Renewable Energy Installer February 2014 | Page 17

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Bare necessities
Heat pump expert Bob Long asks why a buffer cylinder/thermal
store is essential in any system
B
uffer cylinders have a large role to
play in the operation of air source
heat pumps, but can also be an
asset in ground source systems too. Accurate
selection of a buffer cylinder is essential
to obtain the best result. Too large and the
cylinder will waste valuable space inside the
property, and too small will prove ineffective.
A buffer cylinder increases the total
hydraulic volume of the heating system,
storing useful amounts of thermal energy and
creating a stable heating effect.
A buffer cylinder promotes the longevity
of the heat pump itself, by reducing the
number of stop-starts the heat pump will
endure in its lifetime.
Soft start devices and inverter driven
variable-speed drives, available in high-end
heat pumps, have alleviated much of this
problem but even these units will benefit from
a more stable operating environment.
A buffer cylinder also plays an important
role in defrosting the heat exchanger of an air
source heat pump (ASHP). A suitably sized
buffer cylinder will ensure an adequate supply
of thermal energy is available to perform
essential defrost cycles, as required by all
ASHPs.
Defrost cycles use a large amount of
energy over short periods of time, and rely
upon the thermal storage capacity of the
heating system to supply this energy.
When selecting a suitable buffer cylinder,
a number of factors need to be considered.
As an example, we could imagine a 10kW
heat pump operating an underfloor heating
system with a circulating water temperature
of approximately 35°C. To maintain this
condition, the heat pump controller will
probably be adjusted to cycle between low
and high limits of 38°C to 41°C, and maintain
the buffer cylinder within this temperature
range.
The minimum size of the buffer cylinder
is generally defined according to size of the
heat pump, plus the rate at which the heat
pump will affect the temperature of a specific
volume of water.
During a defrost period, the example
10kW heat pump should be capable of
removing a similar amount of energy from the
thermal store, for the duration of the defrost
cycle. Ideally, the energy transfer required to
complete a defrost cycle should be available
without reducing the temperature of water
inside the thermal store to below +30°C.
During defrost periods it is advisable to
stop the circulating pump feeding the emitters
to avoid any negative heating effect, or
impose additional load on the thermal store.
An ASHP’s defrost cycle will not generally
exceed 10 minutes, during which time a buffer
cylinder of 300 litres capacity might expect a
temperature drop of around 8°C.
In this particular instance, a system
operating an average water temperature of
+40°C would finish the defrost with a residual
temperature in the buffer cylinder of about
32°C.
Heat pump manufacturers employing
inverter-drive technology may say a buffer
tank is not required, due the power matching
potential of inverter control. The absence of
a buffer cylinder can be detrimental during
defrost periods, however, where the energy to
complete a defrost cycle must be extracted
from a limited, low volume of water contained
in the emitter circuit.
Under this circumstance, the heat pump
is likely to complete the defrost cycle with a
water temperature significantly lower than
ideal.
Incorporation of additional energy
sources can be easily achieved with the
assistance of a buffer cylinder/thermal store.
The buffer cylinder can be fitted with a
number of direct and indirect input/output
connections to accept energy from other
renewable energy sources, such as solar
thermal panels or log burners.
A suitably designed passive heat
exchanger, embedded in the buffer cylinder
can also provide economical pre-heat for the
DHW systems.
Although a buffer cylinder represents an
added cost, it provides good system stability,
together with all the flexibility needed to
incorporate other renewable energy sources at
a later stage.
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