The most commonly used domestic water heaters are solar water heaters. It converts solar energy ‘sunlight’ into heat energy for water heating. Solar water heater consists of solar panel “Solar thermal collector”, Storage tank and working fluid.
Water circulation across solar panel can be active or passive. Active solar panel uses pump for circulating water. The pump requires power to operate and thus active solar panel system requires electricity to operate.
Passive solar panel works with the principle of natural convection known as thermosyphon, The temperature difference across solar water circulation helps in circulating water.
Passive solar system are less expensive due to non requirement of pump and control system, this system also require no power for water circulation as required in case of active solar.
Type of solar system
1. Direct Circulation (Open Loop)
In direct circulation water for heating is directly circulated and heated directly in the solar thermal collector and hot water is consumed in home or industry.
This type of heaters can be installed where water quality is good, having low TDS.
2. Indirect Circulation( Closed loop)
Separate fluid generally distilled water is used in solar thermal collector for generating hot water in indirect circulation type solar system. Heat of hot water generated is exchanged though heat exchanger for water consumption. In this case circulation fluid with qood water quality keeps on circulating in collector hence life of collector increases.
Indirect circulation type water heaters are famous in the area where water quality is poor. These heaters are also popular in low temperature areas to avoid freezing issues of water inside solar thermal collector.
Direct and indirect circulation system can be designed with both both active and passive system.
Solar water storage tanks are insulated to avoid heat loss to atmosphere.
Another most important component on solar heater is solar thermal collector panel. Solar thermal collector panel receives and converts solar energy into heat energy.
Types of solar thermal collector:-
1. Flat plate collector
The Flat plate collectors are in use from last many years and still it is used for domestic application.Contruction of Flate plate tube is simple, It consists of insulated metallic box with toughened glass on the top, Absorber plate, Virtical tubes also called as raiser tube, bottom and top header.
Metallic Box consists of coated absorber sheet for the absorption of solar energy. Vertical tubes made of copper ‘raiser tubes’ are attached to the absorber and carry the water for heating. Raiser tubes are connected with two horizontal tubes at bottom and top as shown in picture of flat plate collector.
Bottom tubes are used for water feeding and top tubes are used for hot water withdrawal.
2. Evacuated solar tube collector
The Evacuated Tube Collector consists of two co-axial glass tubes fused at both ends with air between the tubes evacuated to create vacuum that works as insulation. The outer surface of the inner tube forms the collector area and it has selective coating to increase the absorption of solar energy.
Cold water enters the storage tank, fills it up and then flows into the individual tubes until all the tubes are full. At the time of Sun shine, heat energy is absorbed and transferred to the water in the inner tube.
Evacuated tubes are more efficient type of solar heater and occupy 33% less area for similar duty of flat plate solar collector.
Solar heaters may be installed along with electric heater for consistent supply of hot water.