Earth gets the equivalent of 84TWh/day in solar power at the surface, but current technology doesn’t allow us to collect it all, which is a shame, because the energy consumption of the entire planet is just 12TWh/day.
Part of the problem is that solar power is intermittent, that is, it can only generate electricity during the day. The US Advanced Research Projects Agency – Energy [ARPA-E] estimates that, because of this factor, solar power might be limited to providing just 5% of energy consumption here in the US.
Grid energy storage solutions could help to fill the gap, taking up extra energy when solar power exceeds demand, releasing that stored energy when supply drops below demand, but these have proven to be limited in their own right, as well as expensive.
Today, solar power typically runs along one of two lines. First, photovoltaic solar panels convert sunlight directly to electricity. Photovoltaic solar power is not overly efficient, 13% for commercially-available panels, and is the most susceptible to intermittency, due to cloud cover and daily cycles. The rest of the the solar energy is lost as heat or simply passes through, or is reflected off, the panel.
The second form, concentrated solar power [CSP], uses concentrated sunlight to heat a tank of liquid, which bursts into steam to drive a turbine, generating electricity. The liquid can hold this heat for a few hours after sundown, so CSP plants can continue generating electricity for a few hours after sundown or during cloudy afternoons.
ARPA-E is devoting $30 million to develop what seems to be a combination of these two solar power technologies. There are a number of approaches, perhaps splitting up the solar spectrum, part of which would go to a photovoltaic cell, specially-tuned for that wavelength, and the rest of which could heat water for a steam-driven generator. Another method could be to store the heated water in insulated containers to run the generators later, when the sun isn’t shining.