Solar Water Heating

Flat Plate Solar Collectors

Description:  A flat-plate collector is a large, shallow box—typically mounted on a roof—that heats water using the sun’s energy.

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A typical flat-plate collector is a metal box with a glass or plastic cover (called glazing) on top and a dark-colored absorber plate on the bottom. The sides and bottom of the collector are usually insulated to minimize heat loss.

Sunlight passes through the glazing and strikes the absorber plate, which heats up, changing solar energy into heat energy. The heat is transferred to liquid passing through pipes attached to the absorber plate. Absorber plates are commonly painted with "selective coatings," which absorb and retain heat better than ordinary black paint. Absorber plates are usually made of metal—typically copper or aluminum—because the metal is a good heat conductor. Copper is more expensive, but is a better conductor and less prone to corrosion than aluminum. In locations with average available solar energy, flat plate collectors are sized approximately one-half- to one-square foot per gallon of one-day's hot water use.

Applications:  The main use of this technology is in residential buildings where the demand for hot water has a large impact on energy bills. This generally means a situation with a large family, or a situation in which the hot water demand is excessive due to frequent laundry washing.

Commercial applications include laundromats, car washes, military laundry facilities and eating establishments. The technology can also be used for space heating if the building is located off-grid or if utility power is subject to frequent outages. Solar water heating systems are most likely to be cost effective for facilities with water heating systems that are expensive to operate, or with operations such as laundries or kitchens that require large quantities of hot water.

unglazed liquid collectors are commonly used to heat water for swimming pools. Because these collectors need not withstand high temperatures, they can use less expensive materials such as plastic or rubber. They also do not require freeze-proofing because swimming pools are generally used only in warm weather or can be drained easily during cold weather.

While solar collectors are most cost-effective in sunny, temperate areas, they can be cost effective virtually anywhere in the country so should be considered.

Performance/Costs:  To compare performance ratings, look for a Solar Rating & Certification Corporation (SRCC) or Florida Solar Energy Center (FSEC) sticker on the equipment you are considering. Paybacks - (The amount of time required - usually in years - for positive cash flows to equal the total investment costs. This is often used to describe how long it will take for energy savings resulting from using more energy-efficient equipment to equal the premium paid to purchase the more energy-efficient equipment.) - vary widely, but for a well-designed and properly installed solar water heater, you can expect a simple payback of 4 to 10 years, depending on climate and utility costs. FSEC found that solar water heaters offer potential savings, compared to electric water heating, of as much as 50% to 85% in the water heating portion of the utility bill.

Flat plate water heating systems range in price from about $2,000 to $4,000 installed for residential systems (for 40 to 80 gallons per day usage), and $2,000 to $50,000 for commercial systems (for 40 to 1700 gallons per day usage). The following chart compares the percent of water heating energy that solar can provide in various cities for a 48-square-foot flat plate solar hot water system based on average water usage for four persons.

Availability:  Flat plate solar water heating systems are available in most areas of the United States and many other countries. FSEC currently lists 192 solar collector panel models and 280 solar systems that they certify. They also provide a list of manufacturers of flat plate collectors and systems. In addition, six mail-order catalogs sell solar water heating systems.

Material from: Washington State University Energy Program / WSUCEEP00-121
Diagram and geographic performance data from FEMP Solar Water Heating Technology Alert