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View Subscription Details: Letter ( 03/07/2006 ) - HTML Format

Investment in energy efficiency

I heard from several chief engineers and managers that they made significant investment in energy efficiency of refrigeration plants, but their energy savings were much less than expected. Why did that happen?

There are 2 parts of energy saving process in industrial refrigeration:

  • designing energy savings
  • operating energy savings

We can get designing energy savings by investing in sophisticated PLC, variable frequency drives, refrigeration system improvements, new compressors, additional or new condensers and evaporators. This part requires significant investment, usually around 90% of total investment.

To obtain operating energy savings, we have to optimize operating set points and operating strategies. Implementation of this part requires investment around 10% of total investment, because it usually includes only consulting fees. What savings can we get from each part of energy efficiency project?

Surprisingly, operating savings can give us 60-90% of total savings and designing savings can give us just 10-40% of total savings.

Many companies invested in energy efficiency, but they only received designing energy savings. I believe that this is a major reason of unsuccessful energy efficiency projects. Investment in energy saving equipment gives us an opportunity to implement the best set points and operating strategies. Companies have invested in this equipment, but they continue to run refrigeration plants at old set points and operating strategies.

. Optimum head pressure for our refrigeration plant at given moment is 120 psig. Head pressure set point is 140 psig.

Operating energy savings. We know that optimum head pressure is 120 psig, but we can not keep it, because our plant has regular pressurestat (with differential 20 psig) to control the head pressure. This pressure will fluctuate from 110 psig to 130 psig.
We can get some energy savings by changing set point from 140 psig to 120 psig, but we will not get the maximum energy savings.

Designing energy savings. We have bought sophisticated PLC, but we do not know the optimum head pressure. Our plant is operating at 140 psig, at the same setting that we had before buying PLC. Our energy savings is not significant.

Good full-scale energy efficiency project. We know that optimum head pressure is 120 psig. We have PLC to keep it.

We have got maximum energy savings.

Shifting of set point from140 psig to 120 psig can give us 70-80% of total energy savings. Eliminating of head pressure fluctuating from 110 psig to 130 psig, by using PLC, will give us additional 20-30% of energy savings.

As I mentioned early, full-scale energy efficiency project is pretty expensive. Do we have alternative to this project?

Yes. This is implementation of operating energy savings.


Example. You have opportunity to invest $200,000 in energy efficiency. Good full-scale energy efficiency project can give us 2 years payback or $100,000 in energy savings. Investment of $20,000 in operating savings can give us $70,000 in energy savings. Investment of $20,000 in operating savings of 10 refrigeration plants (total investment is $200,000) will give us $700,000 in energy savings. It means that return of our investment is 7 times better for operating savings than for full-scale energy efficiency project.

I believe that optimization of operating set points and operating strategies is an excellent first step to improve efficiencies of refrigeration plants. This step does not require major capital investment and it gives you opportunity to test the waters.

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