There are two types of solar water heating systems, based on type of circulation:
All domestic and industrial systems upto 4000 LPD (liters per day) 60 deg.C,; works on the principle of thermosyphon. Industrial systems beyond 4000 LPD to any capacity will work on forced circulation principle.
The system works on a principle called "THERMOSYPHON" (Natural circulation).Due to density difference between the cold & hot Water, the lighter hot water flows up into the tank and the heavier cold water enters the collector.
Solar radiation incident on the collector panels heats the absorber. This heat is transferred to the water contained in the absorber & becomes less dense than the water in the storage tank. Hot water starts rising and settles at the top of the storage tank. Simultaneously cold water descends to the collector absorber tubes, gets heated up and the cycle repeats.
The circulation in the system during the heating process is purely due to the difference of density only. There is no other mechanical moving part, hence the system reliability is very high.
At the end of the day when radiation is zero (after dark / sun set) the collector will contain heavier cold water and the tank will have lighter hot water. This will ensure that there will be no circulation during night & hence no conduction losses on account of this.
The hot water storage tank is well insulated to retain the hot water temperature and to minimize the overnight temperature drop / heat losses.
The Sacrificial Anode (Only for SS tank) prevents the tank from galvanic corrosion. For the monsoon / cloudy days the system is-fitted with Electrical back up heater with thermostat.
Forced circulation system:
In forced circulation systems, two pumps in tandem are used for circulation of water between collector bank & system insulated tank. The pump operation is controlled by OTC (Differential Temperature Controller), which is fitted Inside the Control panel. OTC senses the cold & hot water temperature through two sensors, one fitted at the bottom of the tank to sense cold water temperature & the other fitted at the top of the last collector of the row in the collector bank, to sense the hot water temperature inside the collector
The difference in temperature between the two sensor points is the criteria for the operation of the pump that is controlled through DTC. This temperature difference can be set between 0 to 10 deg, C.
Solar radiation falling on the collector heats up the water inside the absorber. The temperature of water starts gradually increasing compared to the water temperature inside the tank. When this temperature difference between the sensor points reaches the preset temperature difference in DTC, the pump is switched "ON". This will result in entry of cold-water form the system-insulated tank into the collector bank & discharge of hot water from the collector bank to the system insulated tank.
The sensor fitted at the collector will sense the temperature of cold water entering into the collector & puts "OFF" the pump when this temperature difference between the sensor points falls below the preset temperature difference in DTC.
The process of switching ON & QFF of the pump & circulation of water continues till the end of the day. By the end of the day, the entire quantity of water in the tank reaches the system-designed temperature, provided the radiation level is at minimum of 800 watts per sq.mtr.