| There are basically three types of thermal solar collectors:
flat-plate, evacuated-tube and concentrating.
Flat-Plate collectors comprise of an insulated, weatherproof
box containing a dark absorber plate under one or more transparent
or translucent covers. Water or heat conducting fluid passes
through pipes located below the absorber plate. As the fluid
flows through the pipes it is heated. This style of collector,
although inferior in many ways to evacuated tube collectors,
is still the most common type of collector in many countries.
Evacuated Tube solar water heaters are made up of rows of
parallel, glass tubes. There are several types of evacuated
tubes (sometimes also referred to as Solar Tubes).
Type 1 (Glass-Glass) tubes consists of two glass tubes which
are fused together at one end. The inner tube is coated with
a selective surface that absorbs solar energy well but inhibits
radiative heat loss. The air is withdrawn ("evacuated")
from the space between the two glass tubes to form a vacuum,
which eliminates conductive and convective heat loss. These
tubes perform very well in overcast conditions as well as
low temperatures. Because the tube is 100% glass, the problem
with loss of vacuum due to a broken seal is greatly minimized.
Glass-glass solar tubes may be used in a number of different
ways, including direct flow, heat pipe, or U pipe configuration.
Apricus uses a high efficiency heat pipe and heat transfer
fin design to conduct the heat from within the evacuated tube
up to the header. For more information about heat pipes, click
here.
Type 2 (Glass-Metal) tubes consist of a single glass tube.
Inside the tube is a flat or curved aluminium plate which
is attached to a copper heat pipe or water flow pipe. The
aluminium plate is generally coated with Tinox, or similar
selective coating. These type of tubes are very efficient
but can have problems relating to loss of vacuum. This is
primarily due to the fact that their seal is glass to metal.
The heat expansion rates of these two materials. Glass-glass
tubes although not quite as efficient glass-metal tubes are
generally more reliable and much cheaper.
Type 3 (Glass-glass - water flow path) tubes incorporate
a water flow path into the tube itself. The problem with these
tubes is that if a tube is ever damaged water will pour from
the collector onto the roof and the collector must be "shut-down"
until the tube is replaced.
Concentrating collectors for are usually parabolic troughs
that use mirrored surfaces to concentrate the sun's energy
on an absorber tube (called a receiver) containing a heat-transfer
fluid, or the water itself. This type of solar collector is
generally only used for commercial power production applications,
because very high temperatures can be achieved. It is however
reliant on direct sunlight and therefore does not perform
well in overcast conditions.
Types of Solar Water Heating Systems
Solar water heating systems (SWHS) can be either active or
passive. An active system uses an electric pump to circulate
the fluid through the collector; a passive system has no pump
and relies on thermo-siphoning to circulate water. The amount
of hot water a solar water heater produces depends on the
type and size of the system, the amount of sun available at
the site, installation angle and orientation. SWHS are also
characterized as open loop (also called "direct")
or closed loop (also called "indirect"). An open-loop
system circulates household (potable) water through the collector.
A closed-loop system uses a heat-transfer fluid (water or
diluted antifreeze) to collect heat and a heat exchanger to
transfer the heat to the household water. A disadvantage of
closed looped system is that efficiency is lost during the
heat exchange process.
Active Systems
Active systems use electric pumps, valves, and controllers
to circulate water or other heat-transfer fluids through the
collectors. They are usually more expensive than passive systems
but generally more efficient. Active systems are often easier
to retrofit than passive systems because their storage tanks
do not need to be installed above or close to the collectors.
If installed using a PV panel to operate the pump, an active
system can operate even during a power outage.
Open-Loop Active Systems
Open-loop active systems use pumps to circulate household
potable water through the collectors. This design is efficient
and lowers operating costs but is not appropriate if water
is hard or acidic because scale and corrosion will gradually
disable the system. Open-loop active systems are popular in
regions that do not experience subzero temperatures. Flat
plate open-loop systems should never be installed in climates
that experience sustained periods of subzero temperatures.
Solar water heater can be installed in an open loop in areas
that experience sub-zero temperatures as long as the solar
controller has a low temperature fuction.
Closed-Loop Active Systems
These systems pump heat-transfer fluids (usually a glycol-water
antifreeze mixture) through the solar water heater. Heat exchangers
transfer the heat from the fluid to the water that is stored
in tanks. Double-walled heat exchangers or twin coil solar
tanks prevent contamination of household water. Some standards
require double walls when the heat-transfer fluid is anything
other than household water. Closed-loop glycol systems are
popular in areas subject to extended subzero temperatures
because they offer good freeze protection. However, glycol
antifreeze systems are more expensive to purchase and install
and the glycol must be checked each year and changed every
few years, depending on glycol quality and system temperatures.
Drainback systems use water as the heat-transfer fluid in
the collector loop. A pump circulates the water through the
solar water heater. When the pump is turned off, the solar
water heater drains of water, which ensures freeze protection
and also allows the system to turn off if the water in the
storage tank becomes too hot. A problem with drainback systems
is that the solar water heater installation and plumbing must
be carefully positioned to allow complete drainage. The pump
must also have sufficient head pressure to pump the water
up to the collector each time the pump starts. Electricity
usage is therefore slightly higher than a sealed closed or
open loop.
Passive Systems
Passive systems move household water or a heat-transfer fluid
through the system without pumps. Passive systems have the
advantage that electricity outage and electric pump breakdown
are not issues. This makes passive systems generally more
reliable, easier to maintain, and possibly longer lasting
than active systems. Passive systems are often less expensive
than active systems, but are also generally less efficient
due to slower water flow rates through the system.
Thermosiphon Systems
A thermosiphon system relies on warm water rising, a phenomenon
known as natural convection, to circulate water through the
solar absorber and to the tank. In this type of installation,
the tank must be located above the absorber tubes/panel. As
water in the absorber heats, it becomes lighter and naturally
rises into the tank above. Meanwhile, cooler water in the
tank flows downwards into the absorber, thus causing circulation
throughout the system. This system is widely used with both
flat plate and evacuated tube absorbers. The disadvantages
of this design are the poor aesthetics of having a large tank
on the roof and the isses with structural integrity of the
roof. Often the roof must be reinforced to cope with the weight
of the tank.
Batch Heaters
Batch heaters are simple passive system consisting of one
or more storage tanks placed in an insulated box that has
a glazed side facing the sun. Batch heaters are inexpensive
and have few components, but only perform well in summer when
the weather is warm. Evacuated tube solar collectors are now
an affordable and much more efficient alternative to either
batch or flat plate collectors.
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