Hu Chen

Zhenshan Li

Xinglei Liu

Weicheng Li

Ningsheng Cai

Stéphane Bertholin

Sina Tebianian

Aoling Zhang

Solid circulation study in a 1.5 MWth cold flow model of chemical looping combustion

Solid circulation in chemical looping combustion (CLC) is very important and affects the mass and heat balance and auto-thermal operation of a CLC system. A key task in developing CLC technology is to control the solid circulation. In this work, the solid circulation characteristic of a 1.5 MWth CLC cold flow model is reported. The solid circulation between the fuel reactor and the simplified air reactor riser is controlled by the overflow method. Three kinds of quartz sands are selected as fluidized particles, and their median particle diameters are 392 μm, 249 μm and 122 μm, respectively.

Reasonable pressure profile is obtained in the 1.5 MWth CLC cold flow model. The effects of operational parameters, including the fuel reactor gas velocity, loop seal gas velocity, simplified riser gas velocity, particle size and static bed height, on the solid circulation and hydrodynamic characteristics are measured and analyzed. The maximum solid circulation rate can approach 130 kg/(m 2 •s), and this value satisfies the

requirements of mass and heat balance in CLC system. The static bed height in the fuel reactor should be higher than the overflow port to prevent it from becoming a constraint factor on the solid circulation rate. An overflow model is developed to predict the solid circulation rate, and the relative errors between the predicted result and the experimental data are within 25%.

Highlights:

1. A 1.5 MWth CLC cold flow model is designed and constructed; 2. The maximum solid circulation rate can approach 130 kg/(m 2 •s), and this value satisfies the requirements of CLC auto-thermal operation;

3. The static bed height should be higher than the overflow port to obtain a high solid circulation rate;

4. An overflow model is developed to predict the solid circulation rate, and the relative errors are within 25%.

Introduction

CO2 is considered to be a main greenhouse gas causing climate change and global warming [START_REF] Abanades | Emerging CO2 capture systems[END_REF] . The International Energy Agency has estimated that energy-related CO2 emissions have risen 1.4% and have reached a historic high of 32.5 Gt 2 . This worldwide concern has continuously aroused extensive research towards developing more efficient and economical technology for CO2 capture in term of environmentally sound approaches [START_REF] Kanniche | Precombustion, post-combustion and oxy-combustion in thermal power plant for CO2 capture[END_REF][START_REF] Boot-Handford | Carbon capture and storage update[END_REF] . Recently, chemical looping combustion (CLC) has been regarded as a promising technology because it enables CO2 capture in a fossil fuel combustion process with nearly no energy penalties [START_REF] Adanez | Chemical looping combustion of solid fuels[END_REF] . A CLC unit is primarily composed of a fuel reactor (FR) and an air reactor (AR), and an appropriate solid metal oxide as an oxygen carrier (OC) is circulated between the two reactors to transfer lattice oxygen to fossil fuel.

Richter and Knoche [START_REF] Richter | Reversibility of combustion processes[END_REF] first proposed the concept of CLC in 1983, and since then it has been widely applied to the combustion field [START_REF] Lyngfelt | A fluidized-bed combustion process with inherent CO2 separation; application of chemical-looping combustion[END_REF][START_REF] Berguerand | Design and operation of a 10 kWth chemical-looping combustor for solid fuels -Testing with South African coal[END_REF][START_REF] Stroehle | Design and operation of a 1 MWth chemical looping plant[END_REF] . For a CLC unit, choosing an appropriate reactor type is important because it is related to the chemical reaction and operation of the CLC unit. Presently, based on gas-solid hydrodynamic characteristics, CLC units have three reactor types, namely, fluidized bed as air reactor-moving bed as fuel reactor, moving bed as air reactor-fluidized bed as fuel reactor, and fluidized bed as air reactor-fluidized bed as fuel reactor which is also known as two interconnected fluidized beds [START_REF] Cheng | Experimental investigation on coal-fired chemical cooping combustion coupled with an annular carbon stripper[END_REF] . For the fluidized bed as air reactor-moving bed as fuel reactor, only Ohio State University has adopted this approach in a 25 kWth CLC unit [START_REF] Kim | Coal direct chemical looping combustion process: Design and operation of a 25 kWth sub-pilot unit[END_REF] . For the moving bed as air reactor-fluidized bed as fuel reactor, only Southeast University has adopted this approach in a 20 kWth CLC unit [START_REF] Wang | Experimental evaluation of a novel 20 kWth in situ gasification chemical looping combustion unit with an iron ore as the oxygen carrier[END_REF] . The two interconnected fluidized beds approach has been employed by many research institutes as a CLC pilot plant reactor type, such as the 10 kWth [START_REF] Berguerand | Design and operation of a 10 kWth chemical-looping combustor for solid fuels -Testing with South African coal[END_REF] and 100 kWth [START_REF] Markstrom | Chemical-looping combustion of solid fuels -Design and operation of a 100 kW unit with bituminous coal[END_REF] units at Chalmers University of Technology, the 50 kWth unit at Instituto de Carboquímica [START_REF] Abad | Design and operation of a 50 kWth chemical looping combustion (CLC) unit for solid fuels[END_REF] , the 10 kWth unit at Southeast University [START_REF] Shen | Reactivity deterioration of NiO/Al2O3 oxygen carrier for chemical looping combustion of coal in a 10 kWth reactor[END_REF] , the 10 kWth unit at IFP Énergies Nouvelles [START_REF] Sozinho | Petcoke chemical looping combustion in a continuous 10 kWth pilot plant[END_REF] , the 50 kWth unit at Huazhong University of Science and Technology [START_REF] Ma | Performance of a 50 kWth coal-fuelled chemical looping combustor[END_REF] , the 10 kWth unit at Tsinghua University [START_REF] Chen | Coal-fired chemical looping combustion coupled with a high-efficiency annular carbon stripper[END_REF] , the 225 kWth unit at University of Utah [START_REF] Lighty | Chemical looping with oxygen uncoupling with coal[END_REF] , the 1 MWth unit at Darmstadt University of Technology [START_REF] Ohlemueller | Chemical-looping combustion of hard coal: autothermal operation of a 1 MWth pilot plant[END_REF] , and the 3 MWth unit at Alstom Power [START_REF] Abdulally | ALSTOM's chemical looping combustion prototype for CO2 capture from existing pulverized coal fired power plants[END_REF] . Therefore, the two interconnected fluidized beds are the dominant reactor type used for CLC technology.

In the two interconnected fluidized beds, the gas-solid hydrodynamic characteristics, especially solid circulation control, are significant to the operation of the CLC pilot plant. The temperature difference between the AR and FR, chemical reactions and heat and mass balance are affected by the solid circulation characteristics. Solid circulation must be right for the two interconnected fluidized beds. If the solid circulation rate is too low, the heat balance cannot be satisfied, resulting in a loss in fuel reactor temperature; if the solid circulation rate is too high, the residence time in the fuel reactor will be decreased, resulting in a loss in CO2 capture. There are three types of solid circulation principles for the two interconnected fluidized beds: double loop control, hydraulic connection and overflow exit from the fuel reactor [START_REF] Markstrom | Operation of a 100 kW chemical-looping combustor with Mexican petroleum coke and Cerrejon coal[END_REF][START_REF] Stroehle | Chemical looping combustion of hard coal in a 1 MWth pilot plant using ilmenite as oxygen carrier[END_REF][START_REF] Proell | A novel dual circulating fluidized bed system for chemical looping processes[END_REF] . Some cold flow models have been built to investigate the hydrodynamic characteristics and solid circulation of the two interconnected fluidized beds. Stollhof et al. [START_REF] Stollhof | Fluid dynamic evaluation of a 10 MW scale reactor design for chemical looping combustion of gaseous fuels[END_REF] adopted the hydraulic connection principle to study the effect of four different FR designs on CLC hydrodynamics, and they found that the shape of the conical form of the FR bottom part would affect FR hydrodynamics and led to unstable hydrodynamics in some cases, while AR hydrodynamics was not affected by changes in FR design. Dubey et al. [START_REF] Dubey | Hydrodynamic characteristics in a pilot-scale cold flow model for chemical looping combustion[END_REF] constructed a cold flow model based on the overflow exit from the fuel reactor principle and used Indian ilmenite as an oxygen carrier to investigate the effect of riser gas velocity on the voidage profiles, solid circulation rate, and pressure profiles. Bischi et al. [START_REF] Bischi | Hydrodynamic viability of chemical looping processes by means of cold flow model investigation[END_REF] selected the double loop control and hydraulic connection principle in a 150 kWth cold flow model to study the change trend of CLC hydrodynamics under different loads, gas velocities and solid return styles. To study the feasibility of a proposed dual fluidized bed, Liu et al. [START_REF] Liu | Investigation of a dual cold-flow fluidized bed for calcium looping gasification process[END_REF] constructed a cold flow model based on the hydraulic connection principle to study the effect of the total solid inventory and fluidization number of loop seal (LS) on the solid recirculation flux and solid fraction. Alghamdi et al. [START_REF] Alghamdi | Hydrodynamics similarities in cold flow model of chemical looping combustors: An experimental study[END_REF] constructed a 10 kWth CLC unit and a 200 kWth CLC unit based on the overflow exit from the fuel reactor principle to demonstrate the hydrodynamic similarities. In the two interconnected fluidized beds, controlling solid circulation by using the overflow principle has been widely adopted, such as the 10 kWth 8 and 100 kWth [START_REF] Markstrom | Chemical-looping combustion of solid fuels -Design and operation of a 100 kW unit with bituminous coal[END_REF] units at Chalmers University of Technology, the 25 kWth unit at Hamburg University of Technology [START_REF] Thon | Operational experience with a system of coupled fluidized beds for chemical looping combustion of solid fuels using ilmenite as oxygen carrier[END_REF] , the 10 kWth unit at University of Stuttgart 31 , the 10 kWth unit at Southeast University [START_REF] Shen | Reactivity deterioration of NiO/Al2O3 oxygen carrier for chemical looping combustion of coal in a 10 kWth reactor[END_REF] and the 225 kWth unit at University of Utah [START_REF] Lighty | Chemical looping with oxygen uncoupling with coal[END_REF] . Most of the reported works on CLC hydrodynamics have been conducted with different gas velocities, total solid inventories, particle sizes and solid return styles in small lab-scale setups. However, few studies have focused on the effect of the overflow port fluidization regime on CLC hydrodynamics in large-scale units.

In this work, a 1.5 MWth CLC cold flow model is proposed and constructed to study the overflow hydrodynamics and solid circulation control. The effects of different gas velocities, particle sizes and static bed heights on the solid circulation rate and fluidization characteristics are studied. An overflow model is proposed to predict the solid circulation rate.

2.

Experiments

Experimental setup

The schematic diagram of the 1.5 MWth CLC cold flow model system is shown in

Materials

Three kinds of quartz sands are chosen to investigate the overflow hydrodynamics of different particle sizes. The properties of the three quartz sands are listed in Table 1 32-34 . The particle size distributions (PSDs) are measured by using a Malvern Mastersizer, and the results are shown in Figure 2. Diameter (μm)

p s g s / = + (1 ) P H g g        (1)
where

/ P H  
represents the pressure gradient, Pa/m; g is the gravity acceleration, m/s 2 ; and p  and g  represent the particle density and gas density, respectively, kg/m 3 .

Based on the mass balance, the solid circulation rate of the overflow port is equal to that of the SR. As shown in Figure 1, the solid circulation rate of the SR (hereinafter referred to as the solid circulation rate, Gs) is defined as the total solid circulation rate (Gs,t) minus the FR solid circulation rate (Gs,FR):

s s,t s,FR - G G G  (2)
Both the Gs,t and Gs,FR are measured by the accumulative volume method. When the cold flow model operates steadily, the accumulative height and elapsed time in the dipleg are measured after closing the butterfly valve, and the Gs,t can be calculated by equation (3). Then, turn off the fluidizing gas of the loop seal between FR and SR and measure the Gs,FR using the same method, as shown in equation ( 4). The results of the Gs,FR have been provided in previous work [START_REF] Chen | Cold model study of a 1.5 MWth circulating turbulent fluidized bed fuel reactor in chemical looping combustion[END_REF] . It should be noted that the value of the FR solid circulation rate in the literature [START_REF] Chen | Cold model study of a 1.5 MWth circulating turbulent fluidized bed fuel reactor in chemical looping combustion[END_REF] is calculated based on the sectional area of the annular CS, while the Gs,FR in this work is calculated based on the sectional area of the simplified riser, so the literature value needs to be converted.

b dip dip,t s,t SR dip,t S H G S t   (3) b dip dip,FR s,FR SR dip,FR S H G S t   (4)
where dip

S

is the sectional area of the dipleg, m 2 ; b  is the bulk density, kg/m 3 ;

dip,i H is the accumulative height of solid particles in the dipleg, m; SR S is the sectional area of the simplified riser, m 2 ; and dip,i t is the elapsed time of solid particles in the dipleg, s.

Results

Stable operation results

The pressure profile of the 1.5 MWth CLC cold flow model is depicted in Figure 3. The pressure drops of loop seals (P18-P4, P22-P25) were all positive, and the pressure of the FR and SR decreased with increasing the reactor height. The results indicated that a reasonable pressure balance was obtained in this cold flow model. Most of the particles accumulated in the FR dense phase because the pressure drops (P1-P5, P9-P12) were high, as shown in Figure 3. The pressure drop in the SR dense phase (P23-P25) was relatively low (4.41 kPa), and the sectional area of the SR was 0.025 m 2 , so only a small amount of particles accumulated there. Comparing the pressure drop of the FR dilute phase (P5-P13) with that of the SR dilute phase (P25-P30), it was found that the former was lower than the latter, so the entrainment capacity of the FR was lower than that of the SR.

H s =1.25m u FR =0.78m/s u SR =9.2m/s P22 P31 P30 P29 P28 P27 P26 P25 P23 P24 P21 P20 P19 P18 P17 P16 P15 P14 P13 P11 P10 P9 P12 P8 P7 P6 P5 P4 P3 P2 P1
Height (m) Pressure (kPa)

Effect of operational variables on solid circulation rate

In this section, the effects of five vital variables, including the FR gas velocity (uFR), LS gas velocity (uLS), SR gas velocity (uSR), static bed height (Hs) and particle size, on the solid circulation rate (Gs) are investigated.

The effect of the uFR and Hs on the Gs is shown in Figure 4 m, the tops of the bed level were higher than those of the overflow port. The frequency of bubble coalescence and breakage in the dense phase increased with the increasing uFR, and the particle motion passing through the overflow port was enhanced, so the Gs increased. The Gs decreased with increasing uFR from 0.78 m/s to 1.01 m/s. This is because the uFR of 0.78 m/s is the transition velocity (Uc) for small particles (d50= 122 μm), i.e., when the uFR > 0.78 m/s, the fluidization regime of the TFB changes from the bubbling regime to the turbulent regime, and the fluidization characteristics change accordingly. In the turbulent regime, the entrainment capacity is strengthened, more particles are carried into the annular CS and the height of the TFB dense phase is decreased, so the amount of solid particles passing through the overflow port is reduced.

When the uFR was constant, the Gs increased significantly with increasing Hs increasing Hs, while when the Hs was higher than the overflow port, the Gs increased slightly with increasing Hs. Therefore, to prevent the overflow port from becoming a constraint factor on the Gs, the Hs should be higher than the overflow port. The effect of the particle size and uFR on the Gs is depicted in Figure 5(a). With the increase in the particle size, the change trend of the Gs was different in different uFR ranges. When the uFR was in the range of 0.1 ~ 0.78 m/s, the Gs of small particles (d50 = 122 μm) was the highest among the three different solid particles, but when the uFR > 1), so the uFR corresponding to the decrease in the Gs is different, as shown in Figure 5 The reason is as follows: when the Hs is lower than the overflow port, the Gs is mainly controlled by the Hs rather than the uLS. When the Hs is higher than the overflow port, a large number of particles flow out through the overflow port, and the change in the uLS will affect the Gs. The error bars at different Hs did not intersect, which indicated that the increase rate of the Gs with Hs was higher than the measurement error. In Figure 6(b), it can be seen that the change trend of the pressure drop of the SR dilute phase was the same as that of the Gs. and from 0.00 kg/(m 2 • s) to 106.35 kg/(m 2 • s), respectively. This indicated that the uLS could control the Gs. When the uLS was in the range of 0.06 m/s to 0.31 m/s, the Gs decreased obviously with increasing particle size. This is because the minimum fluidization velocity for different particle sizes is different, the fluidization number corresponding to the same gas velocity is also different, and the smaller particles have a larger fluidization number and a stronger entrainment capacity. When the uLS was in the range of 0.31 m/s to 0.52 m/s, the fluidization number was relatively large for different particle sizes, so the Gs decreased slightly with increasing particle size. With increase in the uLS, the error bars of different particle sizes changed from disjoint to intersect, which also indicated that the Gs difference of different particle sizes decreased, as shown in Figure 7(a). shows the effect of the uSR and particle size on the Gs. The Gs decreased with increasing particle size. For example, when the uSR = 9.2 m/s, with the increase in the particle size from 122 μm to 392 μm, the Gs decreased from 116.52 kg/(m 2 • s) to 83.42 kg/(m 2 • s). This is because the terminal velocity of solid particles increases with increasing particle size, and smaller particles are more easily carried out of the reactor at a constant gas velocity. The average value of error bars decreased from 9.02 kg/(m 2 • s) to 4.22 kg/(m 2 • s) with increasing particle size, which indicated that the material fluctuation of the cold flow model was alleviated with increasing particle size. Figure 9(b) shows the effect of the uSR and particle size on the pressure drop of the SR dilute phase. With the increase in the uSR or particle size, the change trend of the pressure drop of the SR dilute phase was inconsistent, but the variation range was very small. A correlation between the pressure drop of the SR dilute phase and the Gs is shown in Figure 10. The pressure drop of the SR dilute phase was linearly correlated with the Gs at the uSR = 9.2 m/s. When measuring the Gs is inconvenient, the pressure drop of the SR dilute phase can be used to estimate the Gs. Quartz sands with different particle sizes have different physical properties and fluidization characteristics at the same gas velocity. Therefore, the correlations between the pressure drop of the SR dilute phase and the Gs with different particle sizes are different, as shown in equation (5). The negative intercept of equation ( 5) is caused by the pressure drop of the SR fluidizing gas. It should be noted that when the uSR changes, the correlation will also change due to the change in the particle force and fluidization regime. With the increase in the uSR, the change trend of the solid fraction at different SR heights was different. In the range of SR heights from 0 m to 4 m, the solid fraction first decreased significantly and then became moderate with increasing uSR. In the range of SR heights from 4 m to 10.45 m, the solid fraction was basically unchanged with increasing uSR. There are two main reasons: 1) From the perspective of particle force, as the uSR increases, the drag force on particles increases, and the distance required to accelerate from static state to slip velocity decreases, resulting in a decrease in the particle amount in the acceleration region. 2) From the perspective of the fluidization regime, the fluidization regime of the SR gradually transfers from the fast fluidized regime to the transport regime with increasing uSR, and different fluidization regimes have different fluidization characteristics. For different fluidization regimes, the solid fraction of the dense phase or transition phase has a relatively large difference. In this work, when the SR height ranges from 0 m to 4 m, it is in the dense phase or transition phase and the solid fraction displays a large difference, as shown in Figure 11(c). The solid fraction of the dilute phase is generally one order of magnitude lower than that of the dense phase. Compared with the dense phase, the difference in the solid fraction of the dilute phase for different fluidization regimes is very small, so it is basically unchanged.

Figure 11(d) shows the effect of the Hs on the solid fraction profile of the SR. As mentioned above, when the Hs was lower than the overflow port (1.25 m), the Gs increased with increasing Hs; while when the Hs was higher than the overflow port, the Gs remained almost unchanged with increasing Hs. The solid fraction of the SR is positively correlated with the Gs when the uSR remains unchanged. Therefore, the solid fraction profile of the SR increased with increasing Hs from 0.75 m to 1.25 m, while the solid fraction profile of the SR remained almost unchanged with increasing Hs from 1.25 m to 1.45 m.

Figure 11(e) shows the effect of the particle size on the solid fraction profile of the SR. With the increase in the particle size, the change trend of the solid fraction in the SR with different heights was inconsistent. When the SR height was less than 8 m, the solid fraction decreased with increasing particle size, while when the SR height was higher than 8 m, the solid particles accelerated to the slip velocity, so the solid fraction was unchanged with increasing particle size, as shown in Fig. 11(e).

Discussion

Typical experimental results of the Gs and the pressure drop of the overflow port (P4-P19) are shown in Figure 12. The results show that the Gs is positively correlated with the pressure drop of the overflow port. Therefore, an overflow model is proposed to predict the Gs by using the pressure drop of the overflow port. kg/(m•s 2 ).

In Figure 13, it can be seen that H4 and 4

 are equal to H19 and 19  , respectively.

It is assumed that the horizontal direction velocity of port 4 is 0 m/s, i.e., u4 = 0 m/s; therefore, equation ( 6) can be simplified to equation (7):

2 19 4
19 [START_REF] Lighty | Chemical looping with oxygen uncoupling with coal[END_REF] 2

f u P P h     (7)
For ease of expression and calculation, f h is set as

The fluid density of port 19 ( 19 ) depends on the particle density and the solid fraction of the FR dense phase and is calculated by: 19 p sd =    (10) where sd  represents the solid fraction of the FR dense phase, -.

Introducing equation (10) into equation ( 9), the solid velocity of the overflow port (u19) can be expressed as:

(1+ )   c ( 1  c ) (11) 1
The overflow pipe is inclined downward, and the connection between the overflow pipe and FR is an ellipse. The sectional area of the overflow port ( ov S ) can be expressed as:

2 ov ov ov ov 4 cos( / 2 ) 4sin d d d S       (13)
where ov d represents the inner diameter of the overflow pipe, m;  represents the angle between the overflow pipe and the vertical direction, -.

The overflow port may not be fully filled with solid particles. The ratio of the sectional area filled with solid particles to the sectional area of the overflow port is specified as e. The solid volume flux of the overflow port ( ov Q ) is estimated by:

where SR d represents the inner diameter of the simplified riser, m.  is calculated by the pressure drop of the FR dense phase. Therefore, the value of constant  can be obtained by using the experimental results of the cold flow model. Through calculation, the values of  in the bubbling regime and turbulent regime were different, and the results were 0.0537 and 0.102, respectively, as shown in equations ( 17) and (18). The value of  in the bubbling regime is smaller than that in the turbulent regime, which may be because the solid particles in the turbulent regime operate more violently and flow out more easily from the overflow port.

Bubbling regime:

The results of the Gs calculated by equations ( 17) and ( 18) were compared with the experimental data, and the results are shown in Figure 14. The relative errors were mostly within 25%, which indicated that the fitted value of  was reasonable. When measuring the Gs is inconvenient, equations ( 17) and ( 18) can be used to estimate it. In the CLC design process, equations ( 17) and ( 18) obtained in this work can be applied to determine whether the overflow port will become a limiting factor of the Gs or not.

If the Gs of the overflow port is higher than the design value, this indicates that the overflow port is no longer the limiting factor of the Gs. Otherwise, the design of the overflow port is unreasonable and needs to be redesigned. indicated that the overflow model could predict the solid circulation rate accurately. In the CLC design process, this overflow model could be applied to determine whether the overflow port will become a limiting factor of the solid circulation rate.

Nomenclature

Figure 1 .

 1 Figure 1. The fuel reactor is composed of a turbulent fluidized bed (TFB), a FR riser

Figure 1 .

 1 Figure 1. Schematic diagram of the 1.5 MWth CLC cold flow model

Figure 2 . Particle size distributions of three kinds of quartz sands 2

 22 Figure 2. Particle size distributions of three kinds of quartz sands

Figure 3 .

 3 Figure 3. Pressure profile of the 1.5 MWth CLC cold flow model (d50=122 μm) 0 5 10 15 20

  (a). When the uFR was in the range of 0.1 ~ 0.78 m/s, the change trend of the Gs under different Hs was different. For the Hs = 0.75 m, the top of the bed level was lower than the overflow port.With the increase in the uFR, more bubbles were generated, and the amount of solid particles elutriated into the freeboard increased, resulting in an increase in the Gs from 12.72 kg/(m 2 • s) to 36.85 kg/(m 2 • s). For the Hs = 1.05 m, the top of bed level was at the overflow port but not full. The solid particles elutriated into the freeboard increased with increasing uFR, and the solid fraction in the overflow port decreased, so the Gs decreased from 79.45 kg/(m 2 • s) to 67.93 kg/(m 2 • s). For the Hs = 1.25 m and Hs = 1.45

  from 0.75 m to 1.25 m, while the Hs increased from 1.25 m to 1.45 m resulting in only a slight increase in the Gs. As the Hs increased from 0.75 m to 1.45 m, the average value of error bars increased from 2.92 kg/(m 2 • s) to 11.1 kg/(m 2 • s), and this indicated that the material fluctuation of the cold flow model increased with the increasing Hs. Under different uFR, the differences in the Gs of Hs = 1.25 m and Hs = 1.45 m were different, and the average difference was calculated to be 13.9 kg/(m 2 • s), which was slightly larger than the average value of error bars (11.1 kg/(m 2 • s)). The results indicated that when the Hs was lower than the overflow port (1.25 m), the Gs increased rapidly with

  Figure 4(b) shows the change in the pressure drop of the SR dilute phase (P29-P30) with the uFR and Hs. Comparing Figure 4(a) with Figure 4(b), it was found that the changes of the Gs and the pressure drop of P29-P30 were similar.

Figure 4 .

 4 Effect of the uFR and Hs on (a) solid circulation rate and (b) pressure drop of P29-P30

Figure 5 .Figure 6

 56 Figure 6(a) shows the effect of the uLS and Hs on the Gs. With the increase in the

Figure 6 .P30Figure 7

 67 Figure 7(a) shows the effect of the particle size and uLS on the Gs. As the uLS

Figure 7 (

 7 b) shows the change in the pressure drop of the SR dilute phase with the uLS and particle size. As mentioned above, the change trends of the pressure drop of the SR dilute phase and the Gs were basically similar.

Figure 7 .Figure 8 in

 78 Figure 8(a) shows the effect of the uSR and Hs on the Gs. As mentioned above,

Figure 8 .P30Figure 9

 89 Figure 9(a) shows the effect of the uSR and particle size on the Gs. The Gs decreased

Figure 9 .

 9 Effect of the uSR and particle size on (a) solid circulation rate and (b) pressure drop of P29-P30

Figure 10 .

 10 Figure 10. Correlation between the pressure drop of P29-P30 and the solid circulation rate

Figure 11 (

 11 Figure 11(b) shows the effect of the uLS on the solid fraction profile of the SR.

Figure 11 (

 11 Figure 11(c) shows the effect of the uSR on the solid fraction profile of the SR.

Figure 12 .

 12 Figure 12. Correlation of the solid circulation rate and pressure drop of the overflow port (P4-P19): (a) FR in the bubbling regime and (b) FR in the turbulent regime A schematic diagram of the fuel reactor and simplified riser is shown in Figure 13.

iu

  represents the horizontal direction velocity of port i, m/s; and f h represents the resistance loss from port 4 to port 19, Pressure drop of overflow port, P4-P19 (Pa)

Figure 13 .

 13 Figure 13. Schematic diagram of the fuel reactor and simplified riser

Figure 14 .

 14 Figure 14. Comparison of the typical experimental data and the calculated results: (a) FR in the bubbling regime and (b) FR in the turbulent regime

dov 2 GsQ 2 Sov 2 SSRFitting

 222 Inner diameter of overflow pipe, m dSR Inner diameter of simplified riser, m e Ratio of the sectional area filled with the particles to the sectional area of overflow port, -FR Fuel reactor g Gravity acceleration, m/s Solid circulation rate, kg/(m 2 •s) Gs,FR Solid circulation rate of fuel reactor, kg/(m 2 •s) Gs,t Total solid circulation rate, kg/(m 2 •s) hf Resistance loss, kg/(m•s 2 ) Hdip,i Accumulative height of solid particle in the dipleg, m Hi Height of port i, m Hs Static bed height, Solid volume flux of overflow port, m 3 /s Sdip Sectional area of dipleg, m Sectional area of overflow port, m Sectional area of simplified riser, m 2 SR Simplified riser tdip,i Elapsed of solid particles in the dipleg, s TFB Turbulent fluidized bed uFR Gas velocity of fuel reactor, m/s ui Horizontal direction velocity of port i, m/s uLS Gas velocity of loop seal, m/s uSR Gas velocity of simplified riser, m/s Uc Transition velocity, m/s Fluid density of port i, kg/m 3 p Particle density, kg/m 3

Table 1 . The properties of three kinds of quartz sands

 1

	Item	Unit	d50=122 μm	d50=249 μm	d50=392 μm
	Bulk density	kg/m 3	1396	1390	1377
	Particle density	kg/m 3	2715	2716	2702
	Geldart classification	-	B	B	B
	Minimum fluidization velocity 33	m/s	0.01	0.06	0.13
	Terminal velocity 34	m/s	0.85	2.23	3.55
	Transition velocity 32	m/s	0.78	0.95	1.06

  Gs of small particles (d50 = 122 μm) decreased rapidly from 129.98 kg/(m 2 • s) to 67.32 kg/(m 2 • s) and became the lowest. For three different particle sizes, the variation trend was similar with increasing uFR. The transition velocity is different for different particle sizes (as shown in Table

	0.78 m/s, the
	200						500
	50 100 150 G s (kg/(m 2 s)) Solid circulation rate,	u SR =9.2m/s u LS =0.31m/s d 50 =122μm	U c	H s =0.75m H s =1.05m H s =1.25m H s =1.45m	Pressure drop of (Pa) P29-P30	100 200 300 400	u SR =9.2m/s u LS =0.31m/s d 50 =122μm	U c	H H H H	=0.75m s s =1.05m =1.25m s s =1.45m
	0						0
		0.2	0.4 FR gas velocity, u FR (m/s) 0.6 0.8 1.0	1.2			0.2	0.4 FR gas velocity, u 0.6 0.8 FR (m/s) 1.0	1.2

  [START_REF]Global Energy & CO2 Status Report: The latest trends in energy and emissions in[END_REF] • s) to 115.28 kg/(m 2 • s), from 29.10 kg/(m 2 • s) to 101.98 kg/(m 2 • s)

		u SR =9.2m/s						400	u	=9.2m/s	H	=0.75m
	30 60 90 120 Solid circulation rate, G s (kg/(m 2 s))	u FR =0.78m/s d 50 =122μm				H s =0.75m H s =1.05m H s =1.25m	Pressure drop of P29-P30 (Pa)	100 200 300	SR u FR =0.78m/s d 50 =122μm	H H	s s s	=1.05m =1.25m
	0							0
		0.1	0.2	0.3	0.4	0.5				0.1	0.2	0.3	0.4	0.5
		LS gas velocity, u LS (m/s)					LS gas velocity, u LS (m/s)

Figure 11. Solid fraction profiles of the SR at different static bed heights, gas velocities and

											particle sizes
	10					10								10
					u FR =0.11m/s									u LS =0.06m/s	SR u =11.5m/s
					u FR =0.34m/s									u LS =0.10m/s	SR u =9.2m/s
		8			u FR =0.56m/s		8							u LS =0.15m/s	8	SR u =6.9m/s
					u FR =0.78m/s									u LS =0.21m/s	SR u =5.75m/s
					u FR =1.01m/s									u LS =0.31m/s	SR u =4.6m/s
																u	=3.45m/s
	Height (m)	4 6			H s =1.25m u LS =0.31m/s u SR =9.2m/s d 50 =122μm	Height (m)	4 6							H u FR s =1.25m =0.78m/s u SR =9.2m/s d 50 =122μm u LS =0.42m/s u LS =0.52m/s	Height (m)	4 6	SR u FR =0.78m/s u LS =0.31m/s d 50 =122μm H s =1.25m
		2						2								2
		0.00 0	0.04	0.08	0.12	0.16	0.00 0		0.04	0.08			0.12	0.16	0.0 0	0.1	0.2	0.3	0.4
			Solid fraction,  s (-)						Solid	fraction, 	s (-)	Solid	fraction, 	s (-)
				10								10
									H		=0.75m					d 50 =122μm
									H	s	=1.05m					d 50 =249μm
					8				H	s	=1.25m			8		d 50 =392μm
										s
									H		=1.45m
										s						H	s =1.25m/s
				Height (m)	6				u FR =0.78m/s u LS =0.31m/s u SR =9.2m/s d 50 =122μm	Height (m)	6		u SR =9.2m/s u FR =0.78m/s u LS =0.31m/s
					4									4
														2
					2
														0
					0
														0.00	0.04	0.08	0.12	0.16
					0.00	0.04 Solid	0.08 fraction, 	0.12 (-) s	0.16				Solid fraction, 	s (-)

Acknowledgements

This research is funded from National Key Research and Development Plan of China (No 2017YFE0112500), and European Union's Horizon 2020 Research and Innovation Program (No 764697).


Aránzazu Fernández-García

Adel Juaidi

Florian Sutter
email: florian.sutter@dlr.de

Lucía Martínez-Arcos

Francisco Manzano-Agugliaro

Solar Reflector Materials Degradation Due to the Sand Deposited on the Backside Protective Paints

Keywords: concentrating solar thermal technology, solar reflector material, durability, sand particle, water consumption, accelerated aging

Concentrating solar power (CSP) technologies are foreseen to be a crucial actor in the future renewable energy mix. Soil accumulation on the optical surfaces of CSP plants involves significant expenses of the operation and maintenance activities because a high cleanliness level is required to achieve proper plant revenues. Normally, only the front side of the solar reflectors is cleaned to reflect the maximum possible amount of direct solar radiation towards the receiver. However, soil deposited on the backside of the reflector could provoke degradation and might need to be considered in the cleaning strategy. As this possible degradation has never been studied, this work is dedicated to assess if the backside of reflectors should be regularly cleaned. The influence of the sand in the possible paint degradation depends on its chemical composition and the weather conditions. Therefore, several climatic conditions of artificially soiled reflector samples with different types of sand were simulated in accelerated aging tests. Concerning the results obtained, the ambient conditions simulated by the damp heat and thermal cycling tests were the only ones that produced a significant degradation of the backside paints. Also, the sand from Ouarzazate was responsible for higher deterioration.

Introduction

Energy supply might be one of the main financial issues that our society is currently facing. Renewable energies appear to be a meaningful solution because several of the conventional energy resources exploited at present are being depleted [START_REF] Baños | Optimization methods applied to renewable and sustainable energy: A review[END_REF]. In particular, the MENA region (Middle East and North Africa), where non-renewable energy resources are unsustainable, is suffering a significant increase in energy consumption [START_REF] El-Katiri | A Roadmap for Renewable Energy in the Middle East and North Africa[END_REF]. This issue has been intensively addressed in recent years, with the implementation of several renewable energy programs [START_REF] Jäger-Waldau | Photovoltaics and renewable energies in Europe[END_REF][START_REF] Montoya | Renewable energy production in Spain: A review[END_REF][START_REF] Kousksou | Renewable energy potential and national policy directions for sustainable development in Morocco, 2015[END_REF], some of them specifically focused on solar energy [START_REF] Solangi | A review on global solar energy policy[END_REF][START_REF] Ab Kadir | Prospective scenarios for the full solar energy development in Malaysia[END_REF]. Solar energy is leading the renewable energy scene because it is both free and endless [START_REF] Hernández-Escobedo | Solar energy resource assessment in Mexican states along the Gulf of Mexico[END_REF][START_REF] Porras-Prieto | Profitability Variations of a Solar System with an Evacuated Tube Collector According to Schedules and Frequency of Hot Water Demand[END_REF]. This importance is also highlighted by the scientific community as the solar energy topic has been addressed more intensively over last three decades [START_REF] Manzano-Agugliaro | Scientific production of renewable energies worldwide: An overview[END_REF]. Essentially, there are two techniques to directly harvest energy from the Sun: photovoltaic (PV) solar cells, and solar thermal collectors (STC) [START_REF] Kommalapati | Review of the Life Cycle Greenhouse Gas Emissions from Different Photovoltaic and Concentrating Solar Power Electricity Generation Systems[END_REF].

Concentrating solar power (CSP) plants (also named solar thermal electricity, STE, plants) are considered key actors in the renewable energy mix [START_REF] Mills | Advances in solar thermal electricity technology[END_REF][START_REF] Pavlović | A review of concentrating solar power plants in the world and their potential use in Serbia[END_REF][START_REF] Py | Concentrated solar power: Current technologies, major innovative issues and applicability to West African countries[END_REF][START_REF]Technology Roadmap: Solar Thermal Electricity[END_REF], as well as PV technologies [START_REF] Asim | A review on the role of materials science in solar cells[END_REF][START_REF] Vasconcelos Sampaio | Photovoltaic solar energy: Conceptual framework[END_REF] and non-concentrating solar thermal collectors [START_REF] Zhao | Energy-Saving Analysis of Solar Heating System with PCM Storage Tank[END_REF]. According to a report published by the International Energy Agency (IEA) in 2014, 79% of the total electricity generation will be delivered by renewable sources by 2050, and it is expected that STE will represent about 11% [START_REF]Technology Roadmap: Solar Thermal Electricity[END_REF].

Solar reflectors are a key component in CSP plants because they are the first element that the solar radiation intercepts in the energy conversion process. In a CSP plant, more than the 30% of the overall cost is spent on the solar field, and 6% is related to the investment costs of mirrors. Additional costs arise from the labor costs accrued during the mounting, cleaning tasks, replacement of mirrors due to breakage, etc. [START_REF] Kennedy | Optical Durability of Candidate Solar Reflectors[END_REF][START_REF] López Martín | Competitive examination of solar reflectors durability under accelerated aging[END_REF][START_REF]Concentrating Solar Power, Renewable Energy Technologies: Cost Analysis Series[END_REF]. Besides, the area covered by solar reflectors in a solar power plant is significantly high and involves a large land cost (0.58-1.02 ha of reflective area per MW e produced) [START_REF] Fernández-García | Parabolic-trough solar collectors and their applications[END_REF]. The specular reflectance demanded from solar reflectors must be high across the entire solar spectrum [START_REF] Sutter | Spectral characterization of specular reflectance of solar mirrors[END_REF]. The current maximum reflectance for silvered-glass reflectors of with a 4-mm thickness is around 0.945, and the target efficiency in the coming decades (2020-2030) is established at 0.950-0.960, using thinner glass mirrors as first surface reflectors [START_REF]Materials Roadmap Enabling Low-Carbon Energy Technologies[END_REF]. To ensure a high global yield of a CSP plant, this solar reflectance must be maintained during the whole life of the plant [START_REF] Kennedy | Advances in Concentrating Solar Power Collectors: Mirrors and Solar-Selective Coatings[END_REF][START_REF] Kennedy | Optical performance and durability of solar reflectors protected by an alumina coating[END_REF], which is expected to be from 20 to 30 years [START_REF] Kennedy | Development and Testing of Solar Reflectors Technical Report[END_REF][START_REF] Pitchumani | Concentrating Solar Power Program; Sunshot Grand Challenge and Peer Review[END_REF].

The European Union published a materials roadmap enabling low carbon energy technologies, which mentions that solar reflectors are associated with numerous requirements with respect to their optical behavior, durability, cost, and environmental concerns. [START_REF]Materials Roadmap Enabling Low-Carbon Energy Technologies[END_REF][START_REF] García-Segura | Durability studies of solar reflectors: A review[END_REF]. The European Solar Thermal Electricity Association (ESTELA) also published a strategic research agenda (2020-2025), which includes the study of anti-soiling coatings for solar reflectors to reduce water consumption [START_REF]Solar Thermal Electricity: Strategic Research Agenda[END_REF]. This paper is focused on the environmental concerns of solar reflectors, and particularly on the goal of reducing the water consumption required to clean the reflective surfaces, as a result of soil accumulation.

In addition to climate conditions (temperature, humidity, solar irradiance, etc.), soiling has been identified as key issue for solar technology reliability [START_REF] Costa | Dust and soiling issues and impacts relating to solar energy systems: Literature review update for 2012-2015[END_REF][START_REF] Conceição | Organic Soiling: The Role of Pollen in PV Module Performance Degradation[END_REF]. To achieve high direct normal irradiance (DNI), CSP plants are often located in arid and semi-arid locations with harsh weather conditions that can involve severe dust deposition, reducing the reflectance and consequently the CSP plant power production [START_REF] Wolfertstetter | Monitoring of Mirror and Sensor Soiling with TraCS for Improved Quality of Ground based Irradiance Measurements[END_REF][START_REF] Bouaddi | Soiled CSP solar reflectors modeling using dynamic linear models[END_REF]. Moreover, dust can completely terminate the system operation in some cases [START_REF] Sarver | A comprehensive review of the impact of dust on the use of solar energy: History, investigations, results, literature, and mitigation approaches[END_REF]. The absorption and scattering of radiation are the two mechanisms responsible for the reflectance reduction caused by dust and soil deposition on the reflectors [START_REF] Bouaddi | Soiled CSP solar reflectors modeling using dynamic linear models[END_REF]. In particular, the scattering phenomenon indicates that surface dirtiness affects CSP more seriously than PV plants because CSP mirrors only exploit the DNI [START_REF] Sarver | A comprehensive review of the impact of dust on the use of solar energy: History, investigations, results, literature, and mitigation approaches[END_REF][START_REF] Vivar | Effect of soiling in CPV systems[END_REF]. In addition, soil accumulation is the main aspect considered in the cleaning strategy. As a routine activity, the operation and maintenance (O&M) staff of CSP plants measures the reflectance of the solar field and washes it regularly, which leads to significant water consumption [START_REF] Cohen | Final Report on the Operation and Maintenance Improvement Program for Concentrating Solar Power Plants[END_REF][START_REF] Fernández-García | Study of Different Cleaning Methods for Solar Reflectors Used in CSP Plants[END_REF].

The soiling of solar reflectors is a recurrent research topic, recently intensified [START_REF] Costa | Dust and soiling issues and impacts relating to solar energy systems: Literature review update for 2012-2015[END_REF]. While some studies address the dust properties [START_REF] Berg | Heliostat Dust Buildup and Cleaning Studies[END_REF][START_REF] Roth | The Effect of Soiling on Solar Mirrors and Techniques Used to Maintain High Reflectivity[END_REF][START_REF] Morris | Cleaning agents and techniques for concentrating solar collectors[END_REF][START_REF] Biryukov | An Optical System for the Quantitative Study of Particulate Contamination on Solar Collector Surfaces[END_REF], others are focused on the reflectance status over time [START_REF] Pettit | Studies of Dust Accumulation on Solar-Mirror Materials[END_REF][START_REF] Fernández-García | Estudio de la Reflectancia Especular en Plantas de Captadores Solares Cilindroparabólicos[END_REF] or the influence of the location on the soiling level [START_REF] Wolfertstetter | Monitoring of Mirror and Sensor Soiling with TraCS for Improved Quality of Ground based Irradiance Measurements[END_REF][START_REF] Bethea | Environmental effects on solar concentrator mirrors[END_REF][START_REF] Heimsath | Soiling of aluminum and glass mirrors under different climatic conditions and techno-economic optimization of cleaning intervals[END_REF][START_REF] Tahboub | Solar Mirrors Soiling Campaign Abu Dhabi[END_REF]. With respect to soiling modeling, a predictive model was published by Reference [START_REF] Deffenbaugh | The effect of dust accumulation on line-focus parabolic trough solar collector performance[END_REF] and advanced mathematical tools based on dynamic models and time series approaches were proposed in References [START_REF] Bouaddi | Soiled CSP solar reflectors modeling using dynamic linear models[END_REF] and [START_REF] Bouaddi | Comparative analysis of soiling of CSP mirror materials in arid zones[END_REF].

Soil deposits depend on site environment, climatic conditions, and solar field orientation in combination with wind directions [START_REF] Roth | The Effect of Natural Soiling and Cleaning on the Size Distribution of Particles Deposited on Glass Mirrors[END_REF][START_REF] Bouaouadja | Effects of sandblasting on the efficiencies of solar panels[END_REF][START_REF] Nelson | Understanding soil adhesion in Concentrating Solar Power Plants: A novel analysis of soil Characteristics[END_REF]. Consequently, the impact of soiling must be included as a major role when performing the potential exposure site study [START_REF] Karim | Towards the prediction of CSP mirrors wear: Methodology of analysis of influencing parameters on the mirrors surface degradation: Application in two different sites in Morocco[END_REF].

Another effect of soiling on CSP plants is the higher probability of solar materials degradation due to the interaction with airborne particles [START_REF] Karim | Study of the Surface Damage of Glass Reflectors Used in Concentrated Solar Power Plants[END_REF][START_REF] Wiesinger | Sand erosion on solar reflectors: Accelerated simulation and comparison with field data[END_REF][START_REF] Wette | Comparison of Degradation on Aluminum Reflectors for Solar Collectors due to Outdoor Exposure and Accelerated Aging[END_REF][START_REF] Wiesinger | Assessment of the erosion risk of sandstorms on solar energy technology at two sites in Morocco[END_REF][START_REF] Wiesinger | Sandstorm erosion testing of anti-reflective glass coatings for solar energy applications[END_REF]. This damage can be both physicochemical and mechanical (scratches and cracks). Mechanical degradation might be due to dust particles traveling at high wind speeds or cleaning activities with contact means. On one hand, the possible mechanical damage due to the interaction with particles traveling by winds is a function of multiple factors, such as particle parameters (size, form/shape, hardness, and density), surface parameters (type of surface and its elasticity), and the sand transport mechanism towards or across the surface (saltation, shifting), in correlation with weather parameters (wind speed and direction and relative humidity) [START_REF] Karim | Towards the prediction of CSP mirrors wear: Methodology of analysis of influencing parameters on the mirrors surface degradation: Application in two different sites in Morocco[END_REF][START_REF] Bouzid | Effect of impact angle on glass surfaces eroded by sand blasting[END_REF][START_REF] Holze | Sand abrasion testing: Characterization of abrasive resistance of optical surfaces in arid operational areas using standard and adapted methods[END_REF]. On the other hand, contact cleaning is frequently applied to restart solar reflectance, which might provoke mechanical surface degradation [START_REF] Sansom | A comparison of Polymer film and Glass Collectors for concentrating solar power[END_REF][START_REF] Sansom | Predicting the Effects of Sand Erosion on Collector Surfaces in CSP Plants[END_REF][START_REF] Sansom | Soiling and Cleaning of Polymer Film Solar Reflectors[END_REF].

Finally, soiling may cause additional damage to CSP plants through the degradation of the backside protective paints of silvered-glass reflectors due to chemical interaction with the soil deposition. This crucial aspect has never been specifically addressed until now and it is important not only for the durability of solar reflector materials but also for the water consumption of CSP plants. Generally, the backside of the concentrators is not washed during the solar field cleaning task, but it may be recommended if significant degradation is discovered.

This paper examines the effect of the soil deposits on the backside paints of the silvered-glass reflectors. As this influence may depend on the weather conditions and the chemical composition of the sand, several climatic conditions under the effect of different types of sand (from Dubai and Ouarzazate) were simulated in accelerated aging tests. The aim of the testing procedure under accelerated aging conditions was to identify those tests that are more representative to evaluate the possible deterioration of the back-paint layers due to the sand depositions as well as to distinguish which sand is more aggressive.

Materials and Methods

This section is dedicated to describe the material types tested and the methodology applied in this experimental campaign, which was carried out in the Optical Aging Characterization (OPAC) laboratory. This laboratory is located at the Plataforma Solar de Almería (PSA), and is integrated by a research group between German Aerospace Center (DLR) and the Spanish Energy, Environment, and Technology Research Center (CIEMAT). Reflector samples with sand deposited on the backsides were submitted to a set of weathering tests under accelerated aging conditions to analyze the possible degradation of the backside protective paints.

Reflector Samples

Mirror samples from one manufacturer were subjected to a set of accelerated aging tests. The tested samples were silvered-glass reflectors with a thickness of 4 mm and a size of 10 cm × 10 cm. All samples contained the four edges properly protected by the manufacturer (see Figure 1). The total number of samples included in this study was 38.

Sand Samples

Two types of sand were employed in this study. They were taken from sites that are highly representative of CSP plants; the United Arab Emirates (UAE) and Morocco. This section presents a summary of the representative particle size and chemical composition of the two sands considered in this work. This information is useful for the further selection of CSP plant locations because the characteristics of the sand from such locations might be compared with those presented here and so the results achieved in this work could be extrapolated to those sites.

The two sand types are shown in Figure 2. As can be observed, the color of the sand from Morocco is red and the color of the sand from Dubai is light brown. The sand was shifted with a sieve shift, imposing a maximum of 180 µm to simulate the particle sizes that might be expected to be deposited on the backside of the reflectors. The first type of the sand samples was collected from The Sustainable City at Dubai (UAE). Figure 3 shows microscopic pictures taken with a scanning electron microscope (SEM, S-3400N, Hitachi, Chiyoda, Tokyo, Japan), which is described in Section 2.6.4. As can be seen, the typical size of the sand is around 150 µm. The second type of sand was collected by MASEN (Moroccan Agency for Solar Energy) from the Ouarzazate Solar Complex in Ouarzazate, Morocco. Figure 4 shows microscopic pictures taken with the SEM. As can be noted, the typical size of the bigger particles of the sand is around 100 µm, although most particles are smaller. The composition of the two sand types was analyzed by X-ray fluorescence spectrometer (S4 PIONEER, Bruker, Karlsruhe, Germany) and the results obtained are presented in Table 1. It can be seen that the sand from Dubai mainly contains CaO (62.05) and in a second position SiO 2 (26.79%), while the sand from Ouarzazate has a higher content of SiO 2 (51.11%), followed by Al 2 O 3 (14.70%) and CaO (17.76%).

Reflector Samples Preparation

To deposit the sand on the backside of the reflector samples, the first step consisted in spraying demineralized water to wet the surface. After that, a weight of 1 g of sand was deposited with the fingers over the entire surface. Finally, demineralized water was sprayed again on the backside surface and the sand was kept dry for 24 h. The result after the 24 h is presented in Figure 5.

Testing Approach

The experimental campaign started with a first round of four key preliminary weathering tests [START_REF] Sutter | Comparison and evaluation of accelerated aging tests for reflectors[END_REF], aiming to apply an initial pre-damage to the samples. The experiments selected were radiation with temperature and humidity, UV and humidity cycling, damp heat, and temperature cycling (see Sections 2.5.1-2.5.4). For each test, three samples with sand from Ouarzazate, sand from Dubai, and without sand (as a reference) were included. Therefore, nine samples were incorporated for each study, which involved a total of 36 samples. After the tests, sand was removed from the backside of the samples with demineralized water.

After the preliminary weathering test, the samples were divided into three sets of 12 samples. Each set contained one sample type (that is, one replicate with sand from Dubai, one replicate with sand from Ouarzazate, and one replicate without sand) from the four initial weathering tests. The three sets were distributed as follows:

1.

The first set of 12 samples continued with the four weathering tests, which was named the extended weathering test. The testing time was increased to check if a longer time in the accelerated aging chambers could report more relevant results. The sand was again deposited before the long-term weathering tests and then removed with demineralized water after the tests were complete.

2.

The second set of 12 samples was submitted to two mechanical tests (cross-cut and adhesion tests, see Sections 2.5.5 and 2.5.6) that modified the backside paint layers to investigate how the weathering tests might affect the resistance of the backside paint layer and to provoke further weakness in the mechanically pre-damaged area. A reference sample (named Ref. 01) without any previous weathering test was included in the cross-cut and adhesion tests.

3.

The third set of 12 samples was directly exposed to the copper-accelerated acetic acid salt spray (CASS) test (see Section 2.5.7). This is the most suitable test to remark corrosion in the silver layer produced by a weakness point or area in the backside paint layers. A reference sample (named Ref. 02) without any previous weathering tests was also included.

Figure 6 includes a scheme of the testing protocol followed.

Tests Description

This section describes the methodology and the equipment used to carry out the accelerated aging test campaign.

Radiation with Temperature and Humidity Test

There is no standard for this test. The maximum conditions of radiation (1000 W/m 2 ) were selected (employing a metal-halide lamp, which covers the whole solar spectrum), and conditions of temperature, T, and relative humidity, RH, were set at T = 50 • C and RH = 85%. The chamber employed for the experiment was the model SC340 by Vöstch (Reiskirchen-Lindenstruth, Germany) with the radiation module by Atlas. The backside of the mirror sample with the sand deposition faced upwards. In the preliminary weathering tests, nine samples were included (three of each type) for 500 h, while in the extended weathering tests three samples were tested (one of each type) for a further 500 h, accumulating in total 1000 h of testing.

UV and Humidity Cycling

The UV light and humidity test, which is based on the standard ISO 16474-3 (Method A) [START_REF]Paints and Varnishes-Methods of Exposure to Laboratory Light Sources-Part 3: Fluorescent UV Lamps[END_REF], is used to determine the effect of UV radiation in combination with high humidity on the solar reflectors. It consists of several cycles composed of two steps. In the first step the samples are exposed to UV radiation (λ = [290-400] nm) for 4 h at a temperature of T = (60 ± 3) • C. The UV lamps exhibit a peak at 340 nm, with a radiation of 0.83 W/m 2 /nm. Further, the mirror samples are submitted to 4 h of condensation (RH = 100% without irradiation) at T = (50 ± 3) • C. Therefore, one cycle lasts for 8 h. The test was carried out in an UV-Test chamber from Atlas. In the preliminary weathering tests, nine samples were included (three of each type) for 1000 h, while in the extended weathering tests three samples were tested (one of each type) for a further 1000 h, accumulating in total 2000 h of testing.

This chamber was designed to insert the test samples in the support structures located at the two sides of the chamber. This way the samples received the appropriate doses of UV irradiation. However, in this particular case, the samples were located at the bottom of the chamber, in a horizontal position with the backside (paint with the deposited sand) facing upwards, to prevent the sand deposited on the backside reflector paint from become detached during the experiment. Therefore, the testing performed is a modification of the test originally presented in the standard ISO 16474-3 because it cannot be guaranteed that the UV doses received by the samples are the same as those received in the original supports.

Damp Heat Test

The so-called damp heat test applies extreme constant conditions of both T and RH. The combination these two parameters is typically quite aggressive and leads to the corrosion of the silver layer. This experiment is normally applied to test the resistance of solar reflectors against humidity diffusion through the paint coatings at elevated T. It was performed according to the international standard IEC 62108 (test number 10.7, Procedure b) [START_REF]Concentrator Photovoltaic (cpv) Modules and Assemblies-Designed Modification and Type Approval[END_REF]. The samples were submitted to constant conditions of T = (85 ± 2) • C and RH = (85 ± 5)%. The test was carried out in a HCP108 model (Memmbert, Schwabach, Germany) weathering chamber.

The samples were positioned horizontally in the bottom of the chamber with the backside facing upwards. Both weathering tests (preliminary and extended) were performed for 1000 h, with a total testing time of 2000 h. In the preliminary weathering tests, nine samples were included (three of each type), while in the extended weathering tests three samples were tested.

Temperature Cycling

The purpose of this test is to determine the capacity of a solar reflector sample to resist extreme changes in T and RH. This test can detect possible failures due to stress in the component layers of the material from repeated thermal changes. The temperature differences between day and night are expected to be quite high in summer for a desert site. For this reason, the following testing conditions were selected: 32 h at T = 65 • C and RH = 85% + 4 h at T = 85 • C + 4 h at T = -40 • C, according to UNE 206016, method B2 [START_REF]Reflector Panels for Concentrating Solar Technologies[END_REF]. The total duration of one cycle is 40 h. The chamber used in the tests was the model SC340 by Vöstch. The backside of the mirror sample with the deposited sand faced upwards. In the preliminary weathering tests, nine samples were included (three of each type) and 10 cycles were applied (400 h), while in the extended weathering tests three samples were tested (one of each type) for another 10 cycles (400 h), involving a total testing duration of 800 h.

Cross-Cut Test

According to the approach followed (see Section 2.4), after the preliminary weathering tests the second set of samples was subjected to mechanical tests. The cross-cut was the first mechanical test applied. This test was conducted according to ISO 2409 [START_REF]Paints and Varnishes-Cross-Cut Test[END_REF] and consists of cutting the backside paints with a special tool (forming a lattice pattern) and applying tape on the cut area to check if the affected surface is removed.

The spacing and number of the cuts selected were 2 mm and five cuts, respectively, according to the hardness of the substrate and the coating. The cuts were applied to the coating at a constant speed and pressure, assuring a proper penetration up to the substrate surface. This task was repeated with a second set of cuts at 90 • , to create a grid. After the cutting, the surface was brushed with a soft brush. Then, the center of the tape (with a width of 50 mm) was placed over the grid in a parallel direction to one set of cuts. Within 5 min of applying the tape, the tape was removed by pulling it off. The time was between 0.5 s and 1.0 s, and the angle was near 60 • . Finally, the cut area of the test coating was carefully examined to determine the affected area (according to Reference [START_REF]Paints and Varnishes-Cross-Cut Test[END_REF]).

Adhesion Test

The second mechanical test was the adhesion one, based on Reference [START_REF]Paints and Varnishes-Pull-off Test for Adhesion[END_REF]. It was achieved with an automatic adhesion tester, PosiTest AT-A (DeFlesco Corporation, Ogdensburg, NY, USA), and consists of gluing a dolly with a diameter of 20 mm (with a specified adhesive) to the backside of the samples, curing the adhesive for 24 h, and pulling it off with a hydraulic pump. This electronic pump applies the pull-off pressure to remove the dolly in smooth and continuous mode. The result is that the pull-of pressure acts as an indicator of the paint adhesion. Pictures were taken to analyze the effect of these experiments (area affected and paint layer removed).

CASS Test

As explained in Section 2.4, after the preliminary weathering tests, the third set of samples was submitted to the CASS test. According to ISO 9227 [START_REF]Corrosion Tests in Artificial Atmospheres-Salt Spray Tests[END_REF], the samples were exposed to T = (50 ± 2) • C and RH = 100%, with a continuous pulverization of a water solution including (50 ± 5) g/L of NaCl and (0.26 ± 0.02) g/L of CuCl 2 . To achieve a pH between 3.1 and 3.3, HCl, NaOH, or NaHCO 3 were used. This test was selected for this study because, according to Reference [START_REF] Sutter | Durability testing of silvered-glass mirrors[END_REF], it is very aggressive and thus useful to provoke corrosion in the silver of those areas of the reflectors where the backside paint layers are degraded. The chamber used was the model VSC450 by Vöstch. In this study, three time intervals were applied: 24, 48, and 120 h.

The arrangement of the test samples was as follows:

(1) The test samples were located in the chamber room to avoid direct exposure to the spray from the nozzle. (2) The samples were located in the chamber room facing upwards at an angle close to 20 • to the vertical. (3) The test samples were organized to avoid any contact with the chamber walls and to permit free circulation of the spray around the sample surfaces.

Experimental Evaluation

Before and after the weathering tests, an optical reflectance analysis of the samples was performed according to the actual SolarPACES reflectance measurement guideline [START_REF]Parameters and Method to Evaluate the Reflectance Properties of Reflector Materials for Concentrating Solar Power Technology[END_REF], by measuring specular and hemispherical reflectance of the silver layer (and backside paint layer) with a reflectometer and a spectrophotometer, respectively [START_REF] Fernández-García | Equipment and methods for measuring reflectance of concentrating solar reflector materials[END_REF]. Additionally, images of interesting areas (for both weathering and mechanical tests) were taken with a camera and a light microscope. Finally, sand size and composition were analyzed before testing with the SEM. Nomenclature is based on the standard UNE 206009 [START_REF]Solar Thermal Electric Plants[END_REF].

Spectrophotometer

The spectral hemispherical reflectance, ρ λ,h (λ,θ i ,h), was measured in λ = [320, 2500] nm, with 5-nm steps at an incidence angle of θ i = 8 • . The equipment used was a Perkin Elmer Lambda 1050 spectrophotometer, which incorporates a 150-mm diameter integrating sphere accessory. The data were evaluated with a second surface reference reflectance standard (calibrated in the same λ range). Three measurements were taken on each sample, rotating the sample by 90 • each time to check possible anisotropies.

Following ASTM Standard E903 82 (92) [START_REF]Standard Test Method for Solar Absorptance, Reflectance, and Transmittance of Materials Using Integrating Spheres[END_REF], the solar-weighted hemispherical reflectance, ρ s,h ([λ a ,λ b ],θ i ,h), can be calculated by weighting ρ λ,h (λ,θ i ,h), with the solar direct irradiance, G b , on the earth surface for each wavelength, λ i , according to Equation (1).

ρ s,h ([λ a λ b ], θ i , h) = λ b λ a ρ λ,h (λ, θ i , h)G b (λ)dλ λ b λ a G b (λ)dλ (1)
For European and North American latitudes, typical solar direct irradiance spectra are given by the current standard ASTM G173-03 (air mass AM 1.5) [START_REF]Standard Tables for Reference Solar Spectral Irradiances: Direct Normal and Hemispherical on 37 • Tilted Surface[END_REF].

Reflectometer

The monochromatic specular reflectance, ρ λ,ϕ (λ,θ i ,ϕ), within a defined acceptance half-angle, ϕ, of ϕ = 12.5 mrad, was measured with a Devices and Services 15R-USB portable specular reflectometer (Dallas, TX, USA). This instrument uses a parallel beam with θ i = 15 • and λ = [635, 685] nm, with a peak at λ = 660 nm.

The specular reflectance is the relevant value for CSP technology and the ϕ selected is considered to be appropriate for parabolic-trough designs. Each sample was measured in five different positions. To characterize the specular quality of the mirror, the specular reflectance at 660 nm was compared with the hemispherical reflectance at 660 nm, which should show approximately the same value for a mirror without surface scattering effects.

Optical Inspection

The degradation mechanisms were visually analyzed with a camera LUMIX model DMC-F745 by Panasonic (Kadoma, Osaka, Japan). In addition, their microscopic appearance was verified with a three-dimensional (3D) light microscope, model Zeiss Axio CSM 700 (Oberkochen, Germany). Both sides of each sample were checked before and after the tests.

Scanning Electron Microscope

The SEM model S 3400N by Hitachi (Chiyoda, Tokyo, Japan) was employed in this study. This kind of microscope generates high-resolution images of surfaces by scanning it with a focused beam of electrons which interact with atoms and produce several signals that comprise information about topography and composition. The sand composition was analyzed with an X-ray fluorescence spectrophotometer equipment model S4 by Bruker Pionner (Karlsruhe, Germany).

Results and Discussion

This section comprises the results achieved with these experiments performed to examine the effect of the soil deposits on the backside paints of the silvered-glass reflectors.

Preliminary Weathering Tests

Table 2 includes the results of the reflectance for the preliminary weathering tests, applied to all of the samples. In this table, the reflectance decreases before and after the tests, compared to the results of the samples without sand, which was used as a reference level. This means that the reflectance loss of the sample with sand for each test was obtained by subtracting the reflectance loss of the samples without sand for this particular test. With this approach, only the effect of the reflectance degradation due to the sand is considered, and no effect of the test itself is added to avoid any misunderstanding in the conclusions. According to this method, the results of the samples without sand are zero and consequently they are not incorporated in the table. As can be observed, the reflectance loss of the silver layer was not significant in any of the tests, with a maximum decay of -0.002 ppt, which is in the range of the instrument uncertainty. This is the expected behavior because the sand was deposited in the backside of the samples and the possible degradation will appear in the back coating but not reaching the silver layer. Regarding to the paint layer, the sand from Ouarzazate provoked more reflectance decrease of the back coating in all tests. The highest reflectance loss was noticed in the damp heat test followed by the temperature cycling test, with sand from Ouarzazate in both cases. The other two tests also presented a reflectance degradation, but on a lower scale. For a clear understanding, show some illustrative pictures of the lattice created after the cross-cut tests, for the samples previously submitted to the UV and humidity test (Figure 9), the damp heat test (Figure 10), the radiation with temperature and humidity test (Figure 11), and the temperature cycling test (Figure 12). In all figures, the samples with the two types of sand and without sand are presented. As can be seen in these figures, only the damp heat and temperature cycling tests resulted in damage in the lattice area of the cross-cut test.

Adhesion Test

Figures 13141516present the results based on the visual inspection of the affected area after the adhesion test according to ISO 4624 [START_REF]Paints and Varnishes-Pull-off Test for Adhesion[END_REF]. Regarding these results, the UV and humidity test (with a larger area removed by the sand from Dubai) and the damp heat test (with a similar paint layer area affected by both sand types) were the most destructive tests. These results are in accordance with the conclusions already stated about the paint layer reflectance losses in the preliminary and extended weathering tests. Both the temperature cycling and the radiation with temperature and humidity tests did not damage the paint layer for any sample. Once again, the samples tested in the weathering experiments without sand were not affected at all by the adhesion test. This means that the deterioration produced in the backside paint layers in some of the samples after the adhesion test was evidently due to the interaction between the deposited sand and the paint layers during the accelerated aging experiments.

CASS Test

This section includes the results of the third set of samples, which were introduced to the CASS test after being submitted to the preliminary weathering tests. As can be seen in Table 5, no significant reflectance loss was perceived in any of the samples. This means that, although degradation of the paint layer of some samples was noted in other tests, this test is not useful to highlight a reflectance loss of the silver layer of the reflectors and it is not recommended for further studies focused on the backside paints.

Table 5. Results of reflectance differences from the third set of samples, after the copper-accelerated acetic acid salt spray (CASS) test. OU: Ouarzazate; DU: Dubai. humidity), followed by the temperature cycling test with humidity (sharp temperature variations). In these cases, the sand from Ouarzazate was more aggressive than the sand from Dubai. This phenomenon was detected both in the preliminary and extended weathering tests, as well as in the mechanical tests (cross-cut and adhesion tests).

•

The ambient conditions simulated by the UV and humidity test in combination with sand from Dubai also was shown to be destructive. This effect was noticed both in the extended weathering tests and the adhesion test.

•

With respect to the testing protocol proposed in this work, all of the experiments performed generated significant conclusions, except the CASS test, which is not recommended for further studies focused on the backside paints. Therefore, for further similar studies it is advisable to apply the same testing protocol, avoiding the CASS test.

Finally, from the results of this work it can be concluded that a specific cleaning study of the backside paints of the reflectors would be recommended to extend the useful lifetime of the mirrors if climate conditions with high temperature and humidity and extreme temperature changes occur, in locations with sand exhibiting chemical compositions similar to the Ouarzazate sand, and when climate conditions with UV radiation and humidity occur in areas with sand exhibiting chemical compositions similar to the Dubai sand. As most CSP plant locations are not found in such climate conditions, no major issue in this sense is expected.

Nomenclature

Figure 1 .

 1 Figure 1. Mirror sample of second-surface silvered-glass reflectors before any test.

Figure 2 .

 2 Figure 2. Picture of the two sands employed in the study, from Morocco (left) and the United Arab Emirates (UEA) (right).

Figure 3 .

 3 Figure 3. The Dubai sand particles at 50× magnification (a) and 200× magnification (b).

Figure 4 .

 4 Figure 4. The Ouarzazate sand particles at 50× magnification (a) and 200× magnification (b).

Figure 5 .

 5 Figure 5. Picture of the set of samples after the sand deposition, with sand from Ouarzazate (left), with sand from Dubai (center), and without sand (right).

Figure 6 .

 6 Figure 6. Scheme of the test campaign.

Figure 9 .

 9 Figure 9. Appearance of the surface affected by the cross-cut area performed after the UV and humidity test, for (a) the sample with sand from Ouarzazate, (b) the sample with sand from Dubai, and (c) the sample without sand.

Figure 10 .

 10 Figure 10. Appearance of the surface affected by the cross-cut area performed after the damp heat test, for (a) the sample with sand from Ouarzazate, (b) the sample with sand from Dubai, and (c) the sample without sand.

Figure 11 .

 11 Figure 11. Appearance of the surface affected by the cross-cut area performed after the radiation with temperature and humidity test, for (a) the sample with sand from Ouarzazate, (b) the sample with sand from Dubai, and (c) the sample without sand.

Figure 12 .

 12 Figure 12. Appearance of the surface affected by the cross-cut area performed after the temperature cycling test, for (a) the sample with sand from Ouarzazate, (b) the sample with sand from Dubai, and (c) the sample without sand.

Figure 13 .

 13 Figure 13. Appearance of the surface affected by the adhesion test area performed after the UV and humidity test, for (a) the sample with sand from Ouarzazate, (b) the sample with sand from Dubai, and (c) the sample without sand.

Figure 14 .

 14 Figure 14. Appearance of the surface affected by the adhesion test area performed after the damp heat test, for (a) the sample with sand from Ouarzazate, (b) the sample with sand from Dubai, and (c) the sample without sand.

Figure 15 .

 15 Figure 15. Appearance of the surface affected by the adhesion test area performed after the radiation with temperature and humidity test, for (a) the sample with sand from Ouarzazate, (b) the sample with sand from Dubai, and (c) the sample without sand.

Figure 16 .

 16 Figure 16. Appearance of the surface affected by the adhesion test area performed after the temperature cycling test, for (a) the sample with sand from Ouarzazate, (b) the sample with sand from Dubai, and (c) the sample without sand.

Table 1 .

 1 Analysis of Dubai and Ouarzazate sands.

	Compound	Dubai Sand Weight (%)	Ouarzazate Sand Weight (%)
	CaO	62.05	17.76
	SiO 2	26.79	51.11
	MgO	3.30	2.80
	Al 2 O 3	3.08	14.70
	Fe 2 O 3	2.03	8.02
	K 2 O	0.66	3.39
	SO 3	0.58	0.16
	Na 2 O	0.38	0.19
	P 2 O 5	0.38	0.52
	SrO	0.35	0.05
	TiO 2	0.15	0.97
	Cl	0.15	0.03
	Others	0.09	0.29
	Total	100.00	100.00

Table 4 .

 4 Results of the cross-cut experiment applied to the second set of samples after the preliminary weathering tests, according to ISO 2409.

	Preliminary Weathering Test	Sand Type	Classification	Interpretation
	UV and humidity cycling	Ouarzazate Dubai Without sand	0 0 0	No detachment-none of the squares of the grid was detached
		Ouarzazate	2	An area >5%, but <15% was affected
	Damp heat	Dubai	3	Some squares detached partly or totally and an area <15%, but <35% was affected
		Without sand	0	No detachment-none of the squares of the grid was detached
	Radiation with temperature and humidity	Ouarzazate Dubai Without sand	0 0 0	No detachment-none of the squares of the grid was detached
		Ouarzazate	3	Some squares detached partly or totally and an area <15%, but <35% was affected
	Temperature cycling	Dubai	1	A cross-cut area <5% was affected
		Without sand	0	No detachment-none of the squares of the grid was detached
	No test	Without sand [1]	0	No detachment-none of the squares of the grid was detached

Acknowledgments: This project received funding from the European Union's Horizon 2020 research and innovation program under grant agreement No 654479, project WASCOP.

In addition, all of the samples were analyzed microscopically in order to identify and check the aspect of the degradation mechanisms in the tests performed. As indicated before, the microscopic analysis on the back layer of the samples showed that the most aggressive tests were the damp heat and temperature cycling tests, with sand from Ouarzazate in both cases (see Figures 7 and8). Meanwhile, no degradation was detected in the samples without sand. Therefore, the degradation noticed in the paint layer of the samples with sand from Ouarzazate was due to the sand deposition in combination with the weathering tests, not due to the test itself. No significant changes were detected in the rest of the tests.

Extended Weathering Tests

Table 3 summarizes the reflectance results of the extended weathering experiments (applied to the first set of samples after the preliminary weathering tests), that is, the same tests already applied, but for a longer time. As stated in Table 2, reflectance decreased before and after the tests, as indicated by subtracting the result of the sample without sand from the results of the samples with sand. As can be seen in Table 3, no significant reflectance decay of the silver layer was achieved because the resulting values are in the range of the instrument uncertainty. With respect to the paint layer, no reflectance losses were noticed, except on two occasions. The first one is the sample tested with the sand from Ouarzazate in the damp heat test. This result is in accordance with the conclusions already stated in the preliminary test. The only extra information reached with these extended weathering test is that the sample tested with sand from Dubai in the UV and humidity test also suffered a significant reflectance loss (similar to that detected in the sample with sand from Ouarzazate tested in the damp heat test). Therefore, this is the main conclusion from this experiment. , h) (backside paint layer) 0.000 -0.009 -0.009 0.000 0.000 0.000 0.000 0.000

Mechanical Tests

This section includes the results of the two mechanical tests applied to the second set of samples, after the preliminary weathering tests.

Cross-Cut Test

Table 4 summarizes the results of the cross-cut experiment, based on the visual inspection of the affected area and the classification according to ISO 2409 [START_REF]Paints and Varnishes-Cross-Cut Test[END_REF]. According to these results, the damp heat test was the most aggressive (with similar degradation caused by both sand types), followed by the temperature cycling test (with higher degradation caused by the sand from Ouarzazate). Both the UV and humidity and the radiation with temperature and humidity tests did not damage the paint layer for any sample. Also, it is very important to remark that the samples tested in the weathering experiments without sand were not affected at all by the cross-cut test. This means that the deterioration produced in the backside paint layers in some of the samples after the cross-cut test was clearly due to the interaction between the deposited sand and the paint layers during the accelerated aging experiments. , h) (silver layer) 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 ρ λ,h (660 nm, 8 • , h) (silver layer) -0.001 0.000 0.00 0.000 0.000 0.000 0.000 0.000 ρ λ,ϕ (660 nm, 15 • , 12.5 mrad) (silver layer) 0.000 0.000 0.000 -0.002 0.000 0.000 0.000 0.000

Conclusions

A novel research study was accomplished to assess the degradation of the backside paints of solar reflectors for CSP plants due to the interaction of the deposited sand and soil particles under the influence of ambient conditions. According to the results obtained, no reflectance decay of the silver layer was observed in any of the tests performed (including preliminary weathering tests, extended weathering tests, and the CASS test). This is the expected behavior because the sand was deposited on the backside of the reflector samples and the possible degradation will appear in the back coating but not reach the silver layer. However, it is still very important to evaluate the degradation of the paint layers because they are responsible for the protection of the reflective silver layer from further attack under ambient conditions. Regarding to the paint layer degradation due to the interaction with soil deposition, the following conclusions can be made:

•

The representative ambient conditions, simulated by radiation with temperature and humidity tests, did not show any degradation of the paint layers.

•

The samples tested in the weathering experiments without sand were not affected at all by the cross-cut and adhesion tests. This means that the deterioration produced in the back-paint layers after the mechanical tests was clearly caused by the interaction between the deposited sand and the paint layers during the accelerated aging experiments.

•

The accelerated conditions that produced the highest deterioration of the backside layers were those simulated by the damp heat test (high temperature in combination with high

Author Contributions: Aránzazu Fernández-García and Florian Sutter conceived and designed the experiments; Adel Juaidi and Lucía Martínez-Arcos performed the experiments; Adel Juaidi and Francisco Manzano Agugliaro analyzed the data; Aránzazu Fernández-García, Adel Juaidi, and Lucía Martínez-Arcos wrote the paper.

Conflicts of Interest:

The authors declare no conflict of interest.