Solar Water
Heating
Solar Hot Water Primer
Available from Home Power
Learn the basics about solar water heaters in Solar Hot Water: A Primer, a "bonus article"
available on line from
Home Power magazine.
Solar water heaters, sometimes
called solar domestic hot water systems, may be a good investment for you and
your family. Solar water heaters are cost competitive in many applications when
you account for the total energy costs over the life of the system. Although the
initial cost of solar water heaters is higher than that of conventional water
heaters, the fuel (sunshine) is free. Plus, they are environmentally friendly.
To take advantage of these heaters, you must have an unshaded, south-facing location
(a roof, for example) on your property.
These systems
use the sun to heat either water or a heat-transfer fluid, such as a water-glycol
antifreeze mixture, in collectors generally mounted on a roof. The heated water
is then stored in a tank similar to a conventional gas or electric water tank.
Some systems use an electric pump to circulate the fluid through the collectors.
Solar Water Heating
Solar water heaters
can operate in any climate. Performance varies depending, in part, on how much
solar energy is available at the site, but also on how cold the water coming into
the system is. The colder the water, the more efficiently the system operates.
In almost all climates, you will need a conventional backup system. In fact, many
building codes require you to have a conventional water heater as the backup.
First
Things First
Before investing
in any solar energy system, it is more cost effective to invest in making your
home more energy efficient. Taking steps to use less hot water and to lower the
temperature of the hot water you use reduces the size and cost of your solar water
heater.
Good first steps
are installing low-flow showerheads or flow restrictors in shower heads and faucets,
insulating your current water heater, and insulating any hot water pipes that
pass through unheated areas. If you have no dishwasher, or your dishwasher is
equipped with its own automatic water heater, lower the thermostat on your water
heater to 120°F (49°C). For more information on ways to use less energy for water
heating, contact The
Energy Efficiency and Renewable Energy Clearinghouse (EREC—see Source List at
the bottom of this page). You’ll also want to make sure your site has enough available
sunshine to meet your needs efficiently and economically. Your local solar equipment
dealer can perform a solar site analysis for you or show you how to do your own.
You can also contact EREC for more information.
Remember: Local zoning laws or covenants may restrict where you can place your
collectors. Check with your city, county, and homeowners association to find out
about any restrictions.
Solar Water Heater Basics
Solar water heaters
are made up of collectors, storage tanks, and, depending on the system, electric
pumps.
There are basically
three types of collectors: flatplate, evacuated-tube, and concentrating. A flatplate
collector, the most common type, is an insulated, weather-proofed box containing
a dark absorber plate under one or more transparent or translucent covers.
Evacuated-tube collectors are made up of rows of parallel, transparent glass tubes.
Each tube consists of a glass outer tube and an inner tube, or absorber, covered
with a selective coating that absorbs solar energy well but inhibits radiative
heat loss. The air is withdrawn ("evacuated") from the space between the tubes
to form a vacuum, which eliminates conductive and convective heat loss.
Concentrating collectors for residential applications are usually parabolic troughs
that use mirrored surfaces to concentrate the sun's energy on an absorber tube
(called a receiver) containing a heat-transfer fluid. For more information on
solar collectors, contact EREC.
Most commercially available solar water heaters require a well-insulated storage
tank. Many systems use converted electric water heater tanks or plumb the solar
storage tank in series with the conventional water heater. In this arrangement,
the solar water heater preheats water before it enters the conventional water
heater. Some solar water heaters use pumps to recirculate warm water from storage
tanks through collectors and exposed piping. This is generally to protect the
pipes from freezing when outside temperatures drop to freezing or below.
Types of Solar Water Heaters
Solar water heaters
can be either active or passive. An active system uses an electric pump to circulate
the heat-transfer fluid; a passive system has no pump. The amount of hot water
a solar water heater produces depends on the type and size of the system, the
amount of sun available at the site, proper installation, and the tilt angle and
orientation of the collectors.
Solar water heaters are also characterized as open loop (also called "direct")
or closed loop (also called "indirect"). An open-loop system circulates household
(potable) water through the collector. A closed-loop system uses a heat-transfer
fluid (water or diluted antifreeze, for example) to collect heat and a heat exchanger
to transfer the heat to household water.
Active Systems
Active systems
use electric pumps, valves, and controllers to circulate water or other heat-transfer
fluids through the collectors. They are usually more expensive than passive systems
but are also more efficient. Active systems are usually easier to retrofit than
passive systems because their storage tanks do not need to be installed above
or close to the collectors. But because they use electricity, they will not function
in a power outage. Active systems range in price from about $2,000 to $4,000 installed.
Open-Loop Active Systems
Open-loop active systems use pumps to circulate household water through the collectors.
This design is efficient and lowers operating costs but is not appropriate if
your water is hard or acidic because scale and corrosion quickly disable the system.
These open-loop systems are popular in nonfreezing climates such as Hawaii. They
should never be installed in climates that experience freezing temperatures for
sustained periods. You can install them in mild but occasionally freezing climates,
but you must consider freeze protection.
Recirculation systems are a specific type of open-loop system that provide freeze
protection. They use the system pump to circulate warm water from storage tanks
through collectors and exposed piping when temperatures approach freezing. Consider
recirculation systems only where mild freezes occur once or twice a year at most.
Activating the freeze protection more frequently wastes electricity and stored
heat.
Of course, when the power is out, the pump will not work and the system will freeze.
To guard against this, a freeze valve can be installed to provide additional protection
in the event the pump doesn't operate. In freezing weather, the valve dribbles
warmer water through the collector to prevent freezing. Consider recirculation
systems only where mild freezes occur once or twice a year at most. Activating
the freeze protection more frequently wastes electricity and stored heat.
Closed-Loop
Active Systems
These systems
pump heat-transfer fluids (usually a glycol-water antifreeze mixture) through
collectors. Heat exchangers transfer the heat from the fluid to the household
water stored in the tanks.
Double-walled heat exchangers prevent contamination of household water. Some codes
require double walls when the heat-transfer fluid is anything other than household
water.
Closed-loop glycol systems are popular in areas subject to extended freezing temperatures
because they offer good freeze protection. However, glycol antifreeze systems
are a bit more expensive to buy and install, and the glycol must be checked each
year and changed every 3 to 10 years, depending on glycol quality and system temperatures.
Drainback systems use water as the heat-transfer fluid in the collector loop.
A pump circulates the water through the collectors. The water drains by gravity
to the storage tank and heat exchanger; there are no valves to fail. When the
pumps are off, the collectors are empty, which assures freeze protection and also
allows the system to turn off if the water in the storage tank becomes too hot.
Pumps in Active Systems
The pumps in solar water heaters have low power requirements, and some companies
now include direct current (DC) pumps powered by small solar-electric (photovoltaic,
or PV) panels. PV panels convert sunlight into DC electricity. Such systems cost
nothing to operate and continue to function during power outages.
Passive Systems
Passive systems move household water or a heat-transfer fluid through the system
without pumps. Passive systems have no electric components to break. This makes
them generally more reliable, easier to maintain, and possibly longer lasting
than active systems.
Passive systems can be less expensive than active systems, but they can also be
less efficient. Installed costs for passive systems range from about $1,000 to
$3,000, depending on whether it is a simple batch heater or a sophisticated thermosiphon
system.
Batch
Heaters
Batch heaters (also known as "bread box" or integral collector storage systems)
are simple passive systems consisting of one or more storage tanks placed
in an insulated box that has a glazed side facing the sun. Batch heaters are inexpensive
and have few components—in other words, less maintenance and fewer failures. A
batch heater is mounted on the ground or on the roof (make sure your roof structure
is strong enough to support it). Some batch heaters use "selective" surfaces on
the tank(s). These surfaces absorb sun well but inhibit radiative loss.
In climates where freezing occurs, batch heaters must either be protected from
freezing or drained for the winter. In well-designed systems, the most vulnerable
components for freezing are the pipes, if located in uninsulated areas, that lead
to the solar water heater. If these pipes are well insulated, the warmth from
the tank will prevent freezing. Certified systems clearly state the temperature
level that can cause damage. In addition, you can install heat tape (electrical
plug-in tape to wrap around the pipes to keep them from freezing), insulate exposed
pipes, or both. Remember, heat tape requires electricity, so the combination of
freezing weather and a power outage can lead to burst pipes. If you live in an
area where freezing is infrequent, you can use plastic pipe that does not crack
or burst when it freezes. Keep in mind, though, that some of these pipes can't
withstand unlimited freeze/thaw cycles before they crack.
Thermosiphon Systems
A thermosiphon
system relies on warm water rising, a phenomenon known as natural convection,
to circulate water through the collectors and to the tank. In this type of installation,
the tank must be above the collector. As water in the collector heats, it becomes
lighter and rises naturally into the tank above. Meanwhile, cooler water in the
tank flows down pipes to the bottom of the collector, causing circulation throughout
the system. The storage tank is attached to the top of the collector so that thermosiphoning
can occur. These systems are reliable and relatively inexpensive but require careful
planning in new construction because the water tanks are heavy. They can be freeze-proofed
by circulating an antifreeze solution through a heat exchanger in a closed loop
to heat the household water.
Sizing
Your System
Just as you have
to choose a 30-, 40-, or 50-gallon (114-, 151-, or 189-liter) conventional water
heater, you need to determine the right size solar water heater to install. Sizing
a solar water heater involves determining the total collector area and the storage
volume required to provide 100% of your household's hot water during the summer.
Solar-equipment experts use worksheets or special computer programs to assist
you in determining how large a system you need.
Solar storage
tanks are usually 50-, 60-, 80-, or 120-gallon (189-, 227-, 303-, or 454-liter)
capacity. A small (50 to 60 gallon) system is sufficient for 1 to 3 people, a
medium (80-gallon) system is adequate for a 3- or 4-person household, and a large
(120-gallon) system is appropriate for 4 to 6 people.
A rule of thumb
for sizing collectors: allow about 20 square feet (about 2 square meters) of collector
area for each of the first two family members and 8 square feet (0.7 square meter)
for each additional family member if you live in the Sun Belt. Allow 12 to 14
additional square feet (1.1 to 1.3 square meters) per person if you live in the
northern United States.
A ratio of at
least 1.5 gallons (5.7 liters) of storage capacity to 1 square foot (0.1 square
meter) of collector area prevents the system from overheating when the demand
for hot water is low. In very warm, sunny climates, experts suggest that the ratio
should be at least 2 gallons (7.6 liters) of storage to 1 square foot (0.1 square
meter) of collector area. For example, a family of four in a northern climate
would need between 64 and 68 square feet (5.9 and 6.3 square meters) of collector
area and a 96- to 102-gallon (363- to 386-liter) storage tank. (This assumes 20
square feet of collector area for the first person, 20 for the second person,
12 to 14 for the third person, and 12 to 14 for the fourth person. This equals
64 to 68 square feet, multiplied by 1.5 gallons of storage capacity, which equals
96 to 102 gallons of storage.) Because you might not be able to find a 96-gallon
tank, you may want to get a 120-gallon tank to be sure to meet your hot water
needs.
Benefits
of Solar Water Heaters
There are many
benefits to owning a solar water heater, and number one is economics. Solar water
heater economics compare quite favorably with those of electric water heaters,
while the economics aren't quite so attractive when compared with those of gas
water heaters. Heating water with the sun also means long-term benefits, such
as being cushioned from future fuel shortages and price increases, and environmental
benefits.
Economic
Benefits
Many home builders
choose electric water heaters because they are easy to install and relatively
inexpensive to purchase. However, research shows that an average household with
an electric water heater spends about 25% of its home energy costs on heating
water.
It makes economic
sense to think beyond the initial purchase price and consider lifetime energy
costs, or how much you will spend on energy to use the appliance over its lifetime.
The Florida Solar Energy Center studied
the potential savings to Florida homeowners of common water-heating systems compared
with electric water heaters. It found that solar water heaters offered the largest
potential savings, with solar water-heater owners saving as much as 50% to 85%
annually on their utility bills over the cost of electric water heating.
The FSEC analysis
illustrates that the initial installed cost of the solar water heater ($1,500
to $3,000) is higher than that of a gas water heater ($350 to $450) or an electric
water heater ($150 to $350). The costs vary from region to region, so check locally
for costs in your area. Depending on the price of fuel sources, the solar water
heater can be more economical over the lifetime of the system than heating water
with electricity, fuel oil, propane, or even natural gas because the fuel (sunshine)
is free.
However, at the
current low prices of natural gas, solar water heaters cannot compete with natural
gas water heaters in most parts of the country except in new home construction.
Although you will still save energy costs with a solar water heater because you
won't be buying natural gas, it won't be economical on a dollar-for-dollar basis.
Paybacks vary
widely, but you can expect a simple payback of 4 to 8 years on a well-designed
and properly installed solar water heater. (Simple payback is the length of time
required to recover your investment through reduced or avoided energy costs.)
You can expect shorter paybacks in areas with higher energy costs. After the payback
period, you accrue the savings over the life of the system, which ranges from
15 to 40 years, depending on the system and how well it is maintained.
You can determine
the simple payback of a solar water heater by first determining the net cost of
the system. Net costs include the total installed cost less any tax incentives
or utility rebates. (See the box for more information.) After you calculate the
net cost of the system, calculate the annual fuel savings and divide the net investment
by this number to determine the simple payback.
An example: Your
total utility bill averages $160 per month and your water heating costs are average
(25% of your total utility costs) at $40 per month. If you purchase a solar water
heater for $2,000 that provides an average of 60% of your hot water each year,
that system will save you $24 per month ($40 x 0.60 = $24) or $288 per year (12
x $24 = $288). This system has a simple payback of less than 7 years ($2,000 ÷
$288 = 6.9).
For the remainder of the life of the solar water heater, 60% of your hot water
will be free, saving you $288 each year. You will need to account for some operation
and maintenance costs, which are estimated at $25 to $30 a year. This is primarily
to have the system checked every 3 years.
If you are building
a new home or refinancing your present home to do a major renovation, the economics
are even more attractive. The cost of including the price of a solar water heater
in a new 30-year mortgage is usually between $13 and $20 per month. The portion
of the federal income tax deduction for mortgage interest attributable to the
solar system reduces that amount by about $3 to $5 per month. If your fuel savings
are more than $15 per month, the investment in the solar water heater is profitable
immediately.
Long-Term
Benefits
Solar water heaters
offer long-term benefits that go beyond simple economics. In addition to having
free hot water after the system has paid for itself in reduced utility bills,
you and your family will be cushioned from future fuel shortages and price increases.
You will also be doing your part to reduce this country's dependence on foreign
oil. The National Remodelers Association reports that adding a solar water heater
to an existing home raises the resale value of the home by the entire cost of
the system. You may be able to recoup your entire investment when you sell your
home.
Environmental
Benefits
Solar water heaters
do not pollute. By investing in one, you will be avoiding carbon dioxide, nitrogen
oxides, sulfur dioxide, and the other air pollution and wastes created when your
utility generates power or you burn fuel to heat your household water. When a
solar water heater replaces an electric water heater, the electricity displaced
over 20 years represents more than 50 tons of avoided carbon dioxide emissions
alone. Carbon dioxide traps heat in the upper atmosphere, thus contributing to
the "greenhouse effect."
Be
a Smart Consumer
Take the same
care in choosing a solar water heater that you would in the purchase of any major
appliance. Your best protection is to consider only certified and labeled systems.
One such label is put on by the Solar Rating & Certification Corporation (SRCC),
a nonprofit, independent third-party organization formed by the state energy officials,
and consumer advocates to certify and rate solar water heaters.
A national standard
(OG-300) addresses a variety of concerns, including safety and health, durability
and reliability, installation, performance, and operation and maintenance. To
meet this standard, a system is rigorously tested. A certified solar water heater
carries the SRCC OG-300 label, and the system performance is listed in a published
directory. A similar program has been established for Florida by FSEC. Both SRCC
and FSEC provide collector testing and rating programs.
Find out if the
manufacturer offers a warranty, and, if so, what the warranty covers and for how
long. If the dealer you are buying the equipment from goes out of business, can
you get support and parts from the manufacturer, or from a local plumbing contractor?
Make sure that
the workers who are actually installing the system are qualified to do the work.
Ask the installation contractor for references and check them. When the job is
finished, have the contractor walk you through the system so you are familiar
with the installation. And be sure that an owner's manual with maintenance instructions
is included as part of the package.
A
Bright Future
A solar water
heater is a long-term investment that will save you money and energy for many
years. Like other renewable energy systems, solar water heaters minimize the environmental
effects of enjoying a comfortable, modern lifestyle. In addition, they provide
insurance against energy price increases, help reduce our dependence on foreign
oil, and are investments in everyone's future.
You might also
consider other solar energy systems for your home. Systems similar to the solar
water heater are used for space heating and swimming pool heating. In fact, pool
heating is a major market for solar energy systems. For more information on these
systems, contact:
Energy Efficiency
and Renewable Energy Clearinghouse (EREC)
P.O. Box 3048
Merrifield, VA 22116
(800) DOE-EREC (363-3732)
Fax: (703) 893-0400
E-mail: [email protected]
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