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Thermal storage systems for improved flexibility of CHP plants

EnEff:Wärme - Forschung für energieeffiziente Wärme- und Kältenetze

In the autumn of 2013, the most powerful district heating storage system in Germany went into operation in Mannheim.

© Großkraftwerk Mannheim Aktiengesellschaft

Settlement summary

Project status Projektstatus: Phase 1Concept
Project themes

Project description

Greater flexibility of CHP plants can be achieved by integrating thermal storage systems in district heating networks, as this would allow for a decoupling of electricity and heat supply in terms of time. The purpose of this research project is to investigate the conditions under which investments in the construction of a storage system are economically viable, and which improvements would be the result from an environmental policy perspective. This way, the study will be able to assist operators in their investment decisions, and will form a foundation for the future discussion of the political framework. The project is being carried out at TU Berlin in cooperation with Leipzig University and the University of Applied Sciences and Arts Hanover, with consultancy provided by industrial partners.

Project goals, work programme

The aim of this research project is to determine the benefits of thermal storage systems as extensions of district heating systems for various technical and economic framework conditions.

For this purpose, a mathematical model is being developed that reflects the behaviour of different CHP plants, hot water generators and electric heating systems with heating rods or heat pumps in conjunction with thermal storage systems. Electric heat supply (electrical power to heat) is of particular interest when combined with thermal storage systems. The operating mode of each plant is optimised within the system under economic aspects for different input data. Based on the evaluation of results for different system configurations (with and without individual extension technologies), market-driven impacts of individual extension technologies, and combinations thereof, on the energy system can be determined.

The impact of the extension measures on the provision of balancing energy, and the reliability of the results in terms of a potential liberalisation of district heating networks, e.g. for CHP plants, are being investigated. For balancing energy, the impact on the capacity to provide secondary control reserve and minute reserve, as well as on the electrical load ramps of heating plants, are being analysed.

Finally, the individual results are projected onto the German energy system in order to make quantitative statements on the following points:

  • Reduction of CHP must-run generation, and more feed-in from renewable energy sources;
  • Reduction of CHP must-run generation, and more feed-in from renewable energy sources;
  • Feedback of changed feed-in from CHP plants on electricity rates.
Conception, modelling

When adding flexibility options to a district heating system with a thermal storage system and electric heating, an economic optimisation of the system use is crucial. It is imperative to make an optimal use of temporally overlapping degrees of freedom taking into account economic boundary conditions and thermal load. Typical degrees of freedom would currently be point in time and time frame of systems operation, at which load point the system is operated, and when and with which performance the thermal storage system is loaded or unloaded.

Due to the scope of the optimisation model and its desired accuracy, the chosen approach is that of mixed-integer optimisation: at the beginning, representative district heating and CHP plants are analysed, and a typical set of generators and the properties of the network are defined. Then heat demand and operation mode of the flow temperature can be defined. As a basis for optimisation, the economic boundary conditions, the dynamic behaviour of the thermal storage systems, as well as stationary and dynamic CHP characteristics are needed.

Analysis and dynamic modelling of various thermal storage systems is being carried out by the University of Applied Sciences and Arts Hanover, generating input data for the higher-level optimisation model. Based on a foundation model of the European power plant fleet, Leipzig University is creating different scenarios to predict future electricity demand and rates (hourly contracts and balancing energy rates).

The industrial partners are supporting the project with their technical knowledge in the selection of assumptions and input data. In addition, characteristics of various real CHP plants are being made available.

Besides the properties of the existing plants of industrial partners, thermodynamic power plant simulations are being carried out at TU Berlin in order to obtain the necessary input data for further typical CHP plants. With this input data, optimisation calculations are then carried out and the following criteria are considered in greater detail:

  • Increase of the economic efficiency of CHP plants;
  • Improved flexibility of the existing heating plant fleet;
  • Improved flexibility of the existing heating plant fleet;
  • Improved flexibility of the existing heating plant fleet;
  • Increase in overall energy efficiency and
  • Primary energy savings and therefore lower CO2 emissions

Optimisation makes for operation that is optimised down to an accuracy of individual hours and units of all participating plants and storage systems, so the considered criteria can be accurately assessed when compared with the reference case.