Technology pathways for transforming high temperature to low temperature district heating systems in the ENaQ project

Kurzfassung

In Germany there are 1454 district heating systems. Most of them are fossil fuel based and with high temperature levels. The ENaQ project located in Oldenburg proposes a high temperature district heating system (HTDH). This master thesis takes this project as a reference and studies the feasibility of transforming its high temperature to a low temperature district heating system (LTDH). The main objective of this study is to technically understand what is needed in order to perform the system transformation. The results found here are presented as guidelines that can be applied at other systems that aim to reduce its operating temperature. Four steps were carried out to perform this master thesis. The first was to develop and validate a dynamic LTDH model as a reference scenario with the Carnot Toolbox from Simulink in the Matlab software. Then the ENaQ HTDH (initial scenario) was modeled. The third step was to take the data and constrains used in ENaQ HTDH and merge them with the validated LTDH model to preform the high to low temperature system transformation. At this point both models are compared by 5 indicators to observe the difference between them. Since the LTDH model makes use of more space, specially because of the implementation of decentralized storage tanks and a seasonal storage, the last step was to perform a sensitivity analysis of ENaQ LTDH to see how the system performance changes in case there are economic or space constrains that would not allow the transformation 100% to happen. Results show that it is technically possible to perform the district heating system transformation until the very LTDH where the share of renewable energy sources increases from 0.3% to 84%, transport losses and greenhouse gas emissions are reduced from 8.3% to 3.4% and from 135 to 57 tCO2eq respectively. The two key points that allow this transformation to happen are the placement of a heat pump at the outlet of each building, which makes it possible to reduce the return temperature of the district network and consequently the supply temperature, and the implementation of decentralized heat sources on each building that reduces the transported energy through the network. Regarding the sensitivity analysis, the developed guidelines propose that in case the seasonal storage is limited, the transformation is worth doing until it reaches a size of 60% of the proposed nominal value (6198 [m3]). In case the solar collector area is limited, the transformation is still worth it until it reaches 80% of the proposed nominal value (915 [m2] in total). Finally, in case there is space for only one decentralized storage, the transformation is still worth doing, however it is necessary to study if the transformation is hydraulically feasible since there are constant changes in the mass flow direction in the district network.