Boston Medical and Surgical Journal
A massive reduction in CO 2 emissions from fossil fuel burning is required to limit the extent of global warming. However, carbonbased liquid fuels will in the foreseeable future continue to be important energy storage media. We propose a combination of largely existing technologies to use solar energy to recycle atmospheric CO 2 into a liquid fuel. Our concept is clusters of marinebased floating islands, on which photovoltaic cells convert sunlight into electrical energy to produce H 2 and to
... produce H 2 and to extract CO 2 from seawater, where it is in equilibrium with the atmosphere. These gases are then reacted to form the energy carrier methanol, which is conveniently shipped to the end consumer. The present work initiates the development of this concept and highlights relevant questions in physics, chemistry, and mechanics. renewable energy | carbon dioxide recycling | synthetic fuel | maritime structures L imiting anthropogenic global warming to below 2°C, a goal of the Paris Agreement of the United Nations Framework Convention on Climate Change (1), now ratified by 174 countries, will require within the coming decades the phasing out of carbon dioxide emissions from fossil fuel burning. However, in the foreseeable future, carbon-based liquid fuels will continue to play an important role, in particular for aeronautical, marine, and long-haul automotive mobility. It is therefore essential to investigate possibilities of using renewable energy to recycle CO 2 between the atmosphere and synthetic liquid fuel (2). Efforts to photochemically produce synthetic fuel from CO 2 and water (i.e., via artificial photosynthesis) show some promise (3). We propose an approach using more conventional methods, but with important unique aspects. Methanol, CH 3 OH or MeOH, is the simplest carbon-based fuel, which is liquid at ambient conditions (4). With approximately half the energy density of gasoline (15.6 MJ/L vs. 32.4 MJ/L), it can be used to power existing gas turbines, modified diesel engines, and direct methanol fuel cells. Methanol can serve as a feedstock for most petrochemical products, and by simple dehydration it can be converted to dimethyl ether, an attractive substitute for natural gas, and other hydrocarbon fuels. Methanol can be produced (5) by the catalytic hydrogenation of CO 2 [presently the largest methanol production facility using this technique is located in Iceland (6)], and MeOH burns in air to release CO 2 and water: An attractive scenario for the production of synthetic methanol fuel is the recycling of atmospheric CO 2 , the electrolytic produc-tion of H 2 , and their catalytic reaction to CH 3 OH, with all of these processes powered by renewable energy. As a concept for realizing this scenario, the present work proposes the production of H 2 and the extraction of CO 2 from seawater and their catalytic reaction to produce MeOH on clusters of artificial, marine-based photovoltaic (PV)-powered "solar methanol islands" (7) (Fig. 1) . We present an initial implementation plan; in view of many uncertainties, much additional work remains to be done. Seawater as a Source of H 2 and CO 2 Renewable synthetic fuel production on distributed facilities in a marine environment has attractive features, including abundance of insolation and raw materials, avoidance of local CO 2 depletion, convenient ship-based transport to and from the sites, flexible placement close to population centers, and possible combination with aquaculture and other marine activities. The production of 1 mol of CH 3 OH ideally requires 3 mol of H 2 and 1 mol of CO 2 , the production/extraction of which from seawater presents serious electrochemical challenges. Electrolysis of Seawater The enthalpy change in splitting liquid water into gaseous hydrogen and oxygen is 286 kJ/mol H 2 ; industrial electrolysis plants typically require 380 kJ/mol H 2 (11) and a value of 302 kJ/mol H 2 Significance Humankind must cease CO 2 emissions from fossil fuel burning if dangerous climate change is to be avoided. However, liquid carbon-based energy carriers are often without practical alternatives for vital mobility applications. The recycling of atmospheric CO 2 into synthetic fuels, using renewable energy, offers an energy concept with no net CO 2 emission. We propose to implement, on a large scale, marine-based artificial islands, on which solar or wind energy powers the production of hydrogen and the extraction of CO 2 from seawater and where these gases are catalytically reacted to yield liquid methanol fuel. The present work proposes specifications for such facilities and highlights essential challenges in physics, chemistry, and engineering which must be met to realize this ambitious proposal.