• 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 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.

AbstractSpace heating applications account for a high share of global greenhouse gas emissions. To increase the renewable share of heat generation, seasonal thermal energy storage (STES) can be used to make thermal energy from fluctuating renewable sources available in times of high demand. A popular STES technology is pit thermal energy storage (PTES), where heat is stored underground, using water as a storage medium. To evaluate the use of PTES in an energy system, easily adaptable, publicly accessible and tool independent models are needed. In this paper, we improve an existing PTES model developed in the Modelica modeling language. The model is cross-compared with a more detailed and previously validated COMSOL model, considering different amounts of insulation, showing a deviation of 2–13% in the observed annual charged and discharged amount of heat. The results indicate that the presented model is well suited for early design stage and an exemplary case study is performed to demonstrate its applicability in a system context. Dimensions of system components are optimized for the levelized cost of heat (LCOH), both with and without subsidies, highlighting the importance of subsidies for the transition towards climate friendly heating solutions, as the gas boiler use is reduced from 47.6% to 2.7%.

District Heating (DH) systems are often seen as a good practical approach to meet the local heat demand of the districts due to its ability to provide affordable and low carbon energy to the consumers. Yet, under today’s regulations to renovate the buildings into more energy-efficient ones, the local heat demand is decreasing. Therefore, the operation of DH systems is also affected by the changing heat demand profile, which might lead to less profit for the operators of DH systems. Thus, the operators of DH systems strive for an optimal operation at which the heat demand is met and the profits are maximized. Due to the fact that these systems are complex-physical systems, therefore it is difficult to conduct any experimental investigation on them in order to examine the optimal operation. Accordingly, it is crucial to create fundamental models to investigate the optimal operation of such systems. In this paper, a power-based model is built to represent the heating station as part of a DH system. Then, the model is validated using real data from an existing heating station in Freiburg, Germany. The validation results reveal that the goodness-of-fit for the model is held to be good enough to test it for operational optimization cases.

Abstract Nowadays, buildings consume a large amount of conventional energy sources in European countries and subsequently they contribute significantly to fossil fuels emissions. Therefore, many European countries have introduced several policies to minimize this consumption by transitioning buildings into more energy efficient ones, whereas some other policies focus on integrating renewables into energy systems. In this context, solar district heating is one of the promising technologies that reduces the use of fossils and, thereby, leads to fewer CO2 emissions. The main drawback of solar energy, however, is that it fluctuates on daily and seasonal basis in which the highest heat availability is in summer, while the highest demand is in winter. Hence, a seasonal thermal energy storage (STES) is required to bridge the temporal mismatch between renewable energy availability and buildings’ demand. Accordingly, this study reviews briefly the different seasonal thermal energy storage technologies that are feasible for district heating applications. Then, the paper focuses chiefly on large-scale hot water TES (tanks and pits). Construction (geometry and envelope), modeling and design of these TES systems are the primary focus. Next, system performance indicators are also reviewed. A synopsis of the current TES systems is eventually presented as well. The literature review reveals: (1) Tank TES (TTES) and pit TES (PTES) are less subjected to hydro- geological conditions than aquifer TES (ATES) and borehole TES (BTES), (2) TTES and PTES require high construction cost compared to ATES and BTES, (3) TTES and PTES provide higher charging/discharging power than ATES and BTES due to higher operational temperature difference and flowrates, (4) in hot water TES, as the depth decreases, the more the stratification tends to degrade and, therefore, tanks are preferable over pits, (5) no established co-simulation platform between TES envelope and surroundings coupled to energy analysis models and (6) no effective approach or measure has been found to evaluate one TES to another.

Abstract Seasonal thermal energy storage (TES) is envisioned as a major player in the future district heating (DH) systems where large shares of renewables are being integrated. Therefore, in order to fulfill the seasonal tasks, such storage systems are characterized with large volumes. Yet, the integration of such large-scale storage technologies is not easily planned and realized. There exist numerous challenges e.g. TES type, volume and ground conditions, need to be tackled in order to obtain an optimal planning solution for TES integration. Given their promising applications, the scope of this work is limited to tank and pit thermal energy storage. Accordingly, this contribution firstly discusses the modeling of seasonal TES in finite element tools. Then, it examines the influence of a list of parameters i.e. TES construction type, geometry, volume and DH characteristics, on TES performance. Later, the work develops a methodology for construction techno-economic analysis of such technologies. It is revealed that the tank TES has always better performance than pit, but on the other hand it is always characterized with higher capital cost. As TES volume increases, the performance difference between tank and pit starts to vanish. Further, the DH characteristics play a major role in TES performance. It is depicted that lowering DH temperatures will ultimately lead to lower thermal losses from TES. Another important finding is the applicability of the suggested performance indicator for techno-economic analysis as it relates the technology capital cost to the effective volume of TES. The contribution also investigates the influence of insulation level on TES performance and it is found that for volumes larger than 500,000 m3, there is no major performance difference between the tank or the pit in case of insulation enclosing TES envelope. However, it is also revealed that insulation is needed only and solely to preserve the ground quality when large volumes are realized.