One of my first encounters with refrigeration occurred during a summer work term at Scotian Halibut’s land-based marine recirc site. The site had been developed over a period of 4-5 years. Fish had just been put into the final tanks before my work term began in May. As ambient temperatures started to climb past the comfort zone for halibut production, the rush was on to install chillers on the newest recirc system. Nearly six figures later, two 30 horsepower refrigeration compressors were up and running. The performance of the chilling system – I didn’t even think to measure it! Even when watching utility bills triple when all chillers were running, nobody thought to check efficiency o f the chillers.
Fast forward six years. Our refrigeration contractor retired. Chillers at the hatchery were approaching 10 years old and compressors were failing on a regular basis. Water chiller maintenance costs were quickly rising upwards. Yet the refrigeration systems on our two coolers were trouble free and just as old. I began to dig into refrigeration suspecting that the water chillers had some flaws in design.
They certainly did. I finally decided to measure performance. The numbers were not good. Coefficient of performance (COP) on average measured 1.6. Then a new refrigeration contractor (Wilson Titanium Products) came on the scene that specializes in constructing water chillers from scratch. Before he even saw the equipment he told me what the problem was.
At that point we had 60hp of compressors running. Wilson took half of the compressors out of service. Then we connected the remaining 30hp to all the chiller barrels. Performance doubled or better to COP=3.5 on average. Chilling capacity remained the same. The power bill due to chilling was halved.
Coefficient of performance is a form of efficiency measurement that compares the work done with the energy input. Clearly being greater than 100% efficient is not possible. But in refrigeration, the object is not to create energy with the input electrical energy. The work done is energy transferred from one spot to another. In refrigeration, energy transferred should be greater than energy input. In chilling applications, a COP between 3.5-4.5 should be achieved in most aquaculture applications. If heating, the COP should be higher than 4.0. I have recently seen COP measured over 5.0 in new heat pumps built by Wilson Titanium.
The formula for measuring COP is:
COP = (Energy Transfered) / (Electrical Energy Consumed)
To get an exact measurement of COP requires a lot of equipment. Fluid density, flow rate, temperature into and out of chiller barrels, voltage of compressor, amp draw of compressor, power factor of compressor at its current operating point, are all variables in the COP formula. Fortunately there is a general rule of thumb equation to quickly gauge chiller performance.