Salt Tank Research Articles

Numerical investigation on the thermal performance of a molten salt tank under different electric heating method

Two-tank molten salt (MS) thermal energy storage (TES) technology is an efficient and widely used energy storage technology. This study develops a two-dimensional discrete model (2DIM) using the Modelica language to accurately and quickly represent molten salt temperature distribution and heat loss, applicable to system-level analysis in thermal power systems. It also uncovers variations in heat loss, cooling rate, and required auxiliary electric heating power across different tank volumes, liquid levels, and MS temperatures, offering guidance for optimizing design, energy management, and operational strategies. Additionally, the study examined the impact of different electric heater placements on the performance of the thermal storage system. The results show that heating the gas inside the tank is more effective than traditional direct electric heating, reducing molten salt heat loss by 25.89 % and increasing energy storage efficiency by 0.17 %.The research provide a reference for the study of efficient utilization of TES tank in different thermal systems.

Design Basis Document / Owner’s Technical Specification for Nitrate Salt Systems in CSP Projects

Work has begun on a Design Basis Document / Owner’s Technical Specification for parabolic trough and central receiver power plants using nitrate salt as the heat transport fluid and the thermal storage medium. A principal topic of recent interest is the failures of the hot salt tanks in central receiver projects. The paper outlines a hypothesis for the source of the failures, and discusses a range of possible solutions.

Molten Salts Tanks Thermal Energy Storage: Aspects to Consider during Design

Concentrating solar power plants use sensible thermal energy storage, a mature technology based on molten salts, due to the high storage efficiency (up to 99%). Both parabolic trough collectors and the central receiver system for concentrating solar power technologies use molten salts tanks, either in direct storage systems or in indirect ones. But even though this is a mature technology, it still shows challenges in its implementation and operation. This paper underscores the critical importance of stringent design criteria for molten salt tanks in thermal storage technology. Focusing on the potential ramifications of design failures, the study explores various dimensions where an inadequate design can lead to severe consequences, even jeopardizing the viability of the entire technology. Key areas discussed include structural integrity, corrosion, thermal shock, thermal expansions, and others. By elucidating the multifaceted risks associated with design shortcomings, this paper aims to emphasize the necessity of thorough reviews and adherence to robust design principles for ensuring the success, safety, and sustainability of thermal storage technology.

Techno-economic analysis of a new thermal storage operation strategy for a solar aided liquid air energy storage system

The SA-LAES system is an economical and efficient energy storage solution that combines solar energy with LAES. However, the inherent instability of solar energy can result in uncertainty regarding heat storage, thereby compromising the stability of the energy storage system. The development of more economical, efficient, and stable strategies for the SA-LAES system represents a challenging task. The present study puts forward a novel thermal storage operation scheme (Strategy 2) for SA-LAES systems integrated with electric heating during valley periods, thereby achieving long-term stable operation of the energy storage system via the alternating utilization of two full molten salt tanks and one empty molten salt tank. Theoretical and case studies are conducted based on thermodynamic and economical technical indicators, and detailed comparisons are made with the traditional strategy (Strategy 1) of only changing the solar multiple and capacity. The results show the exergy destruction of Strategy 1 mainly occurs in air turbines under off-design operating conditions. The exergy destruction of Strategy 2 is mainly in the electric heater part. The single-day analysis found that the new strategy has a more significant advantage in terms of exergy efficiency than Strategy 1 when the direct normal irradiance (DNI) is below 650 W/m2. In the case study, the levelized cost of energy (LCOE) values for Nanning, Beijing, and Hami regions in Strategy 2 are 0.118 $/kWh, 0.077 $/kWh, and 0.062 $/kWh, respectively. The LCOE values of Strategy 2 are decreased by 10.97 %, 27.49 %, and 3.40 % compared to Strategy 1, respectively. The payback periods for investments in the Beijing and Hami regions have decreased by 67.63 % and 16.41 %, respectively. The research presented in this article provides an economical, efficient, and stable operating strategy for thermal storage in the SA-LAES system.

Self-adaptive heat extraction controller for solar thermal tower operational with molten salt tanks

Molten salts are commonly used in solar thermal power plants to store heat when sunlight is unavailable. However, solidifying the salts can lead to operational interruptions and prevent an optimal energy output. A self-adaptive heat extraction method has been proposed to regulate salt pipeline temperatures, enabling continuous operations that mitigate unplanned shut-downs. Such a method focuses on maintaining operational temperatures and preventing interferences caused by the freezing of the salts. The proposed Self-Adaptive Heat Extraction Controller (SAHEC) relies on fuzzy logic integrated into MATLAB Simulink to ensure optimal heat extraction control utilizing four distinct temperature variables. Modeling this approach through a created MATLAB-based simulation reveals that SAHEC offers an undeniable improvement over standard conventional methods for manipulating temperatures.

Effects of EPCM particle properties on creep damage of the molten salt packed-bed thermal energy storage system

One of the concerned aspects in the design of molten salt packed-bed thermal energy storage (TES) tank with encapsulated phase change materials (EPCMs) is to evaluate the creep damage because of molten salt tanks have to withstand elevated temperature and stress level in charging and discharging processes. This paper aims at estimating the creep damage and investigating the effects of EPCM particle properties on creep damage performance of EPCM-TES tank. First, a commercial scale molten salt EPCM-TES tank applied in 50MWe tower plant is designed. Second, a method to evaluate the creep damage of packed-bed tank in service life is developed based on the integration model coupling FVM and FEM. Then, the effects of particle diameter and melting temperature of EPCMs on creep damage are discussed. Finally, a novel diameter-changed and melting temperature-changed two-layered EPCMs packed-bed structure with lower creep damage is proposed. The results are concluded as follows. (1) The creep damage of bottom wallboard is more significant than that of other wallboards in operation. (2) The EPCMs with smaller particle diameter or higher melting temperature is helpful to reduce the creep damage of tank. When particle diameter decreases from 42 mm to 20 mm or melting point rise from 365 °C to 395 °C, the creep damage of bottom wallboard can be reduced by 4.8 times and 58 %, respectively. (3) The creep damage of novel two-layered EPCM-TES configuration with lower creep damage time and stress is weaker than single-layered form. Compared with single-layered form filling with higher particle diameter and melting temperature of EPCMs, the creep damage of bottom wallboard in two-layered structure can be reduced by 53 %. This work can provide insights on the optimization design of the EPCM packed-bed TES configuration for lower creep damage in service life.

Economic feasibility assessment of a solar aided liquid air energy storage system with different operation strategies

In response to severe environmental problems, the proportion of new energy consumption worldwide is on an unprecedented upward trend, bringing energy storage technologies into focus. Among various energy storage systems, the solar aided liquid air energy storage (SALAES) system shows great prospects for development due to its cleanliness and high efficiency. This paper develops a basic operation strategy based on the idea of peak and valley reduction, considering the temporal fluctuation characteristics of solar energy. Further, operation modes for replacing the oil-air heat exchangers are proposed and an optimized strategy is developed accordingly. Real weather data of Beijing are used to estimate the annual power output of each system, and economic analyses are carried out on this basis using the levelized cost of storage (LCOS) method. The results show that the introduction of solar energy can reduce the LCOS of the liquid air energy storage system by 4.1 %–13.67 % and the proposed optimized operation strategy can increase the annual power output, resulting in a 0.2 %–0.4 % reduction in LCOS compared to the basic operation strategy. In addition, the impact of the molten salt tank capacity on the investment and return of the SALAES system is investigated. The optimal molten salt tank capacity obtained in this paper is 5 h, with the lowest LCOS of 0.1477 $/kWh and the shortest dynamic payback period of 10.29 years when operating with the optimized strategy.

Dynamic Investigation of a Solar-Driven Brayton Cycle with Supercritical CO2

The exploitation of solar irradiation is a critical weapon for facing the energy crisis and critical environmental problems. One of the most emerging solar technologies is the use of solar towers (or central receiver systems) coupled with high-performance thermodynamic cycles. In this direction, the present investigation examines a solar tower coupled to a closed-loop Brayton cycle which operates with supercritical CO2 (sCO2) as the working medium. The system also includes a storage system with two molten salt tanks for enabling proper thermal storage. The sCO2 is an efficient fluid that presents significant advancements, mainly reduced compression work when it is compressed close to the critical point region. The novelty of the present work is based on the detailed dynamic investigation of the studied configuration for the year period using adjustable time step and its sizing for achieving a continuous operation, something that makes possible the establishment of this renewable technology as a reliable one. The analysis is conducted with a developed model in the Modelica programming language by also using the Dymola solver. According to the simulation results, the yearly solar thermal efficiency is 50.7%, the yearly thermodynamic cycle efficiency is 42.9% and the yearly total system efficiency is 18.0%.

Strength analysis of molten salt tanks for concentrating solar power plants

Promoting the development of concentrating solar power (CSP) is critical to achieve carbon peaking and carbon neutrality. Molten salt tanks are important thermal energy storage components in CSP systems. In this study, the cold and hot tanks of a 100 MW CSP plant in China were used as modeling prototypes. The materials and geometric models were determined based on related specifications and engineering experience. Mechanical characteristics of the tanks under steady condition, including the deformation, stress distribution, and stress concentration, were simulated and calculated. Furthermore, the strength of the tank walls was evaluated. The findings can be used as a reference for designing the molten salt storage tank and reducing the risk during the operation.

Experimental and computational optimization of eco-friendly mortar blocks for high temperature thermal energy storage of concentrated solar power plants

New avenues for thermal energy storage (TES) need to be investigated due to the lack of competitiveness of concentrated solar power (CSP) technologies. Solutions must be found to replace molten salt tanks which have a major economic impact and are difficult to maintain due to corrosion problems. In this sense, concrete represented an attractive candidate by proving excellent sensible TES in CSP. However, its main phase, made of Portland cement (PC), has significant environmental consequences. The production of PC is known to emit high levels of polluting gases, particularly the CO2. It is estimated to be responsible for between 5% and 7% of the world's CO2 emissions, making it a major contributor to climate change. This work presents greener cementitious materials, made of alkaline cements and hybrids cements, to be used as alternative eco-friendly TES media in CSP plants. An experimental campaign is presented which shows that these eco-efficient materials can have better mechanical properties, than the ordinary PC mortar, when exposed to high temperatures, in addition, can offer improvements of their thermal properties (thermal conductivity or specific heat). Second part of the work is devoted to Finite Element simulations, with the aim to find the best configuration, in terms of selection of materials and geometry, which are more efficient as TES system. The work is showing the following advancements in CSP technology by using alternative eco-friendly binders: the installation volume can be reduced by 17%, compared to a molten salt tank, while the heat exchanger's surface area can be resized by 29%, compared to the reference system using PC. These improvements enable wider variations in CSP operational efficiency and dynamic capabilities and represent important progress towards developing more efficient and sustainable CSP technologies.

Thermal performance of a hybrid steel-concrete tank section for thermal energy storage in concentrated solar power plants

Several studies of thermocline molten salt tanks have proven to be suitable for thermal energy storage systems. However, there is a lack of experimental investigations performed. The building of a hybrid section and its assessment operating as a thermocline tank are the main objectives of this research work. The novelty is to prove the performance of the hybrid tank concept made of a thick concrete layer and a thin steel liner. The tank section studied comprises the following layers of materials: a steel liner, an air gap interface, a high thermal concrete layer and insulating materials. The thermal response was monitored using thermocouples located between the layers, at different heights and widths of the whole tank section length. The performance during the commissioning (first heat) and the operation (3 heat/cool cycles between 200/500 °C) of the tank section is analysed. The thermal appropriateness of using concrete as part of the thermocline tank has demonstrated the suitability and good thermal response up to 425 °C showing isolating and thermal inertia. The presence of the air gap interface between the steel liner and the concrete generates a heat convection current with local energy gradients along the section height.

Performance analysis of solid heat accumulator used in tower solar thermal power generation system

In recent years, solar energy, wind energy and other new energy sources have been highly praised, especially solar energy, which is considered as one of the cleanest renewable new energy sources, and solar thermal power stations have been built in many countries. Tower solar photothermal power generation is a heat absorber that reflects sunlight to the top of the tower through heliostat field. Molten salt absorbs heat through the heat absorber, heats water supply and promotes thermal power generation. However, solar energy is intermittent and unstable, so the tower solar thermal power station is equipped with heat storage molten salt tank. Solid heat storage technology has the advantages of cheap heat storage medium, no harm to the environment, strong reliability and operability of the system, and can provide cost-effective heat storage solutions for industrial process heat energy and power generation. In this paper, the molten salt tank is replaced by a solid heat accumulator. After the molten salt absorbs heat during the day, part of it heats the heat accumulator through the unit tube in the heat accumulator to store the heat in the heat accumulator, and the other part heats the water to promote the steam power cycle. At night, the heat accumulator heats the molten salt to release heat, which continues to promote the steam power cycle. In this paper, CFD software will be used to numerically simulate the change of temperature characteristics inside the solid regenerator with the time of heat storage and release, and draw the temperature nephogram and change curve.

Experimental investigation of the migration and solidification characteristics of Solar Salt in hot sand layer of tank foundation

In this study, a thermal tank foundation experimental system was built and the migration and solidification characteristics of Solar Salt in the hot sand layer of tank foundation were investigated. The results indicate that after the molten salt leakage, the temperature in the area where molten salt flows first increases rapidly, then decreases rapidly, and finally remains stable. The temperature in the area where molten salt does not flow slowly increases to a stable value. The parameter changes have significant effects on the migration characteristics. As the operating temperature increases or the leakage aperture decreases, the maximum migration depth and width increase and decrease, respectively. As the leakage mass or the filler porosity increases, the maximum migration depth and width both increase. The migration range under the rectangular leakage hole is larger than that under the circular leakage hole. The solidification agglomeration in the sand tank foundation presents the shape of thin at both ends and thick in the middle, and it presents the shape of thin at the upper part and thick at the lower part in the light expanded clay aggregate (LECA) tank foundation. In the sand + LECA tank foundation, when the sand layer is thick enough, the agglomeration shape is approximately the combination of the agglomeration shape in the sand tank foundation and the lower part of the agglomeration shape in the LECA tank foundation. The conclusion of this study provides a useful reference for understanding, preventing and dealing with molten salt tank leakage accidents.

Lessons learned from corrosion of materials with molten salts during molten salt tank preheating

One of the biggest challenges of concentrating solar power (CSP) is achieving cost-reduction in the thermal energy storage (TES) system. Operating conditions of molten salt CSP plants includes intermittent exposure of the TES tanks materials (i.e., carbon steel, stainless-steel) in contact with molten salt (NaNO 3 –KNO 3 ). This manuscript evaluates the lessons learnt in the performance of such tank materials with molten salts under continuous and intermittent exposure in a demo plant for indirect thermal storage at 400 °C. This study shows that the hot tank upper section developed breakaway corrosion phenomena generating oxides layers, which easily delaminates from the base metal. This damage is produced by the tank preheating system, which introduced a non-controlled amount of CO 2 inside the tanks. Then, the carbon steel upper section is exposed in the hot tank to a mixture composed by CO 2 , water vapour, and chlorides at high temperature, producing this type of attack. The materials tested are carbon and low alloyed steels types A516Gr70, A387Gr11 and A387Gr5, Cr–Mo steel A387Gr9, stainless steels types A304L, A316L and A347. The lessons learnt have been implemented in the commissioning of new commercial plants successfully. • Lessons learnt of intermittent exposure of steel in molten salts. • Application in CSP plants with nitrates mixture molten salts at 400 °C. • Breakaway corrosion with easily delaminating oxide layers. • Phenomena produces during tank preheating due to CO 2 presence. • Lessons learnt implemented in the commissioning of commercial plants successfully.

Cooling process of a novel low melting point ternary mixture for thermal energy storage studied experimental and numerically in a pilot-scale tank

The most used thermal energy storage material in concentrated solar power plants is a binary salt of 60%NaNO3 + 40%KNO3. However, the relatively high melting point of this mixture (222 °C) represents a significant risk of local solidification in the operation of these plants during stand-by periods. This research proposes the use of a novel mixture composed by 30%LiNO3 + 57%KNO3 + 13%NaNO3 with a low melting point of 127 °C. The transient cooling process of a molten salt tank working with a considerable amount of 616 kg of the proposed ternary mixture was studied experimentally in a pilot-scale tank for the first time. A computational fluid dynamics (CFD) model was then developed and validated with the experimental data with a maximum temperature relative error below 4 %. Simulation of the cooling process to obtain crystallization times for the proposed ternary salt, solar salt and Hitec molten salts were performed, and it was obtained a higher crystallization time for the proposed ternary salt 125.4 h in comparison with solar salt (56.4 h) and Hitec (124.2 h). Finally, using the model developed, new simulations were performed for a scaled-up model of a state-of-the-art cold tank of a two-tank molten salt CSP thermal storage system. Results revealed that crystallization risk is minimized with the use of the proposed ternary mixture for both considered scenarios of minimum and medium height levels, with crystallization times of 24 days and 7 h and 277 days and 5 h respectively.

A new leakage detection method of molten salt tank and its experimental verification and optimization

Molten salt tank leak detection technology is essential for the timely detection and handling of leaks. Current common thermocouple leak detection technology cannot meet the demand, and more efficient leak detection technology is urgently needed. In this paper, a molten salt tank leakage detection circuit based on the conductivity of high-temperature molten salt is proposed for the first time. Its feasibility is verified by the experimental results of a lab-scale tank foundation experimental system, and a parametric analysis is carried out to optimize the circuit layout. The results show that when molten salt leaks to the measurement point of the circuit, the voltage of the constant resistor in the circuit first rises rapidly, then drops rapidly, and finally, reaches a steady value. The vertical spacing, arrangement depth, and operating temperature significantly influence the detection effect. To ensure the rapid and continuous detection of molten salt tank leakage, the vertical spacing should be less than 50 mm. The arrangement depth should be as close as possible to the bottom of the molten salt tank, preferably within 150 mm. The decrease in the operating temperature significantly deteriorates the continuous detection effect of the circuit. When the detection circuit is used in the cold molten salt tank of concentrating solar power plants, timely circuit detection signal processing and prompt mechanisms are required to ensure that the leakage accident is not missed.

Numerical Simulation Study on Flow Heat Transfer and Stress Distribution of Shell-and-Tube Superheater in Molten Salt Solar Thermal Power Station

The flow heat transfer and stress distribution of the shell and tube superheater of the steam generation system in a 50 MW molten salt tank solar thermal power station are studied by numerical simulation, and the influence of the flow pattern of molten salt and water vapor in the shell-and-tube superheater on the heat transfer efficiency and stress distribution under the heat–fluid–solid coupling condition is deeply studied. When the water vapor is located on the tube side of the superheater, the molten salt is located on the shell side, and the counterflow is used in the superheater, the water vapor outlet meets the inlet temperature of the steam turbine, and the heat exchange efficiency of the superheater can reach 94.2%. The optimum inlet temperatures of molten salt and steam in the superheater are 563 and 345 °C, respectively, and the optimum flow rate of molten salt at the inlet of the superheater is 2.5 m/s. Compared with the stable condition, the heat exchange efficiency can be increased by 2.9%, the equivalent stress value is reduced from 335.63 to 312.60 MPa, and the deformation is reduced by 0.48 mm.

Experimental investigation of solar organic Rankine cycle with parabolic trough concentrator using nitrate salt as heat transfer and storage fluid

Nitrate salt with low melting point as heat transfer and storage fluid simplifies the heat transfer, storage, and exchange processes, and organic Rankine cycle as heat and power conversion system can realize the distributed energy supply. This study experimentally investigated the solar organic Rankine cycle with parabolic trough concentrator using nitrate salt as heat transfer and storage fluid, and the main problems and phenomena during the operation process have been described and analyzed in detail. Experimental results showed that the temperature difference of collector tube changes with direct normal irradiation and lags behind the changes of direct normal irradiation. Molten salt heat storage system can effectively restrain the temperature fluctuation at the collector tube outlet. The abrupt temperature drop at the evaporator inlet was caused by the heat imbalance between R123 and nitrate salt and inappropriate design of the evaporator. The effects of expander inlet pressure and expansion ratio on the ORC electrical power output were obtained at different superheating degrees of the expander inlet. The researches on each component are instructive to the technology of the parabolic trough concentrated solar power generation system with molten salt as heat transfer and storage fluid and provide some guides for the subsequent research. Furthermore, the energy and exergy analyses on overall system were developed, and results indicated the system average efficiency can reach 1.5%. Novelty statement Experimental study on parabolic trough concentrated solar ORC system with nitrate salt as heat transfer and storage medium were firstly analyzed. Single screw expander was firstly applied in a real solar thermal power generation system and showed a stable performance. Dynamic changes on the temperatures of PTC, molten salt tanks, and ORC were shown, and energy and exergy analyses on overall system were carried on.

Experimental and numerical investigation of temperature distribution and heat loss of molten salt tank foundation at different scales

The two-tank thermal energy storage system with a mixture of sodium nitrate and potassium nitrate is one of the most common ways in concentrated solar power plants. However, the unreasonable thermal performance of tank foundations such as excessive heat loss and overheating of concrete slab could result in local molten salt solidification and foundation settlement. This work proposes a novel foundation configuration with road base, light expanded clay aggregate, thermal clay blocks and fire bricks as insulation materials. The temperature distribution and heat loss of the tank foundations are studied at different scales through experimental and numerical methods. A laboratory foundation is set up to prove the feasibility of foundation materials, which is used in the thermal energy storage system of the 1 MWth pilot plant. According to the results, the pilot foundation has a reasonable temperature distribution. Moreover, the foundation configuration and materials are applied to an industrial molten salt tank foundation. The maximum temperature of external surface of the foundation drops from 162.6 °C to 37.3 °C, the maximum temperature of concrete slab decreases from 96.1 °C to 43.2 °C, and the heat loss of foundation reduces by 39.8%. The results indicate that the foundation with the proposed configuration and materials could enhance the thermal performance of the tank foundation and decrease the heat loss through the tank foundation. The proposed foundation is of great significance for guiding the construction of the thermal storage system of commercial concentrated solar power plants.

Measuring heat loss and temperature changes in molten salt storage tanks

Few experimental measurements of heat loss from molten salt tanks are available. A recent study helps to fill the gap with new data.