The Zimbabwean Gardener Issue 3 Summer 2012 | Page 44

Sustainable energy products

Solar water heaters
By Ed Leared

What are solar water
heaters, how do they
work, and how do they
save money compared
to conventional
electrical geysers?

Both flat plate and evacuated tube
heaters work well in Zimbabwe, and
deciding which type of solar heater
to go for is determined by personal
preference and affordability.

In a country like Zimbabwe, where the
sun shines almost every day, using
solar energy makes sense, particularly
when it comes to heating water. The
energy source is completely free, 100%
renewable, almost always available and
environmentally friendly.

Although Zimbabwe’s climate means
there is generally sufficient sunlight
hours all year around for a solar water
heater to work, we do have a few cloudy
spells, which would impede the proper
functioning of a solar water heater.

Types of solar water heaters
There are two main solar water heaters:
the flat plate collector or the evacuated
tube collector. The former consists of a
simple, glass-topped insulated box with
a flat absorber made of metal, which is
attached to metallic pipes. The absorber
is painted black to maximise the
amount of energy from the sun. The flat
plate collector is aesthetically pleasing
and durable with some of the locally
assembled heaters still functioning
some 30 years after they were installed.
However, the main drawback is that the
initial upfront cost is much higher than
the evacuated tube collector.
Evacuated
tube
collector

Flat
plate
collector

The evacuated tube comprises copper
pipes surrounded by a glass vacuum
cylinder. This type of collector is very
efficient at capturing the sun’s energy and
is generally cheaper than the flat plate
collector. However, it is not clear whether
it is as durable as the flat plate collector.

44

Once payback has been reached, you
will see cost savings for the lifespan of
the solar heater, and as electricity costs
continue to increase, these cost savings
will be more pronounced.

What happens when there is not What to consider when buying
enough sun to heat the water? a solar water heater?

However, solar water containers are
far better insulated than conventional
geysers, meaning the water stays
hotter for longer. Like electric geysers,
solar heaters can operate with a
thermostat, so if the water temperature
does drop below the desired level, a
backup electrical element switches on
automatically. The key is to have a timer
on the element so it doesn’t switch
on during periods of peak electricity
consumption, or when the sun is
available to heat the water. This is also
useful when there are guests staying
and the solar geyser might not cope.

How does a solar water heater
save costs?
For an average family of four, a 270
litre geyser is recommended for daily
showering purposes. For the purpose
of this model, it is assumed that an
evacuator tube collector is used and this
costs around $1,600 including VAT and
installation from local suppliers. Further
assuming that an electrical geyser
would normally be in use for four and
a half hours a day for 330 days a year,
turning it off when you go on holiday;
this means approximately 13.5KwH
are used per day at a cost of $0.15 per
KwH. This translates into an annual cost
saving of approximately $668 per year
if using solar, or $56 per month. The
total payback period is calculated at 2
years and 5 months.

Durability of solar heaters is important,
because any cost savings in electricity
will be negated if one has to continue
replacing certain components, or there
is a malfunction with the heater. Linked
to this, it is important to source your
solar heaters from a reputable supplier
who is able to provide a warranty as
well as a decent after sales service for
when things go wrong.
Perhaps the biggest consideration
in purchasing a solar geyser is the
large upfront cost incurred. Once a
solar geyser has been purchased and
installed though, the cost savings will
be immediately appare