Carbon Dioxide Mitigation Using Renewable Power

Combatting global climate change from anthropogenic CO₂ emissions is an active area of discussion and research. One area of considerable excitement is the “utilization” of CO₂ as a raw material for producing valuable products, such as building materials, fuels, and chemical building blocks [1]. Such utilization is challenged due to the energy input required for the chemical conversion of CO₂. There is a growing recognition that, if the energy is provided from fossil resources, the CO₂ produced is greater than that consumed [2]. This problem appears to be overcome if renewable energy is utilized for the conversion of CO₂. However, approaches other than CO₂ “utilization” are available for the use of renewable energy to mitigate CO₂ emissions. Examples include (i) electrolysis of water to hydrogen, using the hydrogen in a fuel cell for transportation, and (ii) direct use of renewable power for battery electric vehicles. These alternate routes mitigate CO₂ by using renewable power to displace the combustion of fossil fuels for transportation.

As we tackle the technical, economic, environmental, and resource management issues related to the use of renewable energy for CO₂ mitigation, it is important to understand the potential “efficiencies” for the conversion of renewable energy in the various approaches. We should view renewable energy, in particular renewable power, as a valuable resource, not as a “cheap” or “free” resource to be used indiscriminately. Both thermodynamic and practical arguments will be made to make realistic estimates of the magnitude of CO₂ consumed or displaced from various schemes that utilize renewable power. Renewable power through the grid to a battery electric vehicle is nearly four times as effective at mitigating CO₂ emissions as using liquid fuel via electroreduction of CO_2. Electrolysis of water with renewable power to produce hydrogen for a fuel cell vehicle is about twice as effective at mitigating CO₂ as electroreduction of CO₂.

This is not an attempt to cover the complete life cycle analysis and greenhouse gas impacts of each scheme discussed. This is an attempt to provide an approximate ranking of the effectiveness of the various schemes that utilize renewable power to mitigate CO₂ emissions. It is believed that inclusion of the thermodynamics for the major energy-consuming steps for each scheme will allow this ranking to be done. It is hoped the perspective this provides will help researchers prioritize their available resources into areas that have the greatest potential impact for CO₂ mitigation via renewable power.

[1]	X. Lim, "How to Make the Most of Carbon Dioxide," Nature, vol. 526, pp. 628-630, 2015.
[2]	S. Stevenson, "Thermodynamic Considerations in CO2 Utilization," AICHE Journal, 2019.