固態氧化物燃料電池動態模擬與運轉控制

Abstract

由於固態氧化物燃料電池(SOFC)具有高效率與多元燃料的優勢，已被視為具有潛力的潔淨能源替代方案，預期將來可推廣應用於車用輔助電力、社區住宅、乃至於廣域型分散式電力系統。近年來，在製程成本降低與耐久性提升方面投入大量的研究，使得具有經濟價值的市場先期產品已逐步開發與驗證中。然而，在有關起機程序與運轉控制策略方面的研究卻是相對缺乏的。本研究運用動態模擬技術進行跨領域整合建模，以探討固態氧化物燃料電池之電池堆響應特性，並進行起機程序驗證與負載跟隨性能評估。由於固態電解質的高溫離子導通特性，固態氧化物燃料電池需在高溫環境下才能進行操作。因此，在運轉前需進行電池堆的昇溫程序。在溫昇過程中，由於電池堆組成元件之間的熱膨脹係數差異，不適當的溫昇程序會造成交互的熱應力產生，促使陶瓷材料既存孔隙或缺陷成長為較大的裂縫，造成組件的洩漏或破損，並降低電池的效率，甚至造成永久損壞。因此，為了發展可靠的固態氧化物燃料電池發電系統，電池堆的溫度與溫升率控制是必要且重要的。為能同時兼顧快速與安全的升溫要求，分別進行電池堆入口溫差與入口流率對溫昇率之特性影響評估，並進行模擬分析。藉由本模型進行參數分析，可獲得在規格條件下的最適溫升參數，在安全的條件下節省時間與耗能。在負載跟隨方面，特別針對可能造成損壞之限制條件，如燃料使用率與空氣過量比等進行控制，並同時兼顧電力需求、效率、耐久性與安全性。對於不同物理量間之響應特性，進行適當之補償，以提升其穩定性。為減低負載變動期間之溫度波動現象，本文提出一種即時修正空氣過量比之機制，該修正機制藉由參考溫度模型進行運作，免除了電池片溫度難以量測之問題。本架構藉由以模型為基礎的設計方式，建構了具有預估能力的參考控制法則，使得進行負載跟隨之溫控更為便利且準確。經由模擬結果顯示，即使在大幅電力變化需求下，本控制策略能適當補償並具有良好的負載跟隨能力。

Solid oxide fuel cells (SOFCs) have been recognized as a promising alternative for clean electrical power generation due to the advantage of higher efficiency and fuel flexibility, that can provide power to a wide variety of utilities, from vehicles to residences, to nation-wide electric grids. As a result of research on SOFC primarily focused on cost reduction, long-term durability improvement, cost competitive pre-commercial applications are becoming viable. However, there has until recently not been much focus on control strategies to comprehend and improve SOFC startup process and dynamic load tracking capabilities. In this dissertation dynamic modeling methodology is utilized to develop the multi-disciplinary integration for assessment of characteristic of dynamic response as well as evaluation of strategy for startup process and of capacity for load tracking. In order to achieve high ion conductivity of ceramic electrolyte, SOFCs should be heated up before being fueled for operation. During heat up process, the thermal stress caused from mismatch of thermal expansion coefficient between components can lead to cracking or permanent damage. For a safe and fast start-up, two factors for stack-heating were investigated; one is the inlet temperature difference from stack and the other is the inlet flow rate. The developed strategy provides a practical perception to optimize stack-heating process minimizing net heating energy and time span based on provided specification of stack. For ease of wide-spread utilities the control strategies for operation tracking to a load profile with magnifying the efficiency and complying with the safety constraints is developed. Particular attention is paid to understand potential constraint violations, control structures, and characteristic time scales. A methodology on regulating fuel utilization (Uf) and air excess ratio (λ) is investigated to validate alleviation of temperature fluctuation. In this configuration, an on-line turning mechanism of air excess ratio is proposed. The mechanism refers temperature from a reference model, obviating the need for thermo-sensing from cell. With the proposed strategy, fluctuation of temperature tracking to a pre-defined load profile is suppressed considerably. Through simulation, transient understanding, and control development this dissertation demonstrates that even though SOFC systems have a wide range operating requirements, integrated controls can be developed and implemented to enhance load tracking capability with lower risk.