Solar water heaters make use of the solar energy to generate hot water. Solar water heaters find their use in a number of applications ranging from home, industrial and business uses.
Solar water heaters can be used in Bungalows and apartments. Commercially they can find their use in hotels, Dormitories and hospitals. Solar water heater can provide for the large scale demand of hot water required in industries like process industries etc. Solar water heaters can be used for heating swimming pools and a solar water heating system can also be used in solar thermal cooling system to promote temperature control in buildings. During cool conditions the system can be used to provide hot water.
Solar water heater designs vary, and the complexity of the design and its type is determined mostly by a number of factors such as the ambient temperature change that take place during night and day cycle, changes in solar radiation and ambient temperature between winter and summer and the water temperature required. Performance of the solar water heater differs from site to site depending upon the level of solar energy available at the place.
The minimum efficiency of the solar water heater is measured by the temperature or the amount of hot water required during the months of winter when the requirement of the hot water is largest. Similarly the maximum efficiency of the solar water heater is measured by the necessity to prevent the water from becoming excessively hot.
Solar water heater systems are usually have solar thermal collectors, interconnecting pipes, a water storage tank and fluid system that transfers the heat to the tank from the collectors. The thermodynamic mechanism used here is different from semiconductor photovoltaic cells which use light to generate electricity, whereas here the process comprises of heating the liquid directly by the sun and there is no generation of electricity.
There can be broadly two types of solar water heaters namely- active and passive.

Passive solar water heaters
Passive systems do not use a pump and use heat pipes for circulation. Integrated Collector Storage is a special type. Here the tank provides for both-as the solar collector as well as the storage. These are usually not pressurized and hence flow depends on the gravity. These have a glass in the front. Passive systems are efficient and simple and less expensive but are suitable for moderate climates only.
Convection Heat Storage unit is another type of passive system which is a level up from the Integrated Collector storage. These are usually evacuated tube collectors or the plate type and have a built-in insulated tank. Convention is used for the movement of water. The advantage of this over Integrated Collector storage is that it can be used with less sunshine as it is more efficient.
Passive systems can also be classified as the direct and indirect passive systems.
In direct i.e. open loop passive system the water circulates between storage and collector tank. The water in the collector rises and flows to the storage due to convention as soon as it becomes warm. However these types are not suitable in cold climates because if the water in the collector freezes at night then it can damage the panels.
In Indirect or closed loop passive system an antifreeze heat transfer fluid (HTF) is used which is non toxic. Once this fluid gets heated, it flows to the tank due to convection. The heat of the HCF is transferred to the water by a passive heat exchanger.
Passive solar water heaters require very less maintenance as they are quite simple and do not have complicated electrical or mechanical parts. However the efficiency of passive heaters is also lower than that of the active heaters. Overheating is generally prevented here.

Active solar water heaters
Active solar water heaters make use of a pump for the circulation of the water. These are again classified into two types: direct and indirect active systems.
Direct systems directly pump water to the collector and storage tank, while the indirect ones use heat transfer fluid. A controller is needed here to turn the pump on and off because the pump should operate only when the fluid present in the collector becomes hotter than the water present in the storage tank.
The advantage of electronically controlled pump is that the storage tank can be located lower than collector which is not feasible in the passive systems. The freedom of location of storage tank enables it to be located at a place where the heat loss is minimized. The pump controllers also prevent overheating. Hot water is pumped through the collector which is cold in night, thus avoiding overheating. Similarly water from the storage tank can be pumped through the collector during cold weather to prevent freezing.
The advantage of active water heater systems is that they can tolerate higher temperatures and therefore are more efficient then passive heaters. But they are more expensive as well as more complex than the passive heater systems. Moreover they have to rely on electricity in order to run the controller and the pump.
Modern active solar water heaters have intelligent controllers to allow a range of different functions.
During good sunshine the water in the collector can attain very high temperatures. Therefore solar water heater designs should be such that allow excessive pressure and excess heat to be relieved through a heat dump.

Similarly in order to prevent damage due to freezing, freeze protection measures must be included. To ensure that there is no damage due to expansion of transfer fluid due to freezing some systems use drain back and drain down measures that is they drain he fluid if the pump stops, while others use anti freeze.

Cost of the solar water heater systems
 In places where there is no need for freeze protection an ICS solar water heater can prove to be extremely cost effective. At higher latitudes the initial cost increases due to complexity of the design, but not the life cycle cost. Since the solar energy is free, the operating cost is significantly reduced in comparison with other energy sources.