District Energy - 2011 1st Quarter

year to year,” says Juan M. Ontiveros, PE, executive director of utilities and energy management at UT Austin. “Our approach was to focus on efficiency. We needed to meet this growth, but we were determined to do it in a way that would pay for itself.”

UT Austin needed to build a new
15,000-ton plant with a footprint
no larger than the 7,800-ton
plant it was replacing.

The UT Austin CHP district cooling system consists of the central utility plant (the Hal C. Weaver Heating and Power Station) plus four chiller plants with 45,000 tons of capacity tied into six miles of chilled-water distribution system piping – the newest of which is located quite literally in the center of the campus. In 2004, UT Austin conducted a chilled-water system master plan study, which determined that the existing plant was facing hydraulic limitations in the distribution system and was within a few years of exceeding available capacity. Chilling Station 2, the oldest plant, consisted of three steam turbine-driven centrifugal chillers totaling 7,800 tons. At 50 years old, it was inefficient and beyond its useful life. In addition, the plant was located in a prime campus location that was needed for a new building that would expand the computer science department.

Faced with this situation, UT Austin’s utilities and energy management team members were tasked with building a new 15,000-ton plant with a footprint no larger than the 7,800-ton plant it was replacing. Not only were they faced with the challenge of a space-constrained campus, they also had a short project timeline of two years, from 2007 to 2009. Since the peak campus cooling load could not be met without the old station still in service, the new station had to be ready by May 2009.

“We really hadn’t built a chilling station from scratch in a very long time,” says Ontiveros. “Chilling Station 2 space was needed for an academic building,

but chilling stations are normally very
difficult to replace – just getting the
chiller built takes a year. So we did it as
design-build, only a little differently.”
Due to the time constraint, the
project was implemented as a design-
build project which required the team
to make the chiller selection up front.
Based on a 30-year lifecycle analysis,
and an emphasis on total plant kilowatt-
per-ton efficiency, the following design
elements were identified:

• A 3,000-ton inlet air coil would be installed adjacent to the new station to serve the 45 MW combustion turbine and provide chilled water from this station to the new 32. 5 MW combustion turbine that was in construction to boost generation capacity and improve the CHP plant heat rate.

• Variable-frequency drives (VFDs) would be used on all rotating equipment to take advantage of the characteristic that kilowatt-per-ton efficiency is improved when the chiller is run at partial loads.

• No control valves would be used to modulate flow through the chiller evaporator or condenser to reduce pressure losses.

• Control valves would only be used
to modulate the three cooling tower
cells so that flow could be balanced
evenly (which would leave only one
modulating valve in the entire system).

Enhanced Control

To ensure that this new chilling station was able to improve efficiency and generate savings that created equal or positive cash flow, UT Austin knew that it needed to further enhance the energy efficiency of the total plant. The university selected OptimumHVAC™ software from Optimum Energy to help meet these goals. This software

Figure 1. Variable-Speed Performance of 5,000-Ton Centrifugal Chiller at Varying Entering Condenser Water Temperatures, The University of Texas at Austin.