Elaboration of spatial current and temperature variations in microtubular solid oxide fuel cells by experimental and numerical techniques
Başlık çevirisi mevcut değil.
- Tez No: 508751
- Danışmanlar: DOÇ. Dr. TATSUMI KITAHARA, Yrd. DOÇ. Dr. HIRONORI NAKAJIMA
- Tez Türü: Doktora
- Konular: Enerji, Mühendislik Bilimleri, Energy, Engineering Sciences
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 2017
- Dil: İngilizce
- Üniversite: Kyushu University
- Enstitü: Yurtdışı Enstitü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 162
Özet
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Özet (Çeviri)
Stemming mainly from the concentration variations of the reactants and products along the respective ow channels, the longitudinal current variations are crucial in SOFCs (solid oxide fuel cells), as they result in the performance and structure degradations (e.g. RedOx cycling). Being related to the current variations, the longitudinal temperature variations are important, because they give rise to the thermal stresses. It is hence required to identify the longitudinal variations in the characteristic properties, for which in-situ spatial characterization techniques are available (e.g. segmentation, etc.). However, it is impractical to apply the existing techniques on various forms of SOFC. Although numerical tools are feasible, they are required to be veri ed with experimental data. For improving the reliability of the numerical SOFC tools, in this Ph.D. study, the longitudinal current and temperature variations are elaborated via applying the segmentation method along with a nite element model on mt-SOFCs (microtubular-SOFCs) utilizing hydrogen and syngas (a mixture of hydrocarbons). In-situ measured current variations along mt-SOFCs are presented under various fuel (hydrogen) ow conditions; a remarkable performance degradation is disclosed. Additionally, a high risk of RedOx cycling is diagnosed at low fuel ow conditions. Substantial temperature variations along mt-SOFCs in both co- and counter- ow con gurations are identi ed. Based on longitudinal impedance measurements, a strong impact of the large temperature gradients on both electrolyte conductivity and reaction kinetics is shown, in uencing the longitudinal current and concentration variations. It is found that longitudinal current (concentration) and temperature variations couple in the counter- ow con guration, exhibiting larger variations in comparison with the co- ow con guration. Analysis of the longitudinal temperature and current variations together under various operation conditions discloses the contributions by current variations, and convective heat transfer between air and the cathode surface to the longitudinal temperature variations. It is shown that the large temperature variations stem mainly from the excess air supplied as a common practice to sweep the waste heat produced in SOFCs. In this regard, the excess air supply should be avoided to minimize temperature variations in the co- ow con guration, which would also reduce the power consumption by the air blower. For evaluating reliability of numerical tools developed to compute spatial variations, a fullycoupled two dimensional nite element model is presented for a mt-SOFC. The model is validated by correlating the conventional current/voltage curve of the cell to the in-situ measured one. This conventional validation process exhibits remarkable deviations among numerical and experimental values (current and temperature). Exclusion of radiant heat transfer makes the deviation among the experimental and numerical temperature values signi cantly larger, revealing the notable impact of the radiant heat transfer on the longitudinal temperature variations. Due to the poor accuracy of temperature computation and substantial impact of temperature on the other processes in such a fully-coupled model, the model is modi ed to capture the in-situ measured temperature variations. This validation shows a positive impact on estimation of the longitudinal current variations, i.e., on reliability of numerical model. Spatial variations in characteristic properties of SOFCs are anticipated to be more remarkable while direct internal reforming of hydrocarbons. Spatial characterization is hence more demanded in direct internal reforming SOFCs. For improving reliability of numerical tools developed for direct internal reforming SOFCs, the impact of the endothermic internal reforming on the longitudinal temperature variations and corresponding rate of the reforming reaction is analyzed while utilizing syngas. It is found that the extent of endothermic cooling changes along the cell, which is attributed to the changing reaction rate related to the concentration variations. It is found that syngas utilization yields remarkable current uctuations in parallel to the rate of the internal reforming. The uctuations are ascribed to the competition among the reforming reaction and the hydrogen oxidation reaction, as they both proceed on the same catalysts. It is concluded that the heat and mass transfer processes should be carefully considered in numerical tools for precisely computing spatial variations. Besides, the rate of the reforming reaction should be accurately de ned to employ numerical tools for internal reforming SOFCs.
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