Solar Heating Systems
A roof does not necessarily have to be orientated exactly to the
south in order to serve as a mounting surface for solar collectors.
Variations from southern orientation of up to 30° lead to only
low losses. Even absolute east or west orientation can be offset
through the use of a corresponding larger collector surface. A
roof's slope can even be between 20° and 60°, whereby a
solar heating system with less slope has a higher energy yield
in summer, and one with more slope has a higher energy yield in
winter. Special stands are recommended for flat roofs.
Smart Dimensions a must
Properly dimensioned solar heating systems offer the best guarantee
for satisfactory operation. Precisely knowing a household's hot
water consumption is required when finding the proper dimensions
but one should also take into consideration the possibilities of
lower consumption. For these questions, the advice of a specialist
is recommended. A tip for deciding the dimensions for a small solar
heating system: daily hot water consumption of 50 liters per person
(at 45° C) yields a collector surface of 1.2 m² by 1.5
m² per person. |
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Choose a Suitable System
Two circuit indirect systems with controlled circulation are most
predominant in Germany. These use heat-transfer fluid that is transported
by pumps to the hot water storage tank. Once there, the solar heat
is transmitted from the heat transfer fluid to the potable water
through a heat exchanger. In order to protect solar heating systems
from freezing damage, there is a water-antifreeze mixture in the
circulation pipes, and, due to separate circuits, the heat-transfer
fluid and the domestic water do not mix. The heated potable water
can then flow to the hot-water faucets. In comparison, one circuit
systems heat water directly in the collector (usually in countries
without danger of freezing).
In thermosiphon systems the regulator and the solar circulation
pumps are not necessary because of convection: The solar radiation
heats the heat-transfer fluid, its density then decreases as its
temperature increases. The fluid becomes lighter and rises inside
the circulation pipes. Therefore, a pump is not necessary. In order
for such a system to function, however, the water tank must be installed
above the collector.
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Two circuit, indirect system with controlled
circulation
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Hot Water Storage Tank and Heat Exchanger
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The purpose of the hot water storage tank is to stockpile energy
for days with poor solar radiation. Its volume capacity should be
1.5 to 2 times more than the daily hot water consumption - that
means 80 to 100 liters per person.
Enameled steel tanks are normally used, such as those known from
conventional heating technology. They need a magnesium-or an external
current-anode for corrosion protection. Stainless steel storage
tanks have a longer life expectancy, but are more expensive.
Good solar storage tanks have a slim, cylindrical form in order
to develop a layering of temperature in the tank. This allows for
optimal usage of the heated potable water in the upper storage region,
thus the entire contents of the tank don't need to be heated to
the desired temperature. Undesired mixing of the tank contents through
incoming cold water is prevented through a special pipe construction
or a baffle plate. The arrangement of the solar circuit heat exchanger
in the lower, colder tank area causes the solar panel to work at
a more economical level of efficiency due to the low incoming water
temperature.
In order that the conventional heater does not have to reheat an
unnecessarily large volume, its heat exchanger is located in the
upper part of the tank.
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Warm Water Storage Tanks with two Heat
Exchangers
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The entire surface of the tank should be tightly fit with a layer
of insulation at least 10 centimeters thick without any gaps. To
further lower heat loss, the connections in the cold lower level
are led out only from one area.
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The Solar Heat Circulation
Within the solar heat circulation, heat is transported from the
collector to the hot water storage tank. In order to minimize heat
loss, the distance from the collector to the tank should be as
short as possible. For systems in one or two family homes, copper
pipes with a circumference of 15 mm to 18 mm are enough to guarantee
an optimal transportation of heat. The pipes are sufficiently insulated
with 30 mm of insulation. For pipes with a circumference of 30
mm, the insulation should have at least the same thickness as the
pipe. The insulation must be able to withstand high temperatures,
and the outdoor section has to be UV- and weather-resistant. The
following materials are used as insulation: mineral wool, polyurethane
pipe wrappers, and foam rubber. |
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Mountings and safety equipment in the solar circuit |
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The prevalent flow rate in small solar heating systems amounts
to 30 to 50 liters per hour per square meter of collector surface.
The solar circulation pump has to be able to guarantee this rate
of flow. As a general rule, conventional pumps with an electric
input between 40 W and 80 W suffice. Also, the pump should always
be installed in the colder reflux of the solar circulation system.
In this way the pump will not be exposed to high temperatures during
operation. Finally, stop valves are mounted in front of and behind
the pump, so that the entire system does not have to be emptied
when replacing a defective pump.
The typical operating pressure of solar heating systems, which
can be controlled by a manometer, lies at approximately 4 bar.
The safety
valve should open at an approximately 0.3 bar triggering pressure.
With a recuperation tank, the heat-transfer fluid can be captured
and then fed back into the solar circuit through one of the refilling
taps. Thermometers fitted in forerun and reflux are used to check
the system's operation. To prevent heat loss out of the tank because
of insufficient solar radiation or at night due to convection
(the
heat-transfer fluid cools in the cold collector, and through the
force of gravity, then circulates towards the storage tank) a
rebound
valve is mounted in the outward flow. The expansion tank keeps
the pressure in the system stable and takes up the volume difference
of the heat-transfer fluid that is caused by the temperature difference.
For safety reasons, the volume of the expansion tank has to be
sufficiently
large. It should be able to take up the entire volume of heat-transfer
fluid. The vent valve serves to ventilate the solar circuit after
it has been filled with heat-transfer fluid. It is to be mounted
on the highest part of the solar circuit.
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The Regulation
Often a simple controller for temperature differences is enough
to regulate a small solar heating system for water heating. Through
the use of two temperature sensors, the regulator ascertains when
the temperature in the collector discharge is higher than the
temperature
of the solar circuit heat exchanger in the tank, and then it activates
the circulation pump. To start the pump, the solar regulator is
usually calibrated so that the necessary temperature difference
between the collector and tank is between 5° C and 8° C.
If this temperature difference sinks to 2° C to 3° C,
then the solar regulator will shut off the solar circuit circulation
pump.
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Anticipatory Planning for Building Construction
If you are building or renovating a house, but still
cannot decide on a solar heating system, just remember that preparations
for future installation of a solar heating system can be made during
construction (ducts for two copper pipes 18' and a quintuple-core
cable from the boiler room to the roof). This will save you a lot
of work and money later.
Text and images used with permission from
the
German Section of the International Solar Energy Society (ISES)
Concise and comprehensible explanations of the basic concepts in
solar heating and photovoltaics can be found in our Solar-Lexicon.
Reports on technology, business
and politics, as well as presentations
on innovative systems and products can be found in the Solar
Magazine
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