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Cool new way to save energy

21 July 2011

Cool new way to save energy

 

As thoughts turn to cooling the harvested crop, FEC Services’ Andrew Kneeshaw looks at how ambient temperature affects refrigeration efficiency and explores a research project that is investigating ground source refrigeration.

Refrigeration cooling is a strange process.  Unlike heating, where you have to supply raw energy to raise temperature, refrigeration uses energy to move heat around – from somewhere you don’t want it to somewhere you do.

So in your store you have a cold coil to extract the heat in the air and then you pump it to a warm outside coil which rejects that heat to waste.  The clever bit with refrigeration is that you can move heat from a cold place to a warm place – a bit like making water flow uphill.

Like all pumping operations, efficiencies change with the volume and pressure of the material to be moved.  That’s also the case with moving heat, and similarly some refrigeration processes are more energy efficient than others

If we take quite low temperature ‘heat’ and move it to somewhere which is very warm, efficiencies are very low.  So for instance, a blast freezer operating on a warm day takes lots of energy to pump a little heat.

However, removing heat from a comparatively warm area and moving to somewhere only slightly warmer is much more efficient.  So an air conditioning system works very efficiently.

The general rule is the closer the ‘chiller’ and rejection temperatures, the less energy will be used to move the heat.

Conventional fridge unitSo what’s all this got to do with potato refrigeration? With a store the cooling temperature is essentially fixed at that required by the potatoes, so not much can be done there.  The temperature of heat rejection is generally determined by ambient weather conditions – again out of our control, so no hope there either.

But what if we could provide a lower rejection temperature by choosing another ‘heat dump’ which is cooler?

A solution has been investigated recently in a project jointly funded by AHDB Potatoes, Defra and Horticultural Development Company and has shown potential refrigeration energy savings of 42%.

GSR unitGround sink refrigeration (GSR) is a technique which uses the large cold mass of the earth as an alternative heat dump to ambient air.  With a consistent ground temperature of 12°C in autumn, spring and summer, cooling can be achieved more efficiently.

GSR uses either closed loop water coils laid in the ground, down a bore hole or a lake/river, or open loop system which use ground water directly.  The later system is cheaper but requires a discharge licence as warm water is classed as a ‘pollutant’.

The GSR trial is on a tomato nursery and is comparing the performance of two coolers, one configured conventionally and the other using water from a reservoir.

Performance is gauged by an efficiency measure called coefficient of performance (COP) – the ratio of heat moved to energy consumed.

Trials have given the following results so far:

 

  Electricity consumed kW Cooling delivered kW COP
Air cooled unit 25 80 3.2
GSR unit 20 110 5.5

This shows that the ground source system is using 42% less energy that the conventional system.

Savings have to be set against capital costs.  In the case of this trial the GSR system came at a premium of £10,000.  With annual cooling energy savings of just under £2,000 paybacks are about 5 to 6 years.

If hot water from the water cooling process can be used, additional savings can accrue from not having to heat the water conventionally.  In the case of this site the glasshouse uses heated irrigation water so savings can be made there.

Data from this project is still being collected and processed so we will be revisiting this subject and presenting the final results early in 2009.

Are you aware of how much it costs to cool your store? Do you take ambient temperature into account? Do you have a question for Andrew or one of the Sutton Bridge team?

E-mail the AHDB Potatoes Energy Hub now at sbeu@ahdb.org.uk – we’ll respond within one working day.