110th Anniversary: Evaluation of CO₂-Based and CO₂-Free Synthetic Fuel Systems Using a Net-Zero-CO₂-Emission Framework

Abstract

This work analyzes quantitatively the energy and exergy efficiencies of storing intermittent renewable energy in chemical fuels. In the future energy system, chemical fuels provide a very effective approach for long-term storage and long-distance transport of renewable electricity. For the sake of completeness and simplicity, we consider both carbon-free fuels, namely, hydrogen and ammonia, and carbon-rich fuels, i.e., methane and methanol, synthesized using CO2 as the precursor. The latter are called CCU fuels as they constitute an application of CO2 capture and utilization (CCU), which is often advocated to be an effective approach toward climate change mitigation (though no consensus exists). Instead of focusing on the CO2 conversion step, we apply a system-oriented perspective, grounded in the net-zero-CO2-emission framework, to quantify merits and drawbacks. In such a framework, we consider eight systems and technology chains where, in the spirit of a circular economy, the only input is renewable electricity and the only output is a service, consisting in delivering either electricity to the grid on demand (power–fuel–power) or a fuel to propel a means of transportation (power–fuel–propulsion); no fossil carbon is used, and no net CO2 release to the atmosphere occurs. Providing the service of storing renewable electricity in chemical fuels obviously results in a loss of primary energy, which differs in the eight cases considered, depending on the chemical nature of the chemical fuel and on the number and efficiency of the individual steps to synthesize them. Power–CCU fuel–power systems exhibit an energy loss from 65% to 86%, whereas the energy loss of power–CCU fuel–propulsion systems increases to 83–94%. The energy loss of the corresponding systems using ammonia as fuel is similar, whereas that obtained when using hydrogen is significantly smaller, namely, 50–65% and 57–69% in the power–fuel–power and the power–fuel–propulsion case, respectively. Compared to hydrogen, the other energy carriers suffer from increased system complexity and consequently lower efficiency. Exergy analysis has shown low efficiency improvement potential for especially the fuel synthesis step, while the other steps in the chain (electrolysis, extraction from air of CO2 or nitrogen, fuel utilization, and associated compression) still exhibit higher improvement potentials.

A Circular Approach for Making Fischer–Tropsch E-fuels and E-chemicals From Biogas Plants in Europe

“…Contrariwise, the circular economy promotes the reuse of carbon material, turning a waste stream into a resource with market potential for producing higher value-added products (Song et al, 2020). In a fully established circularity of carbon material, it is paramount that the C 1 molecule comes from biomass, biogas, or directly capturing CO 2 from the air (Sutter et al, 2019). For this to happen, a period of carbon transition where fossil hydrocarbons are subsequently substituted by renewable hydrocarbon is vital (IEA, 2020).…”

What Does It Take to Go Net-Zero-CO2? A Life Cycle Assessment on Long-Term Storage of Intermittent Renewables With Chemical Energy Carriers

“…This can be done mechanically (high pressure, cryogenic), chemically (via Methane, Methanol or Ammonia) or through physisorption (Fullerenes, Nanotubes) (Niaz et al, 2015). Here, we assumed the same chemical energy carriers for storage as earlier done in Sutter et al (2019): compressed hydrogen (100 bar), methane and ammonia. Methanol was excluded from this research because of the lower net system efficiency compared to methane and the fact that it lacks the inherent benefit of the existing natural gas infrastructure (Sutter et al, 2019).…”

“…Ammonia can be stored as a liquid at only 10 bars of pressure, which may be advantageous from the perspective of storage and energy required for pressurization. These chemical energy carriers can subsequently be used for the production of electricity, as described in Sutter et al (2019), whence they are colloquially called Power-to-X-to-Power (P-X-P) systems.…”

“…The interest for the combination of iRES with P-X-P systems is motivated by their potential to limit environmental impacts compared to existing, fossil-based electricity production systems. To that end, such systems need to have a very low carbon footprint and preferably be net-zero-CO 2 (Davis et al, 2018;Sutter et al, 2019), as well as present limited other environmental burdens. An effective and comprehensive method to analyze the greenhouse gas and other environmental impacts of a technology or system is Life Cycle Assessment (LCA).…”

“…Grinberg Dana et al researched Power-Ammonia-Power (P-A-P) systems, but only on a technical level and did not include an environmental assessment (Grinberg Dana et al, 2016). Sutter et al (2019) performed a first screening assessment, including the system design, efficiency, and exergy analysis of net-zero-CO 2 systems for iRES storage. They included Power-to-Fuel-to-Power, as well as Power-to-Fuel-to-Propulsion and investigated hydrogen, methane, methanol and ammonia as energy carriers.…”