A typical layout of a conventional fuel cell 10 is shown in FIG. 1 which, for clarity, illustrates the various layers in exploded form. A solid polymer ion transfer membrane 11 is sandwiched between an anode 12 and a cathode 13. Typically, the anode 12 and the cathode 13 are both formed from an electrically conductive, porous material such as porous carbon, to which small particles of platinum and/or other precious metal catalyst are bonded. The anode 12 and cathode 13 are often bonded directly to the respective adjacent surfaces of the membrane 11. This combination is commonly referred to as the membrane-electrode assembly, or MEA.
Sandwiching the polymer membrane and porous electrode layers is an anode fluid flow field plate 14 and a cathode fluid flow field plate 15 which deliver fuel and oxidant respectively to the MEA. The fluid flow field plates 14, 15 are formed from an electrically conductive, non-porous material by which electrical contact can be made to the respective anode electrode 12 or cathode electrode 13. At the same time, the fluid flow field plates must facilitate the delivery and/or exhaust of fluid fuel, oxidant and/or reaction product to or from the porous electrodes.
This is conventionally effected by forming fluid flow passages in a surface of the fluid flow field plates, such as grooves or channels 16 in the surface presented to the porous electrodes 12, 13. Hydrogen and/or other fluid fuels or fuel mixes are delivered to the anode channels. Oxidant, typically oxygen or ambient air is delivered to the cathode channels, and reactant product water and/or water vapour is extracted from the cathode channels.
With reference to FIG. 2, usually a large number of fuel cells 10 are arranged in a stack 20, such that the anode 14 of one cell is adjacent to and electrically connected to the cathode 15 of the next cell (preferably using a combined fluid flow field plate 21 as shown), the voltages from each cell successively adding to produce a requisite supply voltage.
There has been considerable interest in fuel cells as an efficient means for providing localised electrical power supplies to domestic and light industrial premises, particularly in remote areas where construction of large power supply networks is costly.
An aspect of the electrochemical fuel cell is that a certain amount of heat is generated within the fuel cell during the electricity generation process. Conventionally, this heat has been regarded as a waste by-product that is extracted together with the water vapour and simply lost.
A certain amount of heat in the MEA and fluid flow field plates is, in fact, desirable to obtain optimum operating conditions, but this must be kept strictly under control, particularly when electrical demand on the fuel cell is high. Control of the heat is existing fuel cell generally utilises one or both of two different cooling mechanisms.
In a first mechanism, liquid phase cooling is used in which water is delivered to and extracted from separate cooling plates located between selected fluid flow plates within the stack 20. Commonly, a cooling plate is positioned between every fourth or fifth anode/cathode field plate pair. Water extracted from the cooling plates is passed through a heat exchanger and recirculated into the cooling plates.
In a second mechanism, vapour phase cooling is used to extract heat from the active fluid flow plates by delivering controlled amounts of water to the MEA 11, eg. directly to the electrode surfaces or into the channels 16 of the fluid flow field plates 14, 15, which water is vaporised and extracted from the cathode exhaust. This technique has the advantage of not only supplying the water to maintain an appropriate membrane water content but it also acts to cool the fuel cell through evaporation and extraction of latent heat of vaporisation.
However, because the water is being delivered into the working MEA of the fuel cell, it is important to use water of adequate purity such that the quality and performance of the membrane 11 is not compromised. In some remote environments, a consistent supply of such water quality is difficult to guarantee and may not be under the control of the fuel cell operator.
In general, the cooling systems for cooling plates and vapour phase extraction from the cathode exhaust are not compatible in that the inlet and outlet temperatures are different, and conventionally, separate heat exchanger circuits are required. This results in increased complexity, cost and size of the overall fuel cell energy system.