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see detailed diagram (new window)
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| HOW IT WORKS |
| Solar water heaters work by the phenomenon of
"Thermosyphon" natural convection circulating the
hot water in the collector panels into the insulated hot water
tank above, (hot water is lighter, it rises while cold water
is heavier, it sinks). No pumps, or motors are involved. The
cold water descends from the bottom of the storage tank to the
bottom of the collector panel where it is heated up by the solar
energy. The water rises up the panel as it heats and returns
to the tank where it rises to the top of the tank ready for
use. |
| EFFICIENT THERMOSYPHON FLOW MEANS MORE HOT WATER
FOR YOU |
Obviously in this weak thermosyphon convection
flow system (there are no pumps), other factors being equal,
the highest water temperatures are achieved by solar water heaters
whose thermosyphon circulation paths are the most efficient
and offer the least drag, i.e.:
1. shortest thermosyphon loop, directly from panel to tank,
no twists and turns,
2. no bottlenecks, freer flowing through greater number of valves
or feeder pipes into tank from panel, no sharing of a common
manifold,
3. minimum or zero horizontal run (hot water wants to go up,
and cold water down, not horizontally), etc. An efficient free
flowing system with minimal drag will start to operate even
under cloudy sky conditions, while a system with a high threshold
of drag resistance (such as single feeder pipe systems) will
operate only under sunny conditions or shut down under cloudy
skies (you will then have to switch on the electric backup heater).
The efficient system will react faster, deliver more hot water
per day, working from sunrise to sunset, while the inefficient
system is more lethargic; it starts work later and shuts down
earlier. |
| DRAWBACKS OF SOME CONVENTIONAL SOLAR WATER-HEATERS
BOTTLENECK - SINGLE FEEDER PIPE |
If you look carefully at a conventional solar
water heater on your neighbor's roof, you will notice the single
feeder pipe running from top corner of the panel to the tank
and the single return flow pipe running from the tank down the
side of panel. Where it enters the tank, this single feeder
pipe system causes a Bottleneck or Constriction m flow diameter.
This is because it collects hot water from numerous (10 to 20)
inclined heating pipes into one small single horizontal feeder
pipe which feeds a large 60 gal tank. This bottleneck can be
compared to 16 traffic lanes squeezed into one single lane -a
traffic jam results. Thermosyphon flow slows down, a hot spot
develops in the panel, more heat is lost through the glass,
and less hot water is collected in the tank.
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| CONVENTIONAL SOLAR HEATERS HAVE HORIZONTAL FLOW
IN THEIR THERMOSYPHON CIRCUIT |
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(very inefficient)
The bottleneck problem is further aggravated by hot water
stagnation due to the horizontal angle of the upper manifold.
The hot water flow stagnates as it is forced to travel horizontally
for up to 2 metres across the top of the panels before it
can enter the tank. The hot water wants to rise, does not
want to move horizontally. It just sits there in the horizontal
manifold. This leads to hot water stagnation, especially at
the far end of the manifold furthest away from the inlet into
the tank.
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| WHY IS HOT WATER STAGNATION IN THE HORIZONTAL
MANIFOLD INEFFICIENT? |
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Because a hot spot develops in the panel – the stagnant
hot water is trapped in the manifold, getting hotter and hotter
but the heat is reradiated back out through the glass. This
heat lost through the glass should have been collected safely
in the insulated hot water tank.
| :: Detail of flow in horizontal
manifold far from feeder pipe to tank |
:: Detail of flow horizontal
manifold close to feeder pipe to tank |
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Hot water is trapped in horizontal
manifold, cannot rise any more (too far from single feeder
pipe into tank). |
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Smooth flowing hot water
in only 3 or 4 heating pipes closest to the single feeder
pipe. This is because the hot water can slowly make its
way to the single feeder pipe nearby and continue rising
into the tank. |
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| WHAT HAPPENS ON CLOUDY DAYS? |
| On cloudy days the thermosyphon force is reduced
because the temperatures are lower. (Thermosyphon/convection
force depends on how hot the water in the panels are, the hotter
the water, the stronger the thermosyphon force). Solar Water
Heaters with a high resistance to thermosyphon flow, such as
conventional single feeder pipe systems with horizontal manifold
may not work as well on cloudy days as the reduced thermosyphon
force may not be sufficient to push the hot water into the tank. |
| HOW DOES IT AFFECT YOU? |
| You may run out of hot water on cloudy days with
an inefficient conventional solar water heater and have to switch
on the electric backup heater (very expensive, goes against
why you purchased a solar system in the first place, i.e. to
save money and not use electricity.) |
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