What Is a Geo-Exchange System?

As a part of U-M’s commitment to carbon neutrality, you may have heard that several new geo-exchange systems are in the process of being planned and implemented on campus: the Leinweber Building geo-exchange project, the new Ginsberg Center, as well as the new Central Campus housing and dining project all incorporate plans to be heated and cooled using geo-exchange. But what is geo-exchange exactly? How does it differ from geothermal? And how is it so much more efficient than our current systems? I’ll try to address those questions in this post.

Geo-exchange Explained

Put super simply: these systems enable us to heat and cool buildings by transferring heat either into or out of the ground depending on the season. During the summer, these systems take excess heat from inside buildings and store it in the ground. This makes the ground function as a sort of thermal battery storing the heat for when it’s needed again. Then in the winter, the reverse happens, and the same system is used to bring the stored heat from the ground into buildings to provide warmth. The system uses the ground (geo) as a heat sink in summer and a heat source in winter (exchange).

If you want to get a bit more specific, the geo-exchange systems proposed for U-M are called “vertical closed-loop” systems. This means that they consist of a lot of vertically drilled holes in the ground, some more than 600 feet deep, and a sealed network of pipes full of water, which serves to transfer that heat energy between the buildings and the depths of the vertical boreholes below. In these systems, the water in the pipes is completely sealed off from the groundwater, and the only thing transferring between the pipes and the ground is thermal energy. 

Graphic showing how water flows through the geo-exchange pipes in summer to store excess heat and in the winter to uptake that stored heat.

Geo-exchange vs Geothermal

While the terms geothermal and geo-exchange are often used interchangeably, they actually refer to different technologies. Without getting too into the weeds, geo-exchange systems are different in that they return the heat to the ground in a renewable cycle. Geothermal systems are purely extractive and don’t include returning the heat to the ground to be used again.

An Efficient Campus Solution

The advantage of a geo-exchange system is that it is extremely efficient compared to a traditional one. A traditional plant runs boilers and chillers simultaneously to both remove heat from buildings and reject it via a cooling tower, while continuing to burn fossil fuels to generate more heat. In contrast, a geo-exchange system simply moves heat from wherever it is being rejected to wherever it is being consumed.

This is particularly relevant in spaces like the University of Michigan that include a variety of building types with diverse energy demands (e.g., science laboratories, residence halls, classrooms). When some types of buildings need heating, others may need cooling.

Hopefully you now have a little better understanding of geo-exchange and how this technology can move our campus forward on our path to carbon neutrality. If you followed along with the PCCN process and read their final report, you may recall that their recommendations included a massive collection of geo-exchange systems for heating and cooling all of our campus buildings on all three campuses—in fact what they called for would be the largest university geo-exchange project in the world. While we might not be drilling those 20,000 boreholes tomorrow, I’m excited that our first geo-thermal projects are approved because they will be a great way for U-M to pilot such a big shift in our building systems. It feels a bit odd as an environmentalist to say this, but there’s a first for everything: drill baby drill! 

P.S. This post was informed by a great resource from Princeton you might want to check out to learn more!