An Overview

Solar water heaters are divided into two kinds of systems:

Active Systems and Passive System.

Active solar systems rely upon moving mechanical parts in order to transport heat, while passive units simply use the sun to accomplish this action. The bulk of systems installed in Southern California are active because they are considered to be more efficient and attractive. However, most of the systems installed worldwide are passive because they are simple and need no auxiliary power (i.e. electricity) to operate.

Active solar systems can further be divided into the following types: open loop, closed loop, drain-down, drain-back, direct or indirect, single tank or two tank. This discussion will concentrate on open loop direct systems, since this type is the most efficient and is also the most prevalent system used in the So Cal region. The collector is usually a copper tube and fin absorber enclosed with an insulated aluminum frame or "box", covered with a low-iron tempered glass glazing. The water contained within the gas or electric water heater is circulated through the solar panels in a single tank system, or a separate tank is used to pre-heat the water before it enters the conventional water heater in a two-tank system. The two-tank system can provide as much as ninety percent of the annual hot water needs and a single tank system can be expected to contribute up to sixty percent (or more if managed). The water being circulated is gradually heated and daily temperatures as high as 160 degrees F can be expected. A reliable automatic control to operate the pump is essential. Fortunately, pumps and electronic controls have evolved and can be expected to provide over twenty years (and counting!) of service.

Passive solar systems can be divided into two types: Thermosiphon and Integral Collector Storage (ICS). Passive solar systems are popular because of their simplicity and inherent reliability. The storage tank is located on the roof and heating effect of the sun causes warm water to circulate within it. Cold water from the city flows directly to the tank on the roof, and then flows to a conventional water heater located on the ground level. It is desirable to keep the distance between the solar system and ground level water heater as short as possible in order to reduce the amount of cold water sitting in the pipe between the two units. In the case of a thermosiphon system, an insulated tank will prevent the loss of stored heat during the night. The ICS or "batch" heater is the most simple kind of solar heater, but the exposure of the storage unit to night air causes significant heat loss and precludes the use of this kind of panel in all but the mildest climates.

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