Molten Salt Thermal Storage System Research Articles

Comparative investigation on the thermodynamic performance of coal-fired power plant integrating with the molten salt thermal storage system

One of the most critical challenges facing China is enhancing the operational flexibility of coal-fired power plants (CFPPs), given the increasing reliance on renewable energy for power generation. This study focuses on analyzing and comparing the molten salt thermal storage systems integrated in CFPP in consideration of peak-shaving capacity, equivalent round-trip efficiency, heat consumption rate, and exergy efficiency. Five charging schemes integrating thermal energy storage (TES), power to heat (P2H) and combination of TES and P2H are proposed and tested via their thermodynamic models. Results show that all five integrated molten salt thermal storage systems can enhance the peak shaving capability of the CFPP. When integrating P2H system, CFPP has the potential to achieve a zero-power output. Furthermore, comparing to the other four schemes, P2H-only integrating scheme has the highest equivalent round-trip efficiency of 36.23 % and the lowest heat consumption rate of 8288.87 kJ·(kW·h)−1. However, its exergy efficiency decreases to 36.51 % due to the electric heater experiencing the highest exergy loss of 132.56 MW, in contrast to the TES-coupled system which shows a higher exergy efficiency of 39.58 %. The P2H-only scheme boasts the largest investment of $33,353,100 and the longest repayment time, while also earning the highest compensation for peak shaving. This study provides a promising approach for molten salt thermal storage system designs in enhancing flexibility for CFPPs.

Thermodynamic analysis of molten salt-based single-tank thermal energy storage system with heat transfer enhanced by gas injection

In this study, the feasibility of gas injection for enhancing heat transfer in a molten salt thermal storage system was experimentally investigated. The convective heat transfer coefficient induced by the gas injection was quantitatively measured. A predictive model was developed to estimate the heat transfer coefficient with respect to the gas injection rate. A comparative analysis between gas-injection and non-injection cases was also performed from a thermodynamic perspective to verify the feasibility of the method. The heat transfer coefficient was within the range of 359–1283 W/m2-K, depending on the gas injection rate. The developed model estimated the measured values within an average absolute error of 2%. From the comparative analysis, the gas injection method improved thermal efficiency from 86.3% to 95.1%. The energy loss induced by the gas injection was within the range of 0.2%–1.1% of the total stored energy. The gas injection also shortened the energy discharge time by 32%–41% owing to the heat transfer enhancement. The proposed method also improved the system performance by enhancing the exergy transfer rate from the salt to the water. These results indicate that a gas injection method is a potential candidate for improving molten salt thermal storage systems.

Thermodynamic analysis and operation strategy optimization of coupled molten salt energy storage system for coal-fired power plant

The growing use of renewable energy requires greater flexibility than existing thermal power units. A steam-extraction system was developed to adjust the power output during a low-load period. The incorporation of molten-salt energy storage enables the decoupling of the boiler from the turbine, thus enabling the regulation of the output power during low-load operation. And the impact of key parameters on the performance of coal-fired units is analyzed to find the suitable operation parameters for the existing coal-fired power plant. The performance parameters of the system are optimized by a multi-objective grey wolf optimization algorithm, which conducts trade-off optimization between roundtrip efficiency and exergy efficiency to identify the Pareto Front for a certain range of extracted steam mass flow rates, hot-tank temperatures, and split ratios. The typical operating conditions of the combined system were analyzed, and a peak capacity of 64.1 MW and a power change factor of 17.33 % were achieved. The discharging process reduced the heat consumption of the unit, while the peak capacity and heat consumption of the combined system were used as objective functions to optimize the charging process. The peak capacity, power enhancement rate, and net profits of the system improved 1.12, 1.42, and 1.27 times after optimization. Heat consumption and efficiency were reduced by 2.61 % and 3.96 %. The molten-salt thermal storage system achieved a synergistic improvement in the operational flexibility and thermal efficiency of the thermal power system.

Performance comparison of three supercritical CO2 solar thermal power systems with compressed fluid and molten salt energy storage

In recent years, the supercritical carbon dioxide (sCO2) Brayton cycle power generation system has gradually attracted the attention of academics as a solar thermal power generation technology. To achieve the stable and effective use of solar energy, three sCO2 solar power generation systems were studied in this paper. These systems included a molten salt thermal storage system, a compressed CO2 energy storage system, and a combined molten salt thermal storage and compressed CO2 energy storage system. The thermodynamic analysis model of the sCO2 power generation system is constructed, and a mathematical model of the solar power tower system is produced. The operating conditions, thermal efficiency, exergy efficiency, and economics of these three systems on typical days are investigated and compared. The results indicate that the power generation system coupled with compressed CO2 storage has higher thermal and exergy efficiencies than the other two systems with the same daily heliostat field. The thermal efficiencies can be as high as 6.56% and 5.91% respectively, and the exergy efficiencies as high as 5.76% and 6.22% respectively. The system coupled with compressed CO2 energy storage is more cost-effective and has a shorter payback period than the system coupled with molten salt thermal storage system and the system with both forms of energy storage.

Dynamic optimization and thermo-economic assessment of a solar parabolic trough combined heat and power system with thermal storage: A case study

The solar-driven combined heat and power system provides an efficient and green approach for the energy demand in remote areas. In this study, a two-tank molten salt thermal storage system is coupled with solar parabolic trough collectors and feeds an organic Rankine unit that produces electricity and heating. Specially, the organic Rankine unit can operate in off-design conditions following the end-user electricity load demand. The configuration of the complete system is parametrically investigated and dynamically optimized aiming to identify the operating conditions for which the net present value is maximized. The optimal arrangement for Lhasa is found to be a 1175-m2 solar parabolic trough field and two 50-m3 molten salt storage tanks. The monthly results show that this optimal case is able to run uninterruptedly the whole month in November and December with a 100% monthly electricity supply reliability rate. Furthermore, the application of this system for typical cities in western China is optimized and compared. The best economical location for the apparatus is in Ejinaqi, with a maximum net present value of 1471 thousand dollars, of which carbon reduction benefit account for 8.8%, and the payback period is only 5.4 years. Moreover, the yearly energy efficiency, exergy efficiency, electricity supply reliability rate and solar abandonment rate for optimal case of Ejinaqi are 55.0%, 20.2%, 93.9% and 25.4%, respectively. The study demonstrates that the proposed apparatus can be a promising option to provide energy for remote areas faced with poorly power grid.

Small modular reactors for green remote mining: A multi-objective optimization from a sustainability perspective

Nuclear energy sources, in particular the small modular reactor (SMR), are being recognized as low-emission clean energy sources. The development of SMRs has focused on minimizing their carbon footprint while ensuring that energy is generated in a safe and cost-competitive manner, which leads to a worldwide growing interest in SMR deployments. In addition, there is a strong demand for renewable energy sources; however, their output can be influenced significantly by weather and location. Therefore, integration of renewable energy sources with SMRs and an energy storage system, which results in a nuclear renewable hybrid energy system (NR-HES), is required to improve power generation reliability. NR-HES represents a promising energy system solution for off-grid communities and industries, especially those involving remote mining sites. In the present study, an off-grid NR-HES to meet electrical and thermal demands is considered for the Victor mine site in northern Ontario, Canada. Off-grid remote mines in Canada primarily rely on diesel generators. The proposed NR-HES is composed of SMR units with a thermal capacity of 15 MWth each, two wind turbines with a capacity of 2.3 MWe each, a two-tank molten salt thermal storage system, a hydrogen storage facility, and peaking diesel generators. To achieve a near-zero-emission mining site, electric vehicles are proposed in the mining site energy demand structure. A transient thermodynamics model is developed to simulate the NR-HES and generate hourly variation of sustainability performance parameters that include power cycle efficiency, SMR capacity factor, round-trip efficiency of the storage systems, thermal storage state of charging, and greenhouse gas emissions. In addition, an economic analysis is performed to determine the additional annualized cost of the system relative to a baseline site. The simulation has a test matrix of 16 runs with various combinations of design input parameters, namely SMR thermal power, storage duration (e.g., daily, seasonally, or monthly), and thermal storage system volume. Grey relational analysis (GRA) is used as a level-based multi-objective optimization technique to determine the nearest optimal design parameter (e.g., decision variable) combination within the test matrix and thus the optimal configuration. In addition, the GRA ranks the design input parameters based on how significantly each parameter can affect the overall performance of the NR-HES. Results revealed that optimal sustainable perspectives have a significant influence on the overall system parameters and performance. For instance, the results showed that there is an opportunity to achieve a reduction in greenhouse gas emissions by 94.8% with a minimal impact to system annualized cost if the cost and environmental effect objectives were prioritized over other sustainability performance parameters.

Using Concentrating Solar Power to Create a Geological Thermal Energy Reservoir for Seasonal Storage and Flexible Power Plant Operation

Abstract We propose a hybrid renewable energy system—a geothermal energy storage system (GeoTES) with solar—to provide low-cost dispatchable power at various timescales from daily, to weekly, to seasonally. GeoTES with solar uses a concentrating solar power collector field to produce hot water that is injected into a sedimentary basin to create a synthetic geothermal resource. The stored geothermal heat can then be dispatched when required by the electrical grid. GeoTES is particularly valuable for a grid with a high penetration of non-flexible renewable technologies such as photovoltaic and wind power. In this work, a sophisticated hybrid model is developed to assess the technical and economic potential of GeoTES by combining IPSEpro, which is a power-cycle simulation tool, and SAM, an economic analysis tool by National Renewable Energy Laboratory (NREL). The analysis shows with proper initial charging period that the heat loss in storage is almost negligible and is a suitable technology for long-term energy storage. Various power-cycle options are evaluated, and the most suitable power cycle is selected for further study. Annual calculations of the GeoTES system indicate that a levelized cost of storage (LCOS) of 12.4 ¢/kWhe can be achieved for seasonal storage of 4000 h; this value is much lower than the existing long-term storage. The LCOS of GeoTES is insensitive to the storage duration above 8 h, unlike battery and molten-salt thermal storage systems. This result demonstrates that GeoTES can be a competitive seasonal storage technology in the future electricity market. The levelized cost of electricity of the GeoTES system is also carefully analyzed and can vary between 10.0 and 16.4 ¢/kWhe, depending on solar-collector prices.

Molten salt composition and composites for improved latent heat thermal energy storage

The growing concern over nonrenewable fuel sources, coupled with the continued increase of global energy demand has incentivised research into numerous new and pre-existing renewable energy sources. Concentrated solar thermal (CST) takes advantage of the high heat capacity of molten salts to provide an alternative solar solution to photovoltaic cells that allows reduced downtime through heat storage for use during suboptimal conditions. This meta-study examines the effectiveness of various eutectic Molten Salt compositions and materials as thermo-physical augmenters, with a focus on improved thermal conductivity when composited with molten salts as a method of enhancing the efficiency of concentrated solar thermal storage technology. The study is based on literature retrieved from scientific databases to investigate information available about enhancing LHTES technology. Research into carbon composites such as Expanded Graphite (EG) exhibits promising results revealing thermal conductivity increases as high as 40% in eutectic salt materials, however inconsistencies in measurements and materials used reveal a need for a greater analysis. Although molten salt thermal storage systems are not optimal in their current state, indication was found that composite storage mediums could potentially solidify their spot as a viable renewable energy source.

Dynamic Multi-configuration Model of a 145 MWe Concentrated Solar Power Plant with the ThermoSysPro Library (Tower Receiver, Molten Salt Storage and Steam Generator)

Abstract Molten salt technology represents nowadays the most cost-effective technology for electricity generation for solar power plant. 0D/1D models are useful to check, validate and improve through simulation the energy performances of existing power plants. They are also used to find the best design that meets required economical criteria and the preparation of the acceptance test for commissioning of the power plant. A dynamic multi-configuration model of a concentrated solar power plant has been developed. The component model is meant to be used for power plant modeling with the ThermoSysPro library developed by EDF and released under open source license. The model is based on momentum and mass/energy conservation equations, as 1-dimensionnal (1D), partial differential equations. The model includes a water/steam cycle (economizer, evaporator, superheater, turbines, aerocondenser, water heaters, pumps, valves, tank, volumes and pipes), molten salt thermal storage system (temperature up to 565 °C with a capacity of 8 hours of electricity production without sunlight) and a cylindrical receiver. The distribution of the heat flux received by the receiver may be calculated by Tonatiuh tool (Ray Tracing tool). The primary aim of this study was to demonstrate the capability of said model to simulate the operating conditions of a solar power plant, and then the capability of the model to check the manufacturers design and targeted performances. In order to be able to answer to many different situations, we created some variables in some of the component model enabling to switch itself on or off. So (turbine stage, water heater, …), the same model can simulate several different plant configurations. This model (Modelica model, translate with Dymola) includes all typical configurations, thus enabling its use through a user-friendly Excel interface. With this interface, comparisons between various architectures according to given criteria and constraints are easy, thus helping the user to define the best design. This interface can be used by a non-specialist in modeling. Several transients are simulated, the objectives are: check the performances and the design (sizing of the components) given by the manufacturers, verify and validate by simulation the scenario of large transients and to reduce the uncertainty of the prediction on the yearly electricity production (simulation with yearly DNI).

The Design and Numerical Study of a 2MWh Molten Salt Thermocline Tank

Abstract The two tank molten salt thermal storage system is widely used in the commercialized solar thermal power plant. However, the thermocline storage system with a low-cost filler material is a more economically feasible option. In this study, a transient two-dimensional and two-temperature model is developed to investigate the heat transfer and fluid dynamics in a molten salt thermocline thermal storage system. After model validation, the effects of inlet flow boundary condition and storage medium properties including fluid and solid materials on the thermal performance of thermocline storage system are investigated. The results show that thermoclne thickness increases slowest with solar salt as heat transfer fluid (HTF) and Cofalit® as solid material in the thermocline tank. Any non-uniformity in the inlet velocity flow would only enhance mixing and widen the thermocline appreciably, which contributes to the loss of thermodynamic availability of stored energy. The thermocline thickness increases with the non-uniformity of the inlet velocity boundary condition. So smaller non-uniformity of inlet flow is better in non-uniform flow though it may causes larger fluctuations in average outlet temperature. Smaller inlet mass flow rate is better for the thermocline storage tank, while it also causes smaller discharging power. With the chosen basic design parameters such as fluid and solid materials, the size of a 2MWh thermocline tank is determined by a simple one-dimensional design method. Tank with larger H/D ratio has higher discharge efficiency. It helps to figure out the thermal stratification mechanism of a storage tank and thereby to determine optimum design and operating conditions.

Object-Oriented Modeling of a Multi-Pass Shell-and-Tube Heat Exchanger and its Application to Performance Evaluation

Molten salt thermal storage systems are the keystone of the dispatchability of demand in solar thermal power plants and many commercial plants use this technology. Nevertheless, there are still open questions related to long-term durability and reliability of components. During experimental campaigns in the CIEMAT-PSA molten salt testing facility, a performance detriment in a multi-pass shell-and-tube heat exchanger was noticed. For this reason and as a tool for analysis and diagnosis, an object-oriented dynamic model is being developed. This paper describes the testing facility and the thermal oil -molten salt heat exchanger particularly, the design of the Modelica model and compares simulation results against manufacturer calculations and experimental data in steady-state operation conditions and at transients in order to evaluate the heat exchanger performance.

Evaluation of Annual Performance of 2-Tank and Thermocline Thermal Storage Systems for Trough Plants

A study was performed to compare the annual performance of 50 MWe Andasol-like trough plants that employ either a 2-tank or a thermocline-type molten-salt thermal storage system. trnsys software was used to create the plant models and to perform the annual simulations. The annual performance of each plant was found to be nearly identical in the base-case comparison. The reason that the thermocline exhibited nearly the same performance is primarily due to the ability of many trough power blocks to operate at a temperature that is significantly below the design point. However, if temperatures close to the design point are required, the performance of the 2-tank plant would be significantly better than the thermocline.