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A solution from a research made in Italy for the soiling problem in photovoltaic plants

The optimization of the performance of a photovoltaic plant is increasingly important. Power losses may be caused by various factors such as electrical problems, degradation of modules, shading, high temperatures and the soiling effect, which is the subject of the present article, as it is a reversible potential factor.

There are lots of scientific literature produced around this phenomenon. There is, however, a clear difficulty in quantifying the performance losses due to soiling, which lies in the fact that it can be quite variable, dependent on different site specifications and polluting environmental factors for each geographic area of the planet. It can reach values of 25%, as calculated by a recent study "Large Reductions in Solar Energy Production Due to Dust and Particulate Air Pollution" published in QualEnergia magazine, especially in desert areas with critical environmental conditions, where sandy winds may cause partial or total darkening of the cells. The biggest economic damage however, falls on larger plants that loose massive amounts of money even in the face of a minimum efficiency loss.Awareness on this issue is perhaps more common among sector operators rather than plant owners, although a distinction must be made between owners of residential plants and those of commercial size and utility scales plants. The O&M market is consolidated and mature and considering the total power of 19 GW of photovoltaic plants installed in Italy by the end of 2016, it becomes clear how maintenance in general is important in order to ensure the correct and efficient operation of the plant overtime. Certainly the owners of commercial size and utility scale plants are the most concerned with efficiency: there is greater attention aimed at obtaining the best performance of photovoltaic installations, optimizing yield production and investment return.

It is possible to restore yield losses caused by agents of environmental impact

There is no right answer to address the problem of maintenance and power recovery in photovoltaic plants: a cleaning planed schedule is generally the most widespread solution. Although it is important, this solution does not provide a full satisfactory answer, as the panels, once washed, enjoy the benefits only for a limited amount of time (see, for example, Graph 1 below). For this reason, many operators choose not to perform periodic cleaning, cutting management costs but settling for a photovoltaic park that produces less than its potential.

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Graph 1. In this chart, we consider a plant in an industrial area of Puglia, south Italy. The gray area represents the structural decay whereas the blue area represents environmental degradation during a two-year period with cleaning performed in the 1st and 13th months. It is possible to observe a decay gradient and, therefore, a more pronounced performance decrease in the immediate months after the washing, followed by a tendency to stabilize the disequilibrium in the next months.

There are a number of products available on the market that claim the ability to minimize dirt accumulation on pv panels by creating a coating to protect the front prism of the panels. Many of them use a silicone based vehicle, others are based on Siliceous precursors, others still use perfluorinated polymers, trying to give the prism, properties of hydrophobicity rather than oleophobicity, acting on the interfacing properties. There are also products that focus on photocatalytic properties of certain oxides, mainly Titanium Dioxide, although they are suitable for a variety of applications without a precise photovoltaic attention.

A research started in 2010 led by Prof. Norberto Roveri of the University of Bologna, developed the nanotechnological product Pannel Plus®, as a result from an innovative, exclusive and patented synthesis of Titanium Dioxide, specifically aimed for photovoltaic application.

Massimo Masetti, Director in charge of Technical-Scientific Dissemination of DIVE Group, says: "The study which was initially theoretical and focused on the synthesis of highly efficient photocatalysts, has immediately married the theme of renewable energies thanks to the intuition of Prof. Norberto Roveri , Honorary Chairman of DIVE Group and Full Professor at the Department of Chemistry at the University of Bologna, and his research team. The product development and improvement continued along with numerous field tests, which enabled Pannel Plus® to be perfected based on real observations and data, which today allows us to speak proudly of scientific innovation and industrial solidity”

The main advantage of the product lies in its bio-mimetic characteristics: its photocatalytic particles are developed, synthesized and stabilized with the precise focus on photovoltaic application. By interacting with light radiation, the product reacts by modifying its surface characteristics, in particular:

energy acquires hydrophilic characteristics from the ultraviolet portion of solar radiation, due to the unconventional synthesis of Titanium Dioxide which is normally unusual to it. Due to the persistent presence of humidity on the surface of the material, during the day it is possible to make soluble pollutants other than capturing the oxydriles produced by the catalytic photo action. During the night, in the absence of light, the surface changes the material tension by regaining hydrophobic traits, where the particles become spherical and slide away from the panel washing off the accumulated dirt (self-cleaning effect). The "structural" innovation of the Pannel Plus® product also consists in its application: the product adheres to the outer glass surface of the module by an electrostatic bond, which does not create chemical interaction of any type with the panel, which is a relevant detail to the coverage validity of module insurance. Furthermore, thanks to its innovative synthesis, the crystalline structure of Titanium Dioxide acquires back-scattering characteristics, reflecting much of the incident infrared radiation and significantly reducing the surface temperature of the photovoltaic panel.

It is also important to note that the synthesis process and the finished product give great importance to sustainability and environmental impact. Dive Group's Chief Commercial Officer, Andrea Pezzoli, adds: "We wanted to propose a product with high environmental sustainability. In addition, we live in an international reality whereby the safety and ease of transportation of products is a competitive advantage". 

In a technology transfer process, DIVE Group has acquired the product rights and will soon launch on the market a software based on a predictive calculation model, which is able to quantify the possibility of yield recovery, based on the environmental characterization and location site of a pv plant.

Restore and maintain the optimal efficiency conditions of pv panels

There are currently numerous treated plants in various parts of Italy, with different conditions of maintenance and environmental pollution. For this reason, the results, that are always positive, differ substantially: some plants have recovered 3-4% of production while others have gone over 18%. It is therefore impossible to standardize the expected data, regardless of the knowledge of the location site and its environmental characterization, which takes into account the many soiling dynamics as well as the technical characteristics of the system. 
"It is precisely for this reason that we have begun to develop a predictive calculation model; in this way we can provide our clients with a more objective answer on how much they could gain and the break even time for the investment made. Certainly, highly polluted areas and plants with performance ratios lower than 80% have shown significantly higher recovery efficiency rates", says Pezzoli.

As an example, we may consider a 1 MW plant, located in Puglia region (south Italy) in an industrial site with 1889 hours of average annual irradiation and an energy rate of 0.31 Euro/kWh. It is possible to estimate, based on its historical data analysis and site-specific environmental characterization, through estimates made by the calculation model, a yield loss due to soiling of about 7.25% per year. Moreover, the model allows us to determine the "star" performance and the expected performance in a two-year period after the treatment with Pannel Plus®, for an increase of + 6.91%, corresponding to +196,04 MWh and an economic gain of Euro 55,130, with break even at the 5th month.

Assuming that Pannel Plus® may only recover 3.5% of the lack of production, this translates into +48.4 MW for an economic gain of around € 5,630.00 in the first year, already deducting the investment needed to the treatment that observes its break even at the 7th month. Considering the two-year period, the expected total gain is approximately Euro 25,190.

Graph 2. This graph represents the effective yield without the soiling in the considered 2-year period (green area) and, the relative production loss (white area up until the red dashed line) 

Graph 3. In this graph, it is possible to observe the expected performance after the application of Pannel Plus® (values on the axis of the ordinates are expressed in KEur).

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