Princeton University pledged to reach net-zero campus carbon emissions without the use of offsets by our 300th anniversary in 2046. To get there we are investing in transformational infrastructure upgrades. The utility infrastructure changes, developed holistically with the Master Planning Study, include campus-wide projects that work together to convert the way we heat and cool every building on campus. 

These six projects will enable us to begin converting from steam to hot water, conserve energy, increase our efficiencies, and decrease our carbon footprint and reliance on fossil fuels:

1. New energy facilities based on heat pumps (TIGER & CUB)
2. Geo-Exchange bore fields
3. Thermal distribution piping campus-wide
4. Building heating & cooling system conversions
5. Solar expansion 
6. Energy conservation initiatives 

History of Campus Energy

Princeton University has operated a campus-wide “district steam” system for over 150 years. The central cogeneration plant, built in the 1990s, has been distributing steam heat to campus buildings for over 25 years with more efficiency than individual building heating systems could. But steam-heating is no longer considered state of the art and much of the existing infrastructure is over a century old and due for replacement.

Princeton’s steam system began operation in the 1860s. There are steam lines serving Nassau Hall that were installed in 1906, and, even where the piping is still intact, the insulation may not be. We estimate that at least 30% of the heat made in the cogeneration plant is lost in the ground before it gets to buildings. The options were to repair the antiquated steam technology or replace them with a newer more efficient technology. Continuing to repair and replace in kind would effectively commit the university to combustion-based heating for another century.

Future of Campus Energy

Utility Master Planning

Our campus conversion started with utility master planning that required thinking about how to deliver energy to the campus for the next 50 to 100 years. It took into account many elements and was a multi-dimensional optimization problem that took a few years to solve. Factors included in our multi-year infrastructure master planning study included analysis of: 

• Existing buildings and adding capacity to support new buildings as the university expands
• Prices of different forms of energy
• Different forms of power generation
• Heating and cooling technology
• First cost and life-cycle cost
• Amount of space and water something requires
• Skills required to operate and maintain 
• Carbon footprint
• Code requirements
• Real-time energy market prices
• Inefficiency of our aging campus-wide "district steam" systems
• Need to replace underground infrastructure of steam distribution pipes
• Drive the entire campus to net carbon neutrality by 2046

After researching global best practices in community-scale heating and cooling, we found that the use of district hot water for heating is vastly more efficient than steam heating. Steam heating also normally involves combustion while hot water heating can be achieved using electric heat pumps paired with geo-exchange. The electricity need to power the heat pumps can be renewably generated with wind or solar energy. A district hot water system offers a path to a renewably-heated and cooled campus. 

The hot water heating system operates at a far lower temperature than steam heating, which requires less energy, and will drastically reduce heat loss. Buildings on campus have traditionally received steam heat at 450°F degrees, and hot water at 180°F degrees, and they will now be supplied with hot water heat at 140°F degrees. The complete plan includes full-campus conversion to hot water heating and it will take us about 20 years to reach every building. 

5x More Efficient

The improvement of the new heating and cooling system is phenomenal. Under the old steam heating system, when we input one unit of energy, we could only move about three-quarters of one unit of energy to a campus building. Using hot water, heat pumps and geo-exchange, when we input one unit of energy, we can move four units of energy to a campus building. The new system is expected to be over five times more efficient. Eventually every building on campus will be converted to work with the new district hot water and geo-exchange system.

The Power of Heat Pumps

Heat pumps are not new technology. They are all around us, in everyday things, such as refrigerators and air conditioners. The general principle of the heat pump is that it moves heat from one place to another. A refrigerator moves heat from inside out to the kitchen, keeping your milk cool. An air conditioner moves heat from inside your home or car to the outside, keeping you cool.

Think of geo-exchange as a giant piggy bank that stores heat. When needed, our heat pumps pull that heat into the new TIGER plant to supercharge the efficiency of the heating system.

Think of geo-exchange as a giant piggy bank that stores heat. When needed, our heat pumps pull that heat into the new TIGER plant to supercharge the efficiency of the heating system. In summer we send cold water to cool campus buildings. The water picks up heat from the buildings and comes back about 15°F warmer. That happens day after day in the summer. 

Now we can send that warmer water into our geo-exchange bore fields through a system of closed loop piping. Through convection, the rock underground picks up the heat from the water and stores it. We expect the temperature of the rock beneath campus to increase from 57°F degrees to about 80°F degrees in the geo-exchange bore fields.  In winter, we reverse the direction. The heat pumps will draw that stored heat out of the ground. We then raise the temperature of the water from an already warm 80°F degrees to 140°F degrees, and pipe that hot water to campus buildings for heat. 

Campus-wide Upgrades

The University committed to a campus-wide steam-to-hot-water conversion project to modernize our campus district heating system and significantly improve its efficiency. The hot water conversion is part of an overall effort that includes more thermal storage, geo-exchange bore fields, new thermal distribution piping, building system conversions, and solar photovoltaic power generation in the context of Princeton's Sustainability Action Plan. New buildings are being designed for hot water heating, and existing buildings will be converted from steam to hot water and analyzed for energy conservation initiatives.