Performance evaluation and degradation assessment of crystalline silicon based photovoltaic rooftop technologies under outdoor conditions

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Performance evaluation and degradation assessment of crystalline silicon based photovoltaic rooftop technologies under outdoor conditions H.I. Dag a, 1, M.S. Buker b, *, 1 a b

Solimpeks Corp, Konya, Turkey Energy and Semiconductors Research Group, Konya NEU University, Turkey

a r t i c l e i n f o

a b s t r a c t

Article history: Received 15 August 2019 Received in revised form 28 October 2019 Accepted 26 November 2019 Available online xxx

Keeping track of field performance and degradation rate of photovoltaic panels in regions with diverse environmental exposures is critical. The objective of this paper is to determine two and half year performance characteristics and degradation rate of poly-crystalline and heterojunction with intrinsic thin layer roof-top photovoltaic units under prevailing weather conditions in the Central Anatolia region. Degradation rates of the photovoltaic units were determined through analysing the effective peak power, and the temperature corrected performance ratio of each technology. Processing of the measured data covering the test period reveals that the thin film technology offers lower degradation rate with nearly
0.1% than the poly-crystalline based technology within the range of
0.67% to
0.83%, respectively. The study allows a better understanding of variations in performance and behaviour of the output power of poly-crystalline and heterojunction with intrinsic thin layer roof-top photovoltaic units after 2.5 years of outdoor exposure. © 2019 Elsevier Ltd. All rights reserved.

Keywords: PV Degradation rate Performance Outdoor test

1. Introduction A photovoltaic (PV) solar cell is a semiconducting device that transforms sunlight directly into electrical energy using the photovoltaic effect. With accelerated technological progress, solar energy has entered a virtuous cycle of falling costs by around 80% since the end of 2009. The rapid amelioration in PV technology has prompted sharp fall in the price of PV panels. According to a market assessment report, spot prices of modules could fall further from $0.30 per watt-DC to $0.18 per watt-DC, experiencing more than 40% decline in the next five years [1]. Therefore, there has been a remarkable growth in utilisation of solar PV technologies recently. Silicon based solar cell technology has dominated solar market by over 90% share since 2015. Besides, thin film materials are very promising for photovoltaic applications. Therefore, heterojunction intrinsic thin layer (HiT) photovoltaic (PV) technology is also extending its market share as they offer

* Corresponding author. Necmettin Erbakan University, Konya, Turkey. E-mail addresses: [email protected] (H.I. Dag), [email protected] (M.S. Buker). 1 Equal contribution.

enhanced efficiencies over 25% [2]. According to a PV report, the market share of all thin film PV technologies constituted about 5% of the overall yearlong production in 2017 [3]. Considering the widespread use of PV technology, long term reliability of PV units has gained enormous importance. The qualification standards set by International Electro-technical Commission (IEC) including IEC61215-1-1 for crystalline silicon PV panels and IEC61215-1-3 for amorphous silicon (a-Si) thin film PV panels are to assure reliability of PV panels in the long-run [4,5]. Although, ideally, those standards are reasonably designed to imitate asperities that PV units go through during a standard life time of nearly 25 years, conforming IEC standards do not guarantee hundred percent performance or service time in the long-term [6]. The most important factor in the long-term performance of any PV system is degradation. After a period of outdoor operation, apparent defects are detected in both appearance and performance of the PV systems including few exceptions. For instance, snail tracks are visible but not in every case linked with a performance loss. This is perhaps the most evident indication that PV panels degrade over time leading to decline in the power output at the cell, module or even system level. In fact, the degradation is an inevitable phenomenon in any type of PV systems. According to NREL,

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Please cite this article as: H.I. Dag, M.S. Buker, Performance evaluation and degradation assessment of crystalline silicon based photovoltaic rooftop technologies under outdoor conditions, Renewable Energy, https://doi.org/10.1016/j.renene.2019.11.141

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Nomenclature Isc Voc G G* RD PM* PDC TG Tm Tm* Im Vm FF PR Y Yr E

Short-circuit current [A] Open-circuit voltage [V] Irradiance [W/m2] Irradiance at STC [W/m2] Degradation rate [%] Effective peak power [W] DC output power [W] Thermal factor [%] PV module temperature [ C] PV module temperature at STC [ C] Current at MPP [A] Voltage at MPP [V] Fill factor [%] Performance ratio [%] Array yield [h] Reference yield [h] DC energy output [kWh]

the cell degradation rate could be as high as 4% annually while average degradation rate is predicted to be 0.8% annually, corresponding decline in power output [7]. PV degradation occurs gradually affecting the panel component characteristics and eventually limits the performance. While, a degraded PV panel could continue producing electricity from sunshine but with increasingly reduced power yield [8]. The performance of PV panels can be downgraded due to various factors including temperature, humidity, ultraviolet (UV) exposure and structural failure [9]. In addition, soiling or dust accumulation on solar surfaces can cause significant performance degradation of PV modules [10,11]. In this context, the performance of silicon based PV panels and thin film technologies are highly affected by climatic conditions [12]. Intense solar radiation and severe temperatures speed up the degradation rate of PV panels exposing to such operational conditions [13]. Although thin film PV panels relative to the crystalline PV systems offer lower production cost and lower temperature coefficients, high degradation rates may frequently pose a main drawback against crystalline PV panels [14]. Degradation on PV panels mainly appears due to natural stresses including severe temperature conditions, humidity level, UV radiation etc. that PV panels are exposed to [15]. In general, degradation mechanisms provide the direct evidences from the physicochemical changes before and after the outdoor exposure [16]. At the cell level, the degradation mechanisms involve gradual loss of performance due to ageing of material [17], corrosion [18], loss of adhesion between cell-cell contacts [19], metal mitigation through the p-n junction [20] and antireflection coating deterioration [21]. At the module level, degradation can occur due to cell crack [22], failure of delamination [23], failure mechanism of cells [24], interconnect failure [25], encapsulant [26] and bypass diode failure [27]. Recently, certain studies have been associated with the degradation assessment of silicon based PV systems. Increasing attention has been given to the degradation behaviour and operational life cycle of various PV types. To exemplify a few, these studies include degradation study of thin film [28] and micro morph silicon under semi-arid climate conditions [29], characterizing the degradation of a-Si under long-term outdoor exposure [30], determination of early life degradation of multi-crystalline silicon PV units under western Himalayan Indian environmental conditions [10] and comparative degradation study of four different

PRcorr Ecorr LT

Performance ratio (corrected) [%] DC energy output (corrected) [kWh] Temperature losses [%]

Abbreviations a-Si Amorphous silicon p-Si Poly-crystalline silicon HiT Heterojunction with intrinsic thin-layer PV Photovoltaic STC Standard test condition MPP Maximum power point Greek letters Temperature coefficient of power [%/ C] Temperature coefficient of voltage [V/ C] Temperature coefficient of current [mA/ C] Efficiency [%] Recoding interval [h]

d b a h t

PV systems namely multi-crystalline, heterojunction intrinsic thin layer (HiT), micro-morph and copper indium gallium diselenide (CIGS) [31]. Hot and humid areas have been said to prompt the degradation of PV systems rapidly. Therefore, adequate information and statistical data on the degradation of PV panels operating under variety of environmental conditions is required to predict long term performance and service life [32]. Therefore, keeping track of field performance and degradation rate of PV panels under diverse climatic zones is critical. In this context, it is important to examine the behaviour of PV units under real outdoor conditions to figure out their performance characteristics and degradation occurrence in the long term. To some extent, there will be sufficient details regarding quantitative assessment of outdoor performance and service life. Considering all aspects reported, this communication offers reporting the degradation profile of selected PV panels in the territory for the first time and intents to grant a comprehensive and quantitative contribution to the knowledge of degradation assessment of PV technologies. Although analysis was performed focusing on the issues of a particular region, the findings will facilitate to ensure long term reliability of PV panels in an international perspective. This work represents the comparative degradation study of two different rooftops installed PV technologies under prevailing weather conditions in Anatolia region. The period of the study cover two and half years from January 2016 to July 2018. In this context, the main ambition of this work is to perform a temperature dependent degradation analysis of poly-crystalline silicon (p-Si) and hetero-junction intrinsic thin layer (HiT) type PV units and compare the degradation rates to determine the long-term electrical performance. Hence, the outcome of the study is expected to be beneficial to quantify the degradation rate of PV technologies under the climatic conditions in mid Anatolian location to map performance characteristics at universal appeal. This communication is organised as follows; a brief survey of recent works is presented to fully appreciate the latest findings and key challenges relating to the topic addressed in this study in Section 2. Technical details of the PV system and degradation analysis methodology are given in Section 3. In Section 4, the results and discussion are provided. The conclusions are drawn in Section 5.

Please cite this article as: H.I. Dag, M.S. Buker, Performance evaluation and degradation assessment of crystalline silicon based photovoltaic rooftop technologies under outdoor conditions, Renewable Energy, https://doi.org/10.1016/j.renene.2019.11.141

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2. Brief overview of PV degradation studies The degradation of PV panels can be categorized by their trace, mechanism and type. Stating this, the performance of PV panel could degrade due to various factors including temperature, humidity level, ultraviolet (UV) exposure and structural damage [33e38]. These factors may cause various defects such as corrosion, stain, delamination and cracking of solar cells [39e44]. Among these factors, temperature is a natural factor that significantly reduces the energy generation performance of a PV panel [45]. Therefore, in this communication, temperature related degradation assessment of crystalline silicon type PV panels will be reported. Several works have been performed in an attempt to analyse the temperature effect on the performance and degradation of the PV units worldwide. Djordjevic et al. [46] examined diverse defects and irregularities in PV units under exposure test of panels to ambient conditions of Western Australia for up till ten years and analysed reported defects comparing with the identified ones specific to the region. However, no specific defect were attained. Ozden et al. [47] carried out long term performance evaluation and degradation examination of poly-Si and thin film silicon type PV units based on measured solar global-horizontal irradiation for about 5 years under the prevailing weather conditions of Ankara city, Turkey. The findings revealed that the average yearly efficiency of poly-Si was reduced from 14% to 11% and thin film from 8% to 6%, respectively. Ogbomo et al. [48] investigated the influence of temperature on the degradation of crystalline Si type PV units. The study focused on relatively high temperature climates ranging from 25  C to 45  C. The cell temperature was simulated between 25  C and 120  C in steps of 1  C with ANSYS simulation environment. The model predicted that the module life was 18.5 years in London while it was attained 9 years in extremely hot climates. The model also advised on enhanced thermo-mechanical design for hot climatic conditions. Dubey et al. [49] stated that PV panels operating under high temperature environments are likely to have disparity between crystalline silicon wafer, solder, silver contacts, coper ribbons and remaining layers in the unit. Consequently, those PV panels actively exhibit higher degradation rates comparing to those in mild climates. Bastidas-Rodriguez et al. [50] focused on the electrical power degradation of PV panels and proposed a quantification method which was successful providing useful information about the degradation and the method was validated with the experimental values. Various studies reported the potential risk that PV panels, even Si wafer type PV panels, were unable to meet the 25-year warranty especially in the tropics [51,52]. Chandel et al. [53] performed degradation analysis of mono-crystalline Si PV panel after 28 years of outdoor operation at western Himalayan region. The findings showed that average power degradation was found to be increasing 1.4% annually while open-circuit voltage was attained at an average increase rate of 2.8% per annum. The study has also revealed that replacement of most degraded sub-panels is required in order to improve the performance and longevity of PV systems beyond the warranty period. Bora et al. [54] found that HIT and a-Si showed improved performance against poly-Si on the temperature related performance measures and degradation analysis of various PV technologies.

Corporation, which is one of the largest PV/T manufacturer in Europe (see Fig. 1). Konya is geographically located at the central plateau of Anatolia with latitude of 37 520 28.707600 N, longitude of 32 290 35.361600 and elevation from sea level of 1028 m (Fig. 2). It is a semi-arid €ppen-Geiger classification [63]. The climate region according to Ko city enjoys relatively hot summers and cold, snowy winters. Fig. 2 also shows the solar radiation potential of the city which is around 1650e1700 kWh/m2-year. Moreover, Table 1 provides the mean irradiation, ambient temperature, wind speed and relative humidity belonging the years under scrutiny. The system is made of six arrays in total as polycrystalline and HiT PV units comprise three arrays each. The PV units are mounted at a fixed inclination of 30 and supported to face the south. Technical parameters of the PV units are presented in Table 2. The PV system employs maximum power point tracking (MPPT) separately for each array. The facility utilizes meteorological instruments linked to data recording system. In-plane irradiance, ambient temperature and module temperature are logged as weather parameters. Voltage, current and DC-AC side output parameters are recorded as electrical parameters as well. Both parameters are collected in 10 min interval. 3.2. Performance evaluation and degradation rate In this part, effective peak power assessment P*M is applied to analyse the characteristics of the output power. This method allows us to obtain the degradation rate of the PV units. The P*M is estimated by Ref. [2].

P *M ¼

G* PDC Tf G

Where PDC corresponds the DC output power from the PV units and G and G* are the actual irradiance and irradiance at STC, respectively. Thermal factor TG can be expressed as [2].

Tf ¼


1   1 þ d Tm
T *m

Where d points the power temperature coefficients of the PV units examined, presented in Table 2 and T*m is the corresponding panel temperature at STC. The degradation rate of PV units, RD (%/year) is obtained via a linear least square fitting method of P*M [2]

RD ¼ 100

12b a

Where b (W/month) indicates the slope of the trend line for P*M and a (W) is the y-intercept [2].

3. Methodology 3.1. Description of the system The deployed grid-connected PV arrays have the total capacity of 48 kWp (24 kWp of p-Si and 24 kWp of HiT) and consist of 96 of 250 Wp polycrystalline and 74 of 325 Wp HiT PV panels, respectively. The PV system is mounted on the rooftop of Solimpeks Solar

3

Fig. 1. View of PV panels installed on the rooftop of Solimpeks Solar [55].

Please cite this article as: H.I. Dag, M.S. Buker, Performance evaluation and degradation assessment of crystalline silicon based photovoltaic rooftop technologies under outdoor conditions, Renewable Energy, https://doi.org/10.1016/j.renene.2019.11.141

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Fig. 2. City of Konya: Latitude: 37 520 28.707600 N, Longitude: 32 290 35.361600 E, Altitude: 1028 m [56,57].

Table 1 Annual mean values of meteorological parameters in Konya over the study period. Year

2016 2017 2018 (½)

Irradiation (kWh/m2)

Ambient temp. ( C)

Wind speed (m/s)

Relative Humidity (%)

Mean

STD

Mean

STD

Mean

STD

Mean

STD

4.79 4.88 4.64

1,69 1,82 1,76

12.66 11.75 11.65

8,86 9,66 6,75

3,04 2,88 3,15

2,35 1,72 1,54

49,75 55,58 51,67

13,67 17,68 10,99

PRð%Þ ¼

Y :100 Yr

where Y indicates the array output which implies the time-period that PV units are required to operate at its rated power. It is characterized by

Y¼

E P STC M

where E marks the energy output and PSTCM specifies the maximum power measured at STC. Yr suggests the reference yield and is found by

Table 2 Technical specifications of the PV units. Parameters

PV type p-Si

HiT

Peak Power (W) Isc (A) Voc (V) Temperature coefficient of power, d (%/C) Temperature coefficient of voltage, b (%/C) Temperature coefficient of current, a (%/C) Efficiency, h (%)

250 W 8.76 A 37.7 V
0.45
0.34 0.05 15.4

325 W 6.03A 69.6 V
0.29
0.174 1.81 19.7

y ¼ a þ bx

Yr ¼

t

P

G

G*

where t is the time interval of data collection. The temperature is one of the key factors influencing the amount of power generated from the PV units. Therefore, so as to compute the temperature corrected performance ratio, PRcorr, the temperature corrected DC energy output, Ecorr, needs to be calculated initially by following equation [2,17].

Ecorr ¼

E 1 þ dðTM
25Þ

The temperature corrected array yield, Ycorr, is given by 3.3. Fill factor The fill factor of a PV stands for the proportion of maximum power achievable and the product of the open-circuit voltage and short-circuit current. It is expressed as [2].

FF ¼

Vm Im VOC ISC

where Vm and Im are the MPP voltage and current, respectively. Voc and Isc are the open-circuit voltage and short-circuit current values of the PV unit, also provided in Table 2.

Ycorr ¼

Ecorr P STC M

Subsequently, the temperature corrected performance ratio, PRcorr, is expressed as [2].

PRcorr ð%Þ ¼

Ycorr :100 Yr

The temperature losses indicate the decrease in efficiency of a PV array. Hence, it can be found by

Ecorr

!,

3.4. Performance ratio and temperature losses

LT ð%Þ ¼ 100:

The degradation study could also be achieved by utilising standard performance criterion. To do so, the temperature corrected performance ratio can be utilised to predict the degradation rate of any PV type. Essentially, all parameters are computed depending on data collected during sunshine. The performance ratio of the PV units, PR, is calculated by Ref. [2].

4. Results and discussion

P STC M


Y

Yr

Fig. 3 illustrates the monthly average change in PV units’ temperature during the monitoring campaign. As shown in Fig. 1,

Please cite this article as: H.I. Dag, M.S. Buker, Performance evaluation and degradation assessment of crystalline silicon based photovoltaic rooftop technologies under outdoor conditions, Renewable Energy, https://doi.org/10.1016/j.renene.2019.11.141

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Fig. 3. Temperature profiles of the panels.

although p-Si units have displayed slightly higher module temperature against HiT units, high temperature difference has not been detected in overall test duration. Fig. 4 demonstrates the progress of P*M attained for p-Si and HiT type PV technologies. The average values obtained for each month were computed in line with the process detailed in methodology of performance evaluation and degradation rate. As the figure indicates that HiT does not show a notable reduction in P*M, with less than 0.05% over the course of the test period. In contrast, p-Si offers a power decline of as much as 1% in the same course. Likewise, RD is estimated by practicing the protocol given in Eqs.

5

(3) and (4) on the trend lines of P*M introduced in Fig. 3. The values attained as a result of this procedure were provided in Table 3. According to the table, p-Si possessed the highest RD of 0.67% as compared to HiT units having 0.1%. Fig. 5 presents the progress in the monthly average output voltage performance corresponding to the MPP and average module temperature. As the figure reveals, the trend of output voltage tracks the seasonal changes due to temperature profile. Therefore, the maximum output voltage values intersect with the minimum values of module temperature recorded, while the minimum output voltage values are observed at the maximum temperature profile. The temperature effect on the output voltage is almost twice for p-Si in comparison to the HiT technologies as given in Table 2. Although p-Si retains the highest voltage temperature coefficient, it shows lower voltage variations because of the temperature effect. This phenomenon probably can be explained by reasoning that the p-Si type PV unit has an open-circuit voltage of 38.7 whereas HiT type has 69.6 at STC, as shown in Table 2. Deviation in the output current of each PV array at the MPP are provided corresponding to the monitored irradiance profile in Fig. 6. It is worthwhile to note that PV units reflect apparent seasonal changes mainly due to not only by irradiance profile but also

Table 3 Degradation rates of the examined PV technologies. PV

Slope (b) Intercept (a) RD (%/year)

p-Si

HiT


0.0938 59.4854
0.67 ± 0.08%


0.02678 74.6436
0.109 ± 0.09%

Fig. 4. Change in P*M.

Please cite this article as: H.I. Dag, M.S. Buker, Performance evaluation and degradation assessment of crystalline silicon based photovoltaic rooftop technologies under outdoor conditions, Renewable Energy, https://doi.org/10.1016/j.renene.2019.11.141

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Fig. 5. Monthly average output voltage from the PV units.

Fig. 6. Monthly average output current of the PV units along with the monitored irradiance profile.

influenced by the deviation in ambient temperature. As expected, p-Si unit has shown greater variation having the highest temperature coefficient almost doubling HiT units. The temperature coefficient values are given in Table 2. Fig. 7 demonstrates the change in the monthly mean values of the FF attained for the studied PV systems during monitoring campaign. As it can be clearly seen from the graph that FF of HiT unit is slightly surpassing the FF of p-Si throughout the test period. The seasonal deviations had clearly more impact on p-Si than HiT. Also, the FF values are mostly higher for HiT during the monitoring campaign and maintains the same trend over the duration. Fig. 8 shows the computed values of PR and its progress within the period for both PV technologies. As the degradation of the PR is

detected, the HiT type PV system offers higher PR while values attained for p-Si is lower. The temperature corrected performance ratio, PRcorr, was calculated following the procedure described in Section 2.4. (see Fig. 9). The annual RD was also attained by conducting linear least square fit method on the extracted trend of the PRcorr by employing Eqs. (3) and (4). The data drawn for the slope, b, and intercept, a, for each PV unit are also presented in Table 4. The table reveals that the RD values calculated through PRcorr are slightly greater than the previously computed RD values based on P*M of both PV technology. It should be, however, underlined that the values are somewhat consistent with the ones given in Table 3. The figure points out that p-Si panels indicate higher RD values than that of HiT panels have.

Please cite this article as: H.I. Dag, M.S. Buker, Performance evaluation and degradation assessment of crystalline silicon based photovoltaic rooftop technologies under outdoor conditions, Renewable Energy, https://doi.org/10.1016/j.renene.2019.11.141

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5. Conclusion

Fig. 7. Fill factor change for the PV units.

The RD value of 0.83% was obtained for p-Si which is within the range of values stated in the literature including 0.52% [58] and 1.27% [59]. However, the RD value attained for HiT panels are much lower than the values obtained in other studies [31,59]. Temperature losses can be decisive for power losses in PV systems [60e62]. The temperature losses, for this study, was computed by using the procedure described in Section 2.4 and the results are shown in Fig. 10. The p-Si panels indicated the higher temperature related power losses as HiT offers lower losses associated to temperature effects. The results are also in line with the temperature coefficients given in Table 2 for both technologies.

In this study, a performance evaluation and degradation assessment of poly-crystalline (p-Si) and heterojunction with intrinsic thin-layer (HiT) PV technologies has been carried out under prevailing weather conditions of Anatolia region in Turkey. The PV units are installed in Konya which is a dry and sunny inland region, and are monitored for around two and half years between January 2016 and June 2018. Based on the results and findings, some concluding remarks can be drawn as follows; The effective peak power, P*M, of HiT was not substantially affected and was almost same while power reduction was less than 1% for p-Si type PV panels over the course of the study period. However, this slight deviation can be attributed to the effect of seasonal oscillation that disturbs the stabilized values of power output. As PR analysis discloses the degradation effect and seasonal deviations, the HiT type panels proposed slightly higher PR values than the p-Si panels attained along the monitoring process. Degradation in the power output can be referred to loss in shortcircuit current (ISC) or fill factor (FF). Degradation rates of the PV units, RD, were determined by analysing the effective peak power, P*M and the temperature corrected performance ratio, PRcorr, of each technology. The HiT type PV panel was better off in both methods as P*M and PRcorr methods yield nearly 0.1% per year, respectively. As for the p-Si panels, RD values of 0.67% and 0.83% were attained from both methods and these values are in the good agreement with the values found in literature. The HiT type PV panels possessed higher FF values than p-Si throughout almost the entire period. Although seasonal variations were detected for both technologies, however, these variations were observed to be having more impact on p-Si type PV units. The PV systems have shown the issue of rapid degradation un-

Fig. 8. Monthly mean PR values.

Please cite this article as: H.I. Dag, M.S. Buker, Performance evaluation and degradation assessment of crystalline silicon based photovoltaic rooftop technologies under outdoor conditions, Renewable Energy, https://doi.org/10.1016/j.renene.2019.11.141

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H.I. Dag, M.S. Buker / Renewable Energy xxx (xxxx) xxx

Fig. 9. Monthly mean PRcorr.

on the physicochemical characteristics of the thermal degradation of PV units and this remains a topic of great interest to the authors.

Table 4 Degradation rates of the PV units from PRcorr analysis. PV

Slope (b) Intercept (a) RD (%/year)

p-Si

HiT


0.0782 56.7483
0.83 ± 0.08%


0.02221 61.47
0.118 ± 0.09%

Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment The authors would like to gratefully acknowledge the support provided by Solimpeks Solar Co. during this research. References

Fig. 10. Temperature losses.

der the dry and sunny climatic conditions. However, higher degradation rate can not only be attributed to the temperature effect and can decelerate on the field, especially in favourable ambient conditions. Consequently, the authors suggest incorporation of a cooling system for the PV panels to avoid temperature related degradation. Moreover, there is scarce information reported

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Please cite this article as: H.I. Dag, M.S. Buker, Performance evaluation and degradation assessment of crystalline silicon based photovoltaic rooftop technologies under outdoor conditions, Renewable Energy, https://doi.org/10.1016/j.renene.2019.11.141