Even at $0.10 per kWh, solar thermal has a very big flaw: The sun only shines half the time, which means the capital cost of a solar thermal array is spread over fewer hours of operation, making it both more expensive than plants that can operate 24/7 and unsuitable for base-load (continuous) power generation.
To challenge coal, solar needs a way to store excess electricity in the daytime and feed it to the grid at night. And here again, solar thermal appears to have the edge on PV in large-scale generation because heat is easier to store than electricity. Whereas a PV array might require giant, as-yet-undeveloped batteries or flywheels or other esoteric devices to store its electricity, solar thermal engineers have come up with several simple but promising heat storage solutions. The furthest along uses most of a thermal solar plant’s daytime heat to generate electricity and the rest to heat a mixture of sodium and potassium nitrate, known as molten salt, that liquefies when heated to between 550 degrees and 1,200 degrees and then retains its heat for 16 hours.
When the sun goes down, the stored heat can be used to run a turbine to keep the power flowing. The current design is a closed loop that doesn’t expose the solution to the air and so doesn’t pollute, while degrading only gradually. This kind of system had yet to be proven in the field as of early 2008, but if the claims now being made for solar thermal and related storage technologies pan out, it might soon be competitive in the parts of the world where 16 hours without sun is a rarity.