# Thermal Effects on Photovoltaic Array Performance: Experimentation, Modeling, and Simulation
## 5 Results  
### Experimental Results  
This section presents the experimental results for different environmental conditions and how these conditions affect PV array performance. Wind speed, temperature, solar irradiance, and PV production data were sampled in five-minute increments during the months in which there was no snow coverage on the array. Data were collected and analyzed from June 2018 to October 2018 and from March 2019 to November 2019 with the exception of April 2019 and July 2019 due to missing data for both months. Hourly averages were computed each day from 5 a.m. to 6 p.m., excluding the early mornings and evening hours when uncertainty in solar irradiance is high. The histogram shown in Figure 4 displays the frequency distribution of wind speeds in which wind data were collected. It can be seen that during this period, most of the wind speeds fall between 1.5 and 4.5 m/s. Figure 5 illustrates the histogram for temperature in the months of interest. As we are focused on the months without snowfall and from dawn to dusk, recorded temperatures lie between 15 and 35 \({}^{\circ}\)C. It is well known that ambient temperature has a significant effect on array temperature, which in turn affects PV performance, and the data collected in this work are used to quantify the effects of array efficiency and ambient temperatures. Figure 4: Wind speed histogram. Figure 3: Mesh (Mesh 3) used in this work. Figure 6 shows the electrical conversion efficiency vs. array temperature. Efficiency is determined using measured AC power outputs and plane of array irradiance. In controlled laboratory experiments, PV efficiency decreases linearly with temperature. Every point in Figure 6a shows the hourly average value for efficiency and temperature during the period of interest. There is a clear general trend that, as array temperature decreases, the efficiency increases. However, as the temperature drops below 10 \({}^{\circ}\)C, we noticed a significant drop in efficiency; this is due to weather events such as rain. We then divided the level of solar irradiance into three levels, low (Figure 6b, 100 to 399 W/m\({}^{2}\)), medium (Figure 6c, 400 to 699 W/m\({}^{2}\)), and high (Figure 6d, 700 to 1100 W/m\({}^{2}\)). This grouping allows us to isolate sunny weather in order to study the effects of wind on power production. In all three groupings, we observe near-linear trends between array temperature and efficiency. The red line is the efficiency line provided by the manufacturer, using an efficiency of 15.23% at an ambient temperature of 25 \({}^{\circ}\)C, using \(\beta=-0.43\%/^{\circ}\)C. However, as shown in Figure 6d, efficiency reported by the manufacturer does not match closely with data measured in the field. It is noteworthy that measuring AC output power encompasses a variety of system losses in addition to temperature effects that will reduce power output. These include AC and DC wiring resistive losses, power mismatch between modules and strings, inverter conversion efficiency loss from DC to AC (dependent on array operating power point), partial shading, dust accumulation, and reflection from the glass. Furthermore, DC to AC inverter conversion efficiency depends on the DC voltage and AC output power. Depending on light levels seen by the array, conversion efficiency losses will change. Under low-light conditions, conversion losses will be larger. However, since we are focused on higher solar irradiance levels, we expect a CEC (California Energy Commission) weighted efficiency of approximately 96%. Thus, the trends reported are expected to be consistent with a small offset due to conversion losses. Figure 5: Ambient temperature histogram. The temperature difference between array temperature and ambient has been reported as a function of wind speed, wind direction, solar irradiance, and ambient temperature [21]. Figure 7 presents the temperature difference between the array and ambient air as a function of wind speed.