Researchers from the Netherlands' Delft University of Technology have sought to embed power electronics in photovoltaic modules with the aim of increasing the shade tolerance of panels via maximum power point tracking (MPPT).
Their work consisted of conducting a feasibility study on whether copper planar air-core inductors can yield the required inductor properties to support sub-module power conversion in PV modules. They claim that implementing MPPT at the sub-module level can ensure that shading on a few cells or parts of the module has minimal impact on the overall performance.
“A common approach to enhance the inductance of an inductor involves using a ferromagnetic core made of material with large magnetic permeability. This study, however, investigates planar inductors without such a core,” the researchers explained. “Since the common PV-module materials all have a relative magnetic permeability close to one – similar to the magnetic permeability of vacuum and air – this design is referred to as air-core inductors.”
They investigated, in particular, how the planar air-core inductor design can be adjusted to achieve the desired inductor performance and evaluate the feasibility of integrating these inductors into a sub-module-level power converter.
The simulated configurations use copper as the inductor's conductor material, with an area fixed at 12.5 cm × 12.5 cm, which is the maximum available area for a 12.7 cm x 12.7 cm solar cell. The inner terminal of the inductor is connected to the metal back-side of a PV cell, while the outer terminal can be connected to an external circuit. The inductor is separated from the PV cell by an electrically insulating layer.
The inductors' performance was simulated using the finite element method simulator COMSOL with various coil design parameters. Namely, they were tested with a different internal diameter or middle gap, track spacing, track width, and thickness. With those size variations, the inductors' AC inductance and AC resistance were measured.
“It was shown that a minimum track spacing of a few millimeters is necessary to limit the proximity effect at frequencies of 50 kHz and above,” the group stated. “Additionally, it was observed that increasing the middle gap size or the number of turns can enhance the inductance, though both approaches come with the drawback of increased inductor resistance. The coil geometries that were simulated yield inductance values between 0.3 ??H and 3.2 ??H.”
Following those simulations, the feasibility of implementing these inductors into an exemplary DC-DC boost converter was evaluated.
The group found that to reduce the current ripple from a solar-cell string with such inductance values, a significant switching frequency of at least several hundred kHz is needed. “Moreover, at 500 kHz, an inductor thickness of around 0.5 mm is necessary to keep the ohmic losses in the inductor below 2% of the total generated power in standard test conditions,” it said.
The system reportedly demonstrated feasible combinations, though with significant challenges for the sub-module integration. “Although the focus of this study is not on manufacturing, it must be noted that the required inductor thickness range surpasses the capabilities of screen printing, the primary technique employed by the PV industry for solar cell metallization,” the team highlighted.
Their findings were presented in “Feasibility study on photovoltaic module-integrated planar air-core inductors to facilitate embedded power electronics,” published in Energy Reports.
