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Solar energy is the source of nearly all the energy that earth receives. (Nuclear and tidal energy are two exceptions.)

It might seem at first that some energy source like petroleum or natural gas is not "solar energy," but the sun allowed algae or other vegetation to grow, which decomposed and transformed into petroleum or natural gas.

The trick, then, is to harvest the sun's energy without passing it through vegetation first.

Forms

There are two primary forms of direct solar energy:

  1. absorption: sunlight heats up a storage medium
  2. photovoltaic: sunlight is converted directly to electricity

Solar absorbtion

Solar absorption can be very simple to implement, or it can be used in complex systems.

Dark surfaces absorb high energy visible light and re-radiate it as low energy heat. So something as simple as a black barrel sitting in the sun can heat water.

More sophisticated absorption systems feature water flowing over black surfaces, to maximize the energy transfer. Although less efficient, solar heat can also be transferred to air or other fluids.

Solar absorption energy is further broken down into pumped or passive. Pumped systems use fans to move air or pumps to move water through the solar collector, whereas passive systems use convection to move the air or water around.

Usually, a thermosiphon needs to have the collector below the storage area, but it is possible to use a heat pipe to move heat from a treetop panel to an underground dwelling if necessary. A suitable fluid is boiled at the high end, and condensed at the bottom.

Concentrating Solar Collectors don't work on cloudy days, when ordinary boxes still get some heat, but when the sun is shining, they can generate high temperatures for steam engines, metalworking, or just boosting the storage medium to run hotter. For the hottest temperatures, careful tracking of the sun is needed, but simple designs can get a mild boost and also operate as a regular collector on cloudy days.

One very elegant unit uses a simple insulated box with a double-glazed cover to heat air, but within the box are reflective cones and solar cells. The expensive cells put out two or three times their usual output, and barely detract from the thermal performance. The cells and their cheap mylar mirrors are protected from wind and dust. With a window-hung installation, the hot air can be circulated through a room directly, perhaps warming thermal mass therein.

Photovoltaics

Photovoltaic panels are what usually comes to mind when one hears of "solar energy." By using thin layers of silicon with minute impurities, photons in sunlight knock electrons loose from the silicon, where they are free to flow through an external electrical circuit.

Currently, solar panels are made using fossil fuels, and have to be used with care for many years to replace it, although solar furnaces could be used instead.

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