Elsevier

Renewable Energy

Volume 152, June 2020, Pages 529-539
Renewable Energy

The effect of dust accumulation on the cleanliness factor of a parabolic trough solar concentrator

https://doi.org/10.1016/j.renene.2020.01.091Get rights and content

Highlights

  • The effect of dust accumulation on reflectivity is investigated in cold regions.

  • The optical performance of dusty reflector is studied with increasing wavelength.

  • Average reflectivity reduces by 10% with increase of 1 g/m2 dust accumulation.

  • A model is developed to predict the cleanliness factor of dusty reflector.

  • The model results are compared to the experiment results.

Abstract

Solar reflectors are exposed to outdoor environments where dust accumulation is a primary degrading factor on optical performance. In this study, the effect of dust accumulation on reflectivity at different positions on the reflector of a parabolic trough solar thermal power plant in Hohhot, China was investigated and analyzed. The physical and chemical properties of dust accumulation were tested using a combination of spectrophotometer, scanning electron microscopy, and X-ray diffraction. The results showed that dust accumulation on the bottom edge of the reflector caused the largest decrease in reflectivity compared to dust on the center and top edge. In addition, dust particles at Hohhot were dominated by quartz (SiO2, 53.5%), followed by calcium oxide (CaCO3, 25.4%), and some minor feldspar minerals (NaAlSi3O8, 21.1%). However, some characteristics of the dust could not be determined by experimental measurements. To address this gap, a physical model was proposed to predict the impact of dust accumulation on light reflectivity of the reflector. Different physical parameters of the model are discussed, such as the size of the particles, diaphaneity, the incidence light angle, and tilt angle. The maximum relative deviation between the mathematical model and the experimental results was only 1%.

Introduction

Pollution is caused by the combustion of fossil fuels, natural disasters, and social industrialization [1], which makes it necessary for researchers to explore new energy resources and improve the efficiency of energy utilization devices [2]. Solar energy is widely applied, as it is easily obtained, environmentally friendly, and there is an inexhaustible supply [3,4]. Thus, the development of solar energy utilization technologies has become significant [[5], [6], [7]]. Concentrating solar power (CSP) and concentrating solar thermal (CST) systems are being implemented to utilize solar power [8,9]. The reflector is one of the main components of the CSP/CST system that delivers thermal from solar energy to fluid [10]. Reflectivity of the solar reflector is affected by external environmental factors, including dust, local solar intensity, temperature, humidity, wind, and rain [11,12]. Among these factors, dust accumulation on the surface of the solar reflector is one of the most important and common causes of decreased efficiency [13,14]. To optimize solar utilization efficiency of the parabolic trough solar concentrator, reflectivity has been investigated in terms of particle size, dust density, mirror position, period of dust accumulation, and shape of dust particles.

Dust accumulation on the surface of a solar collector reduces the amount of solar radiation that reaches the conversion device by decreasing transmittance of the protective cover of non-concentrating collectors [15]. Thus, the effects of dust accumulation on the efficiency of solar photovoltaic (PV) modules and protective transparent covers have been studied [[16], [17], [18]]. Erdenedavaa et al. [19] investigated the effects of dust accumulation on transmittance of the glass tubes in a solar thermal collector in Ulaanbaatar. In addition, effective prediction models of solar radiation transmittance on transparent covers have been established to evaluate the optimal cleaning time [20,21]. Conceicao [22] developed a model to calculate the optimum tilt angle for soiled PV systems instead of using the optimum tilt angle solely according to irradiance. Moreover, Salari and Hakkaki-Fard [23] numerically investigated the effect of dust accumulation on the performance of PV and photovoltaic-thermal systems. Lu and Zhang [24] investigated dust deposition processes and behavior on ground-mounted solar PV arrays using the shear stress transport k-u turbulence model and the discrete particle model. These studies investigated the effect of dust accumulation on optical transmittance loss. The direct and indirect light scattered from dust particles on the glass package is used for conventional non-concentrating panels as long as the light is scattered onto the solar cell. An optical concentrating system is used to collect sunlight. When the surface of the dust collector is contaminated, a large part of the light is scattered and lost. Thus, it is necessary to study concentrator loss. Simultaneously, these research methods of the effects of dust accumulation on optical performance and of the dust deposited progress model as well as numerical investigations play a guiding role when studying the effects of dust on the optical properties of a reflector.

These results emphasize the effects of dust accumulation on solar PV. However, light can interact with dust particles and the losses can be larger on a reflective surface [25]. Hence, understanding the dust depositing mechanism is essential for developing optimized cleaning strategies for the reflector. Cohen [26] proposed that dust accumulation on the solar reflector reduces reflectivity by absorbing and scattering sunlight. The decision to clean the reflectors is initiated when reflectivity is <90%. Roth and Pettit [27] investigated reflectivity loss on mirrors due to deposited dust particles during a 10 month exposure period. The results showed that the average decrease in reflectivity measured at 500 nm was slightly smaller for silvered glass mirrors than aluminized reflective mirrors. The efficiency of the cleaning operation depends on the frequency of cleaning and the properties of the dust attached to the surface of the solar reflector [28]. It is essential to research the characteristics of the dust and soil at different sites to determine the nature of the stickiness between the dust and the mirror. Merrouni et al. [29] developed a mirror sphere with three tilt angles relative to the vertical plane: +45° (facing the sky), 0° (vertical), and −45° (facing the ground). The results showed that the highest average cleanliness drop values per month for the horizontal mirrors were 45% and 33%. To determine the decreased optical capability of the mirror, Vivar [25] used artificial dust on plane mirrors to test reflectivity. In the experiment, losses in reflectivity were 20% on average compared with a clean mirror. The maximum daily loss of reflectivity was 0.7–1.3% per day [30]. Hachicha et al. [31] studied the characteristics of dust particles and their effects on CSP performance under UAE weather conditions and reported a decrease of reflectivity by about 63% after 3 months of exposure.

Current studies have reported some modeling methods to predict the decrease of reflectivity in a dusty mirror. Bouaddi et al. [32] created a model to describe and forecast the loss of reflectivity on solar reflectors used in CSP plants. The approach was based on a time series analysis, using the dynamic linear Gaussian state space time series and included weather parameters as explanatory variables in the analysis. Bouaddi et al. [33] proposed a new approach to simulate the soiling of regularly cleaned reflectors instead of the inaccurate methods undermining the CSP yield using fixed reflectivity assumptions. Biryukov et al. [34] developed a computerized microscopic system to study the physics of dust particles that adhere to various kinds of solar collector surfaces. The device provided the particle size distribution of the dust to calculate the fraction of surface area covered by the dust and to calculate the reduction in optical efficiency (of the solar collector under study) as a function of particle size. Heimsath and Nitz [35] presented a model that predicts mirror reflectivity for different levels of cleanliness. The model applied the Lambert–Beer law and related incidence angle-dependent attenuation of solar radiation to the dust layer. A novel model measured airborne dust concentration and estimated the size distribution, the position of the mirrors, and the recorded wind speed and air temperature [36].

However, few studies have focused on the effect of dust accumulation on reflectivity of different positions of a reflector surface in a parabolic trough solar concentrator. In this study, the effect of dust accumulation on this type of reflector was investigated. Dust deposited on the surface of a parabolic trough was analyzed when the tilt angle was fixed. The results showed that the bottom of the parabolic trough suffered from more pollution when the collector was placed horizontally. This study provides theoretical guidance for parking the parabolic trough collector system and a new method for tracking based on cleaning up the dust accumulation. The characterization of dust particles was investigated in a cold climate zone to optimize the cleaning cost of the solar collectors. The remainder of this study is organized as follows. Section 2 introduces the methods of the prediction on the CF. A description of the experimental set-up and the experimental process is presented in Section 3, which is applied in Section 4 where dust influences on the reflector are presented. Finally, a conclusion is provided in Section 5.

The contributions of this study mainly include the following: (1) the effect of dust deposited on different positions of the concentrator was researched and the particle features and components of dust were measured; (2) a physical model was proposed to predict the CF decrease of a parabolic trough solar concentrator. The particle characteristics and the shape of the parabolic trough solar concentrator, including diaphaneity, size distribution, and the tilt angle of the reflector were considered in the model; (3) the experimental results were compared with the numerical results, and the trend in the experimental results was consistent with the theoretical values.

Section snippets

Dust accumulation mechanism

Various sources contribute to atmospheric dust, such as soil elements lifted by wind (Aeolian dust), factory pollution, the construction of buildings, and feces of flying birds [37]. Dust accumulation on the solar module surface is an issue of great concern, particularly in sandstorm regions. Inner Mongolia is located in the alpine and cold region of northern China, where drought and occasional dust storms occur. Dust accumulation on the surfaces of a parabolic trough solar concentrator leads

Cleanliness factor for the dust accumulation prediction model

The intensity of light propagating through a transparent cover is expressed as follows [38]:II0=exp(3Mγ4ρRA)Where I0 and I are the incidence light intensity and the transmitted intensity on the transparent cover, respectively (cd), and A is the area of the entire surface of the cover (cm2). In addition, R, ρ, and γ are the radius (cm), bulk density (g/cm3), and diaphaneity of the dust, respectively, M is the total weight of the dust on the surface (g). Dust particles are assumed to be spheres

Morphology

Randomly sampled particles were analyzed and characterized based on diameter to obtain the size distribution of the dust particles via SEM. Fig. 5 shows the SEM image of sample particles collected from Hohhot. The range of the dust distribution was <11.182 μm. The diameter of the smallest particles marked by electron microscopy was 4.644 μm in Fig. 5. The literature indicates that the percentage of particles with diameters <4 μm in the total sample is >60%. Dust particles with diameters <4 μm

Conclusions

This study enhanced our understanding of the parabolic trough solar concentrator material optical loss mechanism by accumulating dust at different positions on the reflector surface in an outdoor experiment. Moreover, the physical and chemical properties of dust were analyzed to assess the effect of dust on the optical properties of a parabolic trough solar concentrator. A prediction model was proposed to analyze the effects of the characteristics of dust particles and different tilt angles on

CRediT authorship contribution statement

Ze Wu: Formal analysis, Writing - original draft, Visualization. Suying Yan: Data curation. Zefeng Wang: Writing - original draft, Formal analysis. Tingzhen Ming: Writing - original draft. Xiaoyan Zhao: Formal analysis. Rui Ma: Formal analysis. Yuting Wu: Writing - original draft.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This research was supported by the National Natural Science Foundation of China (No. 51766012), Inner Mongolia Financial Innovation Funding Project in 2017, the Inner Mongolia Science and Technology Major Project (No. 201905) and the Key Project of ESI Discipline Development of Wuhan University of Technology (WUT Grant No. 2017001).

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