In the SimplySolar Yahoo Group someone recently asked if the claim that 100 by 100 miles, 10,000 square miles, of concentrated solar thermal power could provide enough electricity to power all of the US. The claim was made at http://www.100milesofmirrors.com under the banner 100 Miles of Mirrors. To verify the feasibility of the claim I did the research and calculations given below.
A brief intro to solar thermal
Briefly, as the following diagram shows, concentrated solar thermal power involves concentrating the suns energy using parabolic mirrors. For 10,000 square feet of this there would be many, many such rows of mirrors. The mirrors focus all that energy onto a pipe that contains oil, heating the oil. Using a heat exchanger, the heat from the oil turns water into steam. The steam is run through a steam turbine which turns a generator and produces electricity. The steam is next run through a condensor to turn it back into water for use again. The heated oil can also be used to heat salt until it’s molten. This stores the heat for when the sun isn’t shining, at which time the heat in the molten salt acts as the heat source to heat the oil which is then used to turn water to steam to spin the turbine and run the generator.
To do calculations I first needed to find a solar thermal field in Arizona to act as a baseline for calculations. I found this announcement that Albiasa Solar of Spain is building a 200 MW (megawatts, or million watts) concentrated solar themal power field in Arizona and further digging found that the field was 1,400 acres in size.
Step 1. The proposed 100 miles of mirrors is really 100 miles by 100 miles which is a 10,000 square mile solar thermal field (100 multiplied by 100.)
Step 2. 10,000 square miles divided by 0.0015625 square miles per acre means that the field is 6,400,000 acres (1 acre is 0.0015625 square miles.)
Step 3. In 6,400,000 acres you can fit 4,571 solar thermal fields the size of the Albiasa Solar field being built in Arizona (6,400,000 acres divided by 1,400 acres.)
Step 4. Since the Albiasa Solar field produces 200 MW of electricity, that means the production of the 100 miles or mirrors field would be 914,200 MW (4,571 * 200 MW.) That’s 914 GW (gigawatts, or billion watts.)
Step 5. According to Wikipedia in 2007, US summer demand for electricity was 783 GW. That means at any one time the peak demand for solar in the US in 2007 was 783 GW.
Step 6. The 100 miles or mirrors of solar thermal plan would produce 914 GW while the US needs at most 783 GW at any one time. So it looks like the plan is reasonable.
Of course it’s often pointed out that the sun doesn’t always shine. But as said above, the concentrated solar thermal systems store heat in molten salt so that this heat can be used to continue generating power while the sun is no longer shining.
Then there’s the issue of transmission losses. When transmitting electricity over large distances much of that power is lost. Given that the 783 GW is peak demand in the US, that means that at other times the demand is less. So for those non-peak times, enough power may be generated. During peak demand, when 783 GW is needed, it may be necessary to draw on some of the stored energy to add to the 914 GW being produced in order to meet the demand plus make up for transmission losses.
One last thing. 100 miles of mirrors doesn’t sound like a big deal. But remember, it’s 100 miles by 100 miles, or 10,000 square miles. Here’s what that looks like overlaid on a map of Arizona.
But don’t let that scare you. If all the farm land in the US were highlighted instead, the area would be larger. And none of that farm land needs the desert.
In conclusion, from my quick calculations the 100 Miles of Mirrors concentrated solar thermal power plan could generate enough power to meet the needs of the US.