I am currently a PhD student at UC Berkeley, following a 6-year journey working at Apple after my undergrad years at Cornell University. I am a 2025 Paul & Daisy Soros fellow. I grew up in Dhaka, Bangladesh where my interest in electronics was cultivated, resulting in the creation of this blog.
My introduction to working with solar panels goes back a long time. In fact, in 2012, I posted this article when I was working on some charge controllers and inverters. One consistent challenge when doing such work is always having a consistent reliable PV panel outdoors in bright (ideally not-too-hot) sunlight. This becomes difficult with varying weather conditions and limiting development.
Recently I learned about how you can very easily use an external power supply (capable of operating in constant current mode) to use the PV panel indoors and emulate its outdoor behavior! This ends up being great for testing charge controllers and MPPT algorithms!
Fig. 1 - Setup for characterizing PV panel behavior indoors Panel: SLP010-12U Power supply: Rigol DP832 Eload: Rigol DL3031
Fig. 1 shows the test setup used for obtaining PV panel IV curves indoors. The only equipment required for emulating the PV panel itself is the power supply. The electronic load is used to vary the voltage imposed on the panel to obtain the IV curve. You can even see the panel facing down so that light is not incident onto it!
Fig. 2 - Simple PV panel model
We can start by recognizing that a PV panel may be simply modeled as shown in Fig. 2. Iph represents the photocurrent generated by the panel under sunlight. When the panel is indoors, Iph is a miniscule value under typical indoor lighting conditions. What we can do is, instead, use an external power supply to bias the panel with a constant current source. This is represented by Iext in Fig. 3.
Fig. 3 - External power supply used to emulate photocurrent in PV panel
This is as simple as configuring the power supply's voltage at (or higher than) the panel's open-circuit voltage (Voc), and setting the current limit at the panel's short-circuit current (Isc). For the panel I picked, these correspond to 21.6V and 0.68A (at 100% luminosity).
The short-circuit current limit for the PSU can be varied to provide Iext emulating varying luminosities! For example, using 50% of the panel's short-circuit current, a condition of approximately 50% luminosity may be emulated!
This may be taken even further by adding a series voltage source to emulate voltage shifts in the IV curve.
Tests with 10W panel:
To generate the IV curves in a repeatable fashion, I wrote a Python script that imposes different panel voltages by issuing the corresponding SCPI commands to the electronic load. The curve is then plotted in real-time, as shown in Vid. 1 below. The GUI was built by modifying the one from a previous blog post: SmartSinePy, and an example of GUI development with PySide6 ~ Tahmid's blog
Vid. 1 - Characterizing panel IV curve at 100% emulated luminosity
To illustrate the operation at 50% emulated luminosity, the power supply current is set to 0.34A instead of 0.68A. The corresponding curve is generated as shown in Vid. 2.
Vid. 2 - Characterizing panel IV curve at 50% emulated luminosity
These obtained curves are also shown below in Fig. 4 and Fig. 5.
Fig. 4 - Obtained IV curve at 100% emulated luminosity
Fig. 5 - Obtained IV curves at 100% (top, same as Fig. 4) and 50% (bottom) emulated luminosities
Fig. 6 - IV curves provided in the panel datasheet
Fig. 6 shows the IV curves of the panel from its datasheet, showing close agreement between the curves obtained indoors with the 25°C curve! Slight tweaking of the open-circuit voltage and short-circuit current to match the 25°C curve can give slightly better match!
More significantly, this now allows testing with the PV panel indoors without worrying about weather conditions! Of course, all final tests for any charge controller or MPPT algorithm should be run outdoors with the panel under varying weather conditions, but this emulation method can significantly improve development time. I will soon share a demonstration of an MPPT controller developed using this setup!
Characterization of a 100W panel:
The same technique can be used for different panels and power levels. Fig. 6 shows one such IV curve obtained using the techniques outlined here but with a more capable power supply and electronic load. The power supply was set to 24.3V (panel open-circuit voltage) and 5.21A (panel short-circuit current at 100% luminosity).
Fig. 6 - Obtained IV curve for a 100W panel
This simple technique can significantly improve development and testing times for projects using PV panels! For more details about this emulation technique and further enhancements, refer to the following papers which provide much greater detail:
1. S. Qin, K. A. Kim and R. C. N. Pilawa-Podgurski, "Laboratory emulation of a photovoltaic module for controllable insolation and realistic dynamic performance," 2013 IEEE Power and Energy Conference at Illinois (PECI), Urbana, IL, USA, 2013, pp. 23-29, doi: 10.1109/PECI.2013.6506029. https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6506029
2. T. -L. Huang, F. S. Bagci and K. A. Kim, "Indoor Panel-Based Photovoltaic Emulation Method Implementation and Evaluation," 2024 IEEE Workshop on Control and Modeling for Power Electronics (COMPEL), Lahore, Pakistan, 2024, pp. 1-7, doi: 10.1109/COMPEL57542.2024.10613957. https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=10613957
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