Think of direct solar fuels as gas without the death or waiting.

ARPA-E, the advanced projects research group at the Department of Energy, gave out $23.7 million in grants this week to startups and universities experimenting in the relatively new field of direct solar fuels.

What are direct solar fuels? In most cases, the term of convenience explains a way to make fuel where the sunlight serves as an ingredient in chemical reactions. BioCee and the University of Minnesota wants to take sunlight, carbon dioxide and two organisms (cyanobacteria for sunlight capture and shewanella for metabolic transformation) to produce a liquid hydrocarbon. The company's intellectual property in large part revolves around creating platforms out of thin films where two species of organisms can work and thrive in harmony in relatively fixed ratios.

"A question for researchers for the last several years has been how do you work with multiple species in a stable environment," said BioCee CEO and founder Marc von Keitz.

Researchers at Penn State say they can do something similar, but instead of microbes, they mix a membrane of titanium dioxide nanotubes in with sunlight and carbon dioxide.

MIT-spin out Sun Catalytix, meanwhile, captures solar energy and exploits it to split water to produce hydrogen. About half of the companies use a biological catalyst and the other half employ chemical catalysts. Many also include carbon dioxide sequestration in the plan.

The fuels are direct because the energy from the sun doesn't have to pass through an intermediate stage. Coal and oil are indirect forms of solar energy, the end result of what happens when you apply gigantic geological forces to now dead photosynthetic plants over the course of millions of years. The world's largest oil fields came from ancient algal blooms: even if you want to count dinosaurs as part of the fuel system, their food chain revolved around plants. Biomass is effectively coal in a hurry: the plants absorb sunlight to grow and we chop the plants down and burn them.

By contrast, the fuel from direct solar companies will be a byproduct of the organism or chemical reaction, not the husk of the organism itself. Because the microorganism is being milked versus killed to get fuel, the yields potentially can be higher. The same organism, or catalysts, can produce large quantities of fuel without being replenished.

Biological and chemical catalysts are actually related, notes von Keitz. Typically, microorganisms contain trace amounts of metals to spur metabolic functions. "Chlorophyll has magnesium at its core," he said. The difference is that biological catalysts can, potentially, be made less expensively. Chemical catalysts need to be mined, often outside of the U.S.

"We can grow them in a vat," he said. "You might [conceivably] have to switch from a dependency on foreign oil with a dependency on foreign metal."

Similarly, obtaining hydrogen through the electrolysis of water involves getting electricity from a coal-fired power plant. Catalytix requires some outside energy, but says it can be provided from solar cells, a direct form of solar energy. More importantly, cobalt, phosphate and platinum-based catalysts are used to accelerate the reaction. It is similar to photosynthesis. PV panels, solar light and solar thermal energy are also all forms of direct solar energy. Hydroelectric power, wind and wave power are all indirect.

Although the term direct solar is somewhat new, research in the field has gone on for a few years. In 2005, Stanford's James Swartz isolated a microbe that metabolizes sunlight to split hydrogen from water. A few small startups in Israel and the U.S. have experimented with microbial fuel cells. The grants, though, underscore the momentum in the field.