Now, some scientists are testing the notion that sunlight might be optional.
Researchers at the Missouri University of Science and Technology are planning to grow algae for fuel in abandoned mines using light-emitting diodes, or LEDs.
"About this time in the conversation, someone usually raises their hand and says, 'But it's dark,'" said David Summers, a mining engineering professor. "That's not necessarily a disadvantage."
Algae need light to produce lipids, or oil, but they work best when they use only the red and blue parts of the light spectrum and when they are given time in the dark to process the photons, Summers said.
That is where LEDs come in, Summers said. They can be tailored to emit only the needed light frequencies, and they can be set to pulse several times a second at a rate that gives the algae time to absorb and process the light energy without wasting it.
"When it's sunny, plants are totally saturated pretty early on in the day," said D.J. Vidt, a graduate student. "Unless they get shade to process the photons, it's basically wasted energy. We're just shortening ours from hours to milliseconds ... for efficiency."
Using LEDs to grow algae is not a new idea. Researchers have been working on the concept for years, and some startup companies are using the idea as the basis for their business models.
"We like LEDs because they're so efficient," said Riggs Eckelberry, president and CEO of OriginOil Inc., a California-based company using LEDs to grow algae.
But Summers wants to take the concept a step further by placing the photobioreactors, which house the algae, underground in abandoned mines. Using mines allows algae growers to address three problems of open, outdoor ponds: evaporation, contamination and fluctuating temperatures.
"The one thing that underground mines have is constant temperatures," Summers said.
Plus, mines can be inexpensive, since mining companies have already done the most expensive site preparations.
"Something like 47 percent of the capital costs of surface algae ponds are infrastructure," Vidt said. "We're just shifting it from investing in pavements to investing in lighting systems."
Growing the photosynthetic algae underground also addresses concerns about invasive species of algae escaping, Vidt said. Even if the species escape containment from the photobioreactors, they pose little threat as invasive species because they could not survive for long in the dark.
The setup also provides a perk for mining companies faced with the task of cleaning up in the wake of excavation.
"Algae are exceptional at sequestering metals," Vidt said, adding that the biodiesel produced on site could be used in additional mining operations. "This is a [public relations] boon for these guys: It works for them, they get some fuel, and on the flip side, they don't have to pay reclamation."