Master's Thesis Vivien Lengyel


Evaluation of a low temperature district heating system in a city quarter considering dynamic boundary conditions

Westerholt Mine Copyright: EBC Westerholt Mine and the heat supply areas

This master thesis aims to evaluate the impact of dynamic boundary conditions of technical, economic and ecological optimisations for a new low temperature district heating system (DHS).

The DHS introduces innovative solutions as a combination of waste heat utilisation from existing combined heat and power (CHP) plants, heat pump, solar thermal energy system and thermal heat storage. The impact of two superordinate systems, the German electricity market and a local high temperature DHS are examined within the framework of a systemic evaluation. The dynamic boundary conditions include consumption data of the district and meteorological data on hourly basis. In addition to that an electricity market model provides market price, primary energy factors and CO2-emission factors for electricity for the same intervals. The same signals for the district heat were generated according to the merit order for the involved power plants of the local DHS. The results of the electricity market model show that in the future increasing market prices and dispersion are expected due to the growing electricity production of volatile renewable energy sources. Parallelly to that the primary energy factor and CO2-emission factors show a downward trend. The three indicators fluctuate diurnal and seasonal as well.

The heat generation system will be implemented in three phases, in this thesis the initial and final status are regarded.

In the first stage the collaboration of a heat pump, thermal storage system and the exhaust heat of three mine gas cogeneration plants is evaluated, the local DHS makes the heat offtake available on higher temperature for covering the heat demand. Results of the three objective functions for the year 2019 show that the heat demand can be completely covered by waste heat from the CHP plants if the appropriate thermal storage size is chosen. The thermal heat storage reduces the demand-related costs, the expenditures for primary energy and the emitted carbon dioxide. The optimal sizes of the three objective functions are in correspondence, the results under static boundary conditions overlap.

The final stage exemplifies the operation of a solar thermal energy system, a heat pump, a thermal energy storage and the heat offtake of the high temperature DHS. The minimisation of costs, expenditures of primary energy and emission require different sizes for the thermal storage, because the objective functions result in different operation strategies. In comparison to that, the optimisation outcome under static boundary conditions shows independence of the storage size, as the degree of storage utilisation is significant lower.