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Solar Hot Water Heaters
Solar collectors
Solar energy (solar radiation) is collected by the solar collector’s absorber plates.
Selective coatings are often applied to the absorber plates to improve the overall
collection efficiency. A thermal fluid absorbs the energy collected.
There are several types of solar collectors to heat liquids. Selection of a solar collector
type will depend on the temperature of the application being considered and the
intended season of use (or climate). The most common solar collector types are:
unglazed liquid flat-plate collectors; glazed liquid flat-plate collectors; and evacuated
tube solar collectors.
Unglazed liquid flat-plate collectors
Unglazed liquid flat-plate collectors are usually made of a black polymer. They do not
normally have a selective coating and do not include a frame and insulation at the
back; they are usually simply laid on a roof or on a wooden support. These low-cost
collectors are good at capturing the energy from the sun, but thermal losses to the
environment increase rapidly with water temperature particularly in windy locations.
As a result, unglazed collectors are commonly used for applications requiring energy
delivery at low temperatures such as pool heating or make-up water in fish farms. In
colder climates they are typically only operated in the summer season due to the high
thermal losses of the collector.
Glazed liquid flat-plate collectors
In glazed liquid flat plate collectors, a flat-plate absorber (which often has a selective
coating) is fixed in a frame between a single or double layer of glass and an insulation
panel at the back. Much of the sunlight (solar energy) is prevented from escaping due
to the glazing. These collectors are commonly used in moderate temperature
applications for domestic hot water, space heating, and year-round indoor pools.
Evacuated tube solar collectors
Evacuated tube solar collectors have an absorber with a selective coating enclosed in
a sealed glass vacuum tube. They are good at capturing the energy from the sun;
their thermal losses to the environment are extremely low.
Systems presently on the market use a sealed heat-pipe on each tube to extract heat
from the absorber (a liquid is vaporised while in contact with the heated absorber,
heat is recovered at the top of the tube while the vapour condenses, and condensate
returns by gravity to the absorber).
Evacuated collectors are good for applications requiring energy delivery at moderate
to high temperatures (domestic hot water, space heating and process heating
applications typically at 60°C to 80°C depending on outside temperature), particularly
in cold climates.
Balance of systems
In addition to the solar collector, a solar water heating system typically includes the
following “balance of system” components:
1. Solar collector array support structure.
2. Hot water storage tank. However, when there is continuous service hot water flow,
as to a swimming pool, a storage tank is not required.
3. Liquid handling unit, which includes a pump required to transfer the fluid from the
solar collector to the hot water storage tank (except in thermosiphon systems where
circulation is natural, and outdoor swimming pool applications where the existing
filtration system pump is generally used); it also includes valves, strainers, and a
thermal expansion tank.
4. Controller, which activates the circulator only when useable heat is available from
the solar collectors. A controller is not required for thermosiphon systems or if a
photovoltaic-powered circulator is used.
5. Freeze protection, required for use during cold weather operation, typically
through the use in the solar loop of a special antifreeze heat transfer fluid with a low-
toxicity. The solar collector fluid is separated from the hot water in the storage tank by
a heat exchanger.
Typically, an existing conventional water heating system is used for back-up to a solar
hot water heater, with the exception that a back-up system is normally not required
for most outdoor swimming pool applications.
Solar energy (solar radiation) is collected by the solar collector’s absorber plates.
Selective coatings are often applied to the absorber plates to improve the overall
collection efficiency. A thermal fluid absorbs the energy collected.
There are several types of solar collectors to heat liquids. Selection of a solar collector
type will depend on the temperature of the application being considered and the
intended season of use (or climate). The most common solar collector types are:
unglazed liquid flat-plate collectors; glazed liquid flat-plate collectors; and evacuated
tube solar collectors.
Unglazed liquid flat-plate collectors
Unglazed liquid flat-plate collectors are usually made of a black polymer. They do not
normally have a selective coating and do not include a frame and insulation at the
back; they are usually simply laid on a roof or on a wooden support. These low-cost
collectors are good at capturing the energy from the sun, but thermal losses to the
environment increase rapidly with water temperature particularly in windy locations.
As a result, unglazed collectors are commonly used for applications requiring energy
delivery at low temperatures such as pool heating or make-up water in fish farms. In
colder climates they are typically only operated in the summer season due to the high
thermal losses of the collector.
Glazed liquid flat-plate collectors
In glazed liquid flat plate collectors, a flat-plate absorber (which often has a selective
coating) is fixed in a frame between a single or double layer of glass and an insulation
panel at the back. Much of the sunlight (solar energy) is prevented from escaping due
to the glazing. These collectors are commonly used in moderate temperature
applications for domestic hot water, space heating, and year-round indoor pools.
Evacuated tube solar collectors
Evacuated tube solar collectors have an absorber with a selective coating enclosed in
a sealed glass vacuum tube. They are good at capturing the energy from the sun;
their thermal losses to the environment are extremely low.
Systems presently on the market use a sealed heat-pipe on each tube to extract heat
from the absorber (a liquid is vaporised while in contact with the heated absorber,
heat is recovered at the top of the tube while the vapour condenses, and condensate
returns by gravity to the absorber).
Evacuated collectors are good for applications requiring energy delivery at moderate
to high temperatures (domestic hot water, space heating and process heating
applications typically at 60°C to 80°C depending on outside temperature), particularly
in cold climates.
Balance of systems
In addition to the solar collector, a solar water heating system typically includes the
following “balance of system” components:
1. Solar collector array support structure.
2. Hot water storage tank. However, when there is continuous service hot water flow,
as to a swimming pool, a storage tank is not required.
3. Liquid handling unit, which includes a pump required to transfer the fluid from the
solar collector to the hot water storage tank (except in thermosiphon systems where
circulation is natural, and outdoor swimming pool applications where the existing
filtration system pump is generally used); it also includes valves, strainers, and a
thermal expansion tank.
4. Controller, which activates the circulator only when useable heat is available from
the solar collectors. A controller is not required for thermosiphon systems or if a
photovoltaic-powered circulator is used.
5. Freeze protection, required for use during cold weather operation, typically
through the use in the solar loop of a special antifreeze heat transfer fluid with a low-
toxicity. The solar collector fluid is separated from the hot water in the storage tank by
a heat exchanger.
Typically, an existing conventional water heating system is used for back-up to a solar
hot water heater, with the exception that a back-up system is normally not required
for most outdoor swimming pool applications.
Solar hot water heaters use solar collectors and a liquid handling unit to transfer heat
to the load, generally via a storage tank. The liquid handling unit includes the pump(s)
used to circulate the working fluid from the collectors to the storage tank, and control
and safety equipment.
When properly designed, solar water heaters can work when the outside temperature
is well below freezing and they are also protected from overheating on hot, sunny
days. Many systems also have a back-up heater to ensure that all of a consumer’s hot
water needs are met even when there is insufficient sunshine.
Solar hot water heaters perform three basic operations:
Collection: Solar radiation is “captured” by a solar collector;
Transfer: Circulating fluids transfer this energy to a storage tank. Circulation can be
natural (thermosiphon systems) or forced, using a circulator (low-head pump).
Storage: Hot water is stored until it is needed at a later time in a mechanical room, or
on the roof in the case of a thermosiphon system.
to the load, generally via a storage tank. The liquid handling unit includes the pump(s)
used to circulate the working fluid from the collectors to the storage tank, and control
and safety equipment.
When properly designed, solar water heaters can work when the outside temperature
is well below freezing and they are also protected from overheating on hot, sunny
days. Many systems also have a back-up heater to ensure that all of a consumer’s hot
water needs are met even when there is insufficient sunshine.
Solar hot water heaters perform three basic operations:
Collection: Solar radiation is “captured” by a solar collector;
Transfer: Circulating fluids transfer this energy to a storage tank. Circulation can be
natural (thermosiphon systems) or forced, using a circulator (low-head pump).
Storage: Hot water is stored until it is needed at a later time in a mechanical room, or
on the roof in the case of a thermosiphon system.
