Techno-Economic Analysis of Reversible and Paired Solid Oxide Cell Systems for Hydrogen Production

Abstract

The United States (U.S.) Department of Energy (DOE) National Energy Technology Laboratory (NETL) has been pursuing the development of solid oxide cell (SOC) technology to enable future power generation systems with high efficiencies and low emissions. Government and industry net-zero goals are driving interest in approaches to decarbonize challenging sectors of the economy, such as industrial applications. Hydrogen (H2) – including that produced by SOC technology-based high-temperature electrolysis – is one promising route. At present, low prices for low-greenhouse gas intensity electrical power (i.e., renewable sources such as wind and solar) are available intermittently (at low-capacity factors), complicating the economics of clean hydrogen produced by electrolysis. Reversible solid-oxide cell (r-SOC) based power to gas systems have the potential to address this challenge by producing power when electricity prices are high and hydrogen when they are low. Plants with the ability to co-generate power from solid oxide fuel cells (SOFC) and hydrogen from solid oxide electrolysis cells (SOEC) are of particular interest for their dual revenue streams and the flexibility they can provide to the grid.This study investigates the cost and performance of two configurations of co-generating SOC plants. The first includes an r-SOC stack that alternates between power and hydrogen production. The second plant contains paired SOFC and SOEC stacks that are each dedicated to generating a single product when operated. For the purposes of this study, the plants are primarily intended to generate hydrogen, only switching to power production mode when market conditions are favorable. The study aims to elucidate expected tradeoffs between the higher capital cost of the paired SOFC/SOEC plant and the higher degradation rate of the r-SOC.The anticipated tradeoff of a reversible system is an increased degradation rate for lower capital costs. This is due to the paired systems being optimized for either fuel cell or electrolysis operation while the reversible system is not optimized for either. To quantify the impact of this, a base case where the SOFC, SOEC, and r-SOC all degrade performance-wise at the same rate (0.2% per 1000 h). The degradation rate of the r-SOC is then allowed to float to where the levelized cost of hydrogen (LCOH) of the two systems is at parity, which does not occur at the base case assumptions. Additional sensitivity analyses are performed on natural gas price and cost of imported power to determine which system is more favorable under different assumptions.In summary, the results show that the total plant cost of the paired SOFC/SOEC system is about 5% higher than the reversible system. The additional cost of having two separate SOC modules is largely diluted by the balance of plant costs. The LCOH of the r-SOC and paired SOFC/SOEC are ≈$3.60/kg and ≈$3.30/kg H2, respectively as shown in Figure 1. Despite the higher capital cost, the paired SOFC/SOEC has a more favorable LCOH. This is due to the SOFC’s ability to generate electricity cheaper than the assumed $60/MWh grid purchase price. Sensitivity analyses show how the assumed cost of grid power and natural gas price affects the disposition of hydrogen versus electron generation. As an example, the LCOH of the r-SOC becomes more favorable than the paired SOFC/SOEC at natural gas prices above $12/MMBtu and at electricity purchase prices below $27/MWh.In conclusion, this investigation shows that depending on the market situation and final application, reversible SOC units warrant consideration for inclusion in advanced integrated energy systems as grid electricity prices continue to decrease over time.For the purposes of this study, natural gas is used as the fuel for power generation operation, and the systems are equipped with high rate (≈98%) carbon capture. In future work, stored hydrogen may be examined as the fuel for power generation. Future analyses may also examine the use of industrial or other waste heat sources, as well as perform life cycle analyses for the cradle-to-gate greenhouse gas intensity for select system configurations and applications. Figure 1