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Accelerating the Uptake of Large Thermal Energy Storages

  • Task 45
  • Currently running
  • Energy storage in energy systems

Q&A bank

Recordings of Q&A sessions

Q&A session 1.1

Planning, legal, regulatory and societal aspects

No question answered in this category yet

Technical aspects (feasibility, design, tender, monitoring, etc.)

Design (PTES, LTES)
What is the energy storage medium in a PTES?

Treated water is typically the storage medium in PTES systems, with a pH of about 9.7 and has a minimal salt and oxygen content (to prevent corrosion). Using a mixture of water with another fluid would most likely be more expensive

Answered during the first webinar of IEA-ES Task 45, see following link

Are phase change materials used in any LTES?

No. Until now, large-scale thermal energy storage systems have primarily used soil, aquifers, or pure water as the storage medium. They are both more cost effective and simple to handle (heat up and cool down) than PCMs, which are typically applied in compact systems. Another issue is that some LTES are underground and not a perfectly closed system – there is always some leakage of the storage medium (typically water) into the surrounding ground. Using PCM as storage medium would complicate environmental permitting and raise concerns about potential impacts, making implementation difficult.
An exception is when water itself is considered as a PCM (freezing at 0 °C). However, such systems have only been theoretically investigated.

For large-scale systems at higher temperatures, molten salts are used, but do not use the phase change as a part of the energy storage process (but rather maintain the salts in a molten state). These storages fall however outside the scope of typical low-to-medium temperature thermal energy storage systems.

Answered during the first webinar of IEA-ES Task 45, see following link

Is there any PTES system designed to operate in northern climates, such as regions with heavy snowfall and prolonged cold periods?

Although no PTES has yet been implemented with a specific design to withstand very cold climate, 2 kinds of solutions are possible:

1/ Covers with heating/melting elements:
There the aim is to actively melt snow or ice forming on a given surface (which could be the PTES cover/lid). Surface-level heat exchangers embedded in the lid could be used to melt snow, and use some of the heat stored in the storage as a heat source. Such a concept has been studied IEA-ES Task 38, originally designed for de-icing roads using thermal energy storage

2/ Mechanically Improved Covers:
Concepts like those in Austria’s GigaTES project aim to reuse the PTES surface for urban functions (e.g., parking). Structural enhancements include bloating elements as one possible solution. Other concepts are specifically designed to withstand snow-load and support machinery for snow removal if the snow layer increases over a certain threshold. These designs are still under development (e.g. within TREASURE), with a notable project currently being built in Finland (Hyvinkää).

Answered during the first webinar of IEA-ES Task 45, see following link

How are PTES usually integrated in DHNs requiring temperatures above 100°C? What is the typical temperature range for the storage in these cases?

As of 2025, all PTES have operated below 100°C, and this will likely remain the case. Since PTES function at atmospheric pressure, exceeding 100°C would cause the water to boil, which is not desirable. Another important limitation is material durability. To ensure long-term performance and avoid degradation, the storage temperature is typically kept below 90-95°C. On the lower end, the temperature can drop to the freezing point of water, but not below. When higher temperatures are needed for the district heating network, the storage alone may not be sufficient. In such cases, post-heating is required. There are two main approaches:

  • Simple post-heating using a boiler – this could be an electric boiler or another type, which heats up the water after it leaves the storage.
  • A more efficient solution involves using an electric heat pump. This not only raises the temperature of the outgoing water but also cools the lower-temperature water in the storage, improving overall system efficiency.

As an example, a PTES demonstrator project is currently being designed in Vienna as part of the EU-funded Treasure Project. The storage will operate in combination with a heat pump to supply heat to a district heating network that requires a supply temperature of around 120°C. Since the storage itself cannot reach such high temperatures directly, the heat pump is used to boost the temperature of the extracted heat before it is delivered to the grid.

Answered during the first Q&A session of IEA-ES Task 45, see following link

Application (LTES)
In terms of seasonal variability and cost competitiveness, which LTES technologies can be considered most promising in the future?

For purely seasonal storage with stable demand profiles, ATES or BTES are the most appropriate choices – provided there is sufficient heat demand and suitable subsurface conditions, although PTES can also be used as a purely seasonal storage. In cases with more variable demand, PTES or a hybrid approach – combining ATES or BTES with TTES – can help manage daily fluctuations more effectively. The hybrid approach also increases the total investment costs.
The choice of technology depends heavily on local geological and geotechnical considerations. For example, ATES requires the presence of suitable aquifers, while shallow BTES may not be feasible in areas with high groundwater levels. In any case, a preliminary techno-economic assessment is essential to determine the most suitable solution for each specific context. Some specifics about each technology can be mentioned:

  • ATES has the advantage of being able to be used both for heating during winter and cooling during summer
  • ATES and BTES have the advantage of having limited above-ground footprint
  • PTES has the advantage of being more reactive and able to function as a daily/weekly storage

Answered during the first webinar of IEA-ES Task 45, see following link

Cost (ATES, BTES, PTES, LTES)
Between planning and construction of LTES, costs can increase significantly (due to delay(s), inflation, etc.). How can we ensure that heat generation costs remain manageable and that end-user prices for district heating do not increase to the point where the system becomes economically unattractive?

Guaranteeing future energy prices is generally not possible. The energy prices for the charge of a thermal energy storage might vary over time. When it comes to the thermal energy storage itself, once the tendering results are known, future cost of storing heat can be estimated, since most of the investment is made upfront, and operational costs are relatively low. This is similar for solar thermal systems, where the majority of the cost is capital expenditure, giving a predictable cost of heat over time. The same applies when using solar or wind energy combined with heat pumps – the upfront investment defines the long-term cost structure. Other heat sources usually have prices which vary more over time and as such are more uncertain.

As mentionned in the question, between planning and construction, investment costs can also significantly vary, which is why it is important to include a certain margin of error when estimating the required budget, in order to avoid for the budget to explode. A good way to do this is by making a pre-consultation of the market, and include a margin for unforseen costs to the budget. If some delays are encountered, the market pre-consultation should be carried out again, to make sure the estimates are up-to-date. Having a storage can generate benefits/savings, which also might increase with time. So although the investment costs might increase, the entire business case should be re-evaluated before making the final investment decision (see for instance the business case of the PTES in Høje Taastrup as an illustration).

Answered during the first Q&A session of IEA-ES Task 45, see following link

What are the ballpark costs of PTES, BTES and ATES (range in €/kWh)?

The fact sheets made as a part of IEA-ES Task 39 in the Netherlands provide some insights about the specific investment cost range for different LTES technologies:

Typically, investment costs are expressed per kWh of storage capacity or per cubic meter, but these values vary significantly depending on three main factors:

  • Storage size: there’s a substantial difference in cost between a 20’000 m³ and a 200’000 m³ PTES. Larger systems benefit from economies of scale, which significantly reduce the cost per unit of stored energy.
  • Storage concept: whether the system is an ATES, BTES, PTES, or TTES also influences the cost structure. However, when analyzing the financial graph (see slide 5 of the IEA-ES Task 39 introduction brochure), it becomes clear that PTES systems dominate (for now) the larger-scale segment, both in terms of volume and realized projects.
  • Local conditions: the local geological conditions can significantly influence the price of an LTES, but also the expertise of local actors (consultancy, local contractors), which don’t necessarily have the expericence with a given technology.

Another very important factor to look at when considering the investment cost of an LTES is the scope that is included in the cost: is it the standalone LTES? Does it include the district heating connection costs (pipeline, pumping station, heat exchangers, technical building)? Does it include technical consultancy? Does it include the costs for auxiliary elements (such as a buffer tank or a heat pump, if necessary)?

One last thing to have in mind is that all technologies cannot be used for the same application: for instance BTES has a rather slow charge/discharge rate potential, similar to ATES, and might require post-heating to reach the temperature required by the district heating network, compared with TTES or PTES. Another example is that an ATES is ideally designed for combined (winter) heating and (summer) cooling application. This should be weighed when chosing which technology is best suited for a given application.

Answered during the first Q&A session of IEA-ES Task 45, see following link

What companies in Europe deliver ATES, BTES and PTES technologies?
  • ATES: a lot of experience in low and higher temperature ATES systems is present at Dutch companies. These are consultancy, drilling companies, suppliers, developers en ESCO’s.
  • BTES: BTES is a well know technology and many systems have been made in Germany, France, Sweden, etc. There is however few full-scale high-temperature BTES implemented up until 2025. Drilling experts are a key enabler of this technology.
  • PTES: most of the first systems were made in Denmark, where an ecosystem of companies has been formed. The technology is starting to spread out (for instance in China, but also in Germany and other EU countries, as demonstrated in the EU-funded project TREASURE).

Answered during the first Q&A session of IEA-ES Task 45, see following link

Practical experiences

General Task 45 considerations (CTES, LTES)
With several operational systems in the Nordics, have Cavern Thermal Energy Storage (CTES) been considered as part of IEA-ES Task 45? What are the requirements to establish such LTES?

Cavern Thermal Energy Storage (CTES) systems have been successfully developed in countries such as Finland and Scotland, where natural caverns and disused mines are sometimes repurposed for thermal storage. These systems depend on very specific geological conditions that determine whether a site is suitable for long-term energy storage. A critical factor is the ability to reliably contain water within the underground structure, which can vary significantly between regions. As a result, CTES represents a promising yet highly location-dependent technology.

Within Subtask 4 of IEA-ES Task 45, the focus lies on collecting and analyzing information regarding the technical and industrial progress of ongoing and emerging R&D projects in Long-term Thermal Energy Storage (LTES). This work primarily covers concepts and technologies such as Aquifer Thermal Energy Storage (ATES), Borehole Thermal Energy Storage (BTES), Tank Thermal Energy Storage (TTES), and Pit Thermal Energy Storage (PTES). While CTES is not a direct focus area, insights gained from material use and system design in other storage technologies may provide valuable knowledge for future CTES applications.

Answered during the first webinar of IEA-ES Task 45, see following link

Design (PTES)
In PTES projects, have alternate sensible storage media – such as sand, gravel, or rocks – been considered or tested instead of using only water?

Some pilot-scale PTES concepts have tested using a mixture of water and sand as the storage medium. In these setups, the pit contains both materials to potentially enhance thermal performance. However, this approach has not yet been adopted in any large-scale commercial projects. It remains an area of ongoing research, and at least one EU-funded project – Interstores – is currently exploring a similar concept involving a sand-water mixture.

Answered during the first webinar of IEA-ES Task 45, see following link

Application (ATES, PTES, TTES)
How flexible is ATES operation in terms of e.g. daily balancing, compared to TTES or PTES?

Aquifer Thermal Energy Storage (ATES) systems are primarily designed for seasonal storage, enabling large volumes of heat to be stored and reused across different seasons.


While their main function is long-term storage, ATES can also contribute to daily or hourly balancing when combined with heat pumps. For optimal short-term operation, integrating ATES and heat pumps with a small Tank Thermal Energy Storage (TTES) unit is recommended, as this helps to prevent frequent on-off cycling of the heat pumps. In this way, ATES can provide a degree of operational flexibility, although this is not its typical application.


The choice of the most suitable thermal energy storage solution depends on multiple factors, including the specific energy demand, available heat supply, end-user requirements, subsurface conditions, environmental constraints, existing infrastructure, and alternative heat sources or storage capacities. An integrated approach, often combining different storage technologies, can deliver the most effective and adaptable energy solution.

Answered during the first webinar of IEA-ES Task 45, see following link

Have there been any unexpected benefits or emerging opportunities identified after implementing high-temperature ATES systems?

Feedback from a high-temperature ATES user highlights that the system has delivered greater flexibility than originally anticipated. Although flexibility was not a primary design objective, the ability to store surplus heat whenever available has proven to be a significant advantage in practice.

Another emerging consideration is the connection to the electricity grid. With grid congestion becoming an increasing challenge, large-scale thermal energy storage systems could play an important role in balancing electricity supply and demand. Surplus electricity, for example, could be converted into heat and stored using CHP units or other conversion technologies.

This potential is especially relevant in the current European context, where major investments are directed towards electricity infrastructure. While electricity represents only about 20% of total energy demand, heat accounts for more than 50%. Yet, investments in heat-related technologies remain limited. Strengthening the link between thermal storage and electricity systems could therefore not only enhance overall energy system flexibility but also create new funding opportunities and accelerate the rollout of heat storage solutions.

Answered during the first webinar of IEA-ES Task 45, see following link

Is it possible to inegrate heat pumps with thermal storage systems to participate effectively in the electricity flexibility market?

In Denmark, heat pumps are already connected to TTES, and they are actively participating in the flexibility markets. Most of these systems operate based on day-ahead electricity prices, allowing them to charge when prices are low. Additionally, some are also involved in the balancing markets – for example, by offering up-regulation services if they can respond within 15 minutes.
This operational model demonstrates how heat pumps can be flexibly controlled to optimize both cost and grid support. The same approach can be applied when combining heat pumps with PTES, enabling efficient integration with the energy system while taking advantage of market opportunities.

However, it is crucial to be aware that heat pump technologies vary significantly, primarily due to differences in refrigerants and compressor types. Therefore, when specifying or procuring a heat pump, it is essential to have a clear understanding of the intended application. Not all heat pumps are designed to handle the frequent cycling or rapid response times required in flexibility markets, so careful selection is key. Combining the heat pump with an appropriately designed thermal storage is also essential, as the thermal storage provides a heat source and/or sink with a stable operating temperature to the heat pump.

Answered during the first Q&A session of IEA-ES Task 45, see following link

Cost (PTES)
Is there any practical experience with PTES installations in cold climates, and what are the implications for costs and system performance?

Designs for cold-climate PTES are still under development (e.g. within TREASURE), with a notable project currently being built in Finland (Hyvinkää). IEA-ES Task 38 has studied another solution: systems which can actively melt snow or ice forming on a given surface. Although such systems are originally designed for de-icing roads using thermal energy storage, they could also be used on a PTES cover/lid. To the best of our knowledge, no such development has been considered yet.
Not much data is available at this point about performance and cost implication.

Answered during the first webinar of IEA-ES Task 45, see following link

Stakeholders (LTES)
What are the career opportunities in the field of LTES?

Career opportunities in the thermal energy storage field are quite diverse, driven by a growing market and the development of new projects. This creates increasing opportunities for young professionals to enter and contribute to the sector.

The IEA-ES Task 45 group tries to make an effort to distribute vacancies on this topic as widely as possible. One valuable resource for discovering opportunities is the IEA Energy Storage Programme website, which often features openings, including PhD positions.

A practical way to explore work opportunities on this field is by reviewing the active tasks listed on the IEA-ES website. Each task includes a description of the participating organizations, offering insight into involved parties and the nature of their work. For example, the IEA-ES Task 45 webpage provides access to publications, presentations, and project summaries, which help illustrate the current activities and expertise of the partners. This information can be used to identify relevant contacts and reach out directly to individuals or organizations working on topics of interest. Such a network-based approach is often highly effective for discovering research and career opportunities in the field.

Answered during the first Q&A session of IEA-ES Task 45, see following link