Goal & Rationale:

The goal of Heliosphere is to provide a reliable characterization environment to test individual solar cells. For characterization, a reliable environment is defined by the ability to hold a stable power irradiance (typically 1 Sun or 1 kW/m²).

The rationale behind this project is to help provide important data to fuel decision making and/or help our TrackSim team develop accurate simulations of our array. Knowing individual cell IV curves could also help develop custom algorithms to be run on our MPPTs (current R&D project).

Implementation:

Electrical:

To achieve a stable power irradiance, a closed-loop control mechanism featuring an irradiance sensor and PWM-controlled LED is proposed.

Driving an LED with a constant current is a necessary but insufficient condition for achieving constant irradiance at a solar cell. While LED radiant output is approximately proportional to drive current, the proportionality is not fixed and varies with operating conditions.

LED optical output depends on junction temperature, which changes during operation due to self-heating and ambient thermal conditions. Consequently, constant current does not imply constant photon flux at the device under test.

In addition, optical coupling factors, such as LED aging, mechanical alignment, diffuser temperature, and reflective losses, affect the irradiance at the solar cell independently of LED current. These effects introduce slow drift that is not observable through electrical control alone.

For these reasons, a closed-loop control system is required to achieve the goal of this project. An irradiance sensor would be integrated to provide irradiance data feedback to an MCU, which would then control the LED appropriately.

Mechanical

Mechanical integration is an important detail to ensure uniform, repeatable illumination and stable thermal conditions at the device under test. Illumination is provided by a small symmetric array of high-power LEDs attached to a common aluminum heatsink with forced airflow to maintain stable optical output and minimize spectral drift.

The LED array is positioned at a fixed distance of 10 cm above the solar cell, with a diffuser placed between the LEDs and the cell to improve spatial uniformity and reduce local hot spots. LED temperature is monitored using a temperature sensor mounted directly.

The solar cell is mounted on a flat, thermally conductive baseplate to ensure good thermal coupling and repeatable positioning, with one RTD placed directly beneath the active area of the cell to accurately capture cell temperature. An irradiance sensor is located in the same plane as the solar cell and exposed to the same optical path (adjacent), ensuring that measured irradiance accurately represents the illumination experienced by the device under test.

The entire assembly is (ideally) enclosed in a light-tight housing with matte black interior surfaces to suppress reflections and isolate the system from ambient light, with all critical distances mechanically fixed to preserve optical geometry across measurements.

Parts Selection:

LED: 5000K LED COB CRI≥90 High Lumen LED Diode Bridgelux Chip 10W 20W 30W 40W 50W 19*19mm LED Spotlight Downlight repair parts - AliExpress 39 (not on approved vendor’s list. Ideally find another cheap one)

Irradiance Sensor: same as Black Body