Identifying the optimal supply temperature in district heating networks - A modelling approach

The number of low-energy and energy renovated buildings with considerably low heating demand has been continuously increasing in recent years. Combined with utilizing low temperature sources, this development raises the necessity of introducing a new generation of District Heating [DH] Systems with a lowered operational temperature. In order to implement the low temperature concept, adaptation and improvement of existing DH system is required. By lowering the operational temperature, it is further possible to utilize low-grade and renewable energy sources to meet the heating demand.

The main objective of this study is to develop a model for thermo-hydraulic calculation of low temperature DH system. The modelling is performed with emphasis on transient heat transfer in pipe networks. The pseudo-dynamic approach is adopted to model the District Heating Network [DHN] behaviour which estimates the temperature dynamically while the flow and pressure are calculated on the basis of steady state conditions. The implicit finite element method is applied to simulate the transient temperature behaviour in the network. Pipe network heat losses, pressure drop in the network and return temperature to the plant are calculated in the developed model. The model will serve eventually as a basis to find out the optimal supply temperature in an existing DHN in later work.

The modelling results are used as decision support for existing DHN; proposing possible modifications to operate at optimal supply temperature.