• Energy Research

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 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 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.

Abstract In view of the urgent need for energy efficiency measures, renewables-based district heating (R-DH) can prove an efficient approach to meet the heating demand in cities whereby locally-available renewable resources are exploited. Yet, the renewables experience intermittency, which might lead to seasonal mismatch between heat supply and demand. Therefore, large-scale seasonal thermal energy storage (STES) systems are often envisioned as key elements in R-DH. Given their large volumes, these systems are often installed underground whereby groundwater tables are expected to lead to twofold impacts due to the TES-groundwater interaction. This work reports the development of models for the planning and optimization of STES and, then, conducts a calibration study to attain credibility in the models. Next, it examines the planning of STES under such unfavorable hydrogeological conditions whereby a groundwater flow is anticipated. The results indicate that Darcy flow plays a significant role in increasing the thermal losses that result in increasing groundwater temperature. Therefore, it becomes crucial to provide protective measures to maintain acceptable groundwater quality prescribed by national standards. Hence, the work investigates the role of cut-off wall distance and TES insulation quality to mitigate the TES thermal losses, increase the TES efficiency and reduce the groundwater temperature.