Decarbonizing Heating and Cooling with Deep Borehole Technology
Decarbonizing heating and cooling is fundamental to realizing a net-zero carbon emissions energy system. Yet, space heating in the residential and public sectors continues to be sourced largely by natural gas, despite the availability of sustainable alternative heat sources. Geothermal energy has emerged as a feasible alternative, with the potential to deliver baseload electricity generation, direct heating/cooling, and thermal energy storage, independent of weather conditions.
Conventional open-loop geothermal systems face significant barriers, including high initial drilling costs, geological risks, and potential environmental impacts like induced seismicity. Closed-loop systems, such as deep borehole heat exchangers (DBHEs), offer a compelling alternative by removing direct hydraulic interactions with the subsurface. These systems rely on conductive heat transfer between a heat transfer fluid and the surrounding geological formations through the borehole walls, significantly reducing technical risks.
Modeling Methodologies for Deep Borehole Heat Exchangers
Numerical models have become an integral part of DBHE research, providing a flexible approach to simulating complex systems while maintaining accuracy compared to analytical models. These models typically employ a dual-continuum approach, discretizing the DBHE and the surrounding formation using finite element, finite difference, or finite volume methods.
Numerical models generally compare well to analytical models, such as Beier’s solution, while maintaining more flexibility in handling boundary conditions and dynamic thermal plume simulations. However, the increased computational resources required for numerical models can be a limitation. Researchers have explored the use of surrogate models, such as genetic algorithms and one-at-a-time sensitivity analyses, to optimize DBHE design parameters and reduce the computational burden.
In-situ geological parameters, such as thermal conductivity and hydraulic conductivity, cannot be readily modified without resorting to well stimulation techniques (e.g., hydraulic or chemical stimulation). However, engineering system parameters, such as the mass flow rate of the heat transfer fluid, can be optimized to increase thermal yield, overall system performance, and minimize pressure drops.
Heat Extraction Modeling and Performance
Numerical modeling studies have provided valuable insights into the factors influencing DBHE heat extraction performance. Key parameters that positively impact thermal power include increased formation thermal conductivity, depth, geothermal gradient, and grout thermal conductivity. Volumetric heat capacity has a limited influence on thermal power but may impact the system’s thermal recovery.
Groundwater flow can positively influence heat extraction only when the velocity exceeds 1×10^-7 m/s, a threshold unlikely to be encountered at depth in the subsurface. Thermal interference in DBHE arrays has a negative impact on extraction-only systems, with a line array performing better than a square array configuration.
While significant progress has been made, gaps remain in the literature, such as limited studies on the effects of geological heterogeneity on heat extraction and the operation, performance, hydraulics, and system coupling (surface and subsurface) of DBHE arrays.
Deep Borehole Thermal Energy Storage (DBTES)
In addition to heat extraction, the potential for deep borehole thermal energy storage (DBTES) has been explored by several authors, with the Darmstadt SKEWS project providing valuable insights. Modeling studies have shown that the thermal recovery factor (TRF), a common metric for assessing DBTES performance, tends to increase with the length and number of boreholes in the array.
However, increased rock thermal conductivity and hydraulic conductivity can result in lower TRFs due to thermal losses. Shallow BTES arrays typically outperform sparse arrays of deep boreholes for the same total drilled length, as the thermal interaction between boreholes in the shallow array is more productive.
Despite these insights, DBTES projects are incentivized by their reduced surface footprint, the ability to utilize high-temperature storage sources (especially with shallow section insulation), and minimized dependency on geological formations and groundwater flow. Nonetheless, the technical advantage of DBTES over shallow BTES systems appears limited at present, and further research is needed to fully understand the potential of these systems.
Metrics for Assessing Thermal Energy Storage Performance
The literature on DBTES performance suffers from a lack of consistency in the metrics used to report efficiency, making comparisons between studies challenging. To address this, a set of recommended metrics is provided:
- Thermal Recovery Factor (TRF): Ratio of total heat extracted during discharging to total heat injected during charging.
- Thermal Storage Efficiency (TSE): Ratio of the difference in total energy extracted with and without a storage charging period to the total energy injected during charging.
- Thermal Charging Efficiency (TCE): Ratio of total energy injected during charging to the total energy inputted at the wellhead.
- Thermal Recovery Advantage (TRA): Ratio of the difference in total annual energy extracted with and without charging to the total energy extracted without charging.
- Thermal Accumulation Efficiency (TAE): Ratio of annual heat stored to the amount injected during charging.
- Heat Storage Yield Ratio (HSYR): Ratio of the net energy gain in the service life to the electrical power consumed by storage operations.
Consistent use of these metrics will enable more meaningful comparisons of DBTES system performance across studies.
Heating and Cooling Applications of Deep Borehole Heat Exchangers
Conventional DBHE systems are primarily suited for heating applications due to the high bottom-hole temperatures, which can make cooling challenging. To address this, researchers have explored concepts that enable the use of the same DBHE for both heating and cooling, such as employing a check valve or an additional intermediate layer to restrict the cooling mode to the upper section of the DBHE.
Non-uniform central pipe insulation has also been proposed to enable cooling in the summer and heating in the winter. These modifications aim to produce working temperatures more suitable for both cooling and heating, typically in the range of 40°C to 50°C, compared to the very high (85°C – 90°C) or very low temperatures required for conventional DBHE systems.
Real-World DBHE Projects and Case Studies
The active field of DBHE research is generating a growing number of case studies, particularly in areas with low-cost drilling supply chains or abandoned hydrocarbon or geothermal wells suitable for repurposing. Some notable projects include:
- iHarbour, Xi’an, China: A large-scale DBHE array heating system with 91 boreholes, each 2500 m deep, capable of providing 75.69 MW of heating capacity.
- SKEWS Project, Darmstadt, Germany: A medium-deep BTES system with a planned array of up to 37 boreholes, each 750 m deep, investigating the impact of geological parameters on storage performance.
- Newcastle Science Central, UK: A repurposed geothermal exploration well, the Newcastle Science Central Deep Geothermal Borehole, being assessed for its DBHE heat extraction and storage potential.
- Eden Project, UK: A deep DBHE system that is expected to deliver 85°C outlet temperatures.
- Kirby Misperton, UK: A repurposed onshore hydrocarbon well projected to provide 300 kW of thermal power.
These case studies highlight the growing interest and practical applications of DBHE technology across various regions and settings.
Conclusions and Future Research Directions
Deep borehole heat exchangers have emerged as a promising technology for decarbonizing heating and cooling, offering a viable alternative to conventional open-loop geothermal systems. Numerical modeling has played a crucial role in advancing DBHE research, providing insights into heat extraction performance, thermal energy storage, and system optimization.
While significant progress has been made, several areas require further investigation, including the effects of geological heterogeneity, DBHE array configurations, boundary conditions, and modes of operation. Consistency in the reporting of DBHE and DBTES performance metrics is also essential to enable meaningful comparisons across studies.
As the DBHE market continues to evolve, innovative design modifications, such as the use of check valves, intermediate layers, and non-uniform insulation, have shown promise in addressing the challenges of high bottom-hole temperatures and enabling both heating and cooling applications. These advancements, coupled with the growing number of real-world case studies, highlight the potential of deep borehole technology to contribute to the transition towards a sustainable, low-carbon energy future.
