Experiment study of a quartz tube falling particlereceiver

Abstract

This paper presents an experimental evaluation of a speciallydesigned falling particle receiver. A quartz tube was used in thedesign, with which the particles would not be blown away by wind.Concentrated solar radiation was absorbed and converted into thermalenergy by the solid particles flowed inside the quartz tube. Severalexperiments were conducted to test the dynamic thermal performanceof the receiver on solar furnace system. During the experiments, themaximum particle temperature rise is 212°C, with an efficiencyof 61.2%, which shows a good thermal performance with a falling distanceof 0.2 m in a small scale particle receiver. The average outlet particletemperature is affected by direct normal irradiance (DNI) and otherfactors such as wind speed. The solid particles obtain a larger viscositywith a higher temperature while smaller solid particles are easierto get stuck in the helix quartz tube. The heat capacity of the siliconcarbide gets larger with the rise of particle temperature, becauseas the temperature of solid particles increases, the temperature riseof the silicon carbide decreases.

Keywords

solar thermal electricity / central receiver / particle receiver / experimental research

Introduction

Higher efficiency power cycles arebeing pursued to reduce the levelized cost of concentrating solarpower tower. These cycles include combined air-Brayton, supercritical-CO₂ (sCO₂) Brayton, and ultra-supercriticalsteam cycles, which require higher temperatures than those previouslyconstructed using central receivers [1]. Either water/steam or molten nitrate salt has beenused as the heat transfer fluid in subcritical Rankine power cyclesin the current solar thermal power tower plant, such as Ivanpah SolarPlant of the United States and Gemasolar Thermosolar Plant of Spain.The gross thermal-to-electric efficiency of these cycles of the operatingsolar power tower plants is typically between 30% and 40% at turbineinlet temperatures lower than 600°C. Based on the theory of thermodynamics,at higher input temperatures, the thermal-to-electric efficiency ofthe power cycles increases. However, when temperatures are greaterthan 600°C, molten nitrate salt becomes chemically unstable [2]. For water/steam system, it is impracticalfor large scale thermal storage due to very high pressures at criticalconditions and low volumetric heat capacities. Thus, new heat transferfluids should be found to be working under higher temperatures. Solidparticles have been proposed to be the heat transfer fluid in solarreceivers and thermal energy storage material. Particle receivershave the potential to increase the maximum temperature of the heat-transfermedia to over 1000°C. The cost of thermal energy storage can besignificantly reduced by directly storing heat at higher temperatures.In this concept, cold particles that emerge from the cold storagedevice are heated in the solar receiver and then stored in a hot storagedevice. A heat exchanger is located between the hot and cold storagedevices to transfer the stored high temperature heat to water or gasto power a thermodynamic cycle, such as a Rankine or a Brayton cycle.In general, these particle receivers can be categorized as eitherdirect or indirect particle heating receivers. Direct particle heatingreceivers irradiate the particles directly as they fall through areceiver, while indirect particle heating receivers utilize tubesor other enclosures to convey and heat the particles. For indirectparticle heating receivers, the working principle is the same as thatof the molten salt receiver system. The solid particle is the heattransfer fluid and storage material. Flamant et al. [3] have validated the concept withon-sun batch operation of a single-tube experimental solar receiver.During the on-sun testing with the solar facility of a single tube,the outlet temperature of solid particles is less than 300°C.Furthermore, Benoit et al. [4] have tested the dense particle suspension as the heat transferfluid in a high temperature single tube on-sun solar receiver at aFrench solar facility. A powder outlet temperature as high as 750°Cis measured. A correlation has been proposed to predict the dependencyof the heat transfer coefficient and solid mass flux. Marti et al.[5] have investigateddetailed heat-transfer mechanisms in dense gas-particle suspensionsused as heat transfer media for high-temperature concentrated solarpower applications. A two-phase Euler-Euler model for dense gas-particlesystems has been built on the open-source code Open FOAM. Ansart etal. [6] have conductedexperiments using positron emission particle tracking (PEPT) and 3Dnumerical simulation via an Eulerian n-fluid approach with NEPTUNE_CFD code. Lopez et al. [7] have described experiments conductedon a 16-tube, 150 kWth solar receiver using a dense gas-particle suspension(around 30% solid volume fraction) flowing upward as heat transferfluid, in which SiC particles are used. One hundred hours of on-suntests have been performed at the CNRS 1 MW solar furnace in Odeillo.Steady states have been reached during the experiments, with continuouscirculation and constant particle temperatures. The particles areheated up to 700°C at the outlet of the hottest tube, during steadyoperation. The thermal efficiency of the receiver, defined as theratio of power transmitted to the dense particle suspensions (DPS)in the form of heat over solar power entering the receiver cavity,is calculated in the range of 50% to 90% for all the experimentalcases. The direct particle receiver consists of particles fallingthrough a cavity, where the particles are irradiated directly by concentratedsunlight. The particles are released through a storage tank abovethe receiver, producing a thin sheet or curtain of particles fallingthrough the receiver. Lots of studies have been performed on directfree-falling particle receivers since 1980s [8–14]. Those studies focus on modeling the particle hydraulicsand radiant heat transfer to falling particles. Bertocchi et al. [11] have designed and fabricated asolid particle solar receiver with sub-micrometer carbon particlesas the heat absorber, and the outlet gas temperatures exceeding 2100K are obtained with nitrogen, at 1900 K with CO₂, and 2000 K with air. The experimental results show the potentialof solid particle receivers to get higher temperatures. Ho et al.[15,16] have performed on-sun tests ofa 1 MWth continuously recirculating particle receiver with bulk particleoutlet temperatures reaching over 700°C, and thermal efficienciesfrom 50% to 80%. The falling particle receiver appears well-suitedfor scalability. More recently, transparent quartz tubes have beenused in direct solid particles [16–19]. Numericaland experimental studies have been performed on the thermal performanceof an air receiver with silicon carbide particles in transparent quartztubes. Air is blown upward through the particles in quartz tubes whilethe tubes and particles are irradiated with concentrated sunlightfrom a 10 kWth furnace. The results of those tests indicate that theheated air reaches over 600°C for five-tube receivers with minimumtemperature differences between the particles and the air below 10°C,which demonstrates good heat transfer between the air and the particles.For the single quartz tube receiver, the highest temperature is 867°Cin the thermal performance testing experiment. Those research experiencesconfirm the feasibility of solid particle receivers using quartz tubes.Using quartz tubes as the cover of the flowing particles would bevery effective to reduce the influence of the wind. Thus, in thispaper, a solid particle receiver has been designed using the helixquartz tube and the straight quartz tube, respectively. The temperaturerise of the solid particles are measured on the solar furnace experimentalsystem. The objective of this paper is to validate the concept ofthis novel receiver.

Receiver design

The structure of the quartz tubefalling particle receiver is shown in Fig. 1. The receiver is composedof two tanks for solid particles, a heat absorbing cavity, two knifegate valves, a funnel and a quartz tube. There are two kinds of quartztubes used in the receiver, straight pipe and helix pipe. The outerdiameter of the straight quartz tube is 40 mm, the thickness is 3mm and the length is 500 mm. The backlight side of the straight quartztube is filled with thermal insulation, so that the circulation areaof the straight pipe is close to half circle. The bore of the helixquartz tube is 20 mm and the diameter of the helix is 100 mm. Thelength of the helix tube is 500 mm. The quartz tube is processed intohelix type to lengthen the time that the particles are heated by thesolar energy to get a higher outlet temperature. The pitch of thehelix is 167 mm. The receiver uses 0.5 mm and 1 mm silicon carbideparticles as heat absorption materials respectively. The stainlesssteel gauze with a hole is put on the outlet of the quartz tube todecrease the speed of the solid particles in order to extend the timethe solid particles stay in the quartz tube. The flow rate of thesolid particles is controlled by the size of the hole of the stainlesssteel gauze. The quartz tube and solid particle are exhibited in Figs.2 and 3.

The solid particles are put in thefirst tank above the heat absorbing cavity. Once the valve is turnedon, the particles drop freely to the quartz tube inside the heat absorbingcavity. The particles are heated in the quartz tube by concentratedsolar energy. When the particles drop to the second tank below theheat absorbing cavity, the second valve is turned on and the solidparticles can be carried to the first solid particle tank by handfor recycle. The receiver is coated with thermal insulation to reduceheat loss.

Experiment system design

In the experiment, the receiver isplaced on the solar furnace system as shown in Fig. 4. As shown inFig. 5, the furnace consists of a heliostat and a secondary parabolicreflector. The heliostat mirror with a total area of 35.3 m² tracks the sun continuously. The parabolic concentratorfocuses the solar rays, making the focusing spot a nearly 200 mm diametercycle. Type K thermocouples are used to measure the inlet particletemperature and outlet particle temperature. The location of the thermocouplesin the particle box is shown in Fig. 6. All the experimental dataare collected online by a data collection instrument. The equipmentused in the experiments are listed in Table 1.

Results

In the experiment, the reflectanceof the heliostat and parabolic concentrator is measured. The measureddata of 10 mirror units are listed in Table 2.

To evaluate the thermal propertyof the quartz tube particle receiver, the efficiency of the receiveris defined as

(1)\eta ={\displaystyle \frac{Q_{\mathrm{particle}}}{Q_{\mathrm{total}}}}={\displaystyle \frac{{\displaystyle {\int }_{T_{\mathrm{in}}}^{T_{\mathrm{out}}}{c}_{\mathrm{p}}\dot{m}}\mathrm{d}t}{\mathrm{DNI}\cdot \alpha \cdot A_{\mathrm{in}}\cdot \beta }},

where \dot{m}means the mass flux of the solid particles (kg/s), T_out is the averageoutlet particle temperature (°C), T_in is the average inlet particle temperature(°C), a is thetransmittance of quartz tube for solar spectrum, A_in is the light transmitting areaof the quartz tube (m²), β is the optical concentration ratioof solar furnace, and c_p is the specific heat of the solid particles (J/(kg·°C)),respectively.

The specific heat is the functionof the temperature of solid particle, which can be written as [20].

(2){{\displaystyle C}}_{\mathrm{p}}=-\mathrm{7.309}\times {\mathrm{10}}^{-\mathrm{4}}{{\displaystyle T}}_{\mathrm{p}}^{\mathrm{2}}+\mathrm{1.608}{{\displaystyle T}}_{\mathrm{p}}+\mathrm{372.4},

where T_p means the temperature of the solid particles.

The light transmitting area of thehelix tube and the straight tube is 9.04×10⁻³ m² and 6.8×10⁻³ m², respectively. The experimental systemis introduced in detail by Wang et al. [21]. The optical concentration ratios are 391.9 and 590.2,respectively.

Since the outlet particle temperaturesunder different working conditions are similar, some representativeexperimental results are presented.

Experiment 4

Experiment 4 was conducted from 9:12am to 9:51 am on April 23, 2016. The inlet particle temperature is18°C, the particle diameter is 1 mm, and the flow rate of theparticles is 8.12 g/s. The solid particles flow in the helix quartztube. After being heated by the concentrating solar radiation, theaverage temperature of the particles rises from 26°C to 238°C.The efficiency under this condition is 61.2%. The temperature of thethermocouples in the lower particle box and the range of DNI are shownin Fig. 7.

When the height of the solid particlesin the particle box is lower than the position of the thermocouple,the hot solid particles crash on the thermocouple, causing the instabilityof the thermocouple. Once the height of solid particles in the particlebox is higher than the position of the thermocouple, the temperatureof the thermocouple trends to become stable. The measured temperatureof thermocouples close to the outlet of the quartz tube are much higherthan others, because the solid particles stacking in this area sufferless cooling time and achieve higher temperature.

Experiment 5

Experiment 5 was conducted from 10:12am to 10:52 am on April 23, 2016. The inlet particle temperature is166°C, the particle diameter is 1 mm, and the flow rate of theparticles is 7.92 g/s. The solid particles flow in the helix quartztube. After being heated by the concentrating solar radiation, theaverage temperature of the particles rises from 166°C to 294°C.The efficiency under this condition is 38.3%. The temperature of thethermocouples in the lower particle box and the range of DNI are depictedin Fig.8.

Compared with Experiment 4, in Experiment5 with the rises of the inlet particle temperature, the final averagetemperature of the solid particle rises. As the temperature of thesolid particles increases, the flow rate of the solid particles decreases,because the temperature rise of the solid particle leads to the increasingof the viscosity of the particles.

Experiment 8

Experiment 8 was conducted from 16:00to 16:38 on April 22, 2016. The inlet particle temperature is 39°C,the particle diameter is 1 mm, and the flow rate of the particlesis 8.49 g/s. The solid particles flow in the helix quartz tube. Afterbeing heated by the concentrating solar radiation, the average temperatureof the particles rises from 39°C to 221°C. The efficiencyunder this condition is 38.7%. The temperature of the thermocouplesin the lower particle box and the range of DNI are displayed in Fig.9.

Compared with Experiment 4, Experiment8 has a higher average DNI, but the temperature rise is lower becauseof the strong wind. The outlet temperature of the solid particlesis related to the flow rate of the particles, the DNI, and other factorssuch as outside wind speed.

Experiment 9

Experiment 9 was performed from 10:07am to 11:47 am on April 22, 2016. The inlet particle temperature is16°C, the particle diameter is 0.5 mm, and the flow rate of theparticles is 8.74 g/s. The solid particles flow in the helix quartztube. After being heated by the concentrating solar radiation, theaverage temperature of the particles rises from 16°C to 176°C.The efficiency under this condition is 27.8%. The temperature of thethermocouples in the lower particle box and the range of DNI are presentedin Fig.10.

Compared with Experiment 4, Experiment9 has a higher DNI, but the temperature rise is lower, because smallerparticles have a larger viscosity. Once heated, the particles closeto the helix quartz tube stop moving, and the heat transfer processis the heat conduction between the front particles and the back ones.The helix quartz tube after the experiment is shown in Fig. 11. Owingto the long time staying of the solid particles in specific location,the color of the quartz tube of that position is changed.

Experiment 10

Experiment 10 was performed from10:48 am to 11:04 am on April 29, 2016. The inlet particle temperatureis 19°C, the particle diameter is 1mm, and the flow rate of theparticles is 14.92 g/s. The solid particles flow in the straight quartztube. After being heated by the concentrating solar radiation, theaverage temperature of the particles rises from 19°C to 116 C.The efficiency under this condition is 31.1%. The temperature of thethermocouples in the lower particle box and the range of DNI are shownin Fig.12.

Compared with Experiment 4, Experiment10 is carried out in the straight quartz tube, which has a smallerresistance with a higher solid particle speed than that in the helixquartz tube. Therefore, it has a lower average outlet particle temperature.

Experiment 13

Experiment 13 was carried out from13:15 to 13:42 on April 28th, 2016. The inlet particle temperatureis 17°C, the particle diameter is 0.5 mm, and the flow rate ofthe particles is 15.37 g/s. The solid particles flow in the straightquartz tube. After being heated by the concentrating solar radiation,the average temperature of the particles rises from 17°C to 149°C.The efficiency under this condition is 52.0%. The temperature of thethermocouples in the lower particle box and the range of DNI are shownin Fig. 13.

Compared with Experiment 10, in Experiment13, the particle diameter is changed to 0.5 mm. Though the averageDNI is lower in Experiment 13, it has a higher outlet temperature.The solid particles in the straight quartz tube would not be stuckto the tube, so smaller particles get a higher temperature.

Summery of experiments

The results of 15 experiments aresummarized in Table 3. The maximum temperature rise is 212°C inExperiment 4 using the helix quartz tube and 132°C in Experiment13 using the straight quartz tube, with the highest thermal efficiencyof 61.2%.

Conclusions

A single quartz tube falling particlereceiver was designed and manufactured, and experiments were conductedto test the dynamic thermal performance of the receiver. The experimentsfocused on the effect of particle diameter, particle inlet temperature,particle flow rate and type of the quartz tube on outlet particletemperature. From the experiments, the following conclusions can bereached.

The measured temperature of thermocouplesclose to the outlet of the quartz tube are much higher than others,because the solid particles stacking in this area suffer less coolingtime and achieve higher temperature.

The average outlet particle temperatureis affected by DNI and other factors such as wind speed.

The solid particles have a largerviscosity with a higher temperature and smaller solid particles, andare easier to get stuck in the helix quartz tube.

Since the heat capacity of siliconcarbide increases with increasing operating temperature, the highertemperature of the solid particles is working in, the smaller temperaturerise is obtained in the case of the same heat absorption.

The temperature rise of solid particlesdecreases as the working temperature increases while absorbing thesame heat, because the heat capacity of silicon carbide increaseswith temperature.

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