Molten Salt Temperature Research Articles

Synthesis of β-Y2Si2O7 nanowires via a facile molten salt method and their thermal properties

The single phase β-Y2Si2O7 nanowires were synthesized via a facile molten salt method. The phase composition, microstructure, formation mechanism and the phase transformation of the β-Y2Si2O7 nanowires were investigated, as well as the thermal properties of the corresponding β-Y2Si2O7 ceramic. Results show that the molten salt method can effectively decrease the synthesis temperature. Furthermore, the molten salt temperature has a crucial influence on the crystal phase composition and the crystal morphology. The obtained β-Y2Si2O7 nanowires possess low thermal conductivity (1.84 W m−1 K−1) and low thermal expansion coefficient (3.88 × 10−6 K−1), which make them a promising candidate toughening material on coatings.

Full Spectrum Solar System: Hybrid Concentrated Photovoltaic/Concentrated Solar Power (CPV-CSP)

ABSTRACTGas Technology Institute (GTI), together with its partners University of California at Merced (UC Merced) and MicroLink Devices Inc. (MicroLink) are developing a full spectrum solar energy collection system to deliver variable electricity and on-demand heat. The technology uses secondary optics in a solar receiver to achieve high efficiency at high temperature, collects heat in particles for low fire danger, stores heat in particles instead of molten salt for low cost, and uses double junction (2J) photovoltaic (PV) cells with backside infrared (IR) reflectors on the secondary optical element to raise exergy efficiency. The overall goal is to deliver enhancement to established trough technology while exceeding the heliostat power tower molten salt temperature limit. The use of inert particles for heat transfer may make parabolic troughs safer near population centers and may be valuable for industrial facilities.

Thermal performances of molten salt steam generator

Molten salt steam generator is the key technology for thermal energy conversion from high temperature molten salt to steam, and it is used in solar thermal power station and molten salt reactor. A shell and tube type molten salt steam generator was set up, and its thermal performance and heat transfer mechanism were studied. As a coupling heat transfer process, molten salt steam generation is mainly affected by molten salt convective heat transfer and boiling heat transfer, while its energy efficiency is also affected by the heat loss. As molten salt temperature increased, the energy efficiency first rose with the increase of heat flow absorbed by water/steam, and then slightly decreased for large heat loss as the absorbed heat flow still rising. At very high molten salt temperature, the absorbed heat flow decreased as boiling heat transfer coefficient dropping, and then the energy efficiency quickly dropped. As the inlet water temperature increased, the boiling region in the steam generator remarkably expanded, and then the steam generation rate and energy efficiency both rose with the overall heat transfer coefficient increasing. As the molten salt flow rate increased, the wall temperature rose and the boiling heat transfer coefficient first increased and then decreased according to the boiling curve, so the overall heat transfer coefficient first increased and then decreased, and then the steam generation rate and energy efficiency of steam generator both had maxima.

Integrated numerical study on the coupled photon-thermal conversion process in the central solar molten salt cavity receiver

In a solar power tower system, the concentrating and collecting subsystem, including the heliostat field and the receiver, contains some of the most costly and technically challenging components. Many types of solar receivers make up the solar power tower system. Among them, the cavity molten salt receiver has been widely applied due to its lower heat loss. The accurate simulation of the coupled heat transfer process in the cavity receiver and the prediction of receiver performance are of great importance for the receiver design and safe operation. The solar flux distribution on cavity interior surfaces is extremely non-uniform, which has great effect on the receiver thermal performance. Taking the non-uniform solar flux distribution into consideration, the photon-thermal conversion process is simulated integrally in this paper. A hybrid simulation approach, which couples Monte Carlo ray tracing and Gebhart methods, is applied to investigate the complete solar radiation transfer process in the solar power tower system with a cavity receiver and obtain the non-uniform solar flux distribution. On this basis, the coupled photon-thermal conversion process in the molten salt cavity receiver is simulated. The effects of the fluid flow layout on the receiver performance under the non-uniform solar flux distribution are particularly revealed. Also, the variation of the receiver performance with time is analyzed in detail. The results show that the solar flux distribution on the interior surfaces of the cavity receiver is extremely non-uniform, under which the molten salt flow layout has great effects on the receiver performance. In the case of flow layout that the cold molten salt flows into the receiver in the high-flux area, the temperatures of both the absorber wall and the molten salt increase rapidly along the flow path. The temperature of the whole absorber wall is relatively high, which leads to greater heat loss and less amount of hot molten salt, and even seriously, “heat transfer deterioration” near the outlet of the receiver. When the cold molten salt flows into the receiver in the low-flux area, the temperature of both the absorber wall and the molten salt increase slowly along the flow path. The temperatures of the whole absorber wall is relatively low, which results in less heat loss and larger amount of hot molten salt. Although the shape of the solar flux distribution on the interior surfaces varied greatly with time, the evolutions of the molten salt (absorber wall) temperature at different time display similar variation tendency along the flow path. Moreover, the reflective loss varies with time greatly, while the heat loss varies with time slightly.

Cloud-based Generation and Transfer of Solar Power in a Global Scale

One of the most important resources we need today is electricity to carry out comfortably our day-to-day activities. While many nations in the world are abundantly enjoying this resource, more than 1.2 billion people do not have access to electricity. Most of these people are from developing nations typically from Africa and India. They happen to be poor and living in remote villages and mountainous terrains. Several agencies like the World Bank, UNDP, IEA and some developed countries are already taking steps to alleviate this energy poverty with the aim to achieve universal access to electrical power by the year 2030. These efforts are being implemented through well-known methods of electrical power production—both renewable as well as other types, one of which is harvesting the solar energy. As we know, sunlight is not available continuously, 24 hours per day. But what if we could make it so? Since one half the earth’s hemisphere does have sunlight, while the other half experiences night time, what if donor countries having the sunshine store the solar energy so it could be used during the night in receiver countries augmented by other types of electrical power? If one such country has excess solar energy, that portion can be transmitted to the country in need. While the infrastructure to directly transfer the energy may be cost prohibitive, perhaps it can be transformed and shared indirectly. However, this type of power sharing of solar energy from one country to another has not been investigated so far. As an initial step, we discuss in this paper a novel approach to transfer digitized temperature of molten salt, used to generate solar power. Molten salt-based generation of solar power has been proved to be more productive and cost effective compared to other methods. Many countries such as USA, Germany, France and Spain are adopting Molten Saltbased technology. If these countries are willing, the temperature of the molten salt can be accessed in parallel to their operation, digitized and uploaded into a private cloud, set up by these countries along with a developing nation. The developing nation can then download the digitized temperature and convert it back into electrical power for distribution.

Experimental Study on Molten Salt Oxidation of High Salt Content Pharmaceutical Residue

The pharmaceutical residues from chemical synthesis pharmaceutical industry often contain some alkali salts due to the widely use of alkali salts in the manufacturing process, the waste management of such is a significant challenge. Generally, the residues are disposed by landfill and incineration. However landfill is unsustainable and the incineration's efficiency is effected by furnace slag-bonding during combustion, that affects the widely use of incineration. On the other hand, molten salt oxidation is an efficient, flameless thermal process, has the inherent capability of destroying organic constituents of wastes while retaining the inorganic constituents in the molten salt. Therefore, a new access is proposed by disposing the residues using molten salt oxidation. A high salt content pharmaceutical residue that contains 28.82wt% Na has been selected as the sample. Molten salt oxidation experiments have been conducted in a lab-scale molten salt reactor using a ternary salt (Li,Na,K)2CO3. The experimental parameters investigated here are the temperature of molten salt and the excess air factor. The concentrations of the CO, NOx and SOx in the off-gas are monitored on-line. Results show that the concentration of CO decreases with temperature increasing, especially at temperature higher than 600̊C. The SOx in the off-gas has been detected rarely, irrespective of the operating conditions. When the temperature is below 700̊C, the concentrations of NOx in the off-gas are under 200ppm, but as the temperature reached 700̊C, due to the nitrite in salt bath begins to decompose, the concentration increases dramatically, suggests the operating temperature should below 700̊C in order to suppress the emission of NOx. Compared to the temperature's role, the effect on the residue's oxidation of the excess air factor is relative small. In the drained salt no char has been found and the XRD analysis shows that the main content of the salt is still (Li,Na,K)2CO3. The results of our study show molten salt oxidation is a promising alternative technology for the disposal of high salt content pharmaceutical residues from the chemical synthesis pharmaceutical industry, and it may be also suitable for other high salt content residues from the fine chemical industry, however, more research is needed to verify this possibility.

In-situ synthesis of MoSi2 coating on molybdenum substrate by electro-deoxidation of a SiO2 layer in molten salt

MoSi2 coating was fabricated in situ by electro-deoxidation of SiO2 layer pre-fabricated on molybdenum substrate in molten calcium chloride at 900°C. The characteristics of the composition and microstructure of the coating under different conditions were investigated. And the cyclic voltammograms of the silica layer in molten calcium chloride were also analyzed, aiming to achieve a better understanding of the electro-deoxidation and MoSi2 formation processes. It was found that the reduction rate increased with the cell voltage and temperature of molten salt increasing. MoSi2 coating with compact structure was obtained after the electro-deoxidation of the silica layer at 3.1V for 3h. The preliminary study on the mechanism of MoSi2 coating formation suggested that SiO2, CaSiO3 and Ca3Si2O7 were reduced directly on Mo substrate to form Si–Mo alloys. As the process continued, the ratio of Si in Si–Mo alloys increased, and finally the compact MoSi2 coating with a thickness of about 10μm was obtained. The electro-reduction of SiO2 layer was highly irreversible and controlled mainly by the slow rate of electron transfer.

Molten salt synthesis, energy transfer, and temperature quenching fluorescence of green-emitting β-Ca2P2O7:Tb3+ phosphors

Tb3+-doped β-Ca2P2O7 phosphors were successfully prepared via a simple surfactant-free molten salt method for the first time and characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (SEM), and photoluminescence. The results of XRD and SEM suggest that the products belong to pure β-Ca2P2O7 nanoparticles with average particle size of about 88.6 nm. Under the UV light excitation, the as-prepared β-Ca2P2O7:Tb3+ phosphors exhibit green emission, which corresponds to the characteristic emissions of Tb3+ ion. The optimal doping concentration of Tb3+ ions in β-Ca2P2O7 phosphors was confirmed to be abound 10 mol% and the exchange interaction is responsible for energy transfer between Tb3+ ions in β-Ca2P2O7 phosphors. The fluorescent lifetime of 5D4 level of Tb3+ in β-Ca2P2O7 phosphors decreases with the increase of Tb3+ ions concentration because of self-generated quenching process, which was confirmed by Auzel’s model. The thermal quenching behaviors of β-Ca2P2O7:Tb3+ nanophosphors were also studied and the activation energy was deduced to be 0.265 eV.

Enhanced heat transfer performances of molten salt receiver with spirally grooved pipe

The enhanced heat transfer performances of solar receiver with spirally grooved pipe were theoretically investigated. The physical model of heat absorption process was proposed using the general heat transfer correlation of molten salt in smooth and spirally grooved pipe. According to the calculation results, the convective heat transfer inside the receiver can remarkably enhance the heat absorption process, and the absorption efficiency increased with the flow velocity and groove height, while the wall temperature dropped. As the groove height increased, the heat losses of convection and radiation dropped with the decrease of wall temperature, and the average absorption efficiency of the heat receiver can be increased. Compared with the heat receiver with smooth pipe, the heat absorption efficiency of heat receiver with spirally grooved pipe e/d = 0.0475 can rise for 0.7%, and the maximum bulk fluid temperature can be increased for 31.1 °C. As a conclusion, spirally grooved pipe can be a very effective way for heat absorption enhancement of solar receiver, and it can also increase the operating temperature of molten salt.

Preparation of Zircon Whiskers via Non-Hydrolytic Sol-Gel Process Combined with Molten Salt Method

Zircon whiskers were prepared via non-hydrolytic sol-gel process combined with molten salt method, using Si (OC2H5)4 (TEOS) and ZrCl4 as raw materials, LiF as mineralizer, and Na2WO4 as molten salt. The phase composition, morphology and microstructure of the samples were characterized by X-ray diffraction analysis (XRD), field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). The influence of introducing molten salt and calcination temperature on the synthesis and morphology of zircon crystals was investigated. The results show that the introduction of Na2WO4 contributes to the one-dimensional preferential growth of zircon. Zircon whiskers elongated along the c-axis, with diameter of ~100nm and aspect ratio of more than 15, are achieved at the temperature of 850 °C, and neither over low nor over high calcination temperature is conducive to the growth of zircon whiskers.

Assessing thermal energy storage technologies of concentrating solar plants for the direct coupling with chemical processes. The case of solar-driven biomass gasification

Dynamic simulation, design improvements and control issues in solar power plants might compete with special considerations on energy storing techniques. In order to provide the stability in production of power or chemical commodities in spite of discontinuity in the source of energy, i.e., sun, overall concerns in the details of solar power plant, competition and comparison of common storing technologies should be taken into account to ensure the effectiveness and continuity of the supply. This research activity is aimed at extending the study from the power generation purpose to the solar-supplied chemical commodities production, highlighting the limitations of certain well-established thermal energy storage techniques when concentrating solar is directly coupled with chemical processes. The (intrinsically dynamic and closed-loop) simulation of solar power plants and direct thermal energy storage technologies is performed for the direct thermal energy storage technologies and, only for the case of thermocline, it is coupled with computational fluid-dynamic (CFD) studies for the proper assessment of molten salt and steam temperature trends. To investigate benefits/restrictions of the storage technologies, the solar steam generation is integrated with the gasification of biomasses for syngas production. Also, first-principles dynamic model for the biomass gasifier is provided.

Effect of Na<sub>2</sub>WO<sub>4</sub> Content on the Crystallization Temperature of NaCl-KCl-Na<sub>2</sub>WO<sub>4</sub> Molten Salt System

The Liquidus Temperature of Molten Salt Nacl-Kcl-Na2WO4 System was Measured by Differential Thermal Analysis. the Results Show that, in the Molten Salt System, when XNa2WO4≤0.7 Eutectic Temperature is 555 °C and a Phase Transition Temperature is 512.8 °C, Eutectic Composition Become XNaCl= 0.35, XKCl= 0.35, XNa2WO4= 0.3; when XNa2WO4>0.7, Eutectic Temperature and a Phase Transition Temperature are Respectively 613 °C and 574.4 °C, Eutectic Composition Become XNaCl= 0.1, XKCl= 0.1, XNa2WO4= 0.8.

Effect of NaWO<sub>3</sub>F Content on the Liquidus Temperature of NaCl-KCl-NaWO<sub>3</sub>F Molten Salt System

The liquidus temperature of molten salt NaCl-KCl-NaWO3F system was measured by differential thermal analysis. The results show that in the molten salt system when :=1:1, XNaWO3F <0.5 the liquidus temperature decreased with the increase of NaWO3F content; when :=1:1, XNaWO3F>0.5 the liquidus temperature increased with the increase of NaWO3F content; the eutectic temperature is 615.9 °C, the eutectic composition is =0.25, =0.25, XNaWO3F=0.5.

Effect of Na<sub>3</sub>WO<sub>3</sub>F<sub>3</sub> Content on the Liquidus Temperature of NaCl-KCl-Na<sub>3</sub>WO<sub>3</sub>F<sub>3</sub> Molten Salt System

The liquidus temperature of molten salt NaCl-KCl-Na3WO3F3 system was measured by differential thermal analysis. The results show that in the molten salt system when XNaCl:XKCl=1:1, XNa3WO3F3<0.6 the liquidus temperature decreased with increase of Na3WO3F3 content; when XNaCl:XKCl=1:1, XNa3WO3F3> 0.6 the liquidus temperature increased with the increase of Na3WO3F3 content; the eutectic temperature is 612.9 °C, the eutectic composition is XNaCl=0.2, XKCl=0.2, XNa3WO3F3=0.6.

Effect of Na<sub>2</sub>WO<sub>4</sub> Content on the Liquidus Temperature of NaCl-KCl-NaF-Na<sub>2</sub>WO<sub>4</sub> Molten Salt System

The liquidus temperature of molten salt NaCl-KCl-NaF-Na2WO4 system was measured by differential thermal analysis. The results show that in the molten salt system when 0.6 eutectic temperature is 542.1 °C and a phase transition temperature is 515.4 °C, eutectic composition become = 0.28, = 0.28, =0.14, = 0.3; when > 0.6, eutectic temperature and a phase transition temperature are respectively 597.7 °C and 495.8 °C, eutectic composition become = 0.12, = 0.12, = 0.06, = 0.7.

Dynamic thermal performance analysis of a molten-salt packed-bed thermal energy storage system using PCM capsules

We investigate the dynamic thermal performance of a molten-salt packed-bed thermal energy storage (TES) system using capsules filled with high-temperature phase change material (PCM), which is identified as a promising low-cost TES system for concentrating solar power (CSP) plants. A transient two-dimensional dispersion-concentric (D-C) model is modified to account for the phase change process within capsules so as to determine the temperature distribution and phase change front within each capsule. Using the model, detailed characteristics of heat transfer between molten salt and the packed PCM capsules are investigated, and a parametric sensitivity analysis is provided. During the discharging process, different variation trends are found for the capsule temperature due to the existence of the isothermal solidification process. As a result, generally there exists a quasi-isothermal region and two thermocline regions for the molten-salt temperature along the tank height, and the molten-salt temperature at the outlet also shows a quasi-isothermal period, during which the molten-salt outlet temperature is very close to the phase change temperature (PCT) of PCM. It is also found that the effective discharging efficiency of the system can be increased by increasing the PCT, decreasing the molten-salt inlet velocity or decreasing the capsule diameter. These results provide suggestions to optimize the design and operational parameters for the system within practical constrains.

Economic Chances and Technical Risks of the Internal Direct Absorption Receiver

Increased receiver temperatures of solar tower power plants are proposed to decrease the plants levelized electricity costs (LEC) due to the utilization of supercritical steam power plants and thus higher overall plant efficiency. Related to elevated receiver temperatures preliminary concept studies show a distinct LEC reduction potential of the internal direct absorption receiver (IDAR), if it is compared to liquid in tube (LIT) or beam-down (BD) receiver types. The IDAR is characterized by a downward oriented aperture of a cylindrical cavity, whose internal lateral area is illuminated from the concentrator field and cooled by a liquid molten salt film. The objective is the further efficiency enhancement, as well as the identification and assessment of the technical critical aspects. For this a detailed fluid mechanic and thermodynamic receiver model of the novel receiver concept is developed to be able to analyze the IDAR's operating performance at full size receiver geometries. The model is used to analyze the open parameters concerning the feasibility, functionality and performance of the concept. Hence, different system management strategies are examined and assessed, which lead to the proposal of a cost optimized lead-concept. This concept involves a rotating receiver system with inclined absorber walls. The spatial arrangements of the absorber walls minimize thermal losses of the receiver and enhance film stability. The centrifugal forces acting on the liquid salt film are essential to realize the required system criteria, which are related to the maximal molten salt temperature, film stability and droplet ejection. Compared to the state of the art at a 200 MWel power level the IDAR concept can lead to a LEC reduction of up to 8%. The cost assumptions made for the assessment are quantified with sensitivity analysis.

System-level simulation of a solar power tower plant with thermocline thermal energy storage

A thermocline tank is a low-cost thermal energy storage subsystem for concentrating solar power plants that typically utilizes molten salt and quartzite rock as storage media. Long-term thermal stability of the storage concept remains a design concern. A new model is developed to provide comprehensive simulation of thermocline tank operation at low computational cost, addressing deficiencies with previous models in the literature. The proposed model is then incorporated into a system-level model of a 100MWe power tower plant to investigate storage performance during long-term operation. Solar irradiance data, taken from measurements for the year 1977 near Barstow, CA, are used as inputs to the simulation. The heliostat field and solar receiver are designed with DELSOL, while the transient receiver performance is simulated with SOLERGY. A meteorological year of plant simulation with a 6-h capacity for the thermocline tank storage yields an annual plant capacity factor of 0.531. The effectiveness of the thermocline tank at storing and delivering heat is sustained above 99% throughout the year, indicating that thermal stratification inside the tank is successfully maintained under realistic operating conditions. Despite its good thermal performance, structural stability of the thermocline tank remains a concern due to the large thermal expansion of the internal quartzite rock at elevated molten-salt temperatures, and requires further investigation.

Study on the phosphate reaction characteristics of lanthanide chlorides in molten salt with operating conditions

A minimization of waste salt is one of the most important issues for the optimization of pyroprocessing. The separation of fission products in waste salts and the reuse of purified waste salt are promising strategies for minimizing the waste salt amounts. The phosphate precipitation of lanthanide is currently being considered for eutectic (LiCl–KCl) waste salt purification. In this research, the effects of molten salt temperature (400–550 °C) and reaction time (max. 180 min) upon conversion into the phosphate of lanthanides was investigated using 1 and 3 kg of eutectic salt. The conversion efficiency of lanthanides to molten salt-insoluble precipitates and phosphates was increased with an increase in molten salt temperature and operating time until it attained a specific temperature and time. K3PO4 as a precipitant was more favorable than Li3PO4 in terms of reactivity. To obtain over a 99% overall conversion efficiency, about 30 min was required in the case of using K3PO4 at 450 °C, but about 120 min in the case of using Li3PO4 at 550 °C. The lanthanide precipitates formed by a reaction with phosphate were a mixture of monoclinic structures, usually representing a polyhedron structure, and a tetragonal structure, representing a platelet structure.

Evaporation Properties of FLiNaK with Plasma Interaction

Evaporation properties of FLiNaK (LiF 46.5 mol% + NaF 11.5% + KF 42 mol%) were investigated with hydrogen plasma interaction. To prevent massive evaporation of molten salt of flowing wall in fusion device, evaporation property of molten salt should be researched. However vapor pressure of FLiNaK has been studied for liquid state without consideration of interaction with plasma. We measured evaporation property of FLiNaK with hydrogen plasma interaction. Vapor component of FLiNaK were detected with OES(Optical Emission Spectroscopy) and RGA(Residual Gas Analyzer). The film deposited on wafer samples were investigated with EDS(Energy Dispersive Spectrometer) to determine vapor component. Hydrogen plasma was generated with 500W ECR source and the molten FLiNaK was contained with heated crucible of diameter of 46mm and depth of 40mm. OES data showed several peaks of total component of FLiNaK. Those were emission lines of F, Li, Na and K. RGA data also showed FLiNaK components with plasma interaction. Without plasma interaction, the deposited film was mostly KF at molten salt temperature of 973K. The components of deposited film during plasma interaction were similar with the original FLiNaK sample. The evaporated mass became higher with plasma interaction while the remained FLiNaK at crucible still hold similar molar percentage. In this paper, plasma-enhanced-evaporation of FLiNaK was qualitatively discussed.