ReviewA review of integration strategies for solid oxide fuel cells
Introduction
Power generation and the related environmental impacts have become important issues across the world [1], [2]. Today, electrical power is provided mainly by conventional power generation technologies that rely on fossil fuel combustion, which generates soot and sulfur compounds, in addition to other noxious emissions. The combustion of fossil fuels is widely understood to contribute to both global warming and local air pollution. Advanced clean energy systems must be developed urgently, allowing us to make the shift from a fossil fuel-based economy to a new paradigm in a progressive manner. In 1997, the US Department of Energy launched its Vision 21st program [3], [4], which was essentially meant to conduct conceptual feasibility studies to assess high-efficiency fossil fuel power plants and thereby develop the core technologies for a fleet of fuel-flexible, multi-product energy plants with an electricity generation efficiency higher than 75% for gas and 60% for coal. The fuel cell is an emerging alternative to traditional power generation systems; it offers the potential for higher electrical efficiencies and lower emissions [5], [6]. Various fuel cells are available today, differentiated by the electrolyte used and its operating temperature. The electrolyte of a solid oxide fuel cell (SOFC) consists of a solid, fully dense oxide metal (typically yttria (Y2O3) stabilized zirconia (ZrO2) or YSZ). The anode of a SOFC is typically made of a nickel cermet, such as Ni-YSZ, while the cathode is made of strontium (Sr) doped with lanthanum manganite (LaMnO3) [5]. The fact that all the components in a SOFC are solid structures makes it possible for the cells to be constructed in any geometry. The SOFC operates in the range of 600–1000 °C, which allows it to combine with other conventional thermal cycles to yield improved thermal efficiency. The hybrid SOFC system is considered to be a key technology in achieving the goals of Vision 21st [7] on account of the many advantages it offers over other systems.
First, there are no moving components in the fuel cell (except for balance of plant (BoP) components). Noise and vibrations associated with mechanical motion during operation are practically non-existent. This makes it possible to install the system in urban or suburban areas as a distributed power generation plant. Without moving parts, we would expect enhanced reliability and lower maintenance cost. Secondly, SOFCs (by virtue of high-temperature operation) can extract hydrogen from a variety of fuels. SOFC is the most sulfur-resistant (as H2S and COS) fuel cell. It can tolerate sulfur-containing compounds at concentrations higher than other types of fuel cells [8]. In addition, it is not poisoned by carbon monoxide (CO); in fact, CO can be used as a fuel. These features allow SOFCs to be fed with gases derived from either solid or liquid fuels. This advantage is beneficial for coal-based central power generation and in vehicles that are powered by diesel or gasoline fuel [9], [10], [11]. Thirdly, the size of a SOFC module is flexible, thus allowing it to be constructed for use in any power range – from watts to megawatts. Therefore, a SOFC or its hybrid may be built for stationary applications (central power generation and distributed power generation) or as an auxiliary power unit (APU) for vehicles. The attributes of a SOFC and its hybrid system are summarized in Table 1 [9], [12], [13], [14], [15], [16], [17], [18], [19].
Integration strategies for SOFC hybrid systems are generally determined by the application requirements. In most cases, both efficiency and emissions are considered first in the design of these hybrid systems. However, in some cases (such as for aerospace and naval vessel applications), reliability and low noise levels may be more important. In the following sections, we present a thorough review of different integration concepts/strategies for SOFC-based hybrid systems.
Section snippets
SOFC fundamentals
SOFCs convert the chemical energy of a fuel directly into electrical energy through electrochemical reactions that are driven by the difference in the oxygen chemical potential between the anode and cathode of the cell. Oxygen ions migrate through the electrolyte to the anode where they are consumed by fuel oxidation. The electrochemical reaction in a SOFC is generally fuelled by hydrogen. At the cathode side:(1/2)O2 + 2e− → O2−.At the anode side:H2 + O2− → H2O + 2e−.The SOFC consumes the hydrogen that
Basic hybrid integration strategies
Availability, an important thermodynamic term, should be obeyed in plant hybrid systems. Specifically, one system (or cycle) needs to supply enough material or heat to meet the requirements of another system (or cycle). With regard to availability, the hybrid system could be built to operate via either thermal coupling or fuel coupling. Two thermal coupling schemes have been proposed for the exchange of thermal energy between the SOFC system and the objective combined cycle. In one scheme known
Advanced integration cycles for improved power generation
Three basic integration strategies were introduced above to build the integration system based on SOFCs. Quite often, the integration system can be configured using more than one scheme. Table 2 lists typical conceptual integration strategies that use SOFCs. In this paper, we review three advanced conceptual integration systems – the SOFC is integrated with a gas-cooled nuclear reactor cycle, coal gasification cycle, and humid air turbine cycle power generator.
Achievements
SOFC power systems can be classified into three groups: stationary applications – including central power generation (>50 MW) and distributed power generation (usually >10 kW), APUs in vehicle applications, and portable applications. Most of today's installed SOFC units for demonstration purposes are at the sub-MW level. These units are mostly found in Europe, North America and Japan. The success of SOFCs and integrated systems in terms of possible future commercialization depends on their cost
Acknowledgement
This study was financially supported by the Doctor Foundation (No. DFXJTU2005-01) of Xi’an Jiaotong University.
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