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Solar thermal collectors
can potentially gain
energy through radiation,
conduction and convection.
The first law of thermodynamics
states that heat energy
moves from hot to cold,
so when looking specifically
at energy transfer in
solar collectors, conduction
and convection will
almost always mean energy
is being moved from
the hotter collector
to the colder ambient
air. This is because
domestic water needs
to be heated to 60°C
or more. For a solar
thermal collector to
be effective it must
minimize conduction
(by using good insulation)
and convection (by using
a sealing barrier).
A solar thermal collector
therefore can only gain
energy through radiation,
hence the development
of selective coatings
that absorb radiation
but limit the amount
re-radiated.
The table on the left
indicates the ways in
which energy is transmitted
in typical modern Vacuum
Tube and old style Flat
Plate systems, when
the collectors are hotter
than the surrounding
air. As you can see,
both types of collector
will be gaining energy
through radiation. However,
a Flat Plate collector
will lose energy through
conduction, therefore
reducing the amount
of energy that can be
transferred to your
hot water tank. There
is virtually no loss
of energy from the Vacuum
Tube collector as conduction
cannot take place across
the near perfect vacuum.
Because more of the
collected energy is
trapped, more energy
can be transferred into
your domestic hot water
system. A vacuum is
the only effective way
to stop energy being
conducted out of the
system. Conduction will
still take place across
double-glazed and/or
gas-filled Flat Plate
collectors. Insufficient
insulation means a Flat
Plate collector is influenced
by the surrounding air
temperature, wind chill
and evaporation of moisture
from its surface. A
Vacuum Tube collector
works virtually independently
of these influences
as the vacuum acts as
an impassable barrier
that stops the energy
collected from escaping.
A Vacuum Tube collector
will produce almost
as much energy on a
sunny winter's day when
it is 0°C outside, as
it will on a sunny summer's
day.
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The amount of
energy a collector
can gain through
radiation decreases,
as its temperature
increases above
the ambient
air temperature.
The graph on
the left indicates
the efficiency
of typical modern
Vacuum Tube
and old style
Flat Plate collectors
at gaining energy
at different
temperatures
above ambient.
It is easy to
see that the
efficiency of
all collectors
falls as the
temperature
above ambient
increases, but
it is important
to compare the
efficiencies
at the particular
water temperature
to be achieved.
If we assume
that we are
trying to heat
domestic water
to 50°C above
the ambient
temperature
then the Vacuum
Tube system
is 88% efficient,
where as the
efficiency of
the Flat Plate
system has fallen
to just 58%.
These figures
relate to how
efficiently
collectors gain
energy at different
temperatures
above ambient
air temperature.
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Tests have shown that
throughout the year
and per m² of collector,
a high quality Vacuum
Tube system, such as
SolarMaxx Solar Water
Heating, will produce
about twice the energy
to heat water than a
Flat Plate system. Even
though over the year
a 4m² Flat Plate system
would produce a similar
amount of energy as
a 2m² Vacuum Tube system,
the majority of the
energy will be produced
during the summer months
which is not useful
as majority of hot water
requirement is during
winter months. The blue
shaded area in the chart
here indicates the amount
of hot water required
throughout the year.
The coloured lines indicate
the contribution various
systems could make to
this requirement. Any
energy produced in excess
of the hot water requirement
is wasted, so there
is limited benefit of
installing an oversized
Flat Plate System.
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