Using the sun to heat water can be cost-effective and environmentally friendly. Solar water heating is generally more sensible for families that use a lot of hot water. Although it can work anywhere, it is more cost-effective at lower latitudes and in sunnier climates.
A solar water heater needs a backup system. One easy way to provide backup is to add a solar system to an electric or gas storage water heater. A tankless heater can also be used for backup.
There are active and passive solar water heaters. The simplest systems are passive, using nothing but solar energy and gravity to circulate water between the storage tank and the collector, where the water heats up. As water in the collector heats, the hotter water rises into a storage tank placed slightly above the collector, while cooler water runs down to replace it. Active systems are generally more reliable than passive ones, and they can be put in more places. They usually have a pump to move the water from the collector to the storage tank, so the collector can be on the roof, in the yard, or wherever is convenient.
Solar hot water systems can be vulnerable to freezing. If outdoor water lines freeze, their piping can be destroyed. Open-loop systems, which run tap water through the system, need to be protected. But no matter how well protected they are, temperatures below 35°F will keep these systems from functioning. A closed loop, which runs antifreeze or air through the outdoor pipes and then transfers the heat to the tap water, will keep the pipes from freezing.
Open-loop solar systems circulate tap water directly through the collector and store it in the tank. This works best with water that is not hard or acidic. Hard or acidic water corrodes the copper pipes of the collector, causes scale deposition, lowers efficiency and reduces life span of the system.
Closed-loop systems circulate heat transfer fluid, instead of tap water, through the collector. This fluid may be treated water, air, antifreeze solution, or a special oil. It picks up heat in the collector and transfers it to water. These systems are easier to install, but more expensive to operate and maintain.
Especially in open loop systems the points to check are :
Careful assessment of the present and future requirement of hot water for bathing, cooking, cleaning, washing, etc. for the entire family/unit, selection of the system capacity and proper installation and thereafter proper usage will ensure that entire hot water requirements around the year can be practically met by the solar water heating system.
Rule of thumb : Each person uses about 20 litres per day hot water under normal circumstances, for bathing. Hence for a family of 4 a system of 100 lpd/60 deg. c. is sufficient. If tub bath is taken then the requirement rises to 50 litres per person. If hot water is used for washing, cooking and cleaning then system dimensioning must be taken into consideration accordingly. Provision should also be made for unexpected guests!