Closed loop systems are used in areas where
freezing may occur. Closed loop systems usually use
an anti-freeze solution and incorporate a heat
exchanger. Systems installed in warmer climates may
use water as a transfer fluid and may or may not
incorporate a heat exhanger.
Thermosyphon systems use natural convection to
circulate water within a closed loop. These systems can
be used in all type of climates. Heat exchangers with
non freezing fluids are used in colder climates.
Consideration must be take when locating the system
components to ensure proper operation.
Solar Water Heating Systems
Flat plate collectors are commonly used in areas
where freezing weather can occur. Flat plate
collectors, constructed using copper risers and
absorber, can raise water temperatures up to boiling if
the water is left stagnant.
Flat plate collectors made using poly only achieve
temperatures close to 100 F if left stagnant. Constant
expansion and contraction of tubing and connections
can result in failure. Poly collectors are not suitable for
freezing climates and may only be used during summer
months in such areas.
Trough collectors make transfer fluids reach
high temperatures quickly and are usually used in
large commercial heating operations. Transfer
fluids such as oil are used to gain higher
temperatures over water and glycol. Many
troughs are needed to compensate for the low  
flow required to achieve such high temperatures
and the volume of fluid normally needed to be
effective in a system.
Batch collectors heat water in a
tank directly without the use of a
heat exhanger or circulation of fluid.
Evacuated tube collectors are
similar to flat plate collectors in
operation.Though they have the
ability to heat water to high
temperatures, flow rates must be
reduced due to the smaller tubing
arraingments. They are not highly
recommend for areas where freezing
can occur especially those types that
use sealed tubes full of water that
exchange their heat with the fluid
passing through the header. Since
they are heat exchangers themselves,
overall system efficiency is lowered.
Active, closed loop systems incorporate a pump to
circulate heat transfer fluid between the collector and
the tank. Differential temperature controllers or PV are
use to determine when the pump operates.
water heating
Solar Innovations home
Solar pool heating
Drainback systems can use either anti-freeze with a
heat exchanger, or water and are normally considered
to be an open system. Proper piping and controls are
needed to ensure that pumps do not run dry and that
syphoning cannot occur.