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Geo-storage | Hot BTES Explainer Video
Geo-storage | Cool BTES Explainer Video
Borehole Thermal Energy Storage (BTES):
A Scalable Solution for Seasonal Heat Management
01.
The Storage Imperative
in Decarbonized Energy Systems
Borehole Thermal Energy Storage (BTES) is a proven technology for long-term thermal energy management, especially in applications like district heating and cooling. Its ability to provide long-term seasonal storage, heating and cooling using low-cost, off-peak electricity, and aid in reducing greenhouse gases highlights the importance of BTES implementation. Numerous energy reports emphasize the need for both electrical and thermal energy storage, critical for resilient urban energy systems. Seasonal thermal energy storage (STES), particularly BTES, addresses peak load shaving and load shifting, both essential for grid stability in renewable-based systems.
02.
Overview of Energy Storage Technologies
Energy storage technologies can be categorized into electrical and thermal domains. While electrical storage is ideal for short-term peak shaving, thermal storage offers lower costs and is better suited for heating and cooling applications, while also being resilient in long-term operations. Underground Thermal Energy Storage (UTES) systems, such as ATES, PTES, and BTES, store thermal energy in the subsurface environment. While ATES depends on aquifers and is location-dependent, and PTES requires excavation, BTES is relatively versatile and can be implemented in most locations. BTES is a flexible, low-maintenance storage medium with scalable potential.
03.
What is BTES?
The foundations of BTES can be traced back to research done in the 1980s and has only grown as an emerging topic. BTES systems can store both heat and coolth using a network of vertical boreholes drilled into the ground. These boreholes can be filled with single or double U-tube, or coaxial heat exchangers, where a heat transfer fluid flows, allowing for heat to flow between the boreholes and the ground, dependent on the type of BTES used. What makes BTES unique is how the pipes within the ground are connected. A series of radially connected piping loops are connected to create "rings" of flow loops which can be used to control the fluid flow radially within the borehole layout.
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BTES vs. Geo-exchange
While both BTES and geo-exchange systems use the ground as a thermal reservoir, they have distinct objectives and design principles. Geo-exchange focuses on real-time thermal exchange for immediate building loads, aiming to maintain ground temperature balance year-round. In contrast, BTES is designed for seasonal energy storage, actively charged over months with external sources like solar thermal or industrial waste heat, to be discharged when needed. BTES can reach much higher temperatures (80°C for heating) and lower temperatures (0°C for cooling) than geo-exchange (5–25°C). Despite sharing components, their operational strategies, performance metrics, and integration approaches vary significantly. Geo-exchange is driven by immediate demand, while BTES is a strategic, actively charged storage solution. This difference is crucial for system design and control.
04.
Design Considerations in BTES Systems
Designing an efficient BTES system involves optimizing a large set of parameters including borehole spacing, depth, pipe selection, and the surface area-to-volume (SA/V) ratio. These parameters are designed in accordance with the soil conditions where the BTES is operating, with the main design factor being soil thermal conductivity which affects the rate at which heat moves within the soil.
Another crucial factor is the design of a "charged thermal core," which acts as the central reservoir in the borehole field. The outer layers act as a thermal buffer, reducing losses to surrounding soil. This means that when charging the BTES field, either hot or cold, the greatest temperature difference occurs within the center and cascades outwards.
When analyzing performance parameters, some of the most important factors to consider include: soil thermal conductivity, borehole spacing, heat injection temperature, borehole depth, and flow rate.
02.
Current and BTES Installations
BTES is a proven and already implemented design in numerous countries, with many systems operational worldwide. A compiled list of openly available BTES projects indicates 4 installations in Canada, 5 in the United States, 4 in Finland, 12 in Sweden, 5 in Norway, 4 in Germany, 5 in Switzerland, 3 in China, and 7 in other locations. However, this list does not include every installation, as some projects are not readily available or are described under a different name than BTES, meaning the total number of operational systems is greater than what is listed.