We'll get to some of the reasons in a minute. How to Harness the Sun Passive systems consist of collectors which absorb heat and redirect it. In a typical passive system, sunlight passes through a pane of glass onto a dark, heat-absorbing material. The heat then radiates slowly into a building or a tank of water. A common passive solar energy apparatus is something called a thermosiphon system, which uses the natural convection of heated water to circulate liquid from the collector through pipes and along a wall, or past a water heater.
Active systems are similar, but use pumps to circulate the heat absorbed in the collector.
Then there are solar concentrators, which use mirrors to focus the sun's rays. These systems produce enough heat to drive commercial steam turbines and they come in a number of different forms, including parabolic dish collectors, which look like giant satellite dishes lined with aluminum foil. The dish focuses solar radiation at a receiver, where the intense heat drives a small engine that generates current directly, or heats water which is directed to a turbine. Another solar concentrator, called a central receiver system, consists of a tower surrounded by thousands of mirrors, each of which has a tracking mechanism to make sure it is tilted at the proper angle to keep the suns rays focused on the receiver, where liquid is heated. The third version of the solar concentrator is called a trough collector, and it uses long parabolic, trough-shaped mirrors that direct the sun's rays at a tube of liquid.