• Energy Research

For large scale thermal energy storage at temperatures above 300°C, two-tank molten salt systems mark the current state-of-the-art as they are proven technology in parabolic trough and tower solar thermal power plants. Research is focusing on the utilization of molten salts not only as storage medium but also as heat transferring fluid (HTF) in parabolic trough plants [1]. The current two-tank concept offers serveral cost reduction possibilities. Firstly, instead of storing the hot and cold phase in two separate tanks, the salt could be stored inside a single tank to avoid a large gas volume. The separation of both phases can either be achieved by a floating insulated barrier or simply by the different densities of both phases. Secondly, a high share of the total investment costs of a molten salt storage system is caused by the molten salt itself. For the two-tank system in 50 MWel power plants, this can be as high as half of the total TES costs [2]. In the thermocline with filler concept, a large fraction of the molten salt can be substituted by a cost effective solid material, offering a significant potential for further cost reductions [3]. Finally, gaining operational experience of such systems and the ability to derive optimized operation strategies, promise an additional cost reduction potential. The “test facility for thermal energy storage in molten salts” (TESIS:store) has been set up at DLR in Cologne, Germany. An outside view of the plant can be seen in Fig. 1. The facility operates at temperatures up to 560 °C and a maximum molten salt mass flow of 4 kg/s. The storage volume has a length of 5.4 m with a total tank volume of 22 m³. The plant allows the investigation of the thermocline concept with and without filler and gaining widespread operation experience. Heat tracing along the containment walls and the piping ensures adiabatic conditions.

This paper investigates cost reduction potential for thermocline filler (TCF) storage in a direct molten salt parabolic through plant based on system-level simulations in response to weather data for a reference week and corresponding plant control. In a first step the theoretical potential for investment cost reduction of TCF tanks filled with nitrate salts and natural stone as filler material operating in a range of 290°C and 550°C is represented. The effective potential on system-level though is reduced due to a temperature drop at the end of the discharge cycle. Therefor the electricity cost for selected reference days is chosen as a measure for the economic potential of TCF systems over two tank (2T) storage systems. As a base case a 100 MWel direct two tank parabolic trough power plant with 13 hours storage capacity using molten salt as heat transfer fluid and storage medium was chosen. This setup is modelled and simulated as a function of TES capacity and solar field size using the commercial software EBSILON Professional©. Based on best and worst case assumptions for the financial parameters, a total TES investment cost reduction of 30 to 42% can be achieved and a first estimate for the levelized cost of electricity (LCOE) results in 5 to 9% improvement as well as an increased capacity factor and a higher storage capacity.

In this work an overview of the TESIS:store thermocline test facility and its current construction status will be given. Based on this, the TESIS:store facility using sensible solid filler material is modelled with a fully transient model, implemented in MATLAB®. Results in terms of the impact of filler site and operation strategies will be presented. While low porosity and small particle diameters for the filler material are beneficial, operation strategy is one key element with potential for optimization. It is shown that plant operators have to ponder between utilization and exergetic efficiency. Different durations of the charging and discharging period enable further potential for optimizations.

AbstractThe defossilization of the electricity and heat supply in the chemical industry poses a significant challenge. In particular, the intended feed‐in of volatile renewable electricity into the chemical processes may conflict with the need for a constant, secure and affordable electricity and heat supply for chemical plants. Adapted concepts for the operation of the cogeneration plant, which is located at the chemical site, play a central role. The present work defines a so‐called ideal‐typical utility infrastructure (iUI). The iUI is a means to exemplarily investigate the operational behavior of utility infrastructures and furthermore to identify defossilization options for process steam and electricity supply at chemical sites.

Abstract In this work, an extensive parametric study of the molten salt thermocline storage concept with filler is presented. The parametric study is coupled with an optimization routine, allowing a better comparison, since it finds only those storage configurations, which can directly substitute the two-tank system in a given power plant. Results show that, compared to the two-tank molten salt system, the thermocline technology achieves high exergetic efficiency at only slightly increased storage volume size and a huge decrease in salt inventory.

MgCl2-KCl-NaCl molten chloride salt is a promising candidate for thermal energy storage medium and heat transfer fluid for next-generation Concentrating Solar Power (CSP) plants (Gen-3 CSP). The main challenge has yet been the selection of economical yet corrosion-resistant structural materials to be used. Previous work by the authors has demonstrated that simple corrosion control strategies can mitigate corrosion effects, thereby allowing the use of classical stainless steels as structural materials in the hot part (e.g., ≥700 °C) of the CSP system. This study addresses the selection of cold tank materials, which have to withstand corrosion effects up to temperatures of 500 °C. Two cost-effective commercial types of steels, P91 and SS 304, were examined as candidates for structural materials, and controlled corrosion experiments were performed in molten MgCl2-KCl-NaCl salt at 500 °C for 1400 h. Before the exposure tests, the chloride salt was purified using a simple yet effective Mg-doping technique. The results show that the corrosion rates (CRs) of P91 samples are consistently low (<15 μm/year) for both macroscopic and microscopic analysis. The cheaper P91 steel outperforms the more expensive SS 304 in terms of corrosion resistance and also may prove to be beneficial in terms of mechanical properties and economics. Overall, the use of P91 as a cold tank structural material allows for significant cost reduction of the cold tank for chloride-TES system and enhances its competitiveness compared with commercial nitrate-TES.

Solid sensible heat storage is an attractive option for high-temperature storage applications regarding investment and maintenance costs. Using concrete as solid storage material is most suitable, as it is easy to handle, the major aggregates are available all over the world, and there are no environmentally critical components. Long-term stability of concrete has been proven in oven experiments and through strength measurements up to 500 °C. Material parameters and storage performance have been validated in a 20-m3 test module with more than 23 months of operation between 200 °C and 400 °C and more than 370 thermal cycles. For an up-scaled concrete storage design with 1100-MWh capacity in a modular setup for a 50 MWel parabolic trough power plant of the ANDASOL-type, about 50 000 m3 of concrete is required and the investment costs are approximately 38 million euro. The simulation of the annual electricity generation of a 50 MWel parabolic trough power plant with a 1100-MWh concrete storage illustrates that such plants can operate in southern Europe delivering about 3500 full load hours annually; about 30% of this electricity would be generated by the storage system. This number will increase further, when improved operation strategies are applied. Approaches for further cost reduction using heat transfer structures with high thermal conductivity inside the concrete are analyzed, leading to a 60% reduction in the number of heat exchanger pipes required. For implementation of the structures, the storage is build up of precast concrete blocks.