A Solar Water Heater is a device which provides hot water for any purpose using Sun’s thermal energy for heating the water.
A Solar Water Heater comprises of an array of solar collectors to collect solar energy and an insulated tank to store hot water. Both are connected to each other. During the day time, water in solar collectors gets heated which is either pumped or flown automatically on thermosyphon principle to the storage tank. Hot water is then stored in the tank which can be used for various applications.
Two types of Solar Water Heaters are available:
01. Based on flat plate collectors
02. Based on evacuated tube collector
Flat plate collector (FPC) based systems are of a metallic type and have longer life as compared to Evacuated tube collector (ETC) based system because ETCs are made of glass which are of fragile in nature.
Courtesy - hotfrog
Both these systems are available with and without heat exchanger. They can also work with and without pump. Systems without pump are known as thermosyphon systems and those with pump are known as forced circulation systems.
Evacuated Tube Collection based solar water heaters are cheaper than Flat Plat Collection based systems. They perform better in colder regions and avoid freezing problem during sub-zero temperature. FPC based systems also perform good with anti-freeze solution at sub zero temperature but their cost increases. In other regions, both perform equally good.
Systems working on thermosyphon principle are simple and relatively inexpensive. They are suitable for domestic and small institutional applications, provided water quality is good and it doesn’t have large salts / chlorine contents. Forced circulation systems are generally preferred in industries or large establishments.
At places where water is hard and have larger chlorine content, if FPC based system is being installed, it must be with heat exchanger as it will avoid scale deposition in copper tubes of solar collectors which can block the flow of water as well reduce its thermal performance. ETC based systems will not block the flow of water but its performance may go down due to deposition of salt contents on inner surface of glass tubes, which could be cleaned easily once in a year or so.
Can the glass tubes withstand hail stones?
The glass used to make these tubes is not ordinary glass but very strong boro-silicate glass, which enables the tubes to resist hail stones as big as 25 mm (1 inch) diameter size.
Why ‘new’ ETC (Evacuated Tube Collector) Technology is better than the ‘old’ FPC (Flat Plate Collector) Technology?
The ETC technology is almost a decade old and thus not so new! This technology, though much superior and suitable than FPC technology, could not take-off being more expensive than FPC technology. It will be obvious that the only advantage FPC ever had over ETC was lesser price. But NOWADAYS, with recent worldwide advances in mass production techniques of evacuated tubes, ETC based systems are more cost effective than their FPC counterparts. In other words, FPC systems are as obsolete as typewriters in the computer age.
Comparison between Evacuated Tube Collector & Flat Plate Collector
|Sr. No.||Evacuated Tube Collector||Flat Plate Collector|
|01.||Quick heat generation||Flat Plate generation|
|02.||Collector efficiency on higher temperature is high.||Collector efficiency on higher temperature is low.|
|03.||Heat loss in the tubes during the daytime is negligible (evacuated tubes).||Heat loss in the collector & tank during the daytime is high due to convection.|
|04.||Convection and convecting losses are low.||Convection and convecting losses are high.|
|05.||Emissivity is low||Emissivity is high|
|06.||Satisfactory performance even in extreme cold condition (-18 deg. C)||Freezing of water will take place at high altitude causing damage to the collector|
|07.||Temperature range from 600 deg. to 1200 C.||Temperature range from 60 deg. to 80 deg.|
|08.||System hot water tank only is insulated using polyurethane insulation material which does not absorb water or moisture.||Collector & tank insulated with glasswool/rockwool, absorbs moisture as the bore dia is 12.50 mm. giving rise to substantial loss in efficiency of the system.|
|09.||Negligible scaling of tubes which can be cleaned manually (inner tube dia is 37 mm). Loss of efficiency consequently is minimal.||Heavy scaling of the copper / aluminium tubes which cannot be cleaned manually as the bore dia is 12.50 mm giving rise to substantial loss in efficiency of the system.|
|10.||The collector glass tube absorbers being cylindrical the incident sun’s rays on the tubes is at 90 degrees throughout the day. Hence peak heat absorption always.||The collector fins & tubes being flat the incident sun’s rays will be at 90 degrees at noon only for peak absorption|
|11.||In locations with average availability of solar energy over-sizing of the system glass tube collectors is not required.||Higher system sizing is required to get the desired result. Hence added cost.|
|12.||Heat exchanger not required||Heat exchanger required|
|13.||Advanced technology, at competitive prices i.e. System Cost per unit water is low||Old technology at higher prices|
|14.||Hot water availability for 350 days in a year.||Hot water availability for 300 days in a year claimed|
|15.||System life above 15 years||System life above 15 years|
|16.||Water quality will not effect the system||Water quality effects the heating system forming scale over metal tube|
|17.||It has low maintenance.||It requires high maintenance.|
|18.||Grouting of Collectors not required||Grouting of Collectors is required|
Efficiency graph of different types of Solar Collectors:
Efficiency graph showing performance of different types of Solar Collectors indicates evacuated tube collector has highest efficiency at higher temperature. For this reason, evacuated tube collectors are used.