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Of all current proposals for sustainable transportation, the assumption is energy scarcity when there are economically favorable alternatives using existing technology. This paper explores the economics of a sustainable transportation energy pathway that provides carbon-neutral and carbon-negative synthetic fuel derived from seawater as the feedstock and power via Ocean Thermal Energy Cycle (OTEC). Seawater-based synthetic fuel is naturally carbon-neutral - different synthesis processes can yield hydrogen, methane, methanol and ethanol as well as gasoline, diesel or jet fuel - and is carbon-negative when combined with aquaculture. Methanol is favored as a fuel as it requires relatively lower capital investment; can be easily transported and stored; can be used as a feedstock to many chemical processes that currently rely on petrochemicals; and can be coproduced with or converted to dimethyl ether.

Forthcoming transportation carbon dioxide and environment emission regulations favor solutions with the lowest cost of ownership in the open market. Light duty vehicles and especially those used in urban areas benefit the most from increasing electrification, but medium and heavy duty vehicles - including truck, marine, rail and aircraft - have high continuous power requirements that benefit little from electrification until energy storage costs drop significantly. This paper explores energy storage costs whose alternatives will be weighed against increasing payload fraction costs. A conceptual solution is presented that provides low cost of ownership: an engine that operates as a spark-ignited two stroke engine with compounding and light electrification. This engine (named BETE-Max) has a high thermal efficiency across its operating range and is able to meet tailpipe emissions standards with low aftertreatment costs using known technology.