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Soiling-induced transmittance losses in solar PV modules installed in Kathmandu Valley

Soiling-induced transmittance losses in solar PV modules installed in Kathmandu Valley Renewable energy sources are fast emerging as more reliable supplement of conventional energy sources. Among the various renewable sources, solar energy is most sought after in today’s world. Solar PV modules when installed in outdoor environments suffer from various factors which are generally unaccounted in laboratory testing. Energy generation from solar collectors is primarily dependent on the amount of incident radiation on their surfaces. Soiling on modules is known to reduce the transmittance of incident rays to solar cell and cause significant output power degradation. Soiling is closely associated with the various factors such as module tilt angle, site-specific climate, outdoor exposure period, humidity, wind speed, dust characteristics and material properties. This experimental work is aimed to study the transmittance losses encountered by solar PV modules and the corresponding power degra- dation. The experimental results show an alarming reduction in transmittance as high as 69.06% over the dry study period experiencing no rain. The power of dusty solar module decreases by 29.76% compared to the module cleaned on daily basis. Dust deposition density on the PV module accounted to 9.6711 g/m over the study period. Keywords: Solar energy, Soiling, Air pollution, Transmittance loss On the global scenario, due to easy and accessible amount Introduction of resource, solar energy has the significant market over Scope of clean and renewable source of energy in devel- other distributed renewable energy techniques, as denoted oping countries is high. From economic to environmen- by the sharply reducing cost of PV systems all over. tal benefits, renewable sources have a considerable role to Dust is simply defined as a particulate matter less play for the overall development. From mere alternatives than 500  µm in diameter which can comprise various in the race to provide human civilization with required suspended matters in the atmosphere from organic to energy, renewables have now stolen the march and are inorganic particulates (Sarver et al. 2013). Dust is gener- set to become the frontrunners in the coming decades. ated from various sources such as soil elements lifted by Renewable energy sources with the growing share in wind, volcanic eruptions, vehicular movement and pol- the energy mix globally are more than capable of meet- lution (Siddiqui and Bajpai 2012). Deposited particles on ing future energy requirements. Continuous research PV modules interfere with illumination quality by both and development in the various dimensions of renewable attenuating and scattering incident light (Qasem et  al. energy sources are ongoing, and they are touted as the 2011). There is a strong variation in particle shape, size major shareholders for electricity generation in the coming and constituents of dust according to regions throughout future. Commonly known technologies include biomass, the world. Similarly, the deposition patterns, rates and geothermal, solar, tidal, wave and wind energy systems. characteristics are found to vary dramatically in different localities. Ambient conditions such as humidity/moisture gradients, variation in wind velocity direction and mag- *Correspondence: basant.paudyal@gmail.com Department of Mechanical Engineering, Pulchowk Campus, Tribhuvan nitude and seasonal variations affect the properties of University, Kathmandu, Nepal dust as well as deposition rates (Sarver et al. 2013). Dust Full list of author information is available at the end of the article © The Author(s) 2017. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Paudyal et al. Renewables (2017) 4:5 Page 2 of 8 particles attach onto a surface due to gravity, electrostatic platform for power rating comparison. An experiment charge or mechanical effects (wind or water droplets). found the performance ratio decreasing with the dust After deposition, they are held by the variation of elec- accumulation, and the ratio is expected to substantially trical potential near the surface (charge double layer), improve once the modules were cleaned (Adinoyi and surface energy effects and capillary effects, in addition to Said 2013). From various studies, the dust accumulated gravity and electrostatic forces (Qasem et al. 2011). on the PV module surface is found to decrease the trans- One of the major impacts of dust deposition is mittance of incident light and ultimately decrease the observed on the transmittance of solar modules. Trans- solar energy received by the solar cells in PV modules. In mittance is generally known by the degree of solar radia- a study conducted in Baghdad Saidan et  al. (2015), the tion passing through a module encapsulation (generally experimental results show that dust considerably reduces made of plastic or glass). The transmittance reduction the maximum current from 6.9 to 16.4% depending on due to dust deposition eventually leads to reduction on the time period of PV panels’ exposure in dust-affected power generation from modules. Different studies have environment, i.e. from one day to one month. Elminir shown large performance variations from location to et al. (2006) in Egypt investigated the effect of dust on the location as a function of exposure time (Siddiqui and Baj- transparent cover of solar sensor using several sensors pai 2012; Aassem et al. 2012). El-Shobokshy and Hussein and concluded that soiling on glass inclined of an angle of (1993), covered PV module surfaces with different dust 0° and 90° from horizontal causes a reduction in the cor- types (i.e. limestone, cement, carbon) and found the responding transmittance by approximately 52.54 and short-circuit current was reduced to 20% of its initial 12.38%, respectively. This shows that the tilt angle plays value for the carbon accumulation with only 28  g/m , one of the major roles in determining the performance of whereas same reduction was accounted with 73  g/m PV modules. Hegazy (2001) studied dust deposition on 2 2 deposition for cement, 125 g/m for 50 µm, 168 g/m for glass plate surfaces with various tilt angles and also meas- 60  µm and 250  g/m for 80  µm limestone dust. It was ured the transmittance of plate under different weather specifically noted that the material composition of dust conditions and concluded that the degradation in solar also affects PV performance. From the results, carbon transmittance primarily depends upon the tilt angle. Dust particles absorb solar radiation more readily than the accumulation on a tilted glass plate located in Kuwait other dust types. Mailuha et al. (1994) focused the study City was found to reduce the transmittance of the plate on the effects of dust-deposited layer density and from 64 to 17% for the tilt angles ranging from 0° to 60°, included tilt angle and solar intensity, and found that respectively, after 38 days of outdoor environment expo- with the increment of solar intensity, the PV performance sure (Sayigh et al. 1985). Soiling on a glass plate tilted at degraded due to decrement in dust accumulation. At 45° angle decreased transmittance by an average of 8% 700  W/m , the reduction in power output was almost after an exposure period of 10  days in a research per- negligible; however, when the intensity dropped to formed in India (Garg 1973). A study by Cano (2011) on 400  W/m , the reduction was nearly 25% of the initial the effect of tilt angle of PV modules on dust deposition power output. Continuous humid environment causes in Arizona found that during the period of January degradation in solar cell efficiency and causes the trans - through March 2011 there was an average loss due to mittance to decrease (Mekhilef et al. 2012). The results of soiling of approximately 2.02% for 0° tilt angle. Modules study by Jiang et al. (2011), to investigate the output deg- at tilt angles 23° and 33° also have some irradiance losses radation of different types of PV modules with different but do not come close to the module at 0° tilt angle. Tilt surface materials caused by airborne dust pollution angle 23° has approximately 1.05% monthly irradiance experimentally, indicated that dust pollution has a signifi - loss, and 33° tilt angle has an irradiance loss of approxi- cant impact on PV module output. With dust deposition mately 0.96%. The effect of dust deposition is evident at density increasing from 0 to 22  g/m , the corresponding any tilt angle, but the magnitude is different with the reduction in PV output efficiency grew from 0 to 26%. solar module with low tilt angle being bound to more The reduction in efficiency was found to have a linear energy losses. Al-Hasan (1998) investigated the effect of relationship with the dust deposition density, and the dif- the amount of accumulated dust on the efficiency of a PV ference caused by cell types was not obvious. Also the module in the Kuwait climate on almost similar latitude reduction in output power at relatively higher solar den- to Kathmandu (latitude 30°). A linear relation has been sities is much more severe. This phenomenon is probably proposed to correlate the degradation in efficiency with attributed to relatively higher reflection effect of the the amount of sand dust accumulated on the module sur- deposited dust to light. Sometimes PV modules of same face. Paudyal and Shakya (2016) on the similar research and different technologies are known to have a different have derived another regression equation relating the power rating, so performance ratio could be the best impact of various meteorological parameters as well as Paudyal et al. Renewables (2017) 4:5 Page 3 of 8 dust deposition density for Kathmandu. This relation fixed solar panel when compared them with the simula - could help PV system designers to reliably predict the tion results. effect of dust accumulation on PV module efficiency under real environmental conditions. Ndiaye et al. (2013) Experimental set‑up on their investigation on the effect of soiling in the per - Kathmandu Valley lies 1325  m above sea level, and due formance of PV modules have highlighted the impact of to high occurrence of calm and low wind speeds, the dust on the current–voltage and power–voltage charac- dispersion conditions in Kathmandu are poor (Shrestha teristics of PV modules with the advent of the mismatch 2001). The annual average daily global solar radia - effect. The maximum power (P ), the maximum cur- tion for Kathmandu is 3.83  kW/m /day (Poudyal et  al. max rent (I ), the short-circuit current (I ) and the fill factor 2012). The unique topographic features coupled with max sc are the most affected performance characteristics by the high emissions of pollutants make the valley particularly dust deposition on the PV modules surface. P output vulnerable to air pollution. The valley is surrounded by max losses are observed to be from 18 to 78%, respectively, for hills, forming bowl-shaped topography restricting wind the polycrystalline module (pc-Si) and mono-crystalline movement and retaining the pollutants in the atmos- module (mc-Si). I loss can vary from 23 to 80% for, phere. This is especially bad during the winter season max respectively, pc-Si and mc-Si modules. However, the (November–February) when thermal inversion occurs maximum voltage output (V ) and the open-circuit in the valley late night and early morning. Cold air flow - max voltage (V ) are not affected by dust accumulation for ing down from the mountains is trapped under a layer of oc both technologies studied. This shows that mono-crystal - warmer air and acts as a lid. As a result, the pollutants line modules are more prone to efficiency losses due to are trapped close to the ground for extended periods soiling effect. The variation of energy losses during the of time (CANN 2014). The polluting agents generated day depends on the optical transmittance due to the inci- inside Kathmandu cannot be transported during the dence angle of irradiance on tilted plane and refractive winter time and hence settled on the surface of solar index of dust material (Semaouia et al. 2015). Experimen- modules installed in Kathmandu. tal investigations conducted in Indonesia demonstrated a Experimental set-up was installed on the Institute of significant decrease in PV output power in relation to Engineering, Tribhuvan University located in Kathmandu dust accumulation during a long period of dry condi- Valley, Nepal, from the 13 August 2015 to 10 January 2016. tions. Results of experiments show that dust accumula- Two 40 W each polycrystalline solar modules manufactured tion after two-week exposure in the dry season caused a by Rahimafrooz Solar were installed on the Central Cam- PV output power reduction of 10.8%. Two different pus, Pulchowk, with tilt angle of 27° as shown in Fig.  1a. weather conditions were considered to analyse the effect Power generation from those modules was constantly of local weather conditions on PV output power, rainy measured and stored in data loggers. Similarly, 150 micro- and cloudy conditions. Results from the experiment scopic slides of dimension 25.1 mm × 75.2 mm × 1.2 mm under a rainy condition showed that PV output power were placed with an array formation in similar tilt angle as decreased by more than 40% when there was an average shown in Fig.  1b, to the modules to measure dust deposi- relative humidity of 76.32%, whereas during cloudy con- tion density as well as transmittance (Gandhi et  al. 2014). ditions the decrease in output power was more than 45% Dust containing slides were measured for transmittance when there was an average relative humidity of 60.45% before cleaning using Cary 60 UV–Vis Spectrophotometer (Ramli et al. 2016). Analysis from Chin et al. 2011 shows as shown in Fig.  2a, manufactured by Agilent technolo- that the efficiency of solar power system after incorporat - gies. The process was repeated after thorough cleaning of ing the single axis tracker is higher than that of the fixed the slides to gather transmittance value of clean slide. Slides array system and the cost of electricity from a PV system with dust accumulation were measured in electrical bal- is approximately equal to that of a diesel generator and ance shown in Fig.  2b with 0.0001  gm sensitivity, at the cheaper than a grid extension when a single tracking sys- rate of one slide per day. Slides were measured with dust tem is introduced. The completed MATLAB model (Chin first and again measured after the dust was thoroughly 2012) of the solar tracker with external disturbances was cleaned. The difference between two values gave the weight designed to provide a computer-aided design tool to of dust. The quantity, obtained after subtraction of weight determine the efficiency over the fixed solar panel, net of clean slides from weight of dusty slides and divided by current output, power generated and the types of PV sys- area of microscopic slide, gives the dust deposition density. tems that can be combined to give a required level of effi - During the study period, one module was cleaned daily, ciency before actual implementation, where the whereas the other was left to the natural soiling phenom- experimental results show a similar behaviour in the ena throughout. The P of both the solar modules were max power, the efficiency and the current output over the measured simultaneously. The variation in power output Paudyal et al. Renewables (2017) 4:5 Page 4 of 8 Fig. 1 a, b Experimental set-up for data collection Fig. 2 a, b Transmittance and dust deposition density data collection techniques of modules was signalled by the varying values of Imp and towards dust deposition density. The power output is V of both solar modules and recorded into the individual dependent on other factors as well, apart from the irradi- mp data loggers attached on each module. ance, i.e. cell degradation losses. But since the modules Data for temperature, rainfall and humidity were col- are brand new, their performance reduction for a five- lected from Department of Hydrology and Meteorol- month period is not considered. ogy, Kathmandu, from the nearest meteorological site at Kathmandu airport. The values of P were calculated Results and discussion max for each day from 9 am to 3 pm, from which average value Transmittance is generally defined as the ratio of incident per day was calculated. The daily average values of mete - light falling on a body to the light passing through it. As orological variables were taken from the nearest mete- per the observation, the transmittance of the dusty slides orological station. Statistical software NCSS11 was used decreases over time as dust deposition density increases. for multiple regression analysis which was performed to The increment of dust deposition density meant the dust calculate the combined effect of meteorological variables layer on the glass surfaces got thickened with time which Paudyal et al. Renewables (2017) 4:5 Page 5 of 8 in turn blocked the solar energy transmitting through it. Cumulative effect of meteorological parameters on dust So dust deposition density and transmittance are inversely deposition density related to each other. Transmittance was measured for Research by Paudyal and Shakya (2016) shows a nega- the wavelength between 200 and 800  nm. For simplifica - tive correlation between humidity and dust accumula- tion, transmittance value was taken on 3 different values: tion. Humidity and dust deposition density are known 750, 600 and 450 nm. Figure 3 denotes the transmittance to involve more than one variable (Qasem et  al. 2011). reduction graph of dusty samples in 750 nm wavelength. Wind speed can promote raised and suspended dust Another factor to consider is the solar density. It is movement, thus promoting higher rate of dust accumu- found that the reduction in output power at relatively lation due to surface collusion (Qasem et  al. 2011). And lower or higher solar densities is much more severe. other remaining variables such as temperature and rain- This phenomenon is probably attributed to relatively fall are also known to play significant role in dust deposi - higher reflection effect of the deposited dust to light as tion. Here, the relation of all the meteorological variables the certain portion of already low value of irradiance gets under consideration with respect to dust deposition den- reflected by the accumulated dust layer (Jiang et al. 2011). sity is studied using multiple regression analysis. Cell The rainfall, humidity and temperature data which were temperature, humidity, rainfall and ambient temperature available from Department of Hydrology and Meteorol- ogy are shown in Figs.  4,  5  and  6, respectively.  Whereas the Fig. 7 depicts the value of power collected by datalog- gers in a clean and dusty module setup. Dust deposition density increases as the time period of exposition increases. Natural cleaning actions of rain- fall, wind speed and dew are evident in the graph as these agents lowered the dust deposition density as shown in Fig.  8. Some high values of dust deposition density are due to the bird dropping in the dust slide. Number of Days Fig. 5 Temperature variation during study period Transmittance at different Wavelengths 90.00 80.00 70.00 60.00 50.00 40.00 30.00 80 20.00 10.00 0.00 Number of Days Number of Days 750nm 600nm 450nm Fig. 6 Average humidity for the study period Fig. 3 Measured transmittance at different wavelengths P_Clean (W) P_Dusty(W) Numer of Days Number of Days Fig. 4 Rainfall during the study period Fig. 7 Power output from clean and dusty solar modules Percentage Transmittance (%) Rainfall (mm) 8 7 50 49 92 91 Power (W) Relative Humidity (%) Temperature ( C) 37 36 43 43 67 64 97 99 151 148 Paudyal et al. Renewables (2017) 4:5 Page 6 of 8 Number of Days Fig. 8 Measured dust deposition density are the independent variables for regression analysis. The operation resulted in the statistical results as given in Table 1. The regressed model takes the form of the following equation Y = A + β ∗ X + β ∗ X + β ∗ X + β ∗ X , 2 1 1 2 2 3 3 4 4 Fig. 9 Eec ff t of dust deposition density (gm/m ) in Transmittance (%) (1) where Y  =  dust deposition density on modules, A = regression coefficient, X  = rainfall (mm), X  = ambi- 1 2 factor as long expositions to humid environment cause the ent temperature (°C), X   =  humidity (%), X   =  module 3 4 encapsulate delamination and is known to affect the trans - temperature (°C). mittance property of laminating surface. Equation (1) reduces to the form Y = 13.3843 − 0.0064 ∗ X1 − 0.3656 ∗ X2 Percentage transmittance loss (2) Percentage transmittance loss is given by Eq. (3), where τ − 0.0074 ∗ X3 − 0.0642 ∗ X4. c is the transmittance observed for the clean glass samples Eec ff t of dust deposition density on transmittance and τ values depreciate due to dust deposition, that is, by Dust deposition density has adverse effect on the transmit - prolonged exposure tance as shown in Fig. 9. As shown in the numerous exper- iments performed, most notably on the research works of ∗ 100. %τ 1 − reduction= (3) Elminir et  al. (2006) and Gandhi et  al. (2014), the strong dependence of dust deposition on the transmittance and Basic result evident is that transmittance reduction on energy yield is verified. The reduction in transmittance percentage also increases with increasing days of expo- in the glass samples increased before saturating as shown sure. Figure  10 depicts the percentage of transmittance in Fig.  3 (Elminir et  al. 2006). The correlation coefficient loss which was mere 2.52% at the first day of experiment (r) and coefficient of determination (r ) of −0.8690 and which rose up to 69.06% at the final day of experiment at 0.7552 indicate a strong linear relationship between these 750  nm wavelength. The graph shows high variations in parameters. Humidity can be considered another major transmittance loss at the middle, which can be attributed to the formation of dew in prevalent climatic conditions. Dew promotes dust accumulation on flat surfaces and subsequent evaporation reinforces dust adhesion; in fact, Table 1 Statistical parameters of  multiple regression for model it sometimes forms a solid, packed cement-like compos- ite (Elminir et al. 2006). Parameters Value R 0.8632 Eec ff t of transmittance on power output reduction Adjusted R 0.8595 Over the period of study, the power output of modules Coefficient of variation 0.2770 decreased continuously due to dust accumulation, which Mean square error 1.014345 resulted in reduction in transmittance. As shown in Square root of MSE 1.007147 Fig. 11, the correlation coefficient (r) of −0.8904 indicates Dust deposition density (g/m2) 151 Paudyal et al. Renewables (2017) 4:5 Page 7 of 8 to thus reduction in power generation. Voltage gener- Transmittance Loss (%) ated from the modules was not altered to significant 80.00 70.00 levels. The dust deposition density ranging from 0.1047 60.00 to 9.6711  g/m and the power reduction of 29.76% with 50.00 respect to 69.06% transmittance loss can be considered 40.00 high in the span of 5  months. The study enlightens the 30.00 dire need of incorporating a proper cleaning device or 20.00 mechanism in existing solar PV systems installed in the 10.00 areas exposed to relatively high dust deposition condi- 0.00 tions during dry season of Kathmandu to reduce power loss due to soiling. As the laminate encapsulation of the No. of days solar modules contains pores to reduce the reflectance Fig. 10 Percentage loss in transmittance of the solar radiation, some dust may be present in such pores which require careful cleaning. Thus, the proper cleaning mechanism for such cases should be selected and applied. Since the transmittance losses due to soiling are highly pronounced, the research shows the system design of the solar PV systems must incorporate the pos- sible power loss from the transmittance reduction due to soiling. Authors’ contributions BRP is a lead author and corresponding author of the research article. He contributed to performing research activities, data collection, data analysis and writing of the research article. SRS played an integral part of this research process. He contributed to the design of this research and supervision of all the related works. DPP played another integral part of this research. He contributed to the formulation of transmittance measurement, compiling and verification of the manuscript and data analysis. DDM was another prominent contributor of this research as he contributed to the formulation of transmit- tance measurement, analysis of the results and managing the data collection facility. All authors read and approved the final manuscript. Author details Department of Mechanical Engineering, Pulchowk Campus, Tribhuvan Uni- versity, Kathmandu, Nepal. Department of Engineering Science and Humani- ties, Thapathali Campus, Tribhuvan University, Kathmandu, Nepal. Nepal Academy of Science and Technology, Kathmandu, Nepal. Fig. 11 Eec ff t of transmittance on power output reduction Acknowledgements The experimental work of this research was done in Nepal Academy for Sci- ence and Technology (NAST ). The authors are grateful to NAST for providing the research grant for this research. Meteorological data were provided by Department of Hydrology and Meteorology, Kathmandu. strong negative linear relation among these variables. Reduction in transmittance of module surface increases Competing interests the output power reduction percentage of the solar mod- The authors would like to confirm that there are no any competing interests associated with this publication and the research has been carried out for a ules as they are inversely related to each other. As the purely academic purpose. dust layer on the surface of PV module became thicker, the transmittance loss became alarmingly higher which Availability of data and materials The data and materials are available anytime from the author, as per request. triggers higher loss of module output power (Weber et al. 2013). Consent for publication The research contains the data exclusively measured for the academic research. All the contributing authors are the participants of the research and Conclusion hence the article is formulated in the consent of all authors. The correlation of dust deposition density with trans - mittance and eventually correlation of transmittance Ethics approval and consent to participate Not applicable. with power reduction were investigated in this paper. Dust deposition plays a crucial role in obstructing the Funding solar irradiance reaching solar cells and reduces the The research has been funded by Nepal Academy of Science and Technology (NAST ), and the authors would like to thank NAST for the financial support. transmittance of the encapsulate lamination leading Reduction Percentage (%) 151 Paudyal et al. Renewables (2017) 4:5 Page 8 of 8 Mailuha, J., Murase, H., & Inoti, I. (1994). Knowledge engineering based studies Publisher’s Note on solar energy utilization in Kenya. Agriculture Mechanization in Asia, Springer Nature remains neutral with regard to jurisdictional claims in pub- Africa and Latin America, 25, 13–16. lished maps and institutional affiliations. Mekhilef, S., Rahman, S., & Kamalisarvestani, M. (2012). Eec ff t of dust, humidity and air velocity on efficiency of photovoltaic cells. Renewable and Sustain- Received: 6 February 2017 Accepted: 4 August 2017 able Energy Reviews, 16, 2920–2925. Ndiaye, A., Kébé, C. M. 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Soiling-induced transmittance losses in solar PV modules installed in Kathmandu Valley

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Energy; Renewable and Green Energy; Energy Technology; Energy Policy, Economics and Management; Water Industry/Water Technologies
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Abstract

Renewable energy sources are fast emerging as more reliable supplement of conventional energy sources. Among the various renewable sources, solar energy is most sought after in today’s world. Solar PV modules when installed in outdoor environments suffer from various factors which are generally unaccounted in laboratory testing. Energy generation from solar collectors is primarily dependent on the amount of incident radiation on their surfaces. Soiling on modules is known to reduce the transmittance of incident rays to solar cell and cause significant output power degradation. Soiling is closely associated with the various factors such as module tilt angle, site-specific climate, outdoor exposure period, humidity, wind speed, dust characteristics and material properties. This experimental work is aimed to study the transmittance losses encountered by solar PV modules and the corresponding power degra- dation. The experimental results show an alarming reduction in transmittance as high as 69.06% over the dry study period experiencing no rain. The power of dusty solar module decreases by 29.76% compared to the module cleaned on daily basis. Dust deposition density on the PV module accounted to 9.6711 g/m over the study period. Keywords: Solar energy, Soiling, Air pollution, Transmittance loss On the global scenario, due to easy and accessible amount Introduction of resource, solar energy has the significant market over Scope of clean and renewable source of energy in devel- other distributed renewable energy techniques, as denoted oping countries is high. From economic to environmen- by the sharply reducing cost of PV systems all over. tal benefits, renewable sources have a considerable role to Dust is simply defined as a particulate matter less play for the overall development. From mere alternatives than 500  µm in diameter which can comprise various in the race to provide human civilization with required suspended matters in the atmosphere from organic to energy, renewables have now stolen the march and are inorganic particulates (Sarver et al. 2013). Dust is gener- set to become the frontrunners in the coming decades. ated from various sources such as soil elements lifted by Renewable energy sources with the growing share in wind, volcanic eruptions, vehicular movement and pol- the energy mix globally are more than capable of meet- lution (Siddiqui and Bajpai 2012). Deposited particles on ing future energy requirements. Continuous research PV modules interfere with illumination quality by both and development in the various dimensions of renewable attenuating and scattering incident light (Qasem et  al. energy sources are ongoing, and they are touted as the 2011). There is a strong variation in particle shape, size major shareholders for electricity generation in the coming and constituents of dust according to regions throughout future. Commonly known technologies include biomass, the world. Similarly, the deposition patterns, rates and geothermal, solar, tidal, wave and wind energy systems. characteristics are found to vary dramatically in different localities. Ambient conditions such as humidity/moisture gradients, variation in wind velocity direction and mag- *Correspondence: basant.paudyal@gmail.com Department of Mechanical Engineering, Pulchowk Campus, Tribhuvan nitude and seasonal variations affect the properties of University, Kathmandu, Nepal dust as well as deposition rates (Sarver et al. 2013). Dust Full list of author information is available at the end of the article © The Author(s) 2017. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Paudyal et al. Renewables (2017) 4:5 Page 2 of 8 particles attach onto a surface due to gravity, electrostatic platform for power rating comparison. An experiment charge or mechanical effects (wind or water droplets). found the performance ratio decreasing with the dust After deposition, they are held by the variation of elec- accumulation, and the ratio is expected to substantially trical potential near the surface (charge double layer), improve once the modules were cleaned (Adinoyi and surface energy effects and capillary effects, in addition to Said 2013). From various studies, the dust accumulated gravity and electrostatic forces (Qasem et al. 2011). on the PV module surface is found to decrease the trans- One of the major impacts of dust deposition is mittance of incident light and ultimately decrease the observed on the transmittance of solar modules. Trans- solar energy received by the solar cells in PV modules. In mittance is generally known by the degree of solar radia- a study conducted in Baghdad Saidan et  al. (2015), the tion passing through a module encapsulation (generally experimental results show that dust considerably reduces made of plastic or glass). The transmittance reduction the maximum current from 6.9 to 16.4% depending on due to dust deposition eventually leads to reduction on the time period of PV panels’ exposure in dust-affected power generation from modules. Different studies have environment, i.e. from one day to one month. Elminir shown large performance variations from location to et al. (2006) in Egypt investigated the effect of dust on the location as a function of exposure time (Siddiqui and Baj- transparent cover of solar sensor using several sensors pai 2012; Aassem et al. 2012). El-Shobokshy and Hussein and concluded that soiling on glass inclined of an angle of (1993), covered PV module surfaces with different dust 0° and 90° from horizontal causes a reduction in the cor- types (i.e. limestone, cement, carbon) and found the responding transmittance by approximately 52.54 and short-circuit current was reduced to 20% of its initial 12.38%, respectively. This shows that the tilt angle plays value for the carbon accumulation with only 28  g/m , one of the major roles in determining the performance of whereas same reduction was accounted with 73  g/m PV modules. Hegazy (2001) studied dust deposition on 2 2 deposition for cement, 125 g/m for 50 µm, 168 g/m for glass plate surfaces with various tilt angles and also meas- 60  µm and 250  g/m for 80  µm limestone dust. It was ured the transmittance of plate under different weather specifically noted that the material composition of dust conditions and concluded that the degradation in solar also affects PV performance. From the results, carbon transmittance primarily depends upon the tilt angle. Dust particles absorb solar radiation more readily than the accumulation on a tilted glass plate located in Kuwait other dust types. Mailuha et al. (1994) focused the study City was found to reduce the transmittance of the plate on the effects of dust-deposited layer density and from 64 to 17% for the tilt angles ranging from 0° to 60°, included tilt angle and solar intensity, and found that respectively, after 38 days of outdoor environment expo- with the increment of solar intensity, the PV performance sure (Sayigh et al. 1985). Soiling on a glass plate tilted at degraded due to decrement in dust accumulation. At 45° angle decreased transmittance by an average of 8% 700  W/m , the reduction in power output was almost after an exposure period of 10  days in a research per- negligible; however, when the intensity dropped to formed in India (Garg 1973). A study by Cano (2011) on 400  W/m , the reduction was nearly 25% of the initial the effect of tilt angle of PV modules on dust deposition power output. Continuous humid environment causes in Arizona found that during the period of January degradation in solar cell efficiency and causes the trans - through March 2011 there was an average loss due to mittance to decrease (Mekhilef et al. 2012). The results of soiling of approximately 2.02% for 0° tilt angle. Modules study by Jiang et al. (2011), to investigate the output deg- at tilt angles 23° and 33° also have some irradiance losses radation of different types of PV modules with different but do not come close to the module at 0° tilt angle. Tilt surface materials caused by airborne dust pollution angle 23° has approximately 1.05% monthly irradiance experimentally, indicated that dust pollution has a signifi - loss, and 33° tilt angle has an irradiance loss of approxi- cant impact on PV module output. With dust deposition mately 0.96%. The effect of dust deposition is evident at density increasing from 0 to 22  g/m , the corresponding any tilt angle, but the magnitude is different with the reduction in PV output efficiency grew from 0 to 26%. solar module with low tilt angle being bound to more The reduction in efficiency was found to have a linear energy losses. Al-Hasan (1998) investigated the effect of relationship with the dust deposition density, and the dif- the amount of accumulated dust on the efficiency of a PV ference caused by cell types was not obvious. Also the module in the Kuwait climate on almost similar latitude reduction in output power at relatively higher solar den- to Kathmandu (latitude 30°). A linear relation has been sities is much more severe. This phenomenon is probably proposed to correlate the degradation in efficiency with attributed to relatively higher reflection effect of the the amount of sand dust accumulated on the module sur- deposited dust to light. Sometimes PV modules of same face. Paudyal and Shakya (2016) on the similar research and different technologies are known to have a different have derived another regression equation relating the power rating, so performance ratio could be the best impact of various meteorological parameters as well as Paudyal et al. Renewables (2017) 4:5 Page 3 of 8 dust deposition density for Kathmandu. This relation fixed solar panel when compared them with the simula - could help PV system designers to reliably predict the tion results. effect of dust accumulation on PV module efficiency under real environmental conditions. Ndiaye et al. (2013) Experimental set‑up on their investigation on the effect of soiling in the per - Kathmandu Valley lies 1325  m above sea level, and due formance of PV modules have highlighted the impact of to high occurrence of calm and low wind speeds, the dust on the current–voltage and power–voltage charac- dispersion conditions in Kathmandu are poor (Shrestha teristics of PV modules with the advent of the mismatch 2001). The annual average daily global solar radia - effect. The maximum power (P ), the maximum cur- tion for Kathmandu is 3.83  kW/m /day (Poudyal et  al. max rent (I ), the short-circuit current (I ) and the fill factor 2012). The unique topographic features coupled with max sc are the most affected performance characteristics by the high emissions of pollutants make the valley particularly dust deposition on the PV modules surface. P output vulnerable to air pollution. The valley is surrounded by max losses are observed to be from 18 to 78%, respectively, for hills, forming bowl-shaped topography restricting wind the polycrystalline module (pc-Si) and mono-crystalline movement and retaining the pollutants in the atmos- module (mc-Si). I loss can vary from 23 to 80% for, phere. This is especially bad during the winter season max respectively, pc-Si and mc-Si modules. However, the (November–February) when thermal inversion occurs maximum voltage output (V ) and the open-circuit in the valley late night and early morning. Cold air flow - max voltage (V ) are not affected by dust accumulation for ing down from the mountains is trapped under a layer of oc both technologies studied. This shows that mono-crystal - warmer air and acts as a lid. As a result, the pollutants line modules are more prone to efficiency losses due to are trapped close to the ground for extended periods soiling effect. The variation of energy losses during the of time (CANN 2014). The polluting agents generated day depends on the optical transmittance due to the inci- inside Kathmandu cannot be transported during the dence angle of irradiance on tilted plane and refractive winter time and hence settled on the surface of solar index of dust material (Semaouia et al. 2015). Experimen- modules installed in Kathmandu. tal investigations conducted in Indonesia demonstrated a Experimental set-up was installed on the Institute of significant decrease in PV output power in relation to Engineering, Tribhuvan University located in Kathmandu dust accumulation during a long period of dry condi- Valley, Nepal, from the 13 August 2015 to 10 January 2016. tions. Results of experiments show that dust accumula- Two 40 W each polycrystalline solar modules manufactured tion after two-week exposure in the dry season caused a by Rahimafrooz Solar were installed on the Central Cam- PV output power reduction of 10.8%. Two different pus, Pulchowk, with tilt angle of 27° as shown in Fig.  1a. weather conditions were considered to analyse the effect Power generation from those modules was constantly of local weather conditions on PV output power, rainy measured and stored in data loggers. Similarly, 150 micro- and cloudy conditions. Results from the experiment scopic slides of dimension 25.1 mm × 75.2 mm × 1.2 mm under a rainy condition showed that PV output power were placed with an array formation in similar tilt angle as decreased by more than 40% when there was an average shown in Fig.  1b, to the modules to measure dust deposi- relative humidity of 76.32%, whereas during cloudy con- tion density as well as transmittance (Gandhi et  al. 2014). ditions the decrease in output power was more than 45% Dust containing slides were measured for transmittance when there was an average relative humidity of 60.45% before cleaning using Cary 60 UV–Vis Spectrophotometer (Ramli et al. 2016). Analysis from Chin et al. 2011 shows as shown in Fig.  2a, manufactured by Agilent technolo- that the efficiency of solar power system after incorporat - gies. The process was repeated after thorough cleaning of ing the single axis tracker is higher than that of the fixed the slides to gather transmittance value of clean slide. Slides array system and the cost of electricity from a PV system with dust accumulation were measured in electrical bal- is approximately equal to that of a diesel generator and ance shown in Fig.  2b with 0.0001  gm sensitivity, at the cheaper than a grid extension when a single tracking sys- rate of one slide per day. Slides were measured with dust tem is introduced. The completed MATLAB model (Chin first and again measured after the dust was thoroughly 2012) of the solar tracker with external disturbances was cleaned. The difference between two values gave the weight designed to provide a computer-aided design tool to of dust. The quantity, obtained after subtraction of weight determine the efficiency over the fixed solar panel, net of clean slides from weight of dusty slides and divided by current output, power generated and the types of PV sys- area of microscopic slide, gives the dust deposition density. tems that can be combined to give a required level of effi - During the study period, one module was cleaned daily, ciency before actual implementation, where the whereas the other was left to the natural soiling phenom- experimental results show a similar behaviour in the ena throughout. The P of both the solar modules were max power, the efficiency and the current output over the measured simultaneously. The variation in power output Paudyal et al. Renewables (2017) 4:5 Page 4 of 8 Fig. 1 a, b Experimental set-up for data collection Fig. 2 a, b Transmittance and dust deposition density data collection techniques of modules was signalled by the varying values of Imp and towards dust deposition density. The power output is V of both solar modules and recorded into the individual dependent on other factors as well, apart from the irradi- mp data loggers attached on each module. ance, i.e. cell degradation losses. But since the modules Data for temperature, rainfall and humidity were col- are brand new, their performance reduction for a five- lected from Department of Hydrology and Meteorol- month period is not considered. ogy, Kathmandu, from the nearest meteorological site at Kathmandu airport. The values of P were calculated Results and discussion max for each day from 9 am to 3 pm, from which average value Transmittance is generally defined as the ratio of incident per day was calculated. The daily average values of mete - light falling on a body to the light passing through it. As orological variables were taken from the nearest mete- per the observation, the transmittance of the dusty slides orological station. Statistical software NCSS11 was used decreases over time as dust deposition density increases. for multiple regression analysis which was performed to The increment of dust deposition density meant the dust calculate the combined effect of meteorological variables layer on the glass surfaces got thickened with time which Paudyal et al. Renewables (2017) 4:5 Page 5 of 8 in turn blocked the solar energy transmitting through it. Cumulative effect of meteorological parameters on dust So dust deposition density and transmittance are inversely deposition density related to each other. Transmittance was measured for Research by Paudyal and Shakya (2016) shows a nega- the wavelength between 200 and 800  nm. For simplifica - tive correlation between humidity and dust accumula- tion, transmittance value was taken on 3 different values: tion. Humidity and dust deposition density are known 750, 600 and 450 nm. Figure 3 denotes the transmittance to involve more than one variable (Qasem et  al. 2011). reduction graph of dusty samples in 750 nm wavelength. Wind speed can promote raised and suspended dust Another factor to consider is the solar density. It is movement, thus promoting higher rate of dust accumu- found that the reduction in output power at relatively lation due to surface collusion (Qasem et  al. 2011). And lower or higher solar densities is much more severe. other remaining variables such as temperature and rain- This phenomenon is probably attributed to relatively fall are also known to play significant role in dust deposi - higher reflection effect of the deposited dust to light as tion. Here, the relation of all the meteorological variables the certain portion of already low value of irradiance gets under consideration with respect to dust deposition den- reflected by the accumulated dust layer (Jiang et al. 2011). sity is studied using multiple regression analysis. Cell The rainfall, humidity and temperature data which were temperature, humidity, rainfall and ambient temperature available from Department of Hydrology and Meteorol- ogy are shown in Figs.  4,  5  and  6, respectively.  Whereas the Fig. 7 depicts the value of power collected by datalog- gers in a clean and dusty module setup. Dust deposition density increases as the time period of exposition increases. Natural cleaning actions of rain- fall, wind speed and dew are evident in the graph as these agents lowered the dust deposition density as shown in Fig.  8. Some high values of dust deposition density are due to the bird dropping in the dust slide. Number of Days Fig. 5 Temperature variation during study period Transmittance at different Wavelengths 90.00 80.00 70.00 60.00 50.00 40.00 30.00 80 20.00 10.00 0.00 Number of Days Number of Days 750nm 600nm 450nm Fig. 6 Average humidity for the study period Fig. 3 Measured transmittance at different wavelengths P_Clean (W) P_Dusty(W) Numer of Days Number of Days Fig. 4 Rainfall during the study period Fig. 7 Power output from clean and dusty solar modules Percentage Transmittance (%) Rainfall (mm) 8 7 50 49 92 91 Power (W) Relative Humidity (%) Temperature ( C) 37 36 43 43 67 64 97 99 151 148 Paudyal et al. Renewables (2017) 4:5 Page 6 of 8 Number of Days Fig. 8 Measured dust deposition density are the independent variables for regression analysis. The operation resulted in the statistical results as given in Table 1. The regressed model takes the form of the following equation Y = A + β ∗ X + β ∗ X + β ∗ X + β ∗ X , 2 1 1 2 2 3 3 4 4 Fig. 9 Eec ff t of dust deposition density (gm/m ) in Transmittance (%) (1) where Y  =  dust deposition density on modules, A = regression coefficient, X  = rainfall (mm), X  = ambi- 1 2 factor as long expositions to humid environment cause the ent temperature (°C), X   =  humidity (%), X   =  module 3 4 encapsulate delamination and is known to affect the trans - temperature (°C). mittance property of laminating surface. Equation (1) reduces to the form Y = 13.3843 − 0.0064 ∗ X1 − 0.3656 ∗ X2 Percentage transmittance loss (2) Percentage transmittance loss is given by Eq. (3), where τ − 0.0074 ∗ X3 − 0.0642 ∗ X4. c is the transmittance observed for the clean glass samples Eec ff t of dust deposition density on transmittance and τ values depreciate due to dust deposition, that is, by Dust deposition density has adverse effect on the transmit - prolonged exposure tance as shown in Fig. 9. As shown in the numerous exper- iments performed, most notably on the research works of ∗ 100. %τ 1 − reduction= (3) Elminir et  al. (2006) and Gandhi et  al. (2014), the strong dependence of dust deposition on the transmittance and Basic result evident is that transmittance reduction on energy yield is verified. The reduction in transmittance percentage also increases with increasing days of expo- in the glass samples increased before saturating as shown sure. Figure  10 depicts the percentage of transmittance in Fig.  3 (Elminir et  al. 2006). The correlation coefficient loss which was mere 2.52% at the first day of experiment (r) and coefficient of determination (r ) of −0.8690 and which rose up to 69.06% at the final day of experiment at 0.7552 indicate a strong linear relationship between these 750  nm wavelength. The graph shows high variations in parameters. Humidity can be considered another major transmittance loss at the middle, which can be attributed to the formation of dew in prevalent climatic conditions. Dew promotes dust accumulation on flat surfaces and subsequent evaporation reinforces dust adhesion; in fact, Table 1 Statistical parameters of  multiple regression for model it sometimes forms a solid, packed cement-like compos- ite (Elminir et al. 2006). Parameters Value R 0.8632 Eec ff t of transmittance on power output reduction Adjusted R 0.8595 Over the period of study, the power output of modules Coefficient of variation 0.2770 decreased continuously due to dust accumulation, which Mean square error 1.014345 resulted in reduction in transmittance. As shown in Square root of MSE 1.007147 Fig. 11, the correlation coefficient (r) of −0.8904 indicates Dust deposition density (g/m2) 151 Paudyal et al. Renewables (2017) 4:5 Page 7 of 8 to thus reduction in power generation. Voltage gener- Transmittance Loss (%) ated from the modules was not altered to significant 80.00 70.00 levels. The dust deposition density ranging from 0.1047 60.00 to 9.6711  g/m and the power reduction of 29.76% with 50.00 respect to 69.06% transmittance loss can be considered 40.00 high in the span of 5  months. The study enlightens the 30.00 dire need of incorporating a proper cleaning device or 20.00 mechanism in existing solar PV systems installed in the 10.00 areas exposed to relatively high dust deposition condi- 0.00 tions during dry season of Kathmandu to reduce power loss due to soiling. As the laminate encapsulation of the No. of days solar modules contains pores to reduce the reflectance Fig. 10 Percentage loss in transmittance of the solar radiation, some dust may be present in such pores which require careful cleaning. Thus, the proper cleaning mechanism for such cases should be selected and applied. Since the transmittance losses due to soiling are highly pronounced, the research shows the system design of the solar PV systems must incorporate the pos- sible power loss from the transmittance reduction due to soiling. Authors’ contributions BRP is a lead author and corresponding author of the research article. He contributed to performing research activities, data collection, data analysis and writing of the research article. SRS played an integral part of this research process. He contributed to the design of this research and supervision of all the related works. DPP played another integral part of this research. He contributed to the formulation of transmittance measurement, compiling and verification of the manuscript and data analysis. DDM was another prominent contributor of this research as he contributed to the formulation of transmit- tance measurement, analysis of the results and managing the data collection facility. All authors read and approved the final manuscript. Author details Department of Mechanical Engineering, Pulchowk Campus, Tribhuvan Uni- versity, Kathmandu, Nepal. Department of Engineering Science and Humani- ties, Thapathali Campus, Tribhuvan University, Kathmandu, Nepal. Nepal Academy of Science and Technology, Kathmandu, Nepal. Fig. 11 Eec ff t of transmittance on power output reduction Acknowledgements The experimental work of this research was done in Nepal Academy for Sci- ence and Technology (NAST ). The authors are grateful to NAST for providing the research grant for this research. Meteorological data were provided by Department of Hydrology and Meteorology, Kathmandu. strong negative linear relation among these variables. Reduction in transmittance of module surface increases Competing interests the output power reduction percentage of the solar mod- The authors would like to confirm that there are no any competing interests associated with this publication and the research has been carried out for a ules as they are inversely related to each other. As the purely academic purpose. dust layer on the surface of PV module became thicker, the transmittance loss became alarmingly higher which Availability of data and materials The data and materials are available anytime from the author, as per request. triggers higher loss of module output power (Weber et al. 2013). Consent for publication The research contains the data exclusively measured for the academic research. All the contributing authors are the participants of the research and Conclusion hence the article is formulated in the consent of all authors. The correlation of dust deposition density with trans - mittance and eventually correlation of transmittance Ethics approval and consent to participate Not applicable. with power reduction were investigated in this paper. Dust deposition plays a crucial role in obstructing the Funding solar irradiance reaching solar cells and reduces the The research has been funded by Nepal Academy of Science and Technology (NAST ), and the authors would like to thank NAST for the financial support. transmittance of the encapsulate lamination leading Reduction Percentage (%) 151 Paudyal et al. Renewables (2017) 4:5 Page 8 of 8 Mailuha, J., Murase, H., & Inoti, I. (1994). Knowledge engineering based studies Publisher’s Note on solar energy utilization in Kenya. Agriculture Mechanization in Asia, Springer Nature remains neutral with regard to jurisdictional claims in pub- Africa and Latin America, 25, 13–16. lished maps and institutional affiliations. Mekhilef, S., Rahman, S., & Kamalisarvestani, M. (2012). Eec ff t of dust, humidity and air velocity on efficiency of photovoltaic cells. Renewable and Sustain- Received: 6 February 2017 Accepted: 4 August 2017 able Energy Reviews, 16, 2920–2925. Ndiaye, A., Kébé, C. M. 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