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Evaluation of district heating systems based on exergy analysis

Falk, Paul Michael (2018)
Evaluation of district heating systems based on exergy analysis.
Technische Universität Darmstadt
Ph.D. Thesis, Primary publication

Abstract

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.

Item Type: Ph.D. Thesis
Erschienen: 2018
Creators: Falk, Paul Michael
Type of entry: Primary publication
Title: Evaluation of district heating systems based on exergy analysis
Language: English
Referees: Stephan, Prof. Peter ; Schebek, Prof. Liselotte
Date: 2018
Place of Publication: Darmstadt
Refereed: 17 April 2018
URL / URN: http://tuprints.ulb.tu-darmstadt.de/7372
Abstract:

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.

Alternative Abstract:
Alternative abstract Language

In Deutschland werden 22% der eingesetzten Primärenergie zu Heizzwecken und für den Warmwasserbedarf genutzt. Öl und Gas sind dabei die am häufigsten eingesetzten Energieträger, während erneuerbare Energien kaum genutzt werden. Um die Klimaziele der deutschen Bundesregierung zu erreichen und den Energiebedarf zu decken, muss der Primärenergieverbrauch gesenkt und mehr erneuerbare Energien sollten bereitgestellt werden. Gleichzeitig sollte die Effizienz der Energienutzung gesteigert werden. Nahwärmenetze bieten eine gute Gelegenheit erneuerbare Energien in Kombination mit Wärmespeichern zu nutzen. Ein Ansatz zur Primärenergieeinsparung und zur Effizienzsteigerung ist das so genannte Exergiekonzept. Dabei wird versucht die eingesetzte Energie sinnvoll zu nutzen und das Exergieniveau der eingesetzten Energie an das Exergieniveau des Bedarfes anzupassen. Dadurch wird keine hoch exergetische Energie benutzt um niedrig exergetische Bedarfe zu decken. In Veröffentlichungen gibt es zahlreiche Beispiele von angewandter Exergienanalyse von Nahwärmenetzen und Wärmespeichern. Was jedoch nicht bekannt ist, ist ein ganzheitlicher Überblick und eine Bewertung von verschiedenen Technologien für Nahwärmenetze mit energetischer, exergetischer, ökologischer und ökonomischer Analyse. Des Weiteren sind Exergieanalysen von Wärmespeichern eingebunden in Nahwärmenetze nicht öffentlich zugänglich. Deshalb werden in der vorliegenden Arbeit mit der MATLAB/Simulink basierten Toolbox CARNOT dynamische Simulationsmodelle von Nahwärmenetzen entwickelt. Die Modelle werden genutzt um verschiedene Heiztechniken zu untersuchen und einen vergleichenden Überblick zu geben. Außerdem wird eine neue Methode Wärmespeicher zu berechnen entwickelt, die es ermöglicht zwischen Exergieänderung aufgrund veränderlicher Referenztemperatur und Exergieänderung aufgrund von Durchmischung von Temperaturschichten zu unterscheiden. In der Arbeit werden sechs verschiedene Szenarien betrachtet. Diese sind ein Gas Szenario, ein Blockheizkraftwerk Szenario, ein geothermisches Wärmepumpen Szenario, eine Kombination des geothermischen Wärmepumpen und des Blockheizkraftwerk Szenario, ein Solarthermiekollektor Szenario und ein Szenario mit einer Kombination aus einer Luft-Wärmepumpe und einer geothermischen Wärmepumpe. Das untersuchte Nahwärmenetz besteht aus einer Gebäudegruppe mit 11 Gebäuden und einem Jahreswärmebedarf von 263.7MWh. Der berechnete Exergeinhalt des Wärmebedarfes ist 14.0MWh. Die sechs verschiedenen Szenarien werden mit Hilfe energetischer, exergetischer, ökologischer und ökonomischer Analyse bewertet. Die Ergebnisse zeigen, dass das Blockheizkraftwerk Szenario am Kostengünstigsten ist, aber mit einem hohen fossilen Energieaufwand verbunden ist. Das Szenario mit einer Kombination aus einer Luft-Wärmepumpe und einer geothermischen Wärmepumpe hingegen besitzt den niedrigsten fossilen Energieaufwand und Treibhauspotential, ist aber am teuersten. Das geothermische Wärmepumpen Szenario hat den niedrigsten Exergieaufwand und zeigt die beste Gesamtleistung. In Bezug auf das Verhalten eines Wärmespeichers, der in ein Nahwärmenetz eingebunden ist, zeigen die Ergebnisse die Notwendigkeit alle Parameter, die den Wärmespeicher beeinflussen, in die Auslegung mit einzubeziehen, da er sehr sensibel reagiert. Insgesamt zeigt sich, dass die entwickelte Wärmespeicher Berechnungsmethode und die Modelle geeignet sind um Wärmespeicher und Nahwärmenetze zu untersuchen.

German
URN: urn:nbn:de:tuda-tuprints-73723
Classification DDC: 600 Technology, medicine, applied sciences > 620 Engineering and machine engineering
Divisions: 16 Department of Mechanical Engineering
16 Department of Mechanical Engineering > Institute for Technical Thermodynamics (TTD)
16 Department of Mechanical Engineering > Institute for Technical Thermodynamics (TTD) > Analysis of Thermal Energy Systems
Date Deposited: 03 Jun 2018 19:55
Last Modified: 07 Aug 2019 14:29
PPN:
Referees: Stephan, Prof. Peter ; Schebek, Prof. Liselotte
Refereed / Verteidigung / mdl. Prüfung: 17 April 2018
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