In Push to Reach Net Zero By 2046, Princeton Goes Passive

Princeton University is one of the oldest and most well-respected institutions of higher education in North America. It’s also relatively small, with fewer than 9,000 combined undergraduate and graduate students. While the university has certainly grown since relocating from Newark to the borough of Princeton ten years after its 1746 founding, it had remained self-contained within the suburban New Jersey community from where it gets its name until recently.

In December 2021, the university formally broke ground on a 107-acre site across Lake Carnegie in the community of West Windsor known as the Lake Campus Development. The new 85-acre Meadows Neighborhood, the initial phase of the Lake Campus Development, will include three Passive House residential apartment buildings for graduate students designed by the Seattle-based firm Mithun, as well as a racquet center, a softball stadium, additional sports fields, and a facility to house heat pumps and electrical equipment for its newly installed geo-exchange system. The university envisions the new campus as a vibrant, mixed-use community that may be expanded in coming years as the school grows.

These changes also represent a more aspirational and comprehensive transformation occurring on both sides of Lake Carnegie that will see one of the world’s most prestigious universities produce a replicable template for sustainable development that prioritizes building decarbonization and operational efficiency—and includes Passive House performance as one of its guiding lights.

Action Plans

The university adopted its first sustainability plan in 2008. By reducing emissions through a combination of doubling down on conservation efforts, improving the efficiency of energy distribution systems, and increasing on-site renewable energy generation, the university cut absolute emissions by 18% below 2008 levels by 2022. Given that 46% of gross built space was added to the university between 1990 and 2020, this achievement is impressive.

By 2019, the university decided to set their sights higher and enacted a far more ambitious sustainability action plan. Among its many objectives is a multi-faceted approach to decarbonize university operations and achieve net zero greenhouse gas emissions by Princeton’s tricentennial in 2046, or sooner.

Based on estimated future growth, getting to zero will require finding ways to prevent what would have been approximately 130,000 metric tons of annual emission by 2046.

For a growing university with several buildings dating back to the eighteenth and nineteenth centuries and a natural gas-powered steam distribution system that services 180 buildings and was originally designed more than 100 years ago, approaching zero means more than just conservation efforts and additional onsite renewable energy generation; it means abandoning fossil fuels whenever possible, improving building efficiency in new and old buildings, adopting behavioral and technical solutions, and reimagining building operations from the ground up.

Princeton President Christopher L. Eisgruber acknowledges that this is no small task and has given every indication that the university is committed to taking aggressive action to making Princeton operations net zero by 2046. “Princeton can play a leadership role not only by developing innovative solutions through teaching and research,” Eisgruber said when the new action plan was announced in 2019, “but also by establishing best practices in our campus operations and community behaviors that serve as models for the world.”

Innovative solutions include developing tools for tracking embodied carbon in buildings and setting related performance goals. To date, the university has focused on working with local manufacturers to test and procure lower embodied carbon concrete mixes and is replacing structural steel with responsibly sourced mass timber in many of its new buildings. Based on recent pilots, the university is now requiring full-building embodied carbon accounting and goal-setting in the planning stage for future major projects.

The university is also performing a campus-wide, combustion-free conversion of its heating, cooling, and hot water systems as part of their decarbonization effort.

“By 2046, we should have a super energy-efficient campus with a system that’s super reliable, and one that’s fully powered through renewable energy,” said Ted Borer, director of Princeton’s Energy Plant.

Where There’s a Well There’s a Way

Since at least 2021, it’s been difficult to visit Princeton without noticing the construction. Given the scope of the upgrades currently underway, this is no surprise. Throughout the existing campus, the university is converting its fossil-fueled steam plant to an electrified geo-exchange hot water distribution and ground source heat pump system.

The project has required not only retrofitting the systems within buildings throughout campus, but also constructing multiple utility buildings, boring myriad 850-foot-deep wells wherever real estate has been available, and installing over 13 miles of distribution pipes. Finding drillers with the technical skill and capacity to accomplish the task was difficult, as was finding the space to drill the holes while minimizing disruptions to university life. However, these challenges have pushed the team to use land more efficiently—for example, by layering geo-exchange, stormwater management, and programmatic functions on the same sites.

[What Is Geo-Exchange? See below]

Two new buildings, TIGER (Thermally Integrated Geo-Exchange Resource) and CUB (Central Utility Building), will house the new, university-wide geo-exchange system that will provide heating and cooling via underground district energy systems. TIGER will service the existing Princeton campus and utilize approximately 2,900 geo-exchange bores. Princeton’s existing chilled water plant is being converted to work with TIGER so the two can partially backup one another. CUB will provide heating, cooling, and hot water for the Meadows Neighborhood and receive thermal energy from approximately 150 bores located beneath the campus’ new softball stadium. Though CUB is significantly smaller than TIGER, the buildings within the Meadows Neighborhood will be more efficient than those on the other side of Lake Carnegie. Because all the new Meadows Neighborhood student housing will be built to Passive House standards, the system will have the capacity to heat and cool these buildings, the new athletic facilities, and buildings added to the Meadows Neighborhood in the foreseeable future.

In addition to seasonal geo-exchange, these systems also employ hot water and chilled water thermal storage by relying on two tanks located next to each building. The district hot water distribution temperature for the existing campus will be 140°F, while the hot water systems at the more efficient Meadows Neighborhood buildings will operate at about 120°F. For domestic hot water needs, this will be raised to 140°F at the building using small local heat pumps. By comparison, steam from the existing system is generated at 450°F.

The new systems will be almost entirely electric, save for backup boilers that will supplement the heat pump system during peak heating needs. The new geo-exchange’s heat pumps will be powered with 100% renewable energy, much of which will be generated onsite using PV arrays. (Currently about 20% of Princeton’s campus is powered by onsite solar arrays.) Over the coming years, Princeton hopes to add more solar to the campus power grid and procure off-site renewable electricity for any remaining energy needs. This conversion, once complete, will allow Princeton to heat, cool, and electrify its entire campus without the use of fossil fuels.

Most of the construction in the Meadows Neighborhood is expected to be complete by start of 2025, with some projects wrapping up sooner. The building that houses CUB and the Meadows Drive parking garage will be completed by the end of 2023, while the new campus’ graduate housing, softball stadium, and racquet center are scheduled to be completed in 2024.

The Meadows Neighborhood

The Meadows Neighborhood includes 329,000 ft2 of graduate student housing for over 600 individuals spread across 9.3 acres. The project is being undertaken by the university in conjunction with American Campus Communities, the developer, and Thornton Tomasetti, the sustainability consultant. Construction is being overseen by Hunter Roberts. In total, the buildings will contain 379 housing units, as well as a café and community center. The buildings will primarily be three stories tall and outfitted for future PV arrays, but the spaces dedicated to the community center and the café will only be one story in height and each will have green roof systems.

All three of these buildings have already been pre-certified by Phius. The university was ultimately attracted to Passive House because of its high-performance standards, resulting in a combination of high levels of comfort for occupants and extremely low energy use. By minimizing energy use, the university was able to reduce the number of geo-exchange wells needed to provide heating and cooling to the complex (each housing unit will be equipped with fan coil units connected to the geo-exchange system), thereby providing significant cost and embodied carbon reduction benefits.

The team employed a range of solutions to satisfy Phius requirements. Each of the three buildings—known as Building A, Building B, and Building C—needed to be modeled separately in WUFI Passive, with their own set of inputs and conditions, but all needed to stay on track with the Phius criteria. Specific to the modeling, due to height restrictions and existing grade, there was much coordination early on about the air handling units (AHUs) and energy recovery ventilators (ERV) and their locations. Ultimately, the team decided to use a combination of indoor and outdoor units, which required external calculations for each building. For Building C, the units needed to move fully inside the thermal envelope, which impacted the cooling load and made it difficult to offset. The loss of interior space resulted in fewer living units, which impacted the overall project.

Another difficulty concerned the landscaping of the Meadows Neighborhood. There are many planned trees surrounding these buildings, and those had to be modeled as well.

Due to the scale and program of these buildings, they were considered commercial for some Phius criteria so navigating that was interesting. Phius protocol requires dormitories to be modeled as if they are residential buildings, but by code and as far as U.S. DOE’s zero energy ready home (ZERH) program would be concerned, this is a commercial building, so for example the hot water piping and “time-to-hot” test does not apply. This was a challenge because the piping had been optimized as much as possible by the time the team became aware of this exception.

Though the project was designed prior to Princeton’s launch of its building materials embodied carbon analysis and goal setting, the team decided to use wood framed construction rather than steel and concrete construction to help reduce embodied carbon and control costs. Other decisions were more conventional for Passive House construction.

The foundations contain 2 inches of continuous rigid insulation (R-10), installed on the outside of the concrete masonry unit (CMU) foundation wall, as well as four inches of continuous rigid insulation (R-14.4) installed under the slab on grade. The air barrier under the slab is provided by Stego wrap. Continuity of the air barrier is being provided by employing self-adhered flashing at transitions.

For the exterior walls, the team is using siding over ¾-inch furring strips with the Huber ZIP system beneath. The ZIP system’s insulated sheathing panels contain an integrated air and water-resistive barrier, and the joints are being taped with all the fasteners either taped or sealed with liquid flashing. The interior walls are 2x6 wood framing with batt insulation over 5/8-inch-thick gypsum board. For windows, the team ultimately decided on Anderson A-Series Low-E4 SmartSun enhanced triple-pane windows (U-Factor: 0.22). The air and vapor barrier at the roof is a thermoplastic polyolefin membrane by Carlisle adhered directly to the structural wood sheathing, and between the membrane and the timber roof truss is 12 inches of rigid insulation. The R-values for the slab, wall, and roof insulation will be 22, 28, and 60, respectively.

To ensure that construction goes smoothly, a free-standing mockup was used to verify the installation of the envelope details prior to the beginning of construction, which officially started March 2022. Additionally, preconstruction conferences were held with the contractor, subcontractors, manufacture representatives, designers, and the Phius rater, followed by an in-situ mockup of each major element of the envelope system for verification prior to final installation.

“Princeton has the opportunity to lead by example,” said Borer. “Even in our facilities, people are going to look at us and say, ‘Wow, now that I see what Princeton has done, I get it.’ We can influence thousands, tens of thousands, even millions of others by our actions on campus.”

This article was developed in coordination with the Princeton University Facilities Organization, design partners Mithun and Thornton Tomasetti and development partner American Campus Communities (ACC).

The rendering at the top of the page is courtesy of Plomp.

What Is Geo-Exchange?

Instead of burning fossil fuels to produce heat, as a conventional furnace does, geo-exchange systems transfer heat from one place to another. During summers, geo-exchange systems take heat out of buildings and store it in the ground using bores to slightly warm the rock below. During winters, the same geo-exchange bores and warmed rock are used as a heat source. Unlike geothermal systems, which extract heat from the earth, a geo-exchange system relies on electric heat pumps, closed ground heat exchange piping loops, and distribution pipes to deliver heating, cooling, and hot water to buildings.

Most bores at Princeton will be approximately 850 feet deep and once the system is fully online there will be more than 1,000 bores under the university’s campus. Though it is not a brand-new technology by any means, there are only a few geo-exchange systems in the world that have been built to this scale.