THERMAL ENHANCEMENT OF BENTONITE-BASED GROUTING MATERIAL BY USING CARBON FIBERS IN GROUND HEAT PUMP SYSTEM

Abstract

With regard to the global climate changes, green energy and smart resource consumption are highly demanded. There is a great opportunity to make the applications of Ground Heat Pump Systems (GHPS) that take the advantage of green and renewable geothermal resources to save energy consumption, more popular. In GHPS, the thermal performance of bentonite-based grouting material is a key factor that affects the system efficiency. This research focuses on the enhancement of thermal conductivity and specific heat capacity of bentonite-based grouting material by adding different types and volumetric fractions of carbon fiber. A standard bentonite-sand mix set as a baseline grout was used to assess the additives’ effect. Five carbon fiber types supplied by Asbury Carbons, AGM94-0.1mm/3mm/6mm, AGM95-6mm, and AGM99-0.15mm, were investigated in this research. The volumetric fractions of carbon fibers added to the baseline grout ranged from 0.5% to 1.25%. The laboratory research results demonstrated that the grout thermal conductivity could be increased efficiently by adding higher volumetric percentage and longer carbon fibers. Also, it was found that the greater the fiber aspect ratio, the higher the grout thermal conductivity was. AGM94-6mm carbon fiber had the best enhancement effect on grout thermal performance among the five fibers, although shorter carbon fibers performed more consistently. AGM95-6mm carbon fiber had a similar performance to AGM94-3mm fiber due to their close aspect ratio and its relative softness. The addition of carbon fibers was not found affect the specific heat capacity of composite grouting material significantly. A series of on-site bench scale experiments from McClymont & Rak Engineers Inc. was conducted for the potential industrial value of this composite grouting material in GHPS field. Numerical models of this bench scale experiment were also built and simulated using ABAQUS/CAE v6.14. The results were found to be consistent with what is obtained from the field experiments.