Heat Storage: Flexibilities or District Heating

The increasing decarbonisation of district heating is leading to greater sector coupling between the electricity and heating sectors. This in turn increases the influence of volatile electricity prices on the cost of heat production in district heating, thereby increasing the economic advantages of flexible district heating operation. This series of articles therefore deals with the various types of flexibility in district heating and their respective limitations. Furthermore, the flexibilities described are critically examined in light of the real obstacles facing the district heating industry through interviews. This series of articles is aimed at municipal utilities and district heating suppliers who want to gain an overview of the flexibility potential in district heating, as well as anyone interested in district heating supply.

Heat storage as an enabler of energy flexibility in district heating

The main benefit of heat storage systems in district heating lies in the temporal decoupling of heat generation from heat demand. Through this decoupling, heat storage systems enable significantly improved utilisation of energy flexibility potential in the heat supply, allowing heat generation to be better adapted to external factors, such as electricity prices. Furthermore, heat storage systems can smooth out peak loads by providing part of the required heat output. This allows the use of costly peak-load heat generators to be reduced or avoided. When heat storage systems are connected downstream of network bottlenecks, they can also help to relieve the strain on heavily utilised sections of the network. A further advantage arises from the integration of waste heat sources into heating networks, as their feed-in profiles often do not match the network’s heat demand. Through the use of heat storage systems, generation and demand profiles can be synchronised over time.

Compared with other measures for utilising flexibility potential, heat storage systems are characterised by their high storage capacity and their comparatively simple operation. By contrast, adjusting the heat load on the consumer side as part of demand-side management is significantly more complex and, economically speaking, often only viable for very large consumers.

The economic benefits of heat storage systems arise, in a context where heat generation is increasingly linked to the price of electricity, mainly due to the resulting energy flexibility. These economic benefits increase as the homogeneity of a heating network’s generation mix increases. In networks with a heterogeneous generation structure, the demand-driven use of electricity-generating, electricity-consuming and electricity-independent heat generators (see the article Flexibilities in District Heating: Description of possible flexibility potentials) can already enable an operating mode that is partially adapted to the price of electricity. Consequently, the additional benefit arising from the temporal decoupling of heat generation and demand via a heat storage system is reduced to the reduction in the heat load, thereby avoiding the use of heat generators with temporarily higher heat production costs.

With regard to the future development of heat generation, it can be assumed that heat storage systems will continue to gain in importance in the long term. Whilst electricity-generating plants are likely to continue to play an important role during a transitional phase, the share of electricity-consuming technologies is expected to increase in the mid-term. This also increases the need for additional flexibility, which can be provided by heat storage systems.

An overview of heat storage technology

Heat storage systems can be categorised according to a variety of criteria, such as operating principle, storage medium, storage duration or pressure level. The following section provides a classification based on the operating principle. Further sub-classifications are described for each operating principle. In terms of operating principle, heat storage systems are categorised as sensible, latent and thermochemical storage systems. Figure 2 provides an overview of these distinctions.

Sensible heat storage

In sensible heat storage systems, energy is stored by raising the temperature of a storage medium. The amount of energy that can be stored depends on the temperature difference and the specific heat capacity of the storage medium, and increases linearly with temperature. Compared with other storage technologies, sensible heat storage systems have a lower volumetric energy density and a high rate of self-discharge. However, due to their advanced state of development and low investment costs, they represent the most widely used storage technology, accounting for 88.9 % of the market. [2]

Water has become the storage medium of choice due to its low cost, high availability, ease of use, good environmental compatibility and comparatively high specific heat capacity of 4.18 kJ/kg*K. The most common design is the tank storage system (TTES). Other designs that use water as a storage medium include pit thermal energy storage (PTES) and cavern thermal energy storage (CTES), as well as aquifer thermal energy storage (ATES) systems that utilise groundwater. Alternatively, the ground itself can be used in geothermal storage systems, gravel-water mixtures in PTES systems, and various solids such as concrete can be used as storage media.

Latent heat storage

Latent heat storage systems store heat in the form of energy released during the phase change of a storage medium. This enables large amounts of energy to be stored and released at a virtually constant temperature during the transition between different physical states. Even with small changes in temperature, comparatively large amounts of energy can be stored by utilising the energy released during the phase change. In most latent heat storage systems, the phase change between the solid and liquid states is utilised for this purpose. In addition to latent heat, sensible heat can often also be stored below and above the phase-change temperature.

Compared with sensible heat storage systems, latent heat storage systems are characterised by a higher energy density and the ability to supply heat at an approximately constant temperature during the phase change. With sensible heat storage systems, this is only possible with pronounced temperature stratification. Disadvantages include the comparatively higher costs and the fact that many technologies are as yet at an early stage of development and market readiness. Furthermore, the application must be suited to the phase-change temperature in order to utilise the additional storage potential. Storage materials used include, amongst others, water in the form of ice storage systems, salt hydrates, paraffins and various metals.

Thermochemical heat storage

In thermochemical storage systems, heat is stored through reversible endothermic and exothermic processes. Thermochemical heat storage systems can generally be divided into thermochemical reaction storage systems, as well as adsorption and absorption storage systems. In reaction storage systems, heat is stored in chemical bonds and is charged or discharged via a reversible chemical reaction. Examples of this include metal hydrides and reversible hydration reactions.

In thermal storage by adsorption, water vapour adheres to the surface of porous solids such as silica gel or zeolites via intermolecular forces, thereby releasing heat. When heat is applied, the adsorbed substance is desorbed and the storage system is recharged. In absorption, a substance (gaseous or liquid) is absorbed or dissolved within the volume of a solid or liquid medium, which also releases heat. When heat is applied, the absorption or dissolution of the substance is reversed.

Current status of heat storage systems in German district heating networks

According to figures from the AGFW, based on research by Fraunhofer IFAM, heat storage systems with a total capacity of 35.2 GWh were installed in German district heating networks in 2023. This represents an increase of 6.3% compared with the 33.1 GWh recorded in the previous year [1].

Figure 3 shows the trend in the storage capacity of large-scale thermal storage systems between 2019 and 2023.
 

The data from AGFW member companies surveyed in 2022 – published in the AGFW Main Report 2023 – also provide insights into the heat storage technologies in use (see Figure 4) [2]. With a total capacity of 32.4 GWh, the heat storage systems of the companies surveyed account for around 98% of the heat storage capacity installed in German district heating networks in 2022.

At 19.7 GWh, or 61%, unpressurised storage accounts for the largest share. Pressurised storage follows with a capacity of 9.1 GWh, or 28%. PTES systems currently play only a minor role, with a capacity of just 16 MWh, or 0.05%. The remaining storage capacity is accounted for by other, unspecified storage technologies.

A comparison of storage capacity and storage volume reveals an average specific storage capacity of 123 kWh/m³ for pressurised storage, whilst unpressurised storage achieves an average of 59 kWh/m³. Two-zone storage systems, in which the heat-storing water is subjected to a slight overpressure by a second zone filled with water, were classified as pressure storage systems in the analysis. Water was used as the storage medium in all the heat storage systems considered [2].

Outlook

Based on long-term scenarios from Fraunhofer ISI and Consentec, a significant expansion of heat storage capacity in district heating is to be expected. Depending on the scenario, heat storage capacity for 2030 is forecast to range from 624 GWh (T45-Strom) to 766 GWh (T45-PtG/ PtL) and for 2045, from 625 GWh (T45-PtG/PtL) up to 871 GWh (T45-H2). By 2030, this corresponds to an increase in storage capacity by a factor of between 17.7 and 21.7. Whilst a slight decline in storage capacity is expected in the T45-PtG/PtL scenario from 2030 to 2045, it is set to rise in the other two scenarios [3].

In-depth: Sensible heat storage systems

Sensible storage systems are by far the most common form of heat storage technology. In district heating networks, they are also the only significant storage technology. The dominance of sensible heat storage systems is primarily due to their high level of technical maturity and their comparatively simple design. Using water as the heat carrier medium, they generally employ an inexpensive and environmentally friendly medium that can be used directly in the district heating network without the need for an additional heat exchanger. The various designs of sensible heat storage systems are described in more detail in the following sections.

Tank thermal energy storage (TTES)

In TTES, heat is stored by raising the temperature of water in steel tanks, which are usually cylindrical. The storage capacity is determined by the temperature difference between the storage tank’s charged and discharged states. The greater the temperature difference, the more heat can be stored in the tank. In district heating networks, the storage capacity of the TTES system is usually determined by the temperature difference between the supply and return flows. However, as water reaches its boiling point at 100 °C at atmospheric pressure, the maximum volumetric storage capacity – that is, the amount of heat that can be stored per unit volume – is limited in an unpressurised storage tank, also known as an atmospheric storage tank. For higher temperatures, the boiling point must be shifted to higher boiling temperatures by applying excess pressure.

Accordingly, TTES units can be categorised as unpressurised storage tanks, two-zone storage tanks and pressurised storage tanks based on the pressure they operate at. Unpressurised storage tanks operate without excess pressure and are therefore relatively simple in design and cost-effective. They are particularly suitable for district heating networks with flow temperatures below 100 °C. The overpressure in pressurised storage tanks, on the other hand, allows operation in district heating networks with flow temperatures of up to 160 °C. This increases the specific storage capacity, but at the same time also raises the requirements in terms of construction, operation and safety, as well as the costs. Two-zone storage tanks are an intermediate form between unpressurised and pressurised storage tanks. The heat-storing water is subjected to a slight overpressure via a second zone filled with water. This enables storage temperatures of up to approximately 120 °C without the storage tank’s design being as complex as that of a pressurised storage tank.

The greatest advantage of TTES lies in their high level of technical maturity, which is also evident from the large number of projects implemented in German district heating networks. Furthermore, they are easily controllable and relatively simple to integrate into heating networks. Due to their size and controllability, they are particularly well suited to balancing short- to medium-term fluctuations between heat generation and demand, with storage durations ranging from several days to a few weeks. Disadvantages arise primarily from their above-ground construction and the associated space requirements, which can make installation in densely built-up urban areas difficult. For seasonal storage, TTES are therefore generally less suitable than large-volume storage systems such as PTES, ATES and CTES, due to their space requirements and limited size, as their storage capacity is correspondingly limited.

Pit Thermal Energy Storage (PTES)

PTES are large, artificially excavated basins which are sealed off from the ground using liners and filled with water or a mixture of water and gravel. Additional liners on the surface protect the storage facility from environmental influences. Due to their simple construction, they have comparatively low specific costs for large storage volumes and are therefore particularly suitable for large storage volumes and, consequently, for seasonal storage. As solar thermal energy in particular exhibits significant seasonality, ground-based storage tanks are often integrated into district heating networks with a high proportion of solar thermal energy; however, they also demonstrate great potential when combined with large-scale heat pumps. Large heat pumps can also be used to cool the PTES to temperatures lower than the return flow temperature of the heating network, thereby increasing the storage capacity of the system. As with unpressurised storage tanks, the maximum storage temperature is just under 100 °C.

A drawback is the comparatively large amount of space required, which can be reduced in some cases by using deeper pits or by making dual use of the storage area. However, deeper pits increase not only the investment costs but also the complexity of construction. Current research projects are exploring the dual use of storage space by constructing parks, lakes or car parks on top of the pit. A further challenge is the impact on the surrounding groundwater, which must not be heated too much by the thermal storage system. Implementation is therefore subject to planning permission and site-specific conditions. In addition, sealing off the reservoir from the surrounding environment and insulating it can, in some cases, prove difficult.

According to the AGFW analysis, PTES, with a recorded volume of 655 m³ in 2022, have so far played only a minor role in Germany [2]. By contrast, the technology has been used successfully in Denmark for many years, and Denmark is therefore regarded as a pioneer in this field. In Germany, PTES systems are currently being investigated intensively in research projects such as the TREASURE research project.

Aquifer Thermal Energy Storage (ATES)

In ATES systems, heat is stored in water-bearing geological layers. To charge the storage facility, cold water is extracted via wells and warm water is injected. During discharge, the process runs in the opposite direction. The geological layers should have as low a groundwater flow velocity as possible so that the stored heat is not carried away. Furthermore, the geology must meet additional requirements, namely sufficient permeability and thickness of the aquifer, to ensure that a sufficient volume is available for storage. Thickness here describes the vertical extent of the aquifer. Furthermore, a suitable geochemical composition of the subsurface is required to prevent temperature-induced precipitation, i.e. the formation of mineral deposits. By utilising existing volumes, large quantities of energy can be stored, making ATES systems well-suited to seasonal storage. Compared with TTES or PTES, they require only a small surface area, but are highly site-dependent due to their reliance on geological layers. The use of geological layers also entails stringent requirements regarding authorisation and monitoring; furthermore, there may be conflicting objectives with regard to groundwater protection.

Cavern Thermal Energy Storage (CTES)

CTES systems store water as a heat transfer medium in man-made or existing underground cavities. They also offer the potential to provide very high storage capacities and performance whilst requiring little surface area. Similar to ATES systems, CTES systems are also limited to suitable geological sites. However, the geological requirements differ, as there is no need to utilise a naturally water-bearing aquifer. If existing cavities, such as old tunnels, cannot be utilised, CTES facilities require complex and therefore costly development. Whilst the high development costs can be spread over a long service life of more than 100 years, such a long time horizon simultaneously complicates investment planning. The Varanto project by Vantaa Energy can be cited as an example of a CTES system currently under construction. Located beneath the Finnish city of Vantaa, the storage facility will, once operational, have a volume of approximately 1 million m³ and store water under pressure at a temperature of up to 140 °C. This is sufficient for a storage capacity of 90 GWh. The project forms part of the Interstores research project. (Varanto – VECTES – INTERSTORES)

Summary

In summary, it can be said TTES systems are particularly suitable for short- to medium-term flexibility in heating networks, whilst PTES systems, due to their large volumes, are particularly relevant for seasonal applications. ATES and CTES systems also offer significant storage potential whilst requiring little surface area, but are much more dependent on suitable geological conditions and regulatory approvals. Furthermore, the technology is not yet fully mature. Which storage technology is suitable therefore depends in particular on the storage duration, temperature level, availability of land, geology and the requirements of the respective heating network.

More Information

• WARAN – Integration and grid-serving usage of heat
• Towards a common digital infrastructure for the electricity and heat sectors – Use Cases from the Project WARAN
• Flexibilities in District Heating: Description of possible flexibility potentials

Literature

[1] AGFW – Der Energieeffizienzverband für Wärme, Kälte und KWK e. V., „AGFW‑Hauptbericht 2024.“ Frankfurt am Main.. [Online] https://www.agfw.de/zahlen-und-statistiken/agfw-hauptbericht/ (accessed on 17.04.2026)

[2] Energieeffizienzverband für Wärme, Kälte und KWK e. V. (AGFW), „AGFW Hauptbericht 2023“. 2023.  https://www.agfw.de/zahlen-und-statistiken/agfw-hauptbericht/ (accessed on 10.03.2026)

[3] Fraunhofer ISI & Consentec, „Langfristszenarien für die Transformation des Energiesystems in Deutschland“, 2022, [Online] https://www.langfristszenarien.de/enertile-explorer-de/dokumente