4 November 2016


The PV Module Soiling Issue: Best Solutions

Author: By Remo Fagnani, Technical Manager at Moroni & Partners

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The most significant external detriments are soiling losses: the loss in power due to dirt, dust, snow and other particles that cover the surface of the PV module. On average a dust layer is very thin, with particle thickness measuring in at around 10µm. The size, however, varies significantly by location, the environment, the array structure and numerous other factors.

In recent years several studies have been conducted by research programs with the goal of assessing the impact of soiling on photovoltaic modules. These studies unanimously conclude that bad maintenance is responsible for significant production losses for solar PV plants. According to the Mitchell’s study (2006), PV plant efficiency could decline by up to 0.16% per day between significant rainfall events, leading to an annual energy loss between 3-6%. Other studies report even higher annual yield losses, depending both on system location and the specifications of the analyzed plant.

The most important variables in the magnitude of soiling losses are the dust properties (size, shape…), the chemical components in soil, pollution, moisture, rainfall frequency and intensity, wind speed, direction and PV array structural configuration. The last two variables exhibit strong correlation: the more horizontal the surface, the more dust can accumulate when the wind is either not strong or regular enough. For example, the solar PV plants of Saudi Arabia suffer from intense sandy winds and often feature a low tilt angle. Such conditions perfectly illustrate the importance of module cleaning, since in that example soiling losses amounted to 45.8% over three months without module washing! 

In addition to non-optimal cleaning, dust and soil deposition can cause permanent damage to solar PV modules. If even a single cell becomes shaded it acts as a resistance to the current generated from the other cells. Consequently, the shaded cell heats up and becomes a hot spot that can eventually damage the entire module. In regions with both a high level of airborne dust and a cycle of humid and dry days, an even more dangerous phenomenon can occur: soil deposition can result into cementation, a process that can eventually leads to an energy loss of up to 100%. Moreover, it is also possible for acidic particles to stick to the EVA layer and to corrode it over time. 

In countries featuring Feed in Tariff schemes, such as Italy and many others, incentives represent the main drive for grid-connected PV investments. The energy produced is strongly related to the investment’s IRR, thus even a small increase in inefficiency can have disastrous financial consequences. Accurate pollution loss estimates enable technicians to provide reliable yield calculations for the development of dispatch plans. Moreover, the cleaning frequency (typically once or twice a year), period of the year and relevant costs are generally declared and set in the O&M contract.

Due to the numerous different sources of soiling losses, univocal prediction models like prearranged cleaning schedules often end up being unreliable (or even downright misleading), just in order to minimize the O&M contractor’s expenses. 

Why then are PV plant operators not perceiving this as a major issue?

The underestimation of soiling losses is due to a particularly stealthy effect. In most cases, the irradiance sensor suffers from the same amount of dirt that is covering the solar PV panels. Consequently, the measured irradiance level decreases, despite the actual irradiance remaining the same. The decrease in measured irradiance balances out the decrease in electricity generation of the panels, thus the PR does not change, effectively hiding the losses.

PV Cleaning Optimizer helps Energy Producers, Asset Managers and O&M Operators select the optimal period and frequency of cleaning operations for photovoltaic modules. Once the user has set the solar PV plant’s power and technical features, PV Cleaning Optimizer estimates soiling effect and losses, both in terms of energy generation and revenues, and it provides an alert to the user if the cost-benefit ratio becomes disadvantageous.

The internal web-based software monitors the performance of two installed reference PV modules. The first module is washed daily with high pressure brushless technology, according to the best proven cleaning procedures. The second module, however, is left dirty like the remainder of the solar PV plant. through the monitoring of energy yields and environmental conditions, PV Cleaning Optimizer evaluates the optimal cleaning period, considering the present time of year, current incentives, and both the last and the next scheduled cleaning operations.

Estimates for a 3MWp ground mounted PV plant installed in the Southern Italy state that an optimal cleaning schedule can result in an additional 100 MWh/year, which corresponds to a 2.5% PR improvement. These performance gains become even bigger  for plants in arid locations, as well as for larger installations, reaching a PR improvement of about 10% for a country such as Morocco.

Photovoltaic experts worldwide acknowledge that PV plant maintenance is a crucial aspect of the global renewable energy scene. Within this framework, a smart and flexible cleaning management system is an ideal resource for boosting energy production and significantly reducing expenses.

Considering the immense value that is currently being wasted and the revenues from energy production, the PV Cleaning Optimizer’s average payback time is between 2-3 years. The device is easy to install and adaptable to all conditions, the web based software is very smart, and the structure is built with the aim to resist even the most extreme weather conditions. A live demo of the system will be displayed during the Solar Asset Management Europe conference, on 9-10 November 2016 in Milan.

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