Molten Salt Storage System Research Articles

ACWA Power’s CSP Performance Models

CWA Power develops, owns, and operates several CSP projects, including the largest one in the world. One of the primary objectives for companies in the Concentrated Solar Power (CSP) sector is to lower the electricity costs associated with CSP plants. This goal extends not only to existing operational plants but also to future projects. Enhancing plant performance is a key pathway to achieving this cost reduction. CSP performance models play a pivotal role in this journey by allowing us to analyse different configurations, changes, or modifications without the necessity of their physical implementation. In 2015, ACWA Power made a strategic decision to develop its own CSP performance models. This move aimed to enhance ACWA Power’s competitiveness for upcoming projects while also supporting operational plants by identifying avenues for performance improvement. The decision was made to create models for the two prevailing CSP technologies: Parabolic Trough with thermal oil as the heat transfer fluid (HTF) and molten salt storage system, as well as solar tower with molten salts.

Retrofit of a coal-fired power plant with a rock bed thermal energy storage

Power production accounts for about one-fifth of the global final energy consumption and over one-third of all energy-related CO2 emissions. Low-cost, large-scale thermal energy storages are considered as solutions for the decarbonization of fossil-fired power plants by their conversion into power-to-heat-to-power systems, so-called thermal storage power plants. This paper investigates the retrofit of a Chilean coal-fired power plant with an innovative solid media storage from a techno-economic perspective. Selecting a storage capacity of 5.27 GWhth, corresponding to 8 h of discharge, and increasing the inlet steam generator temperature from 590 to 650 °C lead to the highest annual round-trip efficiency of 34.9% and to up to 3.4% lower levelized cost of electricity. Minimum levelized cost of electricity as low as 88.1 €/MWh is attained and considered as close to competitiveness with well-established molten salt storage systems. A sensitivity analysis shows that assuming even five times lower storage costs, initially the strongest driver of capital expenditure, only meant 4% lower levelized costs of electricity on average. On the other hand, the design of electricity purchase for charging, for example with green power purchase agreements, turns out to be the key lever to a successful implementation.

A new solar and wind integrated sodium fast reactor with molten salt to generate multiple useful outputs with hydrogen

The current study proposes the integration of renewable and nuclear energy systems in a hybridized manner to produce multiple useful outputs with synergetic benefits compared to single-generation systems. The proposed integrated energy system is designed to fully meet the power, heating, cooling, freshwater, and transportation requirements of communities and balance the grid, which consists of an offshore wind farm, a floating photovoltaic plant, a sodium-cooled fast nuclear reactor, a molten salt storage system, a steam-based power generation unit, a lithium-bromide absorption chiller, an anion exchange membrane (AEM) electrolyser, a multi-effect desalination unit and a district energy system. The proposed system is also analyzed from the energy and exergy aspects of thermodynamics. Additional modelling and simulations are carried out with time-dependent analysis in a case study for a residential community, which consists of approximately 275,000 of people in California, the United States. The proposed system with 345 MWe, 1,000 MWth sodium-cooled fast nuclear reactor, 100.8 MWe offshore wind farm and 100 MWp floating PV plant is analyzed with changing parameters throughout the year. Furthermore, the overall system is found to be fully sufficient to meet 1,551.4 GWh of non-thermal electricity load, 820.3 GWh of cooling load, and 2,324.8 GWh of heating load. Moreover, to exploit the excess energy and recover the waste energy, to produce more than the triple amount of the freshwater requirement of the community with 39,575,634 tonnes of fresh water and exceeds the transportation fuel requirement of the community with a total of 28,713 tonnes of hydrogen. The highest monthly average energy efficiency is found for the month of December as 72.54%. On the other hand, the highest monthly average exergy efficiency is found for the month of October as 61.11%. The average energy and exergy efficiencies for the overall system are found 63.36% and 52.42%, respectively.

Comparative study of models for packed bed molten salt storage systems

To date various models for packed bed single tank molten salt storage systems have been developed. The question arises in how far certain modelling assumptions can affect the outcome of these models. This can also depend on the applied use case, for example a short validation simulation versus an annual simulation. Thus, a comparative study is performed, considering four one-dimensional models for packed beds: the Schumann model, the continuous-solid-phase model, the single-phase model and a recently developed bidisperse model. Additionally, the impact of heat losses and variable fluid properties are considered as well. The comparison is performed for different use cases and covers the single blow operation, cyclic operation and annual simulations. Common boundary conditions are derived from a 100 MWel concentrating solar thermal power plant with molten salt as heat transfer fluid. A large and a small storage volume, each with a small and a large particle packing are designed to cover a range of practical storage configurations. The aim of this work is to provide guidelines for the selection of a reasonable level of modelling detail depending on the desired accuracy and computational effort. Results indicate that the outcome can significantly differ between the use cases. Effective conductivity has visible impact in stand-by periods and small sized packings with strong thermal gradients. The single-phase model has low computing effort but is applicable only with packings in the range of millimeters. The bidisperse model has significantly improved accuracy when applied to packings with smaller particles in combination with larger particles. Heat losses showed little impact for the considered storage volumes. Temperature dependent fluid properties have an impact within short periods but average out in cyclic and annual simulations, which justifies neglecting them.

Application of an energy storage system with molten salt to a steam turbine cycle to decrease the minimal acceptable load

The paper presents a solution to the problem of large fluctuations of power demand from a fossil-fueled steam turbine cycle. The fluctuations are due to the presence of the renewable energy sources in the power grid. Steams turbines have a minimal load that limits their flexibility, meaning the range of the possible response to the fluctuation in the grid. The solution presented here aims to lower this limit to increase the flexibility. This is achieved by combining a steam turbine cycle with a molten salt storage system. The combined systems allow to store the excessive power and feed the grid with the power output well below the minimal load. Under these conditions a steam turbine becomes much more flexible and is a better fit for the renewables. The paper describes the modifications of existing power generating units required to integrate them with the storage system. The numerical modelling presented in the paper shows the power generation efficiency of the modified cycles for different loads. It also presents the number of hours that the discharging process takes, when the steam for the turbine is generated from the stored energy only.

Performance analysis of a solar based waste to energy multigeneration system

In this study, a solar-powered waste-to-energy multigeneration system is designed to produce various useful outputs, such as hydrogen, ethanol, heating, cooling and power generation, for a sustainable community is developed and analyzed in addition a comprehensive performance assessment. In this regard, thermodynamic analyses using both energy and exergy approaches are conducted to study and examine the effects of system and operation parameters, such as steam feed rate and gasification temperature on the system performance and investigate possible improvements in the amounts of ethanol and hydrogen produced. The present system utilizes the solar tower as a renewable energy source to produce energy and steam. This steam and animal manure are used as a gasification agent for hydrogen and ethanol generation at the gasification unit. The other objective of this study is to investigate the impact of changing system properties and operating conditions to maximize hydrogen and ethanol production as well as energy and exergy efficiency while minimizing detrimental syngas by conducting several parametric studies in a renewable energy-based multigeneration system. Furthermore, a molten salt storage system is available to minimize energy imbalances originating from renewable solar energy. The gas-steam combined cycle in the rest of the system aims to meet the community’s electricity and heating needs. In the last stage, the cooling required by the community is provided by a vapor absorption cooling system. The overall energy and exergy efficiencies are found as 61.1% and 56.4%, respectively.

Thermal energy storage with phase change materials in solar power plants. Economic analysis

Thermal energy storage (TES) increases concentrating solar power (CSP) plant capacity factors, but more important, improves dispatchability; therefore, reducing the capital cost of TES systems is very important to reduce cost of energy and serve as an enabler for commercial solar power plants. After presenting the concept of a novel cascade PCM configuration of CSP and demonstrating it energy efficiency, it is needed to develop a thoughtful economic evaluation of the concept. The goal of this paper was to investigate this system through annual modelling, engineering procurement company price quotes, and levelized cost metric comparison with a baseline case, the commercial two-tank molten salt storage system. Simulation results show that this new PCM concept decrease levelized costs of storage. The baseline of using shell and tube heat exchangers for PCM storage offers a reduction in capital and levelized cost, but this paper also shows that there are further cost reduction possibilities, as shown in the scenarios presented here, such as thermal conductivity enhancement, encapsulation of the PCM, or modification of the material to improve its thermal properties. Therefore, although the basic shell and tube PCM system has been shown to offer a cost savings, the 1.8% reduction in LCOE may not be worth the risk of further development. The shell and tube system should be a jumping off point to more appropriate technologies. The variants studied are likely to yield cost reductions in cost performance quotient (CPQ) as high at 80% and in LCOE as high as 10%, representing an unprecedented and valuable avenue of CSP plant cost reduction.

Development of an integrated system with desalination and heat storage options

In this study, a novel renewable energy-based multigenerational system integrated with desalination and molten salt storage subsystems is developed, thermodynamically analyzed and holistically assessed through energy and exergy efficiencies. The solar power system is integrated with a geothermal energy plant, wind turbine, proton exchange membrane (PEM) and methanol synthesis with CO2hydrogenation reactor. Also, a multi-effect desalination unit is integrated into the system to produce fresh water. During the absence of solar irradiation, a molten salt storage system is employed to provide the required energy to operate the system. The City of Vancouver in Canada is considered for a case study of the proposed integrated system. The proposed system is designed to utilize both geothermal and solar sources for residential applications, including communities with multiple buildings, by producing useful necessities, such as electricity, heat, hot water, freshwater and methanol. The produced methanol as a clean energy carrier is used for energy storage to balance demand and supply in the absence or insufficiency of other renewables, such as solar and wind, besides molten salt heat storage in the integrated system. Designing renewable-based multigenerational system that plays an important role in reducing environmental impacts but increasing efficiency composes the main motivation of this study. The system thermodynamic performance analysis is achieved through energy and exergy approaches. The overall system energy and exergy efficiencies at 3.1 m/s wind speed and 100.000 W/m2 are found as 25.89% and 28.66%, respectively. It is furthermore aimed to design a novel integrated system which appears to be superior to a conventional single-source based system.

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.

An experimental investigation of temperature distribution and heat loss in molten salt tanks in concentrating solar power plants

Two-tank molten salt storage systems, which increase the dispatchability of power, are widely utilized in commercial concentrating solar power plants. In the present work, a pilot solar thermal power plant was created to investigate temperature distribution and heat loss in the molten salt tank. The results showed that the molten salt had no temperature stratification at different liquid levels and operating temperature conditions except at the unprotected position near the electric heater. However, there was an obvious temperature difference in the upper atmosphere: the temperature dropped by 5.5 °C to 6.5 °C with every 400 mm increase in the height. During the cooling process, the temperature gradient in the tank foundation was basically stable. Experiments indicated that the liquid level of the salt affected the cooling rate of the molten salt significantly, while it had a little effect on the total heat loss in the tank. As the liquid level decreased from 2040 to 1039 mm, the temperature difference of the molten salt in the cooling process increased from 22.1 °C to 41.5 °C, but the total heat loss only changed by 4.3%. However, the molten salt operating temperature had a great influence on both the temperature change and the heat loss in the molten salt tank. When the operating temperature decreased from 557.7 °C to 376.9 °C, the temperature difference of molten salt in the cooling process decreased from 37.4 °C to 19.7 °C and the total heat loss in the tank decreased by 48.2%.

Design parameter modelling of solar power tower system using adaptive neuro-fuzzy inference system optimized with a combination of genetic algorithm and teaching learning-based optimization algorithm

Abstract Determining the optimal sizing of a solar power tower system (SPTS) with a thermal energy storage system is subject to finding the optimum values of design parameters including the solar multiple (SM), design direct normal irradiance (DNI) and thermal storage hours. These design parameters are determined for each station separately and have remarkable effects on the thermo-economic performance of the system. This paper aims to demonstrate how artificial intelligence (AI) techniques may play an important role in addressing the above-mentioned need and help determine the optimum design parameters for different stations. For this purpose, we developed a thermo-economic model of a 100 MW SPTS with a molten salt storage system for five stations (two stations in India, and one each in Bangladesh, Pakistan, and Afghanistan). A method-based AI is utilized in this paper to ascertain the design parameters of the system. Additionally, a novel hybrid method based on adaptive neuro-fuzzy inference system optimized with a combination of genetic algorithm and teaching-learning-based optimization algorithm (ANFIS-GATLBO) is employed. The input parameters are latitude, longitude, design point DNI and SM, while the annual energy produced, levelized cost of energy and capacity factor are the target variables. The results of the study show that although the annual energy produced by SPTS rises by increasing the SM and decreasing design point DNI, optimum design parameters should be determined by the economic factors. In addition, it was found that the ANFIS-GATLBO method used in this study successfully predicted the targets with a correlation coefficient close to 1.

Analysis of Optical Efficiency in Heliostat Field Layout with Various Separation Distance

The main aim of this paper is to design the heliostat field layout of solar thermal generation for a concentrated solar tower powerplant, based on the central power tower technology. In this design, the radial staggered pattern is proposed to reduce shadowingand blocking losses. The radial stagger arrangement is the most efficient for a given land area as it cuts down the land usage andatmospheric attenuation losses. By using this pattern, the heliostat can be placed with higher optical efficiency as well as a faster speed due to the fact that each heliostat could move freely and the heliostats standing between two heliostats of the first ring reduce the shadowing and blocking. The field layout consists of 650 heliostats around a 62m tall tower and each heliostat has a 121m2 reflecting area. The power plant is designed to produce an output of 30MWth thermal power with the molten salt storagesystem. The optical efficiency is calculated by the product of the mirror reflectivity, the atmospheric attenuation efficiency, theshadowing and blocking efficiency and the cosine efficiency. The available separation distance of attenuation efficiency is evaluated the optical efficiency. The separation distance of the attenuation efficiency is less; the optical efficiency of the field layout is more. The separation distance of the attenuation efficiency is increased; the heliostat field area of the layout is also increased. By varying the separation distance between the heliostats is achieved the optical efficiency and the heliostat field area for under different conditions. The input data are assigned in MATLAB for testing this proposed method.

Life Cycle Assessment of Air-Rock Packed Bed Storage System and Its Comparison with Other Available Storage Technologies for Concentrating Solar Power Plants

Recently, packed bed of rocks using air as heat transfer fluid has been considered a promising alternative solution for thermal energy storage (TES) able to work in high temperature concentrating solar plants. In fact, the large amounts of mine by-product could be major source of cost-effective and eco-friendly filler materials. The present research work aims to quantify the environmental impact of air-basalt packed bed storage system. A comparative life cycle analyses were performed to give factual criteria to decision-makers for proper choices of TES materials. The embodied energy, GHG emissions and water consumption of the storage system during its whole life cycle have been presented. The following work is in accordance with the international standard series ISO 14040. By comparing with other TES technologies, “two tanks molten salts”, “thermocline/molten salts with recycled ceramic”, “thermocline/molten salts with Mullite”, the “thermocline/air with basalt” showed a significant reduction in environmental impacts. These were reduced by 65% for primary energy demand, 60% for potential climate change and 80% for water consumption compared to the conventional two tanks molten salt storage system. The energy and global warming payback times of the proposed storage technology were very low (about 2 months) for a more than 25 years of expected lifetime.

An innovative small-scale prototype plant integrating a solar dish concentrator with a molten salt storage system

Abstract An innovative plant configuration joining a thermal energy storage device with a 2 kWTh parabolic solar dish collector (PDC) has been designed. The novelty of this small-scale solar plant prototype consists in the use of molten salts (MS) both as fluid carrier and thermal energy storage medium. An eutectic mixture of NaNO3, NaNO2 and KNO3 has been used allowing a 10 kWh energy storage accomplished with two 50-L tanks. An experimental campaign has been planned to test the behavior of each of the assembled components and to provide a preliminary analysis of the plant performances. As first evaluation, the energy efficiency of the system has been calculated in correspondence to selected MS flow rates referred to steady state conditions. This has been achieved by monitoring the MS temperature in correspondence to suitable points of the inlet and outlet sections of the receiver. The preliminary confirmations of this investigation looks promising to integrate the PDC technology within district heating systems for advanced renewable energy exploitations.

A direct steam generation concentrated solar power plant with a decalin/naphthalene thermochemical storage system

This study presents the design and analysis of a new integrated direct steam generation (DSG) concentrated solar power (CSP) plant with a decalin/naphthalene thermochemical storage system. Model simulations were performed in accordance with historical hourly solar radiation data over a year, using a combination of Aspen Plus v10, MATLAB 2016b, and Microsoft Excel VBA. It was found that the proposed plant feasibly stored and discharged energy, based on the solar radiation and chemical storage availability, to maintain base-load power productions (250MWe or 120MWe) with an overall efficiency of 14.6%. The effectiveness of the designed storage system was found to be comparable to a molten salt storage system which is currently used in existing CSP plants. The proposed integrated DSG CSP plant with a decalin/naphthalene thermochemical storage system shows promise for being an alternative to existing CSP plants.

Thermomechanical testing under operating conditions of A516Gr70 used for CSP storage tanks

Thermal energy storage (TES) in molten salts is the storage dominating technology in solar power applications today. In two-tank molten salt storage systems energy density ranges from 30 to 70kWh/m3 are achievable. The salt material used is a binary system, composed of 60% NaNO3 and 40% KNO3. In the 8 MWhth pilot plant built and tested by Abengoa, the storage tanks were made of steel A516Gr.70 using the Appendix M of code API 650 for their design. A specific testing device was developed to evaluate thermo-mechanical properties, and a study was conducted in order to evaluate tensile properties of A516Gr.70 specimens under operation conditions for the hot tank at the pilot plant that is in contact with molten salts at 380°C. Results confirmed the outcomes of the work: the reduction of the yield limit, elongation before fracture, and Young modulus at 380°C after having been 5min immersed in molten salts. Moreover, after a creep-test simulating operating 7 days conditions during, an additional reduction of the yield limit was measured.

Design of a 25 MWe Solar Thermal Power Plant in Iran with Using Parabolic Trough Collectors and a Two-Tank Molten Salt Storage System

Nowadays, parabolic trough solar thermal plants are prevalent around the world. In different areas concerning the amount of solar radiation, their standard size is approximately between 20 and 100 MWe. Certainly, the right size of the solar field is the first selection with regard to nominal electrical power. A vast area will be economically unreasonable whereas a small area will mainly cause the power plant to operate at the part-load condition. This paper presents an economic modeling of a solar parabolic trough plant, operating at 25 MWe in Yazd, Iran. The varying types of collector dimensions have been investigated; then, by selecting autumnal equinox (22 September) at 12:00 PM as the design point, thermal performance of the solar power plant has been featured annually, in all conditions. The total operating time of the power plant is about 1726 hours (1248 hours in full-load condition). In the end, the effect of thermal storage tanks has been analyzed to save extra solar heat and use it at nights in hot months. By implementing a storage system, the total operating time will be increased to 3169 hours (2785 in full-load condition). Moreover, 7974 GJ useful thermal energy can be obtained from the solar field and storage system.

Increasing fossil power plant flexibility by integrating molten-salt thermal storage

Increasing market penetration of renewable energy sources requires measures to stabilize the electric grid. This includes reducing generator output fluctuations as well as providing control reserve. The present study investigates the use of molten-salt storage systems in fossil-fired power plants by conducting a series of numerical process simulations. Two case studies are performed: First, an incinerator representing a heat-controlled facility with fluctuating output due to varying customer steam demand. Second, a small-size lignite-fired power plant for providing control reserve, representing a power-controlled facility.The results show that the concept allows significant attenuation of fluctuations in the heat-controlled facility. For a broad range of customer steam extraction, a steady production of electrical energy could be obtained without substantial modifications to the plant. Positive and negative control reserve in the power-controlled facility can be provided through the use of a storage system with relatively low exergetic losses. However, a number of additional heat exchangers and significant modifications to the steam cycle are required.

Techno-economic heat transfer optimization of large scale latent heat energy storage systems in solar thermal power plants

Concentrated solar power plants with integrated storage systems are key technologies for sustainable energy supply systems and reduced anthropogenic CO2-emissions. Developing technologies include direct steam generation in parabolic trough systems, which offer benefits due to higher steam temperatures and, thus, higher electrical efficiencies. However, no large scale energy storage technology is available yet. A promising option is a combined system consisting of a state-of-the art sensible molten salt storage system and a high temperature latent heat thermal energy storage system (LHTESS). This paper discusses the systematic development and optimization of heat transfer structures in LHTESS from a technological and economic point of view. Two evaluation parameters are developed in order to minimize the specific investment costs. First, the specific product costs determine the optimum equipment of the latent heat storage module, i.e. the finned tube. The second parameter reflects the interacting behavior of the LHTESS and the steam turbine during discharge. This behavior is described with a simplified power block model that couples both components.