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The most powerful district heating storage system in Germany commenced operation in Mannheim in the autumn of 2013.
© Großkraftwerk Mannheim Aktiengesellschaft
Thermal storage system and power-to-heat

The graphic shows the working principle and system integration of a non-pressurized displacement heat storage system. Displacement heat storage systems are used in district heating networks because they have a simple structure and are therefore relatively inexpensive.
© TU Berlin, Institut für Energietechnik (IET)

The researchers analysed in detail possible charging and discharging strategies for heat storage units. The graphic shows the operation of the three back-pressure combined cycle units (model results for 2015). In the reference case without heat storage (left), two of the three cogeneration units are in operation from 2 to 7 November. The peak thermal load is provided by the heating plant as the district heating demand and electricity prices are not high enough to add the third unit. The operation with heat storage is shown on the right (capacity: 1,200 MWh or 2 hours of the maximum heat capacity of the CHP plant).
© TU Berlin, Institut für Energietechnik (IET)

When are flexible district heating systems beneficial?

The integration of thermal storage in cogeneration plants leads to more flexibility in district heating networks because the power generation can be better adapted to the power requirements. Power-to-heat systems couple the electricity and heating sectors to utilise renewable electricity for district heating. Together with partners from science and industry, researchers at the TU Berlin have investigated when the construction of heat storage and power-to-heat systems can be economically and ecologically beneficial.

Two options are available in order to make a district heating supply system more flexible: integrated heat storage and electrical heat provision. A crucial aspect here is how the system deployment can be optimised economically. Here it is important, for example, to optimally utilise the time point and timeframe of the system operation, i.e. when can the heat storage systems be charged or discharged and with what capacity? The economic boundary conditions and the heat load must also be included of course.

To this end, the researchers identified the characteristics of heating plants by using thermodynamic power plant simulations and the advantages of integrating thermal storage and power-to-heat systems. These serve as expansion and flexibility options in district heating systems. Eligible technologies for power-to-heat systems are electrical heat supply systems using heating elements (electric heaters) and heat pumps. Electric heaters convert electrical work directly into thermal energy and can be provided as either electrode boilers or electric instantaneous water heaters.

The plant data is incorporated into an optimisation model for operationally planning district heating systems. It provides an hourly and block-based operation mode for all systems and heat storage units involved. The model therefore supports operators in making investment decisions.

In order to be able to estimate the effects of these expansion technologies on the German energy system, the researchers determined the contribution of various expansion stages for heat storage systems in German district heating networks. The aim is to ensure functional electricity storage and increased power generation from CHP plants in future. They also investigated the benefits of heat storage and power-to-heat systems for providing secondary control reserve and minute reserve, and the contribution they can make to system stability.

In summary, the research revealed the following findings:

  • The operation and economic efficiency of heat storage systems are highly dependent on the flexibility of the cogeneration plant, power-to-heat ratio and electricity production costs. In addition, the volatility and level of the electricity prices must also be taken into account. A precise analysis of the individual case is required.
  • The degree of heat coverage provided by the CHP plants can be increased by using heat storage systems, especially when the CHP plant has a low number of full-load hours.
  • The use of power-to-heat systems is highly dependent on the efficiency (COP) and total cost of electricity.
  • Possible synergies between the heat pump and heat storage system are rather low in the cases investigated.

The project was carried out at the Technische Universität Berlin in cooperation with the University of Applied Sciences and Arts Hanover, Leipzig University and with consultancy provided by industrial partners.

Further information on the research project can be found on the ENERGIEWENDEBAUEN website. The sometimes very detailed analyses on the use of heat storage and power-to-heat systems with corresponding economic considerations can be found in the Final Project Report (PDF, 14 MB).



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Project coordination and research
TU Berlin, IET


Projects, reports, news and analysis from the research initiative ENERGIEWENDEBAUEN