ice-e info pack 12 free cooling
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8/13/2019 ICE-E Info Pack 12 Free Cooling
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Free cooling
Introduction
Free cooling can save energy and operating
expenses in cold stores, where the application of
traditional cooling systems has resulted in very
high energy costs. By using free cooling, the
overall thermal management running
expenditure is expected to be reduced by about
20 to 40 % in a number of cases.
Description
One solution for free cooling is to create an
overpressure in the cold store by venting
ambient air into the cold store. Motorized
dampers can be used to ensure optimum airflow
in the cold store and to prevent unwanted
ambient air from entering the enclosure.
The free cooling can replace the mechanical
cooling at times when the source temperature is
sufficient low.
Application of the
technology
Free cooling can be used in climates where the
ambient temperature in long periods is lower
than the requested indoor temperature of the
cold store or enclosure. Typically free cooling is
used in produce cold stores.
Capacity
The capacity of the system is primarily
dependent on the air flow and the temperature
difference between the source (tsource) and the
Free cooling canprovidesignificantreduction of
energy forcooling
There are manymethods toprovide the free
cooling source
ICE-EINFORMATIONPACK
Potato store showing louvers used to draw ambient air intothe store
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ICE-E INFO PACK
requested indoor temperature (tindoor). There
has to be sufficient temperature difference
between the two temperatures, else the
energy used for transportation of the air will beto large compared with the supplied cooling
energy.
The cooling capacity of 1000 m3/h air entering
at 5C and 90% RH is 730 W per degree
heating.
The number of hours per month that are below
5 C and 0 C are show below for a Danish
climate (labeled with blue colour in the
figures). In these hours it will be possible touse ambient air for cooling of e.g. vegetables
which typically are cooled to a temperature
between 2C and 7C. It can be seen from the
graphs that the potential is reduced if
temperatures below 0C are required.
The different sources can be ambient air, from
the ground or from water (sea, lake or bore
hole). Humidity in the air supplied to the cold
store will generally not be a large problem
since the air is heated when entering the cold
store and thereby the relative humidity
decreases - leaving little risk for condensation.
Control
The temperature of the cold store is controlled
by a sensor inside the cold store. This system
decides how much the store has to be cooled.
An outside temperature sensor control
decides if the free cooling, the refrigeration or
a mix has to be applied.
A system using outside air mixed with
recirculated air can be controlled by the use of
modulating dampers. Dampers can be used at
the ductwork both on the outside air intake,
exhaust air and the recirculated air. In case of
cool outside air the quantity of outside air is
increased and the amount of recirculated air is
reduced to provide the required supply air
temperature. This results in at certain times of
the year, and often at night time, that cooling
by the refrigeration system is avoided.
An outside temperature sensor is used to
determine when the outside air is sufficient
cold and the proportion of the outside air that
should be supplied. In summer, with high
outside temperatures, the ventilation should
be at a minimum to reduce the refrigeration
load. Therefore the dampers should be as
airtight as possible to avoid air entering the
cold store during warm periods.
The enthalpy, often used for regulation in
comfort cooling systems, would in principle be
better to regulate by than the temperatures,
but in practice this is normally not relevant for
free cooling of stores.
Alternatives:
Alternative sources to
free cooling with
ambient air are bore
hole and ground, sea,
river or lake water
Design:
Estimate the
annually number of
hours below the
design temperatureof the cold store
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ICE-E INFO PAC
The work associated with this information pack has been carried out in accordance with the highest academic standards and reasonable endeavours have been made to achieve the degree of reliability andaccuracy appropriate to work of this kind. However, the ICE-E project does not have control over the use to which the results of this work may be put by the Company and the Company will therefore be deemedto have satisfied itself in every respect as to the suitability and fitness of the work for any particular purpose or application. In no circumstances will the ICE-E project, its servants or agents accept liability however
caused arisin from an error or inaccurac in an o eration, advice or re ort arisin from this work, nor from an resultin dama e, loss, ex enses or claim. ICE-E 2012
Installation
In cases where ambient air is used as the
source, many systems use a combination ofmechanical and free cooling, where the free
cooling takes over when the ambient
temperature is sufficiently low.
The inlet to the enclosure should be placed in
a position where there is no obstruction for the
air flow in the enclosure. It will be a little more
efficient to place the inlet from the ambient at
the facade of the building which are
overshadowed or facing north rather than
exposed to solar irradiation. The outlet
(damper) should be placed at a high position
preferably opposite to the inlet due to better
temperature efficiency. The temperature
sensor used for regulating the free cooling
should be placed at the warmest position in
the enclosure.
Alternatives
As mentioned a number of alternatives to
traditional free cooling with ambient air exist.
The source can be cold from the ground
where the energy is extracted from buried
pipes, bore holes or from sea or lake water.
In very cold climates heat from the ground
might also be used for heating of the cold
stores to avoid low temperatures in the store.
If some temperature variation is allowed it may
be possible to reduce store temperatures
overnight and to use the thermal capacity of
the store to maintain temperatures during theday. This is unusual as usually free cooling is
used for produce stores and tight temperature
control is required. One option to overcome
this would be to use a thermal store or phase
change material to act as a battery for the
store and to discharge the cooling from the
store during the day.
The storage room or enclosure should be
placed and constructed to minimize heat gains
from the ambient by use of appropriate
amount of insulation, reduction of solar gains
by shading or by use of surfaces with low
solar absorbance. Use of shading might be
difficult (expensive) but can be done by having
shading of e.g. lamellas above a dark roof that
due to shading will have lower surface
temperatures and a lower maximum cooling
need.
If the ambient temperature is lower than the
cold storage temperature for long periods
consideration should be made to construct the
building envelope with heat pipes or thermo
siphon which could transport cold from theambient to the enclosure. The heat pipes
might be integrated in the traditional
refrigeration system.
Limitations of system
The largest limitation will in most cases be the
climatic conditions which in a number of cases
will limit the possible application of free
cooling to short periods.
In this connection there will often be
coincidence between low cooling needs and
low ambient temperatures but in some cases
there is a cooling need for cooling down of
warm goods place in the cold store.
Consideration should also be taken to the
allowable air velocity. If the system is
designed with a high air velocity this might
influence the thermal comfort of workers and
might have some influence on sensitive goods
as vegetables or plants.Another restrain might be the temperature
variation. If the temperature of inlet air is to
low some goods might be affected. This could
be solved by mixing and regulation of the
incoming air to achieve a suitable inlet
temperature. It could also be solved by
arrangements and manifolds which distributes
the inlet air equally.
For more information, please contact: Lars Reinholdt (lre@teknologisk.dk)
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