Energy balance in solid-oxide fuel assisted electrolyzer cell modules
High temperature electrolysis of water is a well-known method of extracting hydrogen, and has reduced electrical potential requirements when compared to low temperature electrolysis. The reduction in electrical energy cost is useful when a source of high temperature steam (or sufficient waste heat) is available; however, such availability is not the norm in residential and small commercial establishments and the electrical energy cost of producing the steam before electrolysis effectively negates the advantage of the high temperature process. In addition, standard electrolysis has a very high electrical energy cost even under ideal conditions and may not make economic sense for distributed hydrogen generation. The fuel assisted electrolysis technology profiled in this work uses a lower cost chemical energy (in this study it is provided by natural gas and hydrogen) to reduce the electrical potential needed to drive the electrolyzer cells. Natural gas, while a valuable commodity, is significantly less expensive than the electrical energy it is replacing; it is also worth noting that the technology does not, of necessity, need such high-grade fuel. Future studies are likely to use low cost coal gas and near-waste petroleum byproducts (like petroleum coke, i.e., petcoke) to provide the needed chemical potential. This work presents a body of experimental results wherein this new planar solid oxide high temperature electrolysis technology is profiled and characterized. Fuel and steam at various flow rates are fed to the cell module to determine the response of the cells at higher and lower levels of fuel/steam availability under a given electronic load. The resulting data are then contrasted with similar results from a standard solid oxide electrolysis cell (SOEC) module operating under the same loading conditions, with the same steam availability and the same physical cell structure and geometry. Results are then extrapolated to a generalized comparative energy balance showing the possible economic advantages of planar SOFEC modules over comparable planar SOEC modules. The results can also be used to compare the SOFEC method with other hydrogen production technologies.