The integration of volatile, renewable heat sources in district heating networks leads to an increase in temperature changes and thus to a greater load on the materials. The research project is aiming to develop transferable condition assessments of district heating networks. To this end, the researchers are recording their actual loads based on operating data from utility companies and long-term monitoring results. This will enable the material characteristics of the system components to be determined more precisely for planning and designing line sections and entire networks. The project results will be incorporated into concepts that will extend the useful service life of the networks.
The integration of renewable energy into Germany's energy system poses a challenge to district heating networks to a previously unparalleled extent. CHP power plants require a flexible mode of operation to enable them to respond to volatile renewable power generation. At the same time, volatile renewable heat sources will have to be increasingly integrated directly into district heating systems in the long term. For the district heating networks this means an increase in the temperature changes and thus an increase in the loads on the materials, resulting in a shortening of the service life. The influence of increased cycle loads on the networks is unknown. These arise in particular
- through the flexibilisation of the power plant fleet through storage systems,
- through the use of the network as a storage system,
- with volatile solar thermal feed-in,
- through the flexible operation of CHP plants and the temporary shutdown of power blocks and
- with an increase in the connection density through expanding the district heating in smaller line sections.
The project is investigating the effects of real-case cyclic loading on heating networks. The findings will help to qualify heating networks for future challenges and also identify load limits in order to achieve a high level of supply security with increasing volatile heat generation.
The tasks of the research partners are defined by six work packages. In Work Package 1, the real loads, future modes of operation and economic influence parameters will be determined. In Work Packages 2 to 5, qualified test institutes and research facilities will conduct material investigations and develop system components for the mechanical replacement model. These will be brought together in Work Package 6, tested by means of sample calculations and prepared for the relevant standardisation committees.
The starting point of the investigations is to determine the actual loads on buried plastic jacket pipes based on in-situ, long-term measuring points in heating networks. It is assumed that particularly the temperature-induced cyclic loads that arise as a result of the operating modes of the production plants determine the service life of the route sections in networks. Material investigations will determine the load limits of system components such as rigid polyurethane foam, HDPE jackets, expansion pads as well as surrounding soil and jacket pipe joints. This relates to complex loads caused by, for example, temperature, mechanical load cycling or humidity. The results of the material investigations will extend the existing mechanical model for replacing system components. In addition, the researchers are developing a methodology for assessing the condition of line sections using temperature-based modes of operation. These will be incorporated into new engineering tools for integrating renewable energies into existing heating networks.
Central idea and approach
The state of knowledge about the life-limiting factors of district heating systems forms the starting point of the project. This is because the material characteristics of the system components with cyclic, superimposed loads have not yet been investigated. The research results are intended to map real load conditions and to better estimate the technical conditions of existing networks. This will enable an optimised production of the required heat quantities and more considerate handling of district heating networks as a resource. In addition, the project results form a necessary basis for the future technical qualification of district heating networks.
Testing and application
The AGFW industry association is reporting on the project as part of its committee meetings on the technical regulations. It has founded a project-supporting working group entitled “PWG Technical Service Life Analysis”, in which the research partners regularly present the project results achieved to the participants from practice. This allows the research and application to be dovetailed.
The engineering tools developed in the project will be validated and tested at the existing long-term measuring stations as part of example calculations. The members of the project-supporting working group are potential users of these calculation tools.