Solar thermal collectors and applications
Indirect water heating systems (active)
Indirect water heating systems, shown schematically in Fig. 31, circulate a heat transfer fluid through the closed collector loop to a heat exchanger, where its heat is transferred to the potable water. The most commonly used heat transfer fluids are water/ethylene glycol solutions, although other heat transfer fluids such as silicone oils and refrigerants can also be used. When fluids that are nonpotable or toxic are used double-wall heat exchangers
should be employed. The heat exchanger can be located inside the storage tank, around the storage tank (tank mantle) or can be external. It should be noted that the collector loop is closed and therefore an expansion tank and a pressure relief valve are required. Additional overtemperature protection may be needed to prevent the collector heat transfer fluid from decomposing or becoming corrosive.
A variation of indirect water heating systems is the drain - back system. Drain-back systems are generally indirect water heating systems that circulate water through the closed collector loop to a heat exchanger, where its heat is transferred to the potable water. Circulation continues as long as usable energy is available. When the circulation pump stops the collector fluid drains by gravity to a drain - back tank. If the system is pressurised the tank serves also as an expansion tank when the system is operating and in this case it must be protected with a temperature and pressure
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Fig. 31. Indirect water heating system. |
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relief valves. In the case of an unpressurised system (Fig. 32), the tank is open and vented to the atmosphere.
As the collector loop is isolated from the potable water, no valves are needed to actuate draining, and scaling is not a problem, however, the collector array and exterior piping must be adequately sloped to drain completely.
