Elsevier

Journal of Power Sources

Volume 170, Issue 1, 30 June 2007, Pages 130-139
Journal of Power Sources

Analysis of the design of a pressurized SOFC hybrid system using a fixed gas turbine design

https://doi.org/10.1016/j.jpowsour.2007.03.067Get rights and content

Abstract

Design characteristics and performance of a pressurized solid oxide fuel cell (SOFC) hybrid system using a fixed gas turbine (GT) design are analyzed. The gas turbine is assumed to exist prior to the hybrid system design and all the other components such as the SOFC module and auxiliary parts are assumed to be newly designed for the hybrid system. The off-design operation of the GT is modeled by the performance characteristics of the compressor and the turbine. In the SOFC module, internal reforming with anode gas recirculation is adopted. Variations of both the hybrid system performance and operating condition of the gas turbine with the design temperature of the SOFC were investigated. Special focus is directed on the shift of the gas turbine operating points from the original points. It is found that pressure loss at the fuel cell module and other components, located between the compressor and the turbine, shifts the operating point. This results in a decrease of the turbine inlet temperature at each compressor operating condition relative to the original temperature for the GT only system. Thus, it is difficult to obtain the original GT power. Two cell voltage cases and various degrees of temperature difference at the cell are considered and their influences on the system design characteristics and performance are comparatively analyzed.

Introduction

The solid oxide fuel cell (SOFC) is considered a suitable candidate for the electric power plant applications. One of its attractive features is high operating temperature (600–1000 °C), which allows favorable combination with other types of power generators such as gas turbines (GTs). R&D efforts pertaining to SOFC/GT hybrid systems have been initiated worldwide and a few systems are under development for commercialization [1]. The thermodynamic synergy effect of combining the SOFC and the GT can be most effectively realized when the SOFC is designed to operate at an elevated pressure. This kind of system is called a pressurized system where pressurized air from the compressor is delivered directly to the SOFC and the outgoing high pressure gas drives the turbine. This design can be considered to be a modification of a fuel cell only system in that the original air blower is replaced by a compressor which enables a higher operating pressure and thus guarantees higher cell voltage. The turbine is added to recover the high pressure energy of the gas exhausted from the cell. This turbine expansion plays a dominant role in enhancing the efficiency of the entire system [2]. The pressurized system also allows for compact design of auxiliary equipments such as piping due to the relatively high pressure at those parts. Another means of combining the SOFC and GT is an ambient pressure system where the cell operates at a near ambient pressure. This system also has several advantages, the most prominent of which is selection of the GT pressure independently of the cell pressure. Aside from practical cons and pros involved in realizing the two types of system configurations, many basic studies have concluded that a pressurized system may have higher system efficiency over an ambient pressure system from a thermodynamic point of view if equivalent design parameters are assumed [3]. Recent development examples of commercial SOFC/GT hybrid systems have also adopted the pressurized configuration [4], [5].

Recently, diverse analysis results on the design performance of pressurized SOFC hybrid systems considering various options in combining the SOFC and the GT have been reported. Examples include fundamental design parametric analyses [6], [7], a study on the influence of the reforming method [8], an investigation into the influences of both reforming method and temperature constraints on the cell [9]. Most of the design analyses have been performed under the assumption that all components including the SOFC, the GT, and balance-of-the-plant (BOP) parts can be manufactured as predicted. In particular, smoothly matched design of the SOFC and the GT is generally assumed to be possible, that is, a sub-system (GT or SOFC) is assumed to be designed to match the design of the other sub-system. In many studies, parameters of the GT, such as power and main component parameters including turbine inlet temperature, pressure ratio and air flow rate, which match the given design parameters of the SOFC stack (the cell temperature, etc.), have been obtained as a result of analysis. This analysis seems logical in that the SOFC is the major component that generates greater power than the GT. The underlying assumption of this analysis is that the gas turbine, exactly matching the analysis requirements, is available as an off-the-shelf item or the gas turbine is to be newly designed and manufactured to match the design specifications. However, the assumption is rather unreasonable in reality. A gas turbine that exactly matches the required design specifications of the analysis is not usually available as an off-the-self item. Moreover, a new design, i.e. development of a new gas turbine, is generally prohibitive in terms of cost. On the other hand, the capacity (power) of SOFC can be determined more flexibly due to the possibility of modular design of the fuel cell stack, which is one of the most advantageous features of the fuel cell. Accordingly, the design of a hybrid system based on an existing (commercially available) gas turbine is more practical as a short- or mid-term development strategy. An example about predicting system design and operation of a hybrid system based on a commercially available GT can be found in the literature [10].

In a pressurized hybrid system, the turbine inlet state is directly affected by the thermo-fluid dynamic operating conditions of the components located between the compressor and the turbine, such as the SOFC stack and piping. Therefore, the operating conditions of the gas turbine are different from those of the original GT only system. A previous work [11], in which the operational characteristics of an experimental hybrid system simulator were investigated, reported that operation of the compressor during start-up approaches the surge condition. This might have been caused by the change of the GT operation conditions from the original conditions. The possibility of the compressor surge during transient operation for a hybrid system designed at near surge condition has also been reported [12]. Therefore, the design operation condition should be sufficiently far from the surge condition (large surge margin), and this limitation may provide a critical constraint in designing a pressurized hybrid system based on an existing gas turbine.

The aim of this study is to simulate the design of a pressurized SOFC hybrid system using an existing (fixed) gas turbine and to provide useful fundamental design characteristics as well as potential critical problems. The gas turbine is assumed to exist prior to the hybrid system design and all the other components such as SOFC module and auxiliary parts are assumed to be newly designed for the hybrid system. Special focus is given to the shift of the compressor operating points from the original gas turbine design point depending on the design cell temperature. In order to examine the matching characteristic of the gas turbine with different design practices of the fuel cell, a wide range of cell design temperature is used. Thus, this study presents rather general design results using different fuel cell designs. Researchers or designers may refer to the result that includes design conditions closest to their cell design practices. The influence of the cell voltage on the design characteristics and performance is examined as well. Also, since the temperature difference at the cell is an important design parameter that affects not only the SOFC performance but also the hybrid system performance [3], [9], its effect on integration of the SOFC and the existing GT is investigated.

Section snippets

System configuration

Fig. 1 shows the hybrid system configuration analyzed in this study. This system is conceptually similar to that used in a demonstration project [4]. The SOFC module includes a cell stack, a reformer, an afterburner and a preheater. Internal reforming is adopted, and the steam required for the reforming reaction is supplied by the anode gas recirculation. The compressed air is heated consecutively through a recuperator and a preheater and then flows into the cathode of the SOFC. The remaining

Design characteristics

The rotational speed of the gas turbine is fixed at the original design speed for all the hybrid system designs. For a fixed speed, various operating points of the gas turbine (more specifically those of the compressor) are possible according to different settings of design parameters of the SOFC and other parts. The major design parameters are those of the cell stack module such as the cell operating temperature, the cell voltage and the temperature difference at the cell. In this section,

Conclusions

This study has simulated design of a pressurized SOFC hybrid system based on a fixed gas turbine design. The results are summarized as follows:

  • (1)

    As the design cell temperature increases, both the turbine inlet temperature and the pressure ratio increase, and system power and efficiency also increase. Due to the existence of pressure loss at the fuel cell, the turbine inlet temperature at the original design point of the compressor is lower than the design turbine inlet temperature. Thus, at the

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