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The impact of dust on photovoltaic systems
Release time:
2018-02-28 13:26
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Researchers Wen Yan and Zhao Dong from the National Climate Center of the China Meteorological Administration, the Wind and Solar Resource Center of the China Meteorological Administration, and the Public Meteorological Service Center of the China Meteorological Administration. The impact of system power generation efficiency is summarized in related research results at home and abroad.
The analysis shows that the output performance of the photovoltaic panel is affected by the dust deposited on the surface, which reduces the efficiency of the battery; the dust has a direct impact on the solar irradiance and heat dissipation of the panel, and can cause the panel surface to be corroded; the dust accumulation in different areas There is a big difference. In addition, the influencing factors of dust accumulation on photovoltaic panels at home and abroad are also analyzed, and the influence of wind, precipitation intensity, dust fall, air pollution, dust properties and panel inclination on the formation of dust accumulation and power generation is summarized. Dust accumulation model established by factors such as rainfall. Finally, it analyzes the currently widely used photovoltaic panel dust removal methods, and compares their advantages and disadvantages.
For long-running photovoltaic power generation systems, the impact of panel dust on it should not be underestimated. The dust on the surface of the panel has the function of reflecting, scattering and absorbing solar radiation, which can reduce the transmittance of the sun, causing the solar radiation received by the panel to decrease, and the output power is also reduced. Its effect is proportional to the cumulative thickness of the dust.
In addition, because the dust absorbs solar radiation, the photovoltaic panel can heat up, and the dust contains some corrosive chemical components, which also reduces the photoelectric conversion efficiency [1]. The dust on the surface of the panel has varying degrees of impact on the power generation performance and usage of the photovoltaic system under the effects of different solar radiation, ambient temperature, shielding and corrosion, but the relevant theories in this area are not yet complete. Therefore, add dust The impact on photovoltaic system power generation under different working environments is of great significance to photovoltaic system research.
1 The impact of dust accumulation on the efficiency of photovoltaic power generation
Dust is particulate matter, and its sources are divided into natural sources and man-made sources. Including: small particles of soil, sand and rocks formed under the action of wind and biomass of some animals and plants; dust generated by industries, buildings and transportation [1].
During the operation of the solar photovoltaic power generation system, it will be affected by the dust in the environment where it is located. The photoelectric conversion efficiency of photovoltaic cells is related to the intensity of solar radiation. The accumulation of dust on the surface of photovoltaic panels will reduce the light transmittance of the front cover glass. The decline of light transmittance will cause the output performance of the battery to decrease. The greater the deposition concentration, the light transmittance. The lower the value, the lower the amount of radiation absorbed by the panel, and the greater the decrease in its output performance.
When other conditions remain unchanged, the smaller the inclination angle when the panel is installed, the more dust accumulates in the same time, and the less solar radiation the panel absorbs. For a single panel, the output power decline is limited, but for large-scale grid-connected photovoltaic systems, the total output power declines greatly, so the annual power loss is also large [3].
The obtained results show that during periods of low rainfall, due to the accumulation of dirt on the surface of the panel, the battery efficiency loss can reach more than 15%. Dirt and dust on the surface of the panel can reduce the average annual power generation efficiency by 6%. Generally speaking, dust and dirt in the air include: dust, rain, and pollutants, and their presence will lead to a decrease in battery output energy. Because dust particles have a greater impact on the forward scattering of sunlight, the current loss of such centralized modules is usually much higher than that of non-centralized modules.
Salim et al. [4] conducted a study on the dust removal effect of photovoltaic panels in Saudi Arabia. They chose a fixed inclination angle of 24.6° for an 8-month experiment. The results showed that the output power of the panels that were cleaned every day was higher than that of the panels that were never cleaned. 32%.
In the Garg [5] of Roorkee, India, a 45° angle dust shielding experiment was carried out. The results showed that the average transmittance of the photovoltaic panel under the dust shielding effect decreased by 8% after 10 days.
Chen Dongbing et al. [6] conducted experiments with a 2 MW photovoltaic power station in Bengbu, and the results showed that the power generation of 20 solar modules on photovoltaic panels was reduced by 24%, an average of 1.2% per day.
Wang Feng et al. [7] designed a comparative experiment in a distributed photovoltaic power station in Xi'an urban area. The experiment showed that under the influence of "dust falling" weather, urban dust has a great impact on the output power of the photovoltaic system, reaching 15%.
2 The impact of dust accumulation on photovoltaic panels
2.1 Irradiance
The dust covering the surface of the photovoltaic panel blocks the solar radiation, resulting in a reduction in the effective area irradiated on the panel, and a decrease in the transmittance of the glass, which weakens the intensity of the solar radiation received by the panel, and will cause uneven solar radiation, resulting in power generation The amount is reduced, reducing the output power [2]. The greater the dust deposition concentration, the lower the light transmittance of the panel, and the lower the solar radiation absorbed. Cao Xiaoning et al. [8] conducted efficiency test experiments under different irradiation intensities, and the results pointed out that the short-circuit current of the panel is proportional to the incident light intensity, and the voltage and the light intensity have a logarithmic relationship.
Hegazy [9] conducted experiments on the influence of dust on irradiance with photovoltaic panels of 0°-90°. When there is less dust deposition, the irradiance is less affected. When the panel dust increases, the irradiance is also affected accordingly. Increase.
Zhang Feng et al. [3] carried out comparative experiments on the standard hourly power generation of the cumulative irradiation amount of different days, and concluded that the more dust on the surface of the panel, the lower the output power, and the worse the power generation performance (see Figure 1).
2.2 Heat dissipation
The dust covered on the photovoltaic panel reduces the transmittance through the glass plate, which hinders solar radiation, and also affects the heat transfer form of the photovoltaic panel [2]. Dust attaches to the surface of the panel to block heat transfer, which may prevent the panel's own heat from being released, making the temperature higher and higher, and affecting the efficiency of photovoltaic power generation.
Most of the existing photovoltaic power plants use silicon-based solar cells, which are very sensitive to temperature perception. When a certain thickness of dust accumulates on the surface of the panel, it will increase its heat transfer resistance, which will keep the panel warm and reduce its heat dissipation function. Influence. Experimental results show that whenever the battery temperature rises by 1°C, its output power drops by about 0.5% [7].
2.3 Corrosion
The composition of dust is more complicated, some are acidic substances, some are alkaline substances, and the main components of crystalline silicon photovoltaic panels are silica and limestone, etc. If the dust comes into contact with the moisture in the air and becomes humid, it can be produced with the composition of the panel. Acidic or basic reactions [2].
After a period of time, the surface of the photovoltaic panel will gradually undergo corrosion and damage under the erosion of an acidic or alkaline environment, causing the surface to become pitted, causing the optical performance of the photovoltaic panel to attenuate, and the diffuse reflection of solar radiation on the surface of the panel will damage the solar radiation in the photovoltaic The uniformity of spread in the panel.
In addition, other parts of the photovoltaic panel will also be corroded by moist dust, such as joints, brackets and other parts. The materials are mostly various metals. Corrosion can easily lead to problems such as damage and weakening of safety, which may be caused by strong winds, earthquakes, etc. The destruction of natural factors reduces the service life of photovoltaic panels.
2.4 Differences in the impact of dust accumulation in different regions
Dust on the surface of the photovoltaic panel is one of the important factors that affect the efficiency of the photovoltaic system and reduce the power generation of the power station. Its direct effect is to weaken solar radiation and reduce the energy reaching the surface of the panel. The more and thicker the accumulated dust, the more severe the impact on the photovoltaic panel. The accumulation of dust is related to factors such as the amount of dust in the air, air flow and the rate of dust reduction, and the length of time. These depend on weather and environmental factors, and the environment and climate conditions in different regions of my country are quite different, and the dust fall in different seasons in the same region is also different, so the impact on photovoltaic panels is also different [6].
In areas near the sea or near salt lakes, salt spray corrosion has largely affected the power generation efficiency of photovoltaic systems, and photovoltaic panels have also been affected. Therefore, it is necessary to strengthen the salt spray test of photovoltaic panels to evaluate their long-term salt spray corrosion resistance [10]. In high-cold areas with low temperatures throughout the year, wind and wind direction change greatly, and photovoltaic panels need to undergo more stringent and more demanding performance tests to meet the climatic conditions of such special regions.
Due to the different geographical locations, the climate conditions vary greatly from place to place. For example, photovoltaic power plants in northwestern my country are greatly affected by dust shading. Dust can reduce the intensity of solar radiation and reduce the output power of the panel, resulting in a decrease in power generation. In addition, partial dust shading can also cause hot spot effects and generate electricity. Loss of quantity and potential safety hazards [8].
3 Influencing factors of dust accumulation
The deposition and adhesion of dust on photovoltaic panels is affected by many factors, such as weather factors, panel placement locations, angles, environmental factors, and dust properties. Weather factors have a great influence on the power generation of photovoltaic systems. The most common factors are temperature, humidity, cloudy weather, rain and snow, wind and other factors. Among them, temperature has the greatest impact on power generation. In addition, the impact of rain, cloudy, snow and other weather on the photovoltaic system can not be ignored, and it will also cause the weakening of the solar radiation intensity, thereby reducing the power generation of the photovoltaic system.
Seasonal factors affecting the output power of photovoltaic systems are also obvious, and the same is because of changes in radiation intensity that lead to changes in power generation. In addition, experiments have calculated the effect of dust on the surface of the photovoltaic panel on the speed of light irradiated on its surface. The comprehensive factors that affect the speed of light include: the amount of dust per unit area, the scale of dust, the angle and wavelength of sunlight, and it is found that for light propagation, the impact of the incident wavelength is negligible, because the particle size of dust is much larger than the wavelength of light [11].
3.1 Wind
In the current research on the influencing factors of dust accumulation in photovoltaic panels, the influence of wind on dust deposition is rarely involved. In fact, it has a great influence on the dust deposition structure [12]. Under a certain rainfall intensity, the angle between the rain line and the surface of the panel is different, and the degree of dust washing on the surface of the panel by rain is also different, and the wind determines the angle between the rain line and the photovoltaic panel.
Goossens et al. [13] found through long-term observations in the Negev Desert that the maximum dust deposition is usually formed at the highest wind speed. In addition, experiments in the wind tunnel show that the dust formed under the influence of high wind speed will have a higher transmittance; through the influence of different wind directions on the dust deposition process, it is found that the wind blowing to the surface of the panel will increase the dust accumulation effect, and The wind towards the back of the panel hardly affects dust deposition [14].
Neil et al. [15] found that higher wind speeds have a dust removal effect on the non-adhesive dust deposited on the surface of photovoltaic modules.
3.2 The intensity of precipitation and the amount of dustfall
Rainfall has a scouring effect on the dust on the surface of the photovoltaic panel, and different rainfall intensities have different amounts of dust scouring. Normally, the rainfall intensity is proportional to its scouring effect. Some experiments have observed that the current loss is 5% when the rainfall exceeds 5 mm.
The amount of dustfall refers to the mass of particulate matter that naturally settles on a unit area by gravity per unit time, and the amount of dust on the surface of the photovoltaic panel is mainly affected by it. When there is no rain washing, the longer the time, the more dust will be collected, and the dust on the surface of the panel will gradually increase, which will increase the shielding effect of the dust on the photovoltaic panel, which will weaken the light intensity reaching the panel, resulting in a reduction in power generation, which directly affects the photovoltaic system. Power generation efficiency [2].
The accumulation of dust on the surface of the photovoltaic panel is related to the previous accumulation time of dust. The results of the study show that the accumulation of dust occurs between two rainfalls, the fastest in the first few days, and then gradually decrease [16].
3.3 Air pollution
Nowadays, urban environmental pollution is getting more and more serious, especially the haze weather that has often appeared in recent years, which has a great impact on the power generation efficiency of photovoltaic systems [7]. Haze mainly affects the power generation of photovoltaic power plants in two ways: one is to weaken the solar radiation reaching the photovoltaic panel, because the suspended matter in the low altitude will absorb and reflect the sunlight, resulting in a significant reduction in the sunlight received on the surface of the panel. Second, if the haze weather continues for a long time, the particles on the surface of the panel will be blocked on the surface of the panel, which is difficult to clean, causing pollution of the panel surface and further reducing the power generation.
The Wind Energy and Solar Resource Center of China Meteorological Administration selected typical cases to analyze the impact of haze through observational experiments. The preliminary results show that haze has a significant impact on photovoltaic power generation by weakening solar radiation, and mild haze can cause 20% to 30% of daily power generation. In the case of heavy haze, the loss of daily power generation is close to 70%.
Hegazy [9] and Elminir et al. [17] studied the effect of dust accumulation on clean glass samples exposed to the environment. The results were obtained after 30 days. For panels with tilt angles of 0°, 3°, and 90°, Hegazy recorded losses of 27%, 17%, and 3%, respectively.
Elminir recorded losses of 27%, 18% and 6%. Both record points are located in Egypt, the first record point is in a heavily polluted agricultural area, and the second record point is in a heavily polluted industrial area.
3.4 Dust properties
The nature of dust is affected by the climatic and environmental conditions in different regions, resulting in dust in various regions having their own properties and characteristics. For example, the dust in the desert area is composed of sand, red soil and sand particles. The dust in the city mainly comes from automobile exhaust, industrial pollution and urban construction [1].
Affected by the dust of oily substances in the atmosphere, if the panel is not cleaned in time, waiting for the oily dust to accumulate for a period of time, it will bring great difficulties to the cleaning work [18]. In Golmud, Qinghai, the climate is arid, drier, and windy, so the dust is dry dust. The dust will quickly fall on the photovoltaic panel under the action of the wind, and it will adhere to the surface of the panel due to static electricity.
In addition, biomass such as bird droppings has an impact on the nature of dust. Biomass pollution is regarded as one of the sources of dust pollution, especially pollen pollution in spring. Hammond et al. studied the impact of dust accumulation on photovoltaic systems through the measurement of bird droppings on the current reduction, and the results showed that the current was reduced by 2% to 8%.
Haeberlin and Graf conducted a characteristic test of photovoltaic panels placed at a level of 30° in mixed light industrial areas, forest areas and farms. The results showed that due to the pollution of different biomass, the energy output of photovoltaic systems has been reduced to varying degrees [19].
3.5 Panel angle
The angle at which the photovoltaic panel is placed has a great impact on the power generation efficiency of the photovoltaic power station. On the one hand, the amount of solar radiation absorbed by the installation inclination is different, and on the other hand, it has an impact on the accumulation of dust. Goossens et al. [20] found that the most dust deposits on the horizontal panel surface in desert areas. Chen Wei et al. [21] selected photovoltaic panels with different orientations and inclination angles for testing in Guangzhou, and the results showed that dust, rain, etc. have a greater impact on panels placed horizontally and at small inclination angles, and more dust deposited on the panels.
Sayigh et al. [22] conducted a study on the dust accumulation effect of photovoltaic panels with inclination angles of 0°, 15°, 30°, 45°, and 60°. After one month, the solar radiation transmittance was reduced by 64%, 48%, and 38, respectively. %, 30% and 17%, the results show that the greater the inclination of the photovoltaic panel, the more difficult it is for dust to deposit and adhere to its surface.
Elminir et al. [17] conducted dust accumulation experiments on panels with an inclination angle of 0°-90° and concluded that the dust accumulation density gradually decreases with the increase of the panel angle, and the decline in solar radiation transmittance also gradually decreases.
Liu Limin et al. [23] found that in grid-connected photovoltaic power plant systems, photovoltaic panels installed at a 90° due south inclination are more than the cumulative annual power generation of 15°. One is because photovoltaic panels with different angles absorb different solar radiation, and the other is The dust accumulation is more serious than the former, so the choice of different inclination angle installation has a great influence on the power generation efficiency of the photovoltaic power station.
4 Dust accumulation mechanism and model
The theoretical study of dust accumulation is very difficult because it is closely related to the location, the tilt angle of the panel, and other variables. Ju Fali [2] explored the theory of photovoltaic dust accumulation. According to the relationship between dust accumulation and dust fall, rainfall, etc., he established a model of the amount of dust on the photovoltaic surface area, and considered the effects of dust and rain erosion on the surface of photovoltaic panels in three scenarios. The relations are as follows: there is no rainfall, and the amount of dust accumulation is not affected by rainfall erosion; there is rainfall, but the rainfall is not enough to wash away all the dust; the rainfall is enough to wash away the dust.
He also proposed a photovoltaic dust accumulation coefficient, which is used to express the influence factor of dust accumulation on the performance of photovoltaic power generation, that is, under the same light intensity, the ratio of the power generation efficiency of the dust accumulation photovoltaic panel to the power generation efficiency of the clean photovoltaic panel. There are many factors that affect dust deposition, and the establishment of the model is very complicated. The Jufali model only considers the relationship between dust accumulation, dust fall, and rainfall, and other factors have not been considered, so this model has certain limitations.
5 Dust removal method for photovoltaic modules
At present, some photovoltaic power generation systems still only rely on natural effects such as rainfall and wind to remove dust from photovoltaic panels. Some small photovoltaic power stations use manual cleaning methods, generally using mops, rubber scrapers or soft cloths for cleaning [24]. The disadvantage of this method is that the glass panel will inevitably be scratched during the cleaning process, and the glass surface will be scratched. Part of the work requires two people to work together, and the efficiency is relatively low; the advantage is that it does not waste water and electricity resources, and almost only has labor costs.
Manual cleaning of large and medium-sized photovoltaic power plants is difficult, and mechanical cleaning is generally adopted. High-pressure water gun cleaning, the water is pressurized to form a water-vapor mixture to wash away the dust on the surface of the photovoltaic panel. The cleaning effect is good, so it is widely used by many photovoltaic power stations; the disadvantage is that there is a large demand for water and electricity, and a lot of sewage will be formed during the cleaning process. Pollution of the environment; input costs are mainly machine and personnel wages. The cleaning vehicle requires a large amount of water, almost no damage to the panel, and high efficiency. However, it needs to be used in a wide and flat area, and the investment cost is high.
In addition, there is mechanical dust removal technology, which uses mechanized brushes combined with automatic dust removal devices for washing photovoltaic panels with water spray, etc., to sweep away dust by mechanical force [25]. The advantage is strong automation and saving manpower. However, some machines have poor cleaning effect, high cost, and also pollute the environment. Further development and research are needed.
6 Summary
This paper summarizes the impact of dust accumulation on the efficiency of photovoltaic power generation, proposes the impact of dust on the irradiance, heat dissipation and corrosion of photovoltaic panels, and the differences in the impact of dust accumulation in different regions, and explores its influencing factors: wind, precipitation intensity and dust reduction, The influence of air pollution, dust properties and panel inclination angle, and the dust accumulation mechanism and model are summarized, and the following conclusions are drawn:
1) Dust has a great influence on the power generation efficiency of the photovoltaic system, and the power loss caused is very serious, sometimes reaching about 30%.
2) Dust will reduce the intensity of solar radiation received by the panel, and reduce the amount of photovoltaic power generation, resulting in a drop in output power. The greater the deposition concentration and the lower the light transmittance, the lower the amount of radiation absorbed by the component.
3) Dust causes a change in the heat transfer form of the photovoltaic panel, which will increase the heat transfer resistance of the panel and affect its heat dissipation. Studies have shown that when the panel temperature rises by 1 ℃, the output power drops by about 0.5%.
4) Dust is sometimes acidic and sometimes alkaline in different environments. The corrosion of the photovoltaic panel reduces the solar radiation transmittance, the light intensity on the panel weakens, and the power generation decreases.
5) Dust accumulation is affected by wind speed and direction, precipitation intensity, air pollution, dust nature, panel placement location, panel inclination and other factors. Wind speed is proportional to dust deposition; the greater the precipitation intensity, the greater the erosion effect of dust deposition; Haze weather causes increased dust deposition; different types of dust will have a great impact on the difficulty of cleaning photovoltaic panels; the greater the inclination of photovoltaic panels, the more difficult it is for dust to stay and adhere to the surface.
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