G. Santamaria, A. Jathar, M. Simon
RockeTruck, Inc.,
United States
Keywords: gallium nitride, inverter, converter, power electronics, energy conversion
Summary:
RockeTruck has developed an Advanced Fuel Cell Power Converter (AFCPC) to convert direct current (DC) power output from fuel cells into alternating current (AC), using a modular design that can support power levels ranging from 10 kilowatts (kW) to hundreds of kW. The key new technology incorporated into the AFCPC is the use of gallium nitride (GaN) wide bandgap semiconductors, which provide higher electron mobility than silicon-based transistors, enable higher switching frequencies, and improve the electrical and thermal efficiency of the power conversion process. This in turn enables the design of a power converter with smaller transistor structures, filter components, and heat sinks, reducing the overall size, weight, and cost of the converter. To validate these advantages, RockeTruck has designed a power converter rated at 42 kW using GaN-based half-bridge power modules. The first prototype of this new converter, designed to convert 350-440 volt DC power from a fuel cell to 120/208 volt three-phase AC power, is less than half the volumetric size and weight of the smallest commercially available inverter RockeTruck was able to find to perform a comparable function. RockeTruck’s innovative approach to developing AFCPC control software is to develop model-based controls using Matlab and Simulink, which will simplify the task of optimizing AFCPC power controls for different applications. Using this design approach, the AFCPC will not be limited to fuel cell applications, and will be inherently capable of converting power from other DC power sources such as solar photovoltaic power and battery energy storage systems. It is also a bidirectional device which can support charging of batteries from AC power sources such as the grid. One particular challenge addressed by RockeTruck’s controls is accommodating the high switching frequency of GaN semiconductors (up to 100 kHz), which requires execution of the control algorithm up to 25 times faster than a typical silicon-based inverter controller. The basic functionality of this new AFCPC device is in the process of being verified through laboratory bench testing. By mid-2025, it is expected that the AFCPC will be integrated into a Mobile Fuel Cell Generator (MFCG) being developed by RockeTruck in parallel and field tested on the MFCG to demonstrate its power conversion capabilities under real-world operating conditions. The modular design of the AFCPC is intended to enable as many as ten such devices to be connected in parallel, which would support power conversion at levels up to 420 kW. RockeTruck is also developing a variant of the AFCPC capable of delivering 480 volt three-phase AC power, using an innovative “flying capacitor” topology to help overcome the natural voltage limitations of GaN. Eventually, RockeTruck intends to develop a high-power variable frequency traction inverter based on the AFCPC design. Sandia National Laboratories and Stony Brook University are providing technical support for RockeTruck’s development of GaN-based power converters, with financial support from the U.S. Department of Energy (DOE) and Southern California Gas Company.