Outdoor Performance Assessment of New and Old Photovoltaic Panel Technologies Using a Designed Multi-Photovoltaic Panel Power Measurement System

This paper presents a new multi-photovoltaic panel measurement and analysis system (PPMAS) developed for measurement of atmospheric parameters and generated power of photovoltaic (PV) panels. Designed system presented with an experimental study evaluates performance of four new and four 5-year-old PV panel technologies which are based on polycrystalline (Poly), monocrystalline (Mono), copper indium selenide (CIS), and cadmium telluride (CdTe) in real time, under same atmospheric conditions. The PPMAS system with the PV panels is installed in Yildirim Beyazit University, Ankara Province, in Turkey. The designed PPMAS consists of three different subsystems which are (1) photovoltaic panel measurement subsystem (PPMS), (2) meteorology measurement subsystem (MMS), and (3) data acquisition subsystem (DAS). PPMS is used to measure the power generation for PV panels. MMS involves different types of sensors, and it is designed to determine atmospheric conditions including wind speed, wind direction, outdoor temperature, humidity, ambient light, and panel temperatures. The measured values by PPMS and MMS are stored in a database using DAS subsystem. In order to improve the measurement accuracy, PPMS and MMS are calibrated. This study also focuses on outdoor testing performances of four new and four 5-year-old PV panels. Average monthly panel efficiencies are estimated as 8.46%, 8.11%, 5.65%, and 3.88% for new Mono, new Poly, new CIS, and new CdTe PV panels, respectively. Moreover, average monthly panel efficiencies of old panels are calculated as 8.22%, 7.85%, 5.35%, and 3.63% in the same order. Test results obtained from the experimental system are also statistically examined and discussed to analyze the performance of PV panels in terms of monthly panel efficiencies.


Introduction
Increasing energy demand and decreasing fossil-based energy sources accelerate the trend towards renewable energy worldwide. As reported by the International Energy Agency (IEA), total renewable energy-based potential in the world is expected to grow by 50% that corresponds to approximately 1220 gigawatts of increase between 2019 and 2024 [1]. In addition, it is estimated that about 25% and 57% of the renewable energy will be generated using wind tribunes and solar panels, respectively [1]. Hence, the trend and popularity of the renew-able energy are growing rapidly. Note that this study focuses on analyzing various types of photovoltaic (PV) panel technologies which are connected to the proposed measurement system and examining power generation performance of these technologies under varying daily weather parameters.
PV energy is one of the most important renewable energy sources with widespread electricity generation in different places such as solar farm and rooftop [2]. With the growth of renewable energy generation, it is important to provide a trade-off between payback period of renewable energy investment and lifetime of PV panels. According to the PV panel technologies, the payback period of investment is variable because power generation performance of PV panels is affected by two main factors which are (1) panel efficiency and (2) varying weather conditions. By considering these two main factors, the PV panel characteristic parameters must be verified for effective power generation. In order to achieve the best characteristic parameters of PV panels, manufacturers work on developing the PV panels and evaluate the peak power of them, within the scope of standard test conditions (STC) that are defined with (1) 25°C PV cell temperature, (2) 1000 W/m 2 irradiance, and (3) 1.5 air mass (AM1.5) [3]. However, it is highly likely that generated power of manufactured PV panels in STC environment is different than outdoor environment. In other words, the actual power generation of the evaluated PV panels can be achieved in STC environment but it cannot be obtained under real case conditions [4]. This difference may cause power loss in energy generation, daily, weekly, monthly, and yearly, due to the varying meteorological conditions in different locations. In order to decrease the power loss and increase PV panel efficiency, different materials have been used to produce and develop different types of PV panels. For instance, conventional crystalline silicon PV cells have doped junctions such as "emitters" or "back surface fields" [4]. Amorphous silicon PV cells are mostly made by chemically vaporized a thin coat of silicone, placing on a surface such as plastic, metal, or glass [5]. Namely, cadmium telluride PV cells are formed by cadmium telluride and cadmium sulphide p-n junction materials on a glass substrate [6]. Copper, gallium, indium, and selenide materials are deposited on a substrate layer by a sequence of chemical processes to form copper indium diselenide PV cells [7]. Polycrystalline, monocrystalline, thin film produced by CIS material, and thin film produced by CdTe material are widely used for renewable energy generation since these technologies provide several advantages in terms of price, energy efficiency, and lifetime [8,9].
Silicon is a leading material used in the production of monocrystalline, polycrystalline, and thin film modules, which corresponds to more than 90% of the annual solar cell production [10]. In monocrystalline silicon cells, each solar cell is made from a single source of silicon. Besides this, monocrystalline silicon cells are produced by cutting 300 μm thick cylindrical bars of monocrystalline silicon in special ovens [11]. On the other hand, polycrystalline silicon cells are made by combining melted pure silicone blocks [11]. Thin film cells are made from photovoltaic material placed between one or more thin layers, such as glass, plastic, or metal [12]. There are different types of thin film panels which are made by different materials such as CIS, CdTe, and amorphous Si photovoltaic materials. CIS-based thin film cells consist layers of ZnO, CdS, CIS, and Mo, which are organized on a glass, metal, or polymer substrate, whereas CdTe-based thin film modules are formed from transparent conductive oxide (TCO) layers such as SnO2 or Cd2SnO4 [12]. The electricity generation performance of these PV panels may differ according to the locations. For instance, monocrystalline and polycrystalline PV panels have been installed to generate electricity in Bursa, Turkey [13]. The results are obtained with one week experiment, and they show that monocrystalline PV panel generates slightly more electricity than polycrystalline. In [14], monocrystalline (2.35 kW), polycrystalline (2.64 kW), and amorphous silicon (2.40 kW) PV panels are analyzed in 2014, in Duzce (Turkey). Efficiency and generated power of PV panels are compared yearly. Solar radiation, ambient temperature, PV panel temperature, and wind measurements are recorded during the test study. Many other works have been conducted to evaluate performance of various PV panels in different locations such as Slovenia [15], Norway [16], Ireland [17], Morocco [18], Brazil [19], Nigeria [20], Jordan [21], Algeria [22], and Croatia [23]. To the best of our knowledge, this is the first work which assesses outdoor performance of four different types of new and 5-year-old PV panels. In order to achieve this, PV panels are installed in Ankara, Turkey, and it is important to select the most efficient PV panel technology to generate electricity efficiently in this location.
There must be a measurement system to measure both weather conditions and power generation capacity of PV panels. In order to measure the generated power of PV panels, various types of measurement systems have been designed and applied. For instance, Rezk et al. [24] have designed a measurement system based on the DAQ card (NI_USB-6009). In the work, a charge controller with battery unit is used as load, and multicrystalline silicon PV panel is used for test activities. Machacek et al. [25] have implemented a measuring unit (NI_PCI−6023E) and computerbased data acquisition model. A 100 W PV panel is used to perform its current, voltage, and power measurements. Solar radiation, PV panel temperature, and ambient temperature measurements are also measured. Hammoumi et al. [26] have performed a test study, where a PV panel is connected to a variable load and taking the measurements using current and voltage sensors which are connected to the system. Demirtas et al. [12] have demonstrated a long-term performance measurements of CIS and CdTe PV panels. In this work, measurement data includes direct current (DC) and alternating current (AC), voltage, power, and efficiency. Munoz-Garcia et al. [27] have performed a study to obtain I-V curves of CIS, a-Si, and CdTe PV panel technologies using electronic load, both indoor and outdoor conditions at STC. Digital thermometer and pyranometer are used to record external conditions. Indoor and outdoor performance of this measurement system has been investigated. Kivrak et al. [28] have designed a system to obtain the measurements of four same types of PV panels using MOSFET electronic load based on a 4-channel measurement printed circuit board (PCB), tested outdoor. Polverini et al. [29] have realized an experimental indoor study under STC, to test 70 pcs of polycrystalline silicon PV panel that remained about 20 years outdoor. The aforementioned measurement systems have several drawbacks and limitations such as (1) lack of availability of weather measurement sensors, (2) lack of capability of measuring generated power of many PV panels, (3) lack of accurate measuring power generation under varying weather conditions, (4) lack of sensitivity of measured power, and (5) using costly products and circuit boards.
To address the limitations and drawbacks of the other measurement systems, in this study, multi-photovoltaic panel measurement and analysis system (PPMAS) is designed to 2 International Journal of Photoenergy measure the different types of PV panels installed in Ankara, Turkey. Note that the data is collected for six months from the first of December 2019 until the end of May 2020. The overall system consists of three subsystems which are (1) PV panel measurement subsystem (PPMS), (2) meteorology measurement subsystem (MMS), and (3) data acquisition subsystem (DAS). In the PPMS, the P max (maximum power) parameter of PV panel is measured. Besides this, other important parameters of the panels are estimated using the measured P max . Moreover, a measurement maximum peak power (MPP) technique is applied in this subsystem to accurately measure the P max . The second subsystem includes different sensors which measure wind speed, wind direction, outdoor temperature, humidity, ambient light, and panel temperatures. In the last subsystem, the parameters which are measured in the other subsystems are stored in a database and displayed in a computer. The overall system is performed with four new and four 5-year-old PV panel technologies which are based on polycrystalline, monocrystalline, copper indium selenide (CIS), and cadmium telluride (CdTe). Thus, performance of power generation of new and old PV panels is experimentally and statistically investigated for the best PV panel evaluation and selection under varying weather conditions in Ankara, Turkey. The main objectives and contributions of this study are as follows: (1) Multi-photovoltaic panel measurement and analysis system (PPMAS) is designed

Designed Multi-Photovoltaic Panel Measurement and Analysis System
The PPMAS is a system to evaluate power generation capacities of up to eight PV panels in outdoor environment in Ankara, Turkey (see Figure 1). The overall system including eight PV panels and PPMAS is shown in Figure 2.
A block diagram of designed PPMAS is shown in Figure 3. The PPMAS includes three major subsystems, and their working principles are explained with details below.

Photovoltaic Panel Measurement Subsystem (PPMS).
This subsystem is used to obtain P max (maximum power) value of each PV panel in each 1000 milliseconds using MOSFET electronic load. The P max is an important parameter because it is used to evaluate the performances of used PV panels. Moreover, this parameter has also been used in many studies to assess the performance of PV panels [12-15, 19, 22, 30, 31]. Other important parameters of PV panels which are V mp (voltage at maximum power), I mp (current at maximum power), I sc (short circuit current), and V oc (open circuit voltage) are needed to be obtained or estimated. In order to estimate these parameters, the measured P max is used in equations (1) and (2) where FF is fill factor.
After modification of the equation (1), the maximum power P max is estimated using the new equation as follows: Note that, in the system, it is important to accurately obtain the P max because it is used to estimate the other parameters of PV panels such as efficiency. To achieve this, it is necessary to increase sensitivity of the PPMS. In order to increase the sensitivity of the PPMS, the measurement of maximum peak power (MPP) technique is applied in PPMS subsystem [27,28]. Furthermore, PPMS can measure P max of up to eight PV panels and estimate other parameters of these panels, separately.
Efficiency of a PV panel is evaluated by the ratio of amount of energy generated by the PV panel to the amount of solar radiation reflected in its surface area [14]. The efficiency is a useful metric that enables comparison of PV panels which have different nominal power outputs with different surface areas. Efficiency of a PV panel is expressed in percentage. In order to estimate efficiency of the PV panels, the following equation is used: where η daily represents the average daily PV panel efficiency in percentage (%), P daily is the daily average power output in Watt, H daily is the amount of daily solar radiation in Watt/m 2 , and PV area symbolizes the surface area of PV panel (m 2 ). Monthly average PV panel efficiency can be described in the equation (4) as follows, where t is number of days in a month: ) is an interface of microcontroller with relay and switching block, measurement block, and data transmitting block. PPMS has PV panel connection pins and one UART headers for data transmitting block interfacing. In this block, the microcontroller manages PPMS measurement activities. Relay and switching block contains 7 relays which are used to switch the PV panels, and these relays in this block are activated one by one using the microcontroller. Measurement block is used to measure the generated power of the PV panels using MOSFET electronic load (see Figure 5). ACS-712/30A is connected on the serial to drain the leg of the MOSFET to measure the current. R_6 and RV_2 voltage dividers are connected parallel to drain and source legs of the MOSFET to measure the voltage. Voltage (V sense ) and current (I sense ) measurement pins are connected to analog inputs of microcontroller (see Figure 5). Data transmitting block is used for communication purposes between PPMS and DAS computer reliably.
The microcontroller in the PPMS is connected to the seven relays, and each relay is connected to a different PV panel as shown in Figure 5. The purpose of using microcon-troller is to switch each relay to activate related PV panel in each 1000 milliseconds, sequentially. Thus, the generated power of the activated PV panel is measured using MOSFET. For instance, as depicted in Figure 5, the microcontroller activates the fourth relay which is connected to the 140 W monocrystal PV panel, and MOSFET is used to measure the generated power P max . After 1000 milliseconds, the fifth relay is switched by the microcontroller to measure the generated power of 150 W polycrystal PV panel and so on. The measured and estimated PV panel data parameters are transmitted to the DAS subsystem using serial communication continuously.

MOSFET as an Electronic Load and I-V
Characterization of a PV Panel. One of the most significant parameter of a PV panel is the I-V curve, which includes PV panel performance metrics such as efficiency, η; open circuit voltage, V oc ; short circuit current, I sc ; maximum power, P max ; voltage at maximum power point, V mp ; and current at maximum power, I mp [32]. In order to achieve the I-V curve of a PV panel, it is required to control the current generated by a PV panel from zero current point to short circuit point [33]. To control the current and to obtain the I-V curve of a PV panel, MOSFET can be used as an electronic load in the designed PPMS subsystem.
Drain and source of a MOSFET may behave as a variable resistor, when there is a voltage difference on gate and source pins, such that when gate and source voltage difference is high, drain and source behave as low resistance (can be assumed short circuit), and when the gate and source voltage difference is low, drain and source behave as high resistance (can be assumed open circuit) [34]. By utilizing this feature of MOS-FET, I-V characteristics of a PV panel can be extracted.
In   International Journal of Photoenergy (1000 milliseconds). During one measurement period, PPMS microcontroller observes the highest power point recorded in Watts, which is called P max . After finding P max , its components are recorded as I mp and V mp .

Meteorology Measurement Subsystem (MMS).
The main purpose of using MMS subsystem is to measure the meteorological parameters which are ambient temperature, humidity, ambient light, wind speed, wind direction, and PV panel temperatures. These parameters are significantly important because performance of power generation of PV panels is depending on these parameters [30,35,36]. In this subsystem, the parameters are measured at each second from 5:00 a.m. until 9:00 p.m. In order to achieve these measurements, the MMS consists four main blocks as shown in Figure 6. In the MMS PCB block, a microcontroller is employed to measure the meteorological parameters and interconnected with power supply block, sensor array block, and data transmitting block. For instance, the pin connections between MMS PCB block and power supply block are illustrated in

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International Journal of Photoenergy blocks can work without any electrical problem. In this subsystem, data transmitting block is used to transmit the measured meteorological data via connected cable to a data acquisition system (DAS) computer for storing data. Finally, the recorded data in the DAS computer is transmitted to the personal computer for analyzing the purposes when the user requests this data from the DAS computer.

Data Acquisition Subsystem (DAS).
The last subsystem of PPMAS system which is illustrated in Figure 8 manages the real-time data monitoring and data storage activities. In order to achieve this, both measured and estimated parameters in the PPMS and the MMS are sent to DAS computer via connected communication cables. In the last subsystem, the parameters which are measured in the other subsystems are stored in a database in DAS computer, and the recorded data can be transmitted remotely to a personal computer for analyzing and data displaying purposes continuously if a user requests data from DAS. Note that DAS computer has the capability to store and monitor all the parameters of all PV panels which are connected to the PPMS as well as the meteorological parameter measurements obtained in MMS. Besides, DAS has a storage capacity to record a total of 50 years of measurement data in continuous operation.
2.4. One-Cycle Operation of PPMS Subsystem. Measurement circuit of PPMS is depicted in Figure 9. As shown in Figure 5, a microcontroller in the PPMS selects the related relay combinations to activate mono-old PV connection to Hall effect sensor and then to drain MOSFET to measure current pass through in it when the switching signal comes to the gate of MOSFET by the 12-bit DAC signal. DAC signal is generated through the R-2R ladder circuit from the digital outputs of the microcontroller. DAC signal enters the optoisolator to switch 12 V to a totem-pole circuit. The totem-pole circuit is used to drive MOSFET in the linear region. The R_1 potentiometer is an adjustable resistor to adjust the threshold level in terms of voltage for the optoisolator. At the same time PV panel (mono-old PV) connection comes to (IP+) pin of Hall effect sensor (ACS-712/30A) serially (see Figure 5) which is used to calculate the current of (mono-old PV) PV panel. (IP-) pin of the Hall effect sensor goes to the MOSFET's drain pin, so that the current flows from the drain to source by using MOSFET as electronic load. The I sense output pin of the Hall effect sensor goes to the analog input of the microcontroller to measure the current of the PV panel (mono-old PV). Also the V sense pin goes to another analog input channel of teh microcontroller to measure the voltage of the PV panel (mono-old PV), where

Results and Discussion
In the scope of this experimental study, it aimed at analyzing and comparing the power generation performances of polycrystalline, monocrystalline, CIS, and CdTe PV panel pairs, within a specified time, under different weather conditions in Ankara, Turkey. Besides this, the generated power and temperatures of PV panels as well as atmospheric parameters are measured and stored using the developed system PPMAS. In [37], the inclination angles of different locations are examined and discussed, and for Ankara Province, it is about 30°. Therefore, in this study, the inclination angle of the PV panels is set to 30°and facing to the south to get sunlight better based on the location where the PV panels are installed. Furthermore, measurements obtained by PV panels

Calibration of MMS.
It is significantly important to calibrate the MMS subsystem to get the atmospheric measurements accurately. In order to achieve this, the vantage pro2 wireless meteorology station is used to calibrate MMS sensor measurements. The vantage pro2 wireless meteorology station which has ambient temperature, humidity, wind speed, and direction sensors is installed next to the MMS subsystem. The MMS subsystem is calibrated according to the measurements obtained using the vantage pro2 meteorology station. Thus, the atmospheric values are accurately measured after calibrating the MMS subsystem. Moreover, the Max44009 sensor is used in the MMS to estimate solar radiation, where it is installed to get the illuminance of the sun. Note that the angle of the sensor and PV panels is the same, and the sensor is calibrated using Cem DT-1301 Luxmeter. The measurement unit is in Lux. In order to convert Lux to W/m 2 , the measured value is multiplied by 0.00833 [38].

Calibration of PPMS.
Another important subsystem in the PPMAS system is PPMS subsystem which is needed to be calibrated to measure the generated power of PV panels accurately. To achieve this, Tektronix MSO 2024B oscilloscope, Fluke-289 multimeter, and Tt-Technic RXN305D2 power supply with 0-30 V 0-5A are used to calibrate the PPMS subsystem. As a result, the calibrated PPMS subsystem can measure the generated power of PV panels accurately.

PV Panels.
In this study, new and 5-year-old PV panel technologies have been used, and the characteristics of these PV panels are tabulated in Table 1. Thus, in this experiment, totally eight PV panels are used which are new and 5-yearold monocrystalline with 140 W, new and 5-year-old CIS with 125 W, new and 5-year-old polycrystalline with 150 W, and new and 5-year-old CdTe with 60 W. In order to supply power requirement of the experimental system, a solar power station is used. Power station consists one 270 W polycrystalline PV panel, two 12 V/60A batteries as shown in Figure 2. Thus, the PPMAS works properly under insufficient sunlight situations (i.e., cloudy day) up to 3 days.

Climate Data in Ankara
Province. The climate data between the years of 1927 and 2019 in Ankara Province, Turkey, is presented in Table 2 [39]. According to the climate data, the highest average temperature is obtained in July and August with 23.3°C. Moreover, the greatest average maximum and the least minimum temperatures between 1927 and 2019 are estimated with 30.4°C in August and with 0.1°C in January, respectively. The longest and shortest average sunshine durations are 11.3 hours in July and 2.5 hours in December, respectively. Furthermore, the highest amount of average monthly rainfall is observed 44.9 mm in December.

Solar
Radiation. Solar radiation is significantly important parameter which is used to calculate the efficiencies of the PV panels. Figure 10 shows weekly average results of solar radiation. This data is obtained using the MMS sensor box in the 9 International Journal of Photoenergy designed PPMAS system for six months in Ankara, Turkey. In Figure 10, the first week average result is obtained between 1st of December 2019 and 7th of December 2019, and the week 26 represents the date of 30st of May 2020. According to the results, the least and highest average solar radiation results are estimated with 1968 Wm 2 in week 4 and 9459 Wm 2 in week 24, respectively. As a result, these estimations clearly demonstrate that the lowest solar radiation is obtained in December 2019, and the highest solar radiation is obtained in May 2020. There can be different reasons to get variety of weekly solar radiation results such as weather conditions, angle of sunlight, and location of Ankara [40].

Effect of Atmospheric Variations on PV Panel
Temperatures. Figure 11 depicts the average weekly atmospheric temperature, wind speed, and PV panel temperatures for six months between December 2019 and May 2020. These results show that PV panel temperature depends on ambient temperature. For instance, in week 15, the average weekly temperature is 13.05°C, and the average of PV panel temperatures is increased by 1.25°C which becomes 14.30°C, and in week 4, the average weekly temperature is 4.50°C, and the average of PV panel temperatures is 5.73°C. Moreover, based on the six-month measurement, the estimated average temperature is 7.88°C, and the average of PV panel temperatures is 9.06°C. According to the results in Figure 11, the least and highest average temperatures are measured with -1.78°C in week 8 and with 21.60°C in week 24, respectively. Moreover, these results clarify that May is the month with the highest daily average temperature. However, January is the lowest daily average temperature. Furthermore, wind speed is low and nearly constant all the months between December and May in Ankara. The average wind speed of these six months is 2.93 m/s which indicates that Ankara is not a windy city.

Performance Assessment of Different PV Panels for Power
Generation. Four different PV panel technologies which are polycrystalline-based, monocrystalline-based, CIS-based, and CdTe-based have been used for performance assessment. Besides this, each PV technology has one new and one 5-yearold PV panel. Thus, the performance of new PV panels and 5-year-old PV panels is evaluated. The generated power of each PV panel is measured within a specified time period under the same outdoor conditions in Ankara. The measured powers of eight PV panels are stored in the designed PPMAS system. Figure 12 illustrates weekly average power generated by new and 5-year-old PV panels between December 2019 and May 2020. Based on these results, all PV panels generate maximum power between 9.05.2020 and 16.05.2020 and the lowest power between 21.02.2020 and 28.02.2020. Besides this, these results show that the new PV panel technologies provide better power generation performance than the 5year-old PV panel technologies in all the weeks. For instance, between 14.12.2019 and 21.12.2019, mono-new and monoold generate daily average of 327 Wh and 316 Wh, respectively, and CIS-new generates a daily average of 15 Wh more than CIS-old. In terms of power generation performance, mono-new provides 3.10% better performance than monoold, poly-new provides 3.34% better performance than poly-old, CIS-new provides 5.53% better performance than CIS-old, and CdTe-new provides 6.97% better performance than CdTe-old. These results indicate that CdTe PV panel is degraded more than the other PV panel technologies. There can be many factors which may affect the performance of PV panels, and these factors are discussed in [31]. Figures 13(a) and 13(b) depict monthly panel efficiencies of new and 5-year-old PV panels, respectively. Note that monthly efficiencies of PV panels are calculated according to the formula given in equation (4). The results in Figure 13

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International Journal of Photoenergy and the CdTe-new is the worst PV panel in terms of monthly panel efficiency estimation in all the months. Figure 13(b) depicts monthly panel efficiency of 5-yearold PV panels between December 2019 and May 2020. According to the results, 5-year-old monocrystalline (mono-old) and 5-year-old polycrystalline (poly-old) PV panels have the best panel efficiencies in all the months as compared to the other old PV panel technologies. Besides this, 5-year-old CIS (CIS-old) and 5-year-old CdTe PV panels (CdTe-old) have the less panel efficiencies in all the months. Moreover, mono-old, poly-old, and CIS-old PV panels have the highest efficiencies in February whereas the CdTe-old PV panel has the maximum efficiency in December. March is the month that all 5-year-old PV panels have the lowest efficiencies. The average 6-month efficiencies of 5-year-old PV panels are 8.22%, 7.85%, 5.35%, and 3.63% for mono-old, poly-old, CIS-old, and CdTe-old, respectively. Among all 5-year-old PV panels, the mono-old PV panel has the best efficiency estimations in all the months. Furthermore, the average 6-month efficiency of mono-old PV panel is greater than the average 6-month efficiency of poly-new, CIS-new, and CdTe-new PV panels.
Consequently, the results in Figures 13(a) and 13(b) indicate that mono-new and mono-old PV panels are the best panel technologies among all the new and 5-year-old PV panels in terms of monthly panel efficiency, respectively. Even though mono-old PV panel has some degradation in time, it has better average 6-month efficiency than some of new PV panels. Table 3 illustrates monthly average of humidity between December 2019 and May 2020 in Ankara. According to the results, the highest humidity is measured in December with 83.6%, and the lowest humidity is measured in May with 55.1% in Ankara. Humidity affects electricity generating performance of PV panels. For instance, very small water drops on PV panels can reflect the sunlight away from PV panels, and this reduces producing power because less sunlight hits the surface of PV panels. According to Table 3, the least efficiencies of old and new PV panels are obtained in March when the humidity is high. On the other hand, the highest efficiencies of new and old PV panels are achieved in February when the humidity is low. Note that, even though humidity is an important factor which affects the performance of PV panels, there are other important factors that affect the performance of PV panels such as temperature, wind, and solar radiation.

Performance Analysis of PV Panels and Photovoltaic
Panel Measurement System Using Nonparametric Tests. The purpose of using the nonparametric tests is to understand and analyze the performance, stability, and reliability of the PV panels. The main advantage of using the nonparametric tests is that they can handle probabilistic and nonprobabilistic measurements obtained from the PV panels without any limitation.
This section presents and examines different test results obtained using various nonparametric test measurement approaches. In order to achieve the test results, three different nonparametric tests which are Friedman, Friedman Aligned, and Quade are applied to the efficiency values of eight PV panels as shown in Figures 13(a) and 13(b). The aim of applying Friedman, Friedman Aligned, and Quade nonparametric tests is to understand statistically whether there is any difference or similarity between the PV panels used over the given set of data. Note that these tests compute the ranks of the performance of the PV panels, i.e., the best performing PV panel receives the rank of 1, the second best rank 2, etc. The mathematical background and further details of the nonparametric methods can be obtained in [41][42][43][44]. In addition, more statistical experimental results based on Iman-Davenport, Bonferroni-Dunn, Holm, Hochberg, Holland, Rom, Finner, Li, Shaffer, and Bergamnn-Hommel tests as well as adjusted p value has been computed using the publicly available JAVA software 1 [41 -44]. Table 4 shows the average ranking results estimated using the Friedman, the Friedman Aligned, and the Quade nonparametric tests. According to the results, mono-new PV panel is the best performing panel, with the average rank of 1.714, 3.206, and 1.121 for the Friedman, Friedman Aligned, and Quade tests, respectively. Moreover, the p values have been calculated using the statistics of each of the tests applied (23.163, 5.148, and 15.613). Furthermore, the Iman Davenport statistic and p value are computed 12:538 × 10 −7 , 0.4021, and 7:128 × 10 −9 , respectively. Furthermore, the poly-new performs very close to the mono-new with the second ranking place comparing to the others. The CdTeold gives the worst performance with the last ranking place  In the next statistical analysis, multiple comparison post hoc procedures (eight) are applied to evaluate the control of mono-new with the rest of PV panels. The results are illustrated by computing p values for each comparison. Tables 5-7 depict the p value results using the rankings computed by the Friedman, Friedman Aligned, and Quade tests, respectively [41][42][43][44]. These results verify the significant improvement of the usage of the mono-new PV panel over the other PV panels for all the post hoc procedures considered. Moreover, Li's statistical technique reaches the lowest p values in the comparisons. Therefore, it is the most certain one among the others in order to reach the clearest conclusion.     in Cyprus, India, and Poland are estimated based on the data which were collected annually. Therefore, efficiencies of mono and poly PV panels are higher than the efficiencies of mono and poly PV panels in Ankara. The main reason of getting lower efficiency of mono in Ankara is that the data was collected in winter and spring periods, and the efficiencies  15 International Journal of Photoenergy of mono and poly PV panels are with 8.46% and 8.11% for Ankara Province, Turkey, and it is important to emphasize that the efficiencies of the PV panels will be much higher when the summer data is taken into consideration. Furthermore, in other researches, only new PV panels were used for performance comparison. In our study, new and old PV panels have been used for performance assessment and analyses.

Conclusions and Future Work
This paper presents two different studies which are (1) a designed photovoltaic panel measurement and analysis system (PPMAS) and (2) a detailed performance analysis and comparison of four new and four 5-year-old PV panels which are based on polycrystalline, monocrystalline, CIS, and CdTe. The designed PPMAS system consists three different subsystems. In order to accurately obtain the atmospheric measurements and power generation of PV panels, the subsystems are calibrated. The designed PPMAS system with PV panels is installed in Yildirim Beyazit University Campus, Ankara, Turkey, and performed for six months between December 2019 and May 2020. According to the results, PPMAS system performs well to obtain measurements which are ambient temperature, wind speed, and panel temperature as well as the amount of power generation of PV panel. Besides this, eight PV panels, in which four of them are new and other four are 5-year-old PV panels, have been tested under the same weather conditions for 6 months. According to the results, February is the best month that mono, poly, and CIS have the best efficiency measurements whereas CdTe has the best measurement in December. Besides this, CdTe-based PV panel technology has the less efficiency measurements in all the months. Test results obtained from the experimental system are also statistically examined and discussed to analyze the performance of PV panels in terms of monthly panel efficiency.
The current version of the designed PPMAS can measure generated power of up to eight PV panels. As a future work, this system will be modified to measure generated power of more than eight PV panel technologies. Moreover, performance of different PV panel technologies in different locations of Turkey will be assessed and analyzed to understand efficiencies of PV panels in different locations in Turkey. In this work, we discussed the performance of PV panel technologies produced by monocrystalline, polycrystalline, CIS, and CdTe. Other PV panel technologies such as amorphous silicon (a-Si) and gallium arsenide-(GaAs-) based PV panels will also be used for performance assessment. The current PPMAS system can work three days under insufficient sun conditions with the help of its battery system. As a future work, the capacity of battery will be increased; thus, the system can work up to seven days under insufficient sun conditions.

Data Availability
The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
The authors declare that they have no conflicts of interest.