Direct (Open-Loop Or Passive Or Thermosiphon) And Indirect (Closed-Loop Or Active Or Pumped) Configurations
Direct Or Open Loop Configuration is one in which the potable water held in the geyser circulates directly from the tank to the solar collector and back again, in effect the water is heated directly as it passes through the solar collector.
Hence, a passive or thermosiphon configuration is the most basic of the two where the geyser is located above the solar collector, either externally on the roof or on a raised internal frame inside the roof space.
Hot water from the top of the collector rises naturally back into geyser while the denser, colder water from the geyser flows to the bottom of the collector to be heated. This is called the thermosiphon effect and can beseen in the demonstrative diagram.
Indirect (Closed-Loop Or Active Or Pumped) Configuration is one in which the fluid that flows to the solar collector is not the actual geyser water itself but rather a glycol mixture, contained within a closed loop, running from the geyser to the collector and back again.The water in the geyser is heated indirectly as the glycol mixture returns from the solar collector and passes through a heat exchanger (or coil) located inside the geyser.
An active or pumped configuration is one in which the geyser is located below the solar collector, usually inside the roof space where a conventional geyser would be located. In these systems, a small, energy efficient circulation pump is used to pump the water around the solar system from the geyser to the collector and back again.
Pumped systems are technically more advanced than passive systems, giving the homeowner more control and interaction with their solar system through an easy to use digital, solar controller interface. The diagram opposite illustrates a standard pumped system configuration.
Indirect Systems are generally only required in frost prone areas where water in a direct system would be likely to freeze, expand and burst the external copper pipes in the solar system. Indirect systems should also be considered in rural areas where poor water quality can, over time, cause damage to the solar collector.