The solar water heater comes in wide variety of designs; all of the available designs include a collector and a storage tank. It basically uses the Sun’s thermal energy to heat water. These are basically categorized according to the type of collector and the circulation system. A collector is a device for converting the energy in sunlight, or solar radiation, into a more usable or storable form. This energy is in the form of electromagnetic radiation from the infrared (long) to the ultraviolet (short) wavelengths. The quantity of solar energy striking the Earth's surface averages about 1,000 watts per square meter under clear skies, depending upon weather conditions, location, and orientation of the surface.
Solar collectors fall into two general categories: non-concentrating and concentrating. In the non-concentrating type, the collector area (i.e., the area that intercepts the solar radiation) is the same as the absorber area (i.e., the area absorbing the radiation). In these types the whole solar panel absorbs the light.
Flat-plate and evacuated-tube solar collectors are used to collect heat for space heating, domestic hot water or cooling with an absorption chiller.
These are also called as integrated collector storage (ICS) systems. These collectors basically include an tube with an insulated box or a dark tank that is basically makes this collector a storage of water for a long period of times, if the demand of water for the family is low, also making the water Very Hot.
This system is also fitted with a tempering value that becomes your savior against extremely hot water before it is delivered to your tap.
These collectors are generally not recommended for cold climates as they are in compatible with closed loop circulation systems.
Flat Plate Collectors:
These Collectors mainly consists of copper tubes that are fitted to flat absorber plates. The most common configurations a series of parallel tubes connected at each end by two pipes, the inlet and the outlet manifolds. The flat plate assembly is contained within an insulated box and covered with tempered glass.
Flat plate collectors are created mainly to contain nearly 40 gallons of water. Two collectors provide roughly half of the water needed to serve a family of four. These types of collectors are the most common types. The main use of this technology is in solar kits of residential buildings where the demand for hot water has a large impact on energy bills.
Evacuated tube collectors:
Evacuated tube collectors are the most efficient collectors available. Each evacuated tube is similar to a thermos in principle. A glass or metal tube containing the water or heat transfer fluid is surrounded by a larger glass tube. The space between them is a vacuum, so very little heat is lost from the fluid. The vacuum that surrounds the outside of the tube greatly reduces convection and conduction heat loss to the outside, therefore achieving greater efficiency than flat-plate collectors, especially in colder conditions. This advantage is largely lost in warmer climates, except in those cases where very hot water is desirable, for example commercial process water. The high temperatures that can occur may require special system design to prevent overheating.
Direct systems circulate water through solar collectors where it is heated by the sun. The heated water is then stored in a tank, sent to a tank less water heater, or used directly. These systems are preferable in climates where it rarely freezes. Freeze protection is necessary in cold climates.
Closed-loop, or indirect, systems use a non-freezing liquid to transfer heat from the sun to water in a storage tank. The sun's thermal energy heats the fluid in the solar collectors. Then, this fluid passes through a heat exchanger in the storage tank, transferring the heat to the water. The non-freezing fluid then cycles back to the collectors. These systems make sense in freezing climates.
Active, or forced-circulation, systems use electric pumps, valves and controllers to move water from the collectors to the storage tank. These are common in the U.S.
Passive systems require no pumps. Natural convection moves water from the collectors to the storage tank as it heats up.