Smarter storage: how TNO optimises thermal energy storage

Sustainable energy supply

To ensure a sustainable energy supply, further innovation is key—such as increasing storage temperatures, integrating systems more effectively into the subsurface, and managing seasonal fluctuations more intelligently. TNO is developing solutions to address these challenges and unlock the full potential of thermal energy storage.

The growth potential of aquifer thermal energy storage

Aquifer Thermal Energy Storage (ATES) has reliably provided heating and cooling to thousands of Dutch buildings for decades—from offices and hospitals to data centres and greenhouses. This proven technology offers a solid foundation, but the energy transition demands the next step. ATES systems use two or more wells to simultaneously extract and inject groundwater, storing and recovering heat by reversing the flow direction between wells.

Martin Bloemendal, TNO’s lead scientist for thermal energy storage, explains: ‘The potential of the subsurface is vast and still largely untapped. The two main challenges are increasing the temperature range and scaling up deployment. While standard ATES systems operate up to 25°C, higher temperatures open up new possibilities.

Systems operating between 25°C and 50°C are referred to as medium-temperature ATES (MT-ATES), and those above 50°C as high-temperature ATES (HT-ATES). These can store summer heat at 40–90°C for winter use, making them suitable for large-scale heating networks and industrial applications.’

A TNO benchmark study shows that HT-ATES becomes technically and financially viable at sufficient scale—around 300,000 m³ of stored hot water per year. But the benefits go beyond cost: HT-ATES strengthens heating networks by adding extra sources and replacing gas-fired peak boilers, directly contributing to CO₂ reduction.

Innovations in closed loop systems

In addition to open systems, TNO is also innovating in closed loop geothermal systems (CLGS). In 2024, around 130,000 homes in the Netherlands were heated using CLGS. These systems circulate fluid through a closed pipe network to exchange heat with the subsurface, without extracting groundwater.

TNO is exploring how CLGS can be extended to depths of 400–800 metres, potentially increasing capacity tenfold. While CLGS uses the subsurface as a thermal buffer, its storage capacity is more limited than ATES. That’s why TNO focuses on collective applications where groundwater safety is critical. We’re also working to make ATES more affordable for individual homes, though ATES remains the preferred solution for large-scale seasonal storage.

A shift in thinking is needed

To fully harness the potential of thermal energy storage, a shift in mindset is required. Martin continues: ‘Current system designs include large safety margins. Systems are oversized to account for uncertainties in usage and climate, and are spaced further apart than strictly necessary.

In areas with high demand for subsurface energy, this leads to unnecessary limitations. Permits act as virtual boundary markers underground—but these are often too wide.

We’re developing new technical and spatial planning concepts. By using subsurface space more intelligently, we can accommodate more systems without compromising reliability. This calls for innovative design methods, improved monitoring, and more flexible permitting and management.’

TNO also provides insights into which subsurface layers are suitable for energy storage, enabling the use of deeper, previously untapped layers.

Heat transition solutions

TNO is working on various solutions for the heat transition, such as optimising seasonal storage and technology.

Further optimisation of seasonal storage

Thermal energy storage naturally addresses seasonal fluctuations, but the challenge grows as we move away from gas. That’s why TNO is developing smarter ways to manage uncertainty. By linking multiple buildings to multiple storage systems, risks are spread. In an unusually warm winter followed by a cool summer, other users in an integrated system can absorb the fluctuations.

Increasingly, thermal storage is also used to tackle grid congestion. When there’s a surplus of wind or solar energy, power-to-heat systems convert electricity into heat for storage. This relieves pressure on the electricity grid and creates a buffer for periods with less renewable generation. TNO is working to improve power-to-heat systems and storage combinations.

Technical optimisation

Raising storage temperatures is a key research area. However, for systems above 25°C, there is no formal assessment framework due to potential environmental impacts. This makes permitting more complex. TNO is investigating how to increase temperatures without adverse environmental effects.

In Middenmeer, TNO is monitoring the long-term performance of the Netherlands’ first high-temperature storage system. At Utrecht Science Park, the ACCEL-UTES project links thermal storage to solar energy. These projects provide valuable insights for further temperature increases. At temperatures above 100°C, water remains liquid under sufficient pressure underground, opening up opportunities for industrial applications.

TNO is also developing technical innovations in drilling and well design.

Policy support and knowledge sharing

We advise provinces on permitting and contribute to regulatory discussions, such as the conservatively set 25°C threshold. With digital twins of the subsurface, governments can ‘see’ underground without digging, allowing for more flexible permitting.

Knowledge gained is widely shared through open tools such as The Design Toolkit for policymakers, ThermoGIS for assessing potential and risks of thermal storage and DINOloket for subsurface data.

Partnering with TNO

TNO welcomes collaboration with partners working on sustainable heating. We can support:

• Optimising supply and demand in seasonal fluctuations
• Complex subsurface challenges where standard solutions fall short
• Deployment and development of systems above 25°C
• Scaling up to neighbourhood level, where system integration, spatial planning and infrastructure converge
• Integration with heating networks, geothermal energy or power-to-heat
• Preventing grid congestion through smart use of thermal storage and power-to-heat
• New drilling technologies to accelerate the heat transition
• Policy support and regulatory advice

The heat transition calls for new ways of thinking and working. Our expertise in thermal energy storage contributes to this transition. Get in touch with TNO to explore how we can support your heating project.