• Energy Research

Abstract District heating dynamic models arise as an alternative approach to in-situ experimental investigations. The main advantage of dynamic modeling and simulation is the possibility to avoid technical and operational risks that might occur during in-situ experimental investigations (e.g. heat demand is not met, damages in the energy systems etc.). Within this study, the authors present two models for an existing district heating system in Cottbus, Germany. One model is developed using the tool EBSILON Professional, while the other one is developed using the Simscape toolbox for physical modeling in Matlab/Simulink. The models were experimentally validated against measured data from the considered district heating system. The results show that the Simscape model has a better fit and better response than the EBSILON model. Yet, some discrepancies were found between the measured and the simulated data and, therefore, the uncertainties of the models were addressed. A comparative study between both tools is presented. The EBSILON models permit only unidirectional flow, whereas the Simscape toolbox permits reverse flow. Nevertheless, the EBSILON model outperforms the Simscape model in computation time. In addition, this study presents an approach for dynamic thermo-hydraulic modeling of district heating networks. This approach is utilized to examine the role of district heating networks as heat storage as an optimization configuration. The numerical results show less start-ups for additional heat sources. Yet, higher heat losses from the network are observed due to the installation of unburied pipelines.

Abstract District heating dynamic models arise as an alternative approach to in-situ experimental investigations. The main advantage of dynamic modeling and simulation is the possibility to avoid technical and operational risks that might occur during in-situ experimental investigations (e.g. heat demand is not met, damages in the energy systems etc.). Within this study, the authors present two models for an existing district heating system in Cottbus, Germany. One model is developed using the tool EBSILON Professional, while the other one is developed using the Simscape toolbox for physical modeling in Matlab/Simulink. The models were experimentally validated against measured data from the considered district heating system. The results show that the Simscape model has a better fit and better response than the EBSILON model. Yet, some discrepancies were found between the measured and the simulated data and, therefore, the uncertainties of the models were addressed. A comparative study between both tools is presented. The EBSILON models permit only unidirectional flow, whereas the Simscape toolbox permits reverse flow. Nevertheless, the EBSILON model outperforms the Simscape model in computation time. In addition, this study presents an approach for dynamic thermo-hydraulic modeling of district heating networks. This approach is utilized to examine the role of district heating networks as heat storage as an optimization configuration. The numerical results show less start-ups for additional heat sources. Yet, higher heat losses from the network are observed due to the installation of unburied pipelines.

Buildings consume large amount of energy for cooling in summer and heating in winter. A renewable energy-based district heating using seasonal thermal storage can better serve for a lower carbon space heating for buildings. The research objective is to propose a first semi-analytical model of large-scale water tank storage as an efficient and flexible tool for further development of TES. A new idea of “three-zone method” is proposed for detailed heat and mass flow inside water storage with least increase in computational burden and better capture of internal non-uniform thermal distribution. All three modes of charging, discharging and standby are modeled separately with high flexibility. A modified finite cylindrical source model for TES was proposed, for the first time, for transient heat transfer in the ground, which is inspired by analytical model of ground source heat pump. A complete comparison was made between the new semi-analytical model and validated reference data, which shows a good match in temperature profile in different locations. This study will pave a way for a further systematical study on seasonal thermal storage.

Buildings consume large amount of energy for cooling in summer and heating in winter. A renewable energy-based district heating using seasonal thermal storage can better serve for a lower carbon space heating for buildings. The research objective is to propose a first semi-analytical model of large-scale water tank storage as an efficient and flexible tool for further development of TES. A new idea of “three-zone method” is proposed for detailed heat and mass flow inside water storage with least increase in computational burden and better capture of internal non-uniform thermal distribution. All three modes of charging, discharging and standby are modeled separately with high flexibility. A modified finite cylindrical source model for TES was proposed, for the first time, for transient heat transfer in the ground, which is inspired by analytical model of ground source heat pump. A complete comparison was made between the new semi-analytical model and validated reference data, which shows a good match in temperature profile in different locations. This study will pave a way for a further systematical study on seasonal thermal storage.

Abstract The buildings sector is a main player in the decarbonization pathway as it contributes with a share of 40% of the total energy use in which space heating and domestic hot water are responsible for a considerable portion. A key lever to overcome the challenges in the buildings sector related to today’s extensive utilization of fossil fuels is the introduction of renewables-based district heating systems. Yet, most renewables fluctuate based on seasonal and hourly patterns. This pinpoints the significance of large-scale seasonal thermal energy storage (TES) systems. Yet, such large-scale systems require a thorough planning in order to avoid the high investment cost. Consequently, numerical models gain importance as an alternative. Accordingly, this work develops numerical finite element models for large-scale tanks and pits. To obtain credibility in the approach, the models are then validated against measured data from the Dronninglund pit TES in Denmark. The outcomes exemplify that the simulation method is suitable and the models can be calibrated very well. Next, the work examines pit TES performance considering two energetic efficiency indicators and two stratification quality measures. The performance evaluation shows that the Dronninglund pit achieved an efficiency of 90%, whereas only 76% of the pit energy capacity was effectively utilized for the year 2015. Further, the pit maintained a moderate quality of stratification for longer periods. The work later demonstrates the influence of TES geometry on stratification quality by comparing the MIX number between Dronninglund PTES and a corresponding cylindrical TES.

Abstract The buildings sector is a main player in the decarbonization pathway as it contributes with a share of 40% of the total energy use in which space heating and domestic hot water are responsible for a considerable portion. A key lever to overcome the challenges in the buildings sector related to today’s extensive utilization of fossil fuels is the introduction of renewables-based district heating systems. Yet, most renewables fluctuate based on seasonal and hourly patterns. This pinpoints the significance of large-scale seasonal thermal energy storage (TES) systems. Yet, such large-scale systems require a thorough planning in order to avoid the high investment cost. Consequently, numerical models gain importance as an alternative. Accordingly, this work develops numerical finite element models for large-scale tanks and pits. To obtain credibility in the approach, the models are then validated against measured data from the Dronninglund pit TES in Denmark. The outcomes exemplify that the simulation method is suitable and the models can be calibrated very well. Next, the work examines pit TES performance considering two energetic efficiency indicators and two stratification quality measures. The performance evaluation shows that the Dronninglund pit achieved an efficiency of 90%, whereas only 76% of the pit energy capacity was effectively utilized for the year 2015. Further, the pit maintained a moderate quality of stratification for longer periods. The work later demonstrates the influence of TES geometry on stratification quality by comparing the MIX number between Dronninglund PTES and a corresponding cylindrical TES.

Abstract Large-scale thermal energy storage (TES) emerges as key for the expansion of renewables-based district heating (R-DH) as it is able to bridge the seasonal gap between the heating demand and the availability of renewable energy resources (e.g. solar energy). This work develops a framework for techno-economic analysis considering several key performance indicators (e.g. energy efficiency, exergy efficiency). As TES systems integrated in DH are typically stratified, the work also examines the TES by means of stratification number and efficiency. The economic feasibility of the TES options is examined via the TES specific investment cost. Then, the work recommends the levelized cost of stored heat (LCOS) as a practical measure for the TES techno-economic feasibility. The outcomes show that the tank has higher performance in terms of efficiency indicators (energy and exergy) and stratification measures, but it is characterized with high specific cost. Yet, the tank LCOS is lower compared to that of the shallow pit due to its low performance and despite its low specific cost. Thus, in order to take advantage of the tank's better performance and shallow pit's lower specific cost, the work proposes a third TES geometry called as hybrid TES that combines both tank and shallow pit. The results reveal the potential of this geometry as it arises as a promising option. Furthermore, the results indicate that the transition to low-temperature R-DH brings technical and economic advantages as the LCOS tends to be lower compared to that of TES installed in high-temperature R-DH. Moreover, the work reveals that due to the importance of increasing the economic feasibility for large-scale TES, it is of crucial to develop new materials and construction methods to ensure cost-efficient insulation of the buried TES.

Abstract Large-scale thermal energy storage (TES) emerges as key for the expansion of renewables-based district heating (R-DH) as it is able to bridge the seasonal gap between the heating demand and the availability of renewable energy resources (e.g. solar energy). This work develops a framework for techno-economic analysis considering several key performance indicators (e.g. energy efficiency, exergy efficiency). As TES systems integrated in DH are typically stratified, the work also examines the TES by means of stratification number and efficiency. The economic feasibility of the TES options is examined via the TES specific investment cost. Then, the work recommends the levelized cost of stored heat (LCOS) as a practical measure for the TES techno-economic feasibility. The outcomes show that the tank has higher performance in terms of efficiency indicators (energy and exergy) and stratification measures, but it is characterized with high specific cost. Yet, the tank LCOS is lower compared to that of the shallow pit due to its low performance and despite its low specific cost. Thus, in order to take advantage of the tank's better performance and shallow pit's lower specific cost, the work proposes a third TES geometry called as hybrid TES that combines both tank and shallow pit. The results reveal the potential of this geometry as it arises as a promising option. Furthermore, the results indicate that the transition to low-temperature R-DH brings technical and economic advantages as the LCOS tends to be lower compared to that of TES installed in high-temperature R-DH. Moreover, the work reveals that due to the importance of increasing the economic feasibility for large-scale TES, it is of crucial to develop new materials and construction methods to ensure cost-efficient insulation of the buried TES.