Trading for energy savings

Recently, I heard from one chief engineer, that energy savings in industrial refrigeration is not rocket science. I do not completely agree with this statement. Energy savings in our industry is not rocket science, but it is pretty complicated.

We can compare processes of electrical energy savings in different areas of electrical energy consumption.

If we can shut off the light or electrical heater for a certain period of time, our energy savings can be calculated as multiplication of light (heater) power by the shut off time. This is straightforward.

If we can shut off the light in refrigerated space and save 10 KWh of energy, our additional energy savings for refrigeration plant would be around 5 KWh and total savings will be 15 KWh. This is not rocket science.

Three parts of refrigeration plant consume the electrical energy:      

• A condensers
• B compressors
• C evaporator fans

We can get majority of energy savings by trading the energy between these 3 parts.

For example, we spend additional 5 KW of A (condenser) power and condensing pressure have decreased. This step has saved 10 KW of B (compressor) power. It is a good trading. Our net power savings are 

( 10 – 5 )  =  + 5 KW. 

At other conditions, if we increase condenser (A) power by 20 KW, but our compressor (B) savings will be 15 KW, our total power savings would be negative or (- 5) KW. It means that we are going to the wrong direction.

A and B are opposite functions. If A (condenser power) is going up, B (compressor power) is going down and vice versa. Our goal is to get the point (setting), where sum A+B is minimum.

There is a similar relationship between B and C. If we increase suction pressure, efficiency of compressor (B) is going up (less power per unit of refrigeration), but efficiency of evaporator fans is going down (more fan power per unit of

refrigeration).

Finally, we have found the minimum total power for the sum A + B + C.  This is the optimum for certain refrigeration load and ambient conditions. If refrigeration load or/and ambient conditions have changed, we need new optimum set points.

Therefore, our goal is a moving target.

Optimization of refrigeration plant operation is not an easy task because of many variables. Only the knowledge of energy trading laws will give us an opportunity to run refrigeration plant at optimum set points all year around.