Solar Water Heater
Solar hot water heaters use the sun to heat either water or a
heat-transfer fluid in collectors. There are passive systems and active
systems. A typical system will reduce the need for conventional water
heating by about two-thirds. Sometimes the plumbing from a solar heater
connects to a house's existing water heater, which stays inactive as long
as the water coming in is hot or hotter than the temperature setting on
the indoor water heater. When it falls below this temperature, the home's
water heater can kick in to make up the difference. High-temperature solar
water heaters can provide energy-efficient hot water and hot water heat
for large commercial and industrial facilities.
Hot Water Systems
This system uses a pump to circulate potable water from the water storage
tank through one or more collectors and back into the tank. The pump is
regulated by an electronic controller, an appliance timer, or a
In this system, a heat exchanger heats a fluid that circulates in tubes
through the water storage tank, transferring the heat from the fluid to
the potable water.
A thermosiphon solar water heating system has a tank mounted above the
collector. As the collector heats the water, it rises to the storage tank,
while heavier cold water sinks down to the collector.
In cold climates, this system prevents water from freezing in the
collector by using electric valves that automatically drain the water from
the collector when the temperature drops to freezing. "Drainback
systems," a variation of this approach, automatically drain the
collector whenever the circulating pump stops.
Swimming Pool Systems
In solar heated swimming pools, the pool's filter pump pumps water through
a solar collector, and the pool itself stores the hot water.
Solar Energy Collectors
Flat Plate Collectors
The most common collector for solar hot water is the flat plate collector.
It is a rectangular box with a transparent cover, installed on a
building's roof. Small tubes run through the box and carry fluid-either
water or other fluid, such as an antifreeze solution. The tubes attach to
a black absorber plate. As heat builds up in the collector, it heats the
fluid passing through the tubes. The hot water or liquid goes to a storage
tank. If the fluid is not hot water, water is heated by passing it through
a tube inside the storage tank full of hot fluid.
Evacuated Tube Collectors
These collectors consist of rows of parallel transparent glass tubes, each
containing an absorber and covered with a selective coating. Sunlight
enters the tube, strikes the absorber, and heats the liquid flowing
through the absorber. These collectors are manufactured with a vacuum
between the tubes, which helps them achieve extremely high temperatures
(170-350 degrees F); so they are appropriate for commercial and industrial
Parabolic trough-shaped reflectors concentrate sunlight onto an absorber
or receiver to provide hot water and steam, usually for industrial and
Transpired Solar Collectors
A transpired collector is a south facing outside wall covered by a dark
sheet metal collector. The collector heats outside air, which is then
sucked into the building's ventilation system through perforations in the
collector. They have been used for pre-heating ventilation air and crop
drying. They are inexpensive to make, and commercially, have achieved
efficiencies of more than 70 percent.
Batch or Breadbox Heaters
This system is also referred to as a batch heater and a breadbox. It
consists of an approximately 40-gallon insulated tank, lined with glass on
the inside and painted black on the outside. It is mounted on the roof, or
on the ground in the sun. Plumbing from the house supplies the box with
cold water through an inlet that extends down to the bottom of the tank.
The box itself acts like a collector, absorbing and trapping the sun's
heat and heating the water. An outlet supplies the house with heated water
from the top of the tank.
Solar Process Heat
These systems consists of several thousand square feet of ground-mounted
collectors, pumps, heat exchangers, controls, and one or more large
storage tanks. Typically, they provide hot water and hot water space
heating for large institutions such as schools, office buildings, prisons,
and military bases.
Active Solar Cooling
As water evaporates, it cools the air. Evaporative cooling systems,
usually appropriate for hot dry climates, can be powered with solar
technology. In humid climates, desiccant evaporative cooling systems use
the same evaporative concept to cool air, but they also include a
desiccant wheel to dry incoming air. Waste heat from the building, natural
gas, or solar technologies can be used to regenerate the desiccant wheel.
Evaporative cooling is a CFC-free and energy-efficient way to cool
commercial buildings. In absorption solar cooling, an absorption device
uses a heat source, such as natural gas or a large solar collector, to
Certification of Solar Water Heaters
Solar research facilities have developed testing and certification
requirements for solar collectors and design and installation criteria for
solar systems. Thermal performance ratings, derived from testing and
certification programs, are a useful tool in comparing the efficiency and
economics of various flat-plate solar collectors.
Builders, utilities, and the Department of Energy have joined together on
several occasions to offer tax credits and utility rebates to encourage
the use of solar hot water for residential and commercial projects.
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