At UW-Madison, heating and cooling is all about water.
Libraries, offices, dorms, dining halls, hospitals, and other campus locations must be kept warm in the winter and cool in the summer. Meanwhile, sensitive spaces like operating rooms and research labs require precise, steady temperatures. None of this would be possible without a constant flow of chilled water and high-pressure steam.
These utilities, along with compressed air, are generated at three facilities on campus: Charter Street Heating & Cooling Plant, Walnut Street Heating & Cooling Plant, and the West Campus Cogeneration Facility (which is operated by Madison Gas and Electric).
A dedicated team of Facilities Planning & Management (FP&M) employees ensure that the delivery of heating and cooling to campus is reliable and resilient. “We are the heart of the campus for its utilities,” says Travis Thoeny, Plant Manager for the Charter and Walnut Street plants. Just as one’s heart must keep beating to keep the body alive, so too must the three plants operate consistently, with an absolute minimum of downtime, in order to keep UW-Madison running.
From hot to cold
Making steam and chilled water for a large campus requires serious equipment and substantial output. The five natural gas-fired boilers at Charter Street—which replaced far less efficient coal-fired units—can produce a maximum of 1,200,000 lbs/hour of steam. As a backup system for the boilers, the plant also keeps 840,000 gallons of fuel oil in a single tank.
UW-Madison uses the steam for several purposes: heating campus buildings and two hospitals; running a steam turbine generator to produce electricity; and powering four steam turbine chillers at Charter Street as well as two more at Walnut Street. The steam also powers eight auxiliary steam turbine drives to support steam production at both plants.
The Charter Street chillers are the largest of their kind in the world—8,000-ton and 8,500-ton behemoths manufactured by YORK. They provide the 40-degree chilled water that circulates through the coils of air handling units on campus. As it circulates, the chilled water draws heat from the ambient air, making buildings more comfortable before it returns to the plant at approximately 50-55 degrees.
This relatively warmer water then passes through a chiller evaporator, which incorporates between 3,100 and 5,200 tubes (depending on the unit) to transfer heat to a refrigerant. In other words, the water gives up the heat it picked up in the buildings to the refrigerant.
The refrigerant, now carrying that heat, then travels to a compressor, in which it is converted to a hot, high-pressure gas. The gas enters a refrigerant condenser, which uses thousands more tubes to transfer heat from the refrigerant to the cooling tower water.
Having become a liquid again, the refrigerant finally returns to the beginning of the cycle so it can chill new water. Meanwhile, the heat captured by the cooling tower water process is released to the atmosphere as vapor—commonly misinterpreted as “smoke” billowing from the Charter Street Plant.
A more sustainable lake-to-lake system
But where does all this water come from, and how does UW-Madison use its water resources sustainably?
In recent years, UW-Madison has pumped untreated lake water from Lake Mendota to supply certain systems in its heating and cooling plants. Lake water brings many benefits. Compared to city water, lake water costs far less: approximately $0.30-$0.50 per 1000 gallons compared to $5.00 per 1000 gallons for city water.
In addition, using lake water means that Charter and Walnut Street Heating & Cooling plants, along with West Campus Cogeneration Facility, do not rely on groundwater for their operations. Rather, they draw a relatively small amount from Lake Mendota—500-600 million gallons per year, which is the equivalent to about two weeks of evaporation during hot summer weather.
Lake water presents challenges, however, because it carries biological organisms that can accumulate inside pipes and other pieces of machinery.
The downside of the sand ballast clarifier? A lot of waste water was going down the sanitary sewer drain, in order for the bleach to be removed from the water before redistribution into the natural environment.
So the Charter Street Heating & Cooling Plant team decided to try an experiment: they stopped injecting the bleach into the lake water that supplied the closed-loop cooling (CLC) systems and cooling tower. As part of this process, the Charter Street team and UW-Madison’s water treatment vendor, U.S. Water, worked diligently to maximize the Charter and Walnut Street Plants’ performance while minimizing cost and maintenance. After careful testing, they found that residual chlorine was gone from their systems and that no problematic organic buildup was occurring in their machinery.
By making this relatively simple change, the Charter Street plant diverted over 70 million gallons of water from the sanitary sewer drain to the storm sewer between 2016-2017.
This water now flows, effectively, from one lake to another: it begins in Lake Mendota, and is discharged into Lake Monona under Department of Natural Resources (DNR) guidelines. Moreover, avoiding the sanitary sewer drain and thus the wastewater treatment plant reduces both energy usage and emissions.
U.S. Water was so impressed that they gave UW-Madison an award for sustainability, recognizing the reduction in sanitary sewer volume from the plant and an annual savings of $191,970.
“It’s awesome,” said Jeff Pollei, Director of Utilities and Energy Management. “It’s a simple change to save that amount of money.”
Flowing into the future
Travis Thoeny and his team plan to implement more sustainability measures in the near future, with two projects already underway. The first is an automatic cleaning system for those massive chillers. The trial chiller contains 3100 tubes in the evaporator and condenser units, which gradually experience build-up on their inner walls and a corresponding reduction in efficiency and performance. By installing new equipment that circulates hundreds of small, foam balls through these tubes, the entire system is cleaned at least once per day.
Once more, a sustainable choice is leading to both economic and environmental benefits. In a six-month test of the new cleaning method over the summer, the trial chiller at Charter Street saw a $40,000 savings in natural gas. It also gained an equivalent of 400-500 tons of additional cooling capacity compared with the previous year. Once the foam balls are added to the additional three chillers, Thoeny expects that the plant will save $220,000 in natural gas per year—which also translates to fewer greenhouse gas emissions.
Finally, the plant has transitioned to a zero-phosphate chemical in their cooling towers. Thanks to this change, they hope to obtain a wastewater discharge permit from the DNR to switch their cooling tower blowdown water from the sanitary sewer to the storm sewer. This would result in the diversion of an additional 25-30 million gallons/year to the storm sewer, for an additional savings of $68,000-$82,000 per year.
By: Nathan Jandl