Large Thermal Energy Storage (LTES) emerges as a key for the energy transition, facilitating cost-effective and reliable renewable heating and cooling for District Heating and other large-scale applications. Moreover, it enables sector coupling, harnessing excess heat and bolstering electricity grid flexibility. During periods of surplus renewable electricity, heat or cold can be generated, stored, and later deployed during peak demand, curbing additional electricity consumption. Similarly, excess heat from renewable-energy-based combined heat and power plants can be stored in LTES for later use, mitigating demand spikes while generating more profits for the plant operators. This use is key in the business model behind the Pit Thermal Energy Storage (PTES) in Høje Taastrup.
By Geoffroy Gauthier, Energy and Calculation Engineer – Project Manager, PlanEnergi
Published in Hot Cool, edition no. 6/2024 | ISSN 0904 9681 |
LTES are crucial for the energy transition
Heating and cooling constitute nearly half of the world’s energy consumption, underscoring the critical role of thermal energy storage in aligning renewable energy production with the energy demand. Results from IEA-ES Task 39 have unveiled essential insights:
Affordability and Efficiency: Large-scale thermal energy storage (LTES) is cost-effective compared to electrical storage solutions, with specific investment costs plummeting from 4 to less than 1€/kWh capacity. In comparison, large-scale pumped hydro storages run between 100 and 200 €/kWh capacity. Moreover, specific heat losses diminish as storage volume increases, meaning efficiency increases.
Long-term Storage Capability: LTES systems excel in storing heat and cold over extended periods with minimal losses.
Utilization of Low-tech Solutions: LTES technologies leverage readily available and easy-to-produce materials, ensuring accessibility.
Insights from IEA-ES Task 35 (another task from the Energy Storage branch of the International Energy Agency) emphasize the indispensable role of LTES within District Heating systems for optimizing energy system costs and enhancing energy efficiency. One study’s results show that doubling the thermal storage capacity for a reference scenario in Germany would lower primary energy use while reducing total energy production costs.
Advancements in Thermal Energy Storage Technologies
The task’s deliverables are directed to diverse stakeholders, including policymakers, researchers & engineers, and project developers. Deliverables include:
- Online and physical leaflets, serving as an introduction to LTES and providing use cases
- Synthetic reports about how to carry an LTES project from idea to implementation, fostering informed decision-making and facilitating project implementation
- Reports about:
- LTES project development stages
- The development of a materials and components database
- A modeling tools comparison methodology specifically developed for LTES
- List of project references for the main types of LTES technologies
- A database of materials and components
- A policy workshop (recording and presentations are available online)
All of these deliverables are available on the website of IEA-ES Task 39: https://iea-es.org/task-39/deliverables/.
Figure 2. The main stages and main stakeholders of LTES projects identified in Task 39
IEA-ES Task 39, which ended with a policy workshop in December 2023, focused on four primary technologies capable of annually storing over 1 GWh of thermal energy: Tank, Pit, Borehole, and Aquifer Thermal Energy Storages. The task aimed to provide reference materials to accelerate LTES implementation in District Heating (DH) and industrial settings, drawing on expertise in energy system simulations, storage materials, and construction.
The systems studied in Task 39 have been defined as large sensible thermal energy storages designed to store a minimum of 1 GWh/year at atmospheric pressure. The stored heat should be suitable for discharge into District Heating networks, maintaining temperatures between 50°C and 100°C. These technologies offer versatility, serving as daily, seasonal, or multifunctional thermal energy storage adapted to various heat sources and load profiles.
Figure 3. The main types of LTES technologies can store heat or cold in water or the subsurface.
Pit Thermal Energy Storage (PTES): Denmark’s Innovative Solution
Denmark has emerged as a pioneer in Pit Thermal Energy Storage (PTES), particularly in combination with Solar District Heating (SDH) for seasonal storage. PTES, now utilized for short-term thermal storage in district heating networks, holds immense promise for decarbonizing the heating sector.
Figure 4. The concept of seasonal storage can be illustrated with the mismatch between solar thermal energy production, which peaks during the summer, when the heat demand is lowest: an LTES can then be used to store large amounts of heat during the mismatch period to be reused later during the year (hence the term “seasonal” storage).
The PTES project in Høje Taastrup, Denmark, illustrates this new use of the PTES technology, showcasing substantial fuel savings and CO2 emissions reductions. This 70,000 m3 PTES, owned by Høje Taastrup District Heating and VEKS (transmission company for the Western suburbs of Copenhagen), is facilitating the optimization of electricity production in the Copenhagen area and is used in the local distribution DH network for peak shaving, yielding significant environmental and economic benefits. The PTES in Høje Taastrup received the Global District Energy Climate Awards in the Sector Coupling category during the Euroheat & Power Summit on 14-15 November 2023 in Brussels, Belgium.
The list of PTES implemented by Danish Companies and consultancies can be seen in Table 1.
Table 1. Historical table of PTES development in Denmark or initiated by Danish companies and consultancies. The last line is the only storage not used as seasonal storage for solar thermal heat.
Looking Ahead: Accelerating the uptake of LTES
According to a recent study by Aalborg University, the extension of DH to reduce fossil fuel imports and consumption has tremendous potential, for instance, to use the large amounts of available waste and geothermal heat. “District Heating is the next enabling technology for integrating Renewable Energy.” This potential can only be supported by using LTES, which in turn needs to be widely implemented.
Within Task 39, the policy workshop has highlighted the main challenges and opportunities for the vast deployment of LTES:
- raising awareness about LTES technologies, demonstrating their robustness, and illustrating typical implementation process from idea to commissioning
- reducing uncertainties by facilitating the permitting process and increasing standardization
- shortening the realization time of LTES by sharing experiences and involving all stakeholders
- decreasing costs through materials and process development and industrial network expansion
These will be tackled by Task 45, the follow-up of IEA-ES Task 39. This new project will run from 2024 to 2027 and aims to leverage insights from the previous task. In parallel, the EU-funded project “TREASURE” will focus on demonstrating PTES in five European countries, building upon reference initiatives like the PTES in Høje Taastrup.
Figure 5. Function of the PTES in Høje Taastrup. Source Høje Taastrup Fjernvarme and VEKS
What are IEA-ES Task 35 and Task 39?
A Task is an activity in the Energy Storage Technology Collaboration Programme (“TCP”) of the International Energy Agency (“IEA-ES”). It is working on a specific topic: Large Thermal Energy Storages (LTES) for Task 39.
Task 39 gathers 60 experts from 36 institutions and 11 countries: Austria, Canada, Denmark, France, Germany, Italy, the Netherlands, Sweden, Turkey, the UK, and the USA. They can be found here: https://iea-es.org/task-39/institutes-companies-and-experts/. The experts work in the fields of Research and Development and Industry.
1 GWh (1,000 MWh) is enough heat to supply the demand of 62 households for one year. Households in Denmark consume, on average, 16 MWh/year for space heating and hot water in 2020. (Source: Danish Energy Agency)
The PTES in Høje Taastrup serves three CHP plants and three waste-to-energy plants (partners in the project). Optimizing electricity production (sector coupling) and reducing peak production in the Copenhagen area is expected to represent 27.4 TJ of fuel saved/year and a total CO2 reduction of 6,200 tons/year.
The total investment is 10.7 Mio €. The simple payback period is 12 years.
“Large Thermal Energy Sorage (LTES) will help shape the future energy landscape.” was published in Hot Cool, edition no. 6/2024. You can download the article here:
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