An infrastructure master plan, developed beginning in 2016, outlined a shift from steam to district hot water, campuswide geo-exchange borefields, large-scale heat recovery chillers, thermal energy storage and advanced controls. Options were evaluated for life-cycle cost, carbon impact, reliability, constructability and fit with architectural context. The plan then guided the design and construction of TIGER, the Thermally Integrated Geo-Exchange Resource, and the conversion of the West Plant into a coordinated partner facility.

The project team designed TIGER to function as the all-electric primary plant. Ground-source heat pumps were incorporated to move energy to and from deep borefields, supported by two outdoor thermal energy storage tanks sized to manage daily hot- and chilled-water loads. By design, thermal production could be shifted to times that lowered both operating cost and carbon intensity. The West Plant was designed to partially retain conventional systems, providing operational flexibility and backup capability during periods of high demand or extreme weather.

The geo-exchange system was designed to operate as both a seasonal and daily thermal bank. During summer, heat is transferred from buildings into water and stored underground through 850-foot boreholes. In winter, that stored heat is returned to the system to warm buildings.  

The program is producing measurable reductions in Scope 1 and 2 emissions as buildings transition to low-temperature hot water and geo-exchange service. Early plant operation showed the system delivering roughly 4.5 to 5 times more energy for heating and cooling than the electricity it consumed, a performance level expected to improve further during summer operation. The dual-plant approach improves resilience. TIGER carries the electrified thermal load, while the West Plant provides backup capacity that can be called on during extreme weather, grid instability or periods of unusually high demand. Shifting from steam to low-temperature hot water improves efficiency and simplifies operations, which supports staffing and long-term maintenance.

Campus integration is visible in both the construction approach and the finished facility, and TIGER’s architecture reveals how modern utilities function, offering a learning opportunity for visitors and students. Faculty, as well as representatives from peer institutions and public agencies, continue to tour the plant, using real-time data and firsthand views to study applied decarbonization at community scale.

The program continues to expand with additional bores, new distribution piping and building conversions that broaden the electrified network. Ongoing measurement of borefield energy balance, the daily charging and discharging of thermal storage tanks, and overall plant efficiency will guide refinements to system operations and the balance between TIGER and the West Plant. Princeton’s approach shows how widely available technologies, applied through disciplined planning and phased execution, can deliver campus-scale decarbonization while strengthening reliability.

Princeton’s utilities transformation has moved from plan to performance. TIGER is operating and district hot water is replacing steam across the campus. The program is cutting emissions, improving efficiency and supporting resilience, while turning the energy system into a living classroom and a practical model for peer institutions.