TABLE OF CONTENTS The term "portable power system" is not sharply defined. Hoogers [ [34]] attempted to define it as "a small grid-independent electric power unit ranging from a few watts to roughly one kilowatt, which serves mainly a purpose of convenience rather than being a primarily a result of environmental or energy-saving considerations." These devices may be divided into two main categories:
Battery replacements, typically well under 100 Watts
Portable power generators, up to 1kW
The key feature of small fuel cells to be used as battery replacements is a running time without recharging. Obviously, by definition the size and weight are also important. Power units with either significantly higher power densities or larger energy storage capacities than those of existing secondary batteries may find applications in portable computers, communication and transmission devices, power tools, remote meteorological or other observation systems, and in military gadgets. In addition to the size of the fuel cell itself, the critical issue is the fuel and its storage. Hydrogen, although a preferred fuel for PEM fuel cells, is rarely used because of bulkiness or weight of its storage, even in small quantities required by those small devices. Hydrogen may be stored in room temperature metal hydride storage tanks. Some chemical hydrides offer higher energy density; however, they must be equipped with suitable reactors where hydrogen is released in controlled chemical reaction [ [35]]. Most portable fuel cells use methanol as fuel, or more precisely methanol aqueous solutions,...
