How does solar hot water work?
Evacuated solar tubes: are used as the
absorber
for the solar heater. Solar tubes
absorb solar
energy,
converting the heat for use in solar water heating.
Solar tubes have been widely used for many years in
several countries including Germany, Canada, China, UK,
Europe, Japan. There are several different versions of solar
tubes used by the solar industry. Solardirect solar tubes
are the most common, the "twin glass type tube" .
This type of evacuated tube is used for the simple
reasons of its proven reliability ease
of replacement and
manufacturing costs.
Each evacuated tube consists of two glass tubes
made from extremely strong
borosiicate glass. (pyrex)
The outer tube is clear, allowing light rays to pass
through with very low reflection. The
inner tube has
two special separate layers of a
selective coating
which produces spectacular solar radiation
absorbtion
with minimal
reflection properties.
The open end of the two tubes are fused together,
the air in the space between the layers is pumped
out when the glass is exposed to
high temperatures.
The "evacuation" process removes the gasses which
forms the vacuum.
This is the important feature solar tubes have.
Why have a vacuum? As you would know if you have
used a thermos flask the heat is
retained because of the
vacuum layer which in an excellent
insulator.
This is very important because once the solar
tube
receives the radiation from the sun and turns it to
heat
we dont want to loose it! The vacuum helps to do
this.
The insulation properties are so good that while the
inner
part of the tube maybe 160 deg c, the outer part of
the
soalr tube is cool to touch. This of coarse means that
the
solar water heater can perform well even in cold
weather
when flat plate collectors perform poorly due to heat
loss.
To be able to maintain the vacuum between the two layers
of glass, a barium getter is
placed at the bottom of the
solar tube.
During the manufacturing process this barium getter is
exposed to high temperatures which
coates the bottom of
the solar tube with a pure layer of barium. This
barium layer
actively absorbs any CO,CO2,N2,O2,H2O,and
H2
out-gassed from the solar tube from storage or
from
operational use, thus helping to maintain the
vacuum.
The barium layer is a clever indicator as to the operational
status of the solar tube vacuum. The barium layer which
looks like a mirror finish turns white if the vacuum is lost
for any reason. This makes an
operational inspection easy
to determine if in fact the tube
is in good condition.
Getter is located at the Left tube vacuum Right tube faulty
bottom of the evacuated is
present
tube
Pressure relief paths must be
installed to prevent solar water
heater cylinder explosions.
Solar tubes are aligned in parallel, positioning of collector
angle and mounting depends upon the latitude at
your
location.
The efficiency of an evacuated tube water heater is
dependent upon a number of factors, one important
one
being the level of evacuated radiation (insolation)
at
your location, another being the solar collector
angle.
Evacuated solar tubes privide you with more annual
output
than that of a standard flat plate solar hot water collector
of
the same net aperture area,and work
very well in cloudy
and windy conditions, even
temperatures down to minus 30
degrees.
Residential solar collectors can also boil water.
A maximum operating temperature of around 70 degrees c
for the tank is ideal for
a retro fit solar hot water
system.
Solar heater operating temperatures can exceed
70
Degree c with the use of a solar ready
cylinder, or a
cylinder
capable of
higher temperatures.
Diagram shows: How the solar tubes fit into the
solar collector manifold, each end of the manifold is
then connected to your hot water cylinder with a
pump
in the
circuit.
Hot water is then stored in your converted solar
hot
water
cylinder.
Ice Breaker brand solar hot water tubes
Standard evacuated tube heat
pipe:
AL/N/AL absorber coating, aluminium nitride layers,
general use-250 deg c
Cross section of manifold and insulation:
Rockwool: has
excellent insulation qualities for solar
collector manifolds, offering the ability to protect against
heat/cold loss.
Rockwool insulation is made from recycled
glass bottles
is also very light and can withstand
very high temp-
eratures, making it an ideal choice for a
solar collector.
The recycle content is about 80% and can
be recycled
again.
Solar tube clamp system: long life stainless
Solar evacuated
tube collectors: are suitable
for low or mains pressure
systems max 8 bar / 116 psi
Features:
1. Selective coated absorber to optimise
collector
efficiency
2 Copper heat pipes
3. Vacuum works like (thermos flask)
4.* Copper manifold-heavy wall
5. Rock wool insulation to protect and reduce
heat loss
6 Tough borosilicate glass ( Pyrex) 25mm hail
impact resistance
7. Stainless Steel cover and frame
8. UV resistant silicon rubber gasket
9. * No water loss if tube fails, easy for the
handyman to
replace .
10. Solar collector panel is 82% efficient.
* When selecting a solar hot water system,
these
features are well worth consideration.
Potential water leaks are minimised unlike U tube
systems.
Flat plate systems have a long piece of tube with
absorber fins attached, known to discolour in some
models, reducing performance.
Check the reviews at www.rustypanels.blogspot.com
Select only systems with 14mm heat pipes, (std size).
Why do residential solar collectors
perform better than flat plate panels:
1. Because of the circular shape of the evacuated
tube, the sunlight is perpendicular to the tube for
most
of the day.
When comparing to a flat plate collector in a
fixed position the sun is perpendicular to the
collector only at noon
2. An evacuated tube has a vacuum trapped
between the glass layers which prevents the
captured heat from escape.
Flat plate collectors vs evacuated
tube
collectors: judge
for yourself !
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Can you use solar heating anywhere in NZ?
Solar heaters function very well from the far
north to the lower south island, although the solar
contribution varies depending on your latitude and
the time of year.
Solar heating can produce around 75% of your
households hot water heating in summer and
between 25 & 45% in winter.
Solar heating performance curve:
Solardirects collector
efficiency
rating
is 82.00%.
copyright 2010 powered by solar heating nz.
Evacuated solar tubes can boil water.
Evacuated tube systems are extensively used in Europe, Canada, China, Germany, Japan, UK.
Over 30 million chinese solar heating customers cant be wrong,join the movement.
