Solar Water Heating

A solar water heater, sometimes called a solar domestic hot water system, may be a good investment for residences or businesses. Solar water heaters are cost competitive in many applications when one accounts 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, and 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.

Solar water heating systems use the sun to heat either water or a heat-transfer fluid, such as a water-glycol antifreeze mixture, in collectors typically mounted on a roof. The heated water is then stored in a tank similar to a conventional water heater. The tank could also be used as the collector itself. Some systems use an electric pump to circulate the fluid through the collectors.

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. In almost all climates, you will need a conventional system as a backup. The solar system should be sized to optimize the performance of the system throughout the year, not for worst case extremes. This strategy will avoid unnecessary initial expenses associated with oversized equipment.

Solar Water Heating & Space Heating, Figure 1

Diagram of a typical solar water heating system.

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. If considering solar space heating, building envelope improvements and weatherization strategies should be incorporated for the same reasons. The largest upfront cost is associated with the size of the solar collectors required to serve the loads.

You will 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.

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 are not 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 homebuilders 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. Austin's residential energy code does not allow electric resistance heat as the primary source of water heat in new construction.

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 (FSEC-see Resources) 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 percent annually on their utility bills over the cost of electric water heating.

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 will not 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.

Tax Incentives and Rebates

Some local or state governments offer tax incentives to encourage residents to invest in solar energy technologies. Currently there are no tax incentives available in Texas to assist in the initial costs of solar energy systems. However, solar systems are exempted from property taxes. Some electric utilities offer rebates to customers who install solar energy equipment because these installations help utilities reduce peak loads. (Peak loads are periods when the utility must generate extra power to meet a high demand.)

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.

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.

Solar Space Heating

Solar space heating systems are generally not very cost effective in Central Texas, since winter heating requirements are marginal and sporadic in this region. Nevertheless, some people prefer to take advantage of renewable energy as much as possible. The active solar space heating system can use the same operational components as the domestic water heating systems, but ties into a heating distribution system that can use heated fluids as a heat source. The possible distribution systems include hydronic baseboard radiators, floor radiant heat systems, and hydronic heat forced air systems.

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, weatherproofed 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.

Most commercially available solar water heaters require a well-insulated storage tank. Many systems use converted conventional electric or gas water heater tanks or the solar storage tank is plumbed 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 done 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 potable water. An indirect or direct system can be used for heating swimming pools and spas. Lower cost unglazed (no glass cover) collectors are available for this purpose.

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. However, because active systems 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 potable 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 provides 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, if the power goes 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"t operate. In freezing weather, the valve dribbles warmer water through the collector to prevent freezing.

Closed-Loop Active Systems

These systems pump heat-transfer fluids (usually tap water or a glycol-water antifreeze mixture) through collectors. Heat exchangers transfer the heat from the fluid to the potable water stored in the tanks.

Double-walled heat exchangers prevent contamination of potable water. Some codes require double walls when the heat-transfer fluid is anything other than potable 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.

Solar Water Heating & Space Heating, Figure 2

Drainback hot water system.

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 potable 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 that lead to the solar water heater, if they are located in uninsulated areas. 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 cannot withstand unlimited freeze/thaw cycles without breaking.

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 potable water.

Sizing Your System

Sizing a solar water heater involves determining the total collector area and the storage volume required to provide 100 percent of your household's hot water during the summer. Solar-equipment experts use worksheets or special computer programs to determine what size system is needed.

Solar storage tanks are usually 50-, 60-, 80-, or 120-gallon 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. Consult with a solar dealer to size equipment for commercial use.

Here is a rule of thumb for sizing collectors for households in Central Texas: allow about 20 square feet of collector area for each of the first two family members and 8 square feet for each additional family member.

A ratio of at least 2.0 gallons of storage capacity to 1 square foot of collector area prevents the system from overheating when the demand for hot water is low in Central Texas. For example, a family of four in Central Texas would need 56 square feet of collector area and a 112-gallon storage tank. (This assumes 20 square feet of collector area for the first person, 20 for the second person, 8 each for the third and the fourth person. (56 square feet, multiplied by 2 gallons of storage capacity equals 112 gallons of storage.) Because you might not be able to find a 112-gallon tank, you may want to get a 120-gallon tank to be sure to meet your hot water needs.

Solar Space Heating

The tilt of space heating solar collectors is the latitude plus 15 degrees (a total of 45 degrees in Austin). The purpose of the tilt is to align the collectors perpendicular to the sun's rays in the heating season when the optimal performance is needed. The number of collectors used in a space heating application is based on the heat load of the house:

Average heat load ˜ collector rated heat output = number of collectors needed.

Basing the size of the collectors only on the average heat load of the building during the heating season will not provide enough heat during the colder part of the heating season. Since the heat load of the building is dependent upon the extent of its energy conserving features, the greater the energy efficiency of the building, the smaller the system can be.

The space heating system, like the domestic water heating system, must be backed up by an auxiliary heating system. It is not practical to size a solar system to provide all of a home's heat requirement under the worst conditions because the system would become too large and too costly. The solar loop pump is controlled in much the same way as the water heating solar system. When solar heat is available to heat the storage water, the pump comes on.

The hot water storage system should be sized to approximately 2.5 gallons of storage for each square foot of collector area. The fluid that is heated and stored (typically water) can be distributed into the house in one of the methods discussed below.

Solar Water Heating & Space Heating, Figure 3

Solar space heating system.

Air distribution system

Whenever the thermostat calls for heat, the heated water in the storage tank is pumped into a coil located in the return air duct. Most solar water heating systems include an auxiliary heater. These heaters add heat to the solar storage tank to maintain a minimum operating temperature in the storage tank at all times.

Hydronic system with radiators

The heated water is circulated in series with a boiler into radiators located in the living spaces. Modern baseboard radiators operate effectively at 140 degrees. Using the solar system's heated water as the source of water for the boiler will reduce the boiler's energy use.

Radiant floor hydronic systems

The solar heated water is pumped through distribution piping located in the floor of the building. Lower temperatures are used in this type of system-- the slab is not heated above 80 degrees in most cases. The auxiliary heat can be connected in series with the solar system's heated output water or it can be connected to the solar tank to provide a minimum temperature.

In the Austin area, most buildings use an air distribution system that can provide air conditioning as well as heating. The radiant hydronic systems are much less common but are considered highly effective in terms of comfort, efficiency, and health impacts (no blowing air to stir up dust). The air distribution method described above can work quite well with a conventional gas water heater as a backup. (See the Water Heating.)

Another form of solar space heating uses air collectors, but this is more practical in climates with longer and colder winters than Austin's. The investment in storage systems for air collectors is substantial in time, money, and materials. In Central Texas, the use of air collectors to put heat directly into the house can be readily achieved with properly oriented windows. Since daytime temperatures in the winter can be relatively high, the additional hot air from an air collector can overheat a home that does not have extra thermal mass to absorb the heat.

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 solar industry, 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 installers 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. Be sure that an owner's manual with maintenance instructions is included as part of the package. Label the system components and provide a laminated system diagram near the controller for future owners.

Alternative Energy Institute
Vaughn Nelson
Box 248, WTUMA
Canyon, TX 79016
(806) 656-2296
Wind, solar, research, education, consultant

Florida Solar Energy Center
1679 Clearlake Rd.
Cocoa, FL 32922
(407) 638-1000
www.fsec.ucf.edu

Solar Energy International
P.O. Box 715
76 S. 2nd St.
Carbondale, CO 81623-0715
(970) 963-8855
www.solarenergy.org

American Energy Technologies, Inc. (AET)
1057 N. Ellis Rd. Unit 4
Jacksonville, FL 32254
(800) 874-2190
www.AETSOLAR.com
Solar products and systems

Dawn Solar
Tom Kociemba, Sales Engineer
www.dawnsolar.com
tom@dawnsolar.com
(603) 828-7274
Fax: (888) 595-1450 Option 4
Solar thermal products

Jade Mountain Inc.
P.O. Box 4616
Boulder, CO 80306
(800) 442-1972
www.jademountain.com
Complete systems

Osborne Solar
Michael Osborne
909 W. 23rd St.
Austin, TX 78705
(512) 476-3511
Consultant, wind, photovoltaics, solar thermal

Real Goods Trading Co.
555 Leslie St.
Ukiah, CA 95482-5576
(800) 762-7325
www.realgoods.com
Complete systems

Solar System Installations
Phillip Fisher
P.O. Box 736
Coppell, TX 75019
(214) 462-0626
Fax (214) 393-1960
Solar water heating, photovoltaics

Sun Trapper Solar Manufacturing
12118 Radium Dr.
San Antonio, TX 78216
(210) 341-2001
www.suntrapper.com
Complete systems

SunQuest, Inc.
1555 Rankin Ave.
Newton, NC 28658
(704) 465-6805
Complete systems

Sunrise Solar
5114 Balcones Woods Dr., Ste.307
Austin, TX 78759
(512) 243-0628
Solar water heaters for pools

Thermal Conversion Technology
P. O. Box 3887
Sarasota, FL 34230-3887
(813) 953-2177
Passive water heaters

American Solar Energy Society, Inc. (ASES)
2400 Central Ave. G-1
Boulder, CO 80301
(303) 443-3130
www.ases.org

Energy Efficiency and Renewable Energy Clearinghouse
PO Box 3048
Merryfield, VA 22116
(800) 363-3732
www.eren.doe.gov

Florida Solar Energy Center
1679 Clearlake Rd.
Cocoa, FL 32922
(407) 638-1000
www.fsec.ucf.edu

National Renewable Energy Laboratory
1617 Cole Blvd.
Golden, CO 80401-3393
(303) 275-3000
www.nrel.gov

Passive Solar Industries Council (PSIC)
1331 H St. NW, Ste. 1000
Washington, DC 20005
(202) 628-7400
www.psic.org

Solar Energy Industries Association (directory of SRCC ratings)
122 C St. NW, 4th Floor
Washington, DC 20001
(202) 383-2600
www.seia.org

State Energy Conservation Office
200 E. 10th St. Ste. 206
Austin, TX 78701
(512) 463-1931

Texas Solar Energy Society
P. O. Box 1447
Austin, TX 78767-1447
(512) 326-3391
(800) 465-5049
www.txses.org