A solar collector is a device used to capture the heat energy of the sun and convert it into a form more readily usable by humans. Unlike a photovoltaic cell, a solar collector is relatively low-tech, and they can be created and installed for a very low cost. The most basic type of solar collector involves some sort of liquid medium that is heated up by the sun’s rays and then transported to distribute heat elsewhere.
A simple form of solar collector can be seen just by leaving a large black container filled with water outside beneath the hot sun. After only a few hours, the water within the container will have absorbed a great deal of the energy of the sunlight, and will be very hot. This hot water can be used for a simple purpose like taking a shower, to fill a hot water bottle to heat a sleeping bag, or some other form of heat distribution. More complex collectors take this basic concept and expand it, making the heat collection more efficient, and the heat distribution more dynamic.
One of the main ways a solar collector can be made more efficient is by using a transport medium that is different than what is being heated up. While water itself makes a perfectly usable medium, many people prefer to use something different, depending on the environmental situation. In regions where the temperatures drop to below freezing, for example, an anti-freeze solution may be used instead of water to prevent pipes or plates from cracking. Glycol is often also used as a heating medium, as it can be more convenient to handle and is more viscous than water, but remains an excellent heat retainer.
Often a solar collector will make use of piping to push water through in order to heat it up more quickly. These pipes will generally be made of something that is a strong thermal conductor, such as copper, and will be run back and forth across a surface to make the best use of space. Water will be forced through the pipes to heat up beneath the sun, and when it is hot enough, will be run to a reservoir or directly into an application where the heat can be used.
One place where solar collectors are often used is in heating the insides of buildings, generally by running piping through the floors. Water can be heated by the sun and then forced through pipes embedded in a stone slab floor, where the heat then radiates out to heat the air mass of the room. This can be a cheap and renewable source of heating energy in places with sufficient solar exposure.
Generally a solar collector is of seasonal use, with the heat it absorbs being used within a day or two, making it ideal only during summer months in most places. In some cases, however, a sufficient thermal mass can make heat storage feasible over the span of many months. Some large apartment complexes, for example, have begun experimenting with using millions of gallons of water as a thermal mass which can be heated up over the summer, and then used over an entire winter as a source of heating.