Molten-salt Energy Storage Research Articles

Dynamic Process Simulation of a Molten-Salt Energy Storage System

The main objective of this work was the construction of a numerical model using Advanced Process Simulation Software to represent the dynamic behaviour of a thermal storage system (TSS). The storage model was validated by comparing the results with the measured data of the storage process of the Andasol 2 solar power plant. Subsequently, a system analysis and system optimisation were carried out, and the stand-alone concept of the thermal storage system is presented. Stand-alone refers to an isolated use of the storage system without a solar power plant. During power peaks, this storage medium is heated with excess electrical power and later returned to the electrical grid through a steam cycle. Then, the system was optimised by modelling four models based on the type of storage medium and the temperature difference of the storage system. The four models, Andasol 2, SSalt max, Hitec, and Carbonate, were evaluated and compared in terms of the improvement in capacity and efficiency that can be achieved. The comparison shows that the preferred storage medium is carbonate salt due to the increases in both efficiency and capacity. The greatest increase in efficiency in terms of power generation can also be achieved with the Carbonate model (18.2%), whereas the amount of increase was 9.5% and 7.4% for each of SSalt max and Hitec, respectively. The goal of this analysis and system optimisation of a thermal salt storage system is to stabilise and relieve the local power grid.

Ternary molten salt energy storage coupled with graphene oxide-TiN nanofluids for direct absorption solar collector

Owing to the obvious advantages of direct solar collectors, considerable research has been continuously conducted for improving the efficiency of solar energy storage. Due to the instability in the solar energy reaching the Earth and the uneven distribution of solar irradiation during the day and night, it is difficult to continuously output the heat source. In order to further improve collection system performance, we have proposed a direct absorption solar collector, which uses graphene oxide (GO) nanosheets and titanium nitride (TiN) nanoparticles mixed with heat transfer oil as the working fluids, and uses a ternary mixed molten salt (44% Ca(NO3)2, 12% NaNO3, and 44% KNO3) as the heat storage core. Moreover, this new kind of collector has been experimentally investigated, and its solar thermal performance is studied with different proportion of GO to TiN in the nanofluids. The experimental results show that the maximum thermal energy can be stored in the collector is nearly 526.96 J, and the energy retention rate can reach 51.7%. Moreover, the solar thermal efficiency of this collector is up to 56.5%. In addition, the mechanism of heat storage is also studied in detail. This study provides a practical avenue for improving the application potential solar collector, which can effectively promote the heat storage for medium temperature of the collector subjected to incident irradiation.

Performance Analysis of Multi-Energy Hybrid System Based on Molten Salt Energy Storage

This paper briefly summarizes the current status of typical solar thermal power plant system, including system composition, thermal energy storage medium and performance. The thermo-physical properties of the storage medium are some of the most important factors that affect overall efficiency of the system, because some renewable energy sources such as solar and wind are unpredictable. A thermal storage system is therefore necessary to store energy for continuous usage. Based on the form of storage or the mode of system connection, heat exchangers of a thermal storage system can produce different temperature ranges of heat transfer fluid to realize energy cascade utilization. Founded upon the review, a small hybrid energy system with a molten-salt energy storage system is proposed to solve the problems of heating, cooling, and electricity consumption of a 1000 m² training hall at school. The system uses molten-salt storage tank, water tank and steam generator to change the temperature of heat transfer fluid, in order to realize thermal energy cascade utilization. Compared to the existing heating and cooling system, the proposed system needs more renewable energy and less municipal energy to achieve the same results according to simulation analysis. Furthermore, by improving the original heating and cooling system, PMV has been improved. The comprehensive efficiency of solar energy utilization has been increased to 83%.

LATEST WORLDWIDE DEVELOPMENTS IN MOLTEN SALT TECHNOLOGY AND APPLICATIONS

Molten salt (MS) storage systems in the 565°C range can store green solar energy from thermal solar power station, such as the Crescent Dunes solar plant in Nevada. Large containers can be used to store energy and generate electricity for eight hours or more to be used at night or during peak demand hours, depending on the container size. Energy storage can reduce the fluctuation due to weather conditions experienced at thermal solar power stations because stable diurnal energy supply is made available by MS energy storage. Supported by the Office of Naval Research (ONR), the research presented discusses the recent technological developments associated with the use of molten salts for energy storage. In addition to their use for storing excess solar energy, molten salts are starting to be used in nuclear or hybrid power production. One particular aspect of interest is the focus using higher temperature salts to provide even more energy storage than conventional molten salts. One such salt, SaltStream700, allows for the use of molten salts at temperatures of 700°C. A summary of worldwide examples of concentrating solar power (CSP) plants is presented. Commercial solar power stations have been constructed in the United States and overseas, particularly in Spain, with molten salt being considered for use in these facilities.

ALTERNATIVE DESIGNS OF MOLTEN SALT STORAGE SHELLS FOR USE IN SOLAR ENERGY STORAGE

Molten salt (MS) storage systems in the 565°C range can store green solar energy from thermal solar power station, such as the Crescent Dunes solar plant in Nevada. Large containers can be used to store energy and generate electricity for eight hours or more to be used at night or during peak demand hours, depending on the container size. Energy storage can reduce the fluctuation due to weather conditions experienced at thermal solar power stations because stable diurnal energy supply is made available by MS energy storage. Supported by the Office of Naval Research (ONR), the research presented discusses the considerations for designing molten salt storage tanks. An alternate molten salt storage cylindrical tank design layout is presented, including an improved roof design concept. A preliminary heat transfer analysis is presented and discussed for the alternate cylindrical tank design. This preliminary analysis was used to determine the thickness of insulating material in and around the cylindrical tank to reduce heat flux. These insulating materials include the use of firebrick and ceramic insulation to complement the structural carbon steel and the stainless steel that is used for corrosion resistance. This paper also introduces the alternate designs of a semi-buried spherical tank and drop shell tank that can be used storing molten salts.

The Application of Molten Salt Energy Storage to Advance the Transition from Coal to Green Energy Power Systems

The paper presents technical solutions for a power grid that undergoes the elimination of a significant number of coal-based power generating units. The purpose of the solutions is to adapt the existing machines with sufficient lifespans to the new operating conditions. In particular these include steam turbines. The steam turbines’ cycles may be extended with energy storage systems based on a molten salt. This allows to increase the flexibility of the power generating units while maintaining the largest possible efficiency of the power generation. The solutions presented here allow to connect the steam turbines cycles to renewable energy sources and reduce the overall number of the units that create the fundamental layer of the power grid. The analysis of the solutions involves numerical modeling. The paper describes the assumptions and the results of the modeling for chosen cases of the modernization. The researched considered a number of options that differed in the investment costs and the resulting performance.

Using molten-salt energy storage to decrease the minimum operation load of the coal-fired power plant

As the renewable energy fluctuating in the power grid, the traditional coal-fired power plant needs to operate on the extremely low load, so as to increase the share of renewable energy. This paper deals with thermodynamic simulation and exergy analysis of the coal-fired power plant integrated with the molten-salt energy storage system to explore the potential of reducing the minimum operation load. A steady-state simulation was performed to obtain the thermodynamic properties of process streams in a subcritical 600 MW unit. The results indicated that with simple main steam and re-heat steam energy storage plan, the storage efficiencies are 39.4-42.9% and 51.3-51.4%, the minimum operation load would decrease by 27.2 MW, 10.6 MW, respectively. Exergy analysis shows that the exergy loss mainly comes from the throttling process and temperature difference in the phase-changer heat transfer process. With optimized Molten-salt energy storage plans, the storage efficiencies increase to 72.6% and 78% via lead the exhaust drains or steam into higher pressure points; the minimum operation load decreases by 35.1 MW and 3 MW, respectively. Moreover, with the coupled plan including both optimized main steam and re-heat steam energy storage system, the minimum operation load would decrease by 80.7 MW, and storage efficiency is 75.1%.

Study on Safety Characteristics of Gate Valve Transient Operation in High Temperature Molten Salt Thermal Energy Storage System

At present, the new energy industry in the world is thriving, but there are obvious intermittent and random characteristics of various power generation methods such as solar energy and wind power. The molten salt energy storage power generation system can effectively reduce the peak fluctuations of the power grid and greatly improve the overall economic advantage. As an important component of the pipe system, the gate valve is widely used in power generation systems. Because molten salt systems may face faster power changes so during certain transient process, the main gate valve may have to withstand rapid changes in mass flow and temperature, which may cause rapid changes in the temperature field of the valve body, resulting in additional thermal stress. In this paper, the CFD method is used to analysis the flow and temperature fields of the main gate valve during the transient process. The key to success is to describe the geometry as accurately as possible and to build a reasonable grid because of larger calculation quality for transient. Then based on the temperature field of valve body the thermal stress and change law can be obtained, which can provide guidance for the design of the gate valve.

Techno-economic analysis of using three Fresnel solar fields coupled to a thermal power plant for different cost of natural gas

In this paper, the effect of using three solar fields in a solar thermal power plant has been evaluated. Two separate solar direct steam generations are used to provide a portion of the energy required by the high-pressure turbine and low-pressure turbine. The third solar field is used for molten salt energy storage. If the energy stored during the daytime is not enough, the fossil fuel boiler will provide the rest of the energy needed. Given the current price of natural gas, the use of solar energy is not cost-effective, regardless of environmental issues, government incentives or external costs, such as health costs. Furthermore, the use of solar energy for the power plant was unable to make a significant reduction in electricity cost, regardless the 3.5$/MMBTU natural gas price. However, considering the complexity of the solar system, the use of solar energy for the price of the current natural gas is not recommended. By increasing the price of natural gas, the use of solar energy is more economically feasible.The results of the study show that the use of three solar fields (two separate solar fields to produce superheated steam for a high-pressure turbine and low-pressure turbine and a solar field for energy storage using molten salt as thermal energy storage medium) rather than one solar field reduces electricity consumption by 1.83%–13.78% for the price of 9 $/MMBtu natural gas. Although each of the first and second solar fields produce different energy quantities, and therefore, require different field areas, the field coefficient of the field is the same for both solar fields.

Analysis and Efficiency Assessment of Direct Conversion of Wind Energy Into Heat Using Electromagnetic Induction and Thermal Energy Storage

This study deals with thermodynamic analyses of an integrated wind thermal energy storage (WTES) system. The thermodynamic analyses of the proposed system are performed through energy and exergy approaches, and the energy and exergy efficiencies of the components in the system and overall system are determined and assessed. The magnitudes of irreversibilities are determined, and the impacts of different parameters on the performance of the system are identified. The overall energy and exergy efficiencies of the proposed system and its subsystems are computed as well. The energy and exergy efficiencies of the overall system are defined and obtained as 7.0% and 8.6%, respectively. WTES plants with combined molten salt energy storage application can run continuously, and can provide electrical power for both on-grid and off-grid systems. By converting the wind power into a permanent energy source, the WTES offers a practical solution that can meet the electrical demand of the regions where the climate conditions are feasible for consistent, environmentally benign and cost-effective electric power, and it can be considered as a potential energy solution.

First-Principles Study for Thermodynamic Properties of Solid $$\hbox {KNO}_{2}$$ KNO 2 System

To enable us better understand the performance of molten salt energy storage in a solar thermal power system, thermodynamic properties of the solid $$\hbox {KNO}_{2}$$ system at ambient pressure and temperatures between 0 K and 711 K are determined by first-principles simulation based on density functional perturbation theory calculations with plane waves and pseudopotentials. Thermodynamic parameters of the Debye temperature, specific heat capacity at constant volume, phonon transfer speed, phonon mean free path, and phonon thermal conductivity as a function of temperature are estimated. The results show that the calculated phonon thermal conductivity is in good agreement with experimental values, but the calculated specific heat capacity at constant volume is lower than measured values. The isometric specific heat capacity of $$\hbox {KNO}_{2}$$ is $$75.03\,\hbox {J}{\cdot }\hbox {mol}^{-1}{\cdot }\hbox {K}^{-1}$$ , and the phonon thermal conductivity is $$2.37\,\hbox {W}{\cdot }\hbox {m}^{-1}{\cdot }\hbox {K}^{-1}$$ at ambient temperature.

Thermal design of a solar hydrogen plant with a copper–chlorine cycle and molten salt energy storage

In this paper, the operating temperature ranges of various solar thermal energy technologies are analyzed, with respect to their compatibility with solar hydrogen production via thermochemical cycles. It is found that the maximum temperature of 530 °C required by the oxygen production step in the Cu–Cl cycle can be supplied by current solar thermal technologies. The heat requirements are examined for the Cu–Cl cycle and it is found that the heat source must be sufficiently high and above the maximum temperature requirement of the Cu–Cl cycle, in order to match the heat requirements of the cycle. The quantity of molten salt and solar plant dimensions for capturing and storing solar heat for an industrial hydrogen production scale are also estimated for 24 h operation per day. The flow characteristics and heat losses of molten salt transport in pipelines are studied while considering the influences of pipeline diameter, heat load and weather conditions. The heat loss from a solar salt storage tank is also calculated based on various tank diameters and heights. The intermediate product of molten salt produced in the oxygen production step gives the Cu–Cl cycle a significant advantage of linkage with current high temperature solar thermal technologies. This allows flexibility for integration of the Cu–Cl cycle and solar thermal plant. Using a thermal network analysis of the Cu–Cl cycle, the layout options for the integration of a Cu–Cl cycle with various solar thermal technologies are presented and discussed in this paper.