Overview
Seasonal thermal energy storage (STES), also known as inter-seasonal thermal energy storage, is a concept within the energy infrastructure sector focused on the storage of heat or cold for periods of up to several months. The primary fuel or source for this technology is solar energy, and the operational status of STES systems is generally classified as operational in various global implementations. The fundamental principle of STES allows thermal energy to be collected whenever it is available and used whenever needed, such as in the opposing season. This temporal decoupling of energy supply and demand is critical for optimizing energy efficiency in both district heating networks and single building applications.
For example, heat from solar collectors or waste heat from air conditioning equipment can be gathered in hot months for space heating use when needed, including during winter months. Similarly, waste heat from industrial processes can be stored and used much later, or the natural cold of winter air can be stored for summertime air conditioning. This flexibility enables the integration of variable renewable energy sources and waste heat recovery systems into the broader energy mix. The temperature ranges for STES systems typically span from 27 to 80 °C, depending on the specific application and storage medium. These temperature ranges are sufficient for most space heating and cooling needs in residential and commercial buildings.
The general applications of STES include district heating systems, where large-scale storage tanks or underground aquifers are used to store thermal energy for multiple buildings. In single buildings, STES can be implemented using smaller storage units, such as borehole thermal energy storage (BTES) or pit thermal energy storage (PTES). The technology supports the transition to a more sustainable energy system by reducing reliance on fossil fuels and enhancing the efficiency of renewable energy utilization. By storing excess thermal energy during peak production periods and releasing it during peak demand periods, STES helps to balance the energy grid and reduce overall energy costs.
What are the main types of seasonal thermal energy storage?
Seasonal thermal energy storage systems are categorized by their physical location and medium. The primary classifications include underground thermal energy storage (UTES), surface or above-ground storage, and passive systems. These technologies enable the decoupling of thermal energy collection and consumption, allowing heat gathered in summer to be utilized for space heating in winter, or cold stored in winter for summertime air conditioning.
Underground Thermal Energy Storage (UTES)
UTES utilizes the subsurface geology as a thermal mass. Common configurations include Aquifer Thermal Energy Storage (ATES), Borehole Thermal Energy Storage (BTES), and Cavern Thermal Energy Storage (CTES). These systems leverage the relatively stable temperatures of the underground environment to minimize thermal losses over several months.
Surface and Above-Ground Storage
Surface systems typically involve insulated tanks or pit storage facilities. Pit storage often uses large, insulated concrete or earth-covered pits filled with water or phase-change materials. These are suitable for sites with limited subsurface geological uniformity.
Comparison of STES Technologies
| Type | Key Characteristics | Typical Medium |
|---|---|---|
| ATES | Utilizes natural aquifers; requires good hydraulic conductivity | Water |
| BTES | Uses vertical boreholes with heat exchangers; flexible site selection | Ground/Water |
| CTES | Large excavated caverns; high capacity; suitable for rock or salt formations | Water/Air |
| Pit Storage | Surface-level insulated pits; lower capital cost; higher thermal loss potential | Water/PCM |
The efficiency of these systems depends on the thermal conductivity of the storage medium and the insulation quality. For instance, the heat loss in a cylindrical tank can be approximated by Q=UAΔT, where U is the overall heat transfer coefficient, A is the surface area, and ΔT is the temperature difference between the stored medium and the surroundings. Passive systems, such as thermal mass in building walls, also contribute to seasonal balancing but offer less controllable capacity compared to active UTES or pit storage solutions.
Underground thermal energy storage technologies
Underground thermal energy storage technologies utilize the subsurface geology to store thermal energy for seasonal use. These systems are critical for decoupling heat production from consumption, allowing solar or waste heat collected in summer to be used for space heating in winter.
Anticyclic Aquifer Thermal Energy Storage (ATES)
ATES systems exploit natural aquifers to store heat and cold. The technology typically involves two wells: one for injecting warm water and another for extracting it. The aquifer acts as a natural reservoir, with groundwater flowing between the wells to transfer thermal energy. This method is efficient for large-scale district heating and cooling networks, leveraging the thermal inertia of underground water bodies.
Borehole Thermal Energy Storage (BTES)
BTES systems use vertical boreholes drilled into the ground to store thermal energy. The provided technical parameters specify borehole diameters of 155 mm, with depths ranging from 50 to 300 m. The spacing between boreholes typically varies from 3 to 8 m, depending on the geological properties and the desired storage capacity. The temperature limits for these systems can reach up to 85 °C, making them suitable for both heating and cooling applications. The thermal energy is transferred through heat exchangers installed within the boreholes, often using water or glycol mixtures as the working fluid.
Cavern Thermal Energy Storage (CTES)
CTES systems utilize underground caverns, often created in salt domes or rock formations, to store large volumes of thermal energy. These caverns can hold significant amounts of heat or cold, making them ideal for large-scale energy storage projects. The thermal energy is typically stored in the form of hot water or molten salts, which are pumped into the caverns during periods of excess energy production and extracted when needed.
Energy Pilings
Energy pilings integrate thermal storage into the foundation structures of buildings. These pilings consist of vertical pipes embedded in concrete piles, through which a heat transfer fluid circulates. This technology is particularly useful in urban environments where space is limited, as it utilizes the existing foundation infrastructure for thermal energy storage.
Groundwater-Integrated Insulated Tank Storage (GIITS)
GIITS systems combine the benefits of groundwater and insulated tanks to store thermal energy. The insulated tanks are placed underground, reducing heat loss to the surrounding soil. Groundwater is used as a heat transfer medium, circulating through the tanks to absorb or release thermal energy. This hybrid approach offers high efficiency and flexibility, making it suitable for various building types and climate conditions.
Surface and above-ground storage systems
Surface and above-ground storage systems provide flexible alternatives to deep borehole arrays, often leveraging existing topography or modular engineering. Pit storage represents a prominent implementation of this approach, particularly within Danish district heating networks. The Vojens facility exemplifies this scale, utilizing a storage volume of 200,000 m3. These pits typically function by capturing solar heat or industrial waste during warmer months, retaining the thermal energy through insulation and stratification for winter release. Large-scale water tanks offer another above-ground solution. These systems rely on the high specific heat capacity of water, allowing for significant energy density in relatively compact footprints compared to earth-bermed options. Horizontal heat exchangers are frequently integrated into these setups to optimize the charging and discharging cycles, minimizing thermal losses to the surrounding air or ground. Earth-bermed buildings utilize the thermal mass of the ground adjacent to the storage vessel to reduce conductive losses. This technique is particularly effective in climates with stable subsurface temperatures, effectively creating a semi-subterranean storage environment without the excavation costs of full pit systems. Salt hydrate technology introduces a phase-change mechanism to above-ground storage. By utilizing the latent heat of fusion, salt hydrates can store substantial energy within a smaller volume compared to sensible heat storage media. This technology is advantageous where space is at a premium, though it requires careful management of supercooling and phase separation. The economic viability of these systems is a critical factor in their deployment. Cost data indicates that surface and above-ground storage solutions generally range from €0.4 to €0.6/kWh. This cost structure makes them competitive for large-scale district heating applications, particularly when integrated with solar thermal collectors or industrial waste heat recovery. The choice between pit, tank, or phase-change systems depends on local geological conditions, available surface area, and the specific temperature requirements of the end-use application.Worked examples
The concept of seasonal thermal energy storage is demonstrated through several operational case studies that highlight different storage mediums and efficiencies. The Drake Landing Solar Community in Alberta, Canada, serves as a prominent example of borehole thermal energy storage. This system utilizes 144 boreholes, each 37 m deep, to store solar heat collected during the summer for winter space heating. According to project data, the community achieved a solar fraction of 97%, meaning nearly all heating demand was met by stored solar energy. The system operates by circulating fluid through the boreholes, transferring heat into the ground mass. This high efficiency demonstrates the viability of using shallow geothermal storage for district heating applications.
Borehole Efficiency at Richard Stockton College
Another significant implementation is found at Richard Stockton College in New Jersey, which employs a larger-scale borehole thermal energy storage system. This facility uses 400 boreholes extending 130 m deep into the ground. The increased depth allows for greater thermal mass and potentially lower temperature fluctuations compared to shallower systems. Reports indicate that this system experiences a thermal loss of only 2% over the storage period. This low loss rate highlights the effectiveness of deep borehole arrays in maintaining temperature differentials between the stored energy and the surrounding geology. The system captures excess heat during warmer months and retrieves it during cooler periods, optimizing the building's energy consumption.
Pit Storage and ATES Systems
Beyond boreholes, pit storage and aquifer thermal energy storage (ATES) offer alternative methods for seasonal storage. The towns of Marstal and Vojens in Denmark utilize pit storage systems, where insulated pits filled with water store thermal energy. These systems are particularly effective for district heating networks, allowing for large volumes of heat to be stored at relatively low costs. Similarly, the Reichstag building in Berlin employs an ATES system, utilizing underground aquifers to store heat and cold. This approach leverages the natural insulation of the ground and the thermal capacity of water. Other notable examples include the Suffolk One project in the United Kingdom and the Brædstrup district heating system in Denmark. These diverse implementations demonstrate the adaptability of seasonal thermal energy storage across different geographical and climatic conditions.
How is STES used in buildings and greenhouses?
Seasonal thermal energy storage finds practical application in building-scale systems, ranging from passive architectural designs to active water-tank installations and greenhouse climate control. In small, passively heated structures, systems such as Passive Annual Heat Storage (PAHS) and Annual Groundwater Storage (AGS) utilize the thermal inertia of the ground or groundwater bodies to bridge seasonal temperature differentials. These systems rely on the natural stratification of heat and cold within the subsurface, allowing buildings to draw warmth in winter that was stored during summer months, or vice versa for cooling.
Internal Water Tank Systems
Active STES in buildings often employs large internal water tanks to store thermal energy collected from solar collectors or waste heat sources. Notable examples include the MIT Solar House, the Oberburg system, installations in Berlin, and projects in Galway. In these configurations, heat is gathered during peak availability periods—such as sunny summer days—and retained in insulated water volumes for use during winter heating seasons. The thermal capacity of these tanks allows for a decoupling of heat production and consumption, enhancing the efficiency of solar thermal systems. The stored energy can be retrieved via heat exchangers, providing space heating or domestic hot water when solar input is at its lowest.
Greenhouse Climate Control via ATES
In agricultural applications, particularly greenhouses, Seasonal Thermal Energy Storage is frequently implemented using Aquifer Thermal Energy Storage (ATES). This method leverages underground aquifers to store thermal energy for seasonal use. During summer, warm water is injected into a warm well, while cold water is extracted from a cold well for air conditioning. In winter, the process reverses: cold water is injected, and the previously stored warm water is extracted for heating. This cycle allows greenhouses to maintain optimal growing temperatures year-round, utilizing the natural thermal mass of the aquifer. The system effectively balances the heating and cooling loads, reducing reliance on external energy sources and enhancing the energy efficiency of greenhouse operations. The integration of ATES in greenhouses demonstrates the versatility of STES in managing variable thermal demands across different sectors.
History and development of STES
Seasonal thermal energy storage (STES) has evolved from early experimental prototypes to a recognized component of global energy infrastructure. The conceptual foundation for storing heat across months was established in the 1930s, with early prototypes developed at the Massachusetts Institute of Technology (MIT) in 1939. These initial efforts demonstrated the feasibility of capturing solar thermal energy for use during periods of lower solar irradiance, laying the groundwork for future inter-seasonal storage systems.
International Coordination and Early Development
The period between 1978 and 1990 marked a significant phase of international coordination in thermal energy storage. The International Council for Thermal Energy Storage was active during this timeframe, facilitating the exchange of technical data and standardizing approaches to storage efficiency. This era saw the formalization of STES as a distinct engineering discipline, moving beyond isolated academic experiments to coordinated global research efforts.
In 1981, the International Energy Agency (IEA) launched the European Commission for Energy Storage (ECES) conferences, which became a primary forum for STES development. These conferences provided a platform for engineers and researchers to present findings on storage media, heat exchangers, and system integration. The IEA-ECES conferences have continued to influence STES technology, driving advancements in both small-scale residential systems and large-scale district heating networks.
Prototype Implementations and Recent Expansions
A notable milestone in STES history was the development of early prototypes in Berlin in 1997. This project demonstrated the practical application of storing solar heat collected during summer months for use in winter space heating. The Berlin prototype utilized large-scale underground storage tanks, showcasing the potential for reducing fossil fuel consumption in urban heating systems.
Recent expansions in STES technology have focused on integrating solar thermal collectors with advanced storage media. Systems now commonly store waste heat from industrial processes or air conditioning equipment, maximizing energy efficiency. The operational status of STES systems is now well-established, with numerous installations worldwide utilizing solar energy as the primary fuel source. These systems continue to evolve, incorporating new materials and control strategies to enhance storage duration and thermal efficiency.
What distinguishes STES from other thermal storage methods?
Seasonal thermal energy storage (STES) is fundamentally distinguished from other thermal storage methods by its temporal scale. While short-term thermal storage, such as daily tank systems, retains heat or cold for hours or days, STES preserves thermal energy for periods of up to several months. This inter-seasonal capability allows energy to be collected whenever it is available and used whenever needed, often in the opposing season. For instance, heat gathered during hot months can be deployed for space heating during winter months, or the natural cold of winter air can be stored for summertime air conditioning. This decoupling of supply and demand is the core operational differentiator of STES.
Integration with Solar Thermal and Waste Heat
The technology is particularly effective when integrated with variable sources like solar thermal collectors and industrial waste heat. Solar collectors can gather significant heat during summer months, which is then stored for winter use. Similarly, waste heat from air conditioning equipment or industrial processes can be captured and stored for later use. This integration maximizes the utility of otherwise intermittent or underutilized thermal resources. The system allows for the collection of heat whenever it is available, ensuring that energy is not lost due to immediate consumption constraints.
Role of Heat Pumps and Circulation Pumps
The efficiency of STES systems is heavily influenced by the interplay between heat pumps and circulation pumps. Heat pumps are often used to upgrade the temperature of the stored thermal energy, making it suitable for space heating or cooling. Circulation pumps, on the other hand, are responsible for moving the thermal energy through the storage medium and the distribution network. The role of these components is critical in minimizing energy losses during the storage and retrieval processes. Proper sizing and operation of heat pumps and circulation pumps can significantly enhance the overall efficiency of the STES system.
Efficiency Gains with Size
Efficiency gains in STES are closely related to the size of the storage system. Larger storage capacities can reduce the relative impact of heat losses over time, leading to higher overall efficiency. This is because the surface-area-to-volume ratio decreases as the size of the storage medium increases, reducing the rate of heat exchange with the surrounding environment. Consequently, larger STES systems can maintain thermal energy for longer periods with less degradation in quality. This scalability makes STES a viable option for both small-scale residential applications and large-scale district heating and cooling networks.
See also
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