Solar hot water systems
Most people would be surprised to know that 12-17% of their total home
energy use goes towards heating hot water according to the U.S.
Department of Energy. The reason is simple, most of us, at least in
North America, use storage tank type water heaters which waste a lot
of energy heating hot water when no one is using it. Granted newer
units have a bit more insulation than the old ones did but for the
most part conventional natural gas or electric storage water tanks are very inefficient systems for heating hot water.
Solar thermal hot water systems can serve as an effective supplement
or alternative to tank-style hot water
heating systems and can significantly reduce your energy costs. During
the late 1970's and early 1980's there was a big surge in their use
because the Carter administration provided incentives for using them.
However, these early systems were not as efficient as they could be
and were often installed by some less than trustworthy people. All of
this served to give this emerging solar technology a bad name.
Fortunately there is now a new generation of solar water heating
systems which are far more efficient and reliable and which are
installed by experienced and trustworthy contractors.
There are three primary types of solar systems for heating water;
batch collectors, flat plat collectors and evacuated tube collectors.
Each has specific advantages and disadvantages depending upon the
climate you live in and the demands you have for hot water. These will
be explained below.
Batch collector systems are very popular in tropical or warm
climates. They usually consist of one or more water tanks that sits on
your roof instead of your basement or garage and they use the heat of
the sun directly to warm the water. Most batch systems use one or two
hot water tanks of between 20 and 40 gallons. These sit in glass
enclosed containers which capture the heat of the sun. The plumbing
for these types of systems is pretty simple. Cold water enters
the tank on the roof. It is heated by the sun and then is
drained off as hot water is needed. They are sometimes referred to as
direct or open-loop systems.
Batch type solar water heaters tend to be used only in warmer
climates because they are susceptible to freezing. One option is
to drain your tanks during the winter but getting on your roof in the
middle of winter to drain your tanks is not high on most people's
favorite things to do.
If you live in a very warm climate such as California or Arizona,
you can actually find that batch type collectors can heat the water to
too high a temperature (over 160 degrees Fahrenheit). In these
situations you might also need a mixing or tempering valve which
allows the overly heated water to be mixed with cool water in order to
obtain an optimal temperature. Most commercial batch hot water systems
come be purchased with sensor driven valves of this type.
Flat Plate Collectors
A more common type of solar hot water system are those that use
flat black metal plates to collect heat. The heat can then be
transferred to heat the hot water. early solar hot water systems
used an electric pump to circulate hot water over the plate to collect
the heat. The problem with this approach is that the water in
the tubes can freeze in winter. To solve this problem some newer
systems circulate a freeze proof liquid, think antifreeze, over the
solar plates and then use a heat exchanger to transfer that heat to
your hot water tank. Such systems are a bit more expensive but have a
long life cycle.
Some solar hot water systems are actually hybrid systems in that
the use the sun directly to heat the water, but also indirectly by
adding photovoltaic panels to run the pumps that pump the fluid over
the collector plates. These can add even greater efficiency to the
Hot water systems generally require sensors. The simple
reason being that you don't want your pump running if the water in the
collector is cooler than the water in your hot water tank.
Evacuated Tube Collectors
Evacuated tube collects are the newest of the technologies for
collecting hot water. Evacuated tube collectors consist of a series of
long transparent glass tubes. Inside each tube is a copper pipe called
the absorber tube. The absorber tube is covered with an absorbent
material to collect the heat from the sun. Inside the inner tube there
is a fluid designed for heat transfer, usually methanol. When
the tube is manufactured the air in the outer tube is pumped out
creating a vacuum (as in evacuated). Vacuums are very poor conductors
of heat. Therefore the heat in the inner tube once captured
cannot radiate out again and therefore a higher percentage of the
captured heat is retained making them more efficient then conventional
flat plat collectors.
The great thing about evacuated tube collectors is that because of
their greater efficiency they can work well on cloudy days and in
colder climates. Some manufacturers claim they will capture as much as
80% of the available radiant energy. Another advantage of these types
of systems is that the vacuum in the tube prevents condensation from
collecting, which sometimes happens in flat plate collectors.
The disadvantage is that these systems are more expensive, though they
are rapidly becoming cost competitive with more traditional flat plate
collection systems. Being a fairly new technology reliability data is
rather limited but this is definitely a technology to keep your eye