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View Subscription Details: Letter ( 12/11/2006 ) - HTML Format

Verification of energy savings

In industrial refrigeration, energy can be saved by improving efficiency of refrigeration plants. The efficiency of refrigeration system is commonly expressed as total horsepower (including the power input of compressors, evaporator fans, condenser fans and condenser pumps) per ton of refrigeration.

E = HP / R

Efficiency improvement should be verified by field testing of refrigeration plant. Power consumption can be measured before and after improvement. However, in real life it is difficult to measure refrigeration capacity of a refrigeration plant. How can we verify the energy efficiency improvement?

Majority, usually over 90 %, of energy savings are instantaneous. As soon as energy saving measures is implemented, we can see the result in a short period of time. Some measures (hot gas defrosting optimization, load shifting) are not visible within a short period of time, but their share in total energy savings are not significant.

To verify energy efficiency improvement, we have to measure power consumption of refrigeration plant during a short period of time.

The following steps should be done:

1.     Measure power consumption of original refrigeration plant. Time is 1 hour.

2.     Energy efficiency measures are implemented. Transition period is 0.5 hour.

3.     Measure power consumption of improved refrigeration plant. Time is 1 hour.

4.     Compare power consumption in step 1 and step 3.

Major requirement for correct testing is constant refrigeration load. This load should not change within 2.5 hours of testing.

Cold storages usually have a steady refrigeration load. It is not difficult to test this type of refrigeration plants. Production facilities should run a steady production during 2.5 hours of testing. Try to avoid defrosting in this period.

Example. In a previous newsletter I have mentioned about refrigeration plant with 3 condensers. Condenser 1 has fan power 40 HP and it is equipped with fan VFD. Condensers 2, 3 have fan power 80 HP each. Testing of this plant should be done as following:

1.     Run condenser 1 at 100 % speed and cycle condenser fan 2 (3).

     Measure power consumption of the refrigeration plant within 1 hour.

2.     Transition period is 0.5 hour. Change condenser sequence. Run condenser 2 and condenser 3 at 100 % capacity. Condenser 1 (VFD) should run as trimming one.

3.     Measure power consumption of refrigeration plant (new condenser sequence) within 1 hour.

4.     Compare power consumption in step 1 and in step 3.

Certainly, everybody is interested in energy savings during long periods of time (months or years). However, refrigeration load during this period can vary significantly. Correct verification can be done only by testing during a short period of time. Sometimes, several short period testings can be done and average energy savings can be estimated.

 

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