A schematic diagram of a direct circulation system is shown in Figure 5.9. In
this system, a pump is used to circulate potable water from storage to the collectors when there is enough available solar energy to increase its temperature
and then return the heated water to the storage tank until it is needed. Because a
pump is used to circulate the water, the collectors can be mounted either above
or below the storage tank. Direct circulation systems often use a single storage tank equipped with an auxiliary water heater, but two-tank storage systems
can also be used. An important feature of this configuration is the spring-loaded check valve, which is used to prevent reverse thermosiphon circulation energy
losses when the pump is not running.