Industrial Utility Efficiency

Xcel Energy and Northwest Airlines Partner to Save Energy

The Problem: A Lot of Hot Air

How do you test a 747 engine to ensure reliability once it’s airborne at 600 miles an hour? At Northwest Airlines in Minneapolis, among other methods, they force an enormous amount of compressed air into it with a 700 horsepower air compressor. It’s no small task and takes a huge amount of energy.

  The 1.1 million square foot facility uses compressed air to test jet engines and for general plant air    


A 1.1 million square foot facility houses a 700-horsepower air compressor – primarily for those massive compressed air needs - and two 600-horsepower air compressors at different locations in the facility that were mostly used for other shop tools. Over time the needs of the facility changed when much of the work done previously at this site moved elsewhere.


The existing air compressors were designed for a facility that used to average over 3000 cfm but by early 2007, with the reduced activity in the facility, the average demand was down to about 250 cfm. Much of that daily demand surrounded routine maintenance in the hangar but all compressors were running most of the time. Suddenly 1,900 horsepower of compressed air was simply more than they needed.

A reasonable compressed air efficiency should be 5 cfm per kw. Because of the mismatch of the large compressors and the small demand, the system efficiency was only .65 cfm/kw. Much smaller compressors were needed to efficiently supply this lower demand. The complicating factor in the compressed air demand was that occasionally large amounts of air were required to test the jet engines in both the Engine Test Cells and in the hangars.

Ron Toward, Northwest Airlines Senior Facility Manager, knew there was energy and money to be saved. “The compressor system in its existing configuration was costing approximately \$194,384 to operate per year,” says Toward. “I knew we could do better.”

In April of 2007 Ron partnered with Vince Suerth from Ultra Energy, Gary Ruff from Compressed Air Consulting and Patrick Becker from Air Power Equipment to commission an Xcel Energy Compressed Air Efficiency study. Toward decided to use trio because of their expertise in managing and executing large scale energy efficiency projects. The purpose of the study was to investigate alternatives to the compressed-air-supply system and find ways to reduce operating costs. Xcel Energy co-funded a portion of the study cost to provide additional incentive to commission the study and find leaks, as well as identify other opportunities for energy savings.

  The Xcel Energy Compressed Air Efficiency study lead to savings of over 1,900,000 kWh and \$70,000 annually.    

Save Before You Buy – Commission a Study

All of the consultants agree that a best-practices baseline approach is to always consider efficiency first.


The Department of Energy finds that the typical compressed air system uses only 50 percent of its air supply for production while the other 50 percent is lost due to leaks and wasteful measures.

“If you can’t measure it, you can’t manage it,” says Tim Guck of Xcel Energy. “We always recommend commissioning a study to find the leaks and waste first because there’s usually a possibility of doing more with less. Adding more or larger capacity air compressors should really be a last resort.”

Toward understood the numbers and the method. “We’re always looking for ways to run the factory more efficiently,” says Toward. “We knew we had some leaks but the study revealed much more.”

Step One: Find and Fix Leaks

Nine significant air leaks were found totaling an estimated 47 cfm. The study identified that reducing leaks to a constant level of 15 cfm was achievable. Repairing the leaks helped them save \$3,063 annually.


In that very first step alone, NWA increased efficiency more than 30 percent.

“We couldn’t believe it,” says Toward. “It really helped us understand how our systems were working and how the needs had changed.”

Step Two – Downsize

The first phase of the survey showed that there was an opportunity to save significant energy dollars by purchasing new, smaller compressors to supply the normal plant air demands. The challenge was to figure out how to continue to supply the surge demands of engine testing. The second phase of the air study was devised to quantify in more detail the character of the existing supply and distribution system during the engine testing as well as take a more protracted look at the general plant air needs. Equally important was to get more detailed information on the actual testing procedures to more accurately determine the needs from the air supply system.


To facilitate this quantification phase, two engines were tested in the test cells and the results were measured. They found that even the existing system was barely adequate for most large-engine tests.

By reconfiguring the Test Cell System with a larger air compressor and storing compressed air in the existing storage tanks at 125 psig instead of the previous 95 psig, there was a marked improvement to the system. The Test Cell System was connected to the general Plant Air System to supplement supply for large engine starts. However, the live tests showed that this supplement was inadequate so modifications to the Plant Air System connection were recommended to improve the supplemental supply.

  The new system included a 100 hp fixed speed rotary screw compressor and a cycling refrigerant air dryer.    

The recommendation was to replace the older Joy centrifugal compressors in the Main Compressor Room with one new Ingersoll Rand 100 hp Fixed Speed Rotary Screw Compressor and a new Ingersoll Rand Cycling Refrigerant Air Dryer sized for the new compressor. A new Ingersoll Rand Fixed Speed 100 hp Rotary Screw Compressor was also recommended to replace the existing 2-40 hp reciprocating compressors in the Test Cell Compressor Room.

“It’s interesting to note that the new design did not include a variable speed drive (VSD) air compressor,” says Pat Becker of Air Power Equipment. “The study showed that the amount of system storage and capacitance made the air compressors online/offline controls as efficient as a VFD. The cost for the Ingersoll Rand VFD Controlled Nirvana was not justifiable.”

The new air compressors were also configured to be air cooled for heat recovery. Historically, a work area adjacent to the compressor room was a heating challenge during the cold Minnesota winter months. The air compressor exhaust was ducted directly to the area to provide supplemental heat. The new 100HP air compressor now delivers 7300 CFM and 299,000 BTU’s, providing a valuable reduction in plant heating load.

There were various options presented for the upgrade of both the Plant Air Test Cell compressed air systems.

“The 700 hp compressor used to be operated almost 24 hours a day but by early 2007 was only used once or twice a week,” says Vince Suerth of Ultra Energy. “The 100 hp unit was now running the rest of the time. The operating cost of the new compressed air supply systems is approximately \$68,700 per year. This is an annual savings of \$125,684.”

The cost of the new compressed air supply system was \$180,000. Xcel Energy rebates of \$62,970.00 helped lessen the load of the overall cost of improvements. The return on investment was less than two years.

“This was a big project,” says Toward. “We thought the rebate was going to be \$39,000 but it ended up almost doubling when we added extra efficiency measures. The fact that the rebate was 33 percent of the total cost really helped our financial folks understand and ultimately sign off on the project.”

“We were really surprised that we could get by with two 100hp compressors for daily maintenance use,” says Toward. “That was major.” One was initially factored into the equation but Toward opted to purchase a second one to meet the daily needs of the shop. Toward says they also improved the reliability of the system.

  The 700 hp centrifugal compressor used to be operated almost 24 hours a day but by early 2007 was only used once or twice a week.    

Utility Partner

Partnering with their utility made a big difference in this case.


“NWA sought study preapproval,” says Guck. “And after fixing 50 percent of air loss, which is a program requirement, they submitted their report to us. We reimbursed their study cost for the amount of \$10,725. NWA made recommended capital improvements and with custom preapproval they were able to receive a rebate for \$62,970. Their collective efforts saved their company over 1,970,000 kWh annually.”

“Because of this project’s magnitude, the NWA project’s rebate and energy savings were based on actual savings, therefore the equipment had to be metered both before and after installation,” says Guck. “Per Xcel Energy program code, any project over 1,000,000 kilowatt hours requires monitoring. Otherwise we use accepted engineering savings technical assumptions, industry best practices, information provided by the customer and manufacturer data sheets to form our analysis.”

  Average compressed air demand at Northwest Airlines has dropped from 3000 cfm to 250 cfm. The complicating factor was that occasionally large amounts of air were required to test the jet engines.    

Huge Savings

The sheer amount of energy saved combined with the rebate amount and simple payback period made this project a no-brainer for Northwest Airlines. The results speak for themselves.

Financial Summary

Study cost $14,300
Estimated improvement costs \$180,000
Xcel Energy rebates \$73,695 - Total
\$10,725 - Study rebate
\$62,970 - Equipment rebate
Annual energy savings Over $70,000/yr
Kilowatt hours saved Over 1,900,000
Payback term (after rebate) Under 2 years



Compressed Air Efficiency features two steps to help you identify and implement ways to save on energy for compressed air.

Step 1 – Diagnosis

The study includes:

  1. An assessment of operating compressed air systems, including analysis of supply and demand, and airflow and /or electric metering
  2. A written report that identifies leaks and waste, and associated energy costs
  3. A list of system recommendations, paybacks, rebate opportunities and estimated energy savings
  4. Preapproval is required for study funding


Step 2 – Implementation

  1. Earn rebates for equipment updates or system improvements that result in lower energy use or higher production from your system
  2. Customers who participate in a preapproved Compressed Air Efficiency study or whose system is under 50 horsepower can receive up to \$200 per kW saved
  3. Customers who choose not to participate in an Xcel Energy-funded study can receive up to \$50 per kW saved
  4. Preapproval is required prior to equipment purchase and installation


The Big Picture

Toward has partnered with Xcel Energy since 1999 and has implemented a variety of energy saving measures, from energy efficiency studies to equipment upgrades and process improvements. The results have been impressive. To date, NWA has saved more than 18 million kWh – that’s more than 24 million pounds of carbon, or the equivalent of removing 4,000 commuters from our roads – while reducing annual energy costs by more than \$1 million. In addition, NWA has earned more than \$1 million in rebates for choosing energy efficient equipment.


Guck says NWA is a model for embracing energy efficiency.

“Northwest Airlines is one of our larger customers,” says Guck. “Year after year they’re also are among the most active companies involved in our energy efficiency programs. Their continued efforts should serve as a model for organizations across the country.”

“We all realized how important it was to take a systematic approach to our energy use and as facility managers,” says Toward. “We all work together to do that. And because our energy systems – from lighting to heating to cooling – have a dramatic effect on one another, it makes sense for us to continually re-examine our systems to ensure we are taking the best approach.”

Toward also realizes that energy efficiency is responsible business practice and like any organization, he wants his facility to run as efficiently and effectively as
possible. He’s quick to add that the financial impact isn’t the only factor.

“My goal here is to make this facility as environmentally friendly and reduce as much energy use as we can,” says Toward. “We’re now in the process of getting LEED certified for all maintenance buildings on the site.”

Moving Forward

According to the U.S. Department of Energy, compressed air systems account for \$1.5 billion per year in energy costs in the United States, and 0.5 percent of emissions. Many industries use compressed air systems as power sources for tools and equipment used for painting, moving product around the facility, separating and other applications. Optimization of compressed air systems can provide energy efficiency improvements of 20 to 50 percent.

To learn more about improving compressed air efficiency, contact any of the following: