Solar Water Heaters

Solar water heaters concentrate the sun’s thermal energy to heat water for industrial, commercial, and residential use. There are two main types of products: active and passive. Active solar water heaters have circulating pumps and controls. Passive products do not. Both types of solar water heaters mount on roofs or walls and have storage tanks and solar collectors. Solar storage tanks must be well-insulated. They have outlets and inlets that connect to and from the solar collector. In one-tank systems, a backup heater and solar heater are located in a single tank. In two-tank systems, solar water heaters are used to pre-heat the water, which then enters a conventional water heater in a separate tank.

There are two types of active solar water heaters: direct and indirect. Both types are more expensive and more efficient than passive devices. Direct-circulation solar water heaters pump water through a solar collector and then into the building. They are used mainly in climates where temperatures stay above freezing. Indirect-circulation solar water heaters move a heat-transfer fluid through the solar collectors and a heat exchanger. This fluid, which is non-freezing, heats the water that flows into the building. Typically, indirect circulation systems are used in climates with freezing temperatures.

Passive solar water heaters are often more reliable than active heating systems. There are two basic types: integral collector-storage and thermosyphon. Integral collector-storage passive systems are suitable for applications with significant hot-water needs during both the day and evening. These passive solar water heaters work well in warm-weather areas where temperatures stay above freezing. With thermosyphon systems, warm water rises as cool water sinks. The solar collector is installed below the solar storage tank, which may also have a backup water heater.

In residential applications, solar water heaters use one of three solar collector types: flat plate, integral collector-storage, and evacuated tube. Flat-plate collectors may be either glazed or unglazed. Glazed products have a dark absorber plate under a glass or plastic cover. Unglazed collectors also have a dark absorber plate, but do not have an enclosure or cover. Integral collector-storage (ICS) products are batch collectors with one or more tanks or tubes in an insulated box. Cold water passes through the solar collector for pre-heating and then flows to a conventional water heater. Evacuated-tube solar collectors are also available. These solar water heaters have parallel rows of transparent glass tubes.

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