Different countries have ambitious goals for energy and climate change adaptation and mitigation, aiming for 50 % more efficient energy use and 100 % renewable energy generation. Thermal Energy Storage (TES) is a key enabling technology for a realisation of a carbon neutral energy system. District Heating (DH) is a mature technology for the heating of the built environment, especially in large cities. Large-scale Thermal Energy Storage (LTES) systems are necessary to further decarbonise the DH systems and to enable a more flexible operation. LTES are needed, in order to further reduce the specific costs of the storage technology and to have storage capacities that are better suited to the sizes of larger DH systems. Present experience with TES for integration in DH is in the utilisation of Pit Thermal Energy Storage (PTES) systems up to 200,000 m3 and of Tank Thermal Energy Storages (TTES) systems up to 50,000 m3. Also the subject of this task are the TES technologies Aquifer Thermal Energy Storages (ATES) and Borehole Thermal Energy Storages (BTES).
Large-scale thermal energy storages offer more flexibility in DH Systems (also adding operational flexibility to power plants and industrial processes), they enable a higher share of renewables and waste heat, they can provide peak shaving functionality for electricity grids through Power-to-Heat (P2H) thus enabling sector coupling of the power and heating sector.
The market for large thermal energy storages is growing, with new plants built and planned in Denmark and Germany, mostly PTES with volumes in the range of 400,000 to 500,000 m³ (in Denmark). In order to facilitate and accelerate the market uptake of these large storages, better materials and knowledge is needed to improve the service lifetime of storages, better tools are needed for designing, planning and integrating the storages and more knowledge of the potential and integration possibilities of these storages is needed for decision makers.
Aim and Objectives of the Task
The Task aims at determining the aspects that are important in planning, design, decision-making and realising very large thermal energy storages for integration into district heating systems and for industrial processes, given the boundary conditions for different locations and different system configurations.
The key objectives of the Task are:
- Definition of a number of representative application scenarios, the connected boundary conditions and Key Performance Indicators
- Improve LTES materials and materials performance measurement methods
- Prepare guidelines for obtaining proper water quality
- Compare the performance and accuracy of simulation models for LTES
- Derive validation tests for LTES simulation models
- Generate information packages for decision makers and actively disseminate the information
For these purposes, four subtasks work on the content subjects:
- Subtask A: Application Scenarios, Assessment of Concepts, Integration Aspects
- Subtask B: Components and Materials Database
- Subtask C: Round robin simulation
- Subtask D: Knowledge base for decision makers
Subtask Description and current Activities
Subtask A: Application Scenarios, Assessment of Concepts, Integration Aspects
Subtask A aims at building a list of existing hot water seasonal storage in the world and select well documented and instrumented ones to be used as reference cases for Subtask C work. A second target consist in defining a list of relevant technical and economical KPIs to assess and compare the different technologies considered (ATES, BTES, LTES, PTES). A last target consists in building a matrix used as a support for decision makers to guide them in the first steps of a project definition to select suitable technology for their specific use.
The definition of what is a seasonal storage and what conditions it needs to fulfil to enter into the scope of work of this task is currently being refined. A list of existing installations is currently available and quite complete. A consolidated list of KPIs is also available for PTES technology, coming from different national projects.
In the following months, the work will consist in finalizing the scope of work of the task in term of technology, size, level of temperature to make sure all participants are well aligned on the objectives. The list of existing projects will be closed, and a selection of best documented use cases will be performed at the end of 2021. A brainstorming phase for relevant technical and economical KPIs will continue until end of 2021 and the final consolidated list will be delivered in April 2022.
Subtask B: Components and Materials Database
The goal of Subtask B is to define common test procedures for hygrothermal and mechanical tests for materials, to define the water quality and to create a database for TES materials that go beyond the standard values. The activities are grouped as follows:
- Hygrothermal and mechanical tests
- Water condition and corrosion protection
- Materials Database for LTES
Ongoing work: The activities were structured, and system boundaries were defined. The activities of the subtask will be focus on the PTES and the outputs can be applied for other type of long-term thermal energy storage methods. In the coming Expert Meeting (2 and 3 of Nov. 2021), the specific actions for activities 1 and 2 will be decided. University of Birmingham has already a data base and it is possible that the results of the data obtain in activities 1 and 2 can be stored in the existing data base.
Subtask C: Round robin simulation
The aim of Subtask C is to validate and compare numerical simulation models for LTES through Round Robin simulations. Models for the following storage concepts will be considered:
- Aquifer Thermal Energy Storage (ATES)
- Borehole Thermal Energy Storage (BTES)
- Pit Thermal Energy Storage (PTES)
- Tank Thermal Energy Storage (TTES)
The investigations are limited to simulation models for large-scale thermal energy storage (LTES). Other system components like pumps, heat exchangers, buffer tanks etc. are not considered. The work furthermore focuses on accuracy, applicability and usefulness of the considered models. The activities are subdivided in the following parts
- Preparation of an inventory for LTES simulation models
- Round Robin simulations
- Test procedure and recommendations for LTES simulation
At present, it is expected that in total between 30 and 40 different LTES simulations models will participate in the model comparison. Currently the detailed definitions of the test cases for the Round Robin simulations are under preparation. The simulations are planned to be carried out in 2022.
Subtask D: Knowledge base for decision makers
The aim of this subtask is to gather information from the different subtasks to make a knowledge base for decision makers that might have to deal with the evaluation of projects including LTES. This information will be disseminated to different target groups through a bi-annual newsletter, various workshops, and the distribution of leaflets and other electronic information. The subtask’s goal is to create general interest in the target groups for implementing LTES and provide them with generic tools to identify the favourable conditions for it.
Currently, Subtask D is determining which kind of information is needed from decision makers. The first edition of the newsletter is also being prepared and will introduce the work being done in Task 39, together with some LTES-related articles. Content will be available in several languages.
Future work will be focused on gathering information corresponding to the identified information need of decision makers (and subsequent target groups).