Danfoss VLT drives provide more than $1 million in energy savings.
In 1970, the U.S. Steel Corp. built a unique headquarters that still stands 64 stories above the Pittsburgh skyline. Built to last 100 years, the skyscraper now known as the U.S. Steel Tower is architecturally unique. It features a distinctive triangular footprint using U.S. Steel-developed Cor-Ten steel to form an external girding system that allows each story to contain an acre of floor space. While ahead of its time in the 1970s, the building fell behind with mechanical equipment that was installed when kilowatts cost pennies and oil was $3 a barrel. That’s why Winthrop Management, Pittsburgh (600grantstreet.com),the building’s property manager, began a series of retrofits using Danfoss, Baltimore (danfoss.com), VLT variable-frequency drives to cut energy costs—resulting in more than $1 million in energy savings and a greener reputation that’s attracting tenants.
“We have been applying Danfoss VLT drives in various retrofit projects for nearly 15 years,” said Gary Sechler, engineering manager for Winthrop Management. “After every retrofit project phase, we’ve found the energy savings on pump motors and fans have been outstanding. So we would embark on another phase. As it stands now, we’ve installed more than 150 VLT drives, with more to come.”
The 841-ft. U.S. Steel Tower, once known as the USX Tower, provides more than 2.3-million sq. ft. of leasable space in downtown Pittsburgh. It is the city’s tallest skyscraper and the highest commercial building between Chicago and Philadelphia —with major tenants including U.S. Steel and the Univ. of Pittsburgh’s Medical Center (UPMC), which occupies 40% of the space.
“We’re moving a lot of water and air up, down, and around this building,” said Sechler. “Water is supplied by two redundant water mains. In addition, there are four redundant, 100-hp water pumps in the building. Each one can serve the entire building, if needed. There are also two boilers on the sixty-fourth floor and two centrifugal chillers on the sixty-third floor to provide redundant heating and cooling. So there is a lot of pumping needed for domestic-water circulation and for the chilled-water loops, all of which consume a lot of energy.”
The first drive retrofit project was in the year 2000 when VLT drives were applied to four 100-hp pump motors responsible for the building’s domestic water supply.
“The old drives were two-step drives like they used in steel mills back in the day,” said Jim Rice, owner of M&R Affiliates, Pittsburgh, (URL?) the Danfoss sales representative who has been working with Sechler since he has been in charge. “They weren’t true variable-frequency drives. We replaced them with four Danfoss VLT model FC102 drives that delivered 100 hp at 460 V and provided a true soft start.”
According to Sechler, the soft start eliminated a lot of wear and tear on the motors —and also saved energy. “We’re talking big motors to pump water to a 300-gal. cushioning tank on the sixty-fourth floor. From there, gravity feeds the water down fountains, sinks, and toilets on the floors below. Only two of the four pumps run at any given time in a lead-lag sequence that alternates weekly. But the old motor speed controls were obsolete and parts were no longer available. I don’t have a record of energy savings from that time. But I do know with the soft start on VLT drives, pump motor rebuilds have been zero.”
The next retrofit opportunity presented itself after the state of Pennsylvania passed legislation in 2008 requiring Electric Distribution Companies (EDCs) to reduce electricity consumption and peak demand. In response, Duquesne Electric, Pittsburgh (duquesnelight.com), provided a rebate program for businesses that install variable-frequency drives to replace old-style motor speed-throttling technology.
“We jumped on this program,” said Sechler. “We knew what VLT drives did for our house water pumps. So in 2010, we looked at what they could do for our large 200- to 250-hp fan motors. These fans circulate conditioned air in large office areas at a given static pressure to satisfy the temperature setpoint. We ended up applying about 40 more FC102 drives for motors ranging from 30 hp to 250 hp.
“We were absolutely delighted with the energy savings, because the drives cut electricity costs by $535,000 annually. And along with those savings, we got rebates that produced a one-year payback. So naturally, we kept looking for more places to apply drives.”
Rice explained that the astounding electricity savings are derived from the physics of “affinity laws,” which state that reducing the speed of a pump or fan motor reduces power consumption exponentially. For example, applying a VLT drive that can reduce pump speed by 20% results in potential energy savings of as much as 50%.
In 2011, Sechler embarked on Phase Two of the retrofit project. Once again, the VLT drives were applied to pump motors—but this time for the chilled water and pre-heat water loops.
“These pump motors are considerably smaller than those used for the domestic water pumps,” said Sechler, “but there are more of them.” For this project, VLT drives were applied to 40 pump motors ranging from 50 hp to 200 hp. And once again, the savings were astounding: annual electric costs were reduced another $138,000.
In 2012, a Phase Three project added 16 drives for 250-hp motors. Phase Four in 2013 applied about 40 VLT drives to smaller 7.5- to 60-hp pump and fan motors. After each phase, the electric savings amounted to $317,000 and $152,000 a year respectively.
“In 2009, our electric consumption averaged 65 million kilowatts hours,” said Sechler. “Now it’s down to 53 million kilowatts. Our peak demand was 16 to 17 megawatts; now it’s 12 megawatts. This is a huge savings that goes right to the bottom line. Altogether, nearly 150 Danfoss VLT drives are producing $1,142,000 in documented annual energy savings. Plus, improved energy efficiency makes the property more attractive to tenants. We’ve been up to 98% occupancy, which is really great in today’s commercial real estate market.”
To manage the installation, each drive incorporates Apogee FLN as a software-selectable communications protocol that connects with the building automation system (BAS). The pump drives are controlled using an in-house Direct Digital Control (DDC), which measures pressure differential across the pump to regulate drive speed. The BAS logs drive performance data and energy consumption, including drive condition. Sechler’s in-house engineering team is also able to track operational status—and they are delighted that there has been practically no drive downtime since the first one was installed nearly 15 years ago.
He also notes that the energy savings from the VLT drives is helping give the building a greener reputation. UPMC recently qualified 17 of the floors it occupies for Silver LEED certification and six for Gold LEED certification through the services of evolveEA, Pittsburgh (evolveea.com), a sustainable architecture and consulting firm. In addition, Winthrop Management recently signed the U.S. Steel Tower onto the Green Building Alliance, Pittsburgh, (usgbc.org/organizations/green-building-alliance) 2030 District Challenge—a public-private partnership for the downtown Pittsburgh building district. The Challenge commits the U.S. Steel Tower to reducing energy usage 50% by 2030.
“Thanks to Danfoss VLT drives, we’ve already cut energy usage by 20%,” Sechler says enthusiastically. “Jim Rice and SSI Inc., Cranberry Twp., PA (ssiinc.org), the local Danfoss service center, has worked closely with us year after year to phase in the installation flawlessly. Combine the energy savings, robust quality, and rebates that reduce paybacks under one year, I couldn’t be happier. Plus, the tenants are happy, and the building owners are delighted.”