1. That such fuels are renewable and can be used in the existing energy infrastructure;

2. That solar fuels—liquid hydrocarbon fuels produced from CO₂, water and solar energy—provide a synergistic solution to the problem of overabundance of CO₂ and the abundance of renewable energy on the one hand, and the scarcity of hydrocarbon fuels on the other.

3. That solar fuels offer the promise of solar energy storage—a key challenge in a world predominantly relying on renewables.

4. That the costs of carbon capture can be offset by producing valuable fuels or chemical products from CO₂.

If we review these four broad claim areas, we observe that all of them have merit in principle, but all of them call forth immediate questions of utility. These questions often cannot be answered in abstracto but will require some form of (quantitative) life cycle assessment (LCA). Although the importance of a life cycle approach for these questions has been acknowledged, the authors are not aware of any existing LCA studies on CO₂ utilization. It has been the observation of the authors that—in the absence of such evidence—the participants in the public and scientific discourse on solar fuels use any or all of the four claims above to substantiate their argument. In the absence of quantitative evidence for or against, four intuitively persuasive arguments make a very strong case indeed. This is the state of affairs as we perceive it to be. It is the purpose of this paper to hold the claims up to quantitative scrutiny and review the general merit of the idea of solar fuels, subject to the definition above.

—van der Giesen et al.

In the study, the team called fuels produced by the conversion of CO₂ into liquid hydrocarbon fuels using proven technologies “synthetic fuels”, and fuels produced via a production route that uses solar energy “solar fuels”.

The team explored three different CO₂ sources (from natural gas (NG) combustion; from biomass (BM) combustion; and from direct air capture (DAC)); and three sources of electricity (natural gas (NG) combustion; biomass combustion (BM); and solar (PV)). For the study, the hypothetical CO₂ pathways were designated by the combination of abbreviations for CO₂ source and power—for example, DACPV indicated direct air capture for CO₂ and solar power.

Their lifecycle analysis of the fuel production routes put a main focus on the energy and material inputs, using fuel production from source-to-fuel expressed in 1 MJ of liquid fuel as the functional unit. CO₂ emissions from combustion were also taken into account when calculating net GHG emissions.

They compared the outcomes to the performance of existing hydrocarbon fuels such as diesel; bioethanol; from sugar cane; GTL and BTL diesel.

GHG emissions of different liquid fuel production routes related to a functional unit of 1 MJ. Data for diesel, GTL, ethanol, BTL and solar hydrogen cannot be divided into the uptake of CO2 and connected processes and are therefore aggregated. R indicates reference alternatives not explicitly modeled in this study. Credit: ACS, van der Giesen et al. Click to enlarge.

Disaggregated cumulative energy demand (CED) scores of different liquid fuel production routes related to a functional unit of 1 MJ. CED data for diesel, GTL, ethanol, BTL, and solar hydrogen are presented in aggregated form since separate production steps for hydrogen and CO2 cannot be discerned. R indicates reference alternatives not explicitly modeled in this study. Credit: ACS, van der Giesen et al. Click to enlarge.

Overall, they found that an improvement in life cycle CO₂ emissions is only found when solar energy and atmospheric CO₂ are used. With respect to their analysis of “the four questions”, they found:

1. Even the best performing fuel production route (DACPV), both now and in the near future depends on nonrenewable fuels and large amounts of renewable electricity.

2. The showed that using CO₂ from a fossil origin will not provide a carbon-neutral fuel. Carbon neutral fuels might be produced from CO₂, but only in a future energy system completely based on renewable energy.

3. The third claim, that producing fuels from CO₂ presents a route to storing renewable energy could not be completely supported or countered from the results, they concluded. Producing fuels from CO₂ by using solar electricity converts solar energy into easily stored chemical energy. To assess if this is a preferable route to store energy, additional research is needed in which the production and use of fuels from CO₂ needs to be compared with battery and hydrogen storage and possibly others storage options, such as water reservoirs, they concluded.

4. Producing a fuel from CO₂ neglects the primary goal of carbon sequestration. In addition, more energy is needed to produce a fuel from CO₂ than it will deliver when combusted. It would be more profitable to sell the required energy directly on the market than to lose a great deal of energy in the inefficient production of liquid fuels, the researchers found.

In conclusion, we state that the energy demand and climate impacts of using CO₂ to produce synthetic hydrocarbon fuels by using existing technologies are higher than the impacts of existing hydrocarbon fuels. The intuitive claims in the public discourse cannot be supported by our quantitative assessment which shows that producing liquid fuels from CO₂ is not the straightforward panacea as suggested. However, novel technologies (e.g., photocatalytic, solar thermal and enzyme based routes) might show a slightly improved system performance.

In a distant future, we foresee possible niches for hydrocarbon fuels produced from CO₂. Since hydrocarbon fuels offer high energy to mass and volume ratios compared to alternatives like electricity and hydrogen, the application of fuels produced from CO₂ in applications such as airplanes and heavy duty transport may prove to be a valuable but future niche. Another application could be as a storage medium for so-called stranded renewables. Because renewable energy technologies produce difficult-to-store electricity and renewable electricity supply increasingly exceeds demand, research has begun in leveling supply and demand using concepts such as smart grids. Even in smart grid systems it can be expected that some sort of energy storage will still be needed. For storing large amounts of renewable energy, the production of synthetic hydrocarbon fuels is one of the technologies that could be used. However, it would have to compete with other storage technologies such as batteries, hydrogen storage, water reservoirs, or compressed air storage.

—van der Giesen et al.

Resources

• Coen van der Giesen, René Kleijn, and Gert Jan Kramer (2014) “Energy and Climate Impacts of Producing Synthetic Hydrocarbon Fuels from CO₂,” Environmental Science & Technology doi: 10.1021/es500191g