Snow effects on PV Lit review: Difference between revisions

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"hydrophobic coatings"
"nakajima hashimoto thin solid films"
"preparation of hard super hydrophobic visible light"
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"Composites: Design for performance"

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"Recent Studies on Super-Hydrophobic Films"

Part I: Developing Hydrophobic Thin Film Coatings

Schondelmaier, D., Cramm, S., Klingeler, R., Morenzin, J., Zilkens, Ch., Eberhardt, W., Orientation and Self-Assembly of Hydrophobic Fluoroalkylsilanes, Langmuir, Vol. 18, Issue 16, 6242-6245, (2002)

Tag: water-repellent, FAS, film stability and thickness

The film stability and thickness of self-arranging FAS monolayers was investigated. Hydrophobic surfaces can be created by inducing surface roughness, or by reducing surface free energy. When both are combined super-hydrophobic surfaces are created, where water is able to "roll" off the surface. In rolling off the surface, the water can drag dirt from it, thereby self-cleaning. To preserve the underlying microstructure of the surface, precise control of film thickness, orientation and stability of the molecules are necessary. An analysis of the thickness and the orientation of a self-assembled coating of FAS on various substrates obtained by X-ray photoelectron spectroscopy and X-ray absorption near edge spectroscopy measurements, are presented. A strong bond between the first monolayer of ip-coated FAS molecules and the substrate was observed, with the molecular axis predominantly perpendicular to the surface, even within the microchannels of an Al2O3 membrane. Further layers can be applied weakly by dip-coating, and removed easily by rinsing in water or ethanol.

Tadanaga, K., Katata, N., Minami, T., Super-Water-Repellent Al2O3 Coating Films with High Transparency, Journal of the American Ceramic Society, Vol. 80, Issue 4, 1040-1042, (1997)

Tag: transparent, super-water-repellent coating, Al2O3

This paper investigates the synthesis of transparent, super-water-repellent coatings. By the sol-gel method, porous Al2O3 coatings were produced with roughness less than 50 nm, then heat treated. An uneven, 20-50 nm flowerlike Al2O3 structure was produced by immersing the film in boiling water. To yield low surface energies and water-repellency, Heptadecafluorodecyltrimethylsilane (a FAS) was coated on the Al2O3 film, then again heat treated. Water contact angles of 165 degrees, and a transmittance of visible light greater than 92% were achieved. The roughness of the alumina was too small to scatter light, however it was utilized along with chemical modification by FAS, to yield a super-hydrophobic transparent glass coating.

Zhang, X., Jarn, M., Peltonen, J., Pore, V., Vuorinen, T., Levanen, E., Mantyla, T., Analysis of roughness parameters to specify superhydrophobic antireflective boehmite films made by the sol-gel process, Journal of the European Ceramic Society 28, 2177-2181, (2008).

Tag: Snow and PV, super-hyrdophobic, antireflective

In this paper, superhydrophobic antireflective boehmite (AlOOH) films were made with different optical transmittances by varying the heat-treatment temperatures and the film thicknesses. A set of roughness parameters was determined to characterize the superhydrophobic property and the antireflective property. The aim is not only to identify the superhydrophobic property but also to specify the antireflective property by the roughness parameters. The superhydrophobic antireflective boehmite films were made from aluminum tri-sec-butoxide, isopropyl alcohol, ethyl acetoacetate and (heptadecafluoro-1,1,2,2-tetrahydrodecyl) trimethoxy-silane. Two different sols were produced and spun coated on glass slides. By varying the heat-treatment temperatures and the film thicknesses the superhydrophobic and antireflective properties were tuned. Three slides with sol-A were produced and heat treated at 100C, 300C and 500C. Contact angles were 103, 152, 152 degrees respectively will optical transmittance of all three greater than that of reference glass (92-96%). Two slides with sol-B coatings were treated at 300C. They both produced contact angles of 154 degrees and excellent optical transmittance. B1-300 yielded 95-96% transmittance over the whole wavelength range.

Scmidt, D., Coburn, C., Dekoven, B., Potter, G., Meyers, G. & Fischer, D., Water-based non-stick hydrophobic coatings,Nature 368, 39 - 41, (03 March 1994).

Tag: reactive perfluoroalkyl polymeric surfactants

This article investigates the suitability of RPPS as non-stick coatings when cross linked and cured with reactive ionic functionalities such as carboxylate and phenate. They found an effective an effective crosslinking approach was to react ammonium-carboxylate functional RPPS with a poly-IPO based crosslinking agent. Solutions with both compounds were stable until applied to the surface and loss of ammonia and solvent, then amide-ester crosslinks were formed. Various RPPS' were prepared by copolymerizing acrylate or methylacrylate esters of fluoroalkyl alcohols with carboxylic acid functional vinyl monomers. Crosslinking of final film was varied by controlling the initial mole ratios of carboxylate to oxazoline in poly-IPO when reacted with RPPS. The wettability of these films was compared using tilt angle measurements (more sensitive than theta c)....

Ma, Y., Cao, X., Feng, X., Ma, Y., Zou, H., Fabrication of super-hydrophobic film from PMMA with intrinsic water contact angle below 90 degrees

Tag: hydrophobic surfaces without low surface energy materials, polymer modification, selective solvent treatment

A super-hydrophobic film was created without the surface modification of low surface energy materials such as fluorides by synthesizing a polystyrene (PS)-PMMA film and treating with warm cyclohexane. The mixture of PMMA and PS was dissolved in THF to 5 wt%, then films were spin-cast to glass slides. Finally, the coatings were exposed to warm cyclohexane. The spin-cast films were initially transparent, but turned translucent following cyclohexane exposure, indicating the possible formation of micro-structures in the film by removal of PS. CAs with water as high as 154 degrees were reported.

Nakajima, A., Saiki, C., Hashimoto, K., & Wantanabe T., Processing of roughened silica film by coagulated colloidal silica for super-hydrophobic coating,Journal of Materials Science Letters 20, 1975-1977, (2001).

Tag: super-hydrophobic surfaces

This article discusses the production of two different silica films coagulated with collodial silica suitable for super-hydrophobic coatings. Tetraethyl orthosilicate (TEOS) and 36% hydrochloric acid solution were mixed with ethanol and stirred for 19 hours, then collodial silica were added. Two different silicas were employed: methanol-based collodial silica (MA-ST), and methylethylketone-based (MEK-ST). Pyrex plates were spin coated five times to produce ~500nm silica films. Next, an ethanolic solution of heptadecafluorodecyltrimethoxysilane was hydrolyzed by water. This silane solution was then evaporated and deposited on the silica films, producing hydrophobic coatings on glass plates. MEK-based collodial silica yielded lower degrees of sliding at all water drop masses. The transmission of 500nm light through the MEK-based coating was approximately 90%.

Nakajima, A., Abe, K., Hashimoto, K., Watanabe, T., Preparation of hard super-hydrophobic films with visible light transmission,Thin Solid Films 376, Issues 1-2, 140-143, (2000).

Tag: surface roughness, fluoroalkylsilanes

This article investigates super-hydrophobic coatings (contact angles greater that 150 degrees) due to the inhibition of snow sticking or water resistance. Surface roughness is necessary for hydrophobicity, but it induces scattering of light. In order to satisfy both hydrophobicity and light transmission, precise control of surface roughness is thus required. This requirement was achieved by the utilization of a sublimable compound and a fluoroalkylsilane coating.

Nakajima, A., Abe, K., Hashimoto, K., Watanabe, T., Invited Review Recent Studies on Super-Hydrophobic Films, Chemical Monthly 132, 31-41 (2001).

Tag: review article

Nakajima, A., Design of a transparent hydrophobic coating, Journal of the Ceramic Society of Japan 112 [10], 533-540, (2004).

Tag: review article

Part II: Snow Dynamics and array effects

Ross, Michael., [http://www.rerinfo.ca/documents/trPVSnowandRime.pdf Snow and Ice Accumulation on Photovoltaic Arrays:

An Assessment of the TN Conseil Passive Melting Technology], Division Report EDRL 95-68 (TR), (1995)

A very comprehensive study into the effects of snow on PV performance. This study was performed with the focus of validating a new snow removal technology aimed at reducing snow losses in remote northern sites. It includes a excellent reveiw of snow types, accumulation methods and physical properties, and a full thermal model of a panel which was used to predict melt times for the panels. This model requires further validation in order to be universally applied, but handles well the many variables associated with the melting of snow from a panel.

D. Thevenard, K. Haddad Ground reflectivity in the context of building energy simulation, 2005----

A great overview of snow albedo and specularity. Done in the context of buildings simulations. -Includes models for specular albedo on sloped suefaces -Did a preliminary analysis and found that the snowfall can increase array performance by ~2.63% -For a building, can decrease heating load by 10.9% in a year

Use as reference for snow albedo numbers and correlations

---K.Hosokawa & T.Tomabechi, A study on the optimal installation of PhotoVoltaic systems in snowfall areas ----

they found 90 degrees to be the optimal angle for a system that was located at a similar latitude to us (actually slightly south). I think that it is hard to make any hard conclusions, however as they are mising a siginificant chunk of data for their system. Also, the author seems to confuse power and energy in some of the graphs. Its also interesing that his measured values for albedo on a vertical plane as a fraction of total global irradiation are close to those that I predicted theoretically in the report (25% predicted, 30% measured by the paper) however, they never explained how they measured this, so it is a bit dubious.

I do have a problem with the insolation vs. power (or energy?) curves, as they are very non linear, levelling off at at around 400w/m^2. This means that those panels that are operating near the optimal angle in the summer time do not have the abillity to correctly utilizse the greater insolation at these times, which means that the losses of the 90 degree panel will not be as great in the summer time (becasue the optimal panels are not operating well either) According to PV syst you should be loosing close to 45% of your summertime yeild and 32% of your wintertime yield at an agle of 90 degrees, which is not shown in the collected data. It can be seen that they never reached their rated yeild of 70W, even at 1200w/m^2

Spectral effects

---[ http://www.sciencedirect.com/science?_ob=MImg&_imagekey=B6V51-44SJ4C3-8-1B&_cdi=5773&_user=1025668&_pii=S0927024801000952&_orig=search&_coverDate=02%2F15%2F2002&_sk=999289996&view=c&wchp=dGLzVtb-zSkzk&md5=f7dbb6403d8f480d8b74b5163435426a&ie=/sdarticle.pdf%7C Ricardo Rüther, Gerhard Kleiss, and Kilian Reiche, “Spectral effects on amorphous silicon solar module fill factors,” Solar Energy Materials and Solar Cells 71, no. 3 (February 15, 2002): 375-385. ]---

• Isc mismatch calculated from Isc at standard AM 1.5 and Isc for arbitrary spectrum E(lambda) extrapolated to 1000W/m^2, assuming spectral response is not effected by point of operation
• This mistatch can be large for a-Si (Possiblity that the spectrum for albedo reflection from old snow can cause poor performance in a:si due to albedo?)
• Used CM11 diffuse and global and a filtered glass to measure 'redness index' of the light
• Modules were stabalized by degredation for six months
• Fill factor showed a large amount of scatter in the low ranges of Isc, it is not at all constant across the range
• Scatter could also be due to environmental issues, eg. Snow
• Fill factor cannot be correlated to panel temperature
• Correlates to spectrum as the reason for the difference
• the diffuse data comes primarily at times of lower irradiance, so the same correlation could be shown between low and high irradiance. There is an interesting bit about scatter in the mid ranges of this curve, which might suggest some advantage of a bluer spectrum in this region, but there was no statistical analysis to back this up.