Water Heater - Solar
Solar Water Heater
Solar water heating (SWH) is the conversion of sunlight into heat for water heating using a solar thermal collector. A variety of configurations is available at varying costs to provide solutions in different climates and latitudes. SWHs are widely used for residential and industrial applications.
A sun-facing collector heats a working fluid that passes into a storage system for later use. SWH is active (pumped) and passive (convection-driven). They use water only, or both water and a working fluid. They are heated directly or via light-concentrating mirrors. They operate independently or as hybrids with electric or gas heaters. In large-scale installations, mirrors may concentrate sunlight into a smaller collector.
DESCRIPTION
Freeze protection
Freeze protection measures prevent damage to the system due to the expansion of freezing transfer fluid. Drainback systems drain the transfer fluid from the system when the pump stops. Many indirect systems use antifreeze (e.g., propylene glycol) in the heat transfer fluid.
In some direct systems, collectors can be manually drained when freezing is expected. This approach is common in climates where freezing temperatures do not occur often, but can be less reliable than an automatic system as it relies on an operator.
A third type of freeze protection is freeze-tolerance, where low pressure water pipes made of silicone rubber simply expand on freezing.
Overheat protection
When no hot water has been used for a day or two, the fluid in the collectors and storage can reach high temperatures in all non-drain-back systems. When the storage tank in a drain-back system reaches its desired temperature, the pumps stop, ending the heating process and thus preventing the storage tank from overheating.
Some active systems deliberately cool the water in the storage tank by circulating hot water through the collector at times when there is little sunlight or at night, losing heat. This is most effective in direct or thermal store plumbing and is virtually ineffective in systems that use evacuated tube collectors, due to their superior insulation. Any collector type may still overheat. High pressure, sealed solar thermal systems ultimately rely on the operation of temperature and pressure relief valves. Low pressure, open vented heaters have simpler, more reliable safety controls, typically an open vent.
