Collector Sizing
When determining what size collector you need, you must consider
two key factors: insolation
level and energy requirements. Energy requirement will usually
take into consideration the volume of water and rise in temperature
required. Once you know
these factors you can determine the size
collector you require. The bigger the collector
you have, the more
hot water, but you should make an economically sound decision.
Generally it is
wise to select a size which will provide you with
90% of your hot water
needs in the
summer.
Although it may
seem strange to use a value of only 90% for summer solar
contribution,
it is for good reason. It is normal to size based on
100% of your summer hot water
energy needs, with a percentage
provided throughout other months, lowest obviously
in winter. That
is based on normal water usage, but often, and particularly in the
summer, water
usage patterns may not be that normal, with cooler
than normal
showers taken in
hot weather, and greater possibility of
the house being vacant for
one or two days each
week (weekends). As
such, using a target value of 90% will
probably actually result in
a
system that is able to supply more than 100% of your
hot water needs
in the summer, without excessive heat production, which can lead
to
water loss via pressure release
and a waste
of energy.
The calculator below can help to determine how many evacuated
tubes you require
given your energy requirements. Solar collectors
come in a set of standard sizing of
10, 20, 25
or 30, depending on
your region. Of course you can also combine collectors
to increase
the size. If you get an answer that is not a standard size, as a
general rule, select the
next size down - this will prevent having
too much heat in the summer.
Depending on your preference, either Metric or
Imperial values may be used to calculate
the number of tubes
required. Please note: 1 kWh/m2/day = 317.1 Btu/ft2/day
*Water Volume = This should represent the
actual volume of hot water used at the tap
in total each day.
Although most hot water systems have target temps of 60oC / 140oF, when showering
a temperature of
between 42oC / 107oF and 45oC / 113oF is normally used. Therefore
300L
of hot water at the tap may only draw 220L of hot water (at 60oC / 140oF) from
the storage tank.
**Temperature Rise = target tap hot water temp - average mains
cold water temp.
Target hot water temp should usually be around
42oC / 107oF to 45oC / 113oF
Cold water usually fluctuates
by about 10oC / 18oF between winter and summer. A
check
of your local weather records should provide you with an idea of
average cold
water temperatures (normall about 10oC / 50oF in winter and 20oC / 68oF in summer,
in mild regions).
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