(274e) Techno-Economic System Analysis for SOFC/GT Hybrid System Accounting for Degradation Effects

Techno-economic
System Analysis for SOFC/GT Hybrid System Accounting for Degradation Effects

Haoxiang
Lai*, Thomas A. Adams II

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 [1]. 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¡ªthe most common way of
baseload power production¡ªthe 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 [2].

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 [2]. 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 [3]. 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.

Figure
1. Scheme of the Solid Oxide Fuel Cell and Gas Turbine Hybrid 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.

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[1]
T. A. Adams II, J. Nease, D. Tucker, and P. I. Barton, ¡°Energy Conversion with
Solid Oxide Fuel Cell Systems: A Review of Concepts and Outlooks for the Short-
and Long-Term,¡± Ind. Eng. Chem. Res., vol. 52, no. 9, pp. 3089¨C3111, Mar. 2013.

[2]
D. Tucker, N. F. Harun, and M. Abreu-Sepulveda, ¡°SOFC Lifetime Assessment in
Gas Turbine Hybrid Power Systems,¡± p. V001T03A003, Jun. 2014.

[3]
D. Tucker et al., ¡°Evaluation of Methods for Thermal Management in a Coal-Based
SOFC Turbine Hybrid Through Numerical Simulation,¡± J. Fuel Cell Sci. Technol.,
vol. 9, no. 4, pp. 041004-041004-9, Jun. 2012.