Einfluss von Mineralstäuben auf keramische Solarabsorber

Transcription

1 15. Kölner Sonnenkolloquium Einfluss von Mineralstäuben auf keramische Solarabsorber Martin Schmücker Deutsches Zentrum für Luft- und Raumfahrt Institut für Werkstoff-Forschung Köln

2 Honeycomb SiC absorber (new) Honeycomb SiC absorber (used, few 10 h) Folie 2

3 Outline Model dust Accelerated dust deposition on SiC absorber components Temperature dependent dust adhesion Estimation of dust deposition under real conditions Melting of dust particles and SiC corrosion Influence on light absorbance and efficiency Conclusions Folie 3

4 Elements Inorganic dust compositions Lithosphere Silica-rich "Arizona road dust" SiO 2 61, Al 2 O 3 15, Fe 2 O 3 /FeO 2,36/3, Na 2 O 3,2 3 1 CaO 5, MgO 3,7 1 6 TiO 2 0, K 2 O 2,40 2 Lime-rich (= desert dust close to Cairo) Commercial available powder mixtures Folie 4

5 Both methods, IR spectroscopy and XRD analysis, show that all investigated samples contain quartz, feldspars and clay minerals in different compositions. In addition, the samples from Cairo and Morocco contain significant amounts of calcite, dolomite and gypsum From: Optical properties and mineralogical composition of different Saharan mineral dust samples: a laboratory study C. Linke, O. Mohler, A. Veres, A. Mohacsi, Z. Bozoki, G. Szabo, and M. Schnaiter Folie 5

6 Sintering of model oxide dust and mineral dust 6 Shrinkage [%] Mineral phases Oxid phases Temperature [ C] Mineral phases Oxide phases Mineral constituents Calcite Quartz Dolomite Clay minerals Folie 6

7 Outline Motivation Model dust Accelerated dust deposition on SiC absorber components Temperature dependent dust adhesion Estimation of dust deposition under real conditions Melting of dust particles and SiC corrosion Influence on light absorbance and efficiency Conclusions Folie 7

8 Test rig for accelerated deposition of mineral dust on SiC honeycomb structures turbulent fluidised bed Particle transport by air flow Air flow corresponds to realistic conditions Cylindrical specimen within tube furnace Folie 8

9 Outline Motivation Model dust Accelerated dust deposition on SiC absorber components Temperature dependent dust adhesion Estimation of dust deposition under real conditions Melting of dust particles and SiC corrosion Influence on light absorbance and efficiency Conclusions Folie 9

10 Temperature dependent dust adhesion Degree of particle adhesion must be known to estimate dust deposition for extrapolated service time Folie 10

11 Temperature dependent dust adhesion Degree of particle adherence [%] Silica-rich mineral dust Temperature [ C] Degree of particle adherence [%] Lime-rich mineral dust melting Temperature [ C] Folie 11

12 Particle adherence and sintering activity Degree of particle adherence [%] Sintering shrinkage Particle adherence Temperature [ C] Temperature Folie 12

13 Particle adherence and channel choking Degree of particle adherence [%] Mineral dust can be removed easily by suitable brushes as long as no melt formation occured Temperature [ C] T < 750 C: Deposition only on cell connecting bars T > 750 C: Choking of channels Folie 13

14 Outline Motivation Model dust Accelerated dust deposition on SiC absorber components Temperature dependent dust adhesion Estimation of dust deposition under real conditions Melting of dust particles and SiC corrosion Influence on light absorbance and efficiency Conclusions Folie 14

15 Estimation of dust deposition under real conditions 15 l air / s, 47 x 47 channels 25 l / channel h 1 m 3 air: ca. 100 µg mineral dust (literature review) Degree of adherence: 25% 4mg 250h 22mg 1400h 43mg 2700h Folie 15

16 Estimation of dust deposition under real conditions, Validation 0.4mg 25 h (estimated for real conditions) in service, few 10 h Folie 16

17 Solid state mineral dust deposition: Relatively poor particle cohesion. Dust particles can be removed easily. Folie 17

18 Outline Motivation Model dust Accelerated dust deposition on SiC absorber components Temperature dependent dust adhesion Estimation of dust deposition under real conditions Melting of dust particles and SiC corrosion Influence on light absorbance and efficiency Conclusions Folie 18

19 Melting of dust particles and SiC corrosion Silica-rich 1200 C Lime-rich Folie 19

20 Melting of dust particles and SiC corrosion Weight gain by SiSiC oxidation [mg] DTg (mg) 1,600 1,400 1,200 1,000 0,800 0,600 0,400 0,200 0, C time (h) Covered by lime-rich melt SiSiC-Reference Covered by silica-rich melt Folie 20

21 Melting of dust particles and SiC corrosion Silica-rich melt Corrosive attack! Cristobalite Lime-rich melt SiSiC Cristobalite SiSiC Folie 21

22 Influence of mineral dust melts on SiSiC strength 4-point bending strength [MPa] SiSiC-Reference SiSiC covered by silicarich melt SiSiC covered by limerich melt Folie 22

23 Thermal shock testing of absorber units by solar dish facilities Folie 23

24 Outline Motivation Model dust Accelerated dust deposition on SiC absorber components Temperature dependent dust adhesion Estimation of dust deposition under real conditions Melting of dust particles and SiC corrosion Influence on light absorbance and efficiency Conclusions Folie 24

25 Influence on light absorbance and efficiency Thermal efficiency [%] 10 percentage points Dust deposited (Silica-rich) Reference Air outlet temperature Folie 25

26 Outline Motivation Model dust Accelerated dust deposition on SiC absorber components Temperature dependent dust adhesion Estimation of dust deposition under real conditions Melting of dust particles and SiC corrosion Influence on light absorbance and efficiency Conclusions Folie 26

27 Conclusion Significant deposition of mineral dust can be expected during service Reduction of efficiency due to lower radiation absorbance Choking of channels may occur at high service temperatures Adhesion (sintering) between dust particles and SiC is poor as long T <1200 C (silica-rich dust) or 1150 C (lime-rich dust), respectively dust deposition can be removed by suitable brushes If molten, mineral coatings have high adherence on SiC structures Lime-rich mineral melts cause corrosive attack on SiSiC structures Folie 27