Using solar radiation for energy / electricity generation is certainly a great idea. In terms of solving current CO2 production (if current concerns prove true), eliminating environmentally devastating coal mining operations and a dependence upon a depleting supply of raw crude oil, it’s almost one of those stupid simple solutions that somehow we’ve neglected for hundreds of years, even though it was there, shining on us each day. A strong case to support using direct solar energy (versus, say, fossil fuels) can be made quickly and easily, and might go something like this:
Efficiency of solar panels or solar trough / Rankin cycle technology for electrical generation (electrical generation): ~15%
Efficiency of fossil fuel electrical generation: (E = E_ecosys * E_organic matter to fossil fuel * E_mining and processing * E_burning and electrical generation)
Since this number is hard to calculate, lets simplify it to the favor of fossil fuel: E = E_ecosys * E_burning and electrical generation, taking E_ecosys as 2% and E_burn and electrical generation = 60% we find:
E_fossil fuel electrical generation : <<1.2%
So, use the sun’s energy with 15% eff, or 1.2% eff? The choice seems obvious. Best of all, using the direct method is FREE! You don’t have to pay to mine, process, transport and burn the fossil fuel, you just have to collect solar radiation! But wait, everyone knows you are not supposed to used the “F” word, because the “F” word violates one of the few time tested truths of humanity “there is no such thing as a free lunch”. But what does using the sun cost, and why do some (Ken Zweibel, James Mason and Vasilis Fthenakis) suppose that the US could solely rely on solar electrical generation by as early (or late) as year 2100?
The cost of solar is land, because the issue at hand is energy density. Though expectantly much less environmentally damaging than say, coal mining, solar harvesting requires an unheard of amount of land for el. gen. Coal and oil mining undoubtedly occupy many thousands of square miles in the U.S. and the world over, but likely less than an equivalent area capable of producing the same energy content via direct solar el. gen. This is a bold statement, solar would use more land than coal and oil mining? But there is so MUCH solar energy, how could this be?! 4500 quadrillion BTU’s (4500,000,000,000,000,000) of solar radiation is dumped down on just the southwestern United States every year (Zweibel et. al.)! Or almost 45x MORE energy than the U.S. uses each year! So the answer is simple, just cover 1/45th of the southwest United States with solar technology like PV cells and trough / Rankin systems! And voilà, the world’s (or at least the U.S.’s) problems are solved- besides constructing a nation wide grid and the pesky and relentless enormous necessity of land! Complaints aside, how much land will be required for the U.S. to be direct solar dependent? Using a current U.S. energy consumption of 100 quads per year, and a 15% solar to electricity conversion efficiency, and a 6.5 kWh/m^2 / day solar intensity in the southwest we need:
Energy use: 100E15 BTU / yr = 2.93E13 kWh / yr = 8.027E10 kWh / day
Southwestern solar energy available: 6.5 kWh/m^2 / day
Land required is:
Energy Use / Energy Available per m^2 = 1.23E10 m^2 = ~5000 sq. miles.
However, this number assumes 100% efficiency, so accounting for eff.: 5000sq. mi. / .15 eff = 33,300 sq. mi.
Therefore, at a minimum, we would need something on the order of 33,000 square miles, which is on the order of magnitude of the size of a large state, every square inch of an entire state! Surprisingly, the proponents of the “Solar Grand Plan”, by Zweibel et. al., determine through presumably more realistic calculations that an overwhelming 165,000 square miles is actually required to support the U.S.’s energy needs. Now like I said at the beginning, solar is very attractive for many reasons, but can we expect it to be feasible to cover 165,000 square miles? That is this big, by the way (orange):
and would be even larger if you wanted the ground to receive sunlight so that plants could grow, or you wanted a city every so often within this massive area for civilization (which the power is for, remember), or you wanted any open space (like a park or forest) to get away from the dense arrays of solar collectors or panels. At any rate, it just intuitively feels infeasible to cover such a vast expanse of land with equipment for solar power electricity generation. Don’t get my wrong, I support the use of solar, but I am unsure if it is feasible for the United States to rely 100% on solar energy power production due to the shear square miles required.
Lastly, the cost of building something of this magnitude is said to be in the $400B range, and would require national policy, not unlike the American Recovery and Reinvestment Act, but maybe more direct in assisting construction. Alternatively, a $.05/kWh tax could pay for the construction says Zweibel et. al., which seems reasonable- and also leading to my conclusion that it’s not the cost of building solar facilities, but the cost of land usage that will prevent the “free” fuel from succeeding at supplying 100% of the U.S.’s energy needs.