A brief review of liquid heat transfer materials used in concentrated solar power systems and thermal energy storage devices of concentrated solar power systems

Solar power generation is an effective approach to promote the achievement of carbon neutrality. Heat transfer materials (HTMs) are important for concentrated solar power (CSP) systems and their accessary thermal energy storage (TES) devices. The performances of HTMs can influence the operation behaviors of CSP systems and TES devices. On the whole, the HTMs can be divided into three categories according to their physical states, which are the liquid, gaseous, and solid HTMs, respectively. This article presents a brief review of research works on liquid HTMs used in CSP systems and TES devices of CSP systems, mainly including different kinds of molten salts, heat transfer oils, nanofluids, liquid metals, and water. Some typical practical applications, experimental and simulation studies on different kinds of liquid HTMs used in CSP and TES systems are introduced, and the advantages, disadvantages, and further research and development work prospects of different kinds of HTMs are briefly summarized. This article aims to providing a reference for researchers in solar thermal power field.

presents the potential pathways for the CSP Generation III technology, 9 which was proposed by the National Renewable Energy Laboratory of the US. When a CSP system operates, the solar concentrator (or solar mirror field) will reflect the incident solar radiation to the solar receiver. Then the heat transfer material (HTM) flowing in the solar receiver will absorb the concentrated solar radiation through the receiver wall and then get a temperature increase. After that, the high-temperature HTM will transfer heat to the working fluid in the heat exchanger and augment the temperature and pressure of the working fluid. Then the high-temperature and high-pressure working fluid drives the turbine to generate electric power. This is the working principle of a typical CSP system. As the final energy source of CSP systems is sunlight, CSP systems are subject to the periodic timeliness of solar energy. The solar radiation intensity is low during cloudy and rainy weather, and there is no solar radiation during the night. These conditions will lead to the operating unstability of CSP systems. The thermal energy storage (TES) system can solve this problem to a certain degree as it can provide heat for CSP systems when the solar radiation is insufficient. Hence, the TES devices are very important for CSP systems as they can ensure the long-time stable solar power output.
HTMs are important for CSP systems and their accessary TES devices. The performances of HTMs can impact the operation behaviors of CSP systems and TES devices. On the whole, the HTMs can be divided into three categories, which are the liquid, gaseous, and solid HTMs, respectively. The liquid HTMs are now the most used and studied ones for CSP systems and mainly include water, heat transfer oil, molten salt, liquid metal, and nanofluid, in which the technologies related to heat conduction oil, molten salt and water are currently more mature. The solid HTM is mainly the granular flow material. The gaseous HTM for CSP systems mainly refers to air, but in general, the relevant publicly reported studies are few compared with the liquid and solid HTMs.
Due to the rapid developments of CSP and TES technologies as well as the importance of HTMs for CSP and TES systems, a review of liquid HTMs used in CSP and TES systems will be meaningful. This article will presents a brief review of research works on liquid HTMs used in CSP systems and TES devices of CSP systems, mainly including different kinds of molten salts, heat transfer oils, nanofluids, liquid metals, and water. Typical applications, experimental and simulation F I G U R E 2 Potential pathways for the CSP Generation III technology. Source: Reproduced from Reference 9, with permission from National Renewable Energy Laboratory studies on different kinds of liquid HTMs in CSP and TES systems are introduced, aiming to providing a reference for researchers in solar thermal power field.

MOLTEN SALTS
Some inorganic molten salts have advantages of high thermal stability, high specific heat capacity, high convective heat transfer coefficient, low viscosity. and low saturated vapor pressure, which make them good HTMs for CSP systems. Table 1 presents the melting and boiling (or decomposition) temperatures of some molten salts. 10 According to the difference of anions, molten salts can be mainly divided into the nitrate, carbonate, chloride, fluoride. and sulfate salts. One disadvantage of molten salt materials is that molten salt pipeline blockage is easy to occur due to their high melting temperatures. In addition, some simple substance molten salt materials have relatively low boiling temperatures, making them easy to decompose. Different kinds of multi-component molten salts are developed to extend the operating temperature ranges of molten salts 11,12 (see Table 2). But the multi-component molten salts with low melting temperature as well as high decomposition temperature are still being explored. Currently, the decomposition phenomenon of some common multi-component molten salts at high temperature still cannot be avoided. For instance, when the multiple nitrate salt temperature reaches about 811 K or higher, the nitrite slowly converts to the nitrate, or the nitrate decomposes into the nitrite and O − free radicals. 13

Nitrate salts
The most iconic multi-component molten salt developed for solar thermal power generation technology is the Solar Salt (60% NaNO 3 -40% KNO 3 ), which has been used in many CSP plants (e.g., the Solar Two, Gemasolar, and Cresent Dunes). Its melting and decomposition temperatures are 493 and 858 K, respectively. In order to improve the performances of nitrate molten salts for CSP systems and make them more responsive to engineering needs, in recent years, researchers have launched their works in several aspects of molten salts, which mainly focus on reducing the melting temperature, increasing the decomposition temperature, extending the operating temperature range, augmenting the specific heat capacity and thermal conductivity, improving the stability, and decreasing the corrosiveness by inserting different additives. Many kinds of new nitrate salts were developed, for instance, the Hitec (53% KNO 3 -7% NaNO 3 -40% NaNO 2 ). Zou et al. 14 developed a kind of quaternary nitrate salt with the melting temperature of 357 K and decomposition temperature of 901 K. They studied the thermal properties and corrosiveness of the quaternary nitrate salt by using the experimental method. Zhao and Wu 15 carried out the thermal property measurements of the ternary nitrate salt comprised by LiNO 3 , Ca(NO 3 ) 2 , and KNO 3 . Mantha et al. 16 conducted the melting temperature, decomposition temperature, and stability tests of the quaternary nitrate salt consisting of LiNO 3 , NaNO 3 , KNO 3 , and NaNO 2 . Xu et al. 17 extended the operating temperature range of the ternary nitrate salt (LiNO 3 -KNO 3 -Ca(NO 3 ) 2 ) by inserting NaCl. The results show that the melting temperature of the molten salt was decreased by 7.9 K (from 390.2 to 382.3 K) and the stable upper temperature limit was increased by 50 K (from 723 to 773 K).
Hu et al. 18 conducted an experimental study on enhancing the specific heat capacity of binary nitrate salt by inserting Al 2 O 3 nanoparticles. The results indicate that the maximum specific heat capacity increase was 8.3% when the nanoparticle concentration of the molten salt was 2%. Zhang et al. 19 studied the effect of salt purification on the thermal properties of the binary nitrate salt (KNO 3 -NaNO 3 ) and found that the purified binary nitrate salt had a lower melting temperature, a higher phase change latent heat and a higher stable upper temperature limit. Chieruzzi et al. 20 carried out a study on improving the heat capacity of NaNO 3 -KNO 3 salt by using different nanoparticles under different concentration conditions. Figure 3   There are also many corrosion studies on nitrate salts. For instance, Zhai 23 launched corrosion behavior experiments on 304, 316L, 321H, P91, and 12CrMoVG materials in the KNO 3 -NaNO 3 salt at 500 • C. The results show that 316L, 304, and 321H stainless steel materials had relatively better corrosion resistance, followed by P91 and 12CrMoVG materials orderly. The surface morphologies and EDS X-ray test results of 316L steel are presented in Figure 4. Fernandez et al. 24 studied the corrosion behaviors of low-Cr steel, 304 and 430 and stainless steels in 60%NaNO 3 -40%KNO 3 salt at 390 and 550 • C, respectively. The results reveal that the corrosion resistance of stainless steels was better than that of the low-Cr steel at 550 • C. Gomes et al. 25 conducted the corrosion experiments of 316L and 321H stainless steels in 60%NaNO 3 -40%KNO 3 salt at 550 • C. The corrosion rates and corrosion mechanism of the two stainless steel materials in the nitrate salt were revealed.

Carbonate salts
Common simple substance carbonate salts are K 2 CO 3 , Li 2 CO 3 , and Na 2 CO 3 . They and their mixture are suitable for high-temperature heat transfer and heat storage systems due to their high decomposition temperatures (see Table 1). But as they also have high melting temperatures and large viscosities, the pipeline blockage is easy to happen when they are used as HTMs for CSP systems. Hence, even for high-temperature CSP systems, the development works of low-viscosity carbonate salt materials are still in need. For published studies on carbonate salts, a large part of them are about the tests, 26,27 improvements, [28][29][30][31][32][33] and predictions 34-37 of their thermal properties. Araki et al. 26 measured the thermal diffusivities, specific heat capacities and densities F I G U R E 4 Surface morphologies and EDS X-ray test results of 316L steel in the KNO 3 -NaNO 3 salt at 500 • C. Source: Reproduced from Reference 23, with permission from Nanchang Hangkong University of different mixtures comprised by K 2 CO 3 and Li 2 CO 3 . The thermal conductivities of different mixtures were calculated according to the measurement results. Shin and Tiznobaik 28,29 carried out a series of experiments on improving the thermal properties of the binary carbonate salt (K 2 CO 3 -Li 2 CO 3 ) by inserting different SiO 2 nanoparticles. Sang and Liu 31 studied the enhancement effects of SiO 2 , CuO, TiO 2 , and Al 2 O 3 on the specific heat capacity of the ternary carbonate salt (K 2 CO 3 -Li 2 CO 3 -Na 2 CO 3 ) by using the experimental method. The results show that the enhancement effect of SiO 2 on the specific heat capacity of ternary carbonate salt was the best compared with the other three nanoparticle materials. Gheribi et al. 34 proposed a model for predicting the thermal conductivities of molten alkali and alkaline earth salts. By using this model, the thermal conductivities of K 2 CO 3 , Li 2 CO 3 , Na 2 CO 3 , and many other kinds of molten salt materials were calculated. Du and Ding [35][36][37] predicted the thermal properties of K 2 CO 3 and Na 2 CO 3 by using the molecular dynamics simulation method.

Chloride salts
There are many kinds of chloride molten salts. They are easy to obtain and their prices are generally very cheap. Myers and Goswami 38 provided extensive data support for the use of chloride molten salts for CSP and TES systems. It has been revealed that chloride salts can be used as HTMs or heat storage materials in high-temperature CSP and TES systems. Some chloride salts have big phase change latent heat, good thermal stability, low melt viscosity and relatively wide operating temperature range, but their melting temperatures are usually high. For instance, the melting temperatures of NaCl, KCl, CaCl 2 , and LiCl are 801, 770, 782, and 605 • C, respectively. The thermal properties of several common chloride salts are shown in Table 3. 39 To meet the requirements of solar thermal power generation, searching multi-component molten salt with good performances (especially the low melting temperature) and determining the right ratio are the one of the important research tasks of chloride molten salts. He 40 prepared 18 kinds of binary chloride salts and 36 kinds of ternary chloride salts and measured the melting temperatures and phase change latent heat of different multi-component chloride salts. The results show that the melting temperatures of eight kinds of binary chloride salts were all close to 340 • C, and those of 29 kinds of ternary chloride salts were all close to 350 • C. For all the prepared binary chloride salts, the salt of 50%KCl-50%LiCl had the maximum phase change latent heat. And for the prepared ternary chloride salts, 10%NaCl-50%KCl-40%LiCl had the maximum phase change latent heat. Wu et al. 41 studied the thermal properties of KCl-LiCl-NaCl salts with different KCl concentrations by using the molecular dynamics simulation method. The predication results and experimental data showed relatively good agreement. Li et al. 42 conducted a study on the melting and boiling temperatures of three chloride salt systems, which were AlCl 3 -KCl-NaCl, ZnCl 2 -NaCl-KCl, and FeCl 3 -KCl-NaCl. The results show that the melting temperature of 58.75%AlCl 3 -15%KCl-26.25%NaCl salt could be 91 • C. But all the three kinds of chloride salt systems will boil when their temperatures were higher than 600 • C under 1 atm.

TA B L E 3
In addition, the solution of strong corrosiveness of chloride salts used in CSP systems is another research focus. Table 4 presents the corrosion rates of different metal materials in three kinds of multi-component chloride salt systems (i.e., NaCl-KCl-MgCl 2 , NaCl-CaCl 2 -MgCl 2 , and NaCl-LiCl) under different environmental and temperature conditions. One suggested solution method for the corrosion problem of chloride salts is using the firebrick as the internal insulation in the pipelines and TES tanks of CSP systems. 9 Some studies on the corrosion problems of chloride salts have also been conducted. For instance, Liu et al. 43 studied the corrosion behaviors of In625 alloy and 316L stainless steel in CaCl 2 -MgCl 2 -NaCl salt at different salt temperatures. The results reveal that In625 alloy had better corrosion resistance compared with 316L stainless steel when they were both in the salt of CaCl 2 -MgCl 2 -NaCl. Some relevant experimental results are shown in Figure 5. 43 Vignarooban et al. 44 investigated the corrosion behaviors of 304 stainless steel, Hastelloy C-267 and Hastelloy C-22 in NaCl-KCl-ZnCl 2 salt. The results show that C-276 had the best corrosion resistance. The corrosion rate of C-276 was 10 μm/year in the ternary chloride salt in the absence of air at 800 • C.

Fluoride salts
Fluoride salts are mainly fluorides of alkali or alkaline earth metals and insoluble fluorides of some other metals. 45 They usually have high melting temperatures and large heat of fusion. They are high-temperature HTMs and TES materials and are often used in high-temperature fields. Table 5 presents the thermal properties of some multi-component fluoride salts. 46 When fluoride salts are used as high-temperature HTMs and TES materials, there are some disadvantages. For instance, the fluoride salts have relatively large volume shrinkages when they change from the liquid state to the solid state. In addition, the thermal conductivities of fluoride salts are relatively low. Currently, the applications and relevant studies of fluoride salts used as HTMs in CSP systems are relatively few. But fluoride salts have been well applied in some other fields, such as the metallurgy, electrolysis and nuclear power industries. 47,48 The typical advantages and disadvantages of different kinds of molten salts are briefly summarized in Table 6. In this article, according to the operating temperature of HTM, the CSP systems are divided into low-(<200 • C), medium-(200-800 • C), and high-temperature (>800 • C) CSP systems. Currently, the nitrate salt technology is the most mature TA B L E 5 Thermal properties of some multi-component fluoride salts

HEAT TRANSFER OILS
Heat transfer oils are also known as heat carrier oils, which are traditional HTMs. They have advantages of good heat transfer effect, big operating temperature range, strong antioxidant activity and very low volatility. Compared with molten salt HTMs, heat transfer oils usually have relatively lower corrosiveness. Heat transfer oils are economical and practical, and the probability of fire or explosion is extremely low under normal operation when they are used in different fields. For the use in solar power field, heat transfer oils are usually used in parabolic trough and linear Fresnel reflector CSP systems. 49,50 According to the chemical compositions, oils can be divided into two mineral and synthetic oils. The source of mineral heat transfer oils is mainly the petroleum. Mineral heat transfer oils can be obtained by the fractional distillation of heavy oil and are mainly comprised by naphthenic and paraffin mixtures. As these compounds have long chemical bonds and are straight-chain structures, they are prone to breakages of chemical bonds. Therefore, the mineral oils obtained by the fractionation method usually have relatively low stability, and their working temperatures are often less than 573 K. In CSP systems, the main advantages of using mineral heat transfer oils as heat carriers are their high safety, low cost, and low product toxicity. But under some extreme conditions (e.g., the very low temperature environment), the viscosity of the heat transfer oil will weaken its fluidity, block the pipeline, or even deteriorate the oil, causing safety accidents. Table 7 presents several typical mineral oils. 51 Different from mineral oils, synthetic oils are mainly symmetric alkyl aromatic compounds with benzene rings. Their production methods include the synthesis, separation and purification of chemical raw materials. Synthetic oils usually have high initial boiling point and short distillation range, and their molecular bond structures are generally complete conjugated structures. Hence, they have better stabilities, higher thermal conductivities, lower viscosities and higher enthalpies. As shown in Figure 6, currently, synthetic oils are mainly biphenyl compounds, including , methylnaphthalene (C 11 H 10 ), and so forth. Table 8 shows some typical synthetic oils. 51 For studies on heat transfer oils used in CSP systems, many works focused on the thermal property improvement of oils by inserting nanoparticles. [52][53][54][55][56][57] For instance, Liu et al. 54 prepared the synthetic aromatic hydrocarbon oil with SiO 2 nanoparticles and tested the viscosity variation of the oil-based nanofluid under different conditions. Saeedinia et al. 55,56 carried out the experimental study on CuO-based oil nanofluid. The specific heat capacities, thermal conductivities and viscosities of oil nanofluid with different nanoparticle concentrations were measured. The effect of CuO nanoparticles on improving the thermal conductivity of heat transfer oil was revealed. Pakdaman et al. 57 studied the thermal properties of multi-walled carbon nanotube heat transfer oil flow inside vertical helically coiled tubes, which provided a potential method for thermal performance improvement of heat transfer oils.
In summary, heat transfer oils have the advantages of strong fluidity, low freezing point, good heat transfer performance and low corrosiveness, and have the disadvantages of high cost, short service life, low applicable temperature, flammability, and explosion hazard. They can serve as the HTMs for medium-temperature CSP systems. The further research and development (R&D) works of heat transfer oils used in CSP and TES systems mainly focus on the enhancement of thermal performances of oil materials.

NANOFLUIDS
With the development of nanoparticle material preparation technology, different nanocomposites have been used in many fields. [58][59][60][61][62][63][64] For CSP systems, nanofluids are rapidly gaining attention as a substance considered as breaking the high temperature limitations of traditional heat transfer fluids. Nanofluids can be prepared by dispersing nanoparticles in solvent materials (e.g., water, molten salt, alcohol, and oil). Currently, the common types of nanofluids include molten salt-based nanofluids, oil-based nanofluids and hydrophilic-based nanofluids. Among them, hydrophilic-based nanofluids are rarely introduced in CSP systems because they cannot meet the high-temperature working requirements of solar power. Nanofluids generally have good thermal conduction performances. The Brownian motion of the nanoparticles in the nanofluid and the compressive liquid layer at the solid-liquid interface are the main reasons for the improvement of the thermal conductivity of the nanofluid. Commonly used nanoparticles include various types of metal or non-metal oxide nanoparticles, including CuO, ZnO, TiO 2 , SiO 2 , various carbon nanotubes, graphene, other mixed metal oxides, and so forth. Some researchers added various types of nanoparticles to molten salts to form molten salt-based nanofluids to optimize the thermal properties of molten salts. Zhang et al. 65 conducted an experimental study on the heat transfer characteristics of quaternary mixed nitrate salt with SiO 2 nanoparticles. The results show that the heat transfer performance of molten salt-based nanofluids was better than that of the pure molten salt, and its convective heat transfer coefficient and Nusselt number were 22.34% and 11.42% higher than those of the pure molten salt, respectively. El-Sayed et al. 66 carried out experiments on extending the operating temperature range of the binary nitrate salt by adding nanodiamonds. Qiao et al. 67 studied the effect of SiO 2 nanoparticle on enhancing the specific heat capacity of KNO 3 molten salt. Jiang et al. 68 investigated the influences of SiO 2 nanoparticle on the specific heat capacities of binary, ternary and quaternary nitrate salts. The results show that the addition ratio of 1% was the optimal addition ratio for all the three kinds of nitrate salts, which can increase the specific heat capacities of the three kinds of nitrate salts by 6.65%-15.99%.
In addition to molten salt-based nanofluids, there are also research works on oil-based nanofluids in the field of solar thermal power generation as mentioned above. For instance, Torres et al. 69 added CuO nanoparticles to the heat transfer oil suitable for the CSP system and tested the stability and thermal properties of the composite material. The results show that the composite material prepared with the lowest energy value was the most stable, and the heat transfer coefficient was increased by about 10%. Abutaleb and Imran 70 mixed CuO nanoflakes with mineral oil, sunflower oil and regular oil, and carried out some relevant studies. The results show that when the concentration of CuO nanoflakes was 0.46 vol%, the thermal conductivities of the three kinds of oils were increased by 15.73%, 20.68%, and 16.14%. Yang et al. 71 tested the thermal stability performance of oil-based CuO nanofluid for CSP systems. The maximum operating temperature and stability under cyclic heating of the oil-based CuO nanofluid were investigated.
In general, the published research works on nanofluid HTMs used in CSP and TES systems mainly include the determination of stability and the study of influencing factors of nanofluids, improvements of thermal conductivity and specific heat capacity of nanofluids, operating temperature range extension of nanofluids, study on influencing factors and model of rheology of nanofluids, and so forth. The preparation of nanofluids with good dispersibility and high stability is the premise and basis for the experimental study of the thermal properties and enhancement of heat transfer performance of nanofluids. But the preparations of different kinds of nanofluids for CSP and TES systems are still in the experimental stage. The mass production of nanofluid HTMs for practical solar thermal power generation engineering remains to be further investigated.

LIQUID METALS
Liquid metals were used as HTMs in nuclear fast reactors at early stages due to their good thermal characteristics. [72][73][74] Typical liquid metals include sodium, sodium potassium alloy, lead, lead-bismuth eutectic (LBE) alloy, and so forth. These materials can have relatively wider operating temperature ranges with low melting temperature as well as very high boiling temperature. Currently, most commercial CSP plants employ the steam Rankine cycle, whose requirements can be met by molten salts or heat transfer oils. For the next generation CSP systems, the working fluids with higher operating temperatures (e.g., >600 • C) will be needed to increase the energy conversion efficiency. In that case, HTMs with higher stable upper operating temperature limit are necessary. Liquid metals are potential alternatives to molten salts and heat transfer oils used in the next generation CSP systems. 75

Sodium
Liquid sodium is a kind of active metal with certain dangers. But benefiting from the rich experience in the nuclear industry, the safety and reliability of sodium as the HTM in CSP systems have been greatly improved. When sodium is used as the HTM in CSP systems, it has many advantages. 76,77 For instance, wide operating temperature range, low melting temperature, high boiling temperature, and high thermal conductivity. In addition, higher solar receiving efficiency can be obtained, wall overheating can be partly avoided, and radiation and convection heat losses can be reduced. Certainly, sodium also has some disadvantages when it serves as the HTM in CSP systems, 78 such as the high corrosiveness under high-temperature condition, relatively high cost, safety challenges due to high chemical activity. In 1980s, the experimental studies of liquid sodium as the HTM in solar power generation have been launched. A 3.6 m 2 sodium receiver experimental facility named CRTF was built by Rockwell International and the US Department of Energy in Albuquerque, New Mexico. 79 A central receiver system using liquid sodium as the HTM was established at the Plataforma Solar de Almeria (PSA), Spain. 80 Although the early research works were interrupted for a time due to the fire accident caused by the leakage of liquid sodium, the research results still show that using liquid sodium as the HTM has great advantages in improving the heat collection efficiency. So far, many experimental and demonstration CPS systems using liquid sodium as the HTM have been built. Figure 7 presents the 1 MW liquid sodium central receiver CSP plant of Vast Solar, which is constructed in New South Wales, Australia.
The research directions of liquid sodium used in CSP systems include the thermodynamic estimate of liquid sodium serving as the HTM in CSP systems, 81 improvement of flow and heat transfer characteristics of liquid sodium in the heat pipes 82,83 and pool boilers 84,85 of parabolic dish CSP systems, applications of liquid sodium in central receiver CSP systems, [86][87][88] and so forth. Coventry et al. 89 carried out a review of liquid sodium HTM technologies for CSP plants, which presented many typical R&D works of liquid sodium used in parabolic dish and central tower CSP systems.

Lead-bismuth eutectic
LBE alloy is a kind of heavy metal material made of lead and bismuth mixed in a certain ratio. LBE has many good physical properties, including low melting temperature (150-200 • C), high boiling temperature (about 1670 • C), wide operating temperature range, low chemical activity, high thermal mobility, strong heat storage capacity, and so forth. But the high corrosiveness of high-temperature liquid LBE is also a key problem which should be solved in the future R&D works. As the HTM, so far, LBE is mostly used in nuclear reactors and thus relevant reported studies mainly focused on the thermo-hydraulics characteristics of LBE. [90][91][92] Currently, experimental studies on liquid LBE used in CSP and TES systems are rarely reported, but some relevant simulation and feasibility evaluation research works were carried out. For instance, Wang et al. 93,94 proposed a hybrid solar-nuclear power system which used liquid LBE as the HTM in the solar island. The technical and economic feasibilities of the hybrid power system were evaluated. Laube et al. 95 studied the effects of several packing-medium parameters on the operation performance of liquid LBE thermocline TES tank. Jiang et al. 96 conducted a performance comparison of TES tanks using different HTMs, including the binary nitrate salt, ternary nitrate salt and liquid LBE. Some typical results can be seen in Figure 8. 96 To estimate the technical feasibility of liquid LBE used in TES systems for CSP plants, Wang et al. 97,98 carried out a series of numerical simulation studies on LBE thermocline TES tanks. The results reveal that the LBE thermocline TES tank can operate stably. The effects of typical factors on the operating and mechanical performances of the LBE thermocline TES tank were demonstrated.
In summary, the feasibility of sodium used in CSP systems has been well revealed and that used in TES systems still needs further investigations. Currently, the feasibility evaluation works of liquid LBE used in CSP and TES systems are mainly based on simulation methods. More experimental studies of liquid LBE for CSP and TES systems should be carried out. In addition, solution methods of the corrosion problem at high temperature as well as the safety problems caused by leakage and high chemical activity of different liquid metals should also be studied.

WATER/STEAM
For solar power generation technologies, when water serves as the HTM, it is mainly used in the direct steam generation CSP systems 99 or some solar-based multi-energy hybrid systems (e.g., integrated solar-gas combined cycle systems 100,101 ). In these CSP systems, water serves as the HTM and working fluid for the steam turbine simultaneously. It will absorb the solar energy in the heat absorber or thermal receiver tube and becomes high-temperature and high-pressure steam directly. Then the steam will drive the steam turbine to generate electric power, which is just a Rankine cycle. That is similar to the coal-fired thermal power generation process. Compared with CSP systems with the heat exchange between other HTMs (e.g., molten salts, heat transfer oils, liquid metals) and water, the direct steam generation CSP systems can have simpler system configurations and lower power generation costs. But the thermal conduction performance of water is not so high as those of molten salts, and the heat F I G U R E 8 Simulation results of thermocline TES tanks using different HTMs: (A) thermocline thickness variations of the TES tank in charging processes, (B) maximum mechanical stress distributions of the TES tank wall. Source: Reproduced from Reference 96, with permission from Elsevier storage capacity of water/steam is not high when it is used in TES systems. Furthermore, water/steam has relatively high requirements for the solar thermal receiver structure.
The HTM initially used by Solar One power plant of the US was water, and the TES materials were heat transfer oil and rocks. In addition to Solar One, there are some other CSP plants using water as the HTM, for instance, CESA-1 (1 MW) and PS10 (11 MW) power plants of Spain, Ivanpah power plant of the US (392 MW, totally three power generator sets), Delhi power plant of China (50 MW) (see Figure 9), and so forth.
Currently, the application experience of water/steam as the HTM of direct steam generation CSP systems is abundant, and the relevant technologies are also relatively mature. But due to the problems and disadvantages of water used as the HTM mentioned above, relevant researchers have conducted many targeted studies. These works include the flow characteristics analysis of water/steam in solar absorber or receiving tubes, 102,103 temperature distribution analysis and optimization of water in solar absorber or receiving tubes, [104][105][106] operation mode evaluation of water in the direct steam generation system, 107,108 and so forth. Further research works of water/steam used in CSP systems may include optimizations of solar absorber or thermal receiving tubes, improvement of operation mode of water/steam in the direct steam generation system and heat storage performance improvement of direct steam generation CSP systems.

F I G U R E 9
The 50 MW Delhi solar power plant using water as the HTM in Qinghai Province, China

SUMMARY AND CONCLUSIONS
HTMs are important for CSP systems and their accessary TES devices. The performances of HTMs can influence the operation behaviors of CSP and TES systems. To provide a reference for researchers in solar thermal power field, this article conducts a brief review of research works on liquid HTMs used in CSP systems and TES devices of CSP systems. Some typical practical applications, experimental and simulation studies on different kinds of liquid HTMs used in CSP and TES systems are introduced, which mainly include different kinds of molten salts, heat transfer oils, liquid metals, nanofluids and water/steam. Table A1 briefly summarizes the scopes of application, advantages, disadvantages and further R&D work prospects of different kinds of HTMs (see the Appendix). The further R&D works of different kinds of HTMs in the future are concluded as follows: (a) For molten salt materials, the further works may include the developments of new molten salts with wider operating temperature range (i.e., with low melting point and high decomposition temperature simultaneously), solution of the corrosion problem of molten salt at high temperature and enhancement of thermal performances of molten salt materials, and so forth. (b) The further studies of heat transfer oils may focus more on improving the thermal performances of oil materials by inserting different additives (e.g., different nanoparticles). (c) For liquid metals used in CSP systems, solutions of the corrosion problem of metal materials at high temperature and safety problems caused by leakage and high chemical activity of some metals will be meaningful. In addition, experimental studies on the applicability of liquid metals used in CSP and TES systems are also necessary. (d) For direct steam generation CSP systems using water as the HTM, the next-step works may include the optimizations of solar absorber or thermal receiving tubes, improvement of operation mode of water/steam in the direct steam generation system and TES performance improvement of direct steam generation CSP systems, and so forth.