Comparative numerical modeling complemented with multi-objective optimization and dynamic life cycle assessment of coaxial ground heat exchangers with oval-shaped and typical circular-shaped configurations

Abstract

Geothermal energy showcases significant potential as a sustainable energy alternative employed for direct applications such as space heating and cooling, industrial processes, and greenhouse heating. However, despite these favorable attributes, conventional Ground-coupled Heat Pumps (GCHPs) can potentially result in significant environmental and economic consequences, including greenhouse gases and high operational costs. The Coaxial Ground Heat Exchanger (CGHE) is the chief role component for harnessing geothermal energy in GCHPs. This investigation provides comprehensive numerical modeling complemented with multi-objective optimization and seasonal life cycle assessment of CGHEs with newly developed oval-shaped (oval-CGHEs) and typical circular-shaped (circle-CGHE). The system design and dispatch of the oval-CGHEs are optimized by considering a novel optimization approach that synergistically encompasses four optimization scenarios, namely minimum Ground Heat Exchanger (GHE) number and minimum GHE length, each with multi-material topology optimization of outer tube selection criteria. The system's Life Cycle Energy Consumption (LCEC), Life Cycle Costs (LCC), and Life Cycle CO₂ emissions (LCCO₂) are introduced in the optimization stage of the oval-CGHEs and compared with typical circle-CGHE. Toward these goals, a series of three-dimensional simulations coupled with a pulse-load finite line source model is executed and validated using the authors' practical field experiment based on combined analyses, including the energy, economic, and environmental multi-criteria, to assess the different objectives and case studies. The optimization results reveal that oval-CGHEs, particularly with inner tube position optimization, consistently outperform the traditional circle-CGHEs in all the investigated scenarios. It is revealed that the most favorable scenario involves substituting traditional circle-CGHEs with oval-shaped CGHEs, utilizing a high-density polyethylene inner tube and a steel outer tube, and adopting a minimum GHE length. This approach remarkably reduces the required length of the GHEs by up to 15.00% and achieves the most substantial reductions in LCEC by up to 13.87%, LCC by up to 10.91%, and LCCO₂ emissions by up to 13.27%, respectively.