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Verification of the energy savings (2)

 

The last step of the energy efficiency project should be verification of the achieved energy savings. The goal of the mentioned project is to improve the efficiency of a refrigeration plant. This means energy use per unit of refrigeration should be improved.

 

                    Efficiency = Energy/Refrigeration

 

To measure energy use, every refrigeration plant should have a separate power meter. Some plants have power meters for the compressors, condensers and evaporators. I think that it is useless to have so many power meters since our goal is to minimize energy use of whole refrigeration plant. Additional condenser power will reduce compressor energy use. At optimum condensing pressure, the condenser and compressor power will be balanced and total energy use will be minimal. It is enough to have one power meter per refrigeration plant.

The second step is to measure refrigeration capacity. To measure refrigeration capacity, operating conditions should be known and refrigerant flow should be measured. It is not easy to measure the refrigerant flow. Typically, this measurement can be done in the laboratory. I do not know any production or cold storage refrigeration plants that have refrigerant flow meters.

Assume that refrigerant flow has been measured. Production of 1 ton of refrigeration at different evaporating temperatures requires different energy use. To produce one ton of refrigeration at - 40 degF require 40% more energy than production of one ton of refrigeration at - 20 degF. The majority of refrigeration plants have more than one evaporating temperature. To compare apples to apples, measured refrigeration capacities at different evaporating temperatures should be brought to common denominator. As you can see, it is complicated to measure refrigeration capacity of the refrigeration plant. What can we do to simplify verification of energy savings? To compare energy use of the refrigeration plant before and after implementation of energy saving measures, refrigeration capacity of this plant should be kept at constant level.

For example.

Run a refrigeration plant at current operating set points for 1 hour and measure energy use. Change set points and run plant for 0.5 hour as transitional period. Run a refrigeration plant at new set points for 1 hour and measure energy use. Compare energy use at different set points. Try to avoid hot gas defrosting during this 2.5 hour of operation. To get a correct result, a production facility should have steady production. Several cycles of the mentioned testing should be done and average energy savings can be determined.

 

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