Research projects

(research projects are partially available in German language only)

selected research projects:

Development of a mathematical model for heat losses at twin pipes – IGF Project No. 1EWN

Utilizations of pre-insulated twin pipe systems for heat distribution offers advantages at various levels in utility companies. Dimensions of trenches for District Heating (DH) piping systems may be reduced, minimizing costs in civil engineering. In addition, DH potentials may be realized within areas offering limited space, e.g. in historic city centers. For the design of DH networks, heat losses of pipe systems are an important criterion. In case of twin pipe systems, network operators report significant differences between heat losses occurring in situ and heat losses that should occur, according to the manufacturer.

Aim of the research project is the development of a mathematical model for thermal losses, which shall be tested by means of numerical simulations, pilot tests and measurements in the field and iterative and continuous adaptions of the mathematical model.

The scientific approach and intermediate results are:

  • Development of a mathematical model for calculating heat loses,
  • Investigation of internal twin pipe temperature fields and resulting internal stresses within the heat insulation and soil,
  • Conceptual development of pilot-plant tests,
  • Conceptual development of field measurements,
  • Execution of pilot tests,
  • Carrying out field measurements,
  • Evaluation of results,
  • Final report.

Benefit of an appropriate method for calculating heat losses is given for small and mediums-sized enterprises (SME) and results in the possibility of an improved design of heating networks, improvement of existing standards EN 15698, EN 15632, EN 13941, EN 253, and an increase in acceptance of twin pipe systems, e.g. in the Scandinavian area. Utilizing twin pipe systems will lead to general reductions of heat losses in heat distribution and reductions of costs caused by civil engineering. Thus, new potentials for DH will be developed.

Operational Lifetime of District Heating Systems – support code: 03ET1335A

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 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

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 5 scientific and 3 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 financed by the federal ministry of economy and energy (FKZ 03ET1063A).

Bedding resistance of district heat pipes under cyclic load

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).

Temperature field in soil around DH pipe
Developing and validating a parameter matrix for the evaluation of TIR-Images for the assessment of district heating (DH) piping systems – AiF/BMWi project 15802 N

The ageing of DH piping systems shall be assessed and evaluated by taking precise and repeated thermographic infrared (TIR) measurements of airborne systems. However, the quality of these measurements is strongly influenced by numerous parameters interfering with the TIR system. Therefore, deviating results of repeated scans are not necessarily connected to a deterioration of the DH piping system. Against this background, a numerical model is developed, mapping the interactions between interfering parameters and TIR measurements. Thus, differentiated statements for evaluating the ageing of DH piping systems are achieved.

Test apparatus for DH pipes driven with cooled fluids
Operational capability of district heating (DH) supply systems within air-conditioning and refrigeration systems AiF – 13849 N

Against the background of a steady increase in cooling demand (air conditioning of a building as well as refrigeration systems for commercial or industrial purposes), the market for "cooling services and concepts" becomes more and more dynamic. Thus, a new market segment for power suppliers and related companies opens, supporting e.g. manufacturing or facility management companies. This trend has led to the utilization and installation of centralized power houses for corresponding realties, supplying electrical power, district heat and cold as well as other media, such as pressurized air, etc. However, the application, installation and advantages of these centralized power houses determine corresponding grids for the distribution of each media to be supplied.

Visualisation of influx processes in an acrylic glass model accumulator
Parameters for the evaluation of construction and management of heat accumulators

In cooperation with the Institute of Power Engineering (TU Dresden), the FFI examined the thermal behavior of sensible heat accumulators in drinking water and heat applications. Main focus of this research was the evaluation and valuation of the energetic and exergetic efficiency of heat accumulators by means of dimensionless parameters. Within this context, internal losses due to vertically oriented heat conductions, the instationary development of thermal mixing zones and exergetic losses within layered heat accumulators were of special interest. Against this background, a systematic approach for describing the performance of heat accumulators has been developed by means of several dimensionless parameters. In order to determine relevant key indicators of heat accumulators, specifications for a systematic testing were developed. In addition, recommendations on the construction and the operation were derived. By applying the methodologies developed for testing and evaluating sensible heat accumulators, a better understanding and generalized evaluation becomes possible with special regards to In addition, a proof regulation for the determination of the indices was created and some recommendations for the construction and the management of heat primary energy savings, efficiencies, economic savings and utilization of thermal heat.

Flow- and return line taken from a 30 years old DH network
Remaining service life of 30 year old bonded pipes

Plastic sheath piping systems of the MVV Energie AG were extracted from the district heating system in order to determine the remaining service life of these components after 30 years of operation. Within this context, the technical parameters of the PUR-foam were examined, still meeting all technological requirements of present regulations. Further analysis unveiled a remaining service life of the plastic sheath pipes extracted of up to 38 additional year of operation. Therefore, the total service life of some pipes sums up to 68 years. Thus, the service life of plastic sheath pipes outruns the expectancies by far posing pressing questions on the economic asset and maintenance of these systems, as well as the transferability of these results to other DH networks.