A fuel cell is an electrochemical device which combines a fuel and an oxidant, typically oxygen from air, to deliver power. Unlike a battery, which is closed, a fuel cell is open on at least one side, the air side being invariably open. Like a battery, individual cells can be combined together to form a stack and hence delivering whatever power is needed for the given application. The fuel at point of use is commonly hydrogen, but can be a hydrocarbon or other hydrogen containing fuel, decomposed by heat, catalytically, or simply stored at pressure. The fuel cell stack combines fuel and air to form water and potentially CO2, cleanly and efficiently. While similar to a battery, a fuel cell system is far more complex, involving pumps, blowers, condensers, etc., (collectively denoted as the balance of plant) which impact on cost and reliability.
Fuel cells can both complement and compete with batteries for portable power systems. At the very small scale (AAA-D cell), batteries are hard to beat, especially for short times as the requirement for the electrochemical converter and associated balance of plant takes up a large share of the power source inventory. As the power level and time requirement increases, the decoupling of fuel and electrochemical converter becomes increasingly advantageous, and is manifest in lightweight power sources with reduced recharge times compared to batteries. The ability to instantly swap out the fuel rather than wait for recharge is particularly advantageous for most portable applications.
While fuel cells offer advantages over batteries, they also bring issues, at least at the present state of development. Some are intrinsic (such as the need to breathe air) and others are being continuously improved (such as start-up from sub-zero temperatures, stack and component lifetimes, cost), while yet more are specific only to certain stack types (such as intolerance of atmospheric CO2 by alkaline fuel cell stacks, or orientation dependence for direct methanol fuel cells).
As they improve, fuel cells will certainly replace batteries in some applications but sealed cells will continue to be needed for start-up, for rapid load following and for short term peaks, where the battery is cheaper than a larger stack. For companies like ABSL, fuel cells offer new opportunities, the PPS Programme being one example of an exploitation path, for soldier power and beyond.
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