# Indoor PV Modeling Based on the One-Diode Model
## 2 Methodology
### Experimental and Validation Procedures  
The performance of PV panel models has been researched extensively, with accuracy evaluated against the available datasheet values under standard test conditions (STCs) corresponding to 1.5 air mass, 1000 W/m\({}^{2}\) irradiance, and a cell temperature of 25 \({}^{\circ}\)C. However, indoor light is primarily reliant on artificial light sources, and it is frequently coupled with natural light. The indoor environment's light intensity is often less than 10 W/m\({}^{2}\), which is significantly lower than in the outdoor environment [38]. Nevertheless, there are currently no defined standards for the indoor environment. This is because theindoor environment is described as having less temperature volatility, much lower light intensity, and a greater spectral range [39]. Therefore, this study is focused on small PV panels that consist of either one or multiple solar cells. Experiments are carried out in an indoor environment at a room temperature of 24 \({}^{\circ}\)C, with regulated lighting, to validate the proposed model's accuracy. This is because detailed I-V curves under indoor conditions are generally not provided by panel manufacturers. To simulate indoor lighting conditions, two types of PHILIPS lights (7 W LED and 12 W CFL) are used [40; 41]. These lights have different spectrum intensities, which can influence the amount of light energy harvested by the PV panel. A STELLARNET Black-Comet-SR concave grating spectrometer [42; 43] is used to measure the spectrum. The spectrometer is capable of executing high-quality spectral analysis in the ultraviolet-visible-near-infrared (UV-VIS-NIR) wavelength range of 200-1080 nm. Moreover, it provides uniform resolution across the spectral spectrum, as the concave grating forms a flat field on the charge-coupled device (CCD) detector. To ensure the accuracy of the light spectrum, when the spectrometer is turned on, a calibration file provided by STELLARNET is used to calibrate the device. Additionally, a KIMO LX200 lux meter is placed alongside the spectrometer to verify its measurement accuracy. The lux meter is used in this experiment to measure the illuminance level from 0 to 1000 lux with a resolution as low as 0.1 lux. The spectrometer receptor's location is fixed beneath the light bulb in an enclosed box, preventing ambient light from penetrating, as shown in Figure 4. To ensure that the light source illuminates the measuring devices directly, a peep hole is included on the side of the box to allow the user to periodically check the setup, as shown in Figure 5. A schematic diagram of the experiment is shown in Figure 6.  
Figure 4: Light source directly illuminating the spectrometer and lux meter. Figure 5: Experimental setup in a confined box. The light texture is placed directly on top of the PV panel at a 90\({}^{\circ}\) angle, as shown in Figure 4, to obtain maximum performance. It is expected that if the angle is changed, the performance of the PV panels will be decreased [44]. A detailed investigation on how the angle of the light fixtures affects the performance of PV panels is an interesting direction that will be considered in future work. The light spectra of the CFL and LED are shown in Figures 7 and 8, respectively, in graphs generated using STELLAR SpectraWiz software. The CFL spectrum is observed to have a narrower bandwidth and spikes at specific wavelengths, whereas the LED spectrum has a smoother distribution. The illuminance value presented in the upper-left corner can be derived by calculating the area under the curve within the wavelength range of 390-830 nm. This range is also referred to as the CIE 2008 photopic luminous efficiency function (\(E(\lambda)\)), which represents the average spectral sensitivity of human vision to light [45]. Thus, effective conversion would enable the use of a spectrometer to evaluate photovoltaic performance under low-illuminance conditions.