A solar collector is a device that transforms solar radiation from the Sun into heat, which is then transferred to some fluid. These devices are primarily used for active solar heating and allow for the heating of water for personal use. These collectors are generally mounted on the roof and must be very sturdy as they are exposed to a variety of different weather conditions.
The use of these solar collectors provides an alternative for traditional domestic water heating using a water heater, potentially reducing energy costs over time. As well as in domestic settings, a large number of these collectors can be combined in an array and used to generate electricity in solar thermal power plants.
There are many different types of solar collectors, but all of them are constructed with the same basic premise in mind. In general, there is some material that is used to collect and focus energy from the Sun and use it to heat water. The simplest of these devices uses a black material surrounding pipes that water flows through. The black material absorbs the solar radiation very well, and as the material heats up the water it surrounds. This is a very simple design, but collectors can get very complex. Absorber plates can be used if a high temperature increase isn't necessary, but generally devices that use reflective materials to focus sunlight result in a greater temperature increase.
It is important to note that solar collectors do not change the amount of sunlight incident on the surface of the Earth. Regardless of whether or not a solar collector is placed in a certain area or not, the solar energy to the Earth is at most 1376 Watts per square meter. The only difference is that if there is a flat solar collector covering a meter, this energy can be collected for human use instead of simply "going away". The way that some solar collectors "absorb more energy" is a result of an increased surface area that the Sun hits.
These collectors are simply metal boxes that have some sort of transparent glazing as a cover on top of a dark-coloured absorber plate. The sides and bottom of the collector are usually covered with insulation to minimize heat losses to other parts of the collector. Solar radiation passes through the transparent glazing material and hits the absorber plate. This plate heats up, transferring the heat to either water or air that is held between the glazing and absorber plate. Sometimes these absorber plates are painted with special coatings designed to absorb and retain heat better than traditional black paint. These plates are usually made out of metal that is a good conductor - usually copper or aluminum.
This type of solar collector uses a series of tubes, similar to fluorescent lamps, to heat water for use. These tubes are evacuated of all air and have a metal strip which acts as the absorber plate in the center of each tube. These tubes use a heat pipe to carry the heat collected from the Sun to the water. This heat pipe is essentially a pipe where the fluid contents are under a very particular pressure. At this pressure, the "hot" end of the pipe has boiling liquid in it while the "cold" end has condensing vapour. This allows for thermal energy to move more efficiently from one end of the pipe to the other. Once the heat from the Sun moves from the hot end of the heat pipe to the condensing end, the thermal energy is transported into the water being heated for use.
These collectors, sometimes known as parabolic troughs, use highly reflective materials to collect and concentrate the heat energy from solar radiation. These collectors are composed of parabolically shaped reflective sections connected into a long trough. A pipe that carries water is placed in the center of this trough so that sunlight collected by the reflective material is focused onto the pipe, heating the contents. These are very high powered collectors and are thus generally used to generate steam for electricity generation and are not used in residential applications. These troughs can be extremely effective in generating heat from the Sun as they can pivot, tracking the Sun in the sky to ensure maximum sunlight collection.
These collectors are large parabolic dishes composed of some reflective material that focus numerous, parallel beams of light on a single focus. Water flows through this focus and is heated in the process. The heat from these collectors is used for steam generation or for driving Stirling engines. Although very effective at collecting sunlight, they must actively track the Sun across the sky to be of any value. These dishes can work alone or be combined into an array to gather even more energy from the Sun.
A device known as a hybrid PV/T concentrator (or CPVT) has been developed that is extremely similar to these parabolic point focus collectors. This device is similar to a parabolic solar thermal collector in its shape and mirror makeup, except that it uses a small number of PV cells instead of a heat transfer fluid. Here, the parabolic dish concentrates solar energy onto a small area covered in PV cells. This concentrating system is advantageous as it requires fewer solar cells while still generating a significant amount of electricity since the sunlight is first concentrated before it hits the solar cells. This system is beneficial as solar cells are more expensive than mirrors, and thus if mirrors can be used to concentrate the sunlight onto a smaller number of solar cells it will save money.