Determining soiling losses on PV modules in a desert climate

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Understanding the impact of soiling on PV modules and thus the most appropriate cleaning regime, is critical to preventing losses in module power and plant yield. David Daßler, Stephanie Malik and Akshayaa Pandiyan of Fraunhofer CSP describe a statistical method they have developed for accurately characterising the losses from soiling in desert conditions. []

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Soiling is the accumulation of dust on solar panels that causes a decrease in the solar photovoltaic (PV) system’s efficiency. The changes in conversion efficiency of 186 residential and commercial PV sites were quantified during dry periods over the course of 2010 with respect to rain events observed at nearby weather stations and using satellite solar resource data. Soiling losses averaged 0.051% per day overall and 26% of the sites had losses greater than 0.1% per day. Sites with small tilt angles (<5°) had larger soiling losses while differences by region were not statistically significant.
Conference Paper
A reduction of series resistance loss due to a declined current in halved cell modules (compared to full cell modules) is applied by several PV module manufacturers. In [1] the outdoor performance of a grid connected array, which used the half-size cell design, was examined. The advantages of half-size cells in PV modules in comparison to full-size cells were also investigated theoretically and experimentally via indoor measurements under standard test conditions by SERIS Institute [2, 3]. In a public funded BMBF project a module design was developed in cooperation with SERIS. After module manufacturing the outdoor characterization, to analyze the performance behavior under real operating conditions, was performed in a second step. For that purpose a half-size cell as well as a full-size cell module in a standard module size were measured outdoor in an open rural test area in Germany. The following paper covers the measuring set-up and a detailed uncertainty analysis. The data evaluation is focused on filtering methods to gain reliable data sets from long-term outdoor tests of half-size and/ full-size cell modules. Keywords: Half-Size Cells, Outdoor Data Evaluation, Module Performance, Energy Yield
Large-scale solar plants are generally located in semi-arid and desert lands where abundant sunlight is available for solar energy conversion. These plants, however, suffer from two major environmental degradation factors: high ambient temperature and high concentration of atmospheric dust. Degradation of solar collectors’ performance caused by soiling results in a considerable loss of energy yield in all solar plants of the world. Dust and other particulate accumulation on solar collectors causes transmission loss. This is true with respect to transmission losses in photovoltaic (PV) and concentrated photovoltaic (CPV) systems, and for reflection losses in concentrated solar power (CSP) systems. We present here a brief review of the energy yield losses caused by dust deposition on solar collectors, with particular emphasis on flat-panel photovoltaic (PV) systems. The review includes some of the major studies reported on energy-yield losses on solar plants in operation in several regions of the world. In addition, laboratory-soiling studies are also included. We report on degradation in the performance of solar plants based on the type of solar collectors, geographical location, local climate, and exposure period of the collectors absent any manual cleaning. An analysis of the advantages of cleaning processes that include natural, manual, automatic, and passive methods is presented. Our objective is to provide solar plant designers with a database for predicting anticipated soiling losses in different parts of the world, and for assessing effective cleaning methods for restoring a system’s energy yield.
The energy delivery of a solar-energy system is generally associated with the sun's available irradiance and spectral content, as well as a variety of environmental and climatic factors and inherent system and component performances. However, other external factors relating to geographical location and conditions can have even greater impacts on system performance. Among these, soiling is a commonly overlooked or underestimated issue that can be a showstopper for the viability of a solar installation. This paper provides a comprehensive overview of soiling problems, primarily those associated with dust (sand) and combined dust-moisture conditions that are inherent to many of the most solar-rich geographic locations worldwide. We review and evaluate key contributions to the understanding, performance effects, and mitigation of these problems. These contributions span a technical history of almost seven decades. We also present an inclusive literature survey/assessment. The focus is on both transmissive surfaces (e.g., those used for flat-plate photovoltaics or for concentrating lenses) and reflective surfaces (e.g., mirrors or heliostats for concentrating power systems).
This work aims to evaluate the effect of soiling on energy production for large-scale ground mounted photovoltaic plants in the countryside of southern Italy. Since the effect of pollution can seriously compromise the yield of solar parks, the results obtained in this study can help the operation and maintenance responsible in choosing the proper washing schedule and method for their plants and avoid wasting money. In order to determine the losses due to the dirt accumulated on photovoltaic modules, the performances at Standard Test Conditions (STC – Irradiance: 1000W/m2; Cell temperature: 25°C; Solar spectrum: AM 1.5) of two 1MWp solar parks before and after a complete clean-up of their photovoltaic modules have been compared. The performances at STC of the two plants have been determined by using a well-known regression model that accepts as an input two climate data (the in-plane global irradiance and the photovoltaic module temperature), while the output results in one electrical parameter (the produced power). A regression model has been preferred to a common performance ratio analysis because this latter is too much influenced by the seasonal variation in temperature and by the plant availability. The results presented in this work show that both the soil type and the washing technique influence the losses due to the pollution. A 6.9% of losses for the plant built on a sandy soil and a 1.1% for the one built on a more compact soil have been found. Finally, these results have been used in order to compare the washing costs with the incomings due to the performance improvement.
Gradual reduction of PV generator yield due to pollution
  • H Haeberlin
  • J D Graf
H. Haeberlin, J.D. Graf, 1998, "Gradual reduction of PV generator yield due to pollution", Second World Conference on Photovoltaic Solar Energy Conversion.
Development of outdoor research platforms and tests on photovoltaic modules in the green energy park in Morocco
  • S Malik
  • M Ebert
  • Z Naimi
  • B Ikken
Malik, S., Ebert, M., Naimi Z., Ikken, B., "Development of outdoor research platforms and tests on photovoltaic modules in the green energy park in Morocco", Proc. of 1st Africa Photovoltaic Solar Energy Conference and Exhibition Africa PVSEC 2014, Durban; WIP Renewable Energies; pp.19-23.