District-Scale Ground Heat Exchange Performance with Fiber Optic Distributed Temperature Sensing

fiber optics;geothermal;distributed temperature sensing;ground heat exchange

McDaniel, Adam LouisTinjum, James M. ;

University of Wisconsin

关键词: fiber optics;    geothermal;    distributed temperature sensing;    ground heat exchange;   

Others  :  https://minds.wisconsin.edu/bitstream/handle/1793/77743/MS_Thesis_McDaniel_Adam.pdf?sequence=1&isAllowed=y

瑞士|英语

来源: University of Wisconsin

【 摘 要 】

Chapter 1: IntroductionGround-source heat pumps (GSHP) systems have great potential as an energy-efficient alternative to conventional space heating and cooling systems. The basic principal behind GSHP technology is cyclical energy storage in the subsurface—energy injection into the ground during summer to cool buildings, and extraction of that same energy during winter to provide space and water heating. When operating properly, efficiently transferring energy within a GSHP system is operationally more efficient than burning fossil fuels to produce single-use heat in the winter, or using low-efficiency air conditioner units when air temperatures are warmer. Due to high initial borefield costs, the installation of a GSHP system is often controlled by its payback period—the time until operational savings has accumulated to equal the additional capital investment. There exists some debate about true GSHP effectiveness and efficiency due to historic case studies where there was difficulty in accurately predicting lifetime performance. This difficulty stems from the inherent complexity of subsurface heat transfer. The subsurface chosen for energy storage is often comprised of distinct geologic units that may vary widely in properties that drive heat transfer; such as mineral constituents, grain orientation, density, porosity, saturation, groundwater flow, fractures, and voids. Due to the multitude of variables in play and the practical difficulties in observing subsurface processes, there is a lack of robust quantification of how heat is transferred in the subsurface.This work begins in Chapter 2 with an Introduction that describes the fundamentals of GSHP systems, overviews the design and predictive methodology, and provides thorough background on the primary tool used in this study to advance subsurface heat transfer mechanisms: fiber optic distributed temperature sensing (DTS). Previous work using DTS as a tool to investigate GSHP performance is described, paying special attention to the development of the distributed thermal response test (DTRT) for in situ measurement of distributed heattransfer. Finally, a short case study is presented that highlights the tangible benefits of understanding the distributed nature of subsurface heat transfer.Chapter 3 presents an in-depth study on the calibration of a fiber optic DTS network installed in a district-scale borefield at Epic Systems in Verona, WI. DTS is a powerful tool with great advantages that also requires a significant investment of time and resources to realize its full potential. Among expanding environmental monitoring applications, the installation of an 11.2-km network of buried fiber optic cables for long-term monitoring is a demanding DTS application that has a unique set of obstacles to achieving consistently accurate temperature data. The design of a dynamic, double-ended, centralized, and remotely accessible calibration routine is carefully described. As an increasingly popular tool, DTS has significant discussion in literature regarding calibration techniques. This paper seeks to build contribute to that discussion to document a novel method of splice and cable end-point location, and to use long-term calibrated results to consider optimal combinations of calibration baths, compare how calibration parameters vary in space in time, and discuss the importance of combing forward and reverse signals in a double-ended configuration.The results of the DTS calibration within Epic’s borefield 4 are presented in Chapter 4. One of the largest borefields of its kind, borefield 4 has exceptional potential for economic and environmental benefits. The heat injection and extraction loads into borefield 4 were monitored from January 2015 through March 2017 and found to be heavily imbalanced towards heat injection. The net heating loads and heterogeneous sedimentary bedrock of borefield 4 with variable thermophysical properties and groundwater conditions provides an optimal field laboratory to study subsurface heat transfer mechanisms. Rock cores from the geologic units found in borefield 4 were analyzed in the laboratory by Meyer (2013) for thermal conductivity using guarded-comparative-longitudinal heat flow experiments (ASTM E1225) and specific heat capacity using ;;coffee-cup calorimetry.” Estimates of bedrock density, porosity, and hydraulic conductivity were taken from the Dane County Regional Flow Model (Parsen et al. 2016). These thermophysical and hydraulic bedrock properties were used as a basis of interpretation for distributed subsurface heat flow rates, as observed by the fiber optic temperature monitoring.Chapter 5 of this thesis focuses on the design and implementation of a DTRT to analyze in situ distributed thermal properties of heterogeneous lithology in central Illinois. A pilot borehole was drilled, cored, and installed with fiber optics within the u-pipe ground heat exchanger (GHX), grouted between the GHX and the borehole wall, and 5 m beneath the GHX. The cores were measured in the laboratory for thermal conductivity using a KD2-Pro Thermal Properties Analyzer (Decagon Devices), gravimetric water content by ASTM D2216-10, and bulk density by ASTM D7263-09. The custom-made rig was designed to conduct both a conventional TRT and a DTRT. The conventional TRT data analysis presented by Raymond et al. (2011) and an error minimization technique were used to find average ;;effective’ subsurface thermal properties. A novel analogy to the Molz et al. (1987) impeller-meter pumping test for distributed hydraulic conductivity was used to calculate thermal conductivity during the heat injection portion of the TRT. Thermal conductivities from the laboratory testing, 1-m-resolution fiber data, and fiber data sectioned by heat transfer rates were compared to known geologic conditions. Distributed heat decay was observed after the conclusion of the test to investigate differential cooling processes.Appendices 1 through 5 document related research that was performed, but not to the extent of a stand-alone paper as in the case of Chapters 3 through 5. Appendix 1 summarizes work to quantify energy flows at Epic’s campus at the building, intermediate campus, and total campus scales. Appendix 2 documents Epic’s total campus energy balance and provides comments on the current state of that research. Appendix 3 investigates the performance of a traditional u-pipe and a coaxial GHX installed nearby Epic’s borefield 4 using fiber optics and traditional ΔT methods. Appendix 4 describes an investigative study performed on the overheated borefield of the Wisconsin Institutes for Discovery (WID) building. Appendix 5 provides documentation of Epic remote connection methodology and calibration Matlab scripts.