solar water-heating
Heat
Your Water
With The Sun
edited by
Diana Young, Liz Merry
and Barry Butler
In most parts of north america, the best bang for your solar energy buck is with domestic solar water heating (DSWh). It’s a no-brainer in the desert Southwest and in semitropical florida and hawaii.
a complete DSWh system can be installed for $3,000 to
$10,000, depending on its size and complexity. These systems are now eligible for the 30 percent federal tax credit. at
today’s energy prices, over the life of the system, the operating cost is about 20 percent lower than a conventional gas
water heater and 40 percent lower than an electric one. as
gas and electricity prices rise, DSWh will look like a better
and better deal.
The price of a system depends on two main factors: sys-
tem size and local climate. a large family uses a lot of hot
water for bathing, laundry and dish washing, while a couple
with no kids living at home can use a much smaller system.
Solar water-heating systems come in two flavors: passive
and active. In warm climates, a simple passive system can
provide plenty of hot water.
Passive Solar Water-heating Systems
aDam Baker, Butler Sun SolutIonS
Passive systems are installed in areas where freeze
protection is not an issue. The most common types are
integral collector storage (ICS), thermosiphon and evacu-
ated tube systems.
In an ICS (or breadbox) system, cold city water flows
into a rooftop collector. The collector holds 30 to 50 gallons
of water in a serpentine pipe with a heat-capturing coating.
hot water from the collector flows directly to a conventional
water heater. In effect, the sun does most of the work usually performed by the water heater’s burner. as hot water is
withdrawn from the water heater, cold water is drawn into
the collector, driven by pressure in the city water pipes.
a thermosiphon system takes advantage of the fact that
water rises as it’s heated. Solar-heated water in the system’s
flat-plate collector rises through tubes and flows into the
top of an insulated storage tank. Colder water at the bottom
of this tank is drawn into the lower entry of the solar collector. Water thus flows in a continuous loop, continually
being reheated during daylight hours. When a hot water
tap is opened in the house, hot water flows from the top of
the storage tank and is replaced with cold city water flowing
into the bottom of the storage tank.
The thermosiphon system is simple. however, it puts a
heavy 800-lb (363-kilogram)
storage tank high on the roof,
which should be reinforced to
support it. Other solar water-
heating systems put the stor-
age tank at ground level or in
the basement, where it’s not a
structural challenge.
both ICS and thermo-
An evacuated-tube water-heating system takes up
very little roof space. This
one uses a solar-powered
electric pump to boost
water circulation.
Energy Star >> In 2009, the Energy Star program (a
joint effort of the U.S. Department of Energy and the Environmental Protection Agency) began rating solar water-heating systems. Comparative performance data for products from a dozen companies are listed at the Energy Star
website at tinyurl.com/EnergyStarSWH. Under the
“Resources” box, click on “Qualified Solar Water Heaters.”
Swimming pools and hot tubs >> One of the most
common uses for solar water heating is to heat pool water.
Pool solar collectors are lighter in weight and less expensive
than DSWH systems. The size should be 50 to 100 percent
of the surface area of the pool. The larger the solar collector
area, the warmer the pool will be in cool weather. The pool
serves as the storage tank, and the filtration pump circulates
the pool water through the collectors. A solar collector can
provide all the heating necessary for a swimming pool, but
hot tubs and spas need a backup or booster heater.
siphon systems can be used in cooler climates if they’re
converted to a drain-down configuration. When temperatures drop near freezing, valves open to drain the collector,
often into a weather-protected indoor storage tank. When
temperatures rise again, the collector system can be filled
again, either from city water pressure or by using an electric
pump to push water back up from the indoor storage tank.
because of the elaborate automatic control systems, drain-down systems are considered active rather than passive.
The evacuated-tube system is more expensive than
other passive systems, but in some climates, much more
efficient, so it takes less space on your roof. Solar heat is
collected in double-walled glass tubes arranged with one
end high. each tube is built like a thermos bottle liner, with
a vacuum between the walls. In the center of the tube is a
copper pipe containing propylene glycol, a non-poisonous
antifreeze. Propylene glycol is the same stuff sold in auto-parts stores as pet-safe antifreeze for your car; it’s also used
in some food products. While the outside of the glass tube
remains cool, the copper pipe turns hot, and the glycol
boils. The steam rises to the top of the pipe, where it heats
a manifold. Water flowing through the manifold is heated
quickly. The glycol steam in the copper pipe condenses
back to liquid and flows to the bottom of the tube, making a
continuous convective loop. evacuated-tube systems work
in both warm and cold climates.
active Solar Water-heating Systems
active systems use an electric pump to circulate water
through the collector. In warm climates, a direct (or open-
loop) system is practical. In these systems, city water goes
into an insulated storage tank. a pump draws water out
of the storage tank to pass through the solar collector
and go back into the tank. hot water for household use is
drawn from the top of the storage tank, sometimes pass-
ing through a booster heater. an automatic control system
starts the pump whenever the collector is warmer than the
storage tank.
In freezing climates, the rooftop part of the system must
be protected either by draining down when the temperature
dips or by running an antifreeze solution. These cold-weath-
er systems require temperature sensors, electric pumps and
