This invention relates to high temperature electrochemical converters and specifically to high performance systems employing such devices and methods.
Electrochemical converters perform fuel-to-electricity conversions in a fuel cell (electric generator) mode or electricity-to-fuel conversions in an electrolyzer (fuel synthesizer) mode. The converters are capable of high efficiencies, depending only on the relation between the free energy and enthalpy of the electrochemical reaction, and are not limited by Carnot-cycle considerations.
The key components in an electrochemical energy converter are a series of electrolyte units onto which electrodes are applied and a similar series of interconnectors disposed between the electrolyte units to provide serial electrical connections. Each electrolyte unit is an ionic conductor having low ionic resistance thereby allowing the transport of an ionic species from one electrode-electrolyte interface to the opposite electrode-electrolyte interface under the operating conditions of the converter. Various electrolytes can be used in such converters. For example, zirconia stabilized with such compounds as magnesia, calcia or yttria can satisfy these requirements when operating at an elevated temperature (typically around 1000.degree. C.). The electrolyte material utilizes oxygen ions to carry electrical current. The electrolyte should not be conductive to electrons which can cause a short-circuit of the converter. On the other hand, the interconnector must be a good electronic conductor. The interaction of the reacting gas, electrode and electrolyte occurs at the electrode-electrolyte interface which requires the electrodes be sufficiently porous to admit the reacting gas species and to permit exit of product species.
The approach of forming electrolyte and interconnector components as free-standing plates was disclosed by the present inventor in U.S. Pat. No. 4,490,445, issued Dec. 25, 1984, which is herein incorporated by reference. However, during operation, the stacks of electrolyte and interconnector plates can experience thermal non-equilibrium. Thus, it is important to reduce thermal gradients across the entire converter assembly by facilitating the transfer of heat to and from the electrolyte elements.
When an electrochemical converter performs fuel-to-electricity conversion in a fuel cell mode, waste energy in the form of heat must be removed from the electrolyte surfaces. Conversely, when the converter performs electricity-to fuel conversion in the electrolyzer mode, the electrolyte must be provided with heat to maintain its reaction. In prior systems, heat exchanging has been achieved primarily by the convective heat transfer capabilities of the gaseous reactants as they travel through the assembly. Such reliance on the heat capacity of the reactants creates an inherent thermal gradient in the system, resulting in non-optimum electrochemical processes.
To rectify this problem, the approach of integrating with the electrochemical converter a series of heat transfer elements was disclosed by the present inventor in U.S. Pat. No. 4,853,100, issued Aug. 1, 1989, which is herein incorporated by reference. The above-mentioned integration system facilitates the heat transfer from the fuel cell stacks by reducing thermal gradients across the converter assembly. However, there still exists a need for further improvements in the thermal control mechanisms within electrochemical energy systems. In particular, an improved electrochemical energy conversion system having the ability to more efficiently regulate the operating temperature within the electrochemical assembly would represent a major improvement in the industry.