Research projects

(research projects are partially available in German language only)

selected research projects:

  Potentials of absorption heat pumps in centralized heat supply networks



District heating is a cross-sectoral key technology for transformation of existing energy supply and energy transition concepts. Lowering of network temperature is a prerequisite for high energy efficiency and economic operation of the fourth district heating generation. An innovative heat supply concept is developed as part of this research project.

The technological solution is integration of a thermally driven absorption heat pump (AHP) in house substation. Energy from DH forward pipe and return pipe is used to drive AHP using this innovative heat supply concept, whereby heat supply of heat consumer direct from DH network is ensured. Operating parameters of AHP are adapted to operation of DH network. Interactions of lowered return temperature on entire DH network, e.g. distribution losses, DH network pump performance and service life of DH network components are determined both experimentally and by simulation.

FFI examines this innovative heat supply concept by simulation calculations. Sub-models heat generator, distribution network with network pumps, house substation with integrated AHP and heat consumer are gradually developed with analytical and static modeling approaches to reflect individual operating characteristics of DH network segments. Finally sub-models are merged to an overall model in order to quantify energy balance as well as integral influences of modified house substation on entire DH operation.

FFI collaborates with "Institut für Gebäudeenergetik, Thermotechnik und Energiespeicherung der Universität Stuttgart" ("Institute for Building Energetics, Thermotechnology and Energy Storage – University of Stuttgart"), which performs laboratory tests for implementation and testing of this innovative heat supply concept. At first time an AHP is designed and constructed whilst taking into account typical operating parameters of DH networks. At various network-side and consumer-side temperatures, Performance ratios of AHP are measured at various DH network-side and heat consumer-side temperatures. A characteristic diagram for steady-state operation is generated from these measurements. Both full and partial load operations are examined based on DH network data in following demonstration mode. Then measured results are incorporated into simulation model of FFI in order to further optimization and calibration of simulation model. Finally simulation calculations are run to quantify following effects of integrated AHP on an entire DH system:

  • Lowering DH return temperature
  • Improving efficiency of heat supply
  • Reduction of heat losses of DH networks
  • Saving power of network pumps
  • Increase of technical service life of DH network components

This research project is supported by the German federal ministry of economic affairs and energy (project number: 19696N).

Publications (FFI):
  • Yang, F., Grage, T.: „Senkung der Rücklauftemperatur im Fernwärmenetz“
    EuroHeat&Power, 48. Jg (2019), Heft 3–4, S. 34-37 (German language)
  Development of empirical methods for determination of thermal conductivity and heat loss based on metrological methods for pre-insulated plastic bonded twin pipe systems

radial sensor application

measured values

simulation (FEM)

test set-up in soil

Usage of pre-insulated twin pipes in district heating (DH) networks gives some advantages against single pipes. Trenches can be sized narrower. This reduces civil engineering costs and gives new DH development potentials in areas with limited space e. g. historic town centres. Branches of service pipe and return pipe take place at the same levels. Wall ducts for building feed are reduced from two to one. Heat loss of a twin pipe is lower than heat loss of two single pipes according to manufacturer's data for single and twin pipes with equal service pipe dimensions and equal thermal insulation series. Some energy utility companies have measured heat losses in their twin pipe DH networks that significantly differs from heat loss according to manufacturer's data. Because heat loss is a crucial criterion for DH network dimensioning, partially significant differences between measured heat losses and manufacturer's data hinder usage of pre-insulated twin pipes in DH networks.

Any unified calculation method doesn't exist for heat loss of twin pipes. One reason for this is, that the more complex geometry of twin pipes compared to single pipes disallows any analytic solution. This applies mutatis mutandis for determination of thermal conductivity of thermal insulation in twin pipes. Also dimensioning of pipe statics is impossible without knowledge of thermal properties in twin pipes. Also this hinder usage of pre-insulated twin pipes in DH networks.

One goal of this research project is development of easy to use calculation methods for heat loss and determination of thermal conductivity of thermal insulation in twin pipes. Hindrances for usage of twin pipes in DH networks shall be removed with the capability of an easy to use and sufficiently accurate calculation method for heat loss.

Simulations with finite element method (FEM) are done on the one hand and measurements in laboratory with repeatable enviroments (air, soil) are done on the other hand to develop calculation methods. Development of calculation methods is finished now. This goal of the research project is achieved. A first implementation of this research result are measurements for determination of thermal conductivity of thermal insulation in twin pipes by customer's order. Furthermore this research result shall be included into standards EN 15698-1, EN 15632 and EN 13941. A first FEM simulation of pipe statics is done based on now known thermal properties in twin pipes.

FFI collaborates with "Institut für Geotechnik – Leibniz-Universität Hannover" ("Institute for Geotechnical Engineering – Leibniz-University Hanover"), which performs the FEM simulations. Additionally some companies of DH sector support this research project with provided twin pipes and services.

This research project is supported by the German federal ministry of economic affairs and energy (project number: 1 EWN).

Publications (FFI):
  • Schuchardt, G.-K., Kraft, S.: „Bestimmung der Wärmeverluste werkmäßig gedämmter Verbundmanteldoppelrohre“
    EuroHeat&Power, 46. Jg (2017), Heft 12, S. 15-18 (German language)
  • Kraft, S.: „Bestimmung der Wärmeleitfähigkeit von werkmäßig gedämmten Doppelrohren“
    3R Fachzeitschrift für sichere und effiziente Rohrleitungssysteme, 09|2018, S. 82-86 (German language)
  • Kraft, S.: “Determination of thermal conductivity of pre-insulated twin pipes”
    EuroHeat&Power, English Edition, I/2019, S. 26-30
  Operational lifetime of district heating systems
90°-bow, set-up for load test
linear water tightness, bending of a flexible pipe before test

The whole district heating (DH) supply system (generation — distribution — consumer) is in an unstable condition once the technical lifetime of the system has passed. The frequency of defects within the system increases jeopardizing the reliability of supply. Therefore, operators of DH systems are bound to know the condition of the DH supply network and its components within as detailed as possible for taking relevant steps to avoid serious damages. For this purpose, investments for the maintenance of the system have to be planned, budgeted and done. These investments are of major importance for the economic operation of DH systems. Therefore, the timing of these measures has to suit to the condition of the DH system in order to allow

  • Market-oriented and more flexible operation of combined heat and power (CHP)-plants and heat sources,
  • Attachments of thermal storages for a more flexible operation of CHP-plants and heat sources attached,
  • flexible operation of CHP and heat sources, e.g. by attaching thermal storages,
  • Utilizing the thermal inertia of the heat distribution system,
  • Integration of most different and renewable heat (solar-, geo-thermal, waste heat, etc.),
  • Densifying the DH grid by attaching additional customers.
  Possibilities and limitations of trenchless district heating supply systems
field test: trenchless laid DH pipe, measuring points nearby fixed point

Under the coordination of the AGFW (German Heat and Power Associating), the FFI evaluated the "Possibilities and Limitations of trenchless district heating supply systems". On cooperation with five scientific and three industrial partners, field tests on 10 different trenchless DH-piping systems have been executed, simulating heat loads of DH systems in situ. Within this context, the FFI supervised the operation and data collection of these most sophisticated field tests and focused on the evaluation of the DH piping systems previous to and after operation. Summarizing, this projects is outstanding concerning the expense, effort and extent of field tests performed. Thus, the uniqueness of this project necessarily leads to additional and most urgent issues carrying the contents of this project into the future of DH research. The project was supported by the federal ministry of economy and energy (FKZ 03ET1063A).

  Bedding resistance of district heat pipes under cyclic load
cyclic load test

Within this research project, some basic knowledge gaps regarding the bedding resistance of district heat pipes due to lateral alternating loads were filled. In cooperation with the Institute for Geotechnical Sciences Hannover (IGtH), the FFI ran systematical test series on a pilot DH piping site in a 1:1 scale for different alternating transversal loads. Project partners in this research association were the geotechnics institute at the Hanover Leibniz University (IGtH) and the district heat - research institute (FFI). The FFI task was to run systematical experiments as pilot-plant testing of 1:1 scale district heating pipe segments under different alternating loads transverse to the axis of line. Applying appropriate instrumentation, the bedding resistance is related to the load intensity and number of cycles. Furthermore, the FFI scheduled laboratory tests for examining material properties of the insulation and plastic sheath pipes under cyclic loads. These experiments are an important element within the context of understanding the load-bearing behavior DH piping systems. This project was supported by the federal ministry of economy and energy (FKZ 03ET1063C).