Thermal and Mechanical Improvement of Filling Mixture for Shallow Geothermal Systems by Recycling of Carbon Fiber Waste

Abstract

The reuse of waste materials such as carbon fiber (CF) as filling additive for closed-loop vertical geothermal probes in shallow geothermal systems has been evaluated as a new grout mixture for the improvement of geothermal energy systems efficiency and a sustainable supply of raw materials from special waste. The study evaluates the improvement in both thermal exchange characteristics and mechanical properties of the filling grout for geothermal purposes through the addition of 5% of CF to standard (ST) materials currently on the market. Uniaxial and flexural tests investigating the material response after 14 and 28 days from sample preparation on samples of both standard and mixed grout material as well as non-stationary hot wire method were used to define the thermal conductivity for both the standard and innovative mixtures. The experimental analysis provides evidence for increasing the thermal conductivity by about 3.5% with respect to standard materials. Even the mechanical properties are better in the innovative mixture, being the compressive strength 187% higher and flexural strength 81% higher than standard materials. The obtained results become useful for the optimization of low enthalpy geothermal systems and mostly for the design of the vertical heat exchange system in terms of depth/number of installed probes. Principally, thermal conductivity improvements result in a reduction of about 24% of the geothermal exchanger’s length, affecting the economic advantages in the implementation of the entire system. A simple analysis of the reuse of CF waste shows the reduction of industrial waste and the simultaneous elimination of disposal costs, defining new perspectives for industrial waste management. This research provides essential elements for the development of a circular economy and is well integrated with the European challenges about the End of Waste process and reduction of environmental impact, suggesting new perspectives for economic development and sectorial work.

Keywords

• shallow geothermal systems
• thermal conductivity
• uniaxial compressive strength
• flexural strength
• carbon fiber
• circular economy