| Title | Workflow for Optimizing the Design of Close-Loop Deep Borehole Heat Exchangers |
|---|---|
| Authors | Martin PUJOL, Dimitri AYMARD |
| Year | 2020 |
| Conference | World Geothermal Congress |
| Keywords | close-loop, deep borehole heat exchanger, FEHM, direct-use, Australia, modelling |
| Abstract | Borehole heat exchangers have been used for heating and cooling of buildings and thermal storage for many years. The majority of systems are constructed of polyethylene pipe at depths that typically do not exceed 200 m. A fluid carrier (typically water) is continually circulated in the pipe allowing heat exchange between the bore and the surrounding rock mass in a geothermal closed loop. The borehole may also operate in heating only or cooling only modes. Then, the temperature is regenerated via heat transfer through conduction with the neighboring rocks in most of the geological contexts. If permeability exists, the heat can also be removed and the temperature conditions regenerated through advection/convection. Some borehole heat exchangers have been constructed at greater depths; up to 3000 m worldwide. Deep borehole heat exchangers rely on circulating the fluid carrier down an open ended tubing inserted to the full depth of the borehole with heated water returning to surface via the annular space between the tubing and the bore casing though the reverse circulation mode is also possible. However, the number of deep borehole heat exchanger projects worldwide is limited and little applied research has been published on the ways to maximize the efficiency of deep borehole heat exchangers, for example the material selection for the tubing or the optimal geometry to meet the energy requirements cost-effectively. Unlike conventional geothermal projects, borehole heat exchangers do not require high permeability values to be successful. This and other characteristics of the technology (such as scalability, retrofitting potential, etc.) have led to a renewed interest into deep borehole heat exchangers. In this study we present a workflow for optimizing the design of deep borehole heat exchangers and answering the basic question of long term operation using a real-world projects the authors are currently working on in Australia. The coupled thermo-hydro-mechanical simulator FEHM (Finite Element Heat and Mass transfer code) was used to model the deep borehole heat exchanger using appropriate python library tools. |