New progress in the next generation of solar thermal power generation and heat storage technology

New progress in the next generation of solar thermal power generation and heat storage technology

Solar thermal power generation technology combined with thermal energy storage is one of the most promising power generation technologies in future renewable energy systems. It can efficiently use resource-rich but intermittent solar energy to provide people with stable, dispatchable and low-cost electricity. In order to further reduce the levelized power generation cost of existing commercial CSP stations, researchers are actively conducting research on a new generation of solar CSP technology with higher operating temperature and power generation efficiency. Molten chloride salt (such as MgCl 2 /NaCl/KCl) has become the most developed in the next generation of molten salt technology due to its excellent thermal properties (such as viscosity, thermal conductivity) , higher thermal stability and lower material cost One of the prospective heat storage/conducting materials.

The Journal of the Chinese Academy of Engineering “Engineering” published “Research and Development Progress of Molten Chloride Salt Technology in Next Generation Solar Thermal Power Station” in the third issue of 2021, introducing the development of the next generation of solar thermal power generation technology and its heat storage technology and future developments. Direction of development. The article focuses on the advanced heat storage technology based on molten chloride salt (such as MgCl 2 /NaCl/KCl mixed salt) , reviews the selection and optimization of mixed chloride salt in the molten chloride salt heat storage technology, and the determination of heat storage related physical properties, and The latest research progress in molten salt corrosion control of structural materials (such as alloys) used in the system .

I. Introduction

Concentrated solar power (CSP) technology with thermal energy storage (TES, hereinafter referred to as heat storage) is one of the most promising power generation technologies in future renewable energy systems . It can efficiently utilize abundant resources but with intermittent Sexual solar energy provides people with stable, dispatchable and low-cost electricity.

According to the research report of REN21 (Renewable Energy Policy Network for the 21st Century), an internationally renowned renewable energy policy research organization , in 2018, more than 550 MW of newly built CSP power plants worldwide started commercial operation, and most of them were equipped with molten salt heat storage. System; From 2008 to 2018, the global installed capacity of CSP increased rapidly from 0.5 GW to 5.5 GW. IEA (International Energy Agency, IEA) under the global organization SolarPACES (Solar Power and Chemical Energy Systems) is committed to promoting international cooperation, promote the development of CSP technology and industry, statistics and published its official website in the world running, construction Or all CSP power plant projects under development ( .

According to statistics, the operating CSP power stations (approximately 5.8 GW installed capacity) in 2019 are mainly distributed in countries and regions such as Spain, the United States, the Kingdom of Morocco and the Republic of South Africa, while the CSP power stations under construction (approximately 2.2 GW) are mainly distributed in the Middle East and North Africa (MENA) and China. In addition, countries and regions such as Europe, the Republic of Chile, the Republic of South Africa, and Australia are also designing and constructing CSP power stations with an installed capacity of more than 1.5 GW.

As shown in Figure 1, CSP technology is mainly divided into four types : Fresnel type, tower type, butterfly type and trough type according to different concentrating methods . Among them, Fresnel type and trough type CSP are linear focusing systems, while tower type and butterfly type are point focusing systems. Compared with the linear focus system, the point focus CSP system has a higher concentration rate, so it can generate higher temperature solar heat and achieve higher thermoelectric conversion efficiency and lower power costs. Most CSP power stations currently in operation use mature trough technology with low construction and low maintenance costs, while most CSP power stations under construction are based on more advanced tower technology.

New progress in the next generation of solar thermal power generation and heat storage technology

Figure 1 The main classification of CSP technology (from left to right): Fresnel, tower, butterfly and trough

The first generation of CSP power stations , such as the solar electric generating system I (SEGS-I) in the United States , did not have an integrated heat storage system and could not generate dispatchable electricity according to electricity demand. In order to improve the competitiveness of traditional power stations and other renewable energy power stations, the second-generation CSP power station integrates low temperature (heat storage temperature 293~393 ℃; such as the Andasol No. 1 trough power station in Spain) and high temperature (heat storage temperature 290). ~565 ℃; such as Gemasolar in Spain and Crescent Dunes tower power station in the United States) molten nitrate heat storage system to achieve dispatchable power supply and greatly reduce the levelized power generation cost (LCOE) of the power station . Compared with a trough-type CSP power station with a maximum operating temperature of about 400 ℃, a tower-type CSP power station with a maximum operating temperature of 565 ℃ has higher power cycle thermoelectric conversion efficiency and can achieve lower power generation costs.

TES technology is mainly divided into sensible heat storage technology based on liquid or solid materials, latent heat storage technology based on phase change materials (PCM) , and thermochemical heat storage technology based on reversible chemical reaction materials.

The mainstream molten nitrate heat storage technology currently used commercially is a sensible heat technology. The review paper gave a comprehensive and in-depth introduction to the various heat storage technologies used in CSP that have been commercialized or researched and developed. Due to space limitations, this article will not discuss it again.

Figure 2 shows the most advanced and representative second-generation CSP power station, which is a commercial tower power station equipped with a direct TES system for molten nitrate . This power station is mainly composed of 4 parts: heliostat, absorption tower, molten salt heat storage system and power cycle power generation system . During the operation of the power station, sunlight is reflected by the heliostats to the receiver on the top of the absorption tower, and the light energy is converted into heat through the receiver, which is stored in the heat storage material flowing through the absorber (ie molten salt from the cold tank) in. The heated molten salt is stored in a high-temperature molten salt tank. When useful electricity is required, the stored thermal energy is transferred to the conventional steam Rankine power cycle through the molten salt heat exchanger for power generation. The molten salt heat storage system can realize low-cost solar thermal storage, enabling the CSP power station to stably supply dispatchable low-cost electricity even in the absence of sunlight.

A common commercial molten salt heat storage material is a non-eutectic molten salt mixed salt mixed with NaNO 3 /KNO (mass fraction 60%/40%) , usually called ” Solar Salt ” (Solar Salt). ) . Figure 3 shows the double-tank molten salt heat storage system in the 50 MW Andasol No. 3 CSP power station in Spain. It uses about 28 500 tons of solar salt, and the stored heat can be used to generate power at full load for about 7.5 hours.

New progress in the next generation of solar thermal power generation and heat storage technology

Figure 2 The most technologically advanced second-generation molten salt tower CSP power station. The molten nitrate in its direct heat storage system can be used as a TES/Heat Transfer Fluid (HTF) material at the same time

New progress in the next generation of solar thermal power generation and heat storage technology

Figure 3 The double-tank molten salt heat storage system in Spain’s 50 MW Andasol No. 3 CSP power station, which stores about 28 500 tons of solar salt, and the stored heat can be used to generate power at full load for about 7.5 hours (picture source: Andasol 3 CSP power station)

The maximum heat storage capacity (Q) of a molten salt heat storage system can be calculated from the temperature difference (∆T) of the hot and cold tanks, the total mass of molten salt in the system (m) and its specific heat capacity (c p ) :

New progress in the next generation of solar thermal power generation and heat storage technology

Molten nitrate used in the second generation of CSP plants, due to thermal decomposition of the problem, which are limited in the maximum operating temperature of about 565 deg.] C, which limits the temperature of the reservoir difference [Delta] T and the heat storage capacity of the thermal storage system Q . Some review papers, such as introducing the latest developments in the research and development of molten nitrate heat storage technology, are limited in space and will not be discussed in this article.

In 2017, the National Renewable Energy Laboratory (NREL) and other American scientific research institutions proposed a next-generation CSP technology with a higher operating temperature (> 700 ℃) and power generation efficiency (the third-generation CSP technology). , Gen3 CSP) development and demonstration roadmap.

Beginning in 2012, the Australian Renewable Energy Agency (Australian Renewable Energy Agency, ARENA) in “Australia photothermal Research Program” (Australian Solar Thermal Research, Initiative, ASTRI) has funded the development of advanced technology within the framework of the CSP. Section 2 of this article will introduce the world’s major research plans and projects on next-generation CSP technology. In these research plans and projects, researchers have made great efforts in the research and development of next-generation CSP and heat storage technology and have made gratifying progress.

Compared with the current commercial molten nitrate technology, the next generation of heat storage technology should have a higher operating temperature and lower capital expenditure (CAPEX) . The main technology currently studied includes inorganic salts based on higher thermal stability. (e.g., chloride and carbonate based) of the next generation technology molten salt , based on the inorganic phase change material (PCM) heat storage techniques and solids techniques (e.g., sintered bauxite particles) . Among these heat storage technologies, the next generation molten salt technology is the most familiar technology, and it is also considered to be one of the most promising heat storage technologies in the next generation of CSP power plants. The next-generation molten salt technology can retain the main design of the current commercial molten salt heat storage tower CSP power station (Figure 2) , which can greatly reduce the risk of R&D and commercialization of the next-generation CSP technology.

Figure 4 is a conceptual diagram of the next-generation CSP technology based on a new molten salt heat storage material proposed by NREL . In the molten salt heat storage next CSP plants, the molten salt heat storage / heat transmitting system (operating temperature of 520 ~ 720 ℃) with supercritical carbon dioxide (SCO 2 ) Brayton power cycle (operating temperature of 500 ~ 700 ℃) combined . Compared with the traditional steam power cycle with a thermal power conversion efficiency of about 40%, the sCO 2 Brayton power cycle has a thermal power conversion efficiency of more than 50% and lower capital expenditure. It is used in the next generation of CSP power plants and other thermal power plants (such as nuclear power plants). ) Has huge application potential. In this article, the sCO 2 power cycle will not be discussed in depth . Interested readers are advised to read a recently published review paper [13] , which focuses on the research and development status and progress of the sCO 2 power cycle used in CSP .

New progress in the next generation of solar thermal power generation and heat storage technology

 Figure 4 Schematic diagram of the next generation molten salt heat storage CSP technology concept-the next generation molten salt heat storage/heat transfer system combined with supercritical carbon dioxide (sCO 2 ) Brayton power cycle. 1000 suns: The concentration of light equivalent to 1000 suns realized by the tower CSP technology on the surface of the absorber

Molten chloride salt (such as MgCl 2 /NaCl/KCl) is one of the most promising heat storage/heat conduction materials in the next generation of molten salt technology . The reason is that it has excellent thermal properties (such as viscosity, thermal conductivity) , relatively High thermal stability (> 800 ℃) and low material cost (< 0.35 USD∙kg –1 ) . In addition, the current development experience of commercial molten nitrate technology can also be used to develop this new type of molten salt technology, which greatly reduces the risk and cost of technology research and development. However, compared with commercial molten nitrate, molten chloride salt is highly corrosive to metal structural materials (ie alloys) at high temperatures , which is one of the most important technical challenges faced in research and development. Therefore, it is very important to find an efficient and low-cost corrosion control technology.

In Section 2 of this article, the author reviews the latest developments in the next generation of CSP technology and its high-temperature heat storage/heat transfer technology; then, in Section 3, it focuses on the latest research developments in molten chloride salt technology, including the development of chloride salts. Selection/optimization, measurement of molten chloride salt performance, and research on corrosion control of structural materials (such as alloys) in molten chloride salt ; in Section 4, the main features of next-generation CSP and high-temperature heat storage/thermal conduction technologies are summarized Research and development progress, and in view of the main technical challenges and problems faced, some suggestions for follow-up research and technical tackling are put forward.

2. A new generation of CSP technology

In order to develop the next generation of CSP technology with higher power generation efficiency and lower power generation cost, in the past 10 years, countries and regions including the United States, Australia, Europe and Asia have proposed different R&D programs or initiated related Research and development projects.

For example, within the framework of the “SunShot Initiative” launched in 2011, the U.S. Department of Energy (DOE) began funding research topics related to the Gen3 CSP research program in 2018.

In Australia, ARENA launched the ASTRI research program in 2012 with the purpose of improving current commercial CSP technology and developing a new generation of CSP technology.

Since 2004, the European Union (EU) has funded a number of EU projects including the next-generation CSP technology through the “Sixth Framework” (FP6) , “Seventh Framework” (FP7) and “Horizon 2020” (H2020) . CSP research and development project. In addition to research and development projects, the EU has also funded projects such as ” Solar Facilities for the European Research Area ” (Solar Facilities for the European Research Area, SFERA) Phase I‒III and “European Concentrated Solar Thermal Utilization Technology Technology Alliance” (STAGE-STE) . , In order to promote the joint collaboration of scientific research institutions in the European Union and promote the development of CSP technology.

China and other countries have also carried out some preliminary research on the next-generation CSP technology. For example, some scientific research institutions in China began to study the next-generation molten salt heat storage technology using molten chloride and carbonate in 2011. In 2020, the Ministry of Science and Technology of China (MOST) also launched the “Supercritical CO 2 Solar Thermal Power Generation” research project through the National Key Research and Development Program .

The following sections will introduce the latest developments in the next generation of CSP technology in the United States, Australia, Europe and Asia, as well as the efforts of the International Renewable Energy Agency (IRENA) to support the development of CSP technology. At present, the highest heat storage/heat transfer temperature of the tower-type CSP power station based on commercial nitrate molten salt technology reaches 565 ℃. The following will discuss how to use the new generation of CSP technology and the new heat storage/heat transfer technology to achieve a higher operating temperature. Improve CSP power generation efficiency and reduce power generation costs.

(1) United States

In 2011, the U.S. Department of Energy launched the 10-year “SunShot Initiative” to provide a large amount of funding to support the research and development of solar energy technologies (ie, solar thermal and photovoltaic technologies) to reduce the cost of solar power generation and make it compatible with conventional power stations and other renewable sources. Compared with energy technology, it is also cost-competitive.

As shown in Figure 5, in 2017, the U.S. Department of Energy announced that it had successfully reduced the LCOE of the base load CSP with energy storage for more than 12 hours to 0.10 USD∙ kW –1 ∙h –1 , which is the same as the 2010 without energy storage function. Compared with CSP, it is reduced by more than 50%. In its follow-up “SunShot Initiative 2030”, the LCOE target of base load CSP is to drop to 0.05 USD∙kW –1 ∙h –1 by 2030 . Such a low LCOE will make CSP power plants more cost-competitive than most conventional power plants based on fossil fuels. In addition, for the peak energy supply CSP with energy storage less than 6 hours, the target of “SunShot Initiative 2030” is 0.10 USD∙kW –1 ∙h –1 .

New progress in the next generation of solar thermal power generation and heat storage technology

Figure 5 CSP technology progress and 2030 goals in the “SunShot Initiative” funded by DOE

In order to achieve the LCOE goals in the “SunShot Initiative 2030”, the U.S. Department of Energy began to provide approximately $72 million in R&D funding for the Gen3 CSP program in 2018. Leading research institutions in the US energy field, such as Sandia National Laboratory (SNL) , NREL, Oak Ridge National Laboratory (ORNL) , Savannah River National Laboratory (SRNL) , Idaho National Laboratory (INL) , Massachusetts Institute of Technology (MIT) , Brayton Energy, Hayward Tyler, Mohawk Innovative Technology and other energy companies have participated in the research project and received project funding. The funded research project is dedicated to reducing the development risk of next-generation CSP technology, and the goal is to make the maximum operating temperature of CSP higher than 700 ℃ through advanced heat storage/heat transfer systems and power cycles. The Gen3 CSP plan has determined to fund the research of the following three development routes.

(1) Molten salt heat sink route: In this route, researchers aim to overcome the main technical problems faced, such as the corrosion of structural materials in contact with molten chloride or carbonate up to 750 ℃. After preliminary research, the chloride salt has been selected for further research and development.

(2) Solid particle heat absorber route: This route stores high temperature (up to 1000 ℃) heat energy in a cheap medium (such as sand-like solid particles) to reduce heat storage costs. Researchers aim to overcome technical problems such as the long-term stability of particles and the development of efficient and low-cost particle receivers.

(3) Gas heat sink route: This route will use cheap gas (such as helium) as a heat transfer medium to transfer heat and generate electricity, and store the heat in heat storage materials such as PCM. The main challenges to be solved in this technical route include the development of a tower receiver that can work stably for a long time under high temperature and high pressure.

In the “SunShot Initiative”, solid particle heat storage/heat transfer technology, sCO 2 Brayton power cycle technology and molten chloride salt technology have made good progress. These technologies have broad application prospects in the next generation of CSP technology and other related energy technologies. In order to test the key components and the entire system process under more realistic conditions, researchers have built (or are building) several test devices, including the third-generation particle test device (Gen 3 Particle Pilot Plant, which uses solid particle heat storage technology) . G3P3) , Supercritical Transformational Electric Power ( STE) that applies sCO 2 Brayton power cycle technology , and Facility to Alleviate Salt Technology Risks (FASTR ) that applies high-temperature molten chloride salt technology to reduce technology research and development risks. ) .

Figure 6 shows the G3P3 device under construction, which can test key components in the system under real conditions, such as high-temperature particle heat sinks. In addition to the design and construction of system test devices, the research and development of materials and components has also made progress. For example, in the aspect of pellet technology, we designed and tested new pellet heat sinks and pellet-sCO 2 heat exchangers; in the next generation of molten salt technology, we collected or measured molten salt engineering data and studied molten chlorine at high temperatures. Corrosion of structural materials by salt and its control. In Section 3, we will introduce the basic research and technological development progress of molten chloride salt technology in more detail.

New progress in the next generation of solar thermal power generation and heat storage technology

Figure 6 G3P3 large-scale test device used to test solid particle heat storage technology under real conditions. 1 ft = 0.3048 m.

 (2) Australia

As one of the countries with the best solar energy resources in the world, Australia has invested a lot of money and energy to develop cost-competitive solar technology in recent years. For example, ARENA launched the 8-year ASTRI in 2012 to promote the replacement and development of CSP technology. Australia’s major solar energy research institutions include Commonwealth Scientific and Industrial Research Organization (CSIRO) , Australian National University (ANU) , University of Queensland (QU) , Queensland University of Technology ( Queensland University of Technology, QUT), etc., as well as start-up companies such as Vast Solar have participated in scientific research projects within the ASTRI framework. Researchers have conducted feasibility studies on the early development of CSP technology, and have developed some demonstration power plants in pilot and commercial environments.

In order to promote the development of next-generation CSP technology, Australia has cooperated with ASTRI and the American Gen3 CSP program introduced earlier. According to ARENA’s CSP development roadmap, Australia’s next-generation CSP technology research and development focuses on the liquid metal route, that is, liquid sodium is used as the heat transfer medium, and different types of materials such as PCM are used as heat storage materials. Compared with molten salt, liquid sodium is a medium with higher thermal conductivity, while PCM is a heat storage material with higher heat storage density.

Research projects within the ASTRI framework have also made good progress , such as liquid metal sodium heat conduction technology, sCO 2 Brayton power cycle technology and new heat storage technologies, including the use of inorganic base PCM and sensible heat storage materials (such as solid particles and Molten chloride salt) , and the corrosion research of alloys in molten salt or inorganic base PCM. As shown in Figure 7, with the funding of ARENA, the Vast Solar CSP test station in New South Wales (with 6 MW of heat storage and 1 MW of power storage) began construction in 2014. According to reports, in 2019, researchers successfully tested liquid sodium metal as a heat transfer medium, and its maximum operating temperature could be higher than 800 ℃.

In addition, researchers have also done a lot of work to test and determine the higher operating temperature PCM heat storage materials suitable for the next generation of CSP technology. The tested PCM materials include NaCl-Na 2 CO 3 and Li 2 CO 3 -K 2 CO 3- Na 2 CO 3 and other mixed salts. Compared with commercial molten nitrate, inorganic base PCM has lower price, higher thermal stability and heat storage density. But at the same time, these inorganic salt mixed salts have low thermal conductivity, limited heat transfer, and severely corrode alloy materials at high phase transition temperatures. Corrosion is usually a key issue affecting the service life of structural materials. Therefore, in order to realize the commercial application of these PCM materials, researchers are studying how to effectively and economically reduce corrosion.

New progress in the next generation of solar thermal power generation and heat storage technology

Figure 7 The Vast Solar CSP test power station in New South Wales, Australia, uses liquid sodium metal as the heat transfer medium. The test power station is designed to store electricity and heat at 1 MW and 6 MW, respectively.

(3) Europe

Europe has a long history in the development of CSP technology and has achieved many results. According to statistics in 2019, Spain is the country with the largest installed CSP capacity in the world (> 2.3 GW) . Since 2004, the EU has supported technology research and development including next-generation CSP technology through the FP7 and H2020 plans. Some CSP research institutions in Europe, such as Spanish Research Center for Energy, Environment and Technology (CIEMAT) , German Aerospace Center (DLR) , and Paul Scherrer Institute of Switzerland (Paul Scherrer) Institute, PSI) , Swiss Federal Institute of Technology in Zurich (ETH Zurich) , Italian National Agency for New Technologies, Energy and Sustainable Economic Development, ENEA ) , French National Center for Scientific Research (CNRS) and other institutions have participated in these plans. More complete information about participating institutions can be found in the SFERA I–III project and the STAGE-STE project.

European CSP research infrastructure, strategies, funding plans and roadmaps are mainly composed of European Association for Storage of Energy (EASE) , European Energy Research Alliance (EERA) , European Electricity Grid Plan Initiative, EEGI) , European Solar Research Infrastructure for Concentrated Solar Power (EU-SOLARIS) , European Research Area Network (ERA-Net), and others European and national associations, such as the German Association for Concentrated Solar Power (DCSP) management.

Compared with the Gen3 CSP plan in the United States, European R&D has adopted a broader development route, which also involves the technology studied in Gen3 CSP . For example, researchers have established the main corrosion mechanism of commercial Fe-Cr-Ni alloys in molten chloride salts, and determined that some corrosion inhibition methods can show good corrosion control effects in laboratory tests. At the same time, some CSP pilot plants for testing new technologies and components under real conditions have been or are being built. DLR researchers have tested solid particle heat storage technology and advanced particle receivers in a CSP pilot plant (Juelich Solar Tower) . They used this technology to achieve efficient heat storage and release at high temperatures above 900 ℃ (Figure 8). ) . In a CSP pilot plant at the Karlsruhe Institute of Technology (KIT) , researchers tested liquid metal used as a high-temperature heat storage/conducting material. In Spain, Abengoa conducted a ternary eutectic Li 2 CO 3 -Na 2 CO 3 -K 2 CO 3 molten carbonate heat storage/heat transfer technology in the Avanza-2 pilot plant at a temperature as high as 700 ℃. Tons of testing. In addition to these work, there are many research and development projects underway in Europe, and have received funding from the European Union and some European countries. Due to space limitations, I will not discuss them here.

(4) Asia

There are many CSP power stations in operation, construction or development in Asia (such as China and India) . In 2016, China announced the first batch of 20 CSP demonstration projects (a total of 1.35 GW) that received state subsidies , including the Zhejiang SUPCON SOLAR Delingha 50 MW molten salt solar thermal power generation project (Zhejiang SUPCON SOLAR Delingha 50 MW moltenha). salt tower project) and Beijing Shouhang IHW Dunhuang 100MW molten salt tower project . In 2019, most of the world’s new CSP power plants (> 1.1 GW) are under construction in China. According to statistics, about 550 MW of new CSP power stations were put into commercial operation in 2018. Among them, China contributed about 550 MW of new CSP power stations through the start-up of Central Control Solar Delingha 50 MW and Shouhang Energy Saving Dunhuang 100 MW Tower Molten 200 MW of electricity.

With the rapid development of the CSP industry in Asia (mainly China) , new CSP technologies are being developed, such as solid particle heat storage/heat conduction technology, molten salt heat storage/heat conduction technology, gas heat conduction and heat storage technology using other materials, sCO 2 Power cycle technology and solar dish Stirling technology.

Recently, the Institute of Electrical Engineering of the Chinese Academy of Sciences (IEE-CAS) and Xi’an Jiaotong University (XJTU) , Zhejiang University (ZJU) , Tsinghua University (THU) and the Shanghai Institute of Applied Physics of the Chinese Academy of Sciences (SINAP-CAS) and several other institutions Funded by the National Key Research and Development Program of the Ministry of Science and Technology of China, a project named ”  Research on Key Basic Issues of Supercritical CO 2 Solar Thermal Power Generation ” was launched. Its main research content includes the design method of the CSP high temperature subsystem and the high temperature receiver. Research and development, research and development of new thermal storage materials and systems, construction of sCO 2  solar thermal power generation demonstration platform, and research topics related to materials, components and pilot plants. In addition, in 2018, Shouhang Hi-Tech Energy Technology Co., Ltd. began to cooperate with the French Electric Power Company (EDF) on the sCO 2 CSP demonstration project, preparing to transform its 10 MW CSP demonstration power station into an sCO 2 power cycle CSP power station.

In India, the research and development of solar energy technology is mainly in charge of the National Institute of Solar Energy (NISE) . According to reports, compared with photovoltaic technology, India is currently facing various challenges in the research and development of CSP technology, such as a lack of experienced labor and insufficient local manufacturing. Therefore, although India’s direct normal irradiance (DNI) is relatively high and the area used for the development of solar energy is also relatively large, the development of the next generation of CSP technology in India has been slow.

Other Asian countries, such as Japan and South Korea, have smaller DNIs and less ground space for CSP development. Therefore, they prefer to develop solar energy technologies that can produce hydrogen overseas (such as Australia) compared to next-generation CSP technologies. . The hydrogen produced can be stored and transported and used in the country for power generation, heating or synthetic chemical substances. For example, Japan has established a concentrating test device to test a two-step water splitting process (800~1400 ℃) using cerium oxide for solar thermal production of hydrogen.

(5) Summary

In the past 10 years (2010-2020) , under the promotion of research programs in various countries and regions, significant progress has been made in the research and development of key technologies for next-generation CSP such as high-temperature heat storage/heat transfer and sCO 2 Brayton power cycle. These technologies have been tested in pilot plants in the United States, Australia, Europe or Asia. The United States has begun preparations for the establishment of a pilot plant to test molten chloride salt heat storage/heat conduction, solid particles and sCO 2 Brayton technologies under sunshine CSP conditions , while Australia has successfully demonstrated liquid metal for CSP in a pilot plant Thermal conductivity technology. Some research institutions and energy companies in Europe are demonstrating molten carbonate, solid particles and liquid metal technologies for CSP. In 2018, China began to build sCO 2 CSP demonstration power stations close to commercial scale and tested the key technologies of sCO 2 CSP.

New progress in the next generation of solar thermal power generation and heat storage technology

Figure 8 DLR’s CSP pilot plant. (A) Juelich Solar Tower; (b) the particle receiver CentRec under test

  • Use new molten salts as heat storage/conducting materials , such as molten carbonate and chloride salts, which are currently mainly used as research objects due to low cost;
  • Using solid particles as heat storage/conducting materials , this technology has been demonstrated in the United States, Europe and China;
  • Use gas (such as helium) for heat conduction , and use other materials (such as solid materials, PCM) for indirect heat storage. Researchers plan to conduct demonstrations in the United States, Europe and China;
  • Use liquid metal as a heat-conducting material , use other materials (such as liquid metal itself, solid materials or PCM) to store heat indirectly, and demonstrate this technology in Australia and Europe.

Table 1 summarizes and compares the advantages of these heat storage/heat conduction technologies, the main challenges they face, and the test equipment available for the test technology. Among them, the molten chloride salt has a suitable melting point and good thermal properties (low vapor pressure, high thermal stability) , and low material prices. In addition, because it is similar to commercial molten nitrate technology, the current design experience of the most advanced tower CSP power station is also applicable to the next generation of CSP power stations using new molten chloride salts.

Compared with molten salt technology, the maximum use temperature of particle technology can reach 1000 ℃, while inorganic base PCM technology has a higher heat storage density. In thermal conductivity technology, the thermal conductivity of liquid metal technology is much higher than other technologies. However, these new heat storage/heat conduction technologies face some technical challenges, such as controlling the corrosion of structural materials by molten salt at high temperatures, improving the cyclic stability of the heat transfer performance of solid particles and PCM materials, and reducing the materials of liquid metal technology, Operation and maintenance costs. In summary, we need more testing and demonstration devices to test under actual lighting conditions to verify the practicability and economics of these new heat storage/heat conduction technologies.

New progress in the next generation of solar thermal power generation and heat storage technology
New progress in the next generation of solar thermal power generation and heat storage technology

Table 1 Comparison of heat storage/heat conduction technology in next-generation CSP technology 

3. Molten chloride salt technology in next-generation CSP power plants

Molten chloride salt has the advantages of high thermal stability and low cost, and is one of the most promising heat storage/conducting materials in the next generation of molten salt technology. As shown in Table 2, mixed salt with carbonate (such as Li 2 CO 3 /Na 2 CO 3 / K 2 CO 3 , 1.3~2.5 USD∙kg –1 ) and mixed salt with nitrate (such as solar salt, 0.5~ 0.8 USD∙kg –1 ) , chloride salt mixed salt (such as MgCl 2 /KCl/ NaCl, less than 0.35 USD∙kg –1 ) has higher thermal stability (> 800 ℃) and suitable thermal physical properties , And the price is much lower.

However, unlike commercial molten nitrate technology, molten chloride salt technology faces another major challenge, that is, it is strongly corrosive to metal structural materials at high temperatures . Therefore, efficient and affordable corrosion control technology is very important for molten chloride technology. There have been many articles on the research progress of molten salt as a heat storage/conducting material. The following sections will focus on the latest research and development progress of molten chloride salt technology, especially in corrosion control.

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Table 2 Comparison of properties and prices of molten salts used as heat storage/heat conduction materials in CSP technology  

(1) Selection and optimization of chloride salts

Some research groups have selected and optimized the mixed salt in the next generation of molten chloride salt heat storage/heat transfer technology through literature review and experimental testing. If the thermal properties and cost of the material are mainly considered, the following chloride salts are more suitable for mixed salts: LiCl, NaCl, KCl, CaCl 2 , MgCl 2 , BaCl 2 , ZnCl 2 and AlCl 3 . The melting point of the mixed salt of the chloride salt is usually lower than that of the single salt, so it is a better heat storage/conducting material. For lower temperature heat storage/heat transfer, the mixed salt of AlCl 3 and ZnCl 2 is more attractive because of its lower melting point.

However, these mixed salts have higher vapor pressures, so they are usually not considered for use at higher temperatures. For example, at the operating temperature of the sCO 2 Brayton cycle in Gen3 CSP mentioned earlier (T> 720 ℃) , ZnCl2 has a very high vapor pressure, which is close to 1 bar at 720 ℃ (1 bar = 1 × 10 5 Pa) , while the vapor pressure of other chloride salts such as MgCl 2 is lower, lower than 0.01 bar at 800 ℃. Low vapor pressure is the main advantage for the application of heat storage/heat transfer technology, because in the heat storage and heat transfer system, the evaporation and condensation of molten salt will be greatly reduced, and the pressure storage tank is not required, which can reduce equipment costs. Similar to the carbonate mixed salt containing Li 2 CO 3 , the mixed chloride salt containing LiCl has a low melting point but a higher cost. Therefore, it is not recommended to use the mixed chloride salt containing ZnCl 2 and LiCl for advanced molten salt technology with higher operating temperature.

The mixed chloride salt mixed with NaCl, KCl, CaCl 2 and MgCl 2 has good performance. Compared with other chloride salts, alkali metal chloride salts (such as KCl and NaCl) have higher heat capacity, lower vapor pressure at high temperatures, and weak hygroscopicity (meaning corrosive impurities generated by crystal water during heating) Less) , and the price is low, but the disadvantage is the high melting point (> 750 ℃) . By mixing with alkaline earth metal chloride salts (such as MgCl 2 , CaCl 2 ) , the melting point of a single alkali metal chloride salt can be significantly reduced.

Among the binary mixed salts formed by NaCl, KCl, CaCl 2 and MgCl 2 , the lowest melting point is KCl-MgCl 2 . The melting point of this mixed salt is 426 ℃, the vapor pressure at high temperature is low, and the material cost is also low. We can also add lower-priced NaCl to the binary mixed salt to further reduce the melting point and cost, while increasing the heat capacity (Table 2) .

Figure 9 shows that by using the commercial software FactSage™ to model and measure by differential scanning calorimetry (DSC) , it can be seen that the melting point of the eutectic ternary mixed salt MgCl 2 /KCl/NaCl is about 383 ℃, and the mass fraction of the eutectic component It is 55%/20.5%/24.5%. After comparison, MgCl 2 /KCl/NaCl is regarded as the most promising next-generation molten salt heat storage material by major international molten salt technology research teams.

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 Fig. 9 The phase diagram of MgCl 2 /KCl/NaCl mixed salt obtained by FactSageTM simulation , in which the mass fraction of the eutectic component is 55%/20.5%/24.5%, which is confirmed by differential scanning calorimetry (DSC). Reprinted from reference, with permission from Elsevier Ltd., ©2018

(2) Determination of important physical properties of molten chloride salt

The physical properties of molten chloride salt, including the lowest melting point, vapor pressure, specific heat capacity, density, thermal conductivity, viscosity and impurity concentration (related to the corrosiveness of the salt) , and the design of the corrosion control system and key components in the molten salt heat storage/heat conduction system Vital. Key components include molten chloride salt storage tanks, pipes, absorbers, pumps, valves, and heat exchangers. In some research projects, such as SFERA II, researchers have determined the test procedures and data analysis standards for the characteristics of the above-mentioned molten chloride salt.

Table 3 summarizes the test procedures and measurement methods for the performance of molten chloride salts. Most of these available molten chloride salt performance test procedures and methods are based on commercial molten nitrate, and it is necessary to pay attention to the problem of measurement inconsistency between various measurement methods. Therefore, these physical property test methods are not all applicable to molten chlorine salt, such as the determination method of the maximum operating temperature. For molten chlorine salt, the determination of the maximum operating temperature should not only consider the thermal stability of molten nitrate, but also the corrosiveness and vapor pressure at high temperature.

INL’s research report summarized the physical properties of MgCl 2 /KCl mixed salt, such as specific heat capacity, density, thermal conductivity, and viscosity. In addition, NREL and ANU used FactSageTM modeling and DSC testing to determine the lowest melting point and composition of MgCl 2 / KCl/NaCl. Wang et al. studied the vapor pressure and viscosity of the eutectic NaCl/KCl/ZnCl 2 mixed salt through a self-made device and a Brookfield viscometer. By comparing the calculation results with the experimental data, Li et al. derive a series of formulas to predict the thermophysical properties of mixed salts containing NaCl, KCl, MgCl 2 , CaCl 2 and ZnCl 2 , including heat capacity, density, thermal conductivity and viscosity Wait. However, the current research on the physical properties of molten chloride salts is still limited, especially for the most promising MgCl 2 /KCl/NaCl mixed salt. In order to achieve commercial applications, a large number of research and testing by scientific researchers are required.

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Table 3 The measurement of the physical properties of molten salt and the thermodynamic simulation method that are essential for the design of heat storage/heat conduction systems 

(3) Research on the corrosion of structural materials by molten chloride and its control methods

1. Research on the corrosion mechanism of molten chloride on structural materials

The strong corrosivity of molten chloride salts to structural materials is the main problem hindering its commercial application. In recent years, the corrosion of structural materials (mainly commercial metal alloys ) in molten chloride salts at high temperatures (> 600 ℃) has been extensively studied. Recently, researchers have published some related review papers. Our review of the corrosion mechanism and control methods of alloy materials in molten chloride salt published in recent years comprehensively introduces the corrosion of molten chloride salt.

Theoretically, pure chlorine salt (such as MgCl 2 /NaCl/KCl mixed salt) itself will not oxidize the metal elements in commercial Cr-Fe-Ni alloys, because MgCl 2 , NaCl and KCl are better than FeCl 2 , CrCl 2 and The thermodynamic properties of NiCl 2 are more stable. The severe corrosion of the alloy is mainly caused by the oxidizing impurities (such as hydrolysate) in the molten chloride salt , which will oxidize the Cr element to form Cr oxide. Unlike contact with air or oxidizing high-temperature gas, Cr oxide can dissolve in the molten chloride after reacting with the chloride ion in the molten chloride salt, so it cannot form a stable oxide protection on the commercial Cr-Fe-Ni alloy Floor.

Studies have shown that if the molten chloride salt contains impurities, it is usually very corrosive at high temperatures. For example, if the structural alloy is in contact with the unpurified MgCl 2 / NaCl/KCl mixed salt at 700 ℃ , even the expensive nickel-based alloy with strong corrosion resistance (such as Hastelloy C-276) cannot meet the 30 requirements for commercial applications. Yearly service life (that is, the corrosion rate should be less than 10 µm∙a –1 ) . Using scanning electron microscope (SEM) and energy dispersive X-ray (EDX) to analyze the microstructure of Cr-Fe-Ni alloy corrosion samples, it is found that during the corrosion process, Cr is oxidized and dissolved before Fe and Ni, thus forming porous Corrosion layer of structure.

Researchers generally believe that the corrosion of metal structural materials in contact with molten chloride salts at high temperatures is caused by corrosive impurities (such as MgOHCl) and gases (such as HCl) in molten chloride salts . As shown in Fig. 10, in the previous work, we proposed the impurity-driven corrosion mechanism of commercial Cr-Fe-Ni alloy in molten MgCl 2 /NaCl/KCl. The main corrosive impurity in the chloride salt mixed salt containing strong hygroscopic MgCl 2 is the hydrolyzed product MgOHCl produced in the dehydration process [Equation (2)] . The hydrolysis reaction is shown in Eqs. (3) and (4):

MgCl2 ∙2H2O → MgCl2 ∙H2O + H2O  (2)

MgCl2 ∙H2O → MgOHCl + HCl (3)

MgCl2 ∙2H2O → MgOHCl + HCl + H2O (4)

A large amount of MgOHCl exists in the molten chloride salt in the form of MgOH+ and Cl- ions. When the temperature is higher than 550 ℃, MgOH + will decompose into MgO and highly corrosive H + ions, thereby reacting with the more active Cr and Si elements in the commercial Cr-Fe-Ni alloy.

In addition to metal alloys, researchers have also studied the corrosion behavior of ceramic structural materials, such as Al 2 O 3 and SiC materials in molten KCl/NaCl at high temperatures (> 600 ℃) , and molten MgCl 2 /NaCl/KCl. Carbon fiber reinforced silicon carbide composite material (C/C-SiC) . The immersion test of C/C-SiC in molten chloride salt shows that it has excellent corrosion resistance, excellent mechanical properties at high temperatures and high fracture toughness. This material can be used as a high-temperature structural material for key components in molten chloride salt technology, such as molten salt pumps and molten salt valves .

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Fig. 10 The impurity corrosion mechanism of Cr-Fe-Ni alloy in molten MgCl 2 /KCl/NaCl under inert atmosphere . Reprinted from reference, with permission from Elsevier BV, ©2018

2. Purification method of molten chloride salt

(1) Thermal purification method

Researchers have studied the use of heating to reduce the corrosivity of molten chloride salts containing strong hygroscopic chloride salts. For example, gradually heating the mixed salt by controlling the temperature to inhibit the aforementioned side reaction of hydrolysis, thereby reducing corrosive impurities. According to the vapor pressure diagram of H 2 O and HCl of MgCl 2 hydrate (Figure 11) , Kipouros and Sadoway used a multi-step heating method to purify MgCl 2 hydrate. By gradually increasing the salt temperature, the MgCl 2 hydrate MgCl 2 ·6H 2 O at room temperature is sequentially dehydrated, and when the temperature is T1~T3, it is sequentially dehydrated into MgCl 2 ·4H 2 O, MgCl 2 ·2H 2 O and MgCl 2 · H 2 O.

Researchers can control the salt temperature between T3 and T4 ( the hydrolysis temperature of MgCl 2 ·H 2 O) , so that more MgCl 2 ·H 2 O is dehydrated to form anhydrous MgCl 2 , and no or only a small amount is formed. MgOHCl. Recently, Vidal and Klammer have studied such a thermal purification process. Our research group uses DSC, thermogravimetric analysis-combined mass spectrometry (TG-MS) and EDX analysis methods, as well as online monitoring of the generated HCl gas, to determine the MgCl 2 /NaCl/KCl containing hydrated MgCl (MgCl 2 ·6H 2 O) The thermal purification method of mixed salt (molar fractions 60%/20%/20% respectively) was studied.

We also tested another salt dehydration method, that is, before heating to the melting point, the solid salt is purged with an inert gas at 350 ℃ below the melting point to reduce the hydrolysis side reaction and the concentration of corrosive impurities such as MgOHCl in the salt. .

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Fig. 11 Based on the vapor pressure diagram of H 2 O and HCl of MgCl 2 hydrate , the salt is purified by stepwise heating. 1 atm = 101 325 Pa. Reprinted from references, with permission from Elsevier Science Ltd., ©2001

However, according to the vapor pressure diagram in Fig. 11, it is impossible to completely avoid the hydrolysis reaction in equations (3) and (4) by gradually heating. After the above-mentioned gradual heating and purification, a small amount of hydroxide impurities still remain in the salt (the mass fraction is usually 0.1% to 1%) , and these impurities can cause severe corrosion of metal structural materials. MgOHCl is dissolved in molten chlorine salt in the form of MgOH + and Cl  . At a temperature higher than 555 ℃, MgOH + is further decomposed into MgO and corrosive H + . In an inert atmosphere, soluble metal hydroxyl ions are considered to be the most critical corrosive impurities in molten chloride salts. Low-concentration corrosive impurities are not easily removed by thermal purification methods. It is recommended to further reduce their concentration by chemical or electrochemical purification methods to slow down the corrosion rate of structural materials.

(2) Chemical purification method

Researchers have used chemical methods to purify molten chloride salts, such as adding Li metal to molten chloride salts containing LiCl or adding Mg metal as corrosion inhibitors to molten chloride salts containing MgCl 2 . The results show that the corrosion rate of commercial Cr-FeNi alloy is significantly reduced under the conditions of molten salt static or thermosiphon dynamic test (Figure 12) . Our research group immersed three commercial Cr-Fe-Ni superalloys (SS 310, Incoloy 800 H and Hastelloy C-276) in MgCl 2 /NaCl/KCl (molar fraction 60%/20%/20%) molten salt In addition, Mg metal with a mass fraction of 1% was added as a corrosion inhibitor, and the test was carried out for 500 h under the conditions of an inert atmosphere and 700 ℃. Compared with not adding Mg, the addition of Mg corrosion inhibitor can significantly reduce the corrosion rate of the alloy by more than 70%. The reason is that the addition of metal Mg can reduce the concentration of the corrosive impurity MgOHCl, thereby reducing the oxidation-reduction potential of molten chloride (ie corrosion Sex) .

Recently, Choi et al. used electrochemical methods such as cyclic voltammetry and open circuit potential to study the corrosion inhibition mechanism of Mg metal in molten chloride salts. Sun et al. used analytical methods such as inductively coupled plasma atomic emission spectroscopy (ICP-AES) , Raman spectroscopy, and infrared spectroscopy to analyze the chemical properties of the molten mixed salt of MgCl 2 /NaCl/KCl after adding metal Mg . Their research results also show that the addition of Mg can remove corrosive impurities such as MgOHCl, thereby reducing the corrosiveness of salt.

In addition to adding corrosion inhibitors, Kurley et al. used a variety of carbon chlorinated organic gases and their mixed gases to purify KCl-MgCl 2 molten salt through the carbon chlorination method . They passed carbon tetrachloride gas through the molten salt and successfully purified the impurity concentration of the kilogram-level molten salt to a very low level (only 42 μmol MgO per kilogram of salt) . As shown in Figure 13, in this purified 700 ℃ molten chloride salt, stainless steel SS 316L has a low corrosion rate similar to that of Hastelloy N. The corrosion rates of the two alloys are less than 30 µm∙a –1 (the alloy mass change after 100 h immersion test is less than 0.2 mg∙cm –2 ) , which is close to the 30-year service life requirement. Therefore, this experiment verifies that the corrosive impurities in the molten chloride salt are the main cause of corrosion. It also shows that if the concentration of impurities is controlled at a low level, then lower-priced structural materials (such as stainless steel) are also allowed to be used in the next-generation molten chloride salt heat storage system to enhance its cost competitiveness.

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Figure 12 Comparison of corrosion rate of Haynes 230 alloy in 850 ℃ MgCl 2 -KCl molten salt with and without Mg corrosion inhibitor under static and thermosiphon corrosion test conditions

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Figure 13 Under inert atmosphere and 700 ℃, with the increase of the purity of KCl-MgCl 2 salt, the quality change of SS 316L (a) and Hastelloy N (b) immersed in it. Reprinted from reference, with permission from Royal Society of Chemistry, ©2019

(3) Electrochemical purification method

Some electrochemical methods are also used by researchers to purify molten chloride salts. Literature [101–103] shows that it is possible to remove most of the impurities by pre-electrolysis (PE) of molten chloride salt by using an inert electrode . However, using an inert electrode will cause the following reactions and produce toxic gases such as Cl 2 .

Cathode (reduction) : 2MgOH + + 2e  = 2MgO(s) + H 2 (g) (5)

Anode (oxidation) : 2Cl  = Cl 2 (g) + 2e  (6)

Total reaction: 2MgOH + + 2Cl  = Cl 2 (g) + 2MgO(s) + H 2 (g) (7)

In addition, the surface of the cathode will be passivated by the generated electrically insulating solid MgO. In order to avoid the generation of toxic gases such as Cl 2 and the passivation of the electrode, our research group used Mg anode and pulse potential to purify MgCl 2 / KCl/NaCl molten salt in electrolysis , thereby reducing its corrosiveness. The specific reaction process is as follows Shown:

Cathode (reduction) : 2MgOH + + 2e  = 2MgO(s) + H 2 (g) (8)

Anode (oxidation) : Mg(s) = Mg 2+ + 2e  (9)

Total reaction: 2MgOH + + Mg(s) = Mg 2+ + 2MgO(s) + H 2 (g) (10)

Figure 14 is a schematic diagram of the above reaction and the phenomenon observed in Mg anode electrolysis. The experimental results show that electrolysis can effectively remove the corrosive impurity MgOHCl. At the same time, the corrosion rate of the commercial superalloy (Incoloy 800 H) immersed in the molten chloride salt was tested by the potential dynamic polarization method (PDP) . The test results showed that the corrosion rate of the alloy was reduced by more than 80% due to the purification of the salt. Moreover, the pulse potential applied during the electrolysis can suppress the cathode passivation and deactivation caused by the precipitation of MgO. Therefore, this electrochemical salt purification method is considered to be promising to control the corrosiveness of molten chloride salt at low cost.

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Figure 14 Electrochemical salt purification of KCl/MgCl 2 /NaCl molten salt at 500 ℃ using a Mg anode in an inert atmosphere . The figure shows the assumed electrolysis reaction and the phenomenon observed in the experiment. Reprinted from reference, with permission from Elsevier BV, ©2019

(4) Measurement and monitoring methods of corrosive impurities

In order to measure/monitor the concentration of corrosive impurities (such as MgOHCl) in molten chloride salt containing MgCl 2 , researchers have proposed several methods, such as acid-base titration and cyclic voltammetry. Kurley et al. used a micropipette and a commercial pH electrode to measure the concentration of oxide impurities dissolved in KCl-MgCl 2 molten salt based on the acid-base titration method . The measurement limit is less than 50 μmol·kg –1 , that is, the MgOH + content is 5 ppm. The research work of Skar and our research group shows that cyclic voltammetry is a promising technique for in-situ monitoring of MgOH + impurities, and the limit of measuring MgOH + content can be less than 100 ppm.

Figure 15 shows the impurity ions with MgOH + MgCI2 the 2 / KCI / NaCI molten cyclic voltammogram. Cyclic voltammetry and titration measurements show that the peak current density ip of reaction B [MgOH + + e  → MgO(s) + (1/2)H 2 (g)] is proportional to the concentration of MgOH + in the molten chloride salt. Proportional. In addition to these methods, other methods, such as the aforementioned Raman spectroscopy and infrared spectroscopy, can also be used to measure and monitor MgOH + impurities. Different from the chemical post analysis method, the advantage of the in-situ monitoring method is that it can be developed as an online monitoring technology for molten chloride salt impurities, and integrated with the above-mentioned salt purification technology to form a molten chloride salt corrosion control system.

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Figure 15 Typical cyclic voltammogram of MgCl 2 /KCl/NaCl molten salt containing MgOH + impurity ions . Reaction A: Mg 2+ + 2e  = Mg(s); Reaction A′: Mg(s) = Mg 2+ + 2e–; Reaction B: MgOH + + e  = MgO(s) + (1/2) H 2 (g); Reaction C: Cl  = (1/2)Cl 2 (g) + e  . T = 500 ℃; working electrode material: tungsten; scanning rate: 200 mV·s –1 . ip(B): The peak current density of response B. Reprinted from references, with permission from Elsevier Ltd., ©2017

3. Other corrosion inhibition methods

In addition to salt purification, researchers have also proposed other corrosion inhibition methods, such as improving and treating structural materials to reduce their corrosion, such as surface coating or the formation of Al 2 O 3 , yttriastabilized zirconia (YSZ) , iron Protective layer such as nickel-based or nickel-based amorphous coating. Experiments show that this method has certain development prospects. Compared with the salt purification method, the alloy protective layer can simultaneously reduce the corrosion of the alloy in the molten chloride salt and the protective gas. Gomez-Vidal et al . pre-oxidize the aluminum alloy at high temperature (such as 1050 ℃) , thereby forming a dense and continuous protective layer of aluminum oxide on the surface of the alloy. Corrosion experiments show that the alloy can effectively protect the alloy from molten chloride corrosion.

In addition, Raiman et al. showed through experiments that iron-based or nickel-based amorphous coatings can improve the corrosion resistance of structural alloys in corrosive molten chloride salts. Our research group has also conducted research in this area, and cooperated with KIT to pre-oxidize the Fe-Cr-Al model alloy (the mass fraction of aluminum is 8%) in the air at high temperature (800 ℃) , thereby forming on the surface of the alloy. A dense protective layer of aluminum oxide. As shown in Figure 16, the aluminum oxide protective layer attached to the alloy surface can effectively inhibit the oxidative dissolution of Cr and Fe and the penetration of corrosive impurities into the alloy to corrode the substrate.

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Figure 16 The SEM image of the cross-section of the Fe-Cr-Al model alloy (the mass fraction of Al is 8%) after corrosion by the MgCl 2 /KCl/NaCl molten salt at 700 ℃ for 500 h in an inert atmosphere And EDS mapping. Reprinted from reference, with permission from Elsevier BV, ©2018

4. Conclusions and prospects

In the past 10 years (2010-2020) , with the support of research projects in many countries/regions, including the United States, Australia, Europe, and Asia (mainly China) , the next generation of CSP technology using new heat storage/heat conduction materials Significant progress has been made in research and development . Researchers develop new heat storage/heat transfer technologies suitable for temperatures above 565°C, and combine them with power cycles with higher thermoelectric conversion efficiency (such as sCO2 power cycles) . They extensively studied 4 promising technical routes, namely solid particles, molten salt, gas and liquid metal technical routes. This article summarizes the latest R&D progress and main challenges of the above technologies. In addition, it also compares and reviews the advantages and disadvantages of these technologies.

Among these candidate new heat storage/heat conduction materials, molten chloride salt is considered to be the most promising heat storage/heat conduction material in the next generation of molten salt technology due to its excellent thermal performance and low material price, and its temperature can be as high as 750 ℃. Use at the same temperature. However, molten chloride salts are highly corrosive to metal structural materials at high temperatures, so researchers need to solve this challenge to ensure the long life and reliability of the heat storage system.

The research and development progress of molten chloride salt heat storage/conduction technology is summarized as follows:

(1) MgCl 2 /KCl/NaCl has been identified as the most promising molten chloride salt heat storage/conducting material;

(2) The physical parameters of molten chlorine salt are very important to the engineering design of molten chlorine salt technology. The measurement methods and recommended values ​​of its important physical properties (ie minimum melting point, vapor pressure, specific heat capacity, density, thermal conductivity, viscosity and corrosivity, etc.) ) Need further confirmation;

(3) The corrosion mechanism of structural materials (such as alloys) in molten chloride salts has been extensively studied. Based on the corrosion mechanism, researchers have also found some promising corrosion control methods.

On the basis of reviewing and summarizing the latest research progress, this article also gives some future research suggestions to promote the maturity of molten chloride salt heat storage/heat transfer technology and realize its application in the next generation of CSP power plants:

(1) Study how to rationally integrate salt purification and corrosion mitigation methods, and develop cost-effective and effective technology to control the corrosiveness of molten chlorine salt;

(2) Research on cost-effective corrosion control methods and systems;

(3) Considering the durability and the corrosivity of molten chloride salt, determine the structural materials that are worth testing at the laboratory level;

(4) Develop all the key components required in the molten salt circuit, such as molten chlorine salt storage tanks, heat exchangers, pipes, pumps and valves, and conduct pilot demonstrations of the molten salt circuit at high temperatures;

(5) Determine the overall technology amplification strategy, including considering the materials and processes required in actual applications.

Note: The content of this article has been slightly adjusted, you can view the original text if necessary .

Adapted from the original text:

Wenjin Ding, Thomas Bauer.Progress in Research and Development of Molten Chloride Salt Technology for Next Generation Concentrated Solar Power Plants[J].Engineering,2021,7(3):334-347.

Posted by:CoinYuppie,Reprinted with attribution to:
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