Cold as Ice: How Downtown Chicago Keeps Cool

Posted by Matt Baker on March 11, 2010 · 2 Comments Print This Page Print This Page

By Matt Baker

Chicago has ice in its veins.

In truth, it’s not ice but 34° water coursing beneath the surface of downtown Chicago. One long stretch runs up LaSalle, another down Dearborn. A search of Lower Columbus might reveal pipes suspended over one’s head. Feeding this network, tucked away in different corners of the city, cooling plants deliver on-demand chilled water. Thermal Chicago is the company behind this district cooling network, and they may be one of the city’s best-kept energy secrets.

District cooling is a mode of distributing thermal energy, usually via chilled water, through an interlinked delivery system. Such networks are usually found in dense metropolitan areas or college campuses. As is the norm with most energy-efficient technologies, the practice is more widespread in Europe than in the U.S. But it’s not a completely alien concept stateside. Several U.S. cities currently have district energy networks, such as Phoenix, New Orleans and Minneapolis. Chicago’s is the largest contiguous system in the nation, if not the world, with fourteen miles of pipe and an eight million gallon capacity.

The key benefit to district cooling is the ability to move grid stress off-peak, where electricity is cheaper. By leveling out the electrical grid, there is less need for infrastructure or for energy production. About half of a building’s use on peak is air conditioning; district cooling abates that need.

Building variety reduces stress to the system even further. Condos and apartment complexes tend to peak in the evening, for instance, while office buildings usually peak in the early afternoon. “That diversity allows us to serve everybody and have margin because they’re not going to peak at the same time,” said Dave Bump, Thermal Chicago President and CEO.

Since Thermal Chicago delivers cooling only, demand goes down in the winter. But the company is far from dormant. Seasonal maintenance ensures that when all the machines are needed, they perform at peak efficiency. Even so, the system never goes completely offline since data centers, switching stations and other uses need cooling year-round.

As an unexpected benefit, district cooling decreases the mechanical footprint of a building, allowing building owners flexibility of use. Some buildings have planted a vegetated roof where cooling towers would have gone. Reduced mechanical systems in the basement can allow for indoor parking. “Where a building performing its own chilling would need all the equipment and infrastructure associated with that,” said Bump, “on this network, all they need is a far smaller heat exchanger.”

If each building on the Thermal Chicago rolls were to chill its own water, “they would need about half of one of our plants.” And that’s not negligible. Thermal Chicago serves around a hundred buildings and upwards of 45 million square feet of space. The company prides itself on its well-trained staff and state of the art refrigeration equipment. One hundred building managers each running separate chillers would never match the efficiency of the district cooling system. Integral to their equipment are variable frequency drives, or VFDs. The VFDs allow the equipment managers to fine tune the system for peak efficiency.

Thermal Chicago has its roots in energy giant Exelon. “In the 90’s, it was very chic for these large utilities to dabble in the deregulated markets,” said Bump. Exelon spent a considerable sum installing the network of pipes. “It probably would not have been done by an entrepreneurial company.”

The first source of chilled water to come online was plant one, or P1, in 1995. Located at State and Adams, P1 could be confused for any other mixed-use Loop building. The first floor is retail, currently a Walgreens store, but the rest is “loaded with machinery, the whole building,” said Bump. “You wouldn’t even recognize that unless you knew what you were looking for it.”

Exelon’s first district cooling venture had just eleven customers at the time. But the following year P2 came online. In a conspicuous location, along Congress and across the river from the old post office, P2 is more obvious as an industrial property, though thousands of commuters pass the building every day without knowing what’s inside.

What’s inside is the largest ice tank of its kind in the world. Forty feet deep, it holds two million gallons of chilled water and can deliver 125,000 ton hours of cooling to the system. Though three more plants have come online for a total of five, P2 still houses a quarter of the network’s capacity.

Thermal Chicago employs three different technologies to cool the city: ice tanks, ammonia chillers and river water chillers. All three are straightforward and time tested. With the first, the plants create ice at night and melt it out slowly the following day. This technique flattens out the clients’ load profiles, reducing their electricity need. Thermal Chicago users also benefit from water usage reductions of up to 25%.
Absorption chillers using chemicals such as ammonia have been around for nearly a century but have seen greater demand since ratification of the Montreal Protocol, due to their lack of ozone-depleting emissions and greenhouse gases. Ammonia is harmful to humans in heavy doses, but prevalent and mostly innocuous to the environment. Even so, every effort is taken to contain the ammonia. “From a past life, I know a little about damage control,” said Bump, in reference to his days aboard a U.S. Navy submarine. Thermal Chicago actually purchased the same leak control devices used by the military.

The final chilling strategy, river water cooling, has been used in many riverside buildings for decades. A plant was installed in the 1950’s for the Merchandise Mart’s air conditioning needs, for example.

The Mart’s plant was expanded in 2000 as P4 and a new river water plant, P5, was installed two years later under the plaza at 330 N. Wabash, formerly known as the IBM Building. Thermal Chicago uses ultraviolet light to sterilize the incoming water, further reducing pollutants that would otherwise end up in the river.

In 1997, Blue Cross/Blue Shield of Illinois finished construction of their headquarters on Randolph Street, overlooking Grant Park. In the basement is P3, housing three large ice tanks and an ammonia chilling plant. Starting in 2008 and finishing up this year, Blue Cross/Blue Shield Tower was vertically expanded, adding twenty-four stories to its height. The insurer had planned for this before the tower’s original groundbreaking, including ground to penthouse atria that could be converted to elevator bays for the addition.

Thermal Chicago’s presence was planned into the expansion as well. By building around the cooling towers on the old roof and providing baffling for airflow, P3 was able to stay online during the entire renovation. Once the new roof was in place, newer, more efficient cooling towers were installed and the old ones dismantled.
The plant itself was upgraded with the building too. Plans to install a new chiller in the basement were altered; because of newer technologies they were able to squeeze in two.

Thermal Chicago is a certified participant in the Clean Air Counts Program. Clean Air Counts, a northeastern Illinois regional initiative to reduce ozone-causing emissions, is a collaboration between the Metropolitan Mayors Caucus, the City of Chicago and the federal and state arms of the Environmental Protection Agency.
They also consult with their clients on strategies to increase energy efficiency. “Believe it or not, we’re not out to sell the most energy that we can,” said Bump. “It’s more long term relationships.”

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