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, 22 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).
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 70%.
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 |
