Liquid-based high-temperature receiver technologies for next-generation concentrating solar power: A review of challenges and potential solutions

Abstract

To reduce the levelized cost of energy for concentrating solar power (CSP), the outlet temperature of the solar receiver needs to be higher than 700 °C in the next-generation CSP. Because of extensive engineering application experience, the liquid-based receiver is an attractive receiver technology for the next-generation CSP. This review is focused on four of the most promising liquid-based receivers, including chloride salts, sodium, lead-bismuth, and tin receivers. The challenges of these receivers and corresponding solutions are comprehensively reviewed and classified. It is concluded that combining salt purification and anti-corrosion receiver materials is promising to tackle the corrosion problems of chloride salts at high temperatures. In addition, reducing energy losses of the receiver from sources and during propagation is the most effective way to improve the receiver efficiency. Moreover, resolving the sodium fire risk and material compatibility issues could promote the potential application of liquid-metal receivers. Furthermore, using multiple heat transfer fluids in one system is also a promising way for the next-generation CSP. For example, the liquid sodium is used as the heat transfer fluid while the molten chloride salt is used as the storage medium. In the end, suggestions for future studies are proposed to bridge the research gaps for > 700 °C liquid-based receivers.

Graphical abstract

Keywords

next-generation concentrating solar power / liquid-based solar receiver / molten salt / liquid metals

1 Introduction

Large-scale utilization of renewable energy is a promising way to meet the global warming challenge [1]. Of different renewable energy resources, solar energy is the most abundant [2]. It is estimated that the solar energy irradiated onto the earth within an hour is larger than the total energy consumed by human society in a year [3]. There are two distinguishing technologies currently in use to utilize solar energy, including photovoltaics (PV) and concentrated solar power (CSP) [4–6]. It is reported by the National Renewable Energy Laboratory that the carbon emission in the life cycle of CSP is lower than that of PV by about 36% [7]. Moreover, CSP can also generate dispatchable electricity because this technology can be easily integrated with a cost-effective thermal storage system [8]. Therefore, CSP has attracted plenty of attention in recent years [9–13]. However, the levelized cost of energy (LCOE) for CSP is still high. He et al. [14] has evaluated the economic performance of the state-of-the-art CSP. It was found that the LCOE of the current commercial solar power tower is still higher than the expected energy cost goal [15]. To improve the solar to electricity efficiency and reduce the LCOE, the operational temperature for the next-generation CSP has to be increased to >700 °C [16,17]. The solar receiver is one of the most important equipment in the CSP plant, where the solar energy is converted to thermal energy in the receiver. Three potential receiver technologies can be used for the next-generation CSP [18], which are the liquid-based receiver [19], the particle-based receiver [20], and the gas-based receiver [21]. Among these receiver technologies, the liquid-based receiver has substantial engineering practices [22,23]. For current commercial CSP plants, a binary nitrate molten salt (NaNO₃ 60 wt.%, KNO₃ 40 wt.%) is used as the heat transfer fluid (HTF). This HTF has a temperature limitation of about 600 °C beyond which it will decompose [24]. Thus, the current commercial nitrate molten salt receiver technology cannot be directly used in the next-generation CSP plant. As alternatives to the current nitrate molten salt receiver, new receivers with high-temperature resistance molten salts and liquid metals as HTFs have received extensive attention [25,26].

The purpose of this paper is to review the current research status of potential liquid-based high-temperature solar receivers, including molten salt receivers and liquid metal receivers, and summarize future studies for the application of liquid-based solar receivers in the next-generation SPT. The framework of this paper is presented in Fig.1. First, key issues of high-temperature molten salt receivers are presented, and corresponding methods which could be used to deal with these issues in literature are classified and reviewed. Thermal properties and heat transfer correlations of HTFs are essential for the flow and heat transfer performance evaluation of the solar receiver. For example, Zhang et al. [27] numerically investigated the response characteristics of an external receiver under cloudy conditions. In the thermal model, thermal properties and heat transfer correlations are needed. Therefore, studies on thermal properties and heat transfer correlations for molten salts are reviewed and summarized. Then, the current research status for liquid metal receivers, including sodium, lead-bismuth, and tin receivers, is reviewed. In the end, future studies for the application of liquid-based high-temperature solar receivers are summarized.

Some important parameters of the receiver that will appear in the review are defined as follows. The receiver efficiency (η_r) is an important parameter of the receiver, which is defined as the ratio between the thermal energy absorbed by the HTF (Q_ab) and the incident solar energy onto the receiver (Q_in), as presented in Eq. (1).

(1){\eta }_{\mathrm{r}}=\frac{Q_{\text{ab}}}{Q_{\text{in}}}\times 100\mathrm{\%}.

2 Molten salt receiver

There are three types of salts having thermal stability of more than 700 °C, including fluoride salts, carbonate salts, and chloride salts [14]. The fluoride salt, such as FLiNaK (46.5 wt.% LiF-11.5 wt.% NaF-42 wt.% KF), has a melting point of 454 °C and a boiling point of 1610 °C. Although fluoride salt has a relatively high thermal conductivity, it also has a higher corrosivity to alloys compared with the other two salts. For instance, Inconel 625 is a typical receiver material for the current CSP plant, and its corrosion rate is (2.8 ± 0.38) mm/a in chloride salt at 650 °C in a nitrogen atmosphere [28]. On the other hand, the corrosion rate of Inconel 625 in FLiNaK at 650 °C is 5.0 mm/a [29], which is nearly twice that in chloride salt. Moreover, fluoride salt also has toxicity. Thus, it is not listed as the candidate salt for the next-generation CSP [30]. Although carbonate salt is the least corrosive to alloys, it contains lithium ions, which makes this salt expensive. For example, the price of a chloride salt candidate (37.5 wt. % MgCl₂, 62.5 wt.% KCl) is only $350 per metric ton, while that of a carbonate salt candidate (33.4 wt.% Na₂CO₃, 34.5 wt.% K₂CO₃, 32.1wt.% Li₂CO₃) is $2500 per metric ton. The price of carbonate salt is more than seven times that of chloride salt. Furthermore, the supply of lithium is also a challenge in the future because it is intensively needed in the battery market. Therefore, chloride salt is considered to be a promising HTF in the next-generation CSP plants due to its abundance and low price [31].

There are two major challenges for the chloride molten salt receiver operated at > 700 °C. The first is the aggressive corrosion rate of the alloys in chloride salt. For the tube used at high temperatures, such as the superheated tube, its corrosion allowance is 0.3 mm [32], which means that the maximum decrease of the tube wall thickness is 0.3 mm in the service life. The commercial SPT plant usually has a service life of 30 years. Thus, the allowable corrosion rate for the receiver tube is 10 μm/a. However, the current receiver material, such as Inconel 625, has a corrosion rate of (2.8 ± 0.38) mm/a in chloride salt at 650 °C [28], far above the allowable corrosion rate for commercial utilization. The second challenge is the decrease of receiver efficiency (η_r) at high temperatures. If the outlet temperature (T_out) of the receiver is increased from 550 °C to 750 °C, η_r will decrease from about 84% to 74% [33], which represents an absolute reduction of 10%. Therefore, for practical application of the next-generation high-temperature central solar receiver, actions must be taken to solve problems related to the receiver material corrosion and the receiver efficiency reduction.

2.1 Corrosion-resistance methods for chloride salt solar receivers

There are two major ways to tackle the corrosion issue of chloride salt, the purification of chloride salt and the development of corrosion-resistance receiver materials, as presented in Fig.2.

2.1.1 Purification of chloride salt

The widely recognized chloride salt for the next-generation CSP plant is the ternary mixture of MgCl₂/KCl/NaCl [31,34,35]. Theoretically, Cr-Fe-Ni alloys cannot be corroded in pure chloride salt because basic chlorides in the chloride salt mixture are more stable than FeCl₂, CrCl₂, and NiCl₂ in thermodynamics [36]. The corrosion of alloys in chloride salts at a high temperature is primarily driven by the corrosive magnesium hydroxychloride (MgOHCl) impurity, as shown in Eq. (2). The MgOHCl impurity is mainly produced by the highly hygroscopic MgCl₂, which usually exists in a hydrate form such as MgCl₂·H₂O, MgCl₂·2H₂O, MgCl₂·4H₂O, and MgCl₂·6H₂O. Equations (3)–(9) give the decomposition processes of magnesium chloride hydrates at different temperatures. Although most of the moisture evaporates at high temperatures, the residual moisture can also cause the corrosion of alloys because it can produce the corrosive MgOHCl impurity by reacting with the salt [37], as shown in Eq. (8). Moreover, if chloride salt is operated at a high temperature in the ambient air, the oxygen will be dissolved, which will accelerate the corrosion by increasing the oxidation potential of the salt. Taking the Hastelloy C-276 as an example, if it is immersed in the chloride salt at 800 °C in the absence of air, its corrosion rate can be reduced from 500 μm/a to 10 μm/a compared with that in the air atmosphere [38]. Thus, removing moisture and oxygen from chloride salt is an effective way to reduce the corrosion of the salt because it can suppress the formation of MgOHCl. For the next-generation molten salt CSP plant, salt purification is necessary to remove or reduce the content of impurities in chloride salt to mitigate its corrosion to housing materials. Currently, different salt purification methods can be found in the literature for the corrosion mitigation of chloride salts, including thermal purification [39, 40], the addition of element Mg [41, 42], electrochemical purification [43], dehydration with ammonium chloride [44, 45], and carbochlorination with CO, Cl₂, and CCl₄ [46, 47]. Among these methods, thermal purification and the addition of element Mg are regarded as cost-effective salt purification methods on a large scale [48].

(2)x\text{MgOHCl}+\text{M}=x\text{MgO}+\text{MC}{\text{l}}_x+\left(x/\phantom{x2}2\right){\text{H}}_2,

where M represents metals, such as Cr, Fe, and Ni.

(3)\mathrm{M}\mathrm{g}\mathrm{C}{\mathrm{l}}_2\cdot 6{\mathrm{H}}_2\mathrm{O}=\mathrm{M}\mathrm{g}\mathrm{C}{\mathrm{l}}_2\cdot 4{\mathrm{H}}_2\mathrm{O}+2{\mathrm{H}}_2\mathrm{O},96-117{}^{\circ }\mathrm{C},

(4)\mathrm{M}\mathrm{g}\mathrm{C}{\mathrm{l}}_2\cdot 4{\mathrm{H}}_2\mathrm{O}=\mathrm{M}\mathrm{g}\mathrm{C}{\mathrm{l}}_2\cdot 2{\mathrm{H}}_2\mathrm{O}+2{\mathrm{H}}_2\mathrm{O},107-180{}^{\circ }\mathrm{C},

(5)\mathrm{M}\mathrm{g}\mathrm{C}{\mathrm{l}}_2\cdot 2{\mathrm{H}}_2\mathrm{O}=\mathrm{M}\mathrm{g}\mathrm{C}{\mathrm{l}}_2\cdot {\mathrm{H}}_2\mathrm{O}+{\mathrm{H}}_2\mathrm{O},153-242{}^{\circ }\mathrm{C},

(6)\mathrm{M}\mathrm{g}\mathrm{C}{\mathrm{l}}_2\cdot 2{\mathrm{H}}_2\mathrm{O}=\mathrm{M}\mathrm{g}\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{l}+\mathrm{H}\mathrm{C}\mathrm{l}(\mathrm{g})+{\mathrm{H}}_2\mathrm{O},193-240{}^{\circ }\mathrm{C},

(7)\mathrm{M}\mathrm{g}\mathrm{C}{\mathrm{l}}_2\cdot {\mathrm{H}}_2\mathrm{O}=\mathrm{M}\mathrm{g}\mathrm{C}{\mathrm{l}}_2+{\mathrm{H}}_2\mathrm{O},304-400{}^{\circ }\mathrm{C},

(8)\mathrm{M}\mathrm{g}\mathrm{C}{\mathrm{l}}_2+{\mathrm{H}}_2\mathrm{O}=\mathrm{M}\mathrm{g}\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{l}+\mathrm{H}\mathrm{C}\mathrm{l}(\mathrm{g}),210-445{}^{\circ }\mathrm{C},

(9)\mathrm{M}\mathrm{g}\mathrm{O}\mathrm{H}\mathrm{C}\mathrm{l}=\mathrm{M}\mathrm{g}\mathrm{O}+\mathrm{H}\mathrm{C}\mathrm{l}(\mathrm{g}),500-568{}^{\circ }\mathrm{C}.

Thermal purification is an effective way to remove O₂ and H₂O impurities by thoroughly drying the chloride salt [40,49,50]. However, the heating procedure should be carefully controlled because an improper heating procedure will cause the formation of highly corrosive MgOHCl at a relatively high concentration [45,51]. Stepwise heating with elaborate temperatures and dwelling time can suppress the formation of MgOHCl to reduce the potential of corrosion of chloride salt. Kipouros and Sadoway [52] adopted a thermal treatment at two different heating temperatures for chloride salts dehydration. Ding et al. [53] dehydrated the chlorides salts at seven different heating temperatures. A comparison of four different thermal treatment methods of a ternary chloride salt (55.1% MgCl₂, 24.5% NaCl, 20.4% KCl) shows that the corrosion rate of the stainless steel AISI 304 at 720 °C in the N₂ atmosphere is reduced by 67.8% when compared with the original salt without thermal treatment [50]. Moreover, the thermal treatment of MgCl₂ before the mixing of single chloride salts can further reduce the formation of impurities in chloride mixtures [35].

The addition of element Mg is another way to reduce MgOHCl [41–43] because the element Mg can serve as a corrosion inhibitor by reacting with MgOHCl, as shown in Eq. (10). This method is also referred to as chemical purification [36,54]. The addition of 1.15 mol.% Mg in a binary MgCl₂/KCl salt can significantly reduce the corrosion rate of Haynes 230 [41]. Mg can also be used as an anode in the electrolysis of molten chloride salt to reduce MgOHCl [42]. Moreover, combining the addition of Mg with thermal decomposition can achieve a better corrosion mitigation for chloride salt. Zhao et al. [54] added 1.7 wt.% of Mg chips after the thermal treatment of a commercial MgKNa chloride salt. Then, the mixture of Mg and thermal purified salt was heated for chemical purification. It was found that the concentration of MgOHCl decreased from about 5 wt.% to less than 0.5 wt.% compared with the thermal treatment only. To reduce the purification time, thermal purification and chemical purification can be simultaneously conducted by mixing Mg chips and chloride salt and then heated together [55]. For chemical purification, excess Mg is usually added for a better purification performance. However, the residual Mg is in a solid state because the melting point of Mg is about 650 °C while the temperatures of the cold salt tank or the inlet of the receiver are about 520 °C for the next-generation CSP plant [30]. Solid Mg is harmful to the salt loop because it may cause the clogging of the heat exchanger channels, and the wear of the salt pump. Thus, the amount of Mg addition should be carefully considered. It is suggested that when there is a higher amount of hydrate in the salt, a higher amount of Mg additive is needed [48].

(10)\text{2MgOHCl + Mg = 2MgO + MgCl +}{\text{H}}_2.

It can be concluded that the combination of thermal treatment and addition of element Mg is a promising method for the purification of MgCl₂/KCl/NaCl, but the addition of Mg is strongly affected by the content of water in the chloride salt. Future studies should be focused on the relationship between the minimum amount of Mg additive and the content of water in the chloride salt. Moreover, the feasibility of such purification methods on a real plant-size scale should be also demonstrated.

2.1.2 Anti-corrosion receiver materials

Nickel-based alloys are believed to be promising in the next-generation CSP plant application because of their excellent mechanical performance and corrosion resistance in the chloride salt at high temperatures. These alloys include Haynes 230, Hastelloy, In740H, and so on. Tab.1 lists the corrosion rate of typical alloys in chloride salts. It seems that Hastelloy C-276 has a higher corrosion resistance in chloride salt compared with Haynes 230. Hastelloy C-276 alloy has a corrosion rate of 10.03 μm/a in 32 mol.% MgCl₂-68 mol.% KCl molten salt at 800 °C, while Haynes 230 has a corrosion rate of 16.14 μm/a under the same condition [56]. Nevertheless, Haynes 230 is still considered the most potential material in the next-generation CSP plants [30,57]. Currently, material and manufacturing cost is relatively high for the massive utilization of nickel-based alloys. To reduce the cost, rolled and welded tube is proposed to replace the seamless tube due to its high material utilization and less cost [58]. It is believed that rolled and welded tube can reduce the manufacturing cost by 30% compared with seamless tube [59]. For future studies, the mechanical and corrosion resistance performance of welded tube in chloride salts at high temperatures should be investigated for the de-risk of the solar receiver. Moreover, the corrosion rate is also influenced by the immersion time of alloys in chloride salt. For the In702 alloy immersed in MgCl₂/KCl (35.59/64.41 wt.%) at 700 °C in an Ar atmosphere, the corrosion rate is 42 μm/a after being immersed for 8 h. However, if the immersion time lasts for 72 h, the corrosion rate will increase to 136 μm/a [60]. It can also be found from Tab.1 that even for the same alloy, the tested corrosion rate varies because of the differences in tested temperature, protection atmosphere, and immersion time. Thus, a standard test condition should be set for the next-generation CSP plant.

Using anti-corrosion coatings is another way to mitigate the corrosion rate of receiver materials in the high-temperature molten salt [53,62–65]. The deposition of Amdry 997-NiCoCrAlYTa coating on SS310 and then pre-oxidation at 900 °C in the air for 24 h can reduce the corrosion rate in 700 °C molten NaCl/KCl (44.53 wt.%/55.47 wt.%) from 4520 to 190 μm/a [64]. For alumina-forming alloys, the pre-oxidation can form a passivation layer on the tube surface, which can act as an anti-corrosion coating to protect the inner alloys from being corroded. There is Al in the alumina-forming alloys. Thus, an alumina layer will be formed after the oxidation of alumina-forming alloys. Ding et al. [53] tested the anti-corrosion performance of two peroxided alumina forming Fe-Cr-Al alloys in a chloride salt at 700 °C in the argon atmosphere. The results showed that the dense α-Al₂O₃ scale generated can reduce the corrosion rate by preventing corrosive impurities from penetrating the alloy. However, the increasing content of Al in the alloy may reduce its mechanical performance at high temperatures. Thus, the content of Al should be optimized. Gomez-Vidal et al. [66] investigated the pre-oxidation of three different alloys, including In702, Haynes 224, and Kanthal APMT. The results indicate that the pre-oxidation of In702 at 1050 °C for 4 h in the 80%N₂-20%O₂ atmosphere has the best anti-corrosion performance. For the anti-corrosion coating used in the next-generation CSP plant, proper coatings should be developed or screened. Moreover, the endurance of these anti-corrosion coatings in the flowing molten chloride salt under high-temperature conditions should be further investigated.

Ceramic and cermet are emerging materials for the high-temperature molten salt receiver. Ceramics has been widely used in volumetric receivers [67–69]. Because of the excellent anti-corrosion performance of ceramics, it is also considered to be one of the potential materials for the molten salt receiver. Researchers in Sandia National Laboratories (SNL) systematically evaluated the feasibility of the application of ceramic tubes made of silicon carbide (SiC) for chloride salt receivers [70,71]. It was found that the mass loss of the SiC tube in the chloride salt at 750 °C was about 90 times lower than that of Haynes 230. However, the mechanical shock resistance test showed that the ceramic tube cracked when it fell from a relatively low height. The fragileness of the ceramic tube makes it almost impractical to be applied in a real receiver. Compared with ceramic materials, ceramic-metal composites have a better mechanical performance. Caccia et al. [72] developed dense ZrC/W-based composites for heat exchangers in the next-generation CSP plant. The results showed that the ceramic-metal composite had a failure strength of 350 MPa at 800 °C. The techno-economic analysis indicated that ZrC/W-based composites had advantages over the nickel superalloy for the printed circuit heat exchanger. The feasibility of ceramic-metal composites for receiver materials should be further demonstrated.

2.2 Energy loss reduction of receiver

When the incident concentrated solar energy (Q_in) hits the solar receiver, part of the energy will be reflected to the ambient, and the rest will be absorbed by the coating and converted to thermal energy. The flowing molten salt inside the tube will absorb most of the thermal energy and the rest of the thermal energy will be lost to the environment by thermal radiation and convection, as depicted in Fig.3. Thus, the energy losses of the molten salt receiver are composed of the reflected solar energy loss (Q_ref), the radiative thermal loss (Q_rad), and the convective thermal loss (Q_conv). Equation (11) expresses the relationship between these energy losses.

(11)Q_{\text{in}}=Q_{\text{ref}}+Q_{\text{ab}}+Q_{\text{rad}}+Q_{\text{conv}},

where Q_in is the solar energy concentrated to the receiver, Q_ref is the reflected solar energy loss of the receiver, Q_ab is the thermal energy absorbed by the heat transfer fluid, Q_rad is the radiative thermal loss of the receiver, and Q_conv is the convective thermal loss of the receiver.

Fig.4 is a comparison of energy losses for the receiver operating at T_out = 565 °C and T_out = 720 °C. For T_out = 565 °C, a binary nitrate salt (NaNO₃ 60 wt.%, KNO₃ 40 wt.%) is adopted as the HTF, and at T_in = 290 °C. While for T_out = 720 °C, a binary chloride salt (MgCl₂ 32 mol.%, KCl 68 mol.%) [73] is used as the HTF, and T_in = 520 °C. Parameters of the heliostat field and the receiver are derived from the Solar Two plant [23,74]. The coating of the receiver is Pyromark2500, whose absorption and emissivity in the solar spectrum and infrared spectrum are 0.95 and 0.88, respectively. The heat flux of the receiver is calculated utilizing the Monte Carlo Ray Tracing (MCRT) method. Then, the heat flux distribution is adopted as the thermal boundary in a finite volume method (FVM) to simulate the heat transfer processes of the receiver. Together, the energy losses of the receiver can be obtained. A detailed description of the combination of MCRT and FVM can be found in Refs. [75,76]. It is observed from Fig.4 that when T_out increases from 565 to 720 °C, the receiver efficiency decreases from 88.2% to 81.9%, an absolute reduction of 6.3%. The proportion of the radiative thermal loss in the total incident solar energy increases from 5.4% to 11.3%, while that of the reflected solar energy loss and the convective thermal loss nearly remain unchanged. This indicates that the receiver efficiency is mainly reduced by the radiative thermal loss. It can also be found in Fig.4 that the radiative thermal loss and the reflected solar energy losses dominate the total energy losses of the receiver. Thus, reducing these two energy losses is of great importance to the improvement of receiver efficiency.

Current methods to reduce energy losses of the receiver can be classified into two types, as presented in Fig.5. The first is to reduce the energy losses directly from the source. This method is mainly focused on increasing the absorptance of the receiver in the solar spectrum and reducing the emittance of the receiver in the infrared spectrum. Thus, Q_rad and Q_ref can be directly reduced. The second is to reduce the energy losses during propagation. This method is mainly focused on the design of the receiver geometry and the development of transparent thermal insulation material. The redesigned receiver geometry can enable the reabsorption of the reflected solar energy and radiative thermal energy, while the transparent thermal insulation material can suppress the radiative and convective thermal loss.

2.2.1 Energy loss reduction from sources

Developing solar selective coatings (SSCs) that have a high absorption in the solar spectrum and a low emissivity in the infrared spectrum is the most effective way to reduce the reflected solar energy loss and radiative thermal loss directly. Currently, silicone based Pyromark2500 coating is widely used in the commercial SPT plant. Although Pyromark2500 coating has an excellent absorption of 0.95 in the solar spectrum, it also has a high emissivity of 0.88 in the infrared spectrum, which causes a high radiative thermal loss for the next-generation high-temperature solar receiver, as presented in Fig.4. Furthermore, as the operating temperature increases, degradation in optical performance will occur for the coating. Studies from SNL indicate that there is a significant decrease in solar absorption for Pyromark2500 coating when the temperature is higher than 750 °C [77], while the outside surface temperature of the next-generation solar receiver may exceed 770 °C and reach about 800 °C [30,78]. Moreover, Pyromark2500 coating may also suffer the delamination problem at a temperature higher than 750 °C [79]. Thus, it is necessary to develop new SSCs for next-generation CSP plants. There are five distinct SSCs according to the working mechanism [78,80,81], as shown in Fig.6. Fig.7 presents the schematic diagram of the receiver tube with SSC and anti-corrosion coating. SSC is used to absorb more solar energy and reduce the radiative thermal loss, while anti-corrosion coating is used to reduce the corrosion of HTF to the tube material. When the solar energy is concentrated onto the receiver, it will be first absorbed by the SSC and then converted to thermal energy. Then, the thermal energy is transferred to the HTF by conduction of the tube wall and anti-corrosion coating. The anti-corrosion coating can prevent direct contact between the corrosive HTF and the tube wall, which can reduce the corrosion rate.

The intrinsic coating is made of materials that have a nature of solar selective absorption, such as W, Si, Ge [82,83]. Intrinsic coatings are not directly used in CSP plants because the coating itself cannot provide a satisfying solar selective absorption performance. For the semiconductor-metal coating, the solar radiation in the spectrum range that has an energy larger than the bandgap energy of the semiconductor will be absorbed. The absorbed solar energy is in the short-wave range. The semiconductor layer is transparent for the unabsorbed energy in the long-wave range, which will be reflected by the underlying metal layer. Metal-dielectric multilayer coatings (MDMCs) are extensively studied in high-temperature SSCs, where the material in each layer can be metals, dielectrics, or metal-dielectric composites. MDMCs usually have satisfying optical characteristics because of the interface effect. The optical characteristics of metal-dielectric multilayer coatings are influenced by the material type and thickness of each layer. These parameters can be optimized by machine learning methods [84]. For ceramic-metal coating, metal nanoparticles are dispersed in dielectrics to enhance the solar absorption due to the surface plasmon resonance. Ceramic-metal coating usually has a good thermal stability at high temperatures. Micro-/nano-structure coating is also a promising SSC for the high-temperature solar receiver. It can enhance the absorption of solar energy because of the light-trapping effect [85]. The absorption of nanostructure coating can be regulated by controlling geometrical parameters. Li et al. [86] adopted Ni as the coating material and fabricated pyramid arrays on the surface, as shown in Fig.8. Its absorption and emissivity are 0.95 and 0.1, respectively. The coating was tested at 800 °C for more than 5 h in the vacuum atmosphere. It was found that the optical performance did not have a significant change, which meant that the coating could keep its nanostructures at 800 °C.

An effective idea for the design of nanostructured coatings with a good optical performance is presented in Fig.9. The refractory material with a low thermal emissivity is chosen first. Then, the nanostructure on its surface is designed to achieve a high absorption. Therefore, the coating will possess a low emissivity and a high absorption simultaneously. The optical performance including absorption and emissivity can be calculated by the finite-different time-domain (FDTD) method. Then, the influences of the coating on the optical-thermal performance of the receiver are evaluated. If the performance of the receiver fails to meet the requirement, the nanostructure will be redesigned by changing the shape or size.

In a previous study, a multi-scale model was developed to evaluate the optical-thermal performance of the receiver with a nano-structured SSC [78]. The framework of the model is presented in Fig.10. Based on this model, the influences of three typical nanostructured coatings on the optical-thermal performance of the receiver are evaluated. Fig.11 gives the schematic diagram of these nanostructured coatings. The results showed that the receiver with cone-structure coating had a maximum receiver efficiency larger than 88%. Compared with the current Pyromark2500 coating, the cone-structure coating can improve η_r by an absolute value of 6%–10%.

Tab.2 summarizes the structures, optical characteristics, and thermal stabilities of different SSCs. It can be seen that the temperature resistance of SSCs developed is usually tested in a vacuum or inert gas environment while the real solar receiver is directly exposed to the air environment. Moreover, studies on SSCs are mainly focused on their optical performance. The influences of SSCs on the optical-thermal performance of the receiver are not fully evaluated. It is suggested that the optical characteristics of SSCs should be tested in a similar environment to that of the real solar receiver and the influences of SSCs on the solar optical-thermal performance of the receiver should be also considered.

2.2.2 Energy loss reduction during propagation

For the current cylindrical receiver, the reflected solar energy and radiative thermal energy are directly lost to the environment. Reabsorbing these energy losses during propagation is another effective way to reduce the energy losses. To reabsorb these energy losses, the current cylindrical receiver geometry should be redesigned. Garbrecht et al. [91] designed a molten salt receiver with many hexagonal pyramid-shaped elements, as presented in Fig.12. The HTF inlets the element from the bottom center and reaches the top, then flows back through the outer hexagonal pyramid, as shown in Fig.12(b). When the concentrated solar radiation hits the receiver and is not absorbed, the reflected solar radiation may be reabsorbed by adjacent elements. The thermal radiation of each element can be also reabsorbed by adjacent elements, which will lead to the reduction of the radiative thermal loss. The CFD simulation shows that the receiver with a hexagonal pyramid shape has a reflected solar energy loss percentage of 1.3% and a radiative thermal loss percentage of 2.8%. The receiver with hexagonal pyramid-shaped elements is designed for external receivers in the beginning [92].

Inspired by the work of Garbrecht et al. [91,92], Slootweg et al. [93] introduced the hexagonal pyramid-shaped element to the cavity receiver, as presented in Fig.13. Elements are deployed at the bottom of the cavity receiver. The optical and thermal characteristics of the receiver were numerically investigated with a nitrate salt as the HTF in the PS-10 heliostat field. The results show that the receiver has a receiver efficiency of 82.39%. But the temperature of the element top exceeds 1100°C because of the bad heat transfer performance, which is too high for the actual application. To make this receiver design practical, the maximum temperature must be lowered, and receiver efficiency should also be improved.

A fin-like receiver design can also reabsorb the reflected solar energy losses by enabling the multi-reflection of sunrays between finned structures, as presented in Fig.14. The fin-like receiver structures can be originated from star receiver geometry [94]. The star receiver has several advantages compared with the cylindrical receiver. Following this idea, a series of studies on the performance of fin-like receivers were conducted by researchers at SNL [95–98]. The numerical simulation shows that the fin-like receiver can reduce the total heat loss by 50% compared with a flat receiver [95]. A mesoscale prototype of a receiver with horizontal fins was designed and fabricated, as presented in Fig.15. The optical and thermal characteristics of the receiver were experimentally studied at the National Solar Thermal Test Facility. Supercritical carbon dioxide (S-CO₂) was used as the HTF (15 MPa, 650 °C). The results indicate that the fin-like receiver can improve the receiver efficiency by 5%–7% [99].

In a previous study, five fin-like external receivers in the real plant scale were developed at the same tube weight [100], as shown in Fig.16. The same tube weight means that the material consumption of fin-like receivers and the cylindrical receiver are the same. The optical simulation in the Gemasolar heliostat field shows that the fin-like receiver with vertical fins can achieve the maximum optical efficiency, which increases the receiver’s optical efficiency by an absolute value of 3.2%. Although the fin-like receiver can increase the optical performance by reabsorbing the reflected solar energy, the same weight design also causes an increase in the exposed area because finned panels dissipate heat from two sides. Thus, the thermal performance of the fin-like receiver design with the same weight may have a lower receiver efficiency compared with the cylindrical receiver due to the larger exposed area. Moreover, the same weight fin-like receiver design also has much higher heat flux on its bottom panels. To solve this problem, a fin-like receiver design that has the same receiver diameter as the cylindrical receiver was further proposed [74]. A parameter sensitivity analysis of the fin-like receiver with the Solar Two heliostat field indicates that a fin number of 12 can achieve the maximum optical-thermal performance. Compared with the original cylindrical receiver of the Solar Two plant, the fin-like receiver with the same receiver diameter can improve receiver efficiency by 3.7% at a T_out of 720 °C. The conceptual design of the fin-like receiver with vertical finned structures is presented in Fig.17.

The combination of SSC and fin-like structures can further improve the optical-thermal performance of the receiver. Fig.18 gives the conceptual design of a multi-scale receiver for the next-generation CSP plant [14].

The macroscale finned structures can enable the multi-reflection of the incident solar energy to reduce the reflected solar energy loss. The mesoscale tubes have turbulators that enhance the heat transfer performance of the HTF flowing in the tube, which can increase the thermal energy absorbed by the HTF. The enhancement of heat transfer performance can also lower the surface temperature of the tube, leading to a decrease in the radiative and convective thermal loss. Moreover, the uneven outside surface of the tube can also increase the effective absorptance of the receiver by producing the macroscale light-trapping effect. The nano-scale SSC with high-temperature resistance has a low emittance and a regulable absorptance by adjusting nano-scale geometrical parameters. Together, the multi-scale structures can achieve a high receiver efficiency. In a previous study [101], the coupled optical-thermal performance of a multi-scale receiver with fin-like structures and nanostructured coatings was evaluated, as presented in Fig.19. The multi-scale receiver has an outlet temperature of 720 °C. It was found that the multi-scale receiver could achieve a receiver efficiency of more than 90% at noon of spring equinox.

Compared with the widely used external receiver, the cavity receiver can reduce the reflected solar energy loss because the receiver tube in the cavity can reabsorb the reflected solar energy. However, the current cavity receiver cannot be used for a surrounding heliostat field. To solve this problem, cavity receivers with multi apertures are proposed [102–104]. Fig.20 gives the schematic diagram of typical multi-aperture receivers. Li et al. [104] optimized the aperture numbers of a multi-aperture receiver that has a concentrator in each aperture, as shown in Fig.21. The results show that increasing the aperture number is good for thermal power but bad for optical efficiency. An aperture number of 4 is suitable for the balance between the thermal power and receiver efficiency. To enable the real application of the multi-aperture receiver, its technical and economic performance should be fully evaluated from the perspective of the whole CSP plant.

Developing transparent thermal insulation materials that can suppress the propagation of thermal radiation propagation is also conducive to improve receiver efficiency. Transparent aerogel is a typical thermal insulation material. Fig.22 shows the energy propagation processes of the receiver covered with the transparent aerogel. The aerogel is transparent in the solar spectrum, which means that most of the incident solar energy can go through the aerogel and be absorbed by the receiver. The aerogel is simultaneously opaque in the infrared spectrum, absorbing the thermal radiation and re-emitting it back to the receiver, which can reduce the radiative thermal loss [105]. Because of the low thermal conductivity, the surface temperature of the aerogel is much lower than that of the receiver, which can also reduce the convective thermal loss. Zhao et al. [106] developed a low scattering and highly transparent aerogel, as presented in Fig.23. The aerogel has a transmission of more than 95% in the solar spectrum. The solar receiver covered with the aerogel can harness heat higher than 200 °C without concentration and evaluation. Li et al. [107] evaluated the application of the transparent aerogel on the parabolic trough collector (PTC). The results indicate that the transparent aerogel can increase the receiver efficiency of PTC by 2.95%. To make the transmission spectrum of the transparent aerogel more suitable for the high-temperature receiver, some oxide nanoparticles can be added to the aerogel to enhance the absorption in the infrared spectrum due to the mechanism of plasmon-enhanced greenhouse selectivity. It was found that a little addition of indium tin oxide (ITO) nanoparticles could reduce the thermal loss by 50% at 700 °C [108]. Although the high-temperature receiver with a transparent aerogel cover has an excellent optical and thermal performance, its fragile nature limits its application in the real CSP plant. Further investigation should pay attention to the improvement of its mechanical performance.

2.3 Thermal properties and heat transfer correlations of chloride salts

The thermal properties of HTFs, including density, heat capacity, thermal conductivity, and viscosity, are essential for the flow and heat transfer performance evaluation of the solar receiver. These thermal properties are usually influenced by temperatures. Researchers have conducted lots of experimental and numerical studies on the acquisition of thermal properties of HTFs. Currently, experimental study is still the most effective and accurate way to obtain the thermal properties of molten salts and liquid metals. Tab.3–Tab.6 give the equations of density, heat capacity, thermal conductivity, and viscosity against temperatures for different chloride salts, respectively. These equations are fitted from experimental data. Moreover, they also give the thermal physical properties of the single salt because properties are necessary for some thermal property prediction methods of binary and ternary salts.

In the absence of experimental data, some prediction methods can also be used for the calculation of the thermal physical properties of molten salts. Li et al. [109] investigated the prediction accuracy of different prediction models by comparing the predicted and tested thermal physical properties of 16 binary and ternary chloride molten salts. The results indicate that for the density prediction of molten salts, the quasi-chemical model [112–114] can be used because its predicted deviation is less than ±5%. For the heat capacity prediction, the Dulong-Petit method and additive principal method can be used, whose prediction deviations are less than 11%. For the thermal conductivity and viscosity prediction, the rough-hard-sphere (RHS) model can be adopted. The RHS model predicts the thermal conductivity and viscosity of NaCl-KCl-ZnCl₂ salt with a deviation < 5%, but its viscosity prediction accuracy for the binary KCl-MgCl₂ salt is relatively low, around 28%. For future work, thermal physical properties prediction methods with high accuracy and simple form for the ternary NaCl-KCl-MgCl₂ system should be proposed. Moreover, some commercial software, such as FactSage and Material Studio, can also be used for the prediction of thermal physical properties [115,116].

For the thermal performance evaluation of the receiver, heat transfer correlations are also needed. Tab.7 listed typical heat transfer correlations for molten salts flowing in pipes. In the previous study [117], the accuracy of different heat transfer correlations in non-uniform heat flux was compared by using four representative molten salts. It was found that classic heat transfer correlations, including Hansen’s, Sider-Tate’s, and Gnielinski’s correlations had a maximum deviation of +25%, +13%, and −15%, respectively. It was also found that the classic friction correlation, Filonenko’s correlation, could well predict the friction factor of molten salts flowing in the tube, with a deviation of only ±2%. In the end, a more accurate heat transfer correlation was fitted, which had a deviation within ±5%. The fitted heat transfer correlation is presented in Eq. (12).

(12)\begin{array}{rl}& N{u}_{\text{salt}}=0.0154R{e}^{0.853}P{r}^{0.35}{\left(\frac{{\mu }_{\text{salt}}}{{\mu }_{\text{w}}}\right)}^{0.14},\\ & Re={10}^4-{10}^5,Pr=3.3-34,{\mu }_{\text{salt}}/{\mu }_{\text{w}}=1.01-1.31.\end{array}

3 Liquid metal receiver

Some liquid metals are also considered to be promising HTFs for the next-generation CSP plant due to their low melting point, high thermal conductivity, and operation temperature [121]. At present, the most promising liquid metals in solar power systems include sodium, lead-bismuth alloy, and tin [122–126]. Key issues and corresponding methods of these receivers are summarized in Fig.24.

3.1 Receiver based on liquid sodium

A liquid sodium receiver is also a promising high-temperature receiver candidate. In the SunShot project initiated by the Department of Energy of the United States, researchers made a comparison between the sodium receiver and the molten-salt receiver for the next-generation CSP plants [127,128]. It was found that the operational benefits of the sodium receiver are due to the superior thermophysical properties of sodium versus chloride salt. Therefore, in the liquid-phase pathway for the next-generation CSP of the SunShot project, sodium will be used as HTF of the solar receiver while chloride salt is adopted as the thermal storage media [128]. The utilization of liquid sodium in concentrating solar power systems can date back to the 1980s. Since the conception was proposed, there have been three significant on-sun tests for sodium receivers. A small sodium receiver with an area of 3.6 m² was constructed and tested by Rockwell International and SNL in the early 1980s [129]. Although this test proved the feasibility of sodium as HTF in CSP plants and gathered the experience of practical fabrication and operating, the test loop only operated for 75 h. Thus, there was not enough experience in dealing with or preventing the accidents caused by the sodium fire. A more famous on-sun test was conducted at Plataforma Solar de Almeria (PSA), in Spain, in 1981 [130]. One receiver of PSA had been running for over 500 days until a sodium spray fire accident occurred in 1986 [131]. Since then, further investigation especially experimental studies of sodium receivers slowed down due to safety concerns. As the technology of next-generation CSP develops, the application of sodium receivers comes back to the sight of scholars again [132]. In 2012, a new test loop with a sodium receiver started to be constructed by Vast Solar in Australia, and commissioned in 2017 [133]. Unfortunately, a sodium fire also occurred at the Vast Solar pilot plant during pre-commissioning tests in June 2015.

Up to the present, most tests on sodium receivers have failed because of the sodium fire, which indicates that the safety issue is the priority when it comes to the fabrication or operation of sodium receivers. The fire is mostly caused by the leakage of the sodium or the failure of the material, which is closely related to the compatibility between sodium and structural materials. Therefore, the main concerns about the safety issues include leakage prevention and compatibility with structural materials.

3.1.1 Leakage prevention and fire extinguishment

The sodium spray fire accident of PSA occurred when operators attempted to repair the valve when the storage tanks are still pressurized. About 12 m³ of sodium leaked through a weld, most of which burned in the fire [134]. The fire accident at the Vast Solar pilot plant also resulted from a sodium leakage from a flange.

All of the above accidents can be attributed to the reaction between sodium and water or air. Since sodium is a strong reducing agent, the contact of sodium with water can lead to a violent exothermic reaction and generate hydrogen [135]. Meanwhile, a secondary explosion may usually be caused by the contact of the released hydrogen with the oxygen in the air, under the tremendous heat produced by the reaction between sodium and water. As for the reaction between sodium and air, since the operating temperature of the sodium receiver exceeds 250 °C and may even reach 720°C in the next generation CSP, which is much higher than its autoignition temperature (115 °C), the exposure of the sodium in the air can easily lead to a sodium fire [134]. Therefore, a key rule to prevent sodium fire or explosion is to keep sodium completely isolated from water and air. Leakage prevention is the main concern when it comes to the fabrication, operation, and maintenance of the sodium receiver.

For the fabrication of the sodium receiver, the place prone to leakage, such as the pipe joint, needs all-welded connections. Meanwhile, welding lines must be designed with a high safety margin to ensure safety [136]. During the operation of the sodium receiver system, an inert cover gas is often used to fill system compartments and components. Fig.25 shows how the cover gas is used in the nuclear industry, which can provide experience for the CSP plant [137]. The cover-gas system should be operated at small positive pressures to avoid air leakage into the system. Normally, helium, argon, and nitrogen are the most acceptable candidates [138]. For leakage prevention during maintenance, freezing the sodium in a section of the tube can be a positive approach to supplement closed valves. The frozen-sodium plug can act as an efficient means of shutoff [136].

When accidents occur, fire extinguishment measures are paramount to minimize the loss and provide security. The core idea of fire extinguishment is insulating sodium from oxygen [139]. According to this idea, methods of extinguishing sodium fire can be divided into the passive method and the active method. In the passive method, by adding auxiliary structures at the place where the sodium is easy to spill, the leaked sodium can be diverted into a safe place and be isolated from the oxygen [140]. The common structures include smothering tanks, funneling floors, and so on [141]. In the active method, spraying flame retardant powder is one of the most useful methods. However, the common fire-extinguishing agents such as CO₂ and dry-chemical-foams are not suitable here, because hot sodium can react with these agents, which may cause explosions and fire. Sodium fires must be extinguished with class D dry chemical extinguishers including powdered graphite, granular sodium chloride, and copper-based powder.

To enable the application of the sodium receiver in a real CSP plant, its safety issues should be fully evaluated, and the industry standards should be further improved in the future. The application of liquid sodium in the nuclear industry has a long history of more than half a century, which has been well developed. Therefore, the experience gained by liquid metals in nuclear power systems can be applied to the next generation CSP. Any of the procedural recommendations from the accidents can be also summarized as standards practice for the present CSP industry practice.

3.1.2 Compatibility with structural materials

Besides leakage prevention and fire extinguishment, the compatibility of sodium with structural materials is another important safety issue [142]. The compatibility problem of structural materials is mainly caused by the corrosion of liquid sodium on these materials. The corrosion behavior can be divided into two main categories [125]. In the first category, the structural material might be dissolved in the liquid metal. In the other category, liquid metals can interact with structural materials and cause the embrittlement of structural materials, leading to the structural failure of components [143]. The corrosion mechanism of liquid sodium belongs to the first category, which means that the structural material will be dissolved in the liquid sodium. Tab.8 presents the corrosion rate of different alloys in liquid sodium at different temperatures [125].

In general, sodium is not aggressive to both ferritic and austenitic stainless steel, as well as Nickel alloys [144]. The major factor affecting the corrosion rates is the presence of oxygen. The corrosion rate of ferritic and austenitic stainless steels can be largely increased due to the formation of binary and ternary sodium oxides [145]. Compared with ferritic and austenitic stainless steels, Nickel alloys are less affected by diluted oxygen. NREL chose nickel alloys 740H as the structural material for the sodium receiver in the next generation CSP [128]. However, its corrosion rates and mechanical performance at different temperatures remain unknown.

Besides the material selection, the purification of sodium is also important for corrosion inhibition. In the nuclear industry, a method named ‘cold trap’ is widely used for sodium purification, which can be a reference for the application in the CSP plant. This method works according to the fact that the solubility of impurities in sodium varies at different temperatures. Thus, cooling the system to a certain temperature which is lower than the saturation temperature of impurities can allow impurities to crystallize, and precipitate out. Then the precipitated impurities can be separated from sodium. Fig.26 shows the schematic of the cold trap [146]. The crystallization reservoir is used to actively reduce the temperature of the sodium, thereby collecting the impurities on the stainless-steel wire mesh. However, studies indicate that impurities are only collected at the lowest temperature region, instead of the full wire mesh. This leads to the low efficiency of the cold trap, varying from 20%–60% [147]. To improve the cold trap efficiency, many new cold traps have been designed and operated. Hemanath et al. [147] set an isothermal zone in the cold trap to extend the resistance time for impurities to crystallize on the wire meshes. Onea et al. [148,149] designed and optimized the layout and the wire mesh dimension of the cold trap. Their designed cold trap was manufactured and installed in the KASOLA facility. Although many efforts have been made to increase the efficiencies of the cold trap, there is still room for efficiency improvements. To inhibit corrosion, the mass concentration of oxygen in liquid sodium should be controlled under 3 × 10^–6 [150]. Therefore, the real-time monitoring of the oxygen is also important to control its amount. However, the present monitorization methods like plugging meters (PMs), Blake meters, freezing point depression techniques, spectroscopy, and electrochemical are complex [146]. Thus, the research facilities are relatively few.

The above analysis indicates that future work and developments should focus on the tests regarding the compatibility with structural materials like 740H and other candidates. Moreover, efficient purification methods and impurities monitoring devices for the liquid sodium still need to be further investigated to control the impurities at an acceptable concentration.

3.2 Receiver based on lead-bismuth

Although sodium is regarded as the first choice among different liquid metals for the next generation CSP, the security problems mentioned above are huge barriers to its experimental investigation and large-scale application [151]. Compared with the sodium receiver, the receiver based on lead-bismuth eutectic (LBE) is a safer selection because LBE is chemically less reactive [152]. Research on the receiver based on LBE is still in the stage of conceptual design and small-scale experimental.

In 2015, a 10-kW_th small CSP system based on LBE was constructed by Karlsruhe Institute of Technology in Germany, named SOMMER, as shown in Fig.27 [153]. The main purpose of this project is to demonstrate the design method for the CSP plant based on liquid metal and gain operation experiences [154]. The on-sun tests in 2020 showed that the LBE receiver in the SOMMER system could work steadily under varying concentrated sunlight. The experimental results of the receiver demonstrated that the LBE receiver could operate at a high heat flux of up to 4 MW/m². The outlet temperature could reach 580 °C for a mass flow of 0.73 kg/s and an inlet temperature of 250 °C [153]. The thermal efficiency ranged from 90% to 93%, which was relatively higher than that of present molten-salt receivers. Although the outlet temperature of the receiver does not achieve the target of the next generation CSP, the successful operation of the LBE receiver is encouraging.

The operation results of SOMMER suggest that the performance of LBE is better than the molten salt due to its higher thermal conductivity. However, the pumping power in the LBE system is also larger than that in the molten salt system, which is a negative factor for the efficiency of the overall system. The reason for this is that the density of LBE (10139 kg/m³) is almost 5 times more than that of molten salts [155]. Mwesigye & Yılmaz [156] compared the thermal and thermodynamic performance of liquid sodium, LBE, and molten salts. The results showed that although LBE gave the second-highest heat transfer performance, its performance degraded as the flow rate exceeded a threshold, owing to the high pumping power. A high-pressure drop penalty for lead-bismuth largely offset its thermal performance gain over molten salt [157,158]. Therefore, under what circumstances is LBE better than the molten salt or other liquid metals needs to be further explored.

In addition to the heat transfer performance, the corrosion and containment of LBE poses another significant challenge since LBE is very corrosive to both nickel-based alloys and steels with a high nickel content, especially at high temperatures [159]. Therefore, the problem of compatibility with structure materials is also important [160]. The present protective methods can be divided into passive methods and active methods. In the active method, adding corrosion inhibitors is commonly used. The corrosion inhibitors like zirconium can interact with carbon and nitrogen and then form stable carbide and nitride films that prevent the dissolution of elements like Fe, Cr, and Ni in structural materials [161]. However, since zirconia films are easy to spall off leading to the contact between structure materials and LBE, it needs a constant supply of zirconium to reform the film. In addition, small pieces of the zirconia film may clog pipes. Thus, the use of zirconium is still technically challenging [162]. An easier active method is adding a suitable amount of oxygen. LBE is chemically less reactive with oxygen than sodium, while the oxygen can help form oxide layers that prevent the dissolution [163]. In passive methods, developing protective layers to avoid the exposure of structural materials to LBE is an important way. For example, using surface aluminized steels forming Fe-Cr-Al compounds can effectively improve the corrosion resistance of the structural materials [164,165]. It also can be observed that adding an appropriate amount of W or Y element can form oxide layers, thereby enhancing the corrosion resistance ability of the structure materials [166,167]. Although studies about protective layers are mostly still in the laboratory, they have shown the potential to be an efficient approach to resist the corrosion of LBE.

From the above discussion, it can be concluded that for the LBE receiver, future work should focus on the performance evaluation of CSP plants with LBE receivers at a system level, and the improvement of compatibility between LBE and candidate structure materials. The corrosion inhibition methods need to be further investigated for the durability of LBE receivers.

3.3 Receiver based on liquid tin

Similar to LBE, liquid tin is also regarded as a potential candidate for the next generation CSP due to its low safety risks and large temperature range (232–2687 °C). The application of liquid tin in the glass industry has gained some experience [168–170], while the operating experience and the knowledge of thermal performance in power systems are still lacking compared with sodium and LBE. Studies on the receiver based on liquid tin are in the stage of feasibility investigation, especially the selection of suitable containment materials.

Although liquid tin is chemically inert like LBE, it is more corrosive than LBE, especially at high temperatures. Even at a temperature of 600 °C, which is much lower than the boiling point (2687 °C) of tin, the corrosion rate with unprotected steel is prohibitively high (> 250 μm/a) [125]. The recommended containment materials are limited to several candidates. The graphite, refractories, quartz glass, tungsten, and rhenium are regarded as suitable containment materials [163]. Moreover, ceramics, such as silicon carbide (SiC) and mullite (Al₆Si₂O₁₃), are also confirmed to serve as effective containment materials for liquid tin [171,172]. Amy et al. [173] designed and fabricated a ceramic pump based on a machinable aluminum-nitride-rich composite. The results show that the ceramic pump can be used to continuously circulate liquid tin at temperatures of around 1473–1673 K. The encouraging work demonstrates that the solar power generation at an extremely high temperature up to 1400 °C may be realized in the future with liquid tin as the HTF.

To achieve the engineering application of liquid tin in the next-generation CSP, the most important work should focus on the development of advanced materials which are compatible with liquid tin. Meanwhile, the design and the thermal performance of the receiver based on liquid tin need to be further investigated.

3.4 Thermal properties and heat transfer correlations of liquid metals

The thermal properties of the above liquid metals are summarized in Tab.9–Tab.12. The melting point of sodium, lead-bismuth, and tin are 98, 125, and 232 °C, respectively. For heat transfer correlations of liquid metals, liquid sodium and liquid lead-bismuth are extensively studied, while the heat transfer correlation for liquid tin is rarely reported. The heat transfer correlations for liquid sodium and liquid bismuth are presented in Tab.13.

4 Summary and conclusions

In this paper, state-of-the-art liquid-based solar receiver technologies for the next-generation concentrating solar power (CSP) are comprehensively reviewed. The conclusions are summarized as follows.

(1) Potential heat transfer fluids (HTFs) for > 700°C liquid-based receivers include molten chloride salts (mixture of MgCl₂, KCl, and NaCl), liquid sodium, liquid lead-bismuth, and liquid tin. Of these candidate HTFs, molten chloride salts and liquid sodium are the most promising ones for next-generation CSP plants. Key challenges for liquid-based receivers include strong corrosivity of the molten chloride salt on metal materials, significant receiver efficiency reduction at high temperatures, and the risk of sodium fire and material compatibility.

(2) The combination of salt purification and anti-corrosion receiver materials is a promising way to deal with the corrosive problems of chloride salts. For salt purification, the stepwise thermal treatment and addition of Mg are cost-effective ways of reducing the corrosion of chloride salts. The purification effect of the thermal treatment is directly influenced by heating temperatures and dwelling times. Moreover, the excessive addition of Mg may cause the abrasion of pumps. For anti-corrosion receiver materials, high-nickel alloys are potential receiver materials for the next-generation CSP plant due to their excellent anti-corrosion performance at high temperatures. However, current high-nickel alloys cannot satisfy the goal of corrosion rate <10 μm/a in the chloride salt. Apart from high-nickel alloys, some other anti-corrosion materials, including ceramics and cermet, are receiving more and more attention. Future studies should be focused on the optimization of heating temperatures and dwelling times for the stepwise thermal treatment method, the determination of the minimum amount of Mg added in different chloride salts, the corrosion characteristics of more high-nickel alloys in chloride salts, the cost-effective processing methods for high-nickel alloys, and the development of cermet materials with a good anti-corrosion and mechanical performance.

(3) The receiver efficiency at high temperatures can be improved by reducing the energy losses of the receiver from sources and during propagation. For the energy loss reduction from sources, the most effective way is to develop solar selective coatings (SSCs) that simultaneously have a high absorption in the solar spectrum and a low emissivity in the infrared spectrum. For the energy loss reduction during propagation, designing receivers that can reabsorb the reflected solar energy, and using transparent aerogels with low thermal conductivity are promising ways to improve the receiver efficiency. The following studies are suggested in the future: developing SSCs that can work at more than 800 °C in the air, evaluating the technical and economic performance of different novel receivers, and improving the mechanical performance of the aerogel with high transparency in the solar spectrum.

(4) The application of liquid metals can be realized by resolving the sodium fire and material compatibility issues. For the liquid sodium and liquid lead-bismuth, the existing projects have demonstrated their feasibility to be used in the next generation CSP, while the application of liquid tin needs to be confirmed. The future study should focus on improving safety measures of liquid sodium receivers, especially for sodium fire, evaluating the thermal performance of the CSP plant with the liquid lead-bismuth receiver at a system level, and developing advanced materials compatible with liquid metals and improving the current anti-corrosion methods.

References

[1] Li M J, Zhu H H, Guo J Q. . The development technology and applications of supercritical CO₂ power cycle in nuclear energy, solar energy and other energy industries. Applied Thermal Engineering, 2017, 126: 255–275

[2] Heywood H. Solar energy: a challenge to the future. Nature, 1957, 180(4577): 115–118

[3] Lewis N S. Toward cost-effective solar energy use. Science, 2007, 315(5813): 798–801

[4] Kraemer D, Jie Q, McEnaney K. . Concentrating solar thermoelectric generators with a peak efficiency of 7.4%. Nature Energy, 2016, 1(11): 16153

[5] He Y L, Wang K, Qiu Y. . Review of the solar flux distribution in concentrated solar power: Non-uniform features, challenges, and solutions. Applied Thermal Engineering, 2019, 149: 448–474

[6] Al-Ashouri A, Köhnen E, Li B. . Monolithic perovskite/silicon tandem solar cell with > 29% efficiency by enhanced hole extraction. Science, 2020, 370(6522): 1300–1309

[7] NREL. Life cycle greenhouse gas emissions from concentrating solar power. Technical Report, National Renewable Energy Laboratories, 2012

[8] Khamlich I, Zeng K, Flamant G. . Technical and economic assessment of thermal energy storage in concentrated solar power plants within a spot electricity market. Renewable & Sustainable Energy Reviews, 2020, 139: 110583

[9] Merchán R, Santos M, Medina A. . High temperature central tower plants for concentrated solar power: 2021 overview. Renewable & Sustainable Energy Reviews, 2021, 155: 111828

[10] Lilliestam J, Labordena M, Patt A. . Empirically observed learning rates for concentrating solar power and their responses to regime change. Nature Energy, 2017, 2: 17094

[11] Pitz-Paal R. Concentrating solar power: Still small but learning fast. Nature Energy, 2017, 2(7): 17095

[12] Wang K, He Y L. Thermodynamic analysis and optimization of a molten salt solar power tower integrated with a recompression supercritical CO₂ Brayton cycle based on integrated modeling. Energy Conversion and Management, 2017, 135: 336–350

[13] Qiu Y, Li M J, He Y L. . Thermal performance analysis of a parabolic trough solar collector using supercritical CO₂ as heat transfer fluid under non-uniform solar flux. Applied Thermal Engineering, 2017, 115: 1255–1265

[14] He Y L, Qiu Y, Wang K. . Perspective of concentrating solar power. Energy, 2020, 198: 117373

[15] Guo J Q, Li M J, He Y L. . A systematic review of supercritical carbon dioxide (S-CO₂) power cycle for energy industries: Technologies, key issues, and potential prospects. Energy Conversion and Management, 2022, 258: 115437

[16] GauchePShultzAStappD, . US DOE Gen3 and SunShot 2030 Concentrating Solar Power R&D: In search of $0.05/kWh autonomy and seasonal storage. Technical Report, Sandia National Laboratories, 2019

[17] Wang K, Li M J, Zhang Z D. . Evaluation of alternative eutectic salt as heat transfer fluid for solar power tower coupling a supercritical CO₂ Brayton cycle from the viewpoint of system-level analysis. Journal of Cleaner Production, 2021, 279: 123472

[18] Ho C K, Iverson B D. Review of high-temperature central receiver designs for concentrating solar power. Renewable & Sustainable Energy Reviews, 2014, 29: 835–846

[19] Wang W Q, Jiang R, He Y L. . Optical-thermal-mechanical analysis of high-temperature receiver integrated with gradually sparse biomimetic heliostat field layouts for the next-generation solar power tower. Solar Energy, 2022, 232: 35–51

[20] Jiang R, Li M J, Wang W Q. . A new methodology of thermal performance improvement and numerical analysis of free-falling particle receiver. Solar Energy, 2021, 230: 1141–1155

[21] Du S, Li M J, Ren Q L. . Pore-scale numerical simulation of fully coupled heat transfer process in porous volumetric solar receiver. Energy, 2017, 140: 1267–1275

[22] BurgaletaJ IAriasSRamirezD. Gemasolar, the first tower thermosolar commercial plant with molten salt storage. In:17th SolarPACES Conference, Granada, Spain, 2011

[23] PachecoJ EBradshawR WDawsonD B, . Final test and evaluation results from the solar two project. Technical Report, Sandia National Laboratories, 2022

[24] Turchi C S, Vidal J, Bauer M. Molten salt power towers operating at 600–650 °C: Salt selection and cost benefits. Solar Energy, 2018, 164: 38–46

[25] Pérez-Álvarez R, González-Gómez P Á, Santana D. . Preheating of solar power tower receiver tubes for a high-temperature chloride molten salt. Applied Thermal Engineering, 2022, 216: 119097

[26] Wetzel T, Pacio J, Marocco L D. . Liquid metal technology for concentrated solar power systems: Contributions by the German research program. AIMS Energy, 2014, 2(1): 89–98

[27] Zhang Q, Cao D, Ge Z. . Response characteristics of external receiver for concentrated solar power to disturbance during operation. Applied Energy, 2020, 278: 115709

[28] Gomez-Vidal J C, Tirawat R. Corrosion of alloys in a chloride molten salt (NaCl-LiCl) for solar thermal technologies. Solar Energy Materials and Solar Cells, 2016, 157: 234–244

[29] Keny S, Gupta V, Kumbhar A G. . Corrosion tests of various alloys in fluorides of lithium, sodium and potassium (FLiNaK) medium for molten salt reactors in the temperature range of 550–750 °C using electrochemical techniques. Indian Journal of Chemical Technology, 2019, 26(1): 84–88

[30] MehosMTurchiCVidalJ, . Concentrating solar power Gen3 demonstration roadmap. Technical Report, National Renewable Energy Laboratories, 2017

[31] Fernández A G, Gomez-Vidal J, Oró E. . Mainstreaming commercial CSP systems: A technology review. Renewable Energy, 2019, 140: 152–176

[32] Wermac. Effects & economic impact of corrosion. 2022-1-19, available at website of Wermac

[33] Wang K, He Y L, Zhu H H. Integration between supercritical CO₂ Brayton cycles and molten salt solar power towers: A review and a comprehensive comparison of different cycle layouts. Applied Energy, 2017, 195: 819–836

[34] WangXXuXElsentriecyH, . Investigation of properties of KCl-MgCl₂ eutectic salt for heat transfer and thermal storage fluids in CSP systems. In: ASME Heat Transfer Summer Conference, Bellevue, USA, 2017

[35] Vidal J C, Klammer N. Molten chloride technology pathway to meet the US DOE sunshot initiative with Gen3 CSP. AIP Conference Proceedings, 2019, 2126(1): 080006

[36] Ding W, Bauer T. Progress in research and development of molten chloride salt technology for next generation concentrated solar power plants. Engineering (Beijing), 2021, 7(3): 334–347

[37] D’Souza B, Zhuo W, Yang Q. . Impurity driven corrosion behavior of HAYNES® 230® alloy in molten chloride salt. Corrosion Science, 2021, 187: 109483

[38] Vignarooban K, Xu X, Wang K. . Vapor pressure and corrosivity of ternary metal-chloride molten-salt based heat transfer fluids for use in concentrating solar power systems. Applied Energy, 2015, 159: 206–213

[39] Ong T C, Sarvghad M, Lippiatt K. . Review of the solubility, monitoring, and purification of impurities in molten salts for energy storage in concentrated solar power plants. Renewable & Sustainable Energy Reviews, 2020, 131: 110006

[40] Cho H S, Van Zee J, Shimpalee S. . Dimensionless analysis for predicting Fe-Ni-Cr alloy corrosion in molten salt systems for concentrated solar power systems. Corrosion, 2016, 72(6): 742–760

[41] Garcia-Diaz B L, Olson L, Martinez-Rodriguez M. . High temperature electrochemical engineering and clean energy systems. Journal of the South Carolina Academy of Science, 2016, 14(1): 11–14

[42] Sun H, Wang J Q, Tang Z. . Assessment of effects of Mg treatment on corrosivity of molten NaCl-KCl-MgCl₂ salt with Raman and infrared spectra. Corrosion Science, 2020, 164: 108350

[43] Ding W, Gomez-Vidal J, Bonk A. . Molten chloride salts for next generation CSP plants: electrolytical salt purification for reducing corrosive impurity level. Solar Energy Materials and Solar Cells, 2019, 199: 8–15

[44] Zhang Z, Lu X, Yan Y. . The dehydration of MgCl₂· 6H₂O by inhibition of hydrolysis and conversion of hydrolysate. Journal of Analytical and Applied Pyrolysis, 2019, 138: 114–119

[45] Kipouros G J, Sadoway D R. A thermochemical analysis of the production of anhydrous MgCl₂. Journal of Light Metals, 2001, 1(2): 111–117

[46] Kurley J M, Halstenberg P W, McAlister A. . Enabling chloride salts for thermal energy storage: Implications of salt purity. RSC Advances, 2019, 9(44): 25602–25608

[47] Chen G S, Sun I W, Sienerth K D. . Removal of oxide impurities from alkali haloaluminate melts using carbon tetrachloride. Journal of the Electrochemical Society, 1993, 140(6): 1523–1526

[48] ZhaoY. Molten chloride thermophysical properties, chemical optimization, and purification. Technical Report, National Renewable Energy Laboratories, 2020

[49] de Bakker J, Peacey J, Davis B. Thermal decomposition studies on magnesium hydroxychlorides. Canadian Metallurgical Quarterly, 2012, 51(4): 419–423

[50] Fernández A G, Cabeza L F. Corrosion evaluation of eutectic chloride molten salt for new generation of CSP plants. Part 1: thermal treatment assessment. Journal of Energy Storage, 2020, 27: 101125

[51] Kashani-Nejad S, Ng K, Harris R. Preparation of MgOHCl by controlled dehydration of MgCl₂·6H₂O. Metallurgical and Materials Transactions B, Process Metallurgy and Materials Processing Science, 2004, 35(2): 405–406

[52] Kipouros G J, Sadoway D R. The chemistry and electrochemistry of magnesium production. Advances in Molten Salt Chemistry, 1987, 6: 127–209

[53] Ding W, Shi H, Jianu A. . Molten chloride salts for next generation concentrated solar power plants: Mitigation strategies against corrosion of structural materials. Solar Energy Materials and Solar Cells, 2019, 193: 298–313

[54] Zhao Y, Klammer N, Vidal J. Purification strategy and effect of impurities on corrosivity of dehydrated carnallite for thermal solar applications. RSC Advances, 2019, 9(71): 41664–41671

[55] Zhao Y, Vidal J. Potential scalability of a cost-effective purification method for MgCl₂-containing salts for next-generation concentrating solar power technologies. Solar Energy Materials and Solar Cells, 2020, 215: 110663

[56] AlkhamisM. Stability of metals in molten chloride salt at 800 °C. Dissertation for the Master’s Degree. Tucson: The University of Arizona, 2016

[57] StoddardLAndrewDAdamsS, . Molten salt: Concept definition and capital cost estimate. Technical Report, Allegheny Science & Technology Corporation, 2016

[58] HuangS YMortzheimJSamarovV, . Low cost HIP fabrication of advanced power cycle components and PM/Wrought inconel 740H weld development—Final technical report. Technical Report, GE Research, 2021

[59] Shingledecker J, de Barbadillo J, O’Donnell D. . Materials improvements for improved economy of high-temperature components in future Gen3 CSP systems. AIP Conference Proceedings, 2019, 2126(1): 020004

[60] Gomez-Vidal J C, Fernandez A, Tirawat R. . Corrosion resistance of alumina forming alloys against molten chlorides for energy production. II: Electrochemical impedance spectroscopy under thermal cycling conditions. Solar Energy Materials and Solar Cells, 2017, 166: 234–245

[61] Ding W, Shi H, Xiu Y. . Hot corrosion behavior of commercial alloys in thermal energy storage material of molten MgCl₂/KCl/NaCl under inert atmosphere. Solar Energy Materials and Solar Cells, 2018, 184: 22–30

[62] Tristancho-Reyes J, Chacón-Nava J, Peña-Ballesteros D. . Hot corrosion behaviour of NiCrFeNbMoTiAl coating in molten salts at 700 °C by electrochemical techniques. International Journal of Electrochemical Science, 2011, 6: 432–441

[63] Fernández A G, Cabeza L F. Anodic protection assessment using alumina-forming alloys in chloride molten salt for CSP plants. Coatings, 2020, 10(2): 138

[64] Gomez-Vidal J C. Corrosion resistance of MCrAlX coatings in a molten chloride for thermal storage in concentrating solar power applications. npj Materials Degradation, 2017, 1(1): 1–9

[65] Ding W, Bonk A, Bauer T. Molten chloride salts for next generation CSP plants: selection of promising chloride salts & study on corrosion of alloys in molten chloride salts. AIP Conference Proceedings, 2019, 2126(1): 200014

[66] Gomez-Vidal J, Fernandez A, Tirawat R. . Corrosion resistance of alumina-forming alloys against molten chlorides for energy production. I: pre-oxidation treatment and isothermal corrosion tests. Solar Energy Materials and Solar Cells, 2017, 166: 222–233

[67] Chavez J M, Chaza C. Testing of a porous ceramic absorber for a volumetric air receiver. Solar Energy Materials, 1991, 24(1−4): 172–181

[68] Patil V R, Kiener F, Grylka A. . Experimental testing of a solar air cavity-receiver with reticulated porous ceramic absorbers for thermal processing at above 1000 °C. Solar Energy, 2021, 214: 72–85

[69] Barreto G, Canhoto P, Collares-Pereira M. Parametric analysis and optimisation of porous volumetric solar receivers made of open-cell SiC ceramic foam. Energy, 2020, 200: 117476

[70] WalkerMArmijoK MYellowhairJ, . High temperature silicon carbide receiver tubes for concentrating solar power. Technical Report, Sandia National Laboratories, 2019

[71] Armijo K M, Walker M, Christian J. . Thermal shock resistance of multilayer silicon carbide receiver tubes for 800 °C molten salt concentrating solar power application. AIP Conference Proceedings, 2020, 2303(1): 150004

[72] Caccia M, Tabandeh-Khorshid M, Itskos G. . Ceramic–metal composites for heat exchangers in concentrated solar power plants. Nature, 2018, 562(7727): 406–409

[73] Xu X, Wang X, Li P. . Experimental test of properties of KCl–MgCl₂ eutectic molten salt for heat transfer and thermal storage fluid in concentrated solar power systems. Journal of Solar Energy Engineering, 2018, 140(5): 051011

[74] Wang W Q, Qiu Y, Li M J. . Coupled optical and thermal performance of a fin-like molten salt receiver for the next-generation solar power tower. Applied Energy, 2020, 272: 115079

[75] He Y L, Xiao J, Cheng Z D. . A MCRT and FVM coupled simulation method for energy conversion process in parabolic trough solar collector. Renewable Energy, 2011, 36(3): 976–985

[76] Qiu Y, He Y L, Li P W. . A comprehensive model for analysis of real-time optical performance of a solar power tower with a multi-tube cavity receiver. Applied Energy, 2017, 185: 589–603

[77] HoC KMahoneyA RAmbrosiniA, . Characterization of Pyromark 2500 for high-temperature solar receivers. In: ASME International Conference on Energy Sustainability, San Diego, CA, USA, 2012

[78] Wang W Q, Li M J, Jiang R. . Receiver with light-trapping nanostructured coating: A possible way to achieve high-efficiency solar thermal conversion for the next-generation concentrating solar power. Renewable Energy, 2022, 185: 159–171

[79] Coventry J, Burge P. Optical properties of Pyromark 2500 coatings of variable thicknesses on a range of materials for concentrating solar thermal applications. AIP Conference Proceedings, 2017, 1850(1): 030012

[80] Zhang K, Hao L, Du M. . A review on thermal stability and high temperature induced ageing mechanisms of solar absorber coatings. Renewable & Sustainable Energy Reviews, 2017, 67: 1282–1299

[81] Xu K, Du M, Hao L. . A review of high-temperature selective absorbing coatings for solar thermal applications. Journal of Materiomics, 2020, 6(1): 167–182

[82] Shah A A, Ungaro C, Gupta M C. High temperature spectral selective coatings for solar thermal systems by laser sintering. Solar Energy Materials and Solar Cells, 2015, 134: 209–214

[83] Dan A, Barshilia H C, Chattopadhyay K. . Solar energy absorption mediated by surface plasma polaritons in spectrally selective dielectric-metal-dielectric coatings: A critical review. Renewable & Sustainable Energy Reviews, 2017, 79: 1050–1077

[84] Zhang W, Wang B, Zhao C. Selective thermophotovoltaic emitter with a periodic multilayer structures designed by machine learning. ACS Applied Energy Materials, 2021, 4(2): 2004–2013

[85] Barshilia H C, Kumar P, Rajam K. . Structure and optical properties of Ag–Al₂O₃ nanocermet solar selective coatings prepared using unbalanced magnetron sputtering. Solar Energy Materials and Solar Cells, 2011, 95(7): 1707–1715

[86] Li P, Liu B, Ni Y. . Large-scale nanophotonic solar selective absorbers for high-efficiency solar thermal energy conversion. Advanced Materials, 2015, 27(31): 4585–4591

[87] Yang J, Shen H, Yang Z. . Air-stability improvement of solar selective absorbers based on TiW–SiO₂ cermet up to 800 °C. ACS Applied Materials & Interfaces, 2021, 13(12): 14587–14598

[88] Wang X, Lee E, Xu C. . High-efficiency, air-stable manganese–iron oxide nanoparticle-pigmented solar selective absorber coatings toward concentrating solar power systems operating at 750 °C. Materials Today. Energy, 2021, 19: 100609

[89] Li Y, Lin C, Wu Z. . Solution-processed all-ceramic plasmonic metamaterials for efficient solar–thermal conversion over 100–727 °C. Advanced Materials, 2021, 33(1): 2005074

[90] Chirumamilla A, Yang Y, Salazar M H. . Spectrally selective emitters based on 3D Mo nanopillars for thermophotovoltaic energy harvesting. Materials Today Physics, 2021, 21: 100503

[91] Garbrecht O, Al-Sibai F, Kneer R. . CFD-simulation of a new receiver design for a molten salt solar power tower. Solar Energy, 2013, 90: 94–106

[92] GarbrechtOAl-SibaiFKneerR, . Numerical investigation of a new molten salt central receiver design. In: 18th SolarPACES Conference, Marrakech, Morocco, 2012

[93] Slootweg M, Craig K, Meyer J P. A computational approach to simulate the optical and thermal performance of a novel complex geometry solar tower molten salt cavity receiver. Solar Energy, 2019, 187: 13–29

[94] FriefieldJFriedmanJ. Technical report No. 1: Solar thermal power systems baded on optical transmission. Technical Report, Rocketdyne Division, Rockwell International, 1974

[95] Ho C K, Christian J M, Ortega J D. . Reduction of radiative heat losses for solar thermal receivers. High and Low Concentrator Systems for Solar Energy Applications IX, 2014, 9175: 917506

[96] Yellowhair J, Ho C K, Ortega J D. . Testing and optical modeling of novel concentrating solar receiver geometries to increase light trapping and effective solar absorptance. High and Low Concentrator Systems for Solar Energy Applications X, 2015, 9559: 95590A

[97] ChristianJ MOrtegaJ DHoC K, . Design and modeling of light-trapping tubular receiver panels. In: ASME International Conference on Energy Sustainability, Charlotte, NC, USA, 2016

[98] HoC KOrtegaJ DChristianJ M, . Fractal-like materials design with optimized radiative properties for high-efficiency solar energy conversion. Technical Report, Sandia National Laboratories, 2016

[99] OrtegaJ DChristianJ MHoC K. Design and testing of a novel bladed receiver. In: ASME International Conference on Energy Sustainability, Charlotte, USA, 2017

[100] Wang W Q, Qiu Y, Li M J. . Optical efficiency improvement of solar power tower by employing and optimizing novel fin-like receivers. Energy Conversion and Management, 2019, 184: 219–234

[101] Wang W Q, He Y L, Jiang R. A multi-scale solar receiver with peak receiver efficiency over 90% at 720 °C for the next-generation solar power tower. Renewable Energy, 2022, 200: 714–723

[102] WilliamBStineM G. Power from the sun. 2022-5-2, available at website of Power from the Sun book

[103] Schmitz M, Schwarzbözl P, Buck R. . Assessment of the potential improvement due to multiple-apertures in central receiver systems with secondary concentrators. Solar Energy, 2006, 80(1): 111–120

[104] Li L, Wang B, Pye J. . Optical analysis of a multi-aperture solar central receiver system for high-temperature concentrating solar applications. Optics Express, 2020, 28(25): 37654–37668

[105] McEnaney K, Weinstein L, Kraemer D. . Aerogel-based solar thermal receivers. Nano Energy, 2017, 40: 180–186

[106] Zhao L, Bhatia B, Yang S. . Harnessing heat beyond 200°C from unconcentrated sunlight with nonevacuated transparent aerogels. ACS Nano, 2019, 13(7): 7508–7516

[107] Li Q, Zhang Y, Wen Z X. . An evacuated receiver partially insulated by a solar transparent aerogel for parabolic trough collector. Energy Conversion and Management, 2020, 214: 112911

[108] Berquist Z J, Turaczy K K, Lenert A. Plasmon-enhanced greenhouse selectivity for high-temperature solar thermal energy conversion. ACS Nano, 2020, 14(10): 12605–12613

[109] Li Y, Xu X, Wang X. . Survey and evaluation of equations for thermophysical properties of binary/ternary eutectic salts from NaCl, KCl, MgCl₂, CaCl₂, ZnCl₂ for heat transfer and thermal storage fluids in CSP. Solar Energy, 2017, 152: 57–79

[110] Li C J, Li P W, Wang K. . Survey of properties of key single and mixture halide salts for potential application as high temperature heat transfer fluids for concentrated solar thermal power systems. AIMS Energy, 2014, 2(2): 133–157

[111] Wang X, Rincon J D, Li P. . Thermophysical properties experimentally tested for NaCl-KCl-MgCl₂ eutectic molten salt as a next-generation high-temperature heat transfer fluids in concentrated solar power systems. Journal of Solar Energy Engineering, 2021, 143(4): 041005

[112] Robelin C, Chartrand P, Eriksson G. A density model for multicomponent liquids based on the modified quasichemical model: Application to the NaCl-KCl-MgCl₂-CaCl₂ system. Metallurgical and Materials Transactions. B, Process Metallurgy and Materials Processing Science, 2007, 38(6): 869–879

[113] Robelin C, Chartrand P. A density model based on the modified quasichemical model and applied to the NaF-AlF₃-CaF₂-Al₂O₃ electrolyte. Metallurgical and Materials Transactions. B, Process Metallurgy and Materials Processing Science, 2007, 38(6): 881–892

[114] Ouzilleau P, Robelin C, Chartrand P. A density model based on the modified quasichemical model and applied to the (NaCl+KCl+ZnCl₂) liquid. Journal of Chemical Thermodynamics, 2012, 47: 171–176

[115] Villada C, Ding W, Bonk A. . Engineering molten MgCl₂–KCl–NaCl salt for high-temperature thermal energy storage: Review on salt properties and corrosion control strategies. Solar Energy Materials and Solar Cells, 2021, 232: 111344

[116] Yu Y S, Tao Y B, He Y L. Molecular dynamics simulation of thermophysical properties of NaCl-SiO₂ based molten salt composite phase change materials. Applied Thermal Engineering, 2020, 166: 114628

[117] Qiu Y, Li M J, Li M J. . Numerical and experimental study on heat transfer and flow features of representative molten salts for energy applications in turbulent tube flow. International Journal of Heat and Mass Transfer, 2019, 135: 732–745

[118] Martinek J, Jape S, Turchi C S. Evaluation of external tubular configurations for a high-temperature chloride molten salt solar receiver operating above 700 °C. Solar Energy, 2021, 222: 115–128

[119] Wang Q, Huang J, Shen Z. . Negative thermal-flux phenomenon and regional solar absorbing coating improvement strategy for the next-generation solar power tower. Energy Conversion and Management, 2021, 247: 114756

[120] Xu L, Stein W, Kim J S. . Three-dimensional transient numerical model for the thermal performance of the solar receiver. Renewable Energy, 2018, 120: 550–566

[121] Pacio J, Singer C, Wetzel T. . Thermodynamic evaluation of liquid metals as heat transfer fluids in concentrated solar power plants. Applied Thermal Engineering, 2013, 60(1–2): 295–302

[122] Benoit H, Spreafico L, Gauthier D. . Review of heat transfer fluids in tube-receivers used in concentrating solar thermal systems: Properties and heat transfer coefficients. Renewable & Sustainable Energy Reviews, 2016, 55: 298–315

[123] Fritsch A, Frantz C, Uhlig R. Techno-economic analysis of solar thermal power plants using liquid sodium as heat transfer fluid. Solar Energy, 2019, 177: 155–162

[124] Lipiński W, Abbasi-Shavazi E, Chen J. . Progress in heat transfer research for high-temperature solar thermal applications. Applied Thermal Engineering, 2021, 184: 116137

[125] Pacio J, Wetzel T. Assessment of liquid metal technology status and research paths for their use as efficient heat transfer fluids in solar central receiver systems. Solar Energy, 2013, 93: 11–22

[126] Flesch J, Niedermeier K, Fritsch A. . Liquid metals for solar power systems. IOP Conference Series. Materials Science and Engineering, 2017, 228(1): 012012

[127] Turchi C S, Libby C, Pye J. . Molten salt vs. liquid sodium receiver selection using the analytic hierarchy process. AIP Conference Proceedings, 2022, 2445: 110016

[128] TurchiCGageSMartinekJ, . CSP Gen3: Liquid-phase pathway to SunShot. Renewable Energy Laboratories Technical Report, 2021

[129] SNL. Final report sodium solar receiver experiment. Sandia National Laboratories Technical Report, 1982

[130] KesselringPSelvageC S. The IEA/SSPS solar thermal power plants volume1: Central receiver system. Springer, Berlin, Germany, 1986

[131] CasalF G. Solar thermal power plants: achievements and lessons learned exemplified by the SSPS project in Almeria/Spain. Springer Science & Business Media, 2012

[132] Heinzel A, Hering W, Konys J. . Liquid metals as efficient high-temperature heat-transport fluids. Energy Technology (Weinheim), 2017, 5(7): 1026–1036

[133] BartosNFisherJWantA. Experiences from using molten sodium metal as heat transfer fluid in concentrating solar thermal power systems. Proceedings of Asia-Pacific Solar Research Conference, Brisbane, Australia, 2015

[134] Coventry J, Andraka C, Pye J. . A review of sodium receiver technologies for central receiver solar power plants. Solar Energy, 2015, 122: 749–762

[135] Deguchi Y, Muranaka R, Kamimoto T. . Reaction path and product analysis of sodium-water chemical reactions using laser diagnostics. Applied Thermal Engineering, 2017, 114: 1319–1324

[136] Armijo K M, Andraka C E. Phenomenological studies on sodium for CSP applications: A safety review. AIP Conference Proceedings, 2016, 1734(1): 040001

[137] Guo Q, Chen Z, Mao L. . Preliminary source term and consequence assessment of primary cover gas leakage accident for CLEAR-I. Progress in Nuclear Energy, 2015, 78: 136–140

[138] BraidTHarperHWilsonR. Operation of cover-gas system during SLSF tests. Argonne National Laboratories Technical Report, 1982

[139] Nur K, Laurent B, Thierry G. . The role of powder physicochemical properties on the extinction performance of an extinguishing powder for sodium fires. Nuclear Engineering and Design, 2019, 346: 24–34

[140] Chikazawa Y, Katoh A, Yamamoto T. . Secondary sodium fire measures in JSFR. Nuclear Technology, 2016, 196(1): 61–73

[141] MaletJ. Ignition and combustion of sodium, fire consequences, extinguishment and prevention. In: International Atomic Energy Agency, International Working Group on Fast Reactors, Vienna, Austria, 1996

[142] Sarvghad M, Delkasar Maher S, Collard D. . Materials compatibility for the next generation of concentrated solar power plants. Energy Storage Materials, 2018, 14: 179–198

[143] LaiG Y. High-temperature corrosion and materials applications. In: ASM international, Ohio, US, 2007

[144] Conroy T, Collins M N, Grimes R. A review of steady-state thermal and mechanical modelling on tubular solar receivers. Renewable & Sustainable Energy Reviews, 2020, 119: 109591

[145] Zhang J, Kapernick R. Oxygen chemistry in liquid sodium–potassium systems. Progress in Nuclear Energy, 2009, 51(4−5): 614–623

[146] Mangus D, Napora A, Briggs S. . Design and demonstration of a laboratory-scale oxygen controlled liquid sodium facility. Nuclear Engineering and Design, 2021, 378: 111093

[147] Hemanath M, Meikandamurthy C, Kumar A A. . Theoretical and experimental performance analysis for cold trap design. Nuclear Engineering and Design, 2010, 240(10): 2737–2744

[148] Onea A, Hering W, Lux M. . Numerical optimization of cold trap designs for the Karlsruhe Sodium Laboratory. International Journal of Heat and Mass Transfer, 2017, 113: 984–999

[149] Onea A, Lux M, Hering W. . Design, optimization and layout of a compact cold trap with high efficient heat recovery by a helical coil for the Karlsruhe Sodium Laboratory. International Journal of Heat and Mass Transfer, 2017, 113: 1000–1011

[150] YvonP. Structural Materials for Generation IV Nuclear Reactors. Woodhead Publishing, 2016

[151] Hering W, Onea A, Jianu A. . Liquid metals, materials and safety measures to progress to CSP 2.0. AIP Conference Proceedings, 2019, 2126(1): 080002

[152] Deng Y, Jiang Y, Liu J. Liquid metal technology in solar power generation-basics and applications. Solar Energy Materials and Solar Cells, 2021, 222: 110925

[153] Müller-Trefzer F, Niedermeier K, Fellmoser F. . Experimental results from a high heat flux solar furnace with a molten metal-cooled receiver SOMMER. Solar Energy, 2021, 221: 176–184

[154] Flesch J, Fritsch A, Cammi G. . Construction of a test facility for demonstration of a liquid lead-bismuth-cooled 10 kW thermal receiver in a solar furnace arrangement-SOMMER. Energy Procedia, 2015, 69: 1259–1268

[155] Alchagirov B B, Shamparov T M, Mozgovoi A G. Experimental investigation of the density of molten lead–bismuth eutectic. High Temperature, 2003, 41(2): 210–215

[156] Mwesigye A, Yılmaz İ H. Thermal and thermodynamic benchmarking of liquid heat transfer fluids in a high concentration ratio parabolic trough solar collector system. Journal of Molecular Liquids, 2020, 319: 114151

[157] Conroy T, Collins M N, Fisher J. . Thermohydraulic analysis of single phase heat transfer fluids in CSP solar receivers. Renewable Energy, 2018, 129: 150–167

[158] Flesch J, Marocco L, Fritsch A. . Entropy generation minimization analysis of solar salt, sodium, and lead–bismuth eutectic as high temperature heat transfer fluids. Journal of Heat Transfer, 2020, 142(4): 042103

[159] Ho C K. Advances in central receivers for concentrating solar applications. Solar Energy, 2017, 152: 38–56

[160] Zhang J. A review of steel corrosion by liquid lead and lead–bismuth. Corrosion Science, 2009, 51(6): 1207–1227

[161] Ilinčev G. Research results on the corrosion effects of liquid heavy metals Pb, Bi and Pb–Bi on structural materials with and without corrosion inhibitors. Nuclear Engineering and Design, 2002, 217(1−2): 167–177

[162] Frazer D, Stergar E, Cionea C. . Liquid metal as a heat transport fluid for thermal solar power applications. Energy Procedia, 2014, 49: 627–636

[163] Lorenzin N, Abanades A. A review on the application of liquid metals as heat transfer fluid in Concentrated Solar Power technologies. International Journal of Hydrogen Energy, 2016, 41(17): 6990–6995

[164] Weisenburger A, Müller G, Heinzel A. . Corrosion, Al containing corrosion barriers and mechanical properties of steels foreseen as structural materials in liquid lead alloy cooled nuclear systems. Nuclear Engineering and Design, 2011, 241(5): 1329–1334

[165] Shi H, Jianu A, Weisenburger A. . Corrosion resistance and microstructural stability of austenitic Fe–Cr–Al–Ni model alloys exposed to oxygen-containing molten lead. Journal of Nuclear Materials, 2019, 524: 177–190

[166] Wei X, Jin J, Jiang Z. . FeCrMoWCBY metallic glass with high corrosion resistance in molten lead–bismuth eutectic alloy. Corrosion Science, 2021, 190: 109688

[167] Fetzer R, Weisenburger A, Jianu A. . Oxide scale formation of modified FeCrAl coatings exposed to liquid lead. Corrosion Science, 2012, 55: 213–218

[168] Ban N, Kamihori T, Takamuku H. A study of the behavior of volatiles in the float process. Journal of Non-Crystalline Solids, 2004, (345−346): 777–781

[169] Li L Y, Lin H J, Han J J. . Influence of spout lip set-height on flow behavior during the glass float process. Journal of Non-Crystalline Solids, 2017, 472: 46–54

[170] Shou P, Hongcan R, Xin C. . Continuous forming of ultrathin glass by float process. International Journal of Applied Glass Science, 2019, 10(3): 275–286

[171] DeAngelis F, Seyf H R, Berman R. . Design of a high temperature (1350 °C) solar receiver based on a liquid metal heat transfer fluid: Sensitivity analysis. Solar Energy, 2018, 164: 200–209

[172] Zhang Y, Cai Y, Hwang S. . Containment materials for liquid tin at 1350 °C as a heat transfer fluid for high temperature concentrated solar power. Solar Energy, 2018, 164: 47–57

[173] Amy C, Budenstein D, Bagepalli M. . Pumping liquid metal at high temperatures up to 1673 Kelvin. Nature, 2017, 550(7675): 199–203

[174] FinkJLeibowitzL. Thermodynamic and transport properties of sodium liquid and vapor. Argonne National Laboratories Technical Report, 1995

[175] Sobolev V. Thermophysical properties of lead and lead–bismuth eutectic. Journal of Nuclear Materials, 2007, 362(2–3): 235–247

[176] Assael M J, Kalyva A E, Antoniadis K D. . Reference data for the density and viscosity of liquid copper and liquid tin. Journal of Physical and Chemical Reference Data, 2010, 39(3): 033105

[177] Humrickhouse P W. An equation of state and compendium of thermophysical properties of liquid tin, a prospective plasma-facing material. IEEE Transactions on Plasma Science, 2019, 47(7): 3374–3379

[178] Chapman T W. The heat capacity of liquid metals. Materials Science and Engineering, 1966, 1(1): 65–69

[179] Savchenko I V, Stankus S V, Agadjanov A S. Measurement of liquid tin heat transfer coefficients within the temperature range of 506–1170 K. High Temperature, 2011, 49(4): 506–511

RIGHTS & PERMISSIONS

Higher Education Press