With the exception of geothermal and tidal energy, most renewable energy is solar at its root. Wind is caused by the sun heating the air. Biomass stems from sun-fed plants. Hydro works because the sun evaporates water from oceans and lakes, which then falls on the land as rain or snow. But when we talk about solar energy, we mainly mean energy derived from the sun without all those intermediary steps. We use two basic methods to harness the energy of the sun that directly: photovoltaics (PV) and solar thermal. In PV, sunlight strikes semiconductors that turn light into electrical power. Solar thermal harnesses solar radiation as heat. The photovoltaic effect isnt a particularly easy thing to explain. (Albert Einstein did so in 1905, work for which he won the 1921 Nobel Prize in Physics.) But the basics are reasonably straightforward: each PV cell is essentially two stacked electrodes composed of semiconductors. Light, made up of photons, hits the electrode in front. Some of those photons is absorbed by the atoms that make up the electrode, increasing the atoms energy. If the photon striking an atom has a certain amount of energy, it knocks an electron off the atom, and the freed electrons migrate to the back electrode. This sets up a difference in electrical potential -- a.k.a. voltage -- between the electrodes, and if the electrodes are connected by wiring, electrical power begins to flow. Thats a vastly oversimplified explanation, but itll do for our purposes. kcet.org/news/redefine/rewire/explainers/explainer-pv-and-solar-thermal.html
Posted on: Mon, 17 Nov 2014 02:14:00 +0000
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