Life cycle techno-economic analysis and optimization framework of hybrid deep ground source heat pump system for low carbon buildings

Abstract

Abstract

This study develops a life-cycle techno-economic analysis and optimization framework for hybrid deep ground source heat pump (GSHP) systems, addressing the limitations of existing evaluations that often neglect subsurface thermal attenuation and rely on simplified heat transfer models. A coupled heat transfer model is established by combining the finite difference method for the borehole interior with the segmented finite line-source model for the surrounding ground, enabling long-term simulation of fluid and ground temperature evolution. The energy model is integrated with a life-cycle economic evaluation to calculate net present value, levelized cost of heating/cooling, payback period (PP), and internal rate of return. A Bayesian optimization algorithm is further employed to identify optimal system configurations under multiple objectives. The framework is applied to a hybrid geothermal system in Xi’an, China, incorporating mid-deep boreholes with auxiliary air-source heat pumps or gas boilers. Results show that long-term thermal attenuation significantly affects economic performance, with a 2 °C increase in fluid temperature extending the PP by approximately 0.3 years. User-side load uncertainty also has a substantial impact, as a 10% deviation from the design load can shift the optimal PP by about 2 years. Sensitivity analysis identifies the GSHP installed capacity ratio and borehole number as the most influential parameters. Compared with pure geothermal systems, the hybrid geothermal–gas boiler configuration achieves the shortest PP, demonstrating the framework’s effectiveness for robust low-carbon system design.