the ground or sea-surface. This concentration of energy is why the available wind power exceeds the solar constant.
The answer, my friend is blowing the wind. The answer is blowing in the wind."
