(590a) Integrating CO2 Electrolyzers with Electricity Systems Including Intermittent Renewables

Electrochemical CO₂ reduction (CO₂R) offers a promising pathway for carbon utilization via the electrolysis of CO₂ to chemical feedstocks. Despite extensive research on scaling up CO₂ electrolyzers, their integration with renewable energy sources and chemical processes remains underexplored. Most assessments of the economic case for CO₂R have assumed continuous, steady-state operation, as is typical for thermal catalysis. However, the variability of wind and solar electricity over multiple timescales may require CO₂ electrolysis to operate under different unsteady state conditions.

Many techno-economic assessments (TEAs) on CO₂R use wholesale renewable electricity prices (≤3 ¢/kWh), while assuming a high capacity factor (≥90%). Here, we present a techno-economic assessment of CO₂R integrated with realistic energy systems. While wholesale wind and solar electricity are cheaper than retail electricity, we find that their low capacity factors make them uneconomical for direct integration with CO₂R today. Even with cheap wholesale solar electricity, operating commercial-scale CO₂R for 27% of the time rather than 97% increases the product cost of CO by 84%, from $1.4/kg to $2.5/kg.¹ Given that CO is a toxic gas that cannot be stored easily, substantial energy storage capacity would be required to operate a CO₂R process powered directly by wind or solar. However, sufficient energy storage to smooth the supply of an average solar farm would triple the capital cost of a 50 t_CO/day plant from $34.4 million to $112.6 million. The capital cost to build this scale of energy storage far exceed the capital expenditure of the CO₂ reduction process itself.

A more promising solution is to construct energy storage outside the process limits, somewhat increasing utility costs without excess capex. Implicitly, the electricity system and electrolysis process must be designed and operated together. Additionally, carbon-neutral CO₂ reduction requires a portion of the electricity mix to be fossil-free (≤0.16 kg CO₂/kWh). Rather than the electrolyzer- or process-level assessments typical in the field, this problem can be represented by an optimization problem balancing variable electrolyzer operation, battery charging and discharging, renewable electricity sold to either the electrolyzer or the grid, or retail electricity powering electrolysis. The resulting flexible CO₂R process can integrate battery systems for supply smoothing, combine renewable and non-renewable electricity, or provide demand response depending on retail electricity pricing. Our approach incorporates current models for electrolyzer dynamics, real-time wind and solar availability, electricity market prices, and grid mixes.

1. Da Cunha, S. C. & Resasco, J. ACS Energy Lett. 9, 5550–5561 (2024).