| Popis: |
In Germany, 22% of the total primary energy usage is used for room heating and domestic hot water. Oil and gas are the most common energy sources for heating, while renewable energies are rarely used. In order to reach the climate targets set by the German government, primary energy input must be reduced and more renewable energy should be supplied to cover the energy demand. At the same time, the efficiency of energy usage should be increased. District heating systems offer a good opportunity for using renewable energies in combination with thermal energy storage. One approach to reducing the primary energy input and increasing the efficiency of usage is the so called exergy concept. It aims at using the energy meaningfully by matching the exergy level of the supply with the exergy level of the demand. Thus, high exergy energy is not used to cover low exergy demand. Numerous examples of applied exergy analysis of district heating systems and thermal energy storages can be found in the literature. However, an integral approach and assessment of different heating technologies for district heating systems by using energy, exergy, ecological and economical analyses is not known. Further, exergy analyses of thermal energy storages integrated into district heating systems are not publicly available. Therefore, in this work, dynamical simulation models of district heating systems are developed with the MATLAB/Simulink based toolbox CARNOT. The models are used to investigate different heating technologies and to give a comparative overview. Further, a new method to calculate thermal energy storage is developed to separate between the exergy change due to a changing reference temperature and the exergy change due to the mixing of temperature layers. Six different heat supply scenarios are considered. These are a gas boiler scenario, a combined heat and power scenario, a geothermal heat pump scenario, a combination of a geothermal heat pump and a combined heat and power plant scenario, a solar thermal collector scenario, and a scenario with a combination of an air source heat pump and a geothermal heat pump scenario. The investigated district heating system consists of a building cluster with 11 buildings and a total annual heat demand of 263.7MWh. The calculated exergy content of the heat demand is 14.0MWh. The six different heat supply scenarios are assessed using energy, exergy, ecological and economical analysis. The results show that the combined heat and power scenario has the highest economical efficiency but a high fossil energy input, while the scenario with a combination of an air source heat pump and a geothermal heat pump has the lowest fossil energy input and global warming potential, but is the most expensive. The geothermal heat pump scenario has the lowest total exergy input and shows the best overall performance. In regards to the performance of a thermal energy storage within a district heating system the findings demonstrate that it is very important to take all parameters affecting the storage into account during the dimensioning of the storage, since it reacts very sensitively. Overall it is shown that the developed thermal energy storage calculation method and the models are suited to study thermal energy storages and district heating systems. |
