TFlex - Temperature-flexibilisation in low-load operation of local district heating systems

Within the research project TFlex it was checked if the losses adherent to small district heating networks during low-load periods can be reduced. One possible solution is by deactivating the network and supplying the customers from previously charged decentralized storages. The optimal clustering of the storages and the possibility of solar-charging the storage were calculated with the aim of a guaranteed one-hundred percent heat supply.

Short Description




Starting point / motivation

It is the aim of this project to present a method to reduce the losses of small district heating networks during low-load periods. Ten years ago losses of about 30 % of the delivered heat were quite common. Today, provided that there is a good quality control, losses account for approximately 15 % of the delivered heat.
Losses are characterized by following parameters:

  • Temperatures of supply and return lines (thermal losses)
  • Diameter of tubes (thermal losses and pumping losses)
  • Insulation of the tubing (thermal losses)
  • Length of the network (thermal losses and pumping losses)

Those parameters have been optimized as far as possible. A further improvement of the losses without changing the network topology and working principle is economically difficult.

Contents and Objectives

With the help of decentralized storage units, which are to be installed at the heat-customers location, the losses can be minimized without expensive changes in the network layout. In low-load periods the network can be shut down and the heat customers will be supplied by the previously charged storage units. This method proves particularly efficient in networks with a high portion of low-load operation.


Methodically the project is split into three parts. The first part deals with data-collection and the energetic modelling of the considered heating-grids. After modeling storage-sizes the appropriate control-algorithms are optimized. A further working package deals with the integration of solar-thermal energy for additional load-relieving in the grid. In the second part questions in regard to grid-technology will be answered. The grid-models and boundary-conditions developed will be implemented into the models used in the first part. Finally, the results are subsequently analyzed according their economic impacts. Also sensitivity and risk analysis are carried out.


Grid losses in the investigated district heat networks can be reduced by 34%because of the installation of decentralized storages and network shutdowns. Dependent on the storage locations 3 to 6 % of the fed in energy can be saved compared to the status quo.

It turned out that the storage size correlates less with the nominal connection power than with the annual energy quantities. With a switch-off period of about 3 to 4 days the highest energy savings can be achieved. The most important optimization criterion is the adaptation storage size. This avoids unnecessary storage volumes and thus heat losses.

A further enlargement of the storages, which at first sight appears to be advantageous and thus should reduce the number of start-up processes, again requires longer charging times and is associated with the problem of higher losses over the storage surfaces. Therefore, an important criterion is not only the duration of the grid off-times, but also the grid operating time. The quotient of these two parameters should be as large as possible in order to minimize the network losses. This can be achieved with increased storage insulation, but also with an increased charging capacity of the storage, which, however, would then exceed the capacities of most networks and heat generators.

The control strategy of the network is directly linked to defined "lower" and "upper"-temperature limits of the single storages, which were designed as a "plug flow model" in the simulation. By clustering several consumers and assigning to one storage, the specific storage losses can be reduced, but parts of the network have to remain in operation. However, it has been shown that clusters do not reduce the network losses this much, because the network sections between the storages and the consumers are always kept in operation. However, this can have an economic advantage due to the lower investment costs.

If embedding solar energy, up to 50 % of energy can be saved, depending on the network configuration and based on the feed-in of the heating plant into the grid. For low-load times in the summer, this can mean that for clustered storage systems the heating plant can be switched off at all and the energy can only be provided via the solar systems. One third of the suitable solar surfaces on the buildings can be sufficient in good constellations.

The TFlex operation is possible with regard to pipe statics for fully compensated pipelines, plastic pipelines and underground pipelines. However, in order to avoid problems due to thermal stresses, the particular installation situation of the pipelines must be taken into account in detail.

For lines which are designed with a unique design fixed-point-floating compensation, there are no restrictions with regard to temperature-flexible operation. In the case of buried plastic sheath pipes, the essential influencing factor is the laying temperature. By an appropriate choice of this temperature, the maximum stresses in the pipelines can be kept significantly below the fatigue strength values.

It is important that the load change number in a temperature-flexible operation (depending on frame conditions approx. 70 -200 load cycles per year) is added up to the life of the lines, stays by a factor of 103 lower than the fatigue loads. All this applies on the condition that the house connection lines and branches are designed according to the applicable installation regulations and standards.
The results of the profitability survey show a payback period of 21 years for the reference scenario. With an amortization period of 22 years (period under consideration of 25 years), the scenario with cluster stores also achieves a positive capital value. The use of decentralized storages, on the other hand, cannot be represented economically. The heat input of the solar thermal systems on all suitable roofs also has a negative capital value in the other scenarios due to the high investment costs.

The evaluation of the eco-efficiency was carried out with the help of the saved fuel quantities. Heat storage clustering reduces biomass use by up to 53 tonnes per year. The saving of resources in the case of solar thermal integration is a much more, for example in the scenario with reduced solar surfaces up to 990 tonnes per year.
The analysis of the scenarios showed that the greatest impact has the heat loss, followed by the biomass fuel costs, the period of observation and the funding rate. On the other hand, the relative changes in storage investment costs, the imputed interest rate or network losses are relatively minor.

For the solar thermal scenarios, investment costs have the greatest impact on profitability. Also important are the heat purchase and the funding rate. The reason for this is the greater impact of the investment costs and the associated investment funding as well as the increased solar coverage. The interest rate has a more significant impact than fuel costs. The result of the risk analysis reflects the development on the energy market of recent years. The greatest impact on the value of the capital has therefore changes in the heat price index and the biomass index.

Prospects / Suggestions for future research

The TFlex operation is able to save minor amounts of carbon dioxide and fuel energy in the pure storage operation, and also considerably larger quantities with the integration of the solar yields. From an economic perspective, however, only the constellation exclusively with the cluster storage on a modest scale makes sense. This is especially the case when the customers are grouped in a narrow space around the cluster stores, when the customer structure is similar (no private customers together with large commercial enterprises or industrial enterprises) and when the spatial distance between heating station and storage is as large as possible, so that as large line sections as possible can be switched off.
This study was intended to find out whether this TFlex operation is energetically and economically feasible and whether this is permissible with regard to the pipe static. However, due to the economic results the project partners do not intend to do a further project, especially their district heating grids do not have those necessary optimal conditions.


Project Partners

Project management

Chair of Energy Network Technology, Montanuniversität Leoben

Project or cooperation partners

  • FH JOANNEUM Gesellschaft mbH

  • Lehrstuhl für Wirtschafts- u. Betriebswissenschaften, Montanuniversität Leoben

  • TB Harald Kaufmann GmbH

Contact Address

Univ. Prof. DI Dr. techn. Thomas Kienberger
Franz-Josef-Straße 18
A-8700 Leoben
Tel.: +43 (3842) 402-5400