A comprehensive review on solid particle receivers of concentrated solar power

Renewable and Sustainable Energy Reviews 116 (2019) 109463

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A comprehensive review on solid particle receivers of concentrated solar power Kaijun Jiang a, Xiaoze Du b, a, *, Yanqiang Kong a, Chao Xu a, Xing Ju a a

Key Laboratory of Condition Monitoring and Control for Power Plant Equipment (North China Electric Power University), Ministry of Education, Beijing, 102206, China b School of Energy and Power Engineering, Lanzhou University of Technology, Lanzhou, 730050, China

A R T I C L E I N F O

A B S T R A C T

Keywords: Solar thermal power Solid particle receiver Particle selection Heat transfer medium Thermal energy storage Solar power conversion efficiency

The receiver is a key component of a concentrated solar thermal power generation system. At present, molten salt is typically used for both heat absorption and as a thermal energy storage medium in commercial tower solar power stations. However, molten salt may decompose and degrade when the temperature exceeds 600 � C, hence affecting the efficiency of the whole thermoelectric conversion system. The solid particle solar receiver can collect heat at very high temperatures (exceeding 1000 � C) and can also function as a thermal energy storage medium, thus providing a new way to improve solar power conversion efficiency and reduce the cost of solar power generation. The present review summarizes progress in research on solid particle receivers. The criteria for particles that can be used in solid particle receivers are discussed. The design and performance characteristics of each particle receiver type are also summarized. Particle receivers are classified according to their structures and operation characteristics. The present review discusses the performance characteristics, applications, and existing issues of solid particle receivers, and it introduces aspects of next-generation concentrated hightemperature particle receivers. This review thus serves as a useful reference for the theoretical research and practical application of photothermic power generation and thermochemical technologies.

1. Introduction Solar power generation technology is divided into two major cate­ gories: photovoltaic power generation and concentrated solar power (CSP). As CSP stations can be equipped with thermal energy storage (TES) systems to ensure continuous operation, they are viewed as promising applications. The world’s first commercial CSP station was built in Spain in 2008. CSP technology has continued to develop rapidly since then. Solar power stations have begun to move toward megawattscale applications due to the progress made in TES technologies. The global installed capacity of CSP reached 5133 MW by 2017. It is ex­ pected that CSP in the United States and Europe will account for 3% of total power generation by 2020. Moreover, global solar power genera­ tion capacity may account for 11.3% of the total global power genera­ tion by 2050 [1]. Spain and the United States are ranked as the top two countries in the world with regard to CSP technology usage. Given the increasing economic benefits of CSP, China, Saudi Arabia, India, Brazil, and other emerging countries are also adopting the technology rapidly [2]. CSP offers the advantage of replacing fossil fuels in power

generation [3]. CSP technologies are classified according to the concentrator and the endothermic system, namely whether it involves a parabolic trough, central tower, dish, or linear Fresnel reflector [4], as shown in Fig. 1. The operating temperature in parabolic trough solar power generation does not exceed 430 � C. Dish solar power generation requires low unit power consumption of 5–10 kW but entails high cost. The efficiency of linear Fresnel solar power generation is only 10% or so. Compared with the other three power generation modes, tower solar power generation of­ fers the advantages of high concentration ratio, high energy conversion efficiency, system stability, high power generation, and low cost. Thus, a considerable amount of research is focused on developing next-generation solar thermal power generation technologies. A central tower solar power station is generally composed of a he­ liostat field, receiver, TES system, particle-to-working fluid heat exchanger, power generating unit and other auxiliary equipment. Fig. 2 display the general working principle of the next-generation CSP system that can achieve the outlet temperature of receiver above 700 � C and uses supercritical carbon dioxide Brayton cycle as the power cycle [5]. The next-generation CSP system can further improve the

* Corresponding author. School of Energy and Power Engineering, Lanzhou University of Technology, Lanzhou 730050, China. E-mail address: [email protected] (X. Du). https://doi.org/10.1016/j.rser.2019.109463 Received 22 May 2019; Received in revised form 2 October 2019; Accepted 2 October 2019 Available online 9 October 2019 1364-0321/© 2019 Elsevier Ltd. All rights reserved.

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safe operation with regard to solar power generation are key issues in both research and application [6]. At present, molten salt or water/steam is used as the heat transfer medium (HTM) in the receivers of commercial solar power tower sta­ tions. The limitation posed by the operating temperature range is the biggest disadvantage of using molten salt receivers. Molten salt will decompose at 600 � C and solidify below 220 � C [7]. In addition, molten salt has strong corrosive properties, causing damage to pipelines. These factors restrict further improvement in the efficiency of molten salt re­ ceivers. When water/steam is used as the HTM for the receiver, it is limited by the low energy density of water and thus cannot be used as a TES medium. Recently developed solid particle solar receivers can achieve high outlet temperatures of above 1000 � C and tolerate temperature differ­ ences as high as hundreds of degrees, thus providing a new approach to improve the efficiency and reduce the cost of solar power generation [8, 9]. The use of solid particles as the HTM also has the following advan­ tages: 1) The solid particles can provide the functions of both heat transfer and TES, 2) the cost of the material is low, 3) the system is highly stable under high temperatures, and 4) the supercritical Rankine cycle or supercritical carbon dioxide Brayton cycle can be driven to improve the power generation efficiency to as high as 40–48%. In addition, combined with fluidized bed technology, solid particle solar receivers can be used for hydrogen production and coal gasification at operating temperatures of 550–2500 � C and 550–2000 � C, respectively [10,11]. Table 1 summarizes comparative data for molten salt and solid particle solar receivers used in a 50 MWth solar power station [12,13]. The table shows that when the solid silicon carbide particles are used as the HTM and TES medium, the cost of the latter can be further reduced

Nomenclature specific heat capacity, J/kg⋅K energy density, J/m3⋅K height, m length, m temperature, K width, m

cp E H L T W

Greek symbols thermal efficiency density, kg/m3

η ρ

Subscripts e in out p sint th

electricity inlet outlet particle sintering thermal

Abbreviations CSP concentrated solar power TES thermal energy storage LCOE levelized cost of electricity HTM heat transfer medium

thermal-to-electric conversion efficiency and reduce the levelized cost of electricity (LCOE). As the key component, the receiver converts the solar radiation energy into thermal energy. Performance enhancement and

Fig. 1. Different concentrator–receiver systems in CSP technologies [4]. (A) Central receiver. (B) Parabolic trough. (C) Linear Fresnel. (D) Dish.

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and the cycle efficiency can be significantly improved. Hence, the overall cost of the solar power station decreases. The more obvious benefit can be achieved when the solid particles be used as the HTM and TES medium in solar power stations of 100 MW or larger capacities. More recently, the next–generation CSP technology has become the focus of cutting-edge research in the field of solar energy, attracting the attention of the SunShot Project in the United States, the Horizon 2020 Plan of the European Union, and the International Energy Agency (IEA) [14]. It is generally accepted that this next–generation CSP technology will entail the efficient use of solar energy resources by using solid particle receivers as the HTM. Many researchers have focused on the exploration of the structures of solid particle receivers, such as those of the downflow, upflow, and horizontal flow types, to obtain higher operating temperatures and drive more efficient thermodynamic cycles [15]. The development of solar receiver using in tower plant has been summarized in some previous review articles (such as the papers of Zhu et al. [16], Tan et al. [11], Ho and Iverson [15], Behar et al. [4].). However, the solid particle receiver is still lack of timely and comprehensive review since rapid development during last few years. Some of the emerging concepts, the principle of particle selection and the development directions for solid particle re­ ceivers were not included in those reviews. Thus, a review focusing particularly on the solid particle receivers is beneficial for us to under­ stand its structure design, performance superiority, application fields, research trend, technical obstacles, and future development route. This review aims to provide an up-to-date comprehensive review on the research and development of solid particle receivers. This review highlights the selection principles of solid particle receivers and the research progress pertaining to different types of receivers in detail. The structure and operation characteristics of various types of solar particle receivers are analyzed. Moreover, the issues faced while using particle receivers are summarized and future development directions are discussed.

Table 1 Comparison of operating parameters of molten salt and solid particle solar receivers. Types Heliostat field Total surface area (m2) Number of heliostats Annual field efficiency (%) Receiver Capacity (MWth) Tin (K) Tout (K) Efficiency (%) Storage system (6 h) Cp (J/kg⋅K) ρ (kg/m3) Energy density (kWhth/m3) Volume of storage tanks (m3) Power cycle parameters Live steam pressure (Pa) Reheat steam pressure (Pa) Live steam temperature (K) Reheat steam temperature (K) Live steam flowrate (kg/s) Gross turbine output (MW) Net efficiency (%) Total construction cost ( � 106 $)

Molten salt

Silicon carbide particles

560,109 4184 59.7

508,264 4629 61.0

50 508 833 81.3

50 �623 �973 86

1560 1680 184 8646

1150 3210 207 6704

1.15 � 107 2.5 � 106 535 535 47.5 54.2 37.7 308.3

2.85 � 107 6.5 � 106 600 620 43.1 53.8 43.2 297.6

Particle selection is a fundamental issue to be considered in the design of the receiver. Solid particles used as HTM should have the following properties to ensure safe and efficient operation [17]: (1) Good thermophysical properties, namely high solid density, high thermal conductivity, and high specific heat capacity, to ensure greater heat transport and heat storage capacities within a certain temperature difference; (2) Good heat resistance such that the particles will not sinter and melt when the temperature exceeds 1000 � C, and lower impurity content to ensure that eutectic melting does not occur at high temperatures;

2. Selection of solid particles Solid particles differ in terms of fluidity and heat transfer properties, which have an important impact on the performance of the receivers.

Fig. 2. The schematic layout of a solar power tower plant with solid particle receiver. 3

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Table 2 Comparison of the physical properties of solid particles. Quartz

Sand

Cristobalite

Bauxite

Alumina

Silicon carbide

CFC

Olivine

ρp (kg/m3)

0.95 2610

0.95 2610

1.05 2330

1.05 3600

1.05 3960

1.05 3210

1.05 2600

1.25 3400

2480 0.4

2480 0.4

2447 0.48

4095 0.65

4198 0.65

3371 0.67

2730 0.55

4284 0.56

Tsint (K) Attrition rate Hardness (Mohs scale) Cost ($/ton)

1600 5 � 10 6 150

1550 10–7 9 400

2072 10–7 9 300

1800 1.6 � 10 9 1900

1200 10–7 8 180

1450 10–7 8 175

Cp (kJ/kg⋅K)

E (kJ/m3) λp (W/m⋅K)

6

750 5 � 10 6 75

6

1714 5 � 10 7 200

6

7

(3) High crack resistance to reduce the wear of particles during high temperature cycles and to improve the stability of heat transport performance; (4) Acceptable health and safety hazards to safeguard the health of operators; (5) Minimized environmental impact, especially when soft particles are selected as they are prone to wear, resulting in dust pollution; and (6) Low cost of solid particles and high storage capacity.

gemstone particles have stable properties and entail low cost at high temperatures, making them suitable for use in indirect heating receivers. Ceramic particles have high absorptivity and do not sinter at high temperatures and pressures. Thus, they are more suitable for receivers with volumetric heat absorption. Silicon-based particles have lower absorptivity and hardness. Hence, they are more suitable for indirect heating receivers [22].

According to the above mentioned criteria, a variety of solid parti­ cles, including sand, quartz, cristobalite, alumina, bauxite, silicon car­ bide, Calcined Flint Clay (CFC) and olivine, can be used as HTM in receivers. The physical properties of these solid particles are listed in Table 2 [17–19]. Several studies have been conducted to understand the influence of particles on the performance of the receiver. Siegel et al. [20] explored the factors affecting the absorptivity of particles and found that it de­ creases due to oxidation on the surface during operation. Knott et al. [21] studied the effects of temperature and pressure on the sintering properties of particles. Their results showed that the sintering of silica sand particles and alumina particles will not occur under actual oper­ ating conditions. The National Renewable Energy Laboratory (NREL) was the first to use calcined coke gemstone particles as the HTM in solar receivers [5]. Their investigation showed that the calcined coke

The residence time and heat transfer coefficient of the solid particles in the receiver directly determine the outlet temperature of the particles, which is the basic parameter in receiver design. The flow direction of the heat transfer fluid in the receiver is closely related to the residence time and heat transfer coefficient, and hence, these properties influence the structural characteristics, operation mode, and arrangement method of the receiver to a considerable extent. Depending on the flow direction of the HTM in the receiver, this review divides receivers into three categories: downflow receiver, upflow receiver, and horizontal flow receiver. A more detailed classifi­ cation of solar particle receivers can be found in Fig. 3.

3. Types of solid particle receivers

Fig. 3. Detailed classification of solar particle receivers. 4

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Fig. 4. Schematic of a free-falling solid particle receiver [24].

3.1. Downflow particle receiver

solar irradiation conditions in 2008. The experimental result showed that the thermal efficiency of the receiver exceeded 50%, and the maximum temperature rise at the particle inlet and outlet was 250 � C. Tan et al. [11] established a three-dimensional numerical model based on the Euler–Lagrange framework, and studied the effects of the particle diameter, quartz glass window, and air curtain on the performance of the free-falling receiver. The results showed that the smaller the particle diameter, the higher the particle outlet temperature. Ho et al. [33] performed an experimental study of a continuous circulation particle endothermic system under the actual (1 MWth) solar irradiation conditions. According to the results, the maximum temper­ ature of the outlet particles was 700 � C, and the thermal efficiency ranged from 50 to 80%. The mass flow rates of the particles and the actual irradiation energy exerted a considerable influence on the outlet temperature of the particles and the thermal efficiency of the receiver. Recently, Ho et al. [34] explored the flow and heat transfer character­ istics of particles in a falling receiver. They noted that the temperature rises of the particles decreased with the increase in particle flow rate, and the thermal efficiency of the receiver increased with the rise in particle mass flow rate. In addition, the Sandia National Laboratory (SNL) conducted experimental and theoretical studies on the effects of TES systems [35], heat exchange systems [36], and different kinds of particles on the

3.1.1. Free-falling particle receiver The free-falling particle receiver is the most basic form of the downflow receiver. Its basic working principle is illustrated in Fig. 4. Under the action of gravity, the particles fall directly from the solid particle release trough on the top to form a thin particle curtain. The falling particles absorb the solar radiation energy directly and are collected at the bottom of the receiver. Then, they are transferred to the heat tank, particle heat exchanger, and cold tank in turn through the conveying device. Finally, the particles are returned to the top particle release trough to complete one cycle. To collect heat at high temperatures through the tower concentrating system and study the solar thermal chemical reaction, Hruby et al. [23] put forward the concept of falling particle receiver for the first time. In the following nearly 40 years, many scholars investigated and improved the design of the downflow particle receiver on the basis of the original one [11,23–46], such as reducing the cosine loss by inclining the lighting hole at a certain angle [24–26], decreasing the convective ra­ diation heat loss by adding quartz glass windows [27–29], and reducing the exergy loss in the heat transfer process by arranging the receiver in a north–south direction [30,31]. Siegel et al. [32] carried out the first experimental test under actual

Fig. 5. Free-falling solid particle receiver with an air curtain [24]. 5

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maintenance are low. However, the following shortcomings persist. (1) The convection heat loss to the environment is large. (2) The particles flow out of the window during the falling process, leading to losses. (3) It is difficult to control the particle flow rate. (4) The particles fall too fast to fully absorb the radiation energy flow. (5) Non-uniform energy flow leads to non-uniform temperature distribution in the particle curtain.

performance of free-falling receivers [37]. The existence of ambient wind can lead to losses in particle circulation and convective heat transfer, seriously affecting the efficient and stable operation of the particle receiver. Other researchers have also studied the flow and heat transfer performance of the free-falling particle receiver under the in­ fluence of ambient wind. Kim et al. [38] analyzed the effects of wind speed and direction on the operation characteristics of the receiver. The result showed that particle circulation loss is the greatest when the wind direction is perpendicular to the window and the depth of the cavity is shallow, and that this loss is almost zero when the wind direction is parallel to the window. To disable the adverse effects of ambient wind and the loss of particle circulation on the performance of particle receivers, researchers have suggested corresponding optimization schemes. As shown in Fig. 5, the adverse effects of ambient wind on particle receivers can be improved by adding air nozzles at the bottom of the outer opening of the cavity and injecting air upflow to form air curtains [39,40]. The experimental result showed that the existence of the air curtain effectively reduced the convective heat loss as well as particle circulation loss. Besides, the air curtain may enter the cavity and cause high turbulence when it com­ bines with the wind flow that results in a low cavity efficiency. This adverse effect can be improved by installing an air guiding device at the top of the air curtain that helps to reduce air jet entering the cavity and the injection condition of the air jet need to be adjusted according to the different attacking wind [24]. Another optimization scheme involves installing a quartz glass window in the opening of the cavity to separate the internal and external flow fields [27–29]. Although the quartz glass window can effectively solve the problems caused by ambient wind, the overall cost is too high (>$10 million) because of the large area requirement of the quartz glass window and the need for high trans­ mittance. Another scheme to reduce the influence of the external ambient wind is to change the tilt angle of the cavity opening [28]. The numerical simulation results showed that adjusting the vertical window to a downflow–facing receiver with a certain inclination angle is bene­ ficial to reduce particle loss and environmental convection loss. How­ ever, the scheme will increase the radiation heat loss of receiver between the environment. In summary, free–falling solid particle receivers have the following advantages. (1) There is no blockage during the particle falling process and the operation process is relatively stable. (2) The structure of the downflow receiver is relatively simple, and the costs of operation and

3.1.2. Choked–flow particle receiver To control the falling speed of particles and increase the temperature of particles at the outlet of the receiver, many researchers have proposed different types of choked–flow receivers. The basic principle of the choked–flow receiver is to increase the resistance of the falling particles by changing the structure of the receiver, thereby prolonging the resi­ dence time of the particles in it. In 1986, Hruby et al. [47] pioneered the idea of choked-flow receivers, which slow down falling particles by adding a suspended ceramic structure to the back wall of the cavity. In terms of structure, choked–flow receivers can be further divided into the following six categories: 1) Λ-plate, 2) porous, 3) gravity-driven, 4) counterflow, 5) inclined, and 6) spiral tubes. 3.1.2.1. Λ–shaped mesh structures. The Λ–plate receiver was devised by Ho while working at SNL [48]. The basic principle is to install a series of continuous staggered Λ-type metal mesh structures on the wall to delay the falling time of the particles and enhance mixing between them, as shown in Fig. 6. The experimental results showed that the peak tem­ perature of the particles could exceed 700 � C, the temperature rise rate of the particles was approximately 30–60 � C/m which is the total tem­ perature rise rate divided by the total length of heating section of the receiver, and the thermal efficiency was 60–90% near the center of the receiver [49]. The results indicated that some Λ–type metal mesh structures fail when the actual energy flow density is 700 kW/m2 after continuous operation for 20 h. Owing to the direct irradiation of concentrated sunlight and particle wear, the grid materials made of stainless steel 316 face problems with regard to overheating, oxidation, and deterioration. The main reason for the failure of the metal mesh is the formation of an oxide film on the metal surface, which increases the brittleness of the mesh. At present, SNL is studying new materials and operational stra­ tegies to reduce grid wear and softening [50,51]. Ansary et al. [52] used red sand as the HTM to carry out actual irradiation experiments. The experimental results showed that red sand is a suitable HTM in falling particle receivers. 3.1.2.2. Porous structures. A novel particle receiver design proposed by researchers at the Georgia Institute of Technology (GIT) and King Saud University (KSU) allows particles to flow downward through a station­ ary porous structure, which facilitates absorption of concentrated solar energy. The cavity of the receiver is filled with connected porous structures to slow the motion of the particles, as shown in Fig. 7. The porous structure reduces the speed of the falling particulate material,

Fig. 7. Metallic porous blocks used in experiments on the cavity of a solid particle receiver [53].

Fig. 6. Particles flowing over a staggered array of Λ–shape structures [52]. 6

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with the wall, the temperature gradient near the wall changed dramatically, which indicated that the heat absorption process of the particles mainly occurred near the wall [58]. The diameters of the particles, their mass flow rates, and the top angle of the hexagonal mesh exert great influence on the heat transfer coefficient [59]. More recently, Martinek et al. [60] established a comprehensive simulation model for assessing the thermal performance of this type particle receiver and investigated its potential application for thermochemical energy stor­ age. Simulation results show that the novel receiver can theoretically achieve high–temperature and high–efficiency operation, however, this depends on several key assumptions––namely, acquirement of sufficient wall–to–suspension heat transfer coefficients and achievability of partially reflective optical coatings that maintain long–term stability when temperature exceed 700 � C.The advantages of the gravity-driven particle receiver include no particle loss during cycles and low heat loss. However, low mass flow rate, a complex structure, and high maintenance cost are the disadvantages of this type of receiver. Fig. 8. Schematic of an enclosed particle receiver with hexagonal heat transfer tubes [59].

3.1.2.4. Counterflow fluidized bed. Miller et al. [61,62] of the Colorado University of Mining and Technology proposed a countercurrent particle receiver and conducted a performance test on a laboratory–level pro­ totype. Its basic structure is shown in Fig. 9. This type of receiver is equipped with a particle storage tank at the top. The particles flow from the top to the bottom, and then flow out from the central orifice at the bottom. The bottom of the receiver is provided with air distribution plates on both sides. The air flows from the bottom to the top through the air distributor plates and flows out through the side orifice plate of the upper part of the receiver. When heated by a xenon lamp, the experimental results showed that the reverse crossflow between solid particles and fluidized air can effectively increase the residence time of the particles in the receiver, enhance the disturbance between them, and facilitate better flow and heat transfer performance. The superficial gas velocity has a considerable influence on the heat transfer coefficient. When the superficial gas velocity is 0.45 m/s, the heat transfer coeffi­ cient is 800–1200 W/(m2⋅K). The wall-to-bed heat transfer coefficient is positively correlated with the particle volume fraction. Jackson et al. [63] carried out an experimental study on the effect of inert oxide particles on the flow and heat transfer performance of a counterflow fluidized bed particle receiver. The results showed that inert oxide particles can reduce the volume of the receiver and the investment cost

hence increasing its residence time within the receiver. A large tem­ perature increment in a single pass can be then achieved. Researchers have carried out experiments and simulations to investigate the dynamic performance of such receivers [53–56]. According to the results, the porosity of the medium produced a significant effect on the particle residence time, and thus, the receiver should be designed according to particle diameter and porous structure. However, the thermal perfor­ mance of this type of receiver has not been tested. Particle receivers with porous structures are advantageous in that the rise in the particle tem­ perature in a single pass may be very high and the flow rate remains stable. However, they also pose a disadvantage in terms of the re­ quirements of porous materials. 3.1.2.3. Hexagonal heat transfer tubes. Ma et al. [57] designed an indi­ rect heating solar particle receiver. As shown in Fig. 8, the particles flow from the top to the bottom through the staggered hexagonal tube walls, driven by gravity. The inner surface of the tube wall receives concen­ trated solar radiation, and the heat is indirectly transferred to the par­ ticles flowing outside the tube by means of heat conduction through the tube wall. The results showed that when particles were heated indirectly

Fig. 9. Configuration of an experimental countercurrent particle receiver [61]. 7

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installed at the top to reduce heat loss and particle circulation loss. The pneumatic control valve at the end of the inclined plate can control the residence time of the particles, ensuring a higher outlet particle tem­ perature. The experimental results showed that when the particle flow rate is ~7.5 g/s and the total incident power is ~8 kW, the maximum particle temperature at the outlet of the receiver can reach ~938 K. Further numerical analysis showed that if the particle flow rate can be controlled at ~5 g/s, the average outlet temperature and efficiency can reach ~1205 K and ~71%, respectively. The advantages of the inclined receiver are as follows. (1) It serves the functions of both heat collection and storage, and is appropriate for use with a dish power generation system. (2) A closed design is adopted for the interior of the receiver, and hardly any particle loss occurs during the circulation process. (3) The particle velocity can be controlled to ensure a higher outlet particle temperature. However, the flow stability of the particles in the receiver needs to be improved. 3.1.2.6. Spiral tubes. To improve the residence time of particles in the receiver, Bai et al. [72–74] designed a spiral tube particle receiver, as shown in Fig. 11. The collector tube is a transparent quartz glass tube with the transmittance of ~93%. The particles fall automatically under gravity and absorb solar radiation energy directly from the tube wall. The particle residence time is increased due to the spiral structure. Bai et al. [74] conducted experiments to investigate the effect of particle inlet temperature, particle diameter, type of quartz tube, and particle flow rate on outlet particle temperature. Their results showed that the outlet particle temperature can reach 511 K when the average direct normal irradiation is 500 W/m2. However, the structural design of the spiral tube heat receiver is complex, the light rejection rate is high, and the particles are prone to blockage under high temperatures. At present, it is difficult to scale up this receiver type for practical application.

Fig. 10. Schematic of an inclined plate solid particle receiver [71].

of energy storage equipment. The advantage of this type of receiver is that the residence time of the particles can be controlled by adjusting the gas velocity, and hence, the heat transfer between the particles and the heated wall can be enhanced. The researches about the gas upward–solids downward countercurrent fluidized flow is very scarce [64–67]. Most of them focus on hydrody­ namics and were applied to chemical industry, especially coal gasifica­ tion. However, the energy loss caused by heat absorption of fluidized air was not considered in the above studies, and more research is required with regard to the influence of particle and gas flow rates on the air outlet temperature.

3.1.3. Centrifugal particle receiver The centrifugal particle receiver was created in the 1980s and used in the field of CSP. The basic principle of this type of receiver is as follows. The particles are transported to the particle distribution trough at the top of the receiver by means of a transport device. They then enter the annular channel on the side through the opening at the top of the receiver. The particles fall slowly along the wall of the receiver under the action of gravity and the rotating centrifugal force of the receiver. The solar radiation energy is focused on the interior of the receiver through the concentrator, heating the internal particles. By changing the rotation speed of the receiver, the outlet particle temperature can be adjusted. Flamant et al. [75] carried out experimental studies on the performance

3.1.2.5. Inclined plate. Koepf et al. [68,69] devised a type of inclined particle receiver in 2012. The vibrator causes the zinc oxide particles to enter the inclined plane from the feed inlet to form a moving bed, and slide from the top to the bottom under the action of gravity. They found that the internal reaction temperature of the receiver can reach 1161 � C. Xiao et al. [70,71] proposed another type of inclined particle receiver. They carried out preliminary experimental tests and numerical simula­ tions on such a receiver. As shown in Fig. 10, the particles from the feeding bin directly absorb the incident radiation energy directly when slowly sliding along the inclined plane. Quartz glass windows are

Fig. 11. Particles flow through the spiral quartz tube in the solar furnace [74].

Fig. 12. Schematic of a rotary kiln receiver [78]. 8

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electromagnet may degauss when the operating temperature is too high. 3.2.2. Upflow fluidized particle-in-tube receiver The fluidization technology in the energy transport process is an important means of particle operation. Traditional fluidization tech­ nology has been widely used in the oil refining, chemicals, metallurgy, and power industries, including coal-fired power generation, as well as in medicine. Among the numerous solar particle receivers, fluidized bed receivers have shown good application prospects due to their strong maneuverability, high wall-to-particle heat transfer coefficient, and low heat loss. A typical fluidized bed particle receiver is composed of many parallel collector tubes. The solid particles are suspended from the bottom distributor of the receiver under the action of fluidized air drag force. They indirectly absorb the solar radiation energy through the heated wall of the receiver and flow out at the top of the receiver. According to the different fluidized gas velocities, the receivers can be divided into the bubbling bed particle receiver with a superficial gas velocity of less than 0.2 m/s and the fluidized bed particle receiver with an apparent gas velocity exceeding 10 m/s. Flamant et al. [75] were the first to propose the application of flu­ idized bed technology to solar receivers in the 1980s. The receiver adopted a transparent quartz glass tube, and they selected solid particles such as zirconia, silicon sand, clinker, and silicon carbide to absorb solar radiation from the bottom to the top under the action of air fluidization. In the experiment, the outlet temperature of silicon sand could reach ~1200 K, and the temperature of the silicon carbide particles could exceed 1400 K. However, the endothermic efficiency was only 20–40%. The feasibility of the concept of fluidized bed solar particle receiver was verified by both numerical simulation and experimental study under actual solar irradiation conditions [86,87]. It was observed that the outlet temperature and heat absorption efficiency of the receiver varied with particle type and particle flow rate under different irradia­ tion intensities. Flamant et al. [88–90] also conducted a series of confirmatory experiments on an upflow single-tube bubbling fluidized bed particle receiver on a 1 MW solar furnace at Center National de la Recherche Scientifique (CNRS) in France, as shown in Fig. 14. The re­ sults showed that when the mass flow rate of the particles was 10.2–45.1 kg/s, their average outlet temperature was 446–723 � C. The experimental results for the parallel operation of multi-tubes showed that when the length of the concentrating heating section was increased to 1 m, the outlet temperature of the solid particles could reach 700 � C, and the overall efficiency of the receiver was 50–90%. This type of receiver is expected to drive solar thermal power stations with a capacity of 50 MWe in the future. At present, CNRS is carrying out experimental tests of a 5 MWe capacity power station using 40 tubes in parallel, with the length of the collector tube being 4 m. However, Zhang et al. [91,92] found that the wall-to-bed heat transfer coefficient first increases and then decreases with the change in fluidized gas velocity. A maximum value exists either in the circulating fluidized bed particle receiver or in the upflow bubbling bed receiver. In addition, when the bed height exceeded 1 m, the coalescence of bubbles in the fluidized tube caused a gas plug and an air block, leading to un­ stable operation [93]. When the multi-tube row is operated in parallel, non-uniformity in fluidization between different tubes can easily occur. These results showed that the engineering scale-up of the upflow flu­ idized bed receiver faces some barriers and improvement is needed. More recently, Zhang et al. [94] comprehensively investigated the po­ tential of the up bubbling fluidized bed solar receiver scale–up from five–fold, including the receiver structure parameters from the property of material and so on. In summary, the advantages of the upflow fluidized bed particle receiver are as follows. (1) The wall-to-bed heat transfer coefficient is large enough, the particles are mixed evenly, and the outlet temperature is high. (2) The bubbling fluidization can reduce the consumption of fluidized gas and consequently reduce energy loss. (3) The particle flow

Fig. 13. Schematic of a spiral particle receiver [83].

of centrifugal receivers. The results showed that when solid particles of calcium carbonate were used in the receiver, the thermal efficiency of the receiver was only 30%, whereas the mass flow rate of the particles could be controlled to 1 kg/s. Recently, Wu et al. [76–79] developed a new type of centrifugal particle receiver, shown in Fig. 12. Bauxite ceramic particles flow into rotary centrifugal receivers with different inclination angles at mass flow rates ranging from 3 to 10 g/s. The particles are irradiated with a 15 kWth solar simulator. When the irradiation power was 670 kW/m2, the outlet particle temperature of the receiver reached 900 � C, and the thermal efficiency was 71–79%. With the increase in rotating speed, the temperature distribution of the particles became more uniform. Prosin et al. [80] proposed a solar–coal complementary power generation system by combining a centrifugal particle receiver with a coal–fired power station, and simulated the system. Their findings indicated that the proposed scheme could reduce coal consumption by 20% per year compared with conventional coal-fired power stations. More recently, Gallo et al. [81] from University of Antofagasta established a lab–scale rotary kiln to investigate the principle of particle temperature control and put forward one–dimensional transient numerical model to analyze the thermal performance of the kiln. The advantage of the centrifugal particle receiver is that the residence time of the particles in the receiver can be controlled via the rotational speed. However, because of the existence of the rotating motor, the additional power requirement is too high, and the particle mass flow rate is small. Thus, it is difficult to scale up the centrifugal particle receiver. 3.2. Upflow particle receiver 3.2.1. Spiral particle receiver In 2014, Xiao et al. [82–84] of Zhejiang University devised a spiral solar particle receiver, as shown in Fig. 13. The particles enter the receiver from the bottom of the spiral track. The electromagnet below the track drives the whole track to vibrate at a certain frequency. Under the effect of vibration, the particles move from the bottom to the top along the spiral track, and the particles are heated by the solar radiation at the top. The experimental results showed that when the particle flow rate was 0.21 kg/s, the one-way final temperature of the particle exceeded 625 � C and the optical efficiency reached ~87%. The thermal efficiency was ~60%, and the residence time of the particle was 15–45 s. According to the theoretical analysis, if the receiver was coupled with the secondary mirror, the final particle temperature and total efficiency would reach 628–673 � C and ~60%, respectively. Spiral particle receivers have the following advantages. (1) The particle residence time is long and controllable. (2) Compared with the fluidized bed type, the transmission power consumption is low. (3) The wear of the particles on the wall is negligible because of their slow moving speed. (4) This receiver type can be used not only in solar power tower systems, but also in dish solar thermal power generation systems. The main problem posed by the upflow spiral particle receiver is that the 9

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Renewable and Sustainable Energy Reviews 116 (2019) 109463

Fig. 14. Schematic of a laboratory-scale upflow fluidized particle-in-tube receiver [85].

Fig. 15. Schematic of a fluidized bed particle air receiver [96].

rate is regulated by fluidized gas velocity, which is easier to control than the direct falling velocity. However, it also suffers from some disad­ vantages, as follows. (1) Practical engineering enlargement entails some difficulties, especially with regard to gas gathering and forming bubbles with increased tube height. When the diameter of the bubbles equals the diameter of the tube, a gas plug will occur. (2) When the multi-row tubes run in parallel, uneven fluidization can easily occur between different tubes.

Fig. 16. Schematic of fluidized bed particle receiver with beam-down solar concentrating optics [105].

3.2.3. Fluidized bed particle air receiver The solid particulate air receiver facilitates solar thermal energy utilization by heating air with fluidized particles. The solid particle endothermic technology transforms the traditional surface absorption into internal absorption, realizes the direct conversion of sunlight and heat, and effectively reduces the surface temperature of the tube wall. 10

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The solid particles remain in the receiver only as the medium to enhance the heat transfer, and the air leaves the receiver as the subsequent HTM. This type of heat receiver can be further divided into a quartz glass tube solid particle air receiver and a fluidized bed particle air receiver with a secondary concentrating system. Bai et al. [95–98] of the Institute of Electrical Engineering, Chinese Academy of Sciences (CAS), designed a quartz glass tube solid particle air receiver and performed experimental and numerical simulation studies on it. The particles are silicon carbide particles with an average size of 5 mm. The air flow enters the receiver through the fluidized fan, and it flows from the bottom to the top with the solid particles while directly absorbing the radiation energy of a solar furnace with an external heating power of 10 kW, as shown in Fig. 15. The experimental results showed that the outlet temperature of the heated air could exceed 600 � C [97], and the maximum temperature difference between the particles and air was less than 25 � C, which indicates that the heat transfer coefficient between the solid and gas phases was large enough. In 2000, Segal and Epstein [99] pioneered the idea of replacing the solar tower with a secondary concentrating system to reduce the cost of power stations and the energy loss generated during transportation. Based on this concept, Matsubara et al. [100–103] proposed the fluid­ ized bed particle receiver shown in Fig. 16. The receiver uses an internal circulating fluidized bed, injecting high-speed and low-speed concentric airflow in the central ring and outer ring, respectively, to fluidize the particles. The receiver is equipped with a guide plate to organize particle circulation, and a transparent quartz glass window is installed at the top. The sunlight from the top heats the fluidized particles inside. The experimental results showed that at an irradiation power of 100 kWth, the solid particle temperature in the center of the receiver and the outlet air temperature could reach 960–1100 � C and 1100 � C, respectively [104]. Numerous researchers have used the numerical simulation method to explore the flow and heat transfer processes inside solid particle re­ ceivers with beam-down solar concentrating optics [106–109]. Matsu­ bara et al. [101] simulated the formation, coalescence, and fragmentation of bubbles in the receiver using the Euler–Euler model. Briongos et al. [107] simulated the flow and heat transfer processes of fluidized bed solid particle air receivers based on a secondary concen­ trating system using the Euler–Euler model and explored the effect of operating parameters on its performance. Sarker et al. [108] used the computational fluid dynamics-discrete element model to investigate the effects of vortex flow and recirculation flow on the flow and heat transfer for this type of receiver, and the numerical results showed that the guide plate can make the mixture between the gas and solid more uniform, thus increasing the heat absorption and outlet temperature, as well as improving the thermal efficiency. The fluidized bed particle air receiver has the following advantages.

(1) Solids absorb the radiation energy flow and strengthen the heat transfer process of the gas, and the outlet gas can attain a very high temperature. (2) The operation process is stable and the particle loss is low. (3) The gas flow rate is easy to control. (4) It can be designed as an integrated heat collection and storage system. The main disadvantage of this receiver, however, is that air cannot be used as the direct TES medium. 3.3. Horizontal fluidized bed particle receiver As mentioned above, because the problem posed by the gas plug in the tube and uneven flow between the heat-collecting tubes destabilizes the operation of the solar particle receiver in the ascending bubble bed, Hernandez et al. [110,111] from Universidad Carlos III de Madrid (UC3M) put forward the idea of a horizontal bed particle receiver. As shown in Fig. 17, the receiver is placed horizontally on the ground. The solar radiation is concentrated on the receiver through the linear Fresnel reflector and the secondary concentrating mirror. The receiver adopts a modular design, allowing multiple units to be installed in series. The fluidized gas flows continuously and horizontally through the multi-stage fluidized bed in series. The particles flow along the hori­ zontal direction while fluidizing, thereby gradually increasing the temperature of the gas and particles. Theoretical calculations showed that the temperature of the gas and particles can reach ~800 � C when the field area of the reflective Fresnel mirror is 9072 m2, the installation height of the secondary concentrating system is 18 m, and the total length (Ltotal) is 140 m. The prospect of horizontal fluidized bed particle receiver integrated into the CSP system and the optimization of the secondary reflector have been initially considered [111,112]. The re­ sults showed that the 150 lines with 38 units per line are needed to produce a net power of 11 MW, with the receiver shows a thermal ef­ ficiency of 78.02% and the optical efficiency of 34.65%. Kong et al. [113] of the Beijing University of Chemical Technology (BUCT) suggested the idea of a horizontal fluidized bed particle receiver, and they carried out cold performance experiments to test the particle residence time distribution for this type of receiver. Their results showed the lack of a flow “dead zone” in the horizontal fluidized bed particle receiver, thus proving the practical feasibility of the receiver. The advantages of using a horizontal bed particle heat receiver are that it does not produce gas plugs and that it can reduce the construction cost and energy loss without building a solar tower. However, it is also disadvantageous as excessive thermal stress occurs in the process of heat collection. In addition, compared with the cavity receiver, the convec­ tive heat loss to the environment is higher with a horizontal bed particle heat receiver. The performance of the receiver under the actual irradi­ ation heating condition needs to be further investigated.

Fig. 17. Layout of the optical system and schematic diagram of a horizontal fluidized bed particle receiver [110]. W: wide of the unit; L: length of the unit; H: height of the unit. 11

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4. Problems to be solved and development directions

arranged on the ground and does not require a high solar tower, which can effectively reduce the investment cost of the system. Moreover, the parasitic power needed to transport particles and the energy losses caused by the transport is reduced during operation. However, solar thermal power generation faces the twin bottlenecks of reducing the cost of TES and improving the efficiency of power gen­ eration. The cost of heat storage depends on the technical and economic performance of the TES materials, whereas the efficiency depends on the increase in outlet temperature of the solar receiver as well as the inte­ gration of a new power cycle such as supercritical Rankine cycle or su­ percritical carbon dioxide Brayton cycle. The key breakthrough

Table 3 summarizes the types of solid particle receivers, including the HTM, outlet temperature ranges, operating efficiencies, research platforms, and their respective advantages and disadvantages. The particles of the downflow receiver are driven by gravity, and the oper­ ation process is relatively stable and reliable. Thus, additional power consumption is low. Therefore, it has certain commercial application prospects. The advantages of the upflow receiver are that the particles stay in the receiver for a considerable amount of time and the heat transfer coefficient is relatively large. The horizontal receiver is Table 3 Comparison of different receivers. Type

Research institute

HSM

Tout/η

Text scale

Advantages

Disadvantages

Reference

Free-falling

NREL/SNL

Air and particles

>700 � C/ 50–80%

1 MW solar furnace

SNL/KSU

Air and particles

>700 � C/ 60–90%

1 MW solar furnace

Porous structure

KSU/NREL/ GIT NREL

Air and particles Air and particles

N.A.

Laboratory prototype Laboratory prototype

Counterflow fluidized bed

Colorado School of Mines

Air and particles

340–380 � C/ N.A.

Laboratory prototype

Heat loss is high, particles will cause losses when falling, particle flow rate is not easy to control, and particle residence time in the receiver is short. The structure is complex, the wear of the particles on the V-plate receiver is high, and the maintenance cost is high. Requirements for the porous material may be strict. Increased heat transfer resistance occurs from the wall to the particle, parts may fail due to high temperature in some areas, and the manufacturing cost is high. Additional power is required to drive the air pump, additional air outlets are required, and the wall-to-particle heat transfer resistance is high.

[11, 23–46]

Λ-plate

Inclined plate

Zhejiang University

Air and particles

>800 � C/ 70%

Laboratory prototype

Downflow spiral tubes

CAS

Air and particles

238 � C/N.A.

10 kW Solar furnace

Centrifugal receiver

Air and particles

900 � C/75%

Laboratory prototype

Upflow spiral receiver

Deutsches Zentrum für Luft- und Raumfahrt Zhejiang University

No blockage in the falling process of particles, the operation process is relatively stable, the structure is relatively simple, and operation and maintenance costs are low. Heating time of particles should be prolonged to increase their outlet temperature, and particle temperature distribution is even. Particle outlet temperature can be high and the flow is stable. Loss of solid particles during heating by the indirect heating method is nil, and the outlet temperature of the particles is theoretically high. The wall-to-particle heat transfer coefficient is large, the heat transfer performance is better compared with that of the upflow receiver, and air plugs do not occur. Particle residence time is high, particle circulation loss is low, particle outlet temperature is high, and the receiver can be transformed into the dish–Stirling type. Particle residence time in the receiver is long, and it is theoretically possible to heat the solid particles to a sufficient temperature. Particle temperatures are high, and particle falling speed can be controlled by the rotating speed.

Air and particles

N.A.

Laboratory prototype

No additional power is required to drive the air pump.

Fluidized particlein-tubes

PROMESCNRS

Air and particles

>750 � C/N. A.

1 WM solar furnace

The wall-to-bed heat transfer coefficient is high, and no particle loss occurs during operation.

Fluidized bed particle air receiver

CAS

Air

>900 � C/ 45–80%

10 kW solar furnace

Fluidized bed with secondary concentrating system Horizontal receiver

Niigata University

Air

>900 � C/N. A.

100 kW solar furnace

UC3M/BUCT

Air and particles

N.A.

Laboratory prototype

The heat transfer coefficient is high with volume absorption, and the outlet temperature of the air is high. The medium’s outlet temperature is high, and the receiver can be laid on the ground using a secondary concentrating system. Air plugs do not occur, system stability is high, and the wall-tobed heat transfer coefficient is high.

Hexagonal heat transfer tubes

N.A.

N.A.: not available. 12

[48–52]

[53–56] [57–60]

[61–63]

Particles are easily blocked during falling due to the accumulation angle, stability is low, and additional air pumps are required to drive the particle flow at the bottom.

[68–71]

Blockages can easily occur during operation, the tube manufacturing cost is high, and the light rejection rate is high.

[72–74]

Additional power is required to drive motor rotation, maintenance cost is high, the structure is complex, and particle flow is low. Electromagnets are prone to failure at high temperature, and arrangement of the receiver is difficult. Air plugs are frequent when the length of the tube is increased, uneven flow distribution occurs in multi-row operation, and additional power is required to drive the air pump. As air is used as the HTM, heat cannot be stored directly, and engineering scale-ups pose certain difficulties. As air is used as the HTM, heat cannot be stored directly, and additional power is required to drive the air pump. The receiver can easily become deformed because of its high thermal stress.

[75–81]

[82–84]

[85–94]

[95–98]

[99–109]

[110–113]

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Renewable and Sustainable Energy Reviews 116 (2019) 109463

requirement for solar thermal power generation technology is to raise the outlet temperature of the concentrating solar receiver to more than 700 � C to develop a highly efficient and low–cost TES medium, accom­ panied by a supercritical carbon dioxide Brayton cycle. Previously, researchers conducted preliminary explorations on different forms of particle receivers and accumulated valuable experi­ ence. Future researches should focus on the following aspects.

(4)

(1) From the viewpoint of experimental measurements, the rule pertaining to the change in receiver performance with time when irradiation intensity fluctuates needs to be studied in detail by considering the thermal inertia of solid particles. The solar simulator can be used to explore continuous variable power heating and the continuous dynamic characteristic response of the receiver when the irradiation conditions change. Moreover, visualization experiments should be carried out to analyze the flow characteristics of particles in the receiver. Secondly, the properties of particles have an important impact on the flow and heat transfer processes of the receiver. Thus, it is necessary to study the effects of different particles on the performance of the receiver. Thirdly, solar radiation has wide spectrum characteris­ tics, most of the particle receivers use one kind of particles as the heat transfer medium in current study. Therefore, the efficiency of solid particle receiver can be further improved by selecting multicomponent particles with different absorption coefficient to form the heat transfer medium. Finally, so far, experimental studies have mainly focused on the heat absorption characteris­ tics of particles. Given that the particles need to release heat in the heat exchanger to increase the temperature of the working medium, it is necessary to explore the heat transfer performance of solid particles in the heat exchanger. (2) From the viewpoint of numerical simulation, two main models are available at this time: the Euler–Lagrange model and the Euler–Euler model. The Euler–Lagrange model can track the flow path of a single particle and explore the heat transfer process between particles, but the maximum number of tracked particles is 106 and the calculations can be tedious. The Euler–Euler model treats both gas and solid as continuous phases, which reduces the cost of calculation. However, it cannot study the interactions between particles and trace the tracks of particles. In addition, all models fail to consider the impact of particle size distribution due to all of them view the diameter of particles as uniform which is not accord with actual situation. At present, the modeling and analysis of multi-phase systems such as fluidized beds are insuf­ ficient. An effective single model has not been developed to capture the details of flow structure and mechanism at all scales. It is necessary to adopt specific models for different time and space scales and develop multi-level analysis methods. In the future, the two methods can be combined and different calcula­ tion methods can be applied for different areas to improve the accuracy of the simulation and consider feasibility. The mesh adaptive technology has been studied to automatically adjust the mesh size according to different accuracy requirements in various computational domains, the main goal being to reduce the amount of computation. (3) From the viewpoint of engineering scale-ups, it is necessary to consider the control of the actual particle flow rate to ensure smooth functioning when designing power stations of 100 MW capacity and higher. In addition, the integrated design of solar power tower generation systems based on solid particles as the TES medium is necessary. This includes correct material selection of the receiver and anti-wear measures; design and selection of the heat exchanger, superheater, and reheater; design, capacity matching, and operational studies of high-parameter steam tur­ bines; and exploration of the overall operation scheme and operation mode of the power station. Moreover, the research and

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development of the auxiliary equipment that guarantees solid particle receivers operate safely is also a tough challenge for re­ searchers, especially how to design a stable mechanical powder conveyor, heat transfer fluid pumps and valves used to control particles flux when temperature exceed 700 � C. From the viewpoint of techno-economic analysis, the majority of the present researches focused on the structure design, and the principles of fluid flow and heat transfer. The study on the techno-economic comparison between different particles re­ ceivers is scarce due to the researches are still at the stage of laboratory development and lack of benchmark tests for com­ parison. A comprehensive techno-economic model needs to consider the following main factors. The design, manufacturing and installation costs of the solid particle receivers. The cost of solid particles, including the price of particles, the losses during circulations and the durability. The fluidized bed particle receiver has merits of little recirculation losses and excellent durability due to slowly rising velocity compared with other solid particle receivers. The auxiliary equipment, mainly including the price of installing particles conveyors, pumps and parasitic power when operating. The upward solid particle receivers may cost higher than other types receivers since it needs auxiliary equipment to help solid particles to rise. The maintenance and depreciation costs of the solid particle re­ ceivers. The type of obstructed solid particle receivers usually need parts to slow the velocity of particles and these parts are more easily damaged. The influence of the solid particle outlet temperature of the re­ ceivers should be considered since different outlet temperature affect the whole thermal cycle efficiency. The cost of constructing the heliostat field need to be considered either. Different types of solid particle receiver match different concentrators, and the demand for the land is different. The horizontal solid particle receiver can match linear Fresnel reflector and can be put on the ground. As a result, the solar tower doesn’t need to construct and the cost of auxiliary equipment and the heliostat field can be further reduced. In that viewpoint, the horizontal solid particle receiver may have the potential to reduce the LCOE further.

In a word, there are many factors need to be considered in building a comprehensive techno-economic model. With the development and maturity of the solid particle receivers, the techno-economic comparison of the different types of receivers needs to be further investigated. 5. Concluding remarks Solar receivers composed of cheap solid particles or granules as the HTM, and which can collect and store thermal energy simultaneously, are a promising solar thermal power generation technology. Solid par­ ticle solar receiver technologies have attracted global attention as they can collect heat at temperatures exceeding 1000 � C. Thus, they are ex­ pected to solve the key problems posed by traditional molten salt, which restrict solar energy development. Among the existing types of receivers, including upflow, downflow, and horizontal flow receivers, the upflow receiver shows low stability and requires additional power to drive the air pump. Thus, the technology needs further research and improve­ ment. The horizontal receiver can be arranged on the ground, thereby reducing the construction and transportation costs of the solar tower. However, detailed knowledge about the photothermal coupling rela­ tionship is lacking. The downflow receiver, especially the free-falling receiver, is a mature technology at present. As the downflow receiver runs stably, it generally does not need additional power drive air. Thus, its commercial application prospects are good. Designing a reasonable 13

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Renewable and Sustainable Energy Reviews 116 (2019) 109463

blocking structure to prolong the residence time of particles remains the most crucial problem to be solved. At present, most studies on particle receivers are in the experimental verification stage and mainly focus on the solar tower system. The key problems posed by solid particles are as follows: heat absorption and storage medium requirements in solar tower thermal power generation systems, flow and heat transfer char­ acteristics of solid particles in the receiver, heat transfer characteristics of solid particles as the HTM in heat exchangers, wear of solid particles in the circulation process and wear on the metal wall, and the regulation and operation optimization of receiver performance. These technical difficulties require further exploration to ensure practical engineering applications of solid particle solar receivers. The present review provides a valuable reference for theoretical research on photothermic power generation and thermochemical tech­ nologies as well as their practical application.

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