www.dlr.de/enerMENA

Fabian Wolfertstetter, DLR

@ SFERA Summer School Hornberg, May 15th – 16th 2013

Reflector Soiling and Cleaning Methods

SFERA Summer School, Hornberg, Germany, May 15th – 26th 2013 Slide 2

Overview

1. Introduction 2. Measurement3. Nature of dirt

1. Microscopic2. Macroscopic3. Weather influences

4. Mitigation techniques1. Natural cleaning2. Artificial cleaning3. Preventing dirt

5. Cleaning strategies6. Summary and outlook

Literature review presentedNo claim to be complete

No best practices availableNo standard procedures for

measurement or cleaning established up to now

SFERA Summer School, Hornberg, Germany, May 15th – 26th 2013 Slide 3

Nomenclature

Soiling is the process of surfaces getting dirty during exposure to the environmentDirt is everything that settles on the surfaces, it includes:

Aerosol Particles, composed ofSalt particles (very small crystals)Mineral Dust (anorganic, from bedrock, travels very long distances)Biological matter like pollen, bacteria, algae

Other biological matter like lichen, algae, bird droppings, etcSand, e.g. in dust storms

Cleanliness is the reduction of reflectivity of a mirror compared to its clean state. Values between 0 and 1Cleaning sets cleanliness to 1 and reflectivity to some 90-95% depending on the mirror material

SFERA Summer School, Hornberg, Germany, May 15th – 26th 2013 Slide 4

Introduction

In general: very little CSP-specific research in the fieldLosses of 1% to 25% per week measured. High variation depending on locationScales at least linearly with reduction of CSP plant efficiency Highly underestimated efficiency limiting factor in CSPMost of research has been done for PhotovoltaicsDifferent optical properties: PV is affected in terms of transmission loss of cover glass (absorption), CSP optics are directional, i.e. also scattering plays a role

CSP more sensitive to soiling than PV In practice constant reflectivity losses of e.g. 5% are assumed – is this

reasonable?!?PV CSP Glass mirror

SFERA Summer School, Hornberg, Germany, May 15th – 26th 2013 Slide 5

Dirt

Everyday-phenomenonVery complex and variable in its natureThere is artistic solutions……and there is serious solutions

SFERA Summer School, Hornberg, Germany, May 15th – 26th 2013 Slide 6

Dirt – basic principles

1. Settling of particles to the surface due to gravity

2. Adhesion with humidity/ Van der Waals/ electrostatics

3. Cementation with salt and other chemicals

4. Other: e.g. bird droppings, melting of dust on absorber windows or glass tubes

5. Washing of mirrors by rain (snow)

Soiling

SFERA Summer School, Hornberg, Germany, May 15th – 26th 2013 Slide 7

Measurement of soiling

Several hand measurement devices for specular reflectivity available on the market:

Devices & Services Reflectometer R15Abengoa´s Condor ReflectometerSOC 410 Solar

For power plants, only punctual measurements availableResearch offers other non-standard-methods

FW2

Slide 7

FW2 nrel paper on comparison of different reflectometersFabian Wolfertstetter; 12.05.2013

SFERA Summer School, Hornberg, Germany, May 15th – 26th 2013 Slide 8

Devices and Services Reflectometer

D & S reflectometer 15 is the reference in most publications Used as the industry standard

Disadvantages:Punctual measurement pointsTime consumingSingle wavelength Measurement results are operator dependent

Advantage:Absolute reflectivity values measured directly in power plant

[Devices and Services Manual]

SFERA Summer School, Hornberg, Germany, May 15th – 26th 2013 Slide 9

Other hand measurement devices

Different models available for hand measurements in a CSP plantAdvantage: measurement directly in power plant

Portable reflectometers from left to right:Condor (Abengoa), Devices and Services 15R,SOC 410 Solar [Crawford 2012]

SFERA Summer School, Hornberg, Germany, May 15th – 26th 2013 Slide 10

Intercomparison of Reflectometers

Optical properties:

Resolution and accuracy

Wavelengths Acceptance angle

Devices & Services R15

0.001 Accuracy not specified

550nm or 660nm 7, 15, 25, 46 mRad

Condor (Abengoa) 0.001 ± 5% 435, 525, 650, 780, 940, 1050 nm

408 mRad

Surface Optics Corporation 410 Solar

0.001 ± 0.03 335-380, 400-540, 480-600, 590-720, 700-110, 1000-1700, 1700-2500 nm

Integrating sphere, diffuse and total

SFERA Summer School, Hornberg, Germany, May 15th – 26th 2013 Slide 11

Intercomparison of Reflectometers

Intercomparison not convincing

[Crawford 2012]

SFERA Summer School, Hornberg, Germany, May 15th – 26th 2013 Slide 12

Automatic device TraCS

Automatic real time measurement of a test mirror with sun spectrum

High time resolution allows for identification of weather influences on soiling

Validated with hand measurement devices

0.5

0.6

0.7

0.8

0.9

1.0

0.5 0.6 0.7 0.8 0.9 1.0Reference cleanliness

TraC

S c

lean

lines

s

[Wolfertstetter, 2012]

0.6

0.7

0.8

0.9

1.0

1.1

16-Jun-12 17-Jun-12 18-Jun-12 19-Jun-12 20-Jun-12 21-Ju

clea

nlin

ess

fact

or

SFERA Summer School, Hornberg, Germany, May 15th – 26th 2013 Slide 13

Microscopic nature of Dirt

Learn about the mechanisms of adhesionOrigin of dustOptical properties of dust particles

SFERA Summer School, Hornberg, Germany, May 15th – 26th 2013 Slide 14

Dirt seen at 50x magnification on a mirrorWhat a beauty!

SFERA Summer School, Hornberg, Germany, May 15th – 26th 2013 Slide 15

Different types of dirt seen in microscope 50x – 100x

Character of single particles: a) Saharan dust ,

compare [Avila 1997]

b) “Local dust”

Character of soiling patterns: c) Dried water dropletd) High salt content

with water forming film

a)

d)c)

b)

SFERA Summer School, Hornberg, Germany, May 15th – 26th 2013 Slide 16

Dust is very different

Sand Particles collected at different sitesa) Sample from Libyab) Iraqc) Libya in SEM (~120 µm)d) Oman in SEM

SFERA Summer School, Hornberg, Germany, May 15th – 26th 2013 Slide 17

Zoom in: Dirt on surfaces is different

Left: sample collected from around NREL research facilityRight: sample washed off mirrors exposed at same siteNote: scales are the same [Nelson 2011]

SFERA Summer School, Hornberg, Germany, May 15th – 26th 2013 Slide 18

Zoom closer with SEM

Left: mirror soil Right: ground soilNote: many coagulated particles [Nelson 2011]

SFERA Summer School, Hornberg, Germany, May 15th – 26th 2013 Slide 19

Optical system for size distribution determination

Scanning microscopic method Size distributions deteminedCompared to airborne dust

[Biryukov 1999, 1996]

SFERA Summer School, Hornberg, Germany, May 15th – 26th 2013 Slide 20

Optical effects of dirt

Specular reflectance as a function of acceptance angle and dust accumulation Beam profile is slightly altered due to scatteringAnother result of Pettits study:

Effect of particles on specular reflectance ~5 times greater than on hemispherical reflectance

[Pettit 1980]

SFERA Summer School, Hornberg, Germany, May 15th – 26th 2013 Slide 21

Composition of Dirt

Dirt components’ compositions measured in many studiesConclusions to influence on soiling not drawn in sufficient manner

[Nelson 2011]

SFERA Summer School, Hornberg, Germany, May 15th – 26th 2013 Slide 22

Macroscopic

The following are general observations, but again, there is very few publications investigating the macroscopic effects for CSP plant types:Dirt is very inhomogeneously distributed on a single mirror panelBack structure of mirrors plays a roleMirrors at the edges of a field soil differently than those in the middle

SFERA Summer School, Hornberg, Germany, May 15th – 26th 2013 Slide 23

Intra-field variation

Bowman compared different soiling rates of CPV elements within the same fieldHigh variations shownComparison to pyrheliometer(NIP) behaviour:

High winds speeds: CPVs soil faster than pyrheliometerLow wind speeds: pyrheliometer soils faster

SFERA Summer School, Hornberg, Germany, May 15th – 26th 2013 Slide 24

Weather influence

Very few publicationsDependency on different mirror inclinations well documented for a few measurement sites

Sandstorms in different regions

SFERA Summer School, Hornberg, Germany, May 15th – 26th 2013 Slide 25

Weather influences

Study from Abu Dhabi comparing weekly measurements of sample mirrors to weather parametersBest correlation with DHI, still far from being proven

[Tahboub 2012]

SFERA Summer School, Hornberg, Germany, May 15th – 26th 2013 Slide 26

Weather influences

[Tahboub 2012]

SFERA Summer School, Hornberg, Germany, May 15th – 26th 2013 Slide 27

Red Rain

Atmospheric research treats red rains as light rains at high atmospheric aerosol concentrations

Red rain event statistics For Southern France

[Avila 1997]

SFERA Summer School, Hornberg, Germany, May 15th – 26th 2013 Slide 28

Red Rain impact

Measurement curve measured in Oujda, Morocco with TraCS instrument during a red rain event

0.6

0.7

0.8

0.9

1.0

1.1

16-Jun-12 17-Jun-12 18-Jun-12 19-Jun-12 20-Jun-12 21-Ju

clea

nlin

ess

fact

or

[Wolfertstetter 2012]

SFERA Summer School, Hornberg, Germany, May 15th – 26th 2013 Slide 29

Mitigation

SFERA Summer School, Hornberg, Germany, May 15th – 26th 2013 Slide 30

Natural cleaning

Rain (and snow) will clean mirrors only above a certain intensityLight rains can have the opposite effect

Light rain washes out the suspended particles in the atmosphere and deposits them on the mirror surfaceIf rain is heavy enough less dirt will stay on the mirrors

Dew can wash the mirrors in some casesDew can also glue smallest particles to surfacesWind has been shown to have no cleaning effect

SFERA Summer School, Hornberg, Germany, May 15th – 26th 2013 Slide 31

Cleaning systems

Many different vehicles and methods for cleaning documentedStudies on best parameters for cleaning

Water pressureWater salt levelScrubbing vs spraying

Some studies on automatic “coulomb – cleaning”

SFERA Summer School, Hornberg, Germany, May 15th – 26th 2013 Slide 32

Cleaning systems

Traditional cleaning method, Mr Twister and Cleaning Truck at Cramer Junction power plant [Cohen 1999]

SFERA Summer School, Hornberg, Germany, May 15th – 26th 2013 Slide 33

Cleaning systems

Brush and pressurerized cleaning systems

SFERA Summer School, Hornberg, Germany, May 15th – 26th 2013 Slide 34

Cleaning systems

“Paris” automatic cleaning robot from Sener[Vicente 2012]

SFERA Summer School, Hornberg, Germany, May 15th – 26th 2013 Slide 35

Cleaning systems

More automized systems:Esolar cleaning robot

Regular heliostat field configurationContinuously moving robot cleans line per line

Novatec cleaning robot for Fresnel systems

Runs automatically along linear concentratorsVery little water consumptionNighttime cleaning

[Schell 2011]

SFERA Summer School, Hornberg, Germany, May 15th – 26th 2013 Slide 36

Coulomb force cleaning

AC voltage induced on the mirror surface can help clean mirrorsParticles are charged so they get pushed around by voltagesDeaggregation effect (on the right)Energy balance ?!?

[Beryukov, 1999]

SFERA Summer School, Hornberg, Germany, May 15th – 26th 2013 Slide 37

Washing optimization

Washing efficiency depends on water temperature and applied pressure in systems with no mechanical force

[Fernández García 2011]

SFERA Summer School, Hornberg, Germany, May 15th – 26th 2013 Slide 38

Washing optimization

SFERA Summer School, Hornberg, Germany, May 15th – 26th 2013 Slide 39

Washing optimization

Kattke and Vant Hull:Scrubbing is typically five times slower than spraying, but requires half the waterScrubbing removes dirt film, spraying only removes dust

Dependency of costs on targetreflectivity

SFERA Summer School, Hornberg, Germany, May 15th – 26th 2013 Slide 40

Cleaning strategies

Only relevant study supplies theoretical formulas for optimizationFocus on dirt film and dirt particle removal

[Kattke 2012]

SFERA Summer School, Hornberg, Germany, May 15th – 26th 2013 Slide 41

Preventing dirt

Nightly stow positions of heliostats and troughs avoid settling of dirtAnti-soiling coatings for solar mirrors Wind fencesPaved roads, not dirt roads in power plantsVegetation, location, etc

SFERA Summer School, Hornberg, Germany, May 15th – 26th 2013 Slide 42

Preventing dirt

Anti soiling coating by Flabeg shows better behaviour than standard mirror surfaces

SFERA Summer School, Hornberg, Germany, May 15th – 26th 2013 Slide 43

Summary

PV literature not applicable to CSPMeasurement techniques questionableSoiling is has very big effectsSoiling is still poorly understood

SFERA Summer School, Hornberg, Germany, May 15th – 26th 2013 Slide 44

Thank you for your attention

Literature sources:Sarver, T., et al “A comprehensive review of the impact of dust on the use of solar energy: history, investigations, results, literature and mitigation

approaches”, Renewable and Sustainable Energy Reviews 22 –(2013) 698-733, 2013Bowman, J., et al “Analysis of soilingrates at the victor valley community college CPV site”, AIP conf. Proc. 1407, 312 (2011); doi:

10.1063/1.3658351Wolfertstetter et al “A novel method for automatic real-time monitoring of mirror soiling rates”, SolarPACES conference proceedings, Marrakech,

Morocco 2012Schwarberg,F., “Enhanced solar mirrors with anti-soiling coating”, SolarPACES conference proceedings, Marrakech, Morocco 2012Biryukov et al “an optical system for the quantitative study of particulate contamination on solar collector surfaces”, Solar Energy Vol 66, No 5, pp

371-378, 1999Biryukov et al “DEGRADATION OF OPTICAL PROPERTIES OF SOLAR COLLECTORS DUE TO THE AMBIENT DUST DEPOSITION AS A

FUNCTION OF PARTICLE SIZE”, Journal of Aerosol Sciences, Vol 27, pp 37-38, 1996Kattke et al “optimum target reflectivity for heliostat mirror washing”, SolarPACES conference proceedings, Marrakech, Morocco 2012Deffenbaugh et al “THE EFFECT OF DUST ACCUMULATION ON LINE-FOCUS PARABOLIC TROUGH SOLAR COLLECTOR PERFORMANCE”,

Solar Energy Vol. 36 No, 2, pp, 139-146, 1986 Nelson et al “UNDERSTANDING SOIL ADHESION IN CONCENTRATING SOLAR POWER PLANTS: A NOVEL ANALYSIS OF SOIL

CHARACTERISTICS”, SolarPACES conference proceedings, Granada, Spain, 2011Crawford et al, “A Comparison of Three Portable Reflectometers for Use in Operations and Maintenance of CSP Plants”, SolarPACES conference

proceedings, Marrakech, Morocco, 2012Cohen et al, “FINAL REPORT ON THE OPERATION AND MAINTENANCE IMPROVEMENT PROGRAM FOR CONCENTRATING SOLAR

POWER PLANTS”, SAND99-1290 Unlimited Release, June 1999Tahboub et al “Solar Mirrors Soiling Campaign – Abu Dhabi” SolarPACES conference proceedings Marrakech, Morocco, 2012Pettit et al “WAVELENGTH DEPENDENT SCATTERING CAUSED BY DUST ACCUMULATION ON SOLAR MIRRORS” Solar Energy Materials 3

(1980) pp 1-20Schell et al “Design and evaluation of esolar’s heliostat fields”, Solar Energy 85 (2011), pp 614-619Vicente et al “PARIS – A Parabolic Trough Autonomous Robotic Cleaning System”, SolarPACES conference proceedings Marrakech, Morocco,

2012Nelson et al, “UNDERSTANDING SOIL ADHESION IN CONCENTRATING SOLAR POWER PLANTS: A NOVEL ANALYSIS OF SOIL

CHARACTERISTICS”, SolarPACES conference proceedings Granada, Spain, 2011