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Estimation of potential energy savings


Many companies promise to end users to save a lot of energy. Very often they give you exaggerated numbers. To make a right decision you should have correct information about potential energy savings. How to get this information? First you should ask these companies to estimate potential energy savings. Second you should check all numbers in this estimation. These numbers should be based on real life operation. I will give couple examples how to estimate potential energy savings.

Example 1. Very often evaporators of the refrigeration plants are over defrosted. How to estimate energy saved by eliminating one defrost cycle of one evaporator?

Assume that we have 20 TR evaporator and it has 30 minute or 0.5 hour hot gas supply for the defrosting. Efficiency of hot gas defrosting is very low. Assume that our evaporator has hot gas defrost efficiency of 10%. It means that 10% of energy supplied by hot gas will go to melt the frost. 90% of this energy will be released into refrigerated room as parasitic refrigeration load. Typically, hot gas supply for the defrost is 1.5 times greater that refrigerant evaporation during cooling mode. For our evaporator, heat released into refrigerated room during one defrost cycle will be 20 x 1.5 x 0.5 x 0.9 = 13.5 TR.  Assume that energy efficiency of the refrigeration plant is 2.5 BHP/TR. For mentioned evaporator elimination of one defrost cycle will save 13.5 x 2.5 x 0.7457 = 25.2 KW energy.

Example 2. During winter operation, lowering condensing pressure can save a lot of energy. Estimation of these potential energy savings can be done based on engineering data from compressor manufacturer. Assume that we have compressor Frick RWB-II-134 and it is 100% loaded. Condensing pressure is 151.7 psig and suction pressure is 23.8 psig. At these operating conditions mentioned compressor has refrigeration capacity of 233.8 TR and energy use of 264.9 BHP. Compressor energy efficiency (energy use per unit of refrigeration) will be 264.9/233.8 = 1.133 BHP/TR. Assume that condensing pressure was reduced to 125.8 psig. At these operating conditions refrigeration capacity will be 240.2 TR and energy use will be 228.7 BHP. Energy efficiency will be 228.7/240.2 = 0.952 BHP/TR. Condensing pressure was reduced by 151.7 - 125.8 = 25.9 psig. This pressure drop has improved compressor energy efficiency by 1.133 - 0.952 = 0.181 BHP/TR. Reduction of condensing pressure from 151.7 psig to 125.9 psig will save 240.2 x 0.181 x 0.7457 = 32.4 KW energy.

These are two simple examples. However, these estimations can be more complicated. Ask your energy efficiency adviser to help you with complicated estimations. Correct estimations can give you real life numbers and they will be very helpful to make right decision.



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