Solar Water Heaters
Solar water heating (SWH) or solar hot water is water heated by solar energy derived from sunlight for residential, commercial or industrial purposes. Solar water heaters are composed of solar thermal collectors, a water storage tank, and a heat transfer fluid (HTF) to transport heat from the collector to the tank.
How Solar Water Heaters Work
Solar water heaters come in two types: active (pumped) or passive (compact). The solar collector, installed on a roof or a wall facing the sun, heats the fluid that is either pumped through an active system or moved by natural convection in a passive hot water system. Collectors are made of a glass topped insulated box with a painted black, flat solar absorber of sheet metal attached to copper pipes, or a set of metal tubes enclosed by an evacuated (near vacuum) glass cylinder. Heat is stored in a hot water storage tank that is well-insulated with an outlet and inlet connected to and from the collector. Active or passive residential solar water heaters include an auxiliary energy source (electric or gas) activated on cloudy days to ensure a constant supply of hot water when water in the tank falls below a certain minimum temperature setting, usually of 55°C.
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Active Solar Water Heaters
Active solar water heating systems use a pump to circulate water or HTF between the collector and storage tank. Coming in two types - direct active systems pump water to the collector and back to the storage tank while indirect active systems circulate HTF through the collector and a heat exchanger. The heat produced is transferred to the water in the storage tank.
Passive Solar Water Heaters
(ICS or batch heater) systems are composed of thin rectilinear, glass tanks fastened on roofs or walls facing the sun. Tanks function as both storage and thermal collectors. The delivery of water depends on gravity flow. Batch heaters are efficient, simple and highly suitable in warm climates.
(CHS or thermosiphon) systems are plate type or evacuated tube collectors with integral insulated tanks. CHS systems use convection to transport water from the collector to the tank. More efficient than an ICS system as the solar collector heats a smaller amount of water that is constantly fed back to the tank. CHS systems can be installed in areas with less sunshine than the ICS.
Direct passive SWH systems take water from the household water supply to circulate between the collector and tank. When water warms up, convection causes it to rise and flow towards the storage tank. Direct passive water heaters are not suited for cold climates as water in the collector can freeze and damage the panels. Indirect passive solar water heaters use a non-toxic, antifreeze HTF in the collector. Once this fluid is heated, convection causes it to flow to the tank where a passive heat exchanger carries the heat to the water in the tank.
While the maintenance of passive solar water heaters is affordable and simple, active systems are much more efficient at heating and retaining hot water. Aside from a higher efficiency rate, active water heating systems are more expensive and more difficult to install, and require electricity to activate the pump and controller.
Solar thermal collectors accumulate and hold on to heat from the sun, transferring this heat to a liquid. Three types of solar collectors are used in residential water heating systems:
The water tank is in a glass-top box with thermally insulated walls. The glass cover allows heat from the sun to reach the water tank, and the insulated walls reduce heat loss from the tank back to the environment. The box also contains a reflective surface that reflects heat lost back to the tank.
A flat box enclosing a network of pipes, with a robust glass top that faces the sun.
Heat loss due to convection cannot cross a vacuum, therefore water pipes in an evacuated tube collector are enclosed by two concentric tubes of borosilicate glass that form a vacuum in between to admit and retain heat inside the collector pipes.
Although flat plate collectors are more efficient than evacuated tube collectors in full sunshine, their energy output decreases in cool conditions in comparison to the energy output of evacuated tube collectors that decrease at a much slower rate.
Solar Water Heater Maintenance
The proper installation of solar water heaters depends on:
- Temperature of hot water required by the system.
- Variations in outside temperature during day and night.
- Changes in outside temperature and solar radiation between summer and winter.
As proper installation ensures the efficiency of the system, have licensed plumbers install your solar water heater. After installation, properly maintaining your system every couple of years will keep it running smoothly. Discuss the maintenance requirements with your solar contractor or consult the manufacturer's manual.
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Solar Water Heaters Brands
- A.O. Smith
- Bradford White
- Solar Panels Plus
- Stiebel Eltron