Corrosion behavior of metallic alloys in molten chloride salts for thermal energy storage in concentrated solar power plants - A review

Recently, more and more attention is paid on applications of molten chlorides in concentrated solar power (CSP) plants as high-temperature thermal energy storage (TES) and heat transfer fluid (HTF) materials due to their high thermal stability limits and low prices, compared to the commercial TES/HTF materials in CSP - nitrate salt mixtures. A higher TES/HTF operating temperature leads to higher efficiency of thermal to electrical energy conversion of the power block in CSP, however causes additional challenges, particularly increased corrosiveness of metallic alloys used as containers and structural materials. Thus, it is essential to study corrosion behaviors and mechanisms of metallic alloys in molten chlorides at operating temperatures (500-800°C) for realizing the commercial application of molten chlorides in CSP. The results of studies on hot corrosion of metallic alloys in molten chlorides are reviewed to understand their corrosion behaviors and mechanisms under various conditions (e.g., temperature, atmosphere). Emphasis has also been given on salt purification to reduce corrosive impurities in molten chlorides and development of electrochemical techniques to in-situ monitor corrosive impurities in molten chlorides, in order to efficiently control corrosion rates of metallic alloys in molten chlorides to meet the requirements of industrial applications

Report on thermo-physical properties of binary NaNO3-KNO3 mixtures in a range of 59-61 wt% NaNO3

This report investigates the impact of defined compositional changes of a Solar Salt mixture on the thermophysical properties and melting properties for a total of three compositions: the first one being the standard mixture (60-40 wt%), the second with a 1 wt% excess of NaNO3, and one with a 1 wt% excess of KNO3. We study the main thermo-physical properties, which are critical for the design of a CSP plant: the heat capacity, viscosity, thermal conductivity and density, as well as the melting properties

Solar Salt - Thermal Property Analysis - Extended Version: Report on thermo-physical properties of binary NaNO3-KNO3 mixtures in a range of 55-65 wt% NaNO3

Thermal Energy Storage (TES) plays a crucial role for the implementation of dispatchable, renewable energy systems world-wide. Molten salt storage has proven advantageous for storage units in modern CSP plants with large power and capacity levels. Solar Salt, a mixture of 60 wt% NaNO3 and 40 wt% KNO3, is the state-of-the-art storage material and heat transfer fluid and is utilized in temperature regimes between 290 °C and 565 °C in a cold- and a hot tank configuration, respectively. Typically, these salts are provided by salt suppliers using big bags (e.g., ~1000 kg) of the single salts. This study considers that SQM offers big bags with pre-mixed Solar Salt, which simplifies the organizational efforts during the TES start-up. The mixing accuracy is however important to ensure that the final mixture exhibits uniform physicochemical properties. This report investigates the impact of these defined compositional changes for binary mixtures containing 55-65 wt% NaNO3. The thermo-physical properties analysed are the heat capacity, viscosity, thermal conductivity and density, as well as the melting properties

Molten Salt Storage for Power Generation

Storage of electrical energy is a key technology for a future climate-neutral energy supply with volatile photovoltaic and wind generation. Besides the well-known technologies of pumped hydro, power-to-gas-to-power and batteries, the contribution of thermal energy storage is rather unknown. At the end of 2019 the worldwide power generation capacity from molten salt storage in concentrating solar power (CSP) plants was 21 GWhel. This article gives an overview of molten salt storage in CSP and new potential fields for decarbonization such as industrial processes, conventional power plants and electrical energy storage

Molten Chloride Salts for Thermal Energy Storage in Concentrated Solar Power Plants

Recently, more and more attention is paid on applications of molten chloride salts in concentrated solar power (CSP) plants as thermal energy storage (TES) and heat transfer fluid (HTF) materials due to their high thermal stability limits (>800°C) and low prices, compared to the commercial TES/HTF materials in CSP - nitrate salts (decomposed at ~550°C). Over the course of the SunShot Initiative, the U.S. Department of Energy (DOE) has supported the molten chloride salt development for the next generation CSP (Gen-3 CSP) [1]. A higher TES/HTF operating temperature in CSP leads to higher efficiency of thermal to electrical energy conversion of the power block. However this causes additional challenges, particularly increased corrosiveness of metallic alloys used as containers and structural materials. This presentation outlines our current development progress of molten chloride salts as TES/HTF materials in the next generation CSP [2-6], including in-depth investigation on corrosion behaviors and mechanisms of metallic alloys in molten chlorides at operating temperatures of 500-800°C, and development of corresponding corrosion mitigation strategies towards realization of commercial applications of molten chlorides in CSP

Molten Halogen Salts as Low-Temperature Electrolytes in Na-Based Liquid Metal Batteries for Low-Cost Large Scale Electricity Storage

Liquid metal battery (LMB) is an intriguing energy storage technology with advantages of low-cost, large-capacity and long-lifespan [1]. Recently, it has been gained great interest as a large scale electricity storage device able to integrate the intermittent renewable energy technologies like wind and solar in to the grid [2-3]. LMB consists of a low-density liquid metal negative electrode (e.g. Na, Li), a medium-density molten salt electrolyte (generally a molten halogen salt mixture), and a high-density liquid metal positive electrode (e.g. Sb, Pb), which self-segregates into three distinct layers by density due to their mutual immiscibility. The strong interaction between the two liquid metals provides the thermodynamic driving force (cell voltage) for the liquid metal batteries [1]. The molten salt electrolyte serves as an isolation layer between the negative and positive electrodes to replace the battery separator existing in traditional batteries. Despite that Li-based LMBs possess excellent electrochemical performance [1,4], the excessive consumption of Li resource (e.g. Li-ion batteries for transport sector) will inevitably lead to a rapidly increasing price, which makes low-cost Na-based LMBs more competitive in large scale energy storage for the future energy system. However, Na halogen salts containing in the molten salt electrolytes of Na-based LMBs lead to high melting temperatures of the molten salt electrolytes and thus high operating temperatures of LMBs. However, a high operating temperature is undesirable because it results in higher rates of corrosion and detracts from overall storage efficiency, which ultimately increases cost of ownership. In our Sino-German research project funded by DFG and NSFC (cooperation with Karlsruhe Institute of Technology (KIT), Germany and Huazhong University of Science and Technology (HUST), China), low-cost low-temperature Na-based LMBs are being developed. One of the major challenges for development of low-cost low-temperature Na-based LMBs is selection of the molten salt electrolytes with the best thermo-chemical properties (melting temperature, corrosivity, Na solubility regarding self-discharge) and the lowest cost. In this work, current research progress on selection of the best molten salt electrolytes for low-cost low-temperature (<450°C) Na-based LMBs will be presented. The commercial software for thermodynamic calculations – FactSage® is used to simulate the phase diagrams of molten halogen salts for screening the promising electrolytes. Furthermore, thermo-analysis via Differential Scanning Calorimetry (DSC) [5], immersion tests of alloy samples [6], and Na solubility measurements are performed to evaluate the selected molten salt electrolytes experimentally. The selected best molten salt electrolyte will be tested in the Na-based LMB test cell

Molten Salt Storage for flexibilization of the Future Energy System – Activities at the German Aerospace Center (DLR)

Thermal Energy Storage (TES) will play a crucial role for the large-scale implementation of renewable energy and the provision of dispatchable electricity in the future. In existing Solar Thermal Power plants, TES systems based on molten salts have been successfully implemented in the GWh-scale and can transform peak-load solar energy into intermediate or even base-load by storing large amounts of energy efficiently. Molten Salt storage systems exhibit an extremely high degree of flexibility in terms of sizing of power and capacity, have very low cost (20 USD/kWh) compared to electric storage solutions, and are inherently compatible with thermal processes. The inherent flexibility opens new fields of applications for Molten Salt systems, such as the flexibilization of Coal-fired power plants into Storage Plants, usage of TES as Carnot Batteries, or its use as a waste-heat recovery system. At DLR, the group "Thermal Systems for Fluids" has investigated molten salts within application-focused R&D activities since more than 30 years. Since almost a decade, research has been focusing on molten nitrate/nitrite salts and molten chloride salts for high temperature storage options and covers the value chain from material aspects to system level integration. The latest developments from materials to components and systems for Molten Salt Storage will be presented

The brittle-to-ductile transition in cold-rolled tungsten sheets: the rate-limiting mechanism of plasticity controlling the BDT in ultrafine-grained tungsten

Conventionally produced tungsten (W) sheets are brittle at room temperature. In contrast to that, severe deformation by cold rolling transforms W into a material exhibiting room-temperature ductility with a brittle-to-ductile transition (BDT) temperature far below room temperature. For such ultrafine-grained (UFG) and dislocation-rich materials, the mechanism controlling the BDT is still the subject of ongoing debates. In order to identify the mechanism controlling the BDT in room-temperature ductile W sheets with UFG microstructure, we conducted campaigns of fracture toughness tests accompanied by a thermodynamic analysis deducing Arrhenius BDT activation energies. Here, we show that plastic deformation induced by rolling reduces the BDT temperature and also the BDT activation energy. A comparison of BDT activation energies with the trend of Gibbs energy of kink-pair formation revealed a strong correlation between both quantities. This demonstrates that out of the three basic processes, nucleation, glide, and annihilation, crack tip plasticity in UFG W is still controlled by the glide of dislocations. The glide is dictated by the mobility of the screw segments and therefore by the underlying process of kink-pair formation. Reflecting this result, a change of the rate-limiting mechanism for plasticity of UFG W seems unlikely, even at deformation temperatures well below room temperature. As a result, kink-pair formation controls the BDT in W over a wide range of microstructural length scales, from single crystals and coarse-grained specimens down to UFG microstructures