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An Overview of Solar Water Heating

10/19/2007 by Andy Walker, PhD PE

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Solar water heating systems are relatively simple extensions to buildings’ plumbing systems, which impart heat from the sun to preheat service hot water.

Water heating accounts for a substantial portion of a building’s energy use, ranging from approximately 9% of total energy use in office buildings to 40% in lodging facilities. Averaged across all buildings, hot water represents 15% of energy use in residential buildings, and 8% in commercial buildings.

Solar water heating systems are usually designed to provide about two thirds of a building’s hot water needs, and more where fuel is very expensive or unavailable. Solar water heating applications include domestic water heating, pool and spa heating, industrial processes such as laundries and cafeterias, and air conditioning reheat in hot, humid climates.

Solar water heating is most effective when it serves a steady water heating load that is constant throughout the week and year (or at a maximum during the summer). For example, a prison that is occupied seven days a week would accrue 40% more cost savings than a school open only five days a week.

In 2004, a total of 14,114 SF of collector area was shipped by suppliers (mostly from New Jersey, California, and Israel) to the U.S. market, up from 8,583,000 SF in 1999. Growth in solar water heating is spurred by federal tax credits, incentives in some states, and the high cost of natural gas. Low-temperature swimming pool heating was by far the largest application, with 13,608,000 SF. Flat-plate collectors to supply service hot water in pumped systems accounted for 383,000 SF, and thermosyphon systems accounted for 118,000 SF. Air heating collectors accounted for 4,000 SF, while evacuated tube collectors totaled 2,000 SF.

Advanced technology and production economies of scale have led to significant cost reductions. The value of shipped low-temperature collectors was $1.80/SF in 2004. The average cost of medium temperature collectors was $19.30/SF in 2004. These values are based on factory revenue divided by output, so retail prices would be roughly double.

Solar water heating can be used effectively in almost any geographic location, but is especially prevalent and effective at low latitudes, where the constant solar resource matches a constant water load. In 2004, 37% of solar thermal collectors were shipped to Florida, 31% to California, 7% to New Jersey, 6% to Arizona, and 3% to Hawaii. Appropriate near-south-facing roof area or nearby unshaded grounds would be required for installation of a collector. System types are available to accommodate freezing outdoor conditions, and systems have been installed as far north as the Arctic and as far south as Antarctica.

There are different types of solar water heating systems; the choice depends on the temperature required and the climate. All types have the same simple operating principle. Solar radiation is absorbed by a wide-area solar collector, or solar panel, which heats the water directly or heats a nonfreezing fluid which, in turn, heats the water by a heat exchanger. The heated water is stored in a tank for later use. A backup gas or electric water heater is used to provide hot water when the sun is insufficient, and to optimize the economical size of the solar system.

Solar water heating systems save the fuel otherwise required to heat the water, and avoid the associated cost and pollution. A frequently overlooked advantage of solar water heating is that the large storage volume increases the capacity to deliver hot water. As one residential system owner described it, “With 120 gallons of solar-heated water and the 40 gallon backup heater, I can take a shower, my wife can take a bath, we can have the dishwasher and the clothes washer going, and we never, never run out of hot water.”

Although solar water heating systems all use the same basic principle, they do so with a wide variety of specific technologies that distinguish different collectors and systems. The distinctions are important because certain types of collectors and systems best serve certain applications in various locations.

The following nomenclature describes types of solar water heating systems:

  • Passive: relies on buoyancy (natural convection) rather than electric power to circulate the water.
  • Active: requires electric power to activate pumps and/or controls.
  • Direct: heats potable water directly in the collector.
  • Indirect: heats propylene glycol or other heat transfer fluid in the collector and transfers heat to potable water via a heat exchanger.

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