Solar energy conversion

energy—enough to power the great oceanic and atmospheric currents, the cycle of evaporation and condensation that brings fresh water inland and drives river flow, and the typhoons, hurricanes, and tornadoes that so easily destroy the natural and built landscape. The San Francisco earthquake of 1906, with magnitude 7.8, released an estimated 1017 joules of energy, the amount the Sun delivers to Earth in one second. Earth’s ultimate recoverable resource of oil, estimated at 3 trillion barrels, contains 1.7 × 1022 joules of energy, which the Sun supplies to Earth in 1.5 days. The amount of energy humans use annually, about 4.6 × 1020 joules, is delivered to Earth by the Sun in one hour. The enormous power that the Sun continuously delivers to Earth, 1.2 × 105 terawatts, dwarfs every other energy source, renewable or nonrenewable. It dramatically exceeds the rate at which human civilization produces and uses energy, currently about 13 TW. The impressive supply of solar energy is complemented by its versatility, as illustrated in figure 1. Sunlight can be converted into electricity by exciting electrons in a solar cell. It can yield chemical fuel via natural photosynthesis in green plants or artificial photosynthesis in human-engineered systems. Concentrated or unconcentrated sunlight can produce heat for direct use or further conversion to electricity.1 Despite the abundance and versatility of solar energy, we use very little of it to directly power human activities. Solar