Development of a Thermally Coupled Hydrogen Storage and Fuel Cell System

ANDREASEN GUSTAVO; RAMOS SILVINA GABRIELA; PERETTI HERNÁN; TRIACA WALTER

Buenos Aires

Congreso; 20th Topical Meeting of the International Society of Electrochemistry; 2017

ISE

A thermally coupled hydrogen storage and fuel cell system, where the heat produced during the H2/air PEM fuel cell operation is used to enhance the release of hydrogen from a metal hydride container, is presented. The design involves a vertical arrangement of the hydrogen storage device and the use of the fuel cell exhaust air as the heat transfer fluid instead of water-circulating loops that are used in most of the reported thermally coupled system, which require additional energy for water pumps, electrovalves and control systems. The hydrogen storage device contains a hydride-forming AB5-type metal alloy (MmNi4.7Al0.3, where Mm stands for ?mischmetal?, a mix of rare earths). The metal alloy absorbs hydrogen forming AB5H6 hydride, which has an equilibrium pressure that makes it suitable for both feeding a H2/air PEM fuel cell and being charged directly from a low pressure water electrolyzer without the need of additional compression.The metal hydride container is made of 304L stainless steel and has a cylindrical shape. It is provided with aluminum extended surfaces to enhance heat exchange with the surrounding medium. These surfaces consist of internal disk-shaped metal foil and external axial fins. A H2/air PEM fuel cell stack (FCgen®-1020ACS fuel cell stack, Ballard Power Systems Inc.) is used. The coolant and oxidant requirements are met with a fan that moves air through the stack. The air is used to cool the stack as well as to provide oxidant for the fuel cell reaction. No external humidification of the air is required. It is designed in a dead-end configuration using dry hydrogen, with no external humidification required.For the experiments, the metal hydride container was connected to the fuel cell stack by a hydrogen-calibrated digital mass flow controller and the hydrogen pressure was measured using a pressure transducer. Two types of measurements were made. In the first case, the metal hydride container was immersed in air at 20 °C, without thermal interaction with the fuel cell, while in the second case there is a thermal interaction between them by the use of a cone positioned at the outlet of the cell, which directs the fuel cell exhaust hot air to the container exchange fins. The hydrogen storage device exhibits a good performance to recover hydrogen from the metal hydride decomposition at the design working conditions, i.e., it is capable of delivering 70 L of hydrogen at 0.5 L min-1 and 20 °C, which allows a power supply of 50 W for 140 min from the H2/air fuel cell. Discharges at a higher hydrogen flow rate, namely 2 L min-1, with the container immersed in air at 20 °C, diminishes the hydrogen recovery in approximately 35 %. When the heat generated at the fuel cell is used to increase the container temperature, hydrogen recovery is 100 %, which allows the fuel cell operation at 150 W for 35 min at a high flow rate of 2 L min-1. Therefore, the thermal coupling of the hydrogen storage device and the fuel cell stack substantially improves hydrogen recovery from the metal hydride at high flow rates by favoring the heat transfer through the metal hydride container external wall, which diminishes thermal gradients in the metal hydride bed and leads to a lower decrease of the hydrogen dynamic pressure. Thus, the total hydrogen stored as metal hydride may be used.