Challenges for Designing Closed-loop Geothermal Systems
Restricted (Penn State Only)
- Author:
- Liu, Sai
- Graduate Program:
- Energy and Mineral Engineering (PHD)
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- January 04, 2024
- Committee Members:
- Jeremy Gernand, Program Head/Chair
John Wang, Major Field Member
Arash Dahi Taleghani, Chair & Dissertation Advisor
Farshad Rajabipour, Outside Unit & Field Member
Derek Elsworth, Major Field Member - Keywords:
- Closed-loop geothermal system
Heat extraction
Dimensionless numbers
Polymer composites
Sealing material - Abstract:
- Closed-loop geothermal systems (CLGS) are a novel heat extraction technique not requiring continuous fluid supply. The mechanism controlling the efficiency of closed-loop systems has not been well analyzed, so it lacks theory basis to design them. Working fluid is circulated only inside the wellbore, so heat extraction efficiency can be limited, which brings challenges to extensive application of closed-loop systems. At high temperatures in geothermal reservoirs, polymer seals may experience thermal deformation and degradation, which could cause leaks of circulating fluid in the wellbore. To address these challenges, a systematic numerical study was first conducted to explore factors affecting heat extraction efficiency of closed-loop systems and approaches to improving the efficiency. Then, experimental work was conducted to develop high-temperature resistant polymer composites for sealing geothermal wells. With the finite element methods, a coupled three-dimensional model was established for a basic closed-loop geothermal system (CLGS) and an enhanced closed-loop geothermal system (ECLGS). Effects of critical design factors on heat extraction of CLGS were investigated quantitatively. Increasing the cement thermal conductivity enhances both produced fluid temperature and thermal power. The effect of cement thickness is correlated with the value of its thermal conductivity. As the circulation rate increases, the temperature of produced fluid decreases while thermal power increases asymptotically. Using a circulating fluid with high heat capacity would result in higher thermal power. A dimensionless analysis was conducted, through which two dimensionless numbers were derived to integrate the effects of different factors. With the dimensionless numbers, effects of different individual factors can be evaluated comprehensively, which brings convenience to parameter design. Heat extraction through the ECLGS was also simulated. The objective of the ECLGS is to increase the heat transfer rate from rock to working fluid by thermally-conductive fractures connected to the wellbore. Simulation results suggest that fractures could improve heat extraction performance of the system noticeably. There exists a critical fluid circulation rate beyond which net power starts to decrease. To further improve heat extraction, a conductive plug along fractures was defined and incorporated in the system. The conductive plug could further improve heat extraction performance significantly. Effects of plug thermal conductivity, plug size, and plug shape were ascertained. Finally, a dimensionless analysis was also performed for ECLGS, through which three dimensionless numbers were derived to integrate different affecting factors. Then, polymer composites for sealing geothermal were developed by compounding treated graphite with ethylene propylene diene monomer (EPDM) rubber. First, the performances of two types of graphite, small-size lamellar graphite (SFG15) and graphite nanoplatelets (GNP), were studied. Second, the performance of high concentrations of treated SFG15 was examined. Experimental results show that treated SFG15 and GNP both enhance the temperature resistance of EPDM by over 80℃. High concentrations (9 wt.%) of treated graphite remarkably enhances the storage modulus of and reduces the loss factor (tan δ) of EPDM at high temperatures. Adding treated graphite significantly enhances the specific heat capacity of EPDM and remarkably increases heat energy required to melt it. The onset degradation temperature of EPDM-SFG15 nanocomposites is up to 30 ℃ higher than that of plain EPDM. In this dissertation, the mechanism controlling performance of closed-loop systems and approaches to improving the performance were explored. The developed polymer nanocomposites, with enhanced mechanical and thermal properties, are a promising candidate for the material used to seal closed-loop geothermal wells. The results and conclusions achieved in this dissertation may provide a reference for optimally designing closed-loop systems.