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Solar Water Heating DOE/GO-10096-050 FS 119 ENERGY March 1996 EFFICIENCY AND RCLEARINGHOUSE ENEWABLE ENERGY Solar Water Heating This publication provides basic informa- cost of solar water heaters is higher than tion on the components and types of solar that of conventional water heaters, the water heaters currently available and the fuel (sunshine) is free. Plus, they are economic and environmental benefits of environmentally friendly. To take advan- owning a system. Although the publica- tage of these heaters, you must have an tion does not provide information on unshaded, south-facing location (a roof, building and installing your own system, for example) on your property. it should help you discuss solar water heating systems intelligently with a solar These systems use the sun to heat either equipment dealer. water or a heat-transfer fluid, such as a water-glycol antifreeze mixture, in Solar water heaters, sometimes called collectors generally mounted on a roof. solar domestic hot water systems, may be The heated water is then stored in a tank a good investment for you and your fam- similar to a conventional gas or electric ily. Solar water heaters are cost competi- water tank. Some systems use an electric tive in many applications when you pump to circulate the fluid through the account for the total energy costs over the collectors. life of the system. Although the initial Homes such as this one use solar water heaters to supply most of the hot-water needs for the household. NT O ME F E T N R E A R P G This document was produced for the U.S. Department of Energy (DOE) by the National Renewable Energy Laboratory (NREL), a DOE national laboratory. E Y D U The document was produced by the Information Services Program, under the DOE Office of Energy Efficiency and Renewable Energy. The Energy Efficiency A N C and Renewable Energy Clearinghouse (EREC) is operated by NCI Information Systems, Inc., for NREL / DOE. The statements contained herein are based on I I T R E information known to EREC and NREL at the time of printing. No recommendation or endorsement of any product or service is implied if mentioned by EREC. D E M S A TAT ES OF Printed with a renewable-source ink on paper containing at least 50% wastepaper, including 20% postconsumer waste Solar water heaters can operate in any cli- more information on ways to use less mate. Performance varies depending, in energy for water heating, contact The part, on how much solar energy is avail- Energy Efficiency and Renewable Energy able at the site, but also on how cold the Clearinghouse (EREC—see Source List at water coming into the system is. The the end of this publication). colder the water, the more efficiently the system operates. In almost all climates, You’ll also want to make sure your site has Using less hot water you will need a conventional backup sys- enough available sunshine to meet your tem. In fact, many building codes require needs efficiently and economically. Your reduces the size you to have a conventional water heater local solar equipment dealer can perform as the backup. a solar site analysis for you or show you (and cost) of the how to do your own. You can also contact First Things First EREC for more information. solar water heater Before investing in any solar energy sys- Remember: Local zoning laws or tem, it is more cost effective to invest in covenants may restrict where you can you will need. making your home more energy efficient. place your collectors. Check with your Taking steps to use less hot water and to city, county, and homeowners association lower the temperature of the hot water to find out about any restrictions. you use reduces the size and cost of your solar water heater. Solar Water Heater Basics Good first steps are installing low-flow Solar water heaters are made up of collec- showerheads or flow restrictors in shower tors, storage tanks, and, depending on the heads and faucets, insulating your current system, electric pumps. water heater, and insulating any hot-water pipes that pass through unheated areas. If There are basically three types of collec- you have no dishwasher, or your dish- tors: flat-plate, evacuated-tube, and con- washer is equipped with its own auto- centrating. A flat-plate collector, the most matic water heater, lower the thermostat common type, is an insulated, weather- on your water heater to 120°F (49°C). For proofed box containing a dark absorber plate under one or more transparent or translucent covers. Flat-Plate Collector Glazing frame Evacuated-tube collectors are made up of Glazing rows of parallel, transparent glass tubes. Inlet connection Each tube consists of a glass outer tube Outlet connection 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. Enclosure Concentrating collectors for residential Flow tubes applications are usually parabolic troughs Absorber plate that use mirrored surfaces to concentrate the sun’s energy on an absorber tube Insulation (called a receiver) containing a heat- transfer fluid. For more information on solar collectors, contact EREC. A flat-plate collector is an insulated, weatherproofed box containing a dark Most commercially available solar water absorber plate. The plate heats up and transfers the heat to the fluid flowing heaters require a well-insulated storage through tubes in or near the absorber plate. tank. Many systems use converted electric 2 closed-loop system uses a heat-transfer Active, Closed-Loop Solar Water Heater fluid (water or diluted antifreeze, for example) to collect heat and a heat exchanger to transfer the heat to house- hold water. Active Systems Active systems use electric pumps, valves, and controllers to circulate water or other Hot water heat-transfer fluids through the collectors. Flat-plate to house collector They are usually more expensive than pas- Cold water sive systems but are also more efficient. supply Active systems are usually easier to retro- Antifreeze fluid in fit than passive systems because their stor- collector loop only age tanks do not need to be installed Solar storage/ backup water heater above or close to the collectors. But because they use electricity, they will not function in a power outage. Active sys- Double-walled tems range in price from about $2,000 to Pump heat exchanger $4,000 installed. Open-Loop Active Systems Open-loop active systems use pumps to circulate household water through the col- An active, closed-loop system heats a heat-transfer fluid (such as water or lectors. This design is efficient and lowers antifreeze) in the collector and uses a heat exchanger to transfer the heat to operating costs but is not appropriate if the household water. your water is hard or acidic because scale water heater tanks or plumb the solar stor- and corrosion quickly disable the system. age tank in series with the conventional water heater. In this arrangement, the These open-loop systems are popular in solar water heater preheats water before nonfreezing climates such as Hawaii. They it enters the conventional water heater. should never be installed in climates that experience freezing temperatures for sus- Some solar water heaters use pumps to tained periods. You can install them in recirculate warm water from storage tanks mild but occasionally freezing climates, through collectors and exposed piping. but you must consider freeze protection. Active systems use This is generally to protect the pipes from freezing when outside temperatures drop Recirculation systems are a specific type to freezing or below. of open-loop system that provide freeze electric pumps, protection. They use the system pump to circulate warm water from storage tanks valves, and controls Types of Solar Water Heaters through collectors and exposed piping Solar water heaters can be either active or to circulate fluid when temperatures approach freezing. passive. An active system uses an electric Consider recirculation systems only where pump to circulate the heat-transfer fluid; a mild freezes occur once or twice a year through the collectors. passive system has no pump. The amount at most. Activating the freeze protection of hot water a solar water heater produces more frequently wastes electricity and depends on the type and size of the sys- stored heat. tem, the amount of sun available at the site, proper installation, and the tilt angle Of course, when the power is out, the and orientation of the collectors. pump will not work and the system will freeze. To guard against this, a freeze Solar water heaters are also characterized valve can be installed to provide addi- as open loop (also called “direct”) or tional protection in the event the pump closed loop (also called “indirect”). An doesn’t operate. In freezing weather, the open-loop system circulates household valve dribbles warmer water through the (potable) water through the collector. A collector to prevent freezing. 3 Closed-Loop Active Systems the collectors are empty, which assures These systems pump heat-transfer fluids freeze protection and also allows the sys- (usually a glycol-water antifreeze mixture) tem to turn off if the water in the storage through collectors. Heat exchangers trans- tank becomes too hot. fer the heat from the fluid to the house- Passive systems hold water stored in the tanks. Pumps in Active Systems The pumps in solar water heaters have are generally more Double-walled heat exchangers prevent low power requirements, and some com- contamination of household water.
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