Fall 2017 Final Report Submitted to the Student Sustainability Committee (SSC)
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Geothermal Test Well (In Progress)
The main objective is to provide comprehensive scientific data and analysis to help our community on evaluating the potential of using ground source heat pump system in a large scale as part of campus green energy policy.
The project will collect baseline subsurface thermal data that is required for evaluating geothermal exchange alternatives. Typically, this data is collected by contractors for a limited time during the design phase of geothermal projects (a week or less). This project will collect data over an entire calendar year to identify impacts of seasonal heating and cooling on the subsurface. In a current geothermal study in the Champaign-Urbana, Lin et al (2015a, b) have found a different thermal profile in the upper 100 m than predicted from the standard thermal transport model (Banks, 2012). We intend to use the same fiber-optic distributed temperature sensing (FO-DTS) system to perform a geothermal test on campus. This project will collect temperature data in high spatiotemporal resolutions (1-m and 0.1°C) using a fiber-optic distributed temperature sensing (FO-DTS) system. Currently, PI Lin is leading a research team from the ISGS that is employing FO-DTS to investigate temperature variations at the surface and in the subsurface in Illinois, and internationally. On-going work for an NSF-funded project in east-central Illinois has demonstrated subsurface temperature is more variable than previously thought (Lin et al., 2015 a;b). Therefore, the analysis conducted by coupling FO-DTS with the heat exchanger is expected to provide greater insight into the feasibility of geothermal energy on campus compared with the current thermal conductivity test. The industry-standard thermal response test uses only a subsurface heat exchanger to provide the average initial temperature and the average ground conductivity for the entire length of one U-bend heat exchanger installed at the depth we are requesting (100 m, in this case). After consulting the University of Illinois Facilities and Services, it was recommended the test site should not be located near the steam tunnel system and somewhere where the technology could be cost effective. Therefore, the University of Illinois Energy Farm on south campus was proposed for the test site. Based on existing expertise on operating a ground heat exchanger and thermal property measurements at UW-Madison, the Wisconsin collaborator will work with the Illinois team on performing heat exchange testing on campus and determining the thermal conductivity of the glacial material from the Energy Farm by using the guarded-comparative-longitudinal heat flow apparatus (or similar column apparatus) in the laboratory at UW.
In addition to a high-resolution temperature profile with depth, we can analyze the drilling mobilization cost and the cost of construction per foot of depth of the installed vertical loop. Therefore, we would be able to determine the optimal depth for the installed vertical loop with respect to the cost of installation (maximize: delta T/(Total well cost/well depth)) or equivalently to state, minimize: cost per ft/delta T. This will allow us to select the optimum depth for a single well or well field and determine if a larger number of shorter wells is more cost effective than a fewer number of deep wells to achieve the same amount of heat transfer based on seasonal heat capacity. Note, if a geologic profile is highly saturated with transient water, the efficiency of heat sink is significantly increased. Therefore, the optimization depth may be controlled by the saturated depth. The shallow geology (to 100 m depth) on campus is mainly clayey, hard glacial till (Burch et al. 1999), which lies below the water table and is saturated. These conditions are ideal for utilizing the earth’s geothermal property, which can provide very high efficient on thermal conductivity like many other areas in Midwest region (Walker et al., 2015; Cartwright 1968). Therefore, the expected outcomes including (1) the comprehensive thermal property measurements, (2) subsurface temperature profile in high spatiotemporal resolutions, and (3) optimal cost analysis of vertical closed loop installation for ground source heat pumps, which can be used to evaluate geothermal energy contribution for future energy planning on campus.