Techo-Economic Analysis of a Metal-Supported Reversible SOC System for Grid-Scale Energy Storage via Hydrogen Production

Metal-Supported Reversible Solid Oxide Cells (MSRSOCs) have experienced considerable interest in both fuel cell (FC) and electrolysis modes due to lower material cost and rapid thermal and redox cycling and rapid start-up capabilities compared to cermet- or ceramic-supported designs. For grid-scale energy storage, MSRSOCs can opportunistically take advantage of unpredictable availability of renewable energy to produce hydrogen in times of low demand or use lower-cost baseload nuclear power at nighttime to produce hydrogen for use at times of higher demand. This work presents a techno-economic analysis (TEA) of an MSRSOC system for production and use of hydrogen for grid-scale electricity storage. The TEA determines a levelized cost of electricity (LCOE) in FC mode and of hydrogen (LCOH) in electrolysis mode. The analysis estimates the CAPEX for the balance of plant (BOP) accompanying the MSRSOC system, the cost of hydrogen storage, and a stack cost estimate. The OPEX is primarily calculated from fuel and utility costs such as electricity for electrolysis, cost of producing steam (considered as MMBtu equivalent of natural gas (NG)), and feedstock involved in the system operation. Balance of Plant (BOP) model The BOP model involves the system design and engineering to facilitate MSRSOC stack operation and encompasses all unit operations for hydrogen generation and compression, power generation, as well as steam generation and heat recovery using ASPEN HYSYS simulations. The operating conditions of the MSRSOC, i.e., temperature, operating voltage, pressure, and target throughput (consumed/produced electrical power for SOEC/SOFC respectively) are the key variables used to calculate the mass and energy flow streams, equipment duty and preliminary size, and cost analysis for each mode of operation. The next key factor is the MSRSOC thermodynamic*voltage efficiency for determining the process parameters for waste heat recovery to preheat the reactants. In the SOEC mode of operation, the operating temperature is 800°C with partial pressures of H2O/H2/O2=0.9/0.1/0.21 atm, respectively and total pressure = 126.5 kPa. Operation near thermoneutral conditions with overall efficiency very close to 1.0 (0.998) was considered, with no waste heat recovery requirement or continuous heat input to the MSRSOC to maintain its temperature at 800°C. For the SOFC mode of operation, PH2O/H2/O2=0.03/0.97/0.21 atm and the operating temperature=800°C. The H2 feed stream is depressurized in the process with considerations of reverse Joule-Thompson effect from stored pressure (450 barg to 700 barg) to 126.5kPa. The calculated overall SOFC efficiency determines the waste heat recovery strategy used for preheating the depressurized H2 in an enclosure. Following the first preheat of the H2 feed stream, it is further preheated with heat exchange from a mixture of produced H2O and unreacted H2 exiting from the SOFC. The exit stream is mixed with the second stage of preheated H2 feed stream to be recycled back to the SOFC inlet. Any additional required heat input to the H2 feed stream is provided with a furnace operated with NG, which is also used for unit start-up. The air feed stream as the source of O2 passes through a blower and is pressurized to 126.5 kPa. It is then preheated in two stages, first with waste energy from the SOFC as for the H2 feed stream, and then with the exit mixture of unreacted O2 and N2. Any additional required heat input to the air feed stream is provided with the NG furnace. The utilization rate for H2O is assumed to be 90%, and it is sourced from a de-ionized water tank stored at atmospheric pressure. The H2O feed stream is pumped to 138.9 kPa in this process, and preheated with compressed H2 exiting from the 1st stage of the H2 compressor unit. Following the first preheat of the H2O feed stream, it is further preheated with heat exchange with the exit mixture of H2 and unreacted H2O from the SOEC. This exit stream is cooled to separate the H2O, and the H2 is compressed in 6 stages with a target maximum exit temperature of less than 200°C. 10% of produced H2 is recycled back to the SOC, mixing with hot water (saturated steam). After each stage of compression, the compressed H2 is cooled down to condense the remaining unreacted H2O. Any additional required heat input to the H2O feed stream is provided with the NG furnace. The unreacted O2 exit stream passes through an aerial cooler to cool it to 25°C before exhausting it to the atmosphere. Process Cost Model The process cost model integrates the direct and indirect costs involved in fabrication and installation of the MSRSOC technology. The process cost model is influenced by the cost of individual cell component materials, cost of interconnects and sealants, fabrication methodology, and the associated utility and labour costs. The manufacturing cost of the SOC technology for this project is considered based on publicly available data from NREL to be $212 per kW when produced at a nominal economy of scale. The MSRSOC utilizes a stainless-steel metal supported design, which introduces additional cost savings in comparison with the traditional electrode- or electrolyte-supported SOCs, offering the potential for further reductions in manufacturing cost when produced on a larger industrial scale. Preliminary cost estimates of the MSRSOC technology are significantly below $200/kW for the stack alone, but as shown below, the utility and fuel costs of hydrogen and electricity production, respectively, remain the dominant factors in the overall economics of the energy storage process using SOCs. The cost of hydrogen storage is based on DOE information available in the public domain. Steam generation is evaluated as equivalent heat input from NG. The MSRSOC enables water recovery in FC mode, which can offset water demand in electrolysis mode. For our evaluation we assumed demineralized water exported to the facility. It should be noted that the actual cost of water was found to be insignificant in the overall economic analysis compared to the other utility costs. FC and electrolysis performance data with hydrogen were generated internally for MS-SOCs at the cell level. A standard power plant output of 5 MW at the design operating condition was chosen for the analysis, being the minimum size of stationary generation unit large enough to contribute to the power grid in Ontario, Canada. The target operating point is 0.8 V per cell, providing a balance between operating efficiency (60% electrical, HHV) and system size. The output power can increase beyond the target value to the peak power density. When using hydrogen as fuel, its cost and source (e,g, reforming vs. electrolysis) can vary widely, affecting the environmental profile. Results and Discussion The impact of both hydrogen cost and operating condition for an SOFC operating at 800 C on hydrogen are shown in Figure 1a. The hydrogen cost has a direct and linear impact on the electricity cost. The choice of operating condition has a more variable effect. At power densities leading to powers below the 5 MW target range, power output has less impact on electricity cost than hydrogen price. At power densities above the target range, the influence of output power becomes stronger as the system is pushed farther below its target voltage. When the power output approaches twice the nominal level, the cost of electricity more than doubles as the operation becomes less and less efficient. Therefore, it is advisable to size the system so that the expected target power output falls near the middle of the range that the system can produce. Figure 1b shows the levelized cost of electricity in $CAD/MWh for a nominally 5 MW SOFC system operating on hydrogen at 800°C, for a range of operating conditions analogous to those shown in figure 1a. The FC stack and the BOP together form the capital expenditures (CAPEX) required to produce the electricity. The fuel cost was considered as the primary operating expense (OPEX), which was combined with the CAPEX to calculate an estimated LCOE when using hydrogen at 800 o C. The OPEX would also include maintenance operations required to keep the system operational. As a relatively new technology, there is insufficient historical data for estimating the MSRSOC maintenance costs. Since the stack has no moving parts, mechanical maintenance would be minimal. However, performance degradation due to chemical processes such as oxidation or material processes such as microstructural coarsening of electrodes would limit stack lifetime in early stages of deployment. Since these mechanisms are not reversible, maintenance time would not be the main mode of intervention to deal with the consequences of these performance losses. Instead, the stack lifetime was estimated to be only 5 years for early generations of the technology, so that the stack hardware cost is incurred every 5 years of operation by replacing the entire stack. The BOP consists of mature technology for hydrogen compression in electrolysis

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