Multidisciplinary team investigates problems with the WID's geofield
In the morning hours of early October 2015, University of Wisconsin-Madison graduate students Christian Herrera and Adam McDaniel could be spotted in an excavated area outside of the Wisconsin Institutes for Discovery (WID) Building, wearing construction hats and maneuvering what appeared to be a high-tech fishing pole.
McDaniel, a graduate student in geological engineering, and Herrera, a graduate student in mechanical engineering, are investigating recent issues with the WID geothermal system. A geothermal system is an innovative, energy-efficient way of heating and cooling a building that pushes heat from the building into the ground during the summertime and absorbs heat from the ground during the winter using a mechanical system referred to as a “heat pump.” The heat exchange with the ground is enabled by a “geofield” and at WID, the geofield consists of a network of deep wells.
Shortly after the building was completed, the UW-Madison campus district cooling system went down. And despite the fact that the WID heat pumps were sized to carry only a fraction of the building's overall heating and cooling load, at that point, the pumps cooled the entire building.
However, they also sent an excessive amount of heat into the ground, causing temperatures in the ground to spike. Since then, it has taken an unusually long time for that ground temperature to return to normal, and the graduate students are part of a team studying the functionality of the system, as well as the geological conditions of the ground surrounding the building.
The students’ "fishing pole" was actually a thermal probe that measures temperature at different sections underground. During a two-week period in early October, crews unearthed a portion of the geofield to enable the students and their faculty mentors to perform a battery of tests to gather data that may help explain the field’s sluggish response.
Among a list of data collected by the students was the ground temperature profile throughout two of the geofield’s excavated bores. “We found that it’s hot in the middle, and also at the very bottom of the two bores we measured. Each bore seemed to cool down faster at the top," says McDaniel. “We’re planning on using this data as part of a model of the WID’s geofield.”
The researchers now know, based on initial ground thermal connectivity tests, that the problem isn’t as much the performance of the various heat pumps or the system design, but of the geofield itself. Yet understandably, it’s not an easy feat to dig up the field every time they need to perform a new test. So during the excavation, the researchers also installed new valves and piping that will give them permanent access to the geofield in the future. This unique feature will enable current and future researchers to investigate a wide range of aspects of this type of geothermal system.
The project has prompted both the geological engineers and the mechanical engineers to puzzle the big question: Why is the high ground temperature persisting?
Douglas Reindl, a professor of mechanical engineering and engineering professional development and Herrera’s mentor, speculates the high temperature has something to do with the innate characteristics of the campus geology, since the geothermal system itself seems to be mechanically sound. UW-Madison sits on an isthmus between two lakes, and the location may have unique ground properties that may work against a geothermal system.
“In collaborating with geologists on the team, one possible explanation of the slow rate of ground temperature recovery is the presence of a huge amount of subsurface perched water that was initially heated up during a period of intense heat pump operation,” Reindl says. “Once a perched reservoir is heated up, it just takes a long time for the absorbed heat to dissipate.”
Now, the researchers are analyzing the geofield with a thermal response test (TRT) rig with a fiber-optic probe attachment, which will yield a much more complete temperature profile of the field. The TRT rig heats fluid and sends it into the ground, measuring the earth’s response and gauging certain properties. The fiber-optic probe will allow them to observe the whole depth of the field, layer by layer. Their approach, though not entirely unprecedented, is an example of collaborative innovation—and they will continue working to diagnose and understand the geofield behavior.
“We want to be able to provide recommendations for how the geothermal heat pump system can sustainably used, because there is more than one option,” says Herrera.
Among those options could be to use the geothermal system only as an emergency system, which is on standby until it's needed. It also could be operated continuously but at a reduced capacity. Ultimately, the team will seek strategies to ensure the system will benefit the WID as it supports its faculty and staff research community as well as the other occupants of the building.
“The WID was made to be a research building where we’re discovering new things,” says McDaniel. “Somewhat unexpectedly, its geothermal system has provided an opportunity for geologists and mechanical engineers to come together and conduct research to enable more effective geothermal designs in the future. If the building itself can tell us something that advances knowledge in the geothermal field, then maybe the system served a much broader purpose, instead of simply heating and cooling the building.”
In addition to Reindl, the research team includes geothermal expert Jim Tinjum, an associate professor of geological engineering and engineering professional development, and John Nelson, an adjunct professor of civil and environmental engineering. Kristin Murray and Erin Badger of the Wisconsin Alumni Research Foundation (WARF), and Todd Kiley from the campus facilities planning and management unit, provided essential support of the research team. WARF also provided funding for the fieldwork.