What Obama Meant by “Converting Sunlight into Liquid Fuel”

Barack Obama wants to convert sunlight into liquid fuel. It was a passing reference in the State of the Union address last night (he highlighted the same technology in his 2011 address). If scientists can figure out how to do this, he said, it could “unleash new jobs.”

Currently there are three main ways to use solar power to propel cars and airplanes, but they all have problems:

1. Put solar panels on your vehicle. Given space limitations on a car, even very efficient solar panels would produce only two kilowatts of power. In comparison, the electric Nissan Leaf uses an 80-kilowatt motor. 
2. Use solar panels to charge batteries or make hydrogen for fuel cells. Battery-powered cars are good for short trips and commuting, but batteries struggle to compete with gasoline, in terms of cost and weight, for longer driving. The weight disparity is an even bigger problem for airplanes. Using solar to split water via electrolysis produces hydrogen. But fuel-cell cars and hydrogen fueling stations are both rare and expensive.
3. Use sunlight to grow plants and convert those plants into biofuels. Plants only convert 1 to 6 percent of the energy in sunlight into sugars and other biomass. So they require a lot of land, as well as tractors and fertilizer involving fossil fuels. The process of converting biomass to liquid fuels also consumes energy. Add concerns about competition for food and heavy water use, and it’s unclear whether biofuels provide much environmental benefit.

Obama is talking about using sunlight to produce energy-dense liquid fuels more directly. There are several ways to do this.

One is to engineer the microörganisms that make biofuels from sugar to get their energy from electricity instead. This could be far more efficient than using plants to produce sugar with 1 percent efficiency, depending on how much electricity you need (see “Making Diesel from CO2 and Sunlight”).

Another option is to use solar panels to generate electricity, then use that electricity to perform electrochemistry. For example, you could split water to make hydrogen and break down carbon dioxide to get carbon, then combine the hydrogen and carbon to make hydrocarbon fuels much like gasoline or diesel.

A variation on this, known as artificial photosynthesis, would involve redesigning solar panels so that the electrons they produce do not generate an external electric current but drive electrochemical reactions inside the panel. This is more or less how leaves produce sugars—using electrons jostled free by the energy in sunlight to manipulate water and carbon dioxide from the air (see “Artificial Photosynthesis Effort Takes Root”).

It all looks good on paper, but in reality there are big hurdles to making the technology work. Researchers have demonstrated parts of these processes, but never put them all together in an efficient, economic package. Practically speaking, the only jobs these efforts will unleash in the near term involve doing the necessary research.