Learn how you can save money by using a solar hot water system to cut water heating costs.
Hot water heating is one of the most efficient and
cost-effective ways of harnessing solar energy. According
to the Department of Energy’s Office of Energy Efficiency
and Renewable Energy, hot water heating accounts for
one-fifth to one-quarter of an average U.S. household’s
energy usage. Installing a solar domestic hot water system
can reduce your hot water heating bill by 60 percent to 95
percent, potentially saving you hundreds of dollars a year.
Invest in a solar hot water system for $3,000 to $6,000
and, in some cases, you might recoup the costs within only
four to six years.
“If someone is really interested in renewable energy, and
they want to get involved somewhere, a solar hot water
system is probably the best place to start,” says Rod
Hyatt, owner of In Hot Water Heat & Power in Eden,
Utah. “It makes the biggest impact at the lowest
investment.” Many states, municipalities and some local
utilities now offer tax incentives; and rebates,
significantly sweetening the solar deal.
Fueled by the sun (think free energy!), instead of
fossil fuels, solar hot water systems emit none of the
pollutants and greenhouse gases — carbon dioxide,
nitrous oxides, sulfur oxides — produced when natural
gas or oil is burned. And solar hot water systems can be
used wherever the sun shines. (Of course, generally the
sunnier your location, the more hot water you’ll reap.)
Hot Household Water
In most homes, domestic hot water (water for washing
dishes and clothes, for bathing or cooking) usually is
provided by an electric or gas-fired water heater, or
boiler or furnace that also heats the home. Heating water
with electricity is expensive and will become even more
costly as the price of fossil fuels used to generate it
continues to climb. And, although heating water with gas is
less expensive than heating with electricity, burning gas
still contributes to pollution and global warming.
In concert with an electric or gas-fired backup unit, solar
domestic hot water can reduce the requirement for
conventional water heating by two-thirds or more. The total
amount the solar hot water system can contribute depends on
your household’s hot water consumption, and the amount of
sunshine the collectors receive daily and throughout the
year. In general, most solar systems are designed to meet
one-half to three-quarters of a family’s domestic hot water
need. During the summertime, the system may usually meet
all of their hot water needs. On average, an efficient
collector in good weather will heat between 1 and 2 gallons
of water per square foot per day. (A solar contractor can
help you correctly size the collectors, storage tank and
backup system.)
Smart Solar Shopping
If you’re haunted by horror stories of the 70s and ’80s,
when fly-by-night installers, more interested in selling
tax credits than reliable systems, left unwitting
homeowners with poorly designed, difficult-to-maintain
systems—it’s time to shed those fears. Today,
performance standards govern most active solar components,
making comparisons between products possible (see Sources,
Page 80), so when you shop, you can shop smart. Most
manufacturers warrant their systems for at least 10 years;
in many cases, the collectors and insulated piping may last
as long as your house. Since it tends to degrade over time,
the circulating fluid should be inspected every few years
by a qualified technician.
Solar System Components
Solar hot water systems consist of collectors connected to
one or more storage tanks by piping. For active systems,
pumps, sensors and controllers may be added.
Flat-plate, evacuated-tube and batch collectors are all
common collectors; each is suited to different needs.
Flat-plate collectors (see illustration at right), with a
“black chrome” or a similar selective-surface coating,
which absorbs the maximum amount of solar heat while
minimizing radiant cooling, are the most common. Small
tubes inside an insulated aluminum box with a tempered
glass face contain either potable water or a heat-transfer
fluid; the collector absorbs and transfers heat energy to
the fluid in the tubes, which is then carried to the heat
storage tank by insulated piping.
Evacuated-tube collectors use rows of parallel, transparent
glass tubes, which are coated with a selective finish. A
vacuum inside the tubes helps the collectors achieve
temperatures between 170 and 350 degrees, which makes
evacuated tube collectors ideal for high temperature water
applications. Batch collectors are simple systems that use
one or more black-painted tanks enclosed in a
well-insulated, glass-topped box. In this design, the
collector and storage tank are combined.
Flat-plate collectors generally perform most consistently
for residential applications, yielding temperatures in a
range of 140 to 160 degrees. Thermal performance ratings
can be used to compare the efficiency and economics of
various flat-plate solar collectors (see Sources, page 80 in this issue).
To maximize their efficiency, collectors need unobstructed
southern solar exposure between the hours of 9 a.m. and 3
p.m., so site them accordingly. The collectors can be
mounted on a building’s roof or walls or set on a frame on
the ground.
Types of Solar Heating Systems
Water heated by the collectors may be moved through the
house with pumps (active systems) or by natural convection
(passive systems). Batch and passive open-loop systems
(which use natural convection to circulate fluid) require
little maintenance but are vulnerable to freezing. More
common active systems that use pumps and sensors to
regulate fluid flow are generally more efficient and
freeze-resistant, but require more maintenance.
In a direct or open-loop active system, a
pump (regulated by an electronic controller, an appliance
timer or photovoltaic panel) circulates heated domestic
water throughout the entire system, from the collectors to
the heat-storage tanks. With fewer components and no heat
exchanger to lose heat, direct active systems tend to be
more efficient. They can be powered by photovoltaic modules
or a simple AC pump and controller. However, because these
systems circulate water instead of antifreeze, they are
vulnerable to freezing and are not recommended in climates
that experience freezing temperatures.
Indirect or closed-loop active systems are
the most freeze-resistant. They circulate a heat-transfer
fluid (most commonly a nontoxic propylene glycol and water
mixture) that transfers heat from the collectors to potable
water held in storage tanks (the antifreeze also keeps the
collectors and exterior piping from freezing). Like direct
active systems, they can be powered by photovoltaics, too.
They have more components and are more complex.
In cold climates where freezing is a concern, solar
contractor Tom Lane, owner of Energy Conservation Services
of North Florida, Inc. in Gainsville. Florida. recommends
consumers choose from one of two closed-loop, active
systems: unpressurized drainback systems or pressurized
glycol systems.
“The most important criteria for both is that they won’t
freeze and burst.”
An unpressurized drainback system uses a
closed- loop of unpressurized water separate from domestic
pressurized water. When solar heat can be collected, a
differential control, which measures the difference in
temperature between two sensors and turns equipment on and
off, turns the AC circulating pump on, and water circulates
through the solar collectors. When the pump stops, the
collectors and pipes drain automatically into a drainback
tank. Hot water from the drainback tank is pumped through a
heat exchanger located in (or sometimes outside) the main
potable water storage tank. Almost all drainback systems
use a differential control and sensors to automatically
drain the water from the collector when the temperature
drops to freezing. In very cold climates, a propylene
glycol solution can be added to the unpressurized solar
loop for added freeze protection. Drainback systems
typically have the least maintenance and are the highest
performing, closedloop solar hot water systems available.
Closed-loop glycol systems are the preferred option for extremely cold climates,
but can be installed anywhere. Instead of using a drainback
system, closed-loop, pressurized glycol systems rely on a
continuous supply of antifreeze that remains in the pipes
and collector. whether the circulating pump is operating or
not. Closedloop glycol systems often rely on a differential
controller, sensors and an AC pump. A small PV panel can be
used to operate a DC circulating pump, which runs while the
sun shines and automatically shuts itself off when night
falls or cloudy weather prevails.
Double-walled heat exchangers prevent contamination of
household water with antifreeze or other heat-exchange
fluids. Heat exchangers can be mounted outside the storage
tank or located inside the tank. Because antifreeze tends
to degrade over time, however, these systems need regular
inspections to ensure that the antifreeze solution is still
viable.
Whatever system you choose, locate the storage tank (and if
practical, the collectors) as close as possible to the
household distribution and backup system to minimize heat
loss from the pipes, and insulate the pipes well.
Real Life Solar Heating Applications
Jay Mead and his wife, Edie Farwell, recently installed a
closed-loop glycol solar hot water system in their home in
Hartland, Vermont. “We live in an environmentally based
cohousing community that has a centralized wood-fired
boiler for all the units,” he says, “and we wanted to have
some kind of non-fossil-fueled augmentation for the
boiler.” A solar hot water system seemed like an obvious
choice.
So far, their solar domestic hot water system has performed
flawlessly. “It’s really quite amazing,” Mead says. “On a
cold winter day, when the temperature outside is around
zero, the temperature of the tank registers between 130 and
150 degrees. We’ve been very impressed.” The system’s
operation is so subtle that Mead says the only way he can
tell that it’s running is the faint sound of the
circulating pump.
Val Roberts and his wife, Rosalie, retired recently to a
remote home in Bancroft, Idaho, where winter temperatures
of 40 degrees below zero are not unusual. But Roberts
didn’t hesitate to install a solar hot water system. The
glycol in the solar system offers freeze protection down to
50 below zero.
Besides offsetting a substantial portion of their domestic
hot water needs, the system provides some space heating.
“There are days when we get water as hot as 200 degrees out
of the solar panels, and we can heat the water tanks up to
180 degrees, which act as a heat sink for the times when
the sun isn’t shining,” he says. The Roberts have a valve
on their domestic hot water line so they don’t get scalded.
“We’re very satisfied with the system,” he says. “We would
definitely recommend it to other homeowners.”
Since 1979, more than 16,000 solar hot water heaters have
been installed in Oregon — certainly not the sunniest
spot in the country! — for domestic hot water, swimming
pools and spas (and many of those early systems are still
working). A typical solar hot water heater there provides
between 50 percent and 60 percent of a home’s water heating
needs. Oregon currently offers a residential energy tax
credit (based on the system’s performance) of up to $1,500
on certified systems. Linking tax credits or other
incentives to system performance is an important new
strategy to minimize tax credit abuses, and has widespread
support in the solar industry.
The Future of Solar Heating
If solar hot water systems are so popular with their
owners, even with those who live in extreme climates, why
aren’t they found in more homes across the United States?
The problem isn’t technical, Lane says; the technology is
time-tested and works well. Part of the problem is the need
to rebuild the industry’s support infrastructure and to
strengthen educational and promotional efforts. The other
part is political.
“We could sure use more vocal support from our
politicians,” Lane says. “It would be extremely helpful if
they would encourage people to adopt solar energy.”
Nevertheless, he is optimistic about the future of solar
hot water. “It’s the best product the solar industry has to
offer, and it’s going to grow in the future,” he says.
“It’s affordable, and from an environmental standpoint,
it’s the responsible thing to do.”
Sources for Solar Domestic Hot Water Systems
American Solar Energy Society
(ASES)
www.ases.org
The ASES can help you find solar contractors and suppliers
in your area. They also publish Solar Today magazine, which
often has articles on solar hot water systems.
The Database of State
Incentives for Renewable Energy (DSIRE)
www.dsireusa.org
A comprehensive web-based source of information on state,
local, utility and selected federal incentives that promote
renewable energy, including tax credits, sales and property
tax exemptions, rebates and loans.
Florida Solar Energy Center
(F SEC)
www.fsec.ucf.edu
Hosts the Solar Rating and Certification Corporation
(SRCC), a nonprofit organization with the primary purpose
of developing and implementing certification programs and
national rating standards for solar energy equipment.
Solar Energy Industries
Association (SEIA)
www.seia.org
Solar contractors and suppliers can be located through the
SEIA website.
Tom Lane, Energy Conservation Services of North
Florida, Inc.
www.ecs-solar.com
Offers a comprehensive, spiralbound book, Solar Hot Water Systems: Lessons Learned, 1977 to Today, available in a black and white edition for homeowners ($35), and a color edition for contractors ($55).
