Scientific description
Just a few weeks ago the European parliament has adopted 35% renewable energy target for 2030, which ensures the stable growth of this industrial sector. Today, solar panels cover more than 4000 square kilometers and are forecasted to be the world’s main electricity source by 2050 (http://www.epia.org). The majority of jobs in the EU- and US-based solar industry are connected to the installation and maintenance projects, processes that are both labor-intensive and hard to automate. But most important, maintenance of the solar parks should be supported by research efforts aiming at understanding the qualitative and quantitative changes solar panel surfaces undergo during their environmental exposure. Furthermore, research energy should be channeled into the development of solar cells that have the potential to reduce the maintenance-connected costs.
Economic viability and exploitation costs of solar plants are directly dependent on understanding and avoiding the processes of soiling. Soiling of photovoltaic (PV) systems compromises their performance causing a significant power loss and demanding periodical cleaning actions. This phenomenon raises great concerns in the solar energy field, thus leading to notable research efforts over the last decades. Soiling is caused by a dual action of airborne dust deposition and biofouling. In this project we will focus on understanding and avoiding the microbiological contribution to PV soiling - an environment-induced factor that is so far often overlooked or underestimated (Martin Sanchez et al., 2018).
The term “soiling” is habitually used to describe the accumulation of dust, dirt, snow, plant debris, pollen and bird droppings on solar panels (Maghami et al. 2016). Surprisingly, despite the well-known prevalence of microorganisms in all environments, including dust and plant debris, their contribution to soiling of solar panels is overlooked or underestimated. To illustrate this fact, it is noteworthy to mention that just one (Shirakawa et al. 2015) of the 198 dust/soiling PV papers published from 2013 to 2015, compiled by Costa et al. (2016) was focused on soiling-causing microorganisms.
Morphologically simple and microbially dominated ecosystems termed “biofilms” have existed and prevailed on Earth for a remarkably long period of biosphere evolution. Such systems form at interfaces of solid materials with gas or liquid phases and represent a unique structure capable of multiple impacts on substrate and element cycles (Gadd, 2017; Gorbushina, 2007). Biofilms that are most relevant for solar panels are microbial communities that form on their surfaces under atmospheric (sub-aerial) exposure (e.g. Noack-Schönmann et al., 2014).
Sub-aerial rock biofilms represent the first stage of primary succession of terrestrial ecosystems on mineral substrates. These microbial structures have occurred throughout the entire geological history of the Earth and actively participate in rock weathering and carbon sequestration in the Earth crust. Today the same biofilms are inherent settlers on the most spread outdoor anthropogenic/ technogenic surfaces: solar panels. Life at the solar panel/ atmosphere interface influences both the underlying substrate and the microclimate zone above and around it - especially because they possess an impressive reactive surface in contact with the atmosphere.
In addition to the PV power loss due to microbial soiling, biofilm development on the surface of PV modules has the potential to cause secondary problems in the underlying glasses, such as corrosion and biodeterioration. These deteriorative processes are well-known in historical and optical glasses (Drewello and Weissmann 1997), being especially relevant in medieval glasses of church windows (Piñar et al 2013). The typical biodeterioration symptoms – etching, pit corrosion, and leaching – were successfully reproduced in laboratory experiments using similar glasses incubated with microorganisms for 6 months (Gorbushina & Palinska 1999; Rodrigues et al. 2014). However, considering the different properties of modern glasses used in PV modules, as well as their relatively short operating life (20-25 years), the expected impact of such deterioration processes on PV systems should be rather low.
Four points are crucial in understanding the connection between soiling and sub-aerial biofilm growth, and these will be addressed in the proposal:
(i) subaerial biofilms develop on solar panel surfaces ubiquitously and in a rather fast fashion. PV modules harbor a highly diverse microbial community of melanised fungi, pigmented bacteria as well as diverse phototrophs (Noack-Schönmann et al., 2014; Shirakawa et al., 2015; Dorado-Morales et al., 2016; Martin Sanchez et al., 2018). Considerable biofouling on PV facilities in tropical environments proceeds very fast. E.g. Shirakawa et al. (2015) showed that PV modules displayed significant power reductions (7% after 6 or 12 months and 11% after 18 months), which were attributed to the dual nature of soiling by both particle deposition and SAB fouling.
(ii) subaerial biofilms can efficiently interact with airborne dust – binding it firmly to the material surface through their extracellular matrix. The airborne compounds and particles can additionally serve as nutrients and thus further support biofouling (sub-aerial biofilm development) of solar panels. Removing of deposited particles is made difficult by the presence of the interacting subaerial biofilm and this fact additionally justifies research on the microbiological components of solar panels soiling.
(iii) subaerial biofilms by themselves consist of pigmented organisms most of which are drought-, heat- and irradiation-adapted bacteria and fungi (Dorado-Morales et al., 2016; Martin Sanchez et al., 2018). These stress-tolerant microbial communities dwelling on solar panels can efficiently absorb/scatter up to 70% of incident light, i.e. the source of energy for energy production (Noack-Schönmann et al., 2014).
(iv) There were attempts to develop coatings for the solar panels that would help to hinder the biofilm growth (Schirakawa et al. 2016). TiO2 coating (self-cleaning glass) inhibited the formation of fungal hyphae, thus reducing the overall biofilm cover. In contrast, silver nanoparticle polymeric coating was not effective in reducing SAB formation (Schirakawa et al. 2016).
The first principal goal of this project is the quantification and characterization of a biofouling ecosystem at a solar panels interface. To reach this goal, laboratory studies will be performed to qualitatively analyze biofilm-forming organisms and extracellular matrix they produce while forming a stable fouling biofilm. It is essential to include the particle deposition into the picture and thus this work will be performed by direct in situ microscopy. Since recently there exist sensitive and reliable methods to quantify the microbial load on PV systems. Such quantitative approach (Martin Sanchez et al., 2018) presents a useful tool to monitor soiling of PV modules and design appropriate cleaning strategies, as well as to investigate the role of microorganisms and biofilms on soiling problems.
This work will concentrate on just a few representative samples from St. Petersburg as well as Berlin and will work towards a delivery of a 3-D model of solar panels biofilm including the air-borne particle deposition. We aim to deliver a model of a unique structure capable of multiple impacts on substrate and element cycles. We intend to investigate processes taking place on the interface between a biofilm and the solar panel surface, also in relation to the biogenic mineral modification that may take place. The study of interdependency between biological colonization and particulate atmosphere-derived deposition will help predict soiling processes. This study will contribute to a fundamental understanding of soiling as well as solar panel deterioration processes.
The second goal is the development of biofilm-hindering surface modifications that are active precisely against the organisms capable of surviving the environmental hostile conditions. With the help of solar panels isolates (fungi as well as phototrophs) we will conduct laboratory experiments to study the impact of photodynamic/photocatalytically active surface modifications on the growth of fouling biofilms. Combining new/novel antifouling mixtures with a laboratory biofilm under defined climatic conditions (fluctuations of temperature and CO2 concentrations can be set in FU labs) allows for the first experiment to be developed in the frame of this short-term proposal.
In this project we aim at conducting laboratory experiments with samples derived from the solar parks of FU and St. Petersburg State University. Prof. Lessovaia (SPSU) can study the examples of solid materials employed for the solar panels to characterize the specificity of porosity (nano- and micro scales) and substances’ changes affected by biota colonization. Surface modifications active against microbial growth proposed by the group of Prof. Bahnemann and Prof. Emeline will be subjected to accelerated in vitro testing with the help of a laboratory model biofilm, established in the laboratory of Prof. Gorbushina (FUB). This weathering-inducing model biofilm combining one heterotroph and one phototroph component is genetically modifiable (e.g. Gorbushina & Broughton, 2009). The broad interdisciplinary consortium (see table 2) will assure synthetic and complete interpretation of the data during a joint workshop in August - September 2018.

Principal components of the planned laboratory experiments
• In this project SPbGU and FU will conduct a joint study of biofilm diversity on solar panels, with a special accent on fouling by stress-tolerant melanised fungi and cyanobacteria. The solar panel biofilms development will be studied qualitatively with respect to cell forms.
• Quantitative analysis of the solar panels biofilm biomass to compare the sites of exposure in Berlin and St. Petersburg.
• Qualitative in situ analysis of solar panel biofilms and their extracellular matrix. Specific accent will be given to studies of cyanobacteria and other phototrophs that grow at interfaces of solar panels and air.
• Additional component would be a search for possible photocatalytically active surface modifications that can counteract the development of biofilms and would be added through the participation of the Bahnemann/Emelin lab. Obtained cultures of stress-tolerant melanised fungi and cyanobacteria will be used in these tests.

Significance of the planned research to the scientific community
Our results will work towards:
(i) qualitative analysis of biofilm-forming organisms present on solar panels.
(ii) Quantification of the microbial load on PV systems in St. Petersburg (by means of a quantitative approach as in Martin Sanchez et al., 2018).
(iii) We will concentrate on just a few representative samples from St. Petersburg as well as Berlin, but will work towards a delivery of a 3-D model of solar panels biofilm including the air-borne particle deposition.
(iv) The second goal is the development of biofilm-hindering surface modifications that are active precisely against the organisms capable of surviving hostile conditions. With the help of solar panels isolates (fungi as well as cyanobacteria/phototrophs) we will conduct laboratory experiments to study the impact of photodynamic/photocatalytically active surface modifications on the growth of fouling biofilms. Combining new/novel antifouling mixtures with a laboratory biofilm under defined climatic conditions allows for the first experiment to be developed in the frame of this short-term proposal.
Quoted literature:
Costa S.C.S., Diniz A., Kazmerski L.L. (2016). Dust and soiling issues and impacts relating to solar energy systems: Literature review update for 2012–2015. Renewable & Sustainable Energy Reviews, 63: 33-61.
Dorado-Morales P., Vilanova C., Peretó J., Codoñer J.M., Ramón D., Porcar M. (2016). A highly diverse, desert-like microbial biocenosis on solar panels in a Mediterranean city. Scientific Reports, 6: 29235, https://doi.org/10.1038/srep29235
Drewello R., Weissmann R. (1997). Microbially influenced corrosion of glass. Applied Microbiology and Biotechnology, 47: 337-346.
Gadd GM (2007) Geomycology: biogeochemical transformations of rocks, minerals, metals and radionuclides by fungi, bioweathering and bioremediation. Mycol Res 111:3–49.
Gadd GM (2017) Fungi, Rocks, and Minerals. Elements 13: 171-176.
Gorbushina AA. Life on the rocks. Environ Microbiol 2007; 9: 1613-31
Gorbushina A., Palinska K. (1999). Biodeteriorative processes on glass: experimental proof of the role of fungi and cyanobacteria. Aerobiologia, 15: 183-192.
Lessovaia S.N., Dultz S., Plotze M., Andreeva N., Polekhovsky Y.. Filimonov A., Momotova O. (2016) Soil development on basic and ultrabasic rocks in cold environments of Russia traced by mineralogical composition and pore space characteristics. Catena, V. 137, 596–604; Impact Factor: 2.612.
Maghami M.R., Hizam H., Gomes C., Radzi M.A., Rezadad M.I., Hajighorbani S. (2016). Power loss due to soiling on solar panel: A review. Renewable and Sustainable Energy Reviews, 59: 1307-1316.
Martin-Sanchez P.M, Gebhardt C., Toepel J., Barry J., Munzke N., Günster J., Gorbushina A.A. (2018) Monitoring microbial soiling in photovoltaic systems: A qPCR-based approach. International Biodeterioration & Biodegradation, DOI: 10.1016/j.ibiod.2017.12.008, in press
Noack-Schönmann S., Spagin O., Gründer K.P., Breithaupt M., Günter A., Muschik B., Gorbushina A.A. (2014). Sub-aerial biofilms as blockers of solar radiation: spectral properties as tools to characterise material-relevant microbial growth. International Biodeterioration & Biodegradation, 86: 286-293.
Piñar G, Garcia-Valles M, Gimeno-Torrente D, Fernandez-Turiel JL, Ettenauer J, Sterflinger K (2013). Microscopic, chemical, and molecular-biological investigation of the decayed medieval stained window glasses of two Catalonian churches International Biodeterioration & Biodegradation, 84: 388-400.
Rodrigues A, Gutierrez-Patricio S, Miller AZ, Saiz-Jimenez C, Wiley R, Nunes D, Vilarigues M, Macedo MF (2014) Fungal biodeterioration of stained-glass windows. International Biodeterioration & Biodegradation, 90: 152-160.
Shirakawa M.A., Zilles R., Mocelin A., Gaylarde C.C., Gorbushina A.A., Heidrich G., Giudice M.C., Del Negro G.M.B., John V.M. (2015). Microbial colonization affects the efficiency of photovoltaic panels in a tropical environment. Journal of Environmental Management, 157: 160-167.
Shirakawa MA, John VM, Mocelin A, Zilles R, Toma SH, Araki K, Toma HE, Thomaz AC, Gaylarde CC (2016). Effect of silver nanoparticle and TiO coatings on biofilm formation on four types of modern glass. International Biodeterioration & Biodegradation, 108: 175-180.
Velde B, Meunier A (2008) The origin of clay minerals in soils and weathered rocks. Springer, Berlin.
Project partners / FUB/SPSU expertise
Prof. Bahnemann and Prof. Emelin lab: photodynamic inhibition of biofilm growth
Drs. Gavrilova, Averina, Velichko (Prof. Pinevich lab): detection of cyanobacteria, isolation and characterization of new strains, detection of different bacterial taxonomic groups by in situ hybridisation
Prof. Gorbushina lab: fungi, isolation of stress-tolerant slow growing ascomycetes, molecular biology and genetics of model rock-inhabiting fungus, geomicrobiology, quantitative deterioration studies (qPCR for determination of fungal and bacterial loads).
Prof. Lessovaia lab: substances’ monitoring of solar panels surfaces which have been in contact with biofilms. The planning methodology based on approaches of Velde and Meunier (2008) includes solid substrate surface visualization using laser and scanning atomic-force microscopy, which was applied to study the specificity of rock porosity of the natural ecosystems (Lessovaia et al., 2016).
Synergies between FUB and SPSU specialists can be best visualised by listing the expertise each specialist will bring into the consortium for analysing the experimental results.
We are planning to prepare a joint research proposal for external funding DFG together with Russian Foundation for Basic Research at the end of 2018 year (November 2018).