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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)