CPD Programme
energy could be sufficient to supply the required to both direct-fired water heater and commercial This ratio between collector array surface
water temperature at the outlets. At these water boiler applications. area and the volume of stored water is one of
outlet temperatures it would be necessary to In order to maximise the SF, the capacity the key factors in delivering the optimum SF
install a thermostatic mixing valve between of the solar cylinder should be matched to the but, at the same time, ensuring the collectors
the hot outlet of the pre-heat cylinder and the daily hot water demand. In so doing, during do not enter into frequent and prolonged
storage water heater, unless there are point-of- the summer months when the available solar periods of stagnation. Even if the correct
use mixing valves used within the building. irradiation is at a maximum, it may be possible volume of storage is selected for a given solar
Another example is where commercial to satisfy the whole hot water load, depending on collector array, there is still the need to be able
boilers are being used for space heating and the the demand profile of the property. This could to discharge this useful energy into the water
generation of hot water via an indirect stored within the indirect cylinder. The
calorifier. For boiler applications, the relationship between the surface area
main principle of generating the solar of the indirect coil and the absorber
thermal energy and the use of an surface area of the collector array is
indirect cylinder are the same as for critical with regard to dissipation of the
direct-fired water heaters. solar energy into the water. Very much
The difference is in the design of like a burner, heat exchanger and water
the cylinder, in that it frequently would flow through a boiler, the solar collector
have two indirect coils (although some array is the heat generator. If the heat
systems do use a separate pre-heat is not discharged into the water, then
cylinder) as in Figure 5. the heat transfer fluid will return to
At times when there is insufficient the collector array and this will affect
solar energy to heat the water to the set collector performance and could result
point (eg 60
O
C) the commercial boilers in collector stagnation.
would provide the additional energy
required to raise the water temperature Conclusion
to the required set-point. The application of solar thermal systems
The control of the transfer of energy employs traditional skills with readily
from the collector array and the indirect
Supplied by
available technology. On new-build
cylinder is conducted in the same
Andrews Water
Heaters
projects there is activity on the roof of
manner, whether the primary heating the building, during which the solar
appliance is a direct-fired water heater
Figure 4: Schematic of an example solar thermal system integrated
collectors may be installed. Beyond the
or a commercial boiler. There is a
with a direct gas-fired water heater
collectors, and in the plant room, the
differential temperature control via a work is predominantly hydraulic and
sensor at the outlet of the solar collector array entirely offset the need to burn fossil fuel in the can be carried out by the mechanical contractor
and a sensor located in the lower portion of the primary appliance to generate hot water. installing the remainder of the equipment.
cylinder. When the temperature differential For glazed flat plate collectors, the rule-of- The presence of the solar collectors on the roof
is greater than, typically, 7K the control unit thumb measure is 50 litres of stored water also offers a visible indication that the end-
switches on the pump, allowing the energy per m
2
of solar collector array (the active area user has taken positive action and invested in
captured within the solar collectors to be rather than the gross physical area). Evacuated a technology that reduces the carbon footprint
circulated and transferred into the water via the tubes are more efficient than glazed flat plates, of the building.
indirect cylinder coil. When the temperature particularly during the colder winter months, as Solar thermal solutions offer an opportunity
differential is typically less than 3K, the pump this type of solar collector is less prone to heat to significantly reduce carbon dioxide emissions
is switched off. loss via convection. For a given surface area – approximately 100 kg CO
2
/m
2
of collector
For commercial applications, the issue of the evacuated tube collectors can hold more array per annum when compared with a
development of legionella bacteria in the solar than twice as much heat transfer fluid. As a natural gas primary heating appliance (based
cylinder is often the subject of much concern consequence, the rule of thumb on sizing and on a gross thermal efficiency of 80 per cent
as the water could be stored at temperatures at selection is 70 litres of stored water per m
2
. with gas 0.193 kg CO
2
/kWh) – contributing to
which the bacteria can develop (20
O
C to around the renewable energy commitment required by
Supplied by Andrews
45
O
C favour growth). This can be overcome
DHW
Water Heaters local authorities and the increased requirement
with appropriate design and properly informed to include LZC technologies. The impact is
Secondary Return
operation. For example, cylinder pasteurisation
Enters Above
Heat Exchanger
Solar Coil
particularly marked as domestic hot water
From Boiler Circuit
can be conducted through the use of shunt Timer becomes an increasingly dominant load as
connection between the storage water heater and building air tightness and insulation levels
the pre-heat cylinder and, for twin coil cylinders, improve and space heating loads reduce.
Solar Heat
a destratification pump can be used.
Exchanger
Cold
Feed
Further Reading
Cylinder selection and design
Combined Cylinder Domestic Hot Water
Outlets
Solar heating design and installation guide,
There are a number of issues to consider when CIBSE 2007
Figure 5: domestic hot water cylinder with two
selecting the capacity of the solar cylinder and
indirect coils (Solar heating design and installation
Solar heating systems for the UK – BRE 1979
the design of the indirect coil. The issues apply
guide, CIBSE 2007)
(old, but still very useful)
www.cibsejournal.com February 2009 CIBSE Journal 75
CIBSEfeb09pp73-76 CPD pages•.indd 75 5/2/09 14:40:51
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