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Some Reserches About The High Tech Self Cleaning Solar Panels
- Aug 27, 2018 -

                                Some reserches about the high tech self cleaning solar panels

 

Recently there is some hot product for solar panel cleaning on the market , they call some magic coat , check the attached report .


‘The outdoor performance of coated and uncoated PV panels and arrays were monitored for several months at different climate conditions (Greece and     China) in order the extra energy produced due to coating to be measured. An average 5–6% gain was found for both cases for the entire period of time. It was established that specific conditions such as intensity and angle of the incident light, occurrence of rain and sand storms influence significantly the power difference (ΔPm) between coated and uncoated PV panels. The increase of ΔPmunder diffused light (cloudy day) and irradiation with high incident angle (morning, evening) reached ∼20% and 30% respectively, that were related to the anti-reflecting property of the glass coating. The coated surface showed better dust removal ability due to its superhydrophilicity (θ = 6°). The superior efficiency of coated panels as well as the low-cost spraying procedure without any post-deposition treatment render the nanocomposite SurfaShield G coating very important especially for northern regions with limited sunlight periods.

The effect of anti-reflective and anti-soiling coating on polycrystalline PV modules exposed outdoors for one year at Spain have been evaluated when using an anti-soiling coating product by Asahi Kasei Corporation (Piliougine et al., 2013). During the one-year exposure period, the coated PV modules demonstrated an average daily soiling loss of 2.5%, while uncoated modules a daily average of 3.3%.

This contribution aims in presenting the optical, self-cleaning properties and stability of the coating determined in laboratory conditions. Also, coated and uncoated PV units functioning in real conditions were monitored within 7 months in Attica (Greece) and 5 months in Neimeng province (China). The recorded variation of energy output difference is discussed in relation with the PV surface properties and the atmospheric conditions.

The outdoor power generation performance was evaluated using two different experimental set-ups. In the one set-up, the evaluation of PV panels (without inverters) was targeted. Two brand new panels were directly connected to two identical resistances able to fully consume the current produced from the PV. The tilt of the investigated panels was fixed at 32°. All the operational parameters were exactly the same except from the covering layer. The experimental analyses were conducted at the Science and Technology Park of Lavrio in Greece (φ = 37°42′). In the second set-up, the evaluation of PV system was targeted. Two arrays, one coated with SSG and one uncoated, consisted of more than 2000 panels each were used. The operational parameters were the same and the measurements were performed following standards requirements. The experimental analyses were conducted at Baoergai PV plant in China (φ = 40°45′) where the tilt of the panels was 34°. The two experimental set-ups are described in details below.



Fig. 1. Experimental set-up of the panel in Greece (a) and calibration results (b).

After the calibration period, both panels were carefully cleaned and one of the panels was sprayed with SSG to create a coating, while the other panel was left untreated. Both panels were exposed to the same ambient environment making sure that not only the solar irradiation but also the wind conditions are exactly the same.

The Baoergai grid-connected PV plant in China was built in 2012 with 30 MWp PV panels (HT60-156P-240) installed. It is located in west China where dry weather prevails and the area is frequently plagued by sandstorms, especially in winter and spring. Two stable 5 kWp PV arrays that had almost the same power generation in 2014 were chosen as experimental group (coated with the SSG) and control group (uncoated). The arrays were exposed to the same ambient environment with similar solar irradiation and wind conditions. Each array (more than 2000 panels) and had a capacity of 500 kW. Each panel comprised of 60 cells with dimensions 156 mm × 156 mm, maximum power Pm = 240 W, maximum voltage Vm = 30.5 V, maximum current Im = 7.87 A, open circuit voltage Voc = 37.5 V and short circuit current Isc = 8.49 A.

The experimental set up of the arrays with uncoated and coated glass surfaces is depicted in Fig. 2. The power generated from the arrays is transferred to the 110 kV grid through combiner boxes and PV inverters by Samlipower (Solar Ocean 500TL). Each array was connected to an inverter and the inverters were manufactured in the same batch to ensure the consistency. The data were collected by data-collector that was connected with PV inverters by PC and remote monitoring system and recorded daily every 15 min. The measured parameter was the output energy as a function of time.


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Fig. 2. Experimental set-up of the panels in China.

The power difference values between the two panels/arrays per day (ΔPd) were calculated using the Eq. (1):

(1)ΔPd(%)=Pd2-Pd1Pd1·100

where Pd1 is andPd2 is the power produced by the uncoated panel 1/array 1, and the coated panel 2/array 2 per day.

The power difference values between the two panels/arrays per minute (ΔPm) were calculated using the Eq. (2):

(2)ΔPm(%)=Pm2-Pm1Pm1·100

where Pm1 is and Pm2 is the power produced by the uncoated panel 1/array 1 and the coated panel 2/array 2 per minute.

It should be mentioned that the type of the cover glass of the PV panels in Greece and China according to the manufacturers was low iron tempered glass. In each case the reference and the treated panels were from the same manufacturer and the results from the two experimental set-ups are evaluated in a complementary way.

The transmission spectra of coated and uncoated glass are comparatively presented in Fig. 4. It can be observed that the light transmittance of the coated glass is significantly enhanced in comparison to the uncoated glass granting antireflective properties of the coated surface. The effect is especially prominent in the visible light range, which is important for better utilization of the incident solar irradiation by the PV units.


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Fig. 4. Transmission spectra of coated and uncoated glass.

The measured water drop contact angle θ on the uncoated and the coated glass surfaces is evident in Fig. 5a and b, correspondingly. The θ value was reduced from 39° on the uncoated glass to 6° on the coated surface demonstrating superhydrophilic property of the latter. This property allows the chemisorbed H2O layer on the TiO2 to attract water molecules through van der Waals forces and hydrogen bonds obstructing thus the contact between the glass surface and the adsorbed contaminants. The impurities deposited on the coated surface can be easily removed by the spreading action of water and consequently, the coated TiO2 glass surface exhibits a self-cleaning effect (Ganesh et al., 2012).


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Fig. 5. Contact angle between water droplet and (a) uncoated glass and (b) coated glass.

The photocatalytic degradation of Methylene Blue on the glass surfaces under UV illumination is shown in Fig. 6. It is important to note that the concentration and the amount of MB deposited on the uncoated (a) and the coated glass (c) is the same. The lighter color on the coated surface (a) is caused by the wider spreading of the liquid due to the superhydrophilic property of the coating. After 30 min irradiation, the color intensity on the coated surface was significantly decreased in comparison to the uncoated one (d). The outcome is attributed to the photocatalytic activity of the nano-sized TiO2 component of the coating. According to the well-established photocatalytic mechanism (Fujishima et al., 1999) the photo-generated electrons and holes reach the surface of the TiO2 nanoparticles and participate in the formation of highly reactive radicals which attack and decompose the organic compound to inorganic CO2 and H2O. The photocatalytic activity tests indicate that the SurfaShield G coated PV panels are expected to exhibit self-cleaning activity towards other organic contaminants reaching the glass surface and consequently increased efficiency in solar light utilization.


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Fig. 6. Images of Methylene Blue on: coated glass before (a) and after (b) irradiation; uncoated glass before (c) and after (d) irradiation. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

The ability of the coating to withstand thermal shock, fatigue and other stresses caused by repeated changes of temperature was investigated by subjecting the coated glass sample to temperature fluctuations from −40 °C ±2 °C to +85 °C ±2 °C. It was established that the multiple dramatic changes of temperature would not severely affect the structure and thus the properties of the coating. Specifically, after the 200 cycles of temperature treatment, the measured average light transmittance of the coated glass sample exhibited only a slight drop from 94.94% to 93.35% in the region 400 nm–1100 nm. Notably, the light transmittance after the thermal treatment was still higher than the respective value of the uncoated glass (92.58%).The exposure of the coated glass on 60 kW/m2 dose UV irradiation decreased the average light transmittance from 95.07% to 92.20%. After the two types of testing, i.e. thermal treatment and UV irradiation, the morphology of the coating was not affected. The nanostructure and the arrangement of the nanoparticles were preserved as evidenced by the SEM images presented in Fig. 7.


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Fig. 7. SEM images of the coated glass before testing (a) and after the thermal cycling (b) and UV irradiation (c) tests.

A comparison of the morphology of coated samples exposed to in-door sandblasting at different conditions is presented in Fig. 8.


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Fig. 8. SEM images of the coated glass after sandblasting tests at Conditions 1–4 (C1-C4).

From the SEM images it can be perceived that the sand with diameter 100 μm did not affect the coating when the wind speed is less than 5 m/s and the testing duration is less than 8 h (Conditions 1). On the contrary, the stability of the coating was significantly reduced when the sandblasting parameters became more severe. Specifically, the structure and the adhesion of the coating appeared disrupted after blasting with sand larger than 100 μm, wind speed higher than 5 m/s and time period more than 8 h (Conditions 4).

3.3. Outdoor power generation performance of PV panels

The power per minute difference (ΔPm) and the intensity of the incident solar irradiation (R) recorded during a sunny day right after coating in Attica Greece (Fig. 9a) revealed that ΔPmbetween the coated and uncoated panels during the day is constant at ∼6%, while the ΔPmreaches an increase up to 30% during the morning and the evening hours. This outcome can be attributed to the antireflective properties of the coated glass surface that utilized the sunlight irradiation with high incident angle (Fig. 9b). Since one of the components of the SSG coatingTiO2 is known to have high refractive index, the increase of the light transmittance can be explained by the relation between the refraction indexes of the two media, i.e. air and glass surface (3):

(3)n1sinθ1=n2sinθ2

where n1 and n2 are the refraction indexes of the air and the glass respectively; θ1 is the angle of incidence and θ2 is the angle of refraction.


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Fig. 9. Outdoor performance of PV panels: (a) power per minute difference measured during the first day after coating the panel; (b) schematic presentation of light transmition through non-coated and coated glass; (c) power per minute difference measured in a cloudy day; (d) power per minute difference measured 16 days and 86 days after coating the panel.

It is known that the refractive index of the air (n1) is 1.00 and the refractive index of uncoated soda lime glass (n2) is 1.51. The measured refractive index of the coating (n2′) was 1.47 which is lower than that of the uncoated glass. Consequently, during the morning and evening, when the incident sunlight angle obtained high values, the smaller θ2caused by lower index of the coating could ensure higher light transmition leading to increased power output.

The antireflective behavior of the coating exhibited during a sunny day in the morning and evening when sunlight is limited, can be noted during a cloudy day as well (Fig. 9c). The explanation is based on the altering of the surface roughness. The uncoated PV panel glass exhibited sub-micron roughness i.e. between 200 and 300 nm, while the roughness of the coated surface was between 40 nm and 60 nm. After application, the nanoscaled particles fill up the gaps on the glass creating nano-roughness and pores which help the panel to trap more scattered and diffused light. Therefore, for the cloudy days with higher component of scattered and diffused light, the transmition of light through the glass is higher.

The effect of dust accumulation on the power generation of the PV panels can be assessed by comparing two sunny days with time difference ∼70 days. The selected days were in June and August that is day 16th and day 83rd after coating application, respectively. The type of the ΔPm curves (Fig. 9d) is similar revealing large ΔPm values for the morning and evening hours and lower ΔPm values stable during the rest of the day. It is evident that the average 5% power difference recorded at 17/06 (day 16th) increased to 9% at 26/08 (day 83th). It must be underlined that there was no rain between the two dates which practically means that the larger difference in power generation between the coated and uncoated panels is caused by dust accumulation on the uncoated panel.

The power difference ΔPd between the two PV panels for each day of the monitored period (from June to December) is presented in Fig. 10. It can be observed that from the very first day of application (1stof June) there is an increase of 2.43%, which is undoubtedly due to the antireflective property of the coating as both of the panels were clean.


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Fig. 10. Outdoor performance of PV panels in Greece: (a) power difference per day for the examined period of time; (inset) picture of the uncoated and coated PV panels after rain; (b) power difference during the day where the maximum value was measured.

It is important to evaluate the change in power ΔPd with respect to the weather conditions. At the beginning of the monitored period (during the summer), the ΔPd values revealed a moderate power increase of ∼5%. Dust accumulation was observed on the uncoated panel only demonstrating the antistatic properties of the coating since no rains occurred meanwhile. Higher power increase was recorded in the beginning of September when the first rains appeared. Rain converted dust into muddy stains on the uncoated panel while it was more efficient in washing off dust on the coated panel due to the hydrophilic nature of the coating (inset in Fig. 10a). During the next months, a satisfactory gain in energy production was recorded. In December, the gained power difference was attributed to the antireflective property of the coating as it was raining continuously and both of the panels were clean.

On the 4th of October which was a cloudy day with sunlight intervals, a spike with high power difference of 19.81% was recorded (Fig. 10b). A closer look at the instantaneous power difference values during this day revealed that the phenomenon can be related to the antireflective property of the coating. Specifically, when the light intensity was low (cloudy intervals), the coated panel produced more energy reaching a maximum of 50%. On the other hand, when the light intensity was high (sunlight intervals) the power difference reached minimum evidencing that the coating transmits better the diffuse radiation.

The monitored period of time covered parts of summer, autumn and winter seasons in Greece and the recorded phenomena are expected to be observed during the rest of the year. An average of 5% power gain due to the coating was calculated for the entire period of time.

The performance of the PV arrays in Qinghai province (China) was monitored for the time period from November to March. The experimental data on the power output difference between uncoated and coated PV panels is presented in Fig. 11a. The average power difference per day (ΔPd) exhibited increasing trend for the entire period. The highest increase of the ΔPd was recorded in January that was attributed to the sandstorm occurred in the PV plant location on January 21st. The ΔPd value reached 12.43% and kept growing to 13.17% for 6 days due to the dust accumulated on the uncoated array. Although the rain on January 29th ended the continuous increase, the muddy stains observed on the untreated panels after the rain (Fig. 11b) made the power difference go up again. The consequent drop of the ΔPd to approximately 2% was associated with cleansing performed at January 31st. It should be mentioned that regular cleansing procedure (every two months) is practiced for simulating the real maintaining conditions in most of the PV plants in western China. Overall, the data collected revealed an average 6% power increase during the entire exposure period.


Fig. 11. Outdoor performance of PV panels in China: (a) power difference per day for the examined period of time; (b) picture of the uncoated and coated PV panels after rain (finger scrubbing marks clearly show the dust accumulated).