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Letters to the Editor: Chiller-Plant Efficiency

Aug. 1, 2007
Questions and answers for the author of 'Real Efficiencies of Central Plants' (May 2007).

Letter No. 1

Dear Editor,

I just read the article “Real Efficiencies of Central Plants” (May 2007) and completely agree that few chiller plants attain the efficiency (kilowatts per ton) that we would like to see. In fact, in most cases, I doubt we can make peak efficiency at full-load operation, let alone at part loads.

I am interested in the statement made at the end of the article: “Every chilled-water-plant designer's goal should be to guarantee annual average energy consumption of 0.5 kW per ton or less.” How did the author arrive at that efficiency when most chillers alone consume 0.5 kW per ton? When you add pumps and cooling towers, you are way above 0.5.

Lastly, what is the cost of all of this efficiency? We always are faced with installation-cost problems. What we need are design engineers to provide life-cycle-cost analysis to determine the payback on these high-efficiency plants. In our situation, we generally need a two-year simple payback on these installations, and, in most cases, the energy savings cannot pay for the increased initial costs.

Does the author have information on the relative cost per ton for the various installations discussed in the article?

Robert Bator
Troy, MI

Author's response:

Regarding the statement made at the end of the article, 0.5 kW per ton is an “annual average chiller-plant efficiency.” And, yes, typically, the full-load efficiency of an energy-efficient chiller plant is 0.72 to 0.79 kW per ton. In the article's example of an ultra-efficient all-variable-speed chiller plant with oil-less compressors (Figures 12 and 13), the kilowatt-per-ton average on June 26 and Oct. 31, 2006, was 0.58 and 0.42, respectively. At part-load conditions, we have variable-speed chillers operating at 0.19 to 0.32 kW per ton and, sometimes, entire plants operating as low as 0.29 kW per ton. When you average all cooling ton-hours and kilowatt-hours, you can achieve an average of less than 0.5 kW per ton. See my March 2006 HPAC Engineering article, “Ultraefficient All-Variable-Speed Chilled-Water Plants.” Also, see the table below. A 10-year-old retrofitted plant averaged below 0.55 kW per ton, for an energy savings of 50 percent.

As for the cost of all of this efficiency, in retrofit applications, we achieve a one- to three-year simple payback on most jobs. For new construction, we can achieve 0.5-kW-per-ton efficiency at less cost (immediate simple payback). It has been well-documented that you can design a primary-only all-variable-flow plant at less cost than a typical primary-secondary one. You have to take into account the reduction in the number of pumps, valves, etc. Also, it is quite costly to achieve the high nominal efficiency of the chillers used in most standard “high-efficiency” plants. With new technology, the nominal efficiency is not as significant as one might expect, especially if the plant has reserve capacity. With demand-based control, 0.58-kW-per-ton variable-speed chillers will operate about the same as chillers with higher nominal efficiency (e.g., 0.54 kW per ton). Variable-speed chillers cost about as much as the slightly higher-efficiency constant-speed chillers used in conventional plants.

Lastly, regarding the cost per ton, this varies greatly from plant to plant. As a result, I have never been able to identify a good rule of thumb.

Ben Erpelding, PE, CEM
Optimum Energy LLC
Seattle, WA

Letter No. 2

In Figure 1, “Average annual chiller-plant efficiency,” of the May 2007 article “Real Efficiencies of Central Plants,” are cooling towers included as part of the systems? I assume they are, but saw no mention of it in the article.

A manufacturer of packaged chiller plants using R-123 refrigerant boasts an efficiency of about 0.79 kW per ton, which is at about the midpoint of the “good” range of efficiency in Figure 1.

John Patton, BSME, CEM
The University of Texas-Pan American
Edinburg, TX

Author's response:

Yes, Figure 1 includes cooling tower fans. I should have made that clearer.

I am very familiar with the manufacturer you mentioned. The 0.79 kW per ton is a full-load efficiency. But as we know, an entire plant should never operate at full load for more than a few hours a year. I took performance curves showing the manufacturer's packaged chiller plant operating at different entering-condenser-water temperatures and laid them on top of one another, getting the curve above. Typically, the average annual central-plant efficiency of these systems is 0.7 to 0.9 kW per ton, depending on the load profile of the facility.

Ben Erpelding, PE, CEM

Letters on HPAC Engineering editorial content and issues affecting the HVACR industry are welcome. Please address them to Rob McManamy at [email protected]