Challenges and Opportunities

The Electrofuels approach to biofuels represents a significant departure from photo­synthetic strategies, and it is important to consider both the merits and challenges inherent to a system built around a chemolithoautotrophic platform. Here, we con­sider the relative advantages and disadvantages of an Electrofuels vs. a photosyn­thetic approach for each of the important drivers of the system, specifically the required resources and the economics of at-scale production.

3.1 Land Resources

The biochemical conversion of solar photons to liquid fuels involves many steps, each with the potential to produce losses of both energy and carbon. Plants harness energy from only a portion of the solar spectrum, and only during the growing sea­son, which in temperate climates ranges from roughly 180 to 250 days per year. Under many conditions, plant growth is limited by access to resources other than light, in particular water, trace nutrients, and CO2. Even the most prodigious dedi­cated energy crops produce harvestable biomass encompassing roughly 1% of the incident solar radiation; additional losses from agriculture and conversion significantly reduce the usable portion of the captured energy. The primary consequence of this low efficiency is that enormous land resources are required to collect sufficient solar energy for US liquid fuel production (Table 2). Several advanced photosyn­thetic biofuels approaches under development claim higher solar photon utilization efficiencies, some as high as 7.2% [49] , although none of these approaches have been demonstrated at scale.

Chemolithoautotrophic approaches to biofuels require land only for solar radia­tion collection, and can thus use nonarable land for fuel production. Any of several currently deployed technologies, for example concentrating solar or solar PV, can capture >20% of incident solar radiation energy. Coupling one of these technologies with a microorganism, up to 13% of total available solar energy could plausibly be captured in a liquid fuel (Supp Calc 4). This solar radiation can be captured year round, significantly increasing the solar yield from a given land area when com­pared to seasonal crops. Further, it is possible to intersperse farmland with renew­able resources such as wind so that fuel production no longer competes with food production.

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