Solar energy refers to using sunlight to generate energy in the form of electricity or heat. Green plants do something like this, turning sunlight and air into sugar. The technology approach is to use large flat panels filled with solar cells, which are silicon wafers (or other semiconductor).
Solar cells work because sunlight knocks electrons loose in the material. The semiconductor material is biased so that the electrons can only flow one direction, hence, electricity. Solar cells work on a small scale (hand-held calculator) or large (acres of panels). Large-scale arrangements are connected together in series and by an inverter to the local power grid.
Solar heating can be done with flat panels that absorb heat, backed by circulating hot water to transfer the heat. Solar heating is usually local. It can heat water, warm a building, or (using a heat exchanger) cool the building. For example, solar panels can heat a swimming pool. A heat reservoir, such as a pit full of hot rocks, can store heat during daytime for use at night.
Engineering and Economics
To date, solar power generation is not as economical as traditional energy sources. The usefulness is strongly dependent on location, with low latitudes and clear skies being preferred. Installation costs are significant. Solar power is quite attractive for isolated locations such as roadside signs, assuming that the intermittent flow of power is tolerable (no solar power at night).
Most solar cells only convert 10-20% of the light energy falling on them. Tricks to up the yield include magnifying lenses, panels that track the sun, and simply using more area. In other words, higher yields cost more. Meanwhile, significantly more efficient cells are being developed.
The investment in solar cells pays back in 10 to 20 years depending on location, technology, and other factors. That can be good enough for governments and large businesses, but is marginal for residences and small businesses. Solar energy is politically popular as a sustainable source with no pollution after the manufacturing stage. Consequently, tax credits and other subsidies encourage solar energy in many jurisdictions.
Solar Energy and Metals
Solar cells make use of metals and near-metals that are often mentioned in discussions of sustainability. Germanium, a semiconductor, is a common replacement for silicon, especially in the recent past when pure silicon crystals were in short supply. Gallium arsenide and indium gallium arsenide are two other material choices. The new copper indium gallium diselenide is the material for solar thin films.