A review of vertical closed-loop geothermal heating and cooling systems with an Emphasis on the importance of the subsurface

Geothermal heat pump (GHP) systems have been established as a proven technology for cooling and heating residential, public and commercial buildings. There is a geothermal solution to the ambitious goal of decarbonizing the space heating and cooling, which is contingent on the successful deployment of the GHP technology. This in turn requires accurate site characterization, sound design methodologies, effective control logic, and short and long-term (life-cycle) performance analysis and optimization. In this article, we review the aforementioned aspects of the vertical closed-loop GHPs specifically focusing on the important role of the subsurface. The basics of GHP technology are introduced along with relevant trends and statistics. GHPs are compared with similar technologies such as air source heat pumps (ASHP) along with the effects of deployment on the grid peak load. We then review the common system architectures and the growing trends for deeper boreholes and the drivers behind it. Various methods for design, sizing, and simulation of GHPs are introduced along with software tools common in research and industry. We then move to subsurface characterization, drilling and well construction of vertical boreholes. Long-term performance monitoring for GHP systems is an important source of information for model validation and engineering design and is garnering increasing attention recently. Data science is another field that is growing rapidly with its methods increasingly utilized in GHP applications. The environmental aspect of GHPs is briefly reviewed. Finally, concluding remarks are given to summarize the review and highlight the potential of petroleum engineering expertise and methods in GHP applications.