Hi Cap!
The 6 percent figure is from a series of 10 lectures in the physics of climate presented by Professor Richard Wolfson, Middlebury College. His professional opinion seems to be supported on wiki, ref:
http://en.wikipedia.org/wiki/Solar_power At the wiki site there are maps showing the area necessary. You can buy audio tape of the lecture series by calling The Teaching Company at 1-800-832 2412. The wiki site gives also the figures in watts for insolation - useful for rough calculations.
Allow me to quote you, in part, to address your post: "...The reasons that solar power plants do not power the world are summed up in two reasons: cost and reliability. Solar plants made of solar cells are extremely expensive to build. Thousands of square feet of solar cells are required to generate enough power for even a very small town. A large city simply doesn't’t have enough room for a solar plant, or enough money. The second main reason is the unreliability of the plants. If the plant has a few cloudy days, there will be no electricity for the town..."
Solar-thermal power plants cost roughly 1.5 times the cost of modern combined cycle GT-HRSG-ST plants. Because comparison demands it, allow me a word here about these modern combined cycle plants, which I have built. GT-HRSG-ST means gas turbine - heat recovery steam generator - steam turbine. For sound engineering and environmental reasons these are almost always operated on NG, though if they were able to run them on coal they would be more efficient, as coal oxidizes to CO2, which does not undergo a phase change in the process and has a fairly low latent heat compared to the CH4 oxidation, which, obviously, produces H2O and CO2. The dry gas flow of a coal burning plant having therefore a greater efficiency as the same chemical energy produces higher a temperature. The "delta-T" is higher in a straight carbon burning boiler or engine. (By the way, the experimental original diesel, which attempted to achieve a near carnot cycle, burned coal for that reason. It had a compression ratio of about 80:1. It failed rather dramatically when it exploded!) Anyway, the point is that NG costs, drilling, exploration, pumping, pipelines, etc, are not added to the price of a NG power plant. Contrawise, in the case of a solar-thermal plant, the "fuel" costs are part of the capital investment. One would be inclined to regard, therefore, the Solar Thermal plant as cheaper, not more expensive. It is true that these plants take up considerable space, but the areas devoted to NG extraction and associated support of a NG plant also takes up considerable space. One would be inclined to regard the overall space requirement as roughly equivalent. Current engineering in solar thermal permits operation 24-7 by means of heat storage. Take a look at:
http://www.solardev.com/SEIA-makingelec.php A further note regarding reliability - any nut with a high-power rifle can shut down a NG plant, and not just at the plant! There are many nuts, it only takes one. Even worse, NG fields by nature give very little warning of depletion and are therefore by nature unreliable. Importing NG as LNG also is an expense and dangerous liability, as well as an efficiency killer.
Solar voltaic power generation is, by nature, a distributed power system, and thus takes up space that is, generally, unused. Rooftops, above parkinglots, along highways, and so forth. Therefore the space requirement for SV systems does not generally argue against the approach. SV systems are expensive. However one knows the cost of the system up front, both the capital cost and the operating cost, unlike the case with an oil, coal, or NG plant. The Germans are paying the cost of SV by a utility tax - about 16%, as I recall. SV systems are very reliable, more so than any other method of power generation, there are no moving parts and system life is often estimated at about 40 years of operation. The German SV goal is 20 % of domestic load by 2012, I think, and they're ahead of schedule and it looks like they'll make 30% by then. The cost of manufacturing SV elements, the PV panels, is forecast to drop by a factor of 5 (ref:
http://www.solaicx.com/ The technical guy there is a distant cousin of mine, by the way.) so the cost of SV may be expected to become fairly minor, absent price gouging.
Your point about unreliability due to clouds would be easier to understand if there were no electrical grid. As things stand today, at least in the industrialized countries, all cities and towns are, as everybody knows, interconnected. A cloudy day is therefore irrelevant so far as keeping the power on in an Nth spot.
As many people do not know the history of solar power systems, it is understandable that they misunderstand the reason why solar plants do not (yet?) power the world. In fact the reasons are primarily two. One is that Britain "won" WW1, the other is that they then found vast amounts of easy to get oil in what had been the Ottoman Empire. Churchill had made the decision to convert the navy to oil, against those who argued for "energy independant" coal. Therefore Britain needed the oil from the Middle East, and they developed those wells. In this context it is interesting to note that American inventors had built and were selling, in California, solar-powered water-pumping systems early in the century, before the Great War. These pumps were on the order of 2500 gallons per minute, serious engines. British engineering expertise combined with the American know-how actually resulted in a solar power plant being installed at Cairo and in operation by 1912. When the war came German saboteurs destroyed the plant. The plant had produced electrical power cheaper than its competition - a coal powered plant. If the Ottomans had not allied themselves with Germany - if they'd stayed out of the war... well, we can only speculate. But it's fair to say that the American Army might not today be in Iraq.
Addressing costs again, let's look at some approximate figures: two thousand million dollars per week spent on the Iraq adventure, which can only be about oil. That could buy something like 200 million solar panels per year for each of the 5 years the business has gone on. If one assumes that each panel works 12 hours per day and makes 100 watts, then the money spent to date would have paid for something like 100,000 megawatts of daily domestic power, speaking very roughly. That's about 100 full-sized power plants, but distributed all over, so that no cadre of "evildoers" could do any material damage at all. As things stand, however, even a small group could "wrench" just about any plant - if they knew what to shoot at. Thank goodness they haven't!
It is true that solar plants are not very efficient. It is also true that efficiency is only really significant when you have to pay for or carry the fuel or have to stuff a great deal of powerplant into a small space. With regard to solar power these considerations generally are minimal. Efficiency is therefore in this context a distraction.
My own tiny plant mimics the real world situation, the political-geography-induced reality. Oil's still cheap so we have the lister type engine, and chose it because of its potential for using waste and vegitable oils. We add solar pannels each year as a "tax" on ourselves. I assure you that the listeroid is loads more trouble than the solar system is. I fully expect the "'loid" to become more and more a silent icon resting in a tomb-like power-house. As that state developes we will add reliability with additional batteries.
Best, Phaedrus
