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Title: Investigating the effect of temperature differences and generated current on dust accumulation process on solar phtovoltaic (PV) modules
Authors: Jiang, Yu
Degree: Ph.D.
Issue Date: 2018
Abstract: Solar Photovoltaic (PV) technology, can directly convert solar energy into utilized electricity with the help of photovoltaic effect by semi-conductor materials. Solar PV modules mainly contain solar cells that can convert receiving solar energy into electricity, glass covers to provide protection for vulnerable solar cell units and other electrical components to transform generated current meet the energy requirements of consumers. Many environmental parameters can greatly affect the energy conversion efficiency of solar PV modules, such as solar irradiance and temperature of solar cells. Additionally, another environmental parameter, dust accumulation, which means that airborne dust from the atmosphere air deposits on solar PV module covers due to different forces, is a significant issue when engineers considering the energy performance of solar PV modules because the accumulated dust can greatly reduce the received solar radiation and affect the energy conversion efficiency of solar PV modules. The formation of accumulated dust is inevitable because solar PV plants are generally located in areas where sunlight is not only abundant for energy conversion process but also has a high concentration of dust particles for dust accumulation. Based on previous studies all over the world, the effect of dust on solar PV modules is significantly negative due to the reduction in received solar irradiation by the refraction and extinction process of deposited dust particles. There are some researches on the dust accumulation on solar PV modules, but mainly focusing on the impact of dust density and property or environmental parameters such as wind and exposure time on the energy performance of solar PV modules without examining the impact of the operating solar PV modules on the dust accumulation process. During sunshine days, when the operating solar PV modules receives solar radiation and converts some irradiance into electricity with efficiency generally lower than 20%, the most received solar radiation will be converted into heat, which results in high temperature of solar cells, especially with high solar irradiance. This will cause obvious temperature difference between solar module surface and its surrounding air. In addition, the generated current by solar PV modules will cause electric field. Both the temperature difference between module surface and ambient air and the generated electric field will affect the particle deposition onto module surface. So, it is significant to study the effect of operational solar PV modules on the surface dust accumulation process. In addition, considering the greatly negative effect of dust accumulation on the energy conversion performance of solar PV modules, studies of approaches for removing or stopping dust accumulation thus have received lots of attention. Even though some studies reported the methods for cleaning or preventing dust accumulation on solar PV modules to maintain the high energy conversion efficiency of solar cells, but not the significant issue of cleaning frequency for cleaning solar PV modules with dust particles, especially for desert areas. Therefore, this work aims to study the effect of characteristics of operating solar PV modules, both temperature differences and generated current, and to investigate cleaning issues, including cleaning frequency and wind cleaning process. Firstly, this thesis investigates the influence of temperature differences between the PV module surface and the ambient air on dust accumulation process by particle deposition experiments with glass samples and solar PV module samples. The measured deposition densities of fine aerosols are from 0.05 to 0.19 g/m² under the experimental conditions and the glass light transmittance ratios increase with the increase in temperature differences and tilt angles, ranging from 0.975 to 0.996. The glass with higher surface temperature has lower particle density due to the effect of thermophoresis arising from temperature differences. In addition, the deposition densities decrease with the increase of sample tilt angles for all conditions due to the combination effect of aerosol gravity and aerosol surface adhesion. The output power ratios of the PV module samples increase from 0.861 to 0.965 with an increase in temperature difference from 0 to 50 °C, and dust particles have a significant impact on the short circuit current and the output power. However, the influence of particle on the open circuit voltage can be negligible.
Secondly, the effect of generated current on dust accumulation process is illustrated. Based on the particle deposition experiments for glass samples with different provided currents, it can be found that the generated current can greatly increase the dust accumulation density due to the electrostatic force by the generated electric field because of the generated current. Particle deposition density results have strongly verified the attractive effect by the generated current. In addition, the influence of different mechanisms for electrostatic force on particle deposition process is analysed, including Columbic force and image force. Columbic force plays a significant role in particle deposition process on the operating solar PV modules and the impact of image force can be neglected. The calculated particle deposition velocity due to Columbic force under strong electric field based on Eq. (4.1) ranges from 7.34×10⁻⁷ m/s to 3.12×10⁻⁴ m/s for different particle diameters. When particle diameters increase from 0.01 μm to 0.5 μm, deposition velocity decreased nearly 75% from 3.12×10⁻⁵ m/s to 7.34×10⁻⁶ m/s because small particle is significantly influenced by Columbic force when compared with large particle in this size range. However, when particle diameters increase from 0.5 μm to 10 μm, the deposition velocity due to electric field is greatly increased 100% from 7.34×10⁻⁶ m/s to 1.51×10⁻⁵ m/s because large charging numbers for large particles. Deposition velocity of image force is increased with the increase of particle diameters because deposition velocity of image force is proportional to the particle charge level and large particles have large particle charging number based on diffusion charging mechanism. In addition, the deposition velocity also increases with the increase of dielectric constant. Thirdly, a resuspension model for rough and spherical particles detaching flat surface is firstly developed to analyse the cleaning effect of wind on the accumulated dust particles on solar PV modules. This model considers adhesion force, hydrodynamic force and torque. The minimum shear velocity and corresponding actual wind velocity for particle removal are calculated for particles with diameters ranging from 0.1 μm to 100 μm based on this model. It is found that large particles could be effectively cleaned by wind due to the low required resuspension velocity compared with small particles. The minimum required shear velocity for particle resuspension process are found to range from 0.23 m/s to 57.56 m/s for different particle sizes, and increases with the increase of particle diameter and the actual wind velocity vary from 0.82 m/s to 2219.8 m/s, and the ratios of shear velocity to wind velocity range 0.04 to 0.06. Therefore, these findings can verify that particle resuspension method is a significant way to analyse the wind cleaning process on deposited particles. In addition, considering the cleaning process of wind, more accurate model to estimate energy output for solar PV modules due to the accumulated dust can be developed for solar energy industry. Finally, a novel method to estimate the cleaning frequency for dirty solar PV modules in desert areas has been established based on the existing particle deposition velocity model. The environmental parameters used are 20 μm for representative average diameter, 0° for tilt angle (horizontal position) and 100 μg/m³ for particle concentration in the ambient air respectively. In addition, the cleaning criterion is 5% reduction in power with accumulated dust density of 2 g/m². The cleaning frequency for solar PV modules in desert regions is about 20 days. Therefore, it’s recommended that the PV modules cleaning can be determined as three weeks to keep the high power output. In addition, the calculated cleaning time based on the present model agrees well with the related experimental results. Therefore, this verified method can be adopted in engineering application to estimate the cleaning frequency of dirty PV modules. Additionally, the average particle diameters and tilt angles can significantly influence the required cleaning time because the deposition velocity is highly dependent on gravity. The cleaning time is decreased to 20 days from 6256 days with the increase of representative average diameter from 1 μm to 20 μm and, is increased to 78.2 days from 20.2 days with the increase of the inclined angle from 0 degree to 75 degree, respectively. In summary, the impact of temperature differences and generated current on dust accumulation process on solar PV modules and cleaning issues (wind cleaning process and cleaning frequency for desert areas) are newly investigated. The main academic contributions of this thesis can be presented as follows: 1) existing temperature differences between module surface and ambient air can greatly decrease the dust accumulation density and thus improve the energy conversion efficiency of solar PV modules; 2) the generated current can significantly increase the dust accumulation density and thus decrease the energy output power of solar PV modules; 3) a novel method to calculate the cleaning frequency for desert areas is developed based on particle deposition velocity model; 4) the method for evaluate the performance of wind cleaning process for dust accumulation is established based on particle resuspension theory.
Subjects: Hong Kong Polytechnic University -- Dissertations
Photovoltaic power generation
Photovoltaic power systems
Pages: xxix, 147 pages : color illustrations
Appears in Collections:Thesis

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