Fuel cell systems based on single conversion stage inverters with a battery back-up energy storage unit

Abstract

Fuel cells (FCs) are a promising alternative energy solution and a reliable technology for niche applications providing high conversion efficiency. FCs are generally classified according to the type of the electrolyte. Proton exchange membrane FCs (PEMFCs) combine the hydrogen fuel with the oxygen from the atmosphere to produce water, heat (at temperature up to 90°C) and electricity. The major advantages of the PEMFCs include: being environmentally friendly, offering high efficiency and low operating temperature while providing increased safety. When low-voltage unregulated FC output is conditioned to generate AC power, two stages are generally required: a boost stage and an inversion one. However, due to the cascaded power stage conventional two-stage FC systems have unavoidable drawbacks such as being bulky, inefficient and having a high cost. Moreover, FC systems require an auxiliary unit to improve efficiency, lifetime and safety by cancelling the ripple current and supporting its slow dynamics. In this thesis, single-stage FC systems are reported to overcome the disadvantages of the conventional FC systems caused by cascaded power stages. The proposed FC systems provide high power conversion efficiency, low-cost and compactness for standalone and grid connected applications in single- and three-phase systems. Analysis, simulation and experimental results taken from 1kW laboratory prototypes operating at 20 kHz are presented to validate the performance of the proposed FC systems. Single conversion stage FC systems based on the boost-inverter and buck-boost inverter with a back-up energy storage unit have been developed in this research as follows: • A single power stage FC system based on a boost-inverter with a battery back-up unit • A grid-connected FC system based on a boost-inverter with a battery back-up unit • A single conversion stage FC system based on a buck-boost inverter with a battery back-up unit • A single conversion stage three-phase FC system based on a boost-inverter with a battery back-up unit • Design and implementation of digital control as implemented in an FC energy system The proposed FC systems has achieved satisfactory performance for delivering boosting and inversion functions within the single power conversion offering high efficiency and compactness. Moreover, an integrated battery back-up energy storage unit has also performed two functions: providing the ramping operation to deal with the slow dynamics of the FC and eliminating the ripple current to increase the efficiency and life time of the FC.