Conference Presentation
Techno-economic System Analysis for SOFC/GT Hybrid System Accounting for Degradation Effects
Haoxiang Lai, Thomas Adams II
October 30, 2018
Solid oxide fuel cells (SOFCs) produce power with higher efficiency and lower greenhouse gas emission than conventional power production systems such as coal/natural gas power plants. However, a major challenge with SOFCs is that they degrade over time, leading to a short lifetime and limiting their commercialization. When operated in constant power mode—the most common way of baseload power production—the lifetime of an SOFC is as short as around 1.5 years. As an SOFC starts to degrade, the fuel rate and current density must increase in order to compensate and keep power production at a constant level. This compounds the problem by actually increasing the rate of degradation further, resulting in an exponentially increasing degradation rate and therefore a short lifetime.
It has recently been found that by operating the SOFC differently with constant voltage instead of power, the degradation rate can be slowed such that the cell lifetime can be increased to around 13-14 years. In this mode, the fuel utilization decreases over time, such that the power produced by the SOFC will also decay over time to 25% of its original output at the end of the 13-14 year period. In addition, the anode exhaust stream contains an ever-increasing amount of unspent fuel. Thus, this mode of operation is unsuitable for the SOFC standalone systems. In order to provide baseload power production using SOFCs with a long lifetime, one potential solution is to integrate the SOFC stack with a gas turbine (GT) in a hybrid system (as shown in Figure 1). The SOFCs operate in constant voltage mode so that their lifetime is longer (13-14 years). The GT is powered by combusting the ever-increasing unspent fuel in the SOFC anode exhaust, thus gradually increasing its power production to mostly make up for SOFC power losses incurred due to degradation over time. However, the GT efficiency is lower than the SOFC stack such that it cannot make up for all of the SOFC losses. Nevertheless, the net effect is a system which can produce electricity over a long (13-14 year) lifetime with only a small amount of decay and a higher efficiency (and thus lower greenhouse gas emissions) than a standalone GT system. To contribute to the large scale adoption of SOFCs by using the SOFC/GT hybrid approach, the question that needs to be addressed is about whether the SOFC/GT hybrid system is economically feasible compared to SOFC standalone system.
Therefore, we present the first techno-economic study to determine if it is better to have a SOFC standalone plant in constant power mode with SOFCs replaced every 1.5 years or a more expensive SOFC/GT hybrid plant with SOFCs in constant voltage mode with SOFCs replaced every 13-14 years. Specifically, a detailed dynamic SOFC model that accounts for degradation developed in Matlab Simulink (in a prior work) was integrated with Aspen Plus steady-state models (developed in this work) of the balance-of-plant for a coal-power hybrid system. The Simulink model considers the spatial degradation over the axial length of the cells over time based on factors such as fuel composition, fuel rate, humidity, utilization, current, voltage, and other factors. The balance of plant model considers aspects such as the gasifier, syngas cleanup processes, combustor, gas turbines, compressor, and heat exchangers. The dynamics over the process lifetimes were modeled using a pseudo-steady state approach with week-long time-steps. By integrating the results of model simulation and the economic analysis, we present the trade-offs between the SOFC/GT hybrid system and the SOFC standalone system in terms of the economics and greenhouse gas emissions.
SOFC systems have been researched for over a century but they are still at their early stage of commercialization due to their high cost as being implemented in an ordinary baseload power plant. This study is valuable because it shows the techno-economic value of a SOFC/GT hybrid system that was designed to meet the needs of ordinary baseload power production with understanding of the fundamental science of how degradation occurs. This work could potentially make large scale adoption of SOFCs economically feasible.
Modeling and simulation, Process design, SOFC/GT Hybrid, Technoeconomic Analysis
Suggested Citation
Lai H, Adams T II. Techno-economic System Analysis for SOFC/GT Hybrid System Accounting for Degradation Effects. (2018). LAPSE:2018.0809
Author Affiliations
Lai H: McMaster University
Adams T II: McMaster University
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Pittsburgh, U.S.
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Oct 30, 2018
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Haoxiang Lai