Most appropriate for regions with mild climates, or
occasional freeze conditions, these systems run the same water that
comes out your household taps directly through the collectors. The systems
consist of one or more collectors, a pump, and a storage tank. Since
the systems have no heat exchanger, they are slightly more efficient
than indirect systems. However, the systems must incorporate special
mechanisms to provide some degree of freeze protection. There are two
main types of active direct systems, and each deal differently with
freeze conditions. When the temperature drops below zero, “draindown"
or "drainback” systems will drain the water from their collectors
into a holding tank to avoid freeze damage. “Recirculating”
systems pump heated water through the collectors when freeze conditions
An indirect system that exhibits effectiveness, reliability,
and low maintenance is the drainback system
The drainback system typically uses distilled water as
the collector circulating fluid.
The collectors in this system will only have water in
them when the pump is operating. This means that in case of power failure
as well as each night, there will be no fluids in the collector that
could possibly freeze or cool down and delay the startup of the system
when the sun is shining.
This system is very reliable and widely used. It requires
that the collectors are mounted higher than the drainback tank/heat
exchanger. This may be impossible to do in a situation where the collectors
must be mounted on the ground.