Effect of Bed Particle Size on Thermal Performance of a Directly-Irradiated

processes

Article Eﬀect of Bed Particle Size on Thermal Performance of a Directly-Irradiated Fluidized Bed Gas Heater

Sae Han Park , Chae Eun Yeo, Min Ji Lee and Sung Won Kim * School of Chemical and Material Engineering, Korea National University of Transportation, Chungju-si 27469, Chungbuk, Korea; [email protected] (S.H.P.); [email protected] (C.E.Y.); [email protected] (M.J.L.) * Correspondence: [email protected]; Tel.: +82-43-841-5228

Received: 18 June 2020; Accepted: 8 August 2020; Published: 11 August 2020

Abstract: There is a growing interest in a fluidized bed particle receiver that directly irradiates sunlight to particles in the fluidized bed as a solar thermal collector for heating. Thermal performance of directly-irradiated fluidized bed gas heater is strongly affected by the physical properties of the particles. The effect of SiC particle size on heat transfer characteristics in the solar fluidized bed gas heater (50 mm-ID 100 mm high) has been determined. The outlet gas temperatures showed a × maximum value with increasing gas velocity due to the particles motion by bubble behavior in the bed, and the maximum values were found at 3.6 times of Umf for fine SiC and less than 2.0 times of Umf for coarse SiC. Heat absorption from the receiver increased with increasing gas velocity, showing with maximum 18 W for the fine SiC and 23 W for the coarse SiC at 4.5 times of Umf. The thermal efficiency of the receiver increased with increasing gas velocity, but was affected by the content of finer particles. The maximum thermal efficiency of the receiver was 14% for fine SiC and 20% for coarse SiC within the experimental range, but showing higher for the fine SiC at the same gas velocity. A design consideration was proposed to improve the thermal efficiency of the system.

Keywords: solar thermal energy; ﬂuidized bed; gas heater; silicon carbide; particle size

1. Introduction To reduce the environmental impact of huge energy-use in industry, companies continue to devote their efforts to using renewable energy. Among the various renewable energies, solar energy has received the most attention as a promising process application [1]. Solar energy is the most abundant renewable and the cleanest source of energy. Solar energy conversion technology is divided into PV (Solar photovoltaic energy) technology, CSP (Concentrating Solar Power) technology and STC (Solar Thermal Collectors for heating and cooling) technology. Much progress has been made in PV technology and the CSP technology to reduce carbon dioxide emissions in the power generation sector, which is responsible for a very high fraction of fossil fuel consumption so far. Recently, there is a growing interest in the STC technology in order to produce medium- and low-temperature energy that is applied to processes below 300 ◦C, such as gas preheating in coal power plant and refinery industries, as well as primary and secondary industries (agriculture, chemical and paper) [2]. The STC technology for utilizing solar energy consists mainly of a solar collection technology for collecting sunlight such as conventional flat plate, parabolic dish and Fresnel lens, and a receiver technology for transferring collected sunlight to an energy carrier. The solar collectors have been continuously developed to improve efficiency; it has recently been found that the performance of a simple flat collector can be greatly improved by optimized configuration with a flat booster bottom reflector [3]. The receiver system can be divided into a direct irradiation method and an indirect irradiation method. The most commonly used method is to indirectly heat the tube for heating the salt or vapor in the tube or to heat the vapor or gas in the transparent tube irradiated

Processes 2020, 8, 967; doi:10.3390/pr8080967 www.mdpi.com/journal/processes Processes 2020, 8, 967 2 of 11 with sunlight. However, the indirect receiver system may have low heat transfer efficiency due to high resistance in heat transfer. The direct irradiation to water vapor or gas also has a difficulty in efficiently transferring the energy because of the ability of the medium to absorb heat [4]. Thus, there is a growing interest in a fluidized bed particle receiver that directly irradiates sunlight to particles in the fluidized bed, because it has good heat transfer characteristics due to smooth mixing between fluidized particles and gas [5]. Many studies have been conducted to develop the fluidized bed particle receiver and the particles for thermochemical heat storage for the CSP applications [6–8]. However, there have been relatively few studies on the gas heating using this system for the STC application. Although both applications use the principle of fluidized bed, the direct solar heat transfer to energy storage particles is important in the CPS applications, while the heat transfer of absorbed energy in the solid to the gas is also very important [1,4]. Thus, the study on particle behavior as an energy transfer medium in the gas–solid fluidized bed is an important part of the STC field. The thermal efficiency of the fluidized bed particle receiver is strongly influenced by the physical properties of the particles at a given operating temperature [9,10]. It has been found that an optimal particle size is ranged between 500 and 1000 µm in mean size diameter for the fluidized bed receiver [7]. However, it is expected that the size depends on the operating temperature [7]. So far, most studies on the effect of particle properties have been carried out in the receiver at high temperature over 600 ◦C, and the research in the medium and low temperature region below 300 ◦C is relatively sparse. [1,7] Moreover, many studies have been conducted using xenon lamps as a heat source in the simulated indoor space, not the outdoor environment [8,11,12]. In this study, thermal performance experiments were carried out in a directly-irradiated fluidized bed of different particles in the outdoor environment for the application of gas heating. The effect of SiC particle size on the thermal efficiency of the preheater has been determined.

2. Experimental Experiments were carried out in a solar energy particle receiver system as in Figure1. The fluidized bed gas heating system has two parts such as a solar collector and a particle receiver. The solar collector system consists of two reflective mirrors (model 1100, 3 M) with reflectance of 93%, Fresnel lens (effective ID = 457 mm, Edmund optics) and focal lens. The first reflective mirror was installed in front of the Fresnel lens in order to chase the direct sunlight of the moving sun as in Figure1a. The Fresnel lens has a light transmittance of 92%. The solar power meter (Procal1333R, TES) was installed on the Fresnel lens, which faces the first reflective mirror to measure the directly nominal irradiance supplied into the fluidized bed through the Fresnel lens. It was checked periodically whether the light intensity changed in radial direction across the Fresnel lens. The fluidized bed particle receiver was made of Pyrex glass and has a main column (50 mm-ID 100 mm high) as shown in Figure1b. The main column has the enlarged top section (110 mm-ID) × to reduce the particles’ elutriation. A highly transparent quartz plate is located at the top surface of the column to get high transmittance for light irradiation. The air as a fluidizing-gas was introduced into the column through a distributor via a mass flow meter (RK1150, Kojima instrument, Kyotanabe Kyoto, Japan). A distributor of sintered plate type was used to inject the air. Pressure drops across the bed were measured at two pressure taps, which were located at column wall around the inlet and outlet to get the information of solid holdup. A total of 5 thermocouples (K-type) were installed to measure the temperatures of inlet air, outlet air and bed particles. Processes 2020, 8, x FOR PEER REVIEW 3 of 11

Processes 2020 8 outlet to get, the, 967 information of solid holdup. A total of 5 thermocouples (K-type) were installed3 of to 11 measure the temperatures of inlet air, outlet air and bed particles.

(a) (b)

Figure 1. Experimental apparatus of solar heat gas heater system: (a) solar light collector, (b) particle Figure 1. Experimental apparatus of solar heat gas heater system: (a) solar light collector, (b) particle receiver, (c) photo of the system in the field test, (d) top view of the particle receiver in the field test. receiver, (c) photo of the system in the field test, (d) top view of the particle receiver in the field test. Two kinds of silicon carbide (SiC) particles, a good absorber of the radiation, were adopted as Two kinds of silicon carbide (SiC) particles, a good absorber of the radiation, were adopted as bed materials to investigate the effect of the particle diameter. The physical properties of the SiC bed materials to investigate the effect of the particle diameter. The physical properties of the SiC particles are shown in Table1. They have almost the same particle density but di fferent mean particle particles are shown in Table 1. They have almost the same particle density but different mean particle diameter and size distribution (PSD) based on sieve analysis as shown in Figure2a. The fine SiC diameter and size distribution (PSD) based on sieve analysis as shown in Figure 2a. The fine SiC particles (F220, Showa denko, Tokyo, Japan) have the particle mean diameter of 0.052 mm and the particles (F220, Showa denko, Tokyo, Japan) have the particle mean diameter of 0.052 mm and the apparent particle density of 2992 kg/m3, corresponding to the Geldart A group based on Geldart apparent particle density of 2992 kg/m3, corresponding to the Geldart A group based on Geldart classification [13]. The coarse SiC particles (F100, Showa denko, Tokyo, Japan) have the particle mean classification [13]. The coarse SiC particles (F100, Showa denko, Tokyo, Japan) have the particle mean diameter of 0.123 mm and the apparent particle density of 3015 kg/m3, corresponding to the Geldart B diameter of 0.123 mm and the apparent particle density of 3015 kg/m3, corresponding to the Geldart group [13]. B group [13].

Table 1. Physical properties of SiC (Silicon carbide) particles used in the study.

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Average Particle Diameter, dp Apparent Density, ρp Particles Reflectivity [%](c) [μm] (a) [kg/m3] (b)

Fine SiC 65.7 1647 11.2

Coarse SiC 68.6 1646 9.0

(a) based on the sieve analysis; (b) based on measurement with pycnometer (ASTM D 854-14 [14]); (c) based on analysis (wave length: 400–2000 nm)) with UV/Vis/NIR Spectrometer (LAMBDA750, PerkinElmer, Waltham, MA, USA). Processes 2020, 8, 967 4 of 11 In comparison of the PSD (Figure 2a), the fine SiC has 25.2 wt% of of fine particles less than 45 microns (F45), but the coarse SiC has no F45. The static bed height was 45 mm with the bed materials of 120 g. The solarTable irradiation 1. Physical was properties concentrated of SiC (Silicon by the carbide)Fresnel particleslens, and used the inlight the study.was directed toward the surface of the SiC particles in the fluidized bed at given gas velocity, as in Figure 1d. The focal Average Particle Diameter, Apparent Density, ρp (c) Particles (a) 23 (b) Reﬂectivity [%] area of the light on the particledp [µ bedm] surface is about 1257[kg mm/m . ]The gas velocity was in the range of 0–0.11Fine m/s. SiC The experiment was 52 performed in an outdoor steady 2992 state [15]. The air was 11.2 injected from a compressor and maintained at a constant temperature. Tests were carried out after determining Coarse SiC 123 3015 9.0 steady state that the air outlet temperature from the particle receiver is constant under the condition (a) based on the sieve analysis; (b) based on measurement with pycnometer (ASTM D 854-14 [14]); (c) based on analysis where(wave the length:solar intensity 400–2000 nm)) did with not UV change/Vis/NIR significantly Spectrometer (LAMBDA750,for a certain PerkinElmer,period. Waltham, MA, USA).

(a)

(b) (c)

Figure 2. (a) Particle size distributions of SiC particles, Magnified images of (b) fine SiC particles and Figure 2. (a) Particle size distributions of SiC particles, Magnified images of (b) fine SiC particles and (c) coarse SiC particles. (c) coarse SiC particles. In comparison of the PSD (Figure2a), the fine SiC has 25.2 wt% of of fine particles less than 45 3. Results and Discussion microns (F45), but the coarse SiC has no F45. The static bed height was 45 mm with the bed materials of

120 g.ItThe is very solar important irradiation to was obtain concentrated the minimum by the Fresnel fluidization lens, and velocity the light (Umf was) for directed determining toward the surfaceoptimal of operating the SiC particles condition in the of ﬂuidizedthe particle bed atreceiver given gasconsidering velocity, asthe in Figuregas–solid1d. Thebehavior focal areain the of 2 thefluidized light on bed the [8]. particle Figure bed 3 surfaceshows variations is about 1257 of mmthe pressure. The gas drop velocity across was the in thebed range with ofvarying 0–0.11 mgas/s. Thevelocity experiment for determining was performed the Umf in. The an outdoor pressure steady drop stateacross [15 the]. Thefixed air bed was increases injected fromlinearly a compressor due to the andincrease maintained in kinetic at aenergy constant loss temperature. and friction Testsloss with were the carried increasing out after of determiningthe gas velocity steady through statethat the the air outlet temperature from the particle receiver is constant under the condition where the solar intensity did not change signiﬁcantly for a certain period.

3. Results and Discussion

It is very important to obtain the minimum fluidization velocity (Umf) for determining the optimal operating condition of the particle receiver considering the gas–solid behavior in the fluidized bed [8]. Figure3 shows variations of the pressure drop across the bed with varying gas velocity for determining the Umf. The pressure drop across the fixed bed increases linearly due to the increase in kinetic energy loss and friction loss with the increasing of the gas velocity through the voidage in the bed. The pressure drop then reaches a constant value that is approximately equal to the weight of the bed material, Processes 2020, 8, x FOR PEER REVIEW 5 of 11

Processes 2020, 8, 967 5 of 11 voidage in the bed. The pressure drop then reaches a constant value that is approximately equal to the weight of the bed material, meaning that all of the bed materials are fluidized. As shown in the meaningFigure 3, thatthe U allmf ofis determined the bed materials from the are intersection fluidized. As of showntwo pressure in the Figuredrop change3, the U linesmf is with determined different fromslopes the [8,16]. intersection From Figure of two 3, pressurethe Umf of drop fine changeSiC particles lines withis about diff 0.006erent m/s. slopes The [8 coarse,16]. From particle Figure Umf3 is, the0.017 Umf m/s,of finewhich SiC is particlesabout three is about times 0.006as high m/ ass. Thethat coarseof fine particle Umf. Itis is 0.017 believed m/s, whichthat the is system about threewith timesfine SiC as highparticles as that is capable of fine particle of operating Umf. Itin is the believed fluidized that bed the at system a lower with gas fine velocity SiC particles range. isIn capableaddition, of it operating is more inlikely the fluidizedthat higher bed gas at atemp lowereratures gas velocity can be range. achieved In addition, compared it is to more the likelycoarse that SiC higherbed due gas to temperaturessufficient fluidized can be bed–gas achieved contact compared time at to lower the coarse gas velocities SiC bed dueand tolarger suffi cientspecific fluidized surface bed–gasarea of the contact particles. time at lower gas velocities and larger specific surface area of the particles.

Figure 3. Pressure drop-versus-velocity diagram for determination of U . Figure 3. Pressure drop-versus-velocity diagram for determination of Umfmf. The performance of the solar energy system depends strongly on the weather, so it is necessary to The performance of the solar energy system depends strongly on the weather, so it is necessary measure the reachable temperature of the system and to analyze the system feasibility and thermal to measure the reachable temperature of the system and to analyze the system feasibility and thermal efficiency based on the results of the temperature variation. The effect of gas velocity on temperatures efficiency based on the results of the temperature variation. The effect of gas velocity on temperatures of bed and outlet gas in the fluidized bed receiver is shown in Figure4. In this study, the temperature of bed and outlet gas in the fluidized bed receiver is shown in Figure 4. In this study, the temperature of the bed material was measured at a location of 60% of the total bed height, which is 2 cm below the of the bed material was measured at a location of 60% of the total bed height, which is 2 cm below surface exposed to solar light. Fine SiC particles showed higher temperature than coarse particles. the surface exposed to solar light. Fine SiC particles showed higher temperature than coarse particles. However, the comparison of the results between the particles is not meaningful here, because the However, the comparison of the results between the particles is not meaningful here, because the obtainable temperature in the receiver depends on the weather condition at the measurement date [17]. obtainable temperature in the receiver depends on the weather condition at the measurement date Figure4 shows well the di fference in the direct normal irradiance (DNI) values, which is the amount [17]. Figure 4 shows well the difference in the direct normal irradiance (DNI) values, which is the of solar radiation received per unit area, due to the different experiment dates of the two particles. amount of solar radiation received per unit area, due to the different experiment dates of the two The outlet gas temperature of both particles was lower than the bed temperature at low gas velocity, particles. The outlet gas temperature of both particles was lower than the bed temperature at low gas indicating a heat loss through the light transmission window at the top of the reactor (Figure1d). velocity, indicating a heat loss through the light transmission window at the top of the reactor (Figure However, as the gas velocity increases, the heat absorption amount increases and the residence time of 1d). However, as the gas velocity increases, the heat absorption amount increases and the residence the gas in the freeboard decreases so that the outlet gas temperature approaches the bed temperature. time of the gas in the freeboard decreases so that the outlet gas temperature approaches the bed The bed temperature and outlet gas temperature showed maximum values with variation of the gas temperature. The bed temperature and outlet gas temperature showed maximum values with velocity, because the particle behavior in the fluidized bed is strongly affected by the bubbles’ formation variation of the gas velocity, because the particle behavior in the fluidized bed is strongly affected by and their motion in the bed. More specifically, the formation of gas bubbles in the fluidized bed and the bubbles’ formation and their motion in the bed. More specifically, the formation of gas bubbles the upward flow of the bubbles give flywheel effect in the fluidized bed, giving better mixing and in the fluidized bed and the upward flow of the bubbles give flywheel effect in the fluidized bed, convective heat transfer effects of the gas-particles in the whole fluidized bed [18]. A gulf streaming in giving better mixing and convective heat transfer effects of the gas-particles in the whole fluidized the bed with downflow of emulsion at the center is induced by the uneven rising bubbles with the bed [18]. A gulf streaming in the bed with downflow of emulsion at the center is induced by the increase in gas velocity [13,19], enhancing the mixing of the hot particles on the bed surface with the uneven rising bubbles with the increase in gas velocity [13,19], enhancing the mixing of the hot other region of the bed and making the temperature distribution in the whole bed uniform. particles on the bed surface with the other region of the bed and making the temperature distribution in the whole bed uniform.

The bubble fraction increases with further increasing of the gas velocity, resulting in increased direct heat transfer efficiency of the particles to gas due to the vigorous mixing in the bed. The outlet gas temperature also increases with the increase in heat transfer efficiency. However, the temperature

Processes 2020, 8, x FOR PEER REVIEW 6 of 11

also decreases with the further increase in gas velocity due to an increase in contacting resistance between the gas in enlarged bubble and the particles, and a reduction in the gas residence-time in the bed of the reactor [18]. In the comparison of the particles, the maximum value was found at 3.6 times of Umf (3.6 times of minimum fluidization velocity) for fine SiC, and the maximum at less than 2.0 times of Umf for coarse particles. The maximum value at different Umf numbers is due to the different fluidization characteristics between the two particles. Coarse SiC belonging to Geldart group B showed a maximum temperature at the initial stage due to the immediate generation of bubbles after the Umf, whereas fine SiC belonging to the Geldart group A generally produces bubbles at 2–3 times of Umf, resulting in the maximum temperature appearing relatively late [20]. Interestingly, the fine SiC exhibited a peculiar phenomenon where the outlet gas temperature was higher than the bed temperature, unlike the coarse SiC, with further increase in the gas velocity above 0.023 m/s. This is because the fine SiC with high contents of fines (F45: particles less than 45 μm [16]) are easily entrained upward at a high gas velocity [16], resulting in further heat transfers to the gas in the expanded column. The coarse SiC with a relatively small amount of fines showed similar bed and outlet gas Processestemperatures2020, 8, 967 as the gas velocity increased. 6 of 11

FigureFigure 4.4. EEffectﬀect of gasgas velocityvelocity onon temperaturestemperatures ofof bedbed andand outletoutlet gasgas inin thethe particleparticle receiver.receiver.

TheAn bubbleanalysis fraction in viewpoint increases withof dimensionless further increasing temperature of the gas with velocity, height resulting is important in increased for directunderstanding heat transfer thermal efficiency performance of the particles of reactor to gasdesign due. [21]. to the In vigorous order to mixingexamine in the the relation bed. The between outlet gasthe temperatureparticle behavior alsoincreases in the bed with and the the increase thermal in pe heatrformance transfer of effi theciency. reactor, However, this study the temperatureintroduces a alsodimensionless decreases withtemper theature further variation increase as in gas velocity due to an increase in contacting resistance between the gas in enlarged bubble and the particles, and a reduction in the gas residence-time in the bed of the reactor [18]. In the comparison of the particles, the maximum value was found at 3.6 T* = (Tbed − Tinlet)/(Toutlet − Tinlet) (1) times of Umf (3.6 times of minimum fluidization velocity) for fine SiC, and the maximum at less than 2.0 timesFigure of U5 mfshowsfor coarse variations particles. of the The dimensionl maximumess value temperature at different (T*) Umf withnumbers gas isvelocity due to and the difluidizationfferent fluidization number characteristics(Ug/Umf). An ideal between fluidized the two bed particles. heat exchanger Coarse has SiC 1.0 belonging of the T* to where Geldart bed group and Boutlet showed temperatures a maximum are temperature the same. atThe the dimensionl initial stageess due temperature to the immediate was higher generation than of 1.0 bubbles at low after gas thevelocity Umf, whereasand decreased fine SiC below belonging 1.0 with to increasing the Geldart gas group velocity. A generally A value produces above 1.0 bubbles at low gas at 2–3velocities times ofmeans Umf, that resulting the gas in theabsorbed maximum heat from temperature the bed appearingmaterial as relatively it passed latethroug [20].h Interestingly,the fluidized bed, the fine but SiCthe exhibitedtemperature a peculiar of the gas phenomenon was lowered where due theto he outletat loss gas through temperature the solar-transmission was higher than window the bed temperature,above the bed unlike surface the (Figure coarse 1d). SiC, This with means further that increase the control in the of gas heat velocity loss in abovethe expanded 0.023 m /columns. This isis becausemore important the fine SiC than with that high inside contents the ofbed fines to (Fimprove45: particles thermal less thanperformance 45 µm [16 at]) arelow easily gas velocities. entrained upwardHowever, at athe high T* gas reached velocity 1.0 [ 16at], a resulting certain gas in further velocity, heat which transfers means to the that gas enhanced in the expanded gas–solid column. heat Thetransfer coarse occurred SiC with over a relatively heat loss small through amount the of wind finesow. showed Interestingly, similar bed the and system outlet showed gas temperatures that both as the gas velocity increased.

An analysis in viewpoint of dimensionless temperature with height is important for understanding thermal performance of reactor design. [21]. In order to examine the relation between the particle behavior in the bed and the thermal performance of the reactor, this study introduces a dimensionless temperature variation as T* = (T T )/(T T ) (1) bed − inlet outlet − inlet Figure5 shows variations of the dimensionless temperature (T*) with gas velocity and fluidization number (Ug/Umf). An ideal fluidized bed heat exchanger has 1.0 of the T* where bed and outlet temperatures are the same. The dimensionless temperature was higher than 1.0 at low gas velocity and decreased below 1.0 with increasing gas velocity. A value above 1.0 at low gas velocities means that the gas absorbed heat from the bed material as it passed through the fluidized bed, but the temperature of the gas was lowered due to heat loss through the solar-transmission window above the bed surface (Figure1d). This means that the control of heat loss in the expanded column is more important than that inside the bed to improve thermal performance at low gas velocities. However, the T* reached 1.0 Processes 2020, 8, 967 7 of 11

Processes 2020, 8, x FOR PEER REVIEW 7 of 11 at a certain gas velocity, which means that enhanced gas–solid heat transfer occurred over heat loss throughparticles the reached window. 1.0 Interestingly,at the fluidization the system number showed of about that both 4.5 particlestimes of reachedUmf, as shown 1.0 at the in fluidization Figure 5b, numberwhere the of aboutfluidization 4.5 times number of Umf corresponds, as shown in to Figure the ve5locityb, where as the fluidizationgas starts tonumber rise homogeneously corresponds tothrough the velocity the bed as [22]. the gas The starts fine SiC to rise having homogeneously large amounts through of easily the entr bedained [22]. Thefines fine exhibits SiC having a relatively large amountslow T* atof a high easily gas entrained velocity fines compared exhibits to a coarse relatively SiC, low confirming T* at a high that gas the velocity additional compared heat transfer to coarse in SiC,the freeboard confirming region that thehas additionala greater effect heat on transfer the overal in thel heat freeboard transfer. region Therefore, has a greaterit is considered effect on that the overallcontrol heatof heat-transfer transfer. Therefore, inside the it bed, is considered such as bu thatbble control size control, of heat-transfer becomes more inside important the bed, suchat high as bubblegas velocity. size control, becomes more important at high gas velocity.

(a) (b)

Figure 5. EffectsEﬀects t of (a) (a) gas velocity and ((b)b) fluidizi ﬂuidizingng number on dimensionless temperature (T*).

Figure6 6 shows shows an an impact impact of of gas gas velocity velocity on on heat heat absorption absorption by gasby ingas the in particlethe particle receiver. receiver. The heat The energyheat energy is obtained is obtained by Equation by Equation (2) as (2) as

Qgas = ρgas Ug ACp,gas (Toutlet Tinlet) (2) Qgas = ρgas Ug A Cp,gas (Toutlet −− Tinlet) (2) where, QQgasgas isis heat energy per unit timetime ofof thethe gas,gas, ρρgasgas is gas density,density, AA isis reactorreactor area,area, and and C Cp,gasp,gas is the heat capacity of the gas. The The heat heat absorption absorption fr fromom the the receiver receiver increased increased as as gas gas velocity velocity increased. increased. As mentionedmentioned earlier, thethe increaseincrease in gasgas velocityvelocity increases the mixing of the bed materials in the fluidizedfluidized bedbed andand the the temperature temperature uniformity uniformity in in the the bed bed due due to theto the increases increases in bubble in bubble frequency frequency [18]. Eventually,[18]. Eventually, this increasesthis increases the directthe direct gas–solid gas–solid heat heat transfer transfer rate, rate, thereby thereby increasing increasing the the amount amount of energyof energy delivered delivered to the to gas.the Thegas. heatThe transferheat transfer rate and rate thus and the thus exit the gas exit temperature gas temperature slightlydecrease slightly asdecrease the bubble as the fraction bubble increases fraction withincreases further with increase further in increase the gas velocity,in the gas but velocity, the resulting but the amount resulting of heatamount absorption of heat increasesabsorption due increases to the increased due to the amount increased of injected amount air. of In thisinjected study, air. maximum In this study, 18 W withmaximum fine SiC 18 W and with 23 Wfine with SiC coarseand 23 SiCW with were coarse obtained SiC were within obtained the experimental within the experimental range. The fine range. SiC wereThe fine also SiC expected were also to obtainexpected higher to obtain amounts higher of heatamounts absorption of heat throughabsorption additional through heat additional transfer heat by thetransfer entrained by the fines entrained in the fines freeboard in the region freeboard as the region gas velocity as the gas increases, velocity but increases, the increase but the in theincrease fines entrainmentin the fines toentrainment exit region causesto exit anregion excessive causes release an excessive of particles, release thus limitingof particles, the experimental thus limiting range the inexperimental the current system.range in It the will current be possible system. to improve It will be this possible by installing to improve particle this recovery by installing systems particle such as internalrecovery cyclones systems [ 13such,23 ].as On internal the other cyclones hand, [13,23 coarse]. SiC On particles the other without hand, coarse the fines SiC could particles obtain without higher amountsthe fines could of energy obtain through higher operation amounts atof higher energy flow through rates. operation at higher flow rates.

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Figure 6. Effect of gas velocity on heat absorption by gas in particle receiver with different SiC. Figure 6. Effect of gas velocity on heat absorption by gas in particle receiver with different SiC. In order to calibrate the daylight difference or DNI variation between test dates as in Figure6, In order to calibrate the daylight difference or DNI variation between test dates as in Figure 6, the thermal efficiency comparison of the receiver is shown in Figure7. The thermal e fficiency is the thermal efficiency comparison of the receiver is shown in Figure 7. The thermal efficiency is obtained by Equations (3) and (4) as obtained by Equations (3) and (4) as

ηthermal = Qgas/Qsolar (3) ηthermal = Qgas/Qsolar (3) Qsolar = IDNI AFresnel ρmirror τl,Fresnel τl,focal τquartz (4) Qsolar = IDNI AFresnel ρmirror τl,Fresnel τl,focal τquartz (4) where, ηthermal is energy efficiency of the system, Qsolar is solar energy per time obtained by the solar where, ηthermal is energy efficiency of the system, Qsolar is solar energy per time obtained by the solar collector system, IDNI is intensity of direct normal irradiance, AFresnel is Fresnel lens area, ρmirror is collector system, IDNI is intensity of direct normal irradiance, AFresnel is Fresnel lens area, ρmirror is reflectance of the second reflect mirror, τl is transmittance of lens, and τquartz is transmittance of reflectancetransparent of quartz the second plate. Thereflect thermal mirror, effi ciencyτl is transmittance increased with of anlens, increase and τ inquartz gas is velocity transmittance due to theof transparentincreased heat quartz transfer plate. in The the thermal bed by efficiency the increased increased bubble with fraction. an increase Maximum in gas thermal velocity eduefficiencies to the increasedof 14% for heat fine transfer SiC and in 20% the forbed coarse by the SiC increase wered obtained bubble fraction. within the Maximum experimental thermal range. efficiencies These are of 14%relatively for fine low SiC values and compared20% for coarse to the previousSiC were report,obtained caused within by athe large experimental heat loss out range. of the These system are in relativelythe field [ 24low]. Invalues comparison compared of theto the two previous particles, repo thert, fine caused SiC showed by a large higher heat thermal loss out e offfi ciencythe system than inthe the coarse field SiC [24]. when In comparison compared at of the the same two gas particle velocitys, the (dotted fine SiC box showed in Figure higher7). This thermal is the opposite efficiency of thanwhat the is reported coarse SiC in the when high compared temperature at receiverthe same [9 ga,10s]. velocity In this receiver, (dotted thebox high in Figure thermal 7). performance This is the oppositeof the fine of SiC what is causedis reported by the in particlethe high properties temperatur ande receiver its behavior [9,10]. such In this as large receiver, specific the area high and thermal small performanceheat transfer resistanceof the fine dueSiC tois caused small bubble, by the whichparticle are properties characteristics and its of behavior Geldart Asuch group as large particles specific [20]. areaHowever, and small the energy heat transfer efficiency resistance of the fine due SiC to particles small bubble, did not which show a are significant characteristics increase of compared Geldart A to groupthe coarse particles SiC, contrary[20]. However, to expectations the energy based efficiency on the of hydrodynamic the fine SiC particles behavior did of not the show particles a significant shown in increase compared to the coarse SiC, contrary to expectations based on the hydrodynamic behavior Figures4 and5. This unexpected result is due to the di fference in fines content (F45) of SiC particles. of the particles shown in Figures 4 and 5. This unexpected result is due to the difference in fines Coarse SiC has no F45, while fine SiC has an F45 content of 25.2 wt%. The fines below 45 µm in the contentfluidized (F45 bed) of generallySiC particles. serve Coarse as additives SiC has tono aid F45, fluidization, while fine SiC such has as an lubricants, F45 content between of 25.2 wt%. particles The finesin the below fluidized 45 μ bedm in [16 the]. However,fluidized bed it was generally observed serve that as the additives fines are to easily aid fluidization, entrained at such low gasas lubricants,velocity [25 between] and attached particles to the in solarthe fluidized light-transmission bed [16]. However, plate by van it derwas Waals observed force that [26– the28] asfines shown are easilyin Figure entrained8a. This at lowerslow gas the velocity light transmissivity [25] and attached of the to the sunlight solar collectedlight-transmission by the Fresnel plate by lens van to theder Waalsreceiver, force resulting [26–28] in as a lowershown thermal in Figure effi ciency8a. This of thelowers system. the light However, transmissivity the effect of thethe finessunlight still collectedseems to predominateby the Fresnel the lens positive to the eff receiver,ect under resu a certainlting conditionin a lower due thermal to the efficiency enhanced heatof the transfer system. in However,the freeboard the regioneffect of as the shown fines instill a comparativeseems to pred experimentominate the carried positive out effect at 0.05 under m/s showna certain in condition Figure8b. dueTherefore, to the itenhanced is desirable heat to reducetransfer the in contentthe freeboa of smallerrd region fines, as which shown easily in a adherecomparative to the transmissionexperiment carriedwindow, out in at order 0.05 tom/s further shown improve in Figure the 8b. system’s Therefore, effi ciency.it is desirable Further to study reduce is the required content to of define smaller the fines,smaller which fines easily and to adhere optimize to th thee transmission PSD of the bed window, considering in order the to eff ectsfurther of theimprove opposing the esystem’sffects of efficiency.the fines. Further study is required to define the smaller fines and to optimize the PSD of the bed considering the effects of the opposing effects of the fines.

Processes 2020, 8, 967 9 of 11 ProcessesProcesses 2020 2020, ,8 8, ,x x FOR FOR PEER PEER REVIEW REVIEW 99 of of 11 11

FigureFigureFigure 7. 7.7. Effect EEffectffect of ofof gas gasgas velocity velocityvelocity on onon the thethe thermalthermal efficien eefficienfficiencycycy of of particle particle receiver receiver with with didifferentdifferentfferent SiC. SiC.

((aa))

((bb))

FigureFigure 8. 8. Effect EEffectffect of of fine finefine powder powder less less than than 45 45 μµ μmm in in bed bed materials materials on on light light transmissivity: transmissivity: ( (aa))) fine finefine power powerpower stuckstuck to to window;window; ( b(b)E)) Effect Effectffect of of finefine fine powderpowder powder onon on thermalthermal thermal eefficiency ffiefficiencyciency atat at UU Uggg = = 0.05 0.05 m/s. mm/s./s.

The design and performance of the fluidized bed heat exchanger has been optimized depending TheThe design design and and performance performance of of the the fluidized fluidized bed bed heat heat exchanger exchanger has has been been optimized optimized depending depending on particle properties and operating conditions such as gas velocity [29]. The results of this study onon particleparticle propertiesproperties andand operatingoperating conditionsconditions suchsuch asas gasgas velocityvelocity [29].[29]. TheThe resultsresults ofof thisthis studystudy could thus provide information for further improvement of the solar particle receiver compatible couldcould thusthus provideprovide informationinformation forfor furtherfurther improvemimprovementent ofof thethe solarsolar particleparticle receiverreceiver compatiblecompatible with solar light collecting system. The small particles in the Geldart A group have positive effects withwith solarsolar lightlight collectingcollecting system.system. TheThe smallsmall particlesparticles inin thethe GeldartGeldart AA groupgroup havehave positivepositive effectseffects such as improved fluidity in the bed and additional heat transfer in the freeboard region in viewpoint suchsuch as as improved improved fluidity fluidity in in the the bed bed and and additional additional heat heat transfer transfer in in the the freeboard freeboard region region in in viewpoint viewpoint of the fluidized bed, but have a negative effect on the light transmittance in viewpoint of the solar ofof thethe fluidizedfluidized bed,bed, butbut havehave aa negativenegative effecteffect onon thethe lightlight transmittancetransmittance inin viewpointviewpoint ofof thethe solarsolar system. In addition, the application of an expanded column above the reactor to reduce the amount of system.system. In In addition, addition, the the application application of of an an expanded expanded column column above above the the reactor reactor to to reduce reduce the the amount amount particles entrained at high gas velocity has a negative effect of providing heat loss due to the increase ofof particlesparticles entrainedentrained atat highhigh gasgas velocityvelocity hashas aa negativenegative effecteffect ofof providingproviding heatheat lossloss duedue toto thethe in gas residence time in the upper region. Therefore, for further improvement of the system’s thermal increaseincrease in in gas gas residence residence time time in in the the upper upper region. region. Therefore, Therefore, for for further further improvement improvement of of the the system’s system’s thermalthermal efficiency, efficiency, further further studies studies on on the the design design of of the the reactor reactor freeboard freeboard section section that that can can reduce reduce the the

Processes 2020, 8, 967 10 of 11 efficiency, further studies on the design of the reactor freeboard section that can reduce the gas residence time as well as the optimum particulate content in the Geldart A particles to minimize the negative effects are required [30]. In addition, an improvement such as a double transmission window will also be required in order to minimize the energy loss through the transmission window in which external insulation is difficult.

4. Conclusions The effect of bed particle size on thermal performance of the directly-irradiated fluidized bed gas heater has been determined. The performance of the system was greatly affected by the physical properties of the particles with an increase in gas velocity. The temperatures of the bed and the outlet gas showed maximum points with gas velocity, and the maximum values were found at 3.6 times of Umf for fine SiC and less than 2.0 times of Umf for coarse SiC. Heat absorption and thermal efficiency from the receiver increased with increasing gas velocity, showing with maximum 18 W and 14% for the fine SiC and 23 W and 20% for the coarse SiC. Design considerations on the fine content in bed materials and reactor geometry were proposed to improve the thermal efficiency of the system.

Author Contributions: S.H.P. conducted the research, wrote the paper, C.E.Y. and M.J.L. analyzed the data, S.W.K. reviewed, edited and supervised. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Ministry of Education (NRF-2019R1A2C1011671). Acknowledgments: This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2019R1A2C1011671). Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Wang, F.; Bai, F.; Wang, T.; Li, Q.; Wang, Z. Experimental study of a single quartz tube solid particle air receiver. Sol. Energy 2016, 123, 185–205. 2. Jia, T.; Huang, J.; Li, R.; He, P.; Dai, Y. Status and prospect of solar heat for industrial processes in China. Renew. Sustain. Energy Rev. 2019, 90, 475–489. 3. Roberto, B.; Frattolillo, A.; Mastino, C.; Curreli, S.; Ghiani, E. A comprehensive optimization model for flat solar collector coupled with a flat booster bottom reflector based on an exact finite length simulation model. Energy Convers. Manag. 2018, 164, 482–507. 4. Li, S.; Kong, W.; Zhang, H.; Sabatier, F.; Ansart, R.; Flamant, G.; Baeyens, J. The fluidized bed air heater exchanger in a hybrid Brayton-cycle solar power plant. In Proceedings of the 6th ASCON-IEEChE, Taiwan, China, 4 November 2018. 5. Alonso, E.; Romero, M. Review of experimental investigation on directly irradiated particles solar reactors. Renew. Sustain. Energy Rev. 2015, 41, 53–67.

6. Gokon, N.; Ono, R.; Hatamachi, T.; Liuyun, L.; Kim, H.; Kodama, T. CO2 gasification of coal cokes using internally circulating fluidized bed reactor by concentrated Xe-light irradiation for solar gasification. Int. J. Hydrogen Energy 2012, 37, 12128–12137. [CrossRef] 7. Flamant, G. Theoretical and experimental-study of radiant-heat transfer in a solar fluidized-bed receiver. AIChE J. 1982, 28, 529–535. [CrossRef] 8. Matsubara, K.; Kazuma, Y.; Sakurai, A.; Suzuki, S.; Lee, S.J.; Kodama, T.; Gokon, N.; Choi, H.S.; Yoshida, K. High-temperature Fluidized Receiver for Concentrated Solar Radiation by a Beam-down Reflector System. Energy Procedia. 2014, 49, 447–456. [CrossRef] 9. Ma, Z.; Mehos, M.; Glatzmaier, G.; Sakadjian, B.B. Development of a concentrating solar power system using fluidized-bed technology for thermal energy conversion and solid particles for thermal energy storage. Energy Procedia 2015, 69, 1349–1359. 10. Ho, C.K. A review of high-temperature particle receivers for concentrating solar power. Appl. Therm. Eng. 2016, 109, 958–969. [CrossRef] 11. Briongos, J.V.; Gomez-Hernandez, J.; Gonzalez-Gomez, P.A.; Serrano, D. Two-phase heat transfer model of a beam-down gas-solid fluidized bed solar particle receiver. Sol. Energy 2018, 171, 740–750. [CrossRef] Processes 2020, 8, 967 11 of 11

12. Tregambi, C.; Chirone, R.; Montagnaro, F.; Salatino, P.; Solimene, R. Heat transfer in directly irradiated fluidized beds. Sol. Energy 2016, 129, 85–100. [CrossRef] 13. Kunii, D.; Levenspiel, O. Fluidization Engineering, 2nd ed.; Butterworth-Heinemann: Waltham, MA, USA, 1991. 14. ASTM D854-14. Standard Test Methods for Specific Gravity of Soil Solids by Water Pycnometer; ASTM International: West Conshohocken, PA, USA, 2014. 15. Roberto, B.; Ubaldo, C.; Setfano, M.; Roberto, I.; Elisa, S.; Paolo, M. Graybox and adaptative dynamic neural network identification models to infer the steady state efficiency of solar thermal collectors starting from the transient condition. Sol. Energy 2010, 84, 1027–1046. 16. Kim, S.W.; Yeo, C.E.; Lee, D.Y. Effect of Fines Content on Fluidity of FCC Catalysts for Stable Operation of Fluid Catalytic Cracking Unit. Energies 2019, 12, 293. 17. Zhang, Y.N.; Bai, F.; Zhang, X.; Wang, F.; Wang, Z. Experimental study of a single quartz tube solid particle air receiver. Energy Procedia 2015, 69, 600–607. [CrossRef] 18. Kim, S.W.; Kim, S.D. Heat transfer characteristics in a pressurized fluidized bed of fine particles with immersed horizontal tube bundle. Int. J. Heat Mass Transf. 2013, 64, 269–277. 19. Kim, S.W.; Kim, S.D. Void properties in dense bed of cold-flow fluid catalytic cracking regenerator. Processes 2018, 6, 80. [CrossRef] 20. Abrahamsen, A.R.; Geldart, D. Behaviour of Gas-Fluidized Beds of Fine Powders Part I. Homogeneous Expansion. Powder Technol. 1980, 26, 35–46. 21. Chinsuwan, A.; Dutta, A. Investigation of the temperature distribution and material limits of membrane water wall tubes of circulating fluidized bed boilers. Can. J. Chem. Eng. 2010, 88, 144–152. 22. Cranfield, R.R.; Gliddon, B.J. Absorption in Shallow Fluidized Beds of Large Particles. Inst. Chem. Engrs. Symp. Series 1976, 38, H4. 23. Yang, W. Handbook of Fluidization and Fluid-Particle Systems; Marcel Deckker Inc: New York, NY, USA, 2003; pp. 1–27. 24. Gomez-Garcia, F.; Gauthier, D.; Flamant, G. Design and performance of a multistage fluidised bed heat exchanger for particle-receiver solar power plants with storage. Appl. Energy 2017, 190, 510–523. 25. Yeo, C.E.; Kim, S.W. Axial holdup distributions of FCC catalysts in a small-diameter riser with abrupt exit in circulating fluidized bed. Adv. Powder Technol. 2020, 31, 1946–1956. 26. Kim, S.W. Effect of height on CNT aggregates size and shape in freeboard region of a fluidized bed. Korean Chem. Eng. Res. 2019, 57, 105–110. 27. Zhu, C.; Yu, Q.; Dave, R.N. Gas fluidization characteristics of nanoparticle agglomerates. AIChE J. 2005, 51, 426–439. [CrossRef] 28. Cocco, R.; Issangya, A.; Karri, S.B.R.; Freeman, T.; Jaeger, H.M.; Knowlton, T.M. Small-Scale Particle Interactions Are Having Significant Effects on Global Fluidized Bed Behavior. KONA Powder Part. J. 2017, 34, 155–167. 29. Chen, J.C.; Grace, J.R.; Golriz, M.R. Heat transfer in fluidized beds: Design methods. Powder Technol. 2005, 150, 123–132. [CrossRef] 30. Ho, C.K. Advances in central receivers for concentrating solar applications. Sol. Energy 2017, 152, 38–56. [CrossRef]

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