Solar water heaters based on Evacuated Tube Collectors (ETC based SWH):

Here the collector is made of double layer borosilicate glass tubes evacuated for providing insulation. The outer wall of the inner tube is coated with selective absorbing material. This helps absorption of solar radiation and transfers the heat to the water which flows through the inner tube.




                              Evacuated Tube Collector based Solar Water Heater


Solar water heating is now a mature technology. Wide spread utilization of solar water heaters can reduce a significant portion of the conventional energy  being used for heating water in homes, factories  and other commercial & institutional establishments. Internationally the market for solar water heaters has expanded significantly during the last decade.  It is estimated that over 107 million sq.m. of collector area has so far been installed world wide for heating water.  In the country, the collector area so far installed for water heating is over 1.00 million sq.m. MNRE has plans to add another 1.00 million sq. m. in next two years.

General Specifications of Solar Water Heating System based on Evacuated Tube Collectors (ETC based SWH):

The Broad Specification of the Solar Water Heating Systems using ETC.

Number of Tubes in the ETC
For tubes with OD: 47 mm, ID: 34 mm and Length: 1500 mm	Minimum 7 numbers for each 50 LPD capacity of system.
For tubes with OD: 58 mm, ID: 47 mm and Length: 1800 mm.	Minimum 5 numbers for each 50 LPD capacity of system
Type of Coating (Selective Absorptive coating.	ALN-SS-Cu (Three Layer)
Tube bottom cap material / Thickness	H.D.PP. / 3 mm
Tube Seal	Silicon Rubber
Storage Tank (Inner tank, directly coupled with the tubes)
Fabricated using Argon arc or metal inert gas (MIG) welding Thickness when fabricated in SS 304 or 316 using Argon arc or metal inert gas (MIG) welding Thickness when fabricated using other type of welding Thickness when fabricated in GI (Internal coating: Epoxy paint stable up to 150ºC)	0.5 mm, minimum up to 300 LPD. 1.2 mm, minimum above 500 LPD. 0.8 mm, minimum up to 300 LPD. 1.5 mm, minimum above 500 LPD. 2.5 mm, minimum up to 500 LPD. 4.0 mm, minimum above 500 LPD.
Storage Tank (Outer cladding)
- Material - Thickness	Al/SS/FRP/GI coated sheets Suitable for handling the load
Storage Tank Insulation
- Material - Characteristic	PUF 55 mm thick / Rock wool 100 mm thick Night temperature drop should not be more than 5-6°C.
Stand / Frames	Should be either of non-corroding materials (viz. SS, Aluminum) or have corrosion resistant protective coating

Measurements of collector area:

Flat Plate Collector: Gross area of solar collector is defined as the total area on which solar radiation is falling (taking into account the gap, if any, between the tubes, and also the frame etc.). It shall be calculated by multiplying width and length of the collector measured from outer edge to the outer edge.
Evacuated Tube Collector: For the solar water heating models using all-glass ETCs, length should be measured from bottom side of the storage tank to the outer edge of bottom tube holder while width should be taken as equal to the length of the bottom tube holder. The measurement accuracy of the gross area should be ± 0.1 %. The volume of the storage tank should be measured to an accuracy of ± 1%.