Overview
Seasonal thermal energy storage (STES), also referred to as inter-seasonal thermal energy storage, is a technology designed to store heat or cold for extended periods, typically spanning several months. This system allows thermal energy to be collected when it is most abundant and utilized when demand peaks, often in the opposing season. For instance, solar heat collected during summer months can be stored for space heating in winter, while natural cold from winter air can be preserved for air conditioning during summer. Industrial waste heat can similarly be captured and deployed later, enhancing overall energy efficiency.
Basic Principles and Temperature Ranges
The core principle of STES involves capturing thermal energy during periods of surplus and releasing it during periods of deficit. This process can involve storing heat from solar collectors or waste heat from air conditioning equipment. The stored energy is retained in various media, such as water tanks, boreholes, or aquifers, depending on the specific application and local geological conditions. The temperature range for STES systems typically spans from 27 to 80 °C, allowing for flexibility in different climatic and operational contexts. This range ensures that the stored energy remains effective for both heating and cooling applications, minimizing thermal losses over time.
Applications in District Heating and Single Buildings
STES is widely applied in district heating networks, where large-scale storage can serve multiple buildings within a localized area. This approach optimizes energy use by balancing supply and demand across a broader geographic scope. Additionally, STES is utilized in single buildings, providing a cost-effective solution for managing thermal energy needs. In these settings, the system can reduce reliance on conventional heating and cooling methods, leading to significant energy savings and reduced carbon emissions. The versatility of STES makes it suitable for diverse architectural designs and climate zones, enhancing its adoption in modern energy infrastructure.
What are the main types of seasonal thermal energy storage?
Seasonal thermal energy storage (STES) systems are classified by their physical location and scale of deployment. The primary categories include underground thermal energy storage (UTES), surface or above-ground storage, and small-scale building-integrated systems. Each type utilizes different thermal media and engineering approaches to minimize heat loss over periods of up to several months.
Underground Thermal Energy Storage (UTES)
Underground systems leverage the thermal inertia of the earth to store heat or cold. Common configurations include borehole thermal energy storage (BTES) and aquifer thermal energy storage (ATES). In BTES, vertical boreholes filled with heat transfer fluid are drilled into the ground. ATES utilizes underground water tables, where warm or cool water is injected and extracted through wells. These systems are typically used for district heating and cooling networks due to their large capacity.
Surface and Above-Ground Storage
Above-ground systems often involve large insulated tanks containing water or phase change materials (PCMs). These are commonly used in industrial settings or large commercial buildings. The thermal energy is collected from sources such as solar collectors or waste heat from air conditioning equipment during hot months and stored for winter use. Similarly, natural cold from winter air can be stored in these tanks for summertime air conditioning.
Small Building Systems
Small-scale STES is integrated directly into individual buildings. This may include phase change materials embedded in walls or floors, or small insulated water tanks. These systems are designed to optimize the energy efficiency of single structures, reducing reliance on mechanical heating and cooling.
| Technology Type | Medium | Typical Scale |
|---|---|---|
| Underground (UTES) | Ground, Aquifer Water | District Heating/Cooling |
| Surface/Above-Ground | Water, Phase Change Materials | Industrial, Large Commercial |
| Small Building Systems | PCMs, Water | Single Buildings |
The efficiency of STES depends on the thermal conductivity of the storage medium and the insulation quality. For water-based systems, the energy stored Q can be approximated by Q=mcΔT, where m is the mass of the water, c is the specific heat capacity, and ΔT is the temperature difference. Proper design ensures that the stored energy is available when needed, such as using summer solar heat for winter space heating.
Underground thermal energy storage technologies
Underground thermal energy storage (UTES) utilizes the subsurface to store thermal energy for seasonal duration. This category includes Aquifer Thermal Energy Storage (ATES), Borehole Thermal Energy Storage (BTES), Cavern Thermal Energy Storage (CTES), Energy Pilings, and Groundwater-Integrated Insulated Tank Storage (GIITS). These technologies leverage the thermal inertia of geological formations to decouple heat production from consumption.
Borehole Thermal Energy Storage (BTES)
BTES systems consist of an array of vertical boreholes equipped with U-tubes containing a heat transfer fluid. The system relies on the thermal conductivity of the surrounding soil or rock. Technical specifications for standard BTES installations include a borehole diameter of 155 mm, depths ranging from 50 to 300 m, and spacing between 3 to 8 m. These parameters are critical for minimizing thermal interference between adjacent boreholes and optimizing the storage density.
| Parameter | Value |
|---|---|
| Borehole Diameter | 155 mm |
| Depth | 50 to 300 m |
| Spacing | 3 to 8 m |
The thermal energy stored, Q, can be approximated by the product of the mass of the storage medium, its specific heat capacity, and the temperature difference. However, in BTES, the effective storage volume is a function of the borehole thermal resistance and the ground's thermal diffusivity. Proper design ensures that the thermal plume expands and contracts within the designated geological zone, reducing heat loss to the surrounding strata.
Other UTES Technologies
ATES utilizes permeable aquifers, injecting warm or cool water to create thermal plumes. CTES employs large underground caverns, often lined with concrete or steel, suitable for high-temperature storage. Energy Pilings integrate thermal loops into foundation piles of buildings, serving a dual structural and thermal function. GIITS combines insulated tanks with groundwater circulation, offering a hybrid approach for sites with specific hydrogeological conditions. Each technology is selected based on local geology, temperature requirements, and capital cost considerations.
Surface and above-ground storage systems
Surface and above-ground storage systems represent a distinct class of seasonal thermal energy storage (STES) technologies, often utilized when subsurface geology is less favorable or when rapid deployment is required. These systems rely on large-scale insulation and strategic placement to minimize thermal losses over months-long storage periods.
Water Tanks and Earth-Bermed Buildings
Large-scale water tanks are among the most straightforward implementations of above-ground STES. These systems typically consist of massive insulated reservoirs where hot or cold water is stored. The thermal energy is collected during peak availability—such as solar heat in summer—and discharged during the opposing season. Insulation quality is critical to maintaining temperature differentials. In some configurations, these tanks are integrated into earth-bermed buildings. By burying a portion of the storage vessel or the building itself into the ground, the earth acts as a natural insulator, reducing the thermal gradient between the stored medium and the ambient environment. This approach leverages the relatively stable temperature of the subsurface to enhance efficiency.
Pit Storage Systems
Pit storage systems involve excavating large depressions in the ground, which are then lined and filled with water or other thermal media. These systems have been notably deployed in Denmark, with examples in Vojens and Marstal. In these installations, the pits are often covered with insulating materials or floating covers to reduce evaporative and convective heat losses. The Vojens and Marstal systems demonstrate the viability of pit storage for district heating networks, where large volumes of heat can be stored at relatively low capital cost compared to deep borehole systems. The performance of these pits depends heavily on the quality of the liner and the insulation layer, as well as the stratification of the water column to prevent mixing of hot and cold layers.
Horizontal Heat Exchangers
Horizontal heat exchangers are another above-ground or near-surface option. These systems typically consist of pipes or tubes laid horizontally in trenches, often filled with gravel or sand to enhance thermal conductivity. Heat is transferred between the fluid in the pipes and the surrounding medium. While less common for large-scale seasonal storage compared to pit or tank systems, horizontal exchangers offer flexibility in site selection and can be integrated with solar thermal collectors or waste heat sources.
Salt Hydrate Technology
Salt hydrate technology offers a higher energy density alternative to water-based storage. Salt hydrates undergo a phase change from solid to liquid (or vice versa) at specific temperatures, absorbing or releasing latent heat in the process. This phase change allows for more compact storage systems compared to sensible heat storage in water. The technology involves encapsulating salt hydrates in containers or within a matrix to prevent segregation and supercooling. While promising, salt hydrate systems can be more complex and costly to implement than simple water tanks or pits, but they offer advantages in space-constrained environments or where higher temperature differentials are required.
Worked examples
Seasonal thermal energy storage systems are deployed globally in diverse configurations, including borehole fields, aquifer pairs, and pit storage. The following table summarizes key operational case studies.
| Project | Location | Technology | Key Metric |
|---|---|---|---|
| Drake Landing Solar Community | Alberta, Canada | Borehole Thermal Energy Storage (BTES) | 97% solar fraction; 144 boreholes |
| Richard Stockton College | New Jersey, USA | BTES | University district heating |
| Reichstag Building | Berlin, Germany | Aquifer Thermal Energy Storage (ATES) | Parliamentary complex cooling/heating |
| Vojens Pit Storage | Denmark | Pit Storage | Large-scale water tank |
| Suffolk One | Suffolk, UK | BTES | Residential district heating |
| Brædstrup | Denmark | Hybrid BTES | Combined heat and power integration |
Drake Landing Solar Community Analysis
The Drake Landing project demonstrates high-efficiency solar integration. The system utilizes 144 boreholes to store excess summer heat for winter use. The reported 97% solar fraction indicates that nearly all heating demand is met by stored solar energy. This high efficiency relies on minimal thermal loss in the ground matrix and precise temperature management during the charging and discharging cycles.
Reichstag Building ATES Configuration
The Reichstag building employs Aquifer Thermal Energy Storage (ATES). This system uses underground water layers to store thermal energy. In summer, waste heat from air conditioning is injected into the warm aquifer. In winter, the stored heat is extracted for space heating. This approach leverages the natural thermal inertia of the groundwater, reducing the need for mechanical compression or expansion for temperature regulation.
How does annualized geo-solar heating work?
Annualized geo-solar heating (AGS) is a specific application of seasonal thermal energy storage that utilizes the ground as a thermal battery. In this system, solar collectors gather heat during summer months, which is then transferred to the subsurface for use during winter. This process relies on the thermal lag of the earth, allowing heat to be stored for approximately six months. The system is particularly effective for passive buildings, where the thermal mass of the structure and the ground work together to maintain stable indoor temperatures.
Passive Annualized Heat Storage (PAHS)
Passive Annualized Heat Storage (PAHS) is a variant of AGS that integrates the building’s foundation with the thermal storage medium. In PAHS systems, the heat from solar collectors is channeled into the ground beneath the building. Insulation skirts are often used to minimize lateral heat loss, directing the thermal energy toward the building’s thermal mass. This setup allows for a more efficient use of the stored heat, as the building itself acts as a heat exchanger.
Thermal Siphons and Heat Transfer
Thermal siphons play a crucial role in the operation of AGS and PAHS systems. These are loops of piping that facilitate the natural convection of heat between the solar collectors and the ground. The efficiency of these siphons depends on the temperature difference between the heat source and the storage medium. The heat transfer can be described by the formula Q=U⋅A⋅ΔT, where Q is the heat transfer rate, U is the overall heat transfer coefficient, A is the surface area, and ΔT is the temperature difference.
Limitations and Evolution
Despite its benefits, AGS and PAHS have limitations. The initial cost of installation can be high, and the system’s efficiency depends on the local climate and soil conditions. Over time, the technology has evolved to include more advanced insulation materials and more efficient heat exchangers. These improvements have helped to reduce costs and increase the overall performance of seasonal thermal energy storage systems.
Applications in greenhouses and small buildings
Seasonal thermal energy storage systems are applied in diverse built environments, ranging from large-scale agricultural facilities to residential prototypes. In greenhouse agriculture, Aquifer Thermal Energy Storage (ATES) is utilized to manage temperature fluctuations. This technology involves pumping water into underground aquifers to store thermal energy, which is then retrieved for heating or cooling the greenhouse environment, thereby stabilizing crop growth conditions across different seasons.
For smaller building scales, internal water tanks and other compact storage media are employed. The MIT Solar House serves as a notable early example of this approach. In this system, thermal energy is collected and stored in water tanks within the building structure. This allows the heat gathered during periods of high solar irradiance to be utilized for space heating during colder months, demonstrating the feasibility of internal storage for residential applications.
Further development of this concept is seen in the Oberburg apartment building. This project integrates thermal storage to enhance energy efficiency in multi-unit residential structures. By storing excess heat, the building reduces its reliance on external energy sources, optimizing the use of collected solar thermal energy for domestic hot water and space heating needs.
The Berlin Zero Heating Energy House represents another advancement in this field. This prototype focuses on minimizing heating energy demand through efficient storage and utilization of thermal energy. The system aims to achieve near-zero heating energy consumption by effectively capturing and storing solar heat, thereby reducing the overall energy footprint of the residential unit.
Additionally, a prototype in Galway explores similar principles. This project investigates the application of seasonal thermal energy storage in residential settings, aiming to optimize energy use and enhance comfort levels. These various implementations highlight the versatility of STES technologies in addressing heating and cooling demands across different building types and scales.
History and organizations
The development of seasonal thermal energy storage (STES) has been significantly advanced by the International Energy Agency (IEA) through its Energy Conservation in Buildings and Communities Programme (ECES). This organizational framework has been instrumental in coordinating global research and standardizing terminology for inter-seasonal thermal energy storage systems. Early dissemination of STES knowledge relied heavily on specialized newsletters, particularly those focusing on Aquifer Thermal Energy Storage (ATES) and general STES technologies. These publications served as the primary mechanism for sharing technical data and case studies among engineers and researchers before the advent of major international conferences.
Evolution of International Conferences
As the field matured, the IEA ECES programme facilitated a transition from printed newsletters to a series of dedicated global conferences. This shift allowed for more dynamic exchange of data regarding solar heat collection and waste heat utilization. The conference series began with EcoStock, establishing a platform for discussing environmental and economic aspects of thermal storage. This was followed by ThermaStock, which further expanded the technical scope of the discussions.
Subsequent events continued to refine the focus of the STES community. InnoStock 2012 highlighted innovative approaches to storing heat or cold for periods of up to several months. GreenStock 2015 emphasized the integration of natural cold from winter air and solar collectors for opposing seasonal use. The series continued with EnerStock 2018, which addressed the operational status and efficiency of systems commissioned in various eras. These conferences have been critical in documenting how waste heat from industrial processes or air conditioning equipment can be gathered in hot months for space heating use during winter. The IEA ECES programme remains a central body in coordinating these efforts, ensuring that technical advancements in STES are systematically recorded and shared across the global energy infrastructure sector.
See also
- Dukovany Nuclear Power Station: Technical Profile and Operational History
- Voerde Powerplant: Technical Profile and Operational Context
- Clean coal technology
- Greenhouse gas inventory: Accounting methods and policy implications
- Climate finance: Mechanisms, flows and the global investment gap