Variable-speed components are everywhere, with applications appropriate in multiple HVAC systems. But how much money could be saved if a chiller system used as much variable-frequency drive (VFD) technology as possible, and controlled it with a system to coordinate the operation to match load demands and tweak performance efficiency as much as possible?

The University of Texas at Austin had an opportunity to find out about the savings possible under such a scenario. Its recent chiller project has already resulted in substantial savings, which means anyone looking to make a significant energy-saving impact should consider using readily available VFD technology.

The opportunity arose in 2008, when the school announced that it needed to expand its campus facilities, and wanted to use land that was already allocated to housing a chilling station in the middle of the campus. When they learned they would be getting rid of an older, inefficient chiller and had to replace it with something completely new, the school’s facilities department decided to really make it count.

“We really hadn’t built a chilling station from scratch in a very long time,” said Juan Ontiveros, PE, executive director of Utilities and Energy Management at the University of Texas at Austin. “Chiller No. 2’s space was needed for an academic building.

“Chilled-water capacity was a very big issue for us,” he continued, and this was a 50-year-old plant. “Chilling stations normally are difficult to replace,” he said. “Our challenge was to get it done in two years. Just getting the chiller built takes a year.

“We did it as design-build, only a little differently,” he said. In short, they selected a chiller vendor based on qualifications, not lowest bid. Eventually the department selected York. “We started off with three all-electric, 5,000-ton chillers with a 30-year lifecycle cost analysis.”


The school district’s cooling system was another driver in the system’s design. “We self produce all energy for campus from natural gas,” said Ontiveros. “We charge for our utilities from auxiliary clients (athletics and housing) and state-funded portions of the institution.

“My challenge in any major capital project is to be revenue neutral, and budget neutral, with a payback that creates equal or positive cash flow from year to year,” he said.

The university’s 350-acre main campus supports 21,000 faculty and staff members, 17 colleges and schools, and more than 50,000 students. A reliable, safe district cooling system is an imperative for the university, which requires cooling 24/7, 365 days a year. But with energy prices tripling in less than 10 years, the university also was challenged to meet the campus’s growing cooling needs using less power.

The district cooling system consists of four central chilling stations serving the campus’s 200 buildings. The school’s electric centrifugal chillers range in capacity from 3,000 to 5,000 tons. Annual chilled water production is more than 145 million ton-hours, and each year the system consumes approximately 109 million kWh (about one-third of the campus’ central power plant output), for an annual average wire-to-water efficiency of 0.75 kW/ton. Peak load is 35,000 tons and growing. A total of 46,000 tons of capacity is provided.

The university’s district cooling optimization project started with Chilling Station 6, a new, all-variable-speed system that replaced the university’s oldest plant, Chilling Station 2, which was retired in order to better utilize its prime location. The new chilling station is intended to increase cooling capacity to keep up with campus growth, and provide the lowest lifecycle cost for the university. As a result, Chilling station 6 was designed with:

• 15,000 tons of cooling capacity.

• A primary-only all-variable-speed system.

• Three 5,000-ton variable-speed electric York chillers with 39°F chilled water design.

• Three variable-speed condenser water pumps (15,000 gpm, 500 hp).

• Three variable-speed chilled water pumps (10,000 gpm, 800 hp).

• Three variable-speed cooling tower cells (15,000 gpm each, 250-hp fans, 85-95° and 78° wetbulb design).

• PLC control system.

• OptimumHVAC™ software.

“We planned on using VFDs from the beginning,” said Ontiveros. “We selected the equipment based on total kW/ton.” The reduction in energy consumption would have a direct impact on the operational cost of the district cooling system. “Our issue was the cost of its fuel.”


“One of the challenges when you are building any kind of plant is that you design for your peak needs, but you don’t need those all the time,” he said. “VFDs allow us to do that,” while allowing for low-load operation of multiple units most of the time. “We decided from the beginning that everything would have VFDs, and no control valves.”

The loads are matched precisely and efficiently using Optimum Energy’s OptimumHVAC™ software, which uses its relational control methodologies to automatically operate all HVAC equipment (including chillers, pumps, and fans) on a variable-speed basis, based on real-time building loads. In addition, online services allow for the tracking of real-time energy performance, therefore assisting the facility team’s ability to detect, diagnose, and correct system faults as they occur to prevent performance degradation.

The software has three components:

• OptimumLOOP™ software reduces energy use of centrifugal chiller HVAC systems in buildings 100,000 square feet or larger, and in district cooling plants.

• OptimumTRAV™ software extends the capabilities of direct digital controls to modulate airflow in air-handling systems for more precise and efficient air temperature management.

• OptimumMVM™, a secure online measurement, verification, and management service, provides real-time and historical performance information to streamline operating processes and help enable sustained energy reductions and cost savings. It also shows real-time plant operating efficiency, daily and monthly dollars saved, and CO2 reduction levels, and it’s accessible to university technical staff and Optimum Energy engineers.

Using OptimumHVAC, the annual performance range for Chilling Station 6 is expected to be 0.33 to 0.78 kW/ton, compared to the design performance range of 0.57 to 0.79 kW/ton. The university is expected to save an estimated $500,000 in the first year. Other benefits include:

Load diversity:The all-variable-speed plant is able to efficiently handle loads between the campus minimum of 4,000 tons up to 12,000 tons without significant staging of chillers and pumps.

Water and chemical savings:Because the plant can efficiently serve the entire campus up to a load of 12,000 tons, the other chilling stations stay off for a significant number of hours per year, reducing cooling tower water and chemical use, and maintenance.

Reliability:Chilling Station 6 is fully dynamic and operable without OptimumLOOP, providing redundancy.


“It’s a self-learning system,” said Ontiveros. “It adjusts its performance as time goes on. And you can see when there’s system degradation, or some other problem.”

He said he is using performance modeling in a practical way to achieve the facility’s energy goals. This is a very realistic goal in new systems. “You’ve got to have great instrumentation,” he said.

“We all know the saying that if you can’t measure it, you can’t control it. Now there’s a new paradigm: If you can’t measure and model it, you can’t control it. With models, you can see what’s happening with the system. These models say, ‘I’m designed to run this way; how am I doing?’” The chilling station’s operational status is updated every 30 seconds.

The modeling aspect opens up still more opportunities to fine-tune the load throughout the campus. “The ability to do what-if scenarios is a big thing. We’re using the [software] system and analysis on other stations.

“While the plant is running, we have algorithms that follow the load and increase the speed of the pumps if the load’s going up; on the back side, they can increase the heat rejection” to maintain lower energy consumption, said Ian Dempster, applications development manager for Optimum Energy. “When things start to cool down, the system backs things down again. It constantly looks at efficiency.”

“It knows what the performance curve is and capitalizes upon this,” said Ontiveros.

“This is the first optimization system I have bought that actually works,” said Kevin Kuretich, PE, associate director of plant operations for the university.

The project started in late 2008 and the station was brought online in October of last year. “Over the winter and up to this point, the system is paid for,” Ontiveros reported.

“I think efficiency is one of the low-hanging fruit for carbon reduction,” he continued. “Anybody that has a central plant can optimize their energy savings. Most campuses across the U.S. have district energy systems. I think the potential is there for residential buildings, too.”

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Publication date:08/02/2010