Design requirements Solar water heating

the type, complexity , size of solar water heating system determined by:

changes in ambient temperature , solar radiation between summer , winter
changes in ambient temperature during day-night cycle
possibility of potable water or collector fluid overheating or freezing

the minimum requirements of system typically determined amount or temperature of hot water required during winter, when system s output , incoming water temperature typically @ lowest. maximum output of system determined need prevent water in system becoming hot.

freeze protection

freeze protection measures prevent damage system due expansion of freezing transfer fluid. drainback systems drain transfer fluid system when pump stops. many indirect systems use antifreeze (e.g., propylene glycol) in heat transfer fluid.

in direct systems, collectors can manually drained when freezing expected. approach common in climates freezing temperatures not occur often, unreliable since relies on operator.

a third type of freeze protection freeze-tolerance, low pressure polymer water channels made of silicone rubber expand on freezing. 1 such collector has european solar keymark accreditation.

overheat protection

when no hot water has been used day or two, fluid in collectors , storage can reach high temperatures in non-drainback systems. when storage tank in drainback system reaches desired temperature, pumps stop, ending heating process , preventing storage tank overheating.

some active systems deliberately cool water in storage tank circulating hot water through collector @ times when there little sunlight or @ night, losing heat. effective in direct or thermal store plumbing , virtually ineffective in systems use evacuated tube collectors, due superior insulation. collector type may still overheat. high pressure, sealed solar thermal systems rely on operation of temperature , pressure relief valves. low pressure, open vented heaters have simpler, more reliable safety controls, typically open vent.