Investigating mixed-circuit injection-extraction strategies between borehole heat exchangers in a cooling dominated system

Underground thermal energy storage (UTES) offers a promising yet underutilized solution for balancing supply and demand in heating and cooling applications. This is especially relevant within the UK’s decarbonization strategy. Applications such as data centres, which have significant cooling demands and generate waste heat, typically rely on grid electricity for cooling while ejecting heat into the atmosphere. However, UTES presents an opportunity to store this excess heat underground whilst meeting cooling demands. The stored energy can later be extracted for heating purposes, addressing both cooling and heating requirements simultaneously.

This study explores an innovative approach to meeting high cooling and heating demands using novel mixed-circuit borehole heat exchanger (BHE) arrays which combine heat exchange and storage. These arrays are particularly suited for applications with continuous cooling needs, such as data centres. This approach is designed to balance the near-constant cooling loads of data centres with the variable heating demands of nearby residential heating networks, using the ground as an energy buffer. The proposed system employs a network of closed-loop BHEs, where fluid circulates through the subsurface, transferring heat via conduction through the borehole wall. Acting as a temporary UTES buffer, the subsurface enables simultaneous heat injection and extraction in a mixed circuit to meet end-user demands.

To test this concept, mixed-circuit BHE arrays will be trialled at the UK Geoenergy Observatories. During a short-duration experiment, heat and cooling energy will be simultaneously injected and extracted within a shared array, recording thermal plume propagation and fluid temperature within the BHEs. The resulting data will be analysed and used to validate numerical models, providing insights into the feasibility of large-scale mixed-circuit BHE arrays for UTES. These models will contribute to optimizing mixed-circuit arrays for energy security, decarbonizing the heating and cooling sectors, and improving the understanding of UTES systems.

By integrating trial data, the study aims to develop scalable solutions for mixed-circuit BHE arrays, offering cost-effective continuous passive cooling while meeting heating demands. It will focus on optimizing system design, controls, and array performance. The key innovation lies in the real-time integration of cooling and heating within a single system, enabling flexible operation that aligns the steady cooling demands of data centres with the variable heating needs of district networks.