Solar water heating
Storage
Tank
Practical Action
Hot Water
Flat plate
collector
Circulation
pump
Figure 2: Active and Direct Solar Thermal System (Illustration by Author, with reference made
to CIBSE Solar Heating Design and Installation Guide, 2007)
Direct vs. Indirect
Solar water heating systems can function as either direct or indirect. In a direct system, the
heat transfer medium is water which is heated up and used directly from the collector/storage
tank. In an indirect system, the heat transfer medium is not necessarily water and does not
come into contact with the water that is used after being heated up through some form of heat
exchange system.
The following are disadvantages inherent in direct systems.
Scalding
Fluid freezing
Deposits
Loss of water quality
Backflow/thermo-
siphoning
Stratification
disturbance
If the water has not been moving through the collector for a while
and therefore becomes stagnant, upon using it again, the water may
be at a dangerously high temperature by the time it leaves the point
of use, i.e. a tap or shower.
This could damage the equipment or block safety vents.
If circulation is reduced, the system becomes blocked or there is a
build-up of bacteria, there is a greater risk of the accumulation of
silt, lime-scale and other debris.
If there is a build-up of debris, the quality of the domestic hot water
is likely to decrease. Another reason for this would be the water
coming into contact with certain fittings and materials when the
water in the system is stagnant and temperatures and pressures are
high
There is a risk that hot water could flow backwards round the circuit
into the cold water cistern.
A reduction in collector efficiency occurs if the return temperature at
the base of the solar storage vessel is increased due to the
circulation pumps in the system.
As well as minimising and/or avoiding the occurrence of the above, another advantage to
installing an indirect system is the capability to introduce certain performance enhancers to
the heat transfer medium without compromising the health of those using the domestic hot
water generated. These enhancements may be in the form of antifreeze or corrosion inhibitors,
which reduce the risk of damage to the equipment during colder months and the frequency of
part replacement and maintenance.
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