This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. IEEE JOURNAL OF PHOTOVOLTAICS 1 Fundamental Studies of Adhesion of Dust to PV Module Surfaces: Chemical and Physical Relationships at the Microscale Lawrence L. Kazmerski, Fellow, IEEE, Antonia Sonia A. C. Diniz, Member, IEEE, Cristiana Brasil Maia, Marcelo Machado Viana, Suellen C. Costa, Pedro P. Brito, Cl´ audio Dias Campos, Lauro V. Macheto Neto, Sergio de Morais Hanriot, and Leila R. de Oliveira Cruz, Member, IEEE Abstract—Photovoltaic (PV) module soiling is a growing area of concern for performance and reliability. This paper provides evaluations of the fundamental interactions of dust/soiling parti- cles with several PV module surfaces. The purpose is to investigate the basic mechanisms involving the chemistry, morphology, and re- sulting particle adhesion to the first photon-incident surface. The evaluation and mapping of the chemistry and composition of sin- gle dust particles collected from operating PV module surfaces are presented. The first correlated direct measurements of the ad- hesive force of individual grains from field-operating collectors on identical PV module glass are reported, including correlations with specific compositions. Special microscale atomic force microscopy techniques are adapted to determine the force between the particle and the module glass surface. Results are presented for samples under dry and moisture-exposed conditions, confirming the effects of cementation for surfaces having soluble mineral and/or organic concentrations. Additionally, the effects of hydrocarbon fuels on the enhanced bonding of soiling particles to surfaces are deter- mined for samples from urban and highly trafficked regions. Com- parisons between glass and dust-mitigating superhydrophobic and superhydrophilic coatings are presented. Potential limitations of this proximal probe technique are discussed in terms of results and initial proof-of-concept experiments. Index Terms—Adhesion, characterization, composition, dust, microscale, mitigation, module, performance, photovoltaics, reli- ability. I. INTRODUCTION S OILING, which is the sedimentation of particulate matter on the exposed surfaces of solar collectors, is a growing Manuscript received July 3, 2015; revised November 30, 2015 and Octo- ber 12, 2015; accepted December 22, 2015. This work was supported by the Coordenac ¸˜ ao de Aperfeic ¸oamento de Pessoal de N´ ıvel Superior (CAPES) (www.capes.gov.br). L. L. Kazmerski is with the Renewable and Sustainable Energy Institute, University of Colorado Boulder, Boulder, CO 80309-0027 USA, and also with the National Renewable Energy Laboratory, Golden, CO 80401 USA (e-mail: solarpvkaz@gmail.com). A. S. A. C. Diniz is with the Energy Center GREEN, Pontifical Catholic University of Minas Gerais, Belo Horizonte 30535-901 Brazil (e-mail: asacd@PUCMinas.br). C. B. Maia, S. C. Costa, P. P. Brito, C. D. Campos, L. V. M. Neto, and S. de Morais Hanriot are with the Pontifical Catholic University of Minas Gerais, Belo Horizonte 30535-901, Brazil (e-mail: cristiana@pucminas.br; suellenc- scosta@gmail.com; pbrito@pucminas.br; compos@pucminas.br; lvilhena@ pucminas.br; hanriot@pucminas.br). M. M. Viana is with the Federal University of Minas Gerais, Belo Horizonte 31270-901, Brazil (e-mail: marcelomachadov@pucminas.br). L. R. de Oliveira Cruz is with the Instituto Militar de Engenharia, Rio de Janeiro 22290-270, Brazil (e-mail: leilacruz@ime.eb.br). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JPHOTOV.2016.2528409 area of concern for solar system performance and reliability. In the case of photovoltaics (PV), the condition of this first surface of interaction of the incident photons is critical for ensuring that the maximum possible light reaches the conversion devices. Historically, this important issue has received intermittent research attention over the past 70 years, with studies mainly focused on the relationship of dust accumulation to electrical and thermal performances [1]. With mounting deployment of this solar technology, especially into areas that have both significant solar resource and ambient dust conditions, studies related to the fundamental nature of the particle adhesion may provide increased understanding and new pathways to the mitigation or prevention of this adverse soiling effects 1 [2], [3]. Dust (or soiling particles) is defined as any particulate matter less than 500 μm in diameter (about 10 times the diameter of a human hair or 50 times that of an optical fiber) [1]. The composi- tion can include amounts of pollen (vegetation, fungi, bacteria), human/animal cells, hair, carpet and textile fibers, other mi- crofiber, and, most commonly, minerals from geomorphic fall- out such as sand, clay, or eroded limestone. The chemistry of dust varies from geographical region-to-region. Sand particles typically dominate in the desert regions of the Middle East, con- trasted with the soil/fertilizer/plant products of the agricultural areas of the U.S. and South America, or possibly the vehicle or power plant fuels that might be pervasive in urban environ- ments. The severity of dust or soiling also depends critically on the climate zone and weather conditions [4]. The fundamental properties of dust and its control are still not fully understood [1], [4], [5]. The science and technology in- vestment in this problem has increased substantially in this past decade—with exceptional work directed at understanding the issue. The purpose of this paper is to investigate the most funda- mental mechanisms and relationships between soiling particles’ chemistry, morphology, and the adhesion of individual particles on the module surface, gaining insights toward possible mitiga- tion. This paper provides evaluations of the interactions of dust particles on several PV module surfaces (glass from the same panels and with dust mitigation coatings). This extends previous studies of the chemical and physical properties of the dust mate- rial from various geographical regions and climate zones world- wide and validates predictions and observations made first in the 1 See www.nrel.gov or http://en.wikipedia.org/wiki/Solar_cell for the latest version of this efficiency chart, which is updated regularly with confirmed performance results. 2156-3381 © 2016 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. 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