Solar Hot Water System Types & Applications: Page 2 of 4

Direct Forced-Circulation

Direct forced-circulation systems (DFC) (aka open loops) are very popular in locations where it never freezes, like Hawaii. The systems heat potable water directly, circulating it through the collector(s). The main components of a DFC system are a collector, tank, and pump; plus a control to energize the pump when there is enough sunshine to add heat to the water in the tank. 

Indirect Forced-Circulation 

Indirect forced-circulation systems (IFC) use a separate HTF that circulates and is heated in the collector. A heat exchanger transfers the heat from the HTF to the potable water in the storage tank. IFC systems all have one or two pumps, depending on the heat exchanger type. A system with an integral storage tank exchanger only needs one pump; an external heat exchange design needs two pumps. The two IFC systems—drainback and antifreeze—are the only truly freeze-protected systems. Both are closed-loop systems.

Drainback systems include a drainback tank, with or without an integral heat exchanger, plus one or two pumps, and a control for the pumps. 

Antifreeze systems are the most complex and installation-intensive of all the SHW systems. These systems need an expansion tank, heat exchanger (internal storage tank or external), pump(s), a control, a temperature gauge, and pressure relief.

The Components

Collectors

The role of a thermal collector is simple—absorb sunlight, transfer heat, and do it reliably for decades. But collectors must absorb lots of heat, while minimizing solar loss from reflection and heat loss to the surrounding environment. Two main SHW collector types—flat-plate and evacuated-tube (ET)—are available to the residential market.

Flat-plate collectors are time-tested, reliable, and currently dominate the market. They have an absorber plate—a sheet of copper, painted or coated black—which is bonded to pipes (risers) that contain the heat-transfer fluid. The risers and copper are enclosed in an insulated metal frame, and topped with a sheet of glass (glazing) to protect the absorber plate and create an insulating air space. High-temperature rigid-foam insulation, low-iron tempered glass, and aluminum frames are most commonly used, and different absorber plate coatings are available, ranging from black paint to proprietary selective-surface coatings that maximize heat absorption and retention.

Flat-plate collectors usually range in size from 24 square feet (3 x 8 ft.) to 48 square feet (4 x 12 ft.) or more, and can weigh more than 150 pounds each. They hold a small volume of circulating HTF—typically less than 3 gallons, even in large collectors.

Evacuated-tube collectors are a more recent technology, introduced in the late 1970s. Several types are available, with the common element being a glass tube surrounding an absorber plate. Because the space inside the tube is a vacuum, which is a far superior insulator than air, these collectors have much better heat retention than the glazing/air space design of flat-plate collectors.

Most use borosilicate glass to maximize solar transmission to the absorber plate, and have similar absorber coatings to flat-plate collectors. Frames and manifolds for paralleling multiple tubes are available that can hold four to 20 tubes or more. As with flat-plate collectors, multiple banks can be plumbed together to increase system capacity. While overall weights and dimensions are similar between the two types, evacuated tubes usually have an advantage in that individual tubes can be carried to the location and then assembled on the roof, rather than lifting an entire collector.

Collector mounts. Almost all collector manufacturers make the racks for their collectors. The mounting systems fit into the aluminum extrusions of the collector and typically will only fit that collector model. The mounting systems are usually aluminum or stainless steel. Many manufacturers can provide engineering reports for the wind-loading capability of their racks.