Princeton University’s geo-exchange/heating hot water system

Designing a Highly Customized Hybrid Geo-Exchange/Heating Hot Water System to Support Princeton’s Goal for Net-Zero Carbon Emissions by 2046

By Salas O'Brien February 25, 2022
Courtesy: Salas O'Brien

Princeton University is well on its way to becoming a sustainability leader among universities and large institutions throughout the world.

It set out to reach net-zero carbon emissions by its 300th anniversary in 2046, and we helped kickstart its progress toward achieving this ambitious goal.

Brandon Dachel, a senior mechanical engineer, explained how Salas O’Brien is helping Princeton accomplish this extensive undertaking.

Courtesy: Salas O’Brien

What is Salas O’Brien’s contribution to the project?

We helped develop the master plan to support the conversion of 180 buildings on campus from a steam-based system to a low-temperature heating hot water system.

We also designed:

  • Mechanical and plumbing systems for the new East Campus energy plant named TIGER (Thermally Integrated Geo-Exchange Resource).
  • Geo-exchange fields for both East and West Campus.
  • Geo design for TIGER and BIG CAT, the West Campus central energy plant.

What are some of the unique aspects of Princeton’s geo-exchange system?

This will be one of the largest closed-loop geo-exchange systems in the United States. The current plan calls for about 900 vertical bores that are 850 feet deep (for a total of about 2,900 bores in the long-range plan) with two new 2.25-million-gallon thermal energy storage (TES) chilled and hot water tanks that support over 9 million square feet of building space. The only other system at a higher education campus that compares is the one we designed for Ball State University.

The geo-exchange system will work in tandem with heat pump chillers at both TIGER and BIG CAT. Heat is a byproduct of the heat pumps, and we’re able to capture that heat and use it in the heating hot water system while simultaneously making chilled water to cool the campus

The combination of geo-exchange with the heat pump chillers and TES tanks allows our client to charge those tanks during nonpeak times when the cost per kW hour is cheaper with a more energy-efficient system than just steam and cooling-only chillers alone.

What influenced our approach to this project?

For every geothermal project, we analyze the variables to determine the most cost-effective approach to achieving maximum efficiency and energy cost savings and determine where the curve starts to flatline on the return on investment (ROI).

For Princeton, one of the constraints to designing a geo-exchange system large enough to handle its peak load is the acreage limitations on campus. There simply wasn’t enough space to fit all of the geo-exchange bores that would be needed to support all of the buildings.

One of the ways we overcame that was by drilling deeper than the typical 400-600 feet. The bores that will support TIGER and BIG CAT are 850 feet deep to minimize the overall space impact on campus and allow for future building development.

Although drilling even deeper would certainly allow for a larger geo-exchange system in the initial phases, doing so would have increased drilling costs and decreased the project’s ROI. The geo-exchange fields are designed to handle a balanced load in which all the energy (BTUs) that go into the ground in the cooling season are extracted back out and used in the heating season. We planned and designed the conversion phases to allow for their current conventional assets, which still have a lot of life left in them, to handle the unbalanced heating and cooling loads during peak demand. This approach drastically reduced the size and capital expenditures required for the geo-exchange fields.

What other costs did we consider for the overall plan?

We also considered the cost of building conversions. We had to find a balance between designing the lowest temperature heating hot water system (low entropy) and the cost to convert buildings that operate on steam and 180-degree heating hot water on average. Our goal was to reduce heat pump chiller lift—the difference between the leaving evaporator water temperature (cool water) and the leaving condenser water temperature (hot water). The lower the lift, the more efficient a system is. Because of our familiarity and expertise in these systems and disciplines, we were able to design a hybrid system that supports 145-degree heating hot water with a future goal of 130 degrees, allowing us to use very efficient heat pump chillers while minimizing conversion costs.

Ultimately, Princeton’s system will be a hybrid that’s entirely unique to their circumstance but will help them achieve net-zero carbon emissions by 2046.

For more details about Princeton’s progress toward becoming a net-zero campus, check out their comprehensive article, where you can find information, infographics, and videos about the entire scope of the project.


This article originally appeared on Salas O’Brien’s website. Salas O’Brien is a CFE Media content partner.

Original content can be found at

Author Bio: For more than 40 years, Salas O’Brien has been at the forefront of designing buildings that are healthy, sustainable, and cost-efficient.