Vol.5 No.4 2013

Research paper : Evaluating Uncertainty for the Standardization of Single Cell/Stack Power Generation Performance Tests for SOFC (A. Momma et al.)−253−Synthesiology - English edition Vol.5 No.4 (2013) SOFC manufacturers to ascertain their views concerning the testing methods they used and the standardization of such testing methods. It was important that the composition of the committee membership be equally distributed among the manufacturers, users, and neutral parties, so that discussions would reflect a balanced perspective. Regrettably, however, because SOFC was still in the development phase and the number of its actual users was limited, the committee debates tended to favor the manufacturers’ views.The second step was to establish the scope of measurements in the proposed standard. As described above, SOFCs came in a wide range of shapes and sizes; it was therefore neither easy nor realistic to set standards on all details of the components and their physical states and testing conditions during the tests. This was one of the most troubling issues that we had to address as we prepared the standard. In the end we selected the following approach: (1) Define the scope of the tests as “cell/stack assembly” to make clear that the standard covers both cells and stacks; (2) Treat the cell/stack assembly as a “black box” so that it can be applicable to any type of SOFC; (3) Define the interface between the measurement instrument system and the cell/stack assembly, and describe the necessary interfaces and the measurement methods of input and output values based on such interfaces.In sum, this approach is based on the view that it is not necessarily advisable to set standards for all details of the composition of test subjects; those components that should not be decided by the standard (i.e. those aspects that should be left to the discretion of manufacturers or test operators) are to be included in the “black box” of the cells/stack assembly, and the standard should only cover the interface between these components and the measurement instrument system. This approach makes it possible for the standard to cover not only the cells but also the stacks. Finally, after deciding to introduce the concept of uncertainty in order to ensure the reliability of measurement results, we needed to consider how to incorporate it into the standard and how to approach the evaluation equation. While a general equation for uncertainty is included in the Guide to the Expression of Uncertainty in Measurement (GUM),[7] we believed it was not appropriate to adopt this directly as the equation for evaluating the uncertainty of the results of the SOFC performance tests. When a general equation is used for a specific application, many aspects are left to the discretion of users. In addition, there was concern that applying the general equation as is would leave too much work to the reviewer of the results, making the standard not very useful (and hence not usable). Thus we believed it was necessary to make the uncertainty equation for the standard as simple as possible, and clearly define and unify the method of using the equation in detail. The following is the approach we took to incorporate the uncertainty equation into the standard. 3 Uncertainty of SOFC performance testing3.1 Approaches to uncertainty evaluationBecause the performance evaluation of SOFC is dependent on a wide range of parameters set for the measurement, calculating the uncertainty of the measurement results is not an easy process. Hence we investigated the approaches to uncertainty evaluation adopted in existing international standards related to fuel cells as well as in those standards currently under preparation. The following are examples of approaches to uncertainty evaluation adopted in the international standards on fuel cells. It is clear from the issued years of the standards that, as of 2007, there were only a very limited number of standards proposing to incorporate uncertainty evaluation into the performance evaluations of fuel cells. (1) In “Stationary fuel cell power systems - Performance test methods,” the Committee Draft for Voting (CDV) circulated in 2010 for IEC 62282-3-200,[8] uncertainty consists of and is evaluated in two categories: systematic uncertainty, which derives from factors such as the accuracy of measurement instruments and calibration errors; and random uncertainty, which derives from data variations. The CDV provides an example of how to combine and evaluate these two in one of the Annexes (including a table of computation results), but specific procedures are to be determined by consulting the GUM. The CDV’s approach to uncertainty is roughly identical to that of the Performance Test Code for Fuel Cell Power System Performance published by the American Society of Mechanical Engineers (ASME)(ASME PTC 50-2002[9]), and in both cases the test is to be conducted in a steady state. Uncertainty is calculated only for the measured efficiency of the system, and the sensitivity coefficients of component gases can be theoretically obtained by calculations. The performance evaluation of the SOFC system as a whole does not require setting the temperatures of the cells or the stack; thus there are fewer input quantities for the uncertainty evaluation of the system than for the uncertainty evaluation of a cell or stack. Only the uncertainties of thermal input and power output, which are dependent on factors such as fuel composition and flow rate, need to be considered. (2) At the end of 2009, the above-mentioned FCTESQA submitted to IEC a Draft for Comment (DC) on the testing method of the current-voltage (I-V) characteristics of Polymer Electrolyte Fuel Cells (PEFC) (An unofficial IEC document; not included in the reference list for the present document). This testing method defines the allowable range


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