United States
Air and Radiation
Environmental Protection
(6202J)
Agency
September 2001
IMPROVE ENERGY E FFICIENCY WITH
SOLAR WATER HEATING
MECHANICAL EQUIPMENT IMPROVEMENTS
DESCRIPTION
Water heating can constitute 15% to 25%, or more,
of the energy use of a home. The approximate
annual cost to operate water heaters ranges from
$200 for a minimum efficiency gas-fired unit to as
much as $500 for a minimum efficiency electric unit.
Solar water heaters are available that can reduce
annual operating costs by 50% to 80% or more
using "free energy" from the sun. These systems
typically are cost-competitive with electric water
heaters and also can be cost-competitive with natural
gas-fired water heaters. Solar water heaters come in
two distinct designs: active and passive systems.
In an "active" system, when sunlight heats one or
more solar collectors sufficient for water heating,
sensors and a controller activate a pump to circulate
a fluid: either potable water from the storage tank or
a food-grade antifreeze solution in climates exposed
to freezing conditions. As shown in Figure 1, the fluid
is drawn from the colder bottom portion of the
storage tank up to the collector for solar heating, and
then circulated back to the top of the storage tank.
Where antifreeze solutions are used, the solar heat is
transferred to water in the storage tank through a
heat exchanger. When the water in the storage tank is
warmer than the collector, the controller and sensor
F IGURE 1 : ACTIVE WATER HEATER
switch off the pump. In many active systems, the
solar system has its own water storage tank, allowing
it to serve as a "pre-heater" for the primary water
heater (gas or electric).
F IGURE 2 : PASSIVE WATER HEATER
A "passive" system combines the solar collector and
storage in one complete unit separate from the
conventional gas or electric water heater storage
tank. The storage can be directly coupled with the
collector (an integral collector/storage system, as
shown in Figure 2) or located a