With the price of solar cells under control, the next problem is to find a way to store the energy collected during the day for use at night. Storage of energy is not the same as storage of fuel. For instance, gasoline does not take much space, but after it is burned, the energy can only be stored in batteries and such, which are large and expensive. Storage is not a big problem for rooftop solar because that energy is only a small supplement to the electrical grid, and large power plants are still needed to supply nighttime energy. If solar farms are to provide backbone power, however, storage is needed to cover nights and cloudy days. The same methods described above for wind power are also available for solar. Capacitors or batteries to serve a GW-size solar farm would be prohibitively large and expensive, and making them would greatly increase the fossil footprint. Pumping uphill to get hydro at night is not practical, since deserts have few hills. For lack of a better idea, what is usually proposed is the unproven concept of compressed air energy storage (CAES), as shown in Fig. 3.17. Excess solar energy is used to force compressed air into under­ground caverns or salt domes. Unlike CO2 storage in such natural structures, the gas does not stay there. It is taken out at night, and its pressure is used to drive turbines to generate electricity. As explained under Wind, there is a large energy loss due to the heating of air when it is compressed.

If energy is so hard to store, what about transmitting it from the southwestern USA to the east coast? A Smart Grid for the USA is under discussion for distribu­tion of renewable energies. This is a huge project that cannot be carried out in less time or for less money than developing fusion reactors. The electrical grid is a complex network of high-voltage lines, ranging from 115 to 765 kV, connecting power generators to user sites. It has to respond to sudden changes in power needs,
and its reliability is tightly regulated. Even without the special needs of renewable energies, it has to be modernized in any case because of aging equipment and the especially stringent requirements of digital circuits [3]. Another publication from the Electric Power Research Institute proposes superconducting transmission lines cooled by liquid hydrogen, which would not only lower transmission losses but also supply hydrogen for cars [4]. Even if it makes sense, it will take many years for such a new idea to reach the design, costing, and building stages. Rights-of-way will be legal roadblocks for new transmission lines. Carrying central-station solar power straight from Arizona to New York or from North Africa to Paris requires changing the whole infrastructure.