During the day, there is a constant supply of radiation coming from the sun. The amount of radiation is considerable, but presently low cost commercial solid state solar cells only convert about 11% of the solar radiation into electricity. There are already laboratory solar cells that are 40% efficient, and in the future even higher efficiencies may be possible. Solid state solar cells are very attractive because they have no moving parts and are very simple. Because sunlight is free, this makes the technology very attractive especially in countries that have difficulty buying fuel. The downside of solid state solar cells is that when the sun goes down, there is no electricity being produced. Batteries can be used, but present batteries are only about 60-80% efficient in storing the electricity. Just as with wind turbines, if solar panels are linked into a large grid system, such fluctuations are not as much of a disadvantage.
A second method of using the sun's radiation is to convert it into high temperature thermal energy and then use conventional steam turbines, gas turbines or Stirling engines to generate electricity. Such methods are already 30-50% efficient in converting the sun's radiation into electricity. There are also efficient solid state thermoelectric converters being researched. If a fluid is heated, a large amount can be stored for operation of the plant during the night or cloudy days. As well, a backup fuel fired heater can be used, but this is only economical when the power plant is highly efficient.
A third desirable method is to use the sun's rays produce a fuel. This fuel could then be used at a later date. Hydrogen could be produced but it is difficult to store. An ideal fuel to produce would be ethanol or natural gas which could be used in a fuel cell at a later date to generate electricity or be used in other applications that require fuel. The solar cell would recycle the carbon dioxide from the atmosphere back into the ethanol or natural gas fuel. Such a solar cell might use genetically engineered bacteria to do the job.
Theoretically much of the thermal energy required in society could come from solar energy. Practically so far it has been considered too intermittent a source. Solar collectors for this purpose can be quite simple, but storage of the thermal energy during periods when the sun isn’t shining has so far been considered expensive compared to cheap fossil fuel. This situation could dramatically change when fuel prices go up in the future.