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 9
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 "0% of your summer hot water
energy needs, with a percentage provided throughout other
months, lowest obviously in winter.; 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.
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. One 1.8m tube = 1.2 x 1.5m
tubes.
*Water Volume = This should represent the actual volume of
hot water used at the tap.
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
- 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.
Metric to Imperial Conversion
kWh/m2/day
=
Btu/ft2/day
Apart from the three key factors
used in the calculation above, you may also need to
consider:
1. Annually/daily shade patterns
2. Angle/direction of
installation (a less than ideal angle will reduce
efficiency)
6. Installation site (Do you have enough room for the
collector(s)?)
The estimations above are
just a guide and are based on a average summer performance
level of 65%. As explained above it is best not to oversize
the system. Your local
distributor may need to complete a onsite inspection to
accurately assess your requirements and design a solar water
heating system which suits your needs. For more information
about performance and heat energy output please
click here.
