Showing papers by "Thomas Bauer published in 2018"

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

Abstract: 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.

Electrochemical measurement of corrosive impurities in molten chlorides for thermal energy storage

Abstract: Molten chlorides are promising alternative thermal energy storage (TES) materials to be applied in concentrated solar power (CSP) plants owing to their higher thermal stability (stable at >800 °C) than the commercial TES materials – nitrate salt mixtures (decomposed at ∼550 °C). Higher operating temperatures of TES can increase efficiencies of thermal into electrical energy conversion for CSP power plants, but cause additional challenges, particularly increase corrosiveness of metal containers and structural materials. Corrosion rates significantly depend on concentration of corrosive hydroxide impurity in the molten chlorides. This paper presents an electrochemical method based on cyclic voltammetry (CV) to in-situ measure the concentration of the hydroxide impurity in molten MgCl2/KCl/NaCl (60/20/20 mol%) salts at 500–700 °C. Before each CV experiment, the concentration of the hydroxide impurity in the molten salts was measured via acid consumption (AC) measurements on the simultaneously collected salt sample. The results of CV and AC experiments show that the peak current densities obtained from CV are proportional to the concentrations of the hydroxide impurity obtained from AC in the studied system. The slopes of peak current densities vs. concentrations of the hydroxide impurity are determined, which compare well with literature data. This electrochemical method for monitoring the corrosive hydroxide impurity is expected to assist the corrosion control on containers and structural materials in the molten chlorides.

Influence of different atmospheres on molten salt chemistry and its effect on steel corrosion

Abstract: Dispatchability of renewable solar energy can be realized by integrating thermal energy storage units. In concentrating solar power plants the use of sensible heat storage based on molten nitrate salts, typically Solar Salt (Na,K//NO3), has proven most beneficial in the last decade. However, a crucial parameter that affects the long-term performance of the TES unit however, is the sustainability and reliability which is directly linked to the material performance of the molten salt and the construction material. Metallic corrosion is one of the biggest concerns due to the harsh conditions provided by the redox active molten nitrate salts. The mechanisms of corrosion are thought to be well understood which is reflected by a significant number of publications in the last decades. Despite, an essential parameter is often ignored or underestimated in many studies – the molten salt chemistry. This work is one of the first addressing directed degradation of Solar Salt by controlling the gas atmosphere in the storage system, thus driving the formation of corrosive impurities, especially oxide species. Austenitic, stainless Cr,Ni-steel and ferritic Cr-steel samples are subjected to the different operating conditions to demonstrate the variations in corrosivity as a function of gas atmosphere and additionally of artificially added chloride impurities. The experimental matrix allows for a comprehensive analysis of the influence of different corrosive species on the stability of different steel types over the course of 1.200h experiments performed at 560 °C.

Parametric study of the thermocline filler concept based on exergy

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.

Semi-empirical Density Estimations for Binary, Ternary and Multicomponent Alkali Nitrate–Nitrite Molten Salt Mixtures

Abstract: For sensible thermal energy storage in Concentrating Solar Power (CSP) plants, a molten salt mixture of 60 wt% sodium nitrate (NaNO3) and 40 wt% potassium nitrate (KNO3), known as Solar Salt, is commonly utilized. The paper presents semi-empirical estimation results of the density of Solar Salt and alternative molten salt mixtures with low melting temperatures in a range from 70 °C to 140 °C. These mixtures are Hitec, HitecXL, LiNO3–KNO3–NaNO3 and a multicomponent mixture. The paper shows that density values of mixtures can be closely predicted from single salt densities. The paper examines different estimation rules for mixtures. The quasilinear volumetric additivity rule (QVAR) is known for ternary reciprocal systems. For the first time, the presented work extends the QVAR to multicomponent mixtures. Temperature-dependent densities of selected salt mixtures of the system Ca,Li,K,Na//NO2,NO3 were estimated. Estimations are motivated by a fast and reliable method compared to time-consuming and error-prone measurements of several mixtures.

Techno-economic assessment for large scale thermocline filler TES systems in a molten salt parabolic trough plant

Abstract: 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.

Experimental and Numerical Investigation of a 4 MWh Single-Tank Thermocline Storage

Abstract: 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.

Corrosion Behaviors and Mitigation Strategies of Metallic Alloys in Molten Chloride Salts for Thermal Energy Storage in CSP plants

Abstract: 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 salt mixtures (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 plants. 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. This presentationOur studies presents corrosion behaviors and mechanisms of metallic alloys in molten chlorides at operating temperatures (500-800°C), as well as corresponding corrosion mitigation strategies to realize the commercial applications of molten chlorides in CSP, based on our works in these fields.