| Water
Heating Basics |
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| 1kWh
of heat energy can heat about 30 Litres / 7.5 US gallons of water
up from cold to showering temperature.
A
household of 5 people will use between 10-15kWh of energy each day
to heat water (not including space heating).
In
the winter more energy is needed to heat water as the cold water is
colder, and people often have longer and hotter showers than in the
summer.
Water
is normally heated by electricity or gas, but solar can be used to
offset between 60-80% of this energy.
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Storing Hot Water |
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| The most common hot
water system involves heating and then storing hot water in a tank.
When hot water is used, it is drawn from the top of the tank, where
the water is the hottest, and fresh cold water is delivered into
the bottom of the tank. Because hotter water is lighter and sits
above colder water, it is realistic to see a tank that has bottom
half cold and the top half hot... this is referred to as thermal
layering, or stratification. Eventually after enough hot water
usage the water suddenly turns cold, which is something everybody
has probably experienced!
Tanks are designed to
enhance thermal layering to maximise hot water supply by reducing
turbulence when the cold water enters the tank.
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| Solar Tank Design |
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The performance of a solar collector is highest when heating cold
water. As the temperature of the water increases the efficiency of
the solar collection will drop, the amount of performance loss
depending greatly on the type and quality of the collector. For
this reason thermal layering is important in a solar storage tank,
ensuring that solar is always heating the coldest water
possible.
Given that the coldest water is in the bottom of the tank, the
solar heating is always in the bottom area, either direct flow or
with a coil heat exchanger.
Ideally a dedicated solar tank should be used which then feeds a
primary tank with solar pre-heated water. This format is
ideal when retrofitting to an existing system as does not require
any major changes to the plumbing layout. The cold supply
is fed into the solar tank, and the solar tank outlet feeds the
cold inlet of the main tank. By have a dedicated volume
of water for the solar to heat to also avoid solar competing with
another energy source, as would happen if the tank was also heated
by electricity, gas etc (unless timer or manual boosting control is
used)
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| How Solar Collectors Heat Water |
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| The question is often
asked,How much hot water will I get, or What temperature is the
water from the solar collector Neither of these questions are
possible to answer directly, as some additional information is
needed.
Firstly, in most
applications, solar does not heat the water in a single pass like a
tankless gas heater would. The solar collector heats the water
gradually, normally by about 10oC / 18oF each pass. The flow rate
through the collector is also quite slow, around 3L/min / 0.8gpm
for an average domestic sized system. So the water in the solar
storage tank is heated gradually throughout the day.
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| How Much Hot Water? |
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| So how much hot water
will a solar collector give you? To answer this question fairly,
one variable must be fixed, either temperature rise or volume of
water.
The following examples
use a nominal heat value of 10kWh. The actual output of the
collector depends on the size, time of the year and other
factors.
Set Temperature
Rise
If raising cold water
by 30oC / 54oF to a suitable showering temperature:
10kWh can provide about
285 Liters / 75 US gallons of hot water.
Set Volume
Temperature rise for
200 Liters / 53 US gallons of cold (15oC / 60oF)
water:
10kWh will heat the
water to about 58oC / 136oF
So a much better
question to ask is "How much showering temperature hot
water will this collector give me in summer?.
That is a questions
that a solar professional should be able to answer confidently
based on a specific collector model, system configuration and you
local climate.
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