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predicted. And oil and gas drillers, using technology advances of their own, learned how to unlock enormous new reserves of fossil fuels.

Dozens of solar, biofuels and battery companies failed, unable to show enough promise to go public or attract the attention of bigger companies. "Investors are still waiting for their cleantech investments to produce returns," says Dallas Kachan, who runs the San Francisco clean technology consulting firm Kachan & Co. "Nobody's seen the stellar home runs they were hoping for five years ago."

Even when clean energy companies have gone public, they haven't fared well. An index of clean energy companies is down 71 percent since it began in 2005. A similar index of traditional energy companies is up 75 percent over the same period.

The value of global clean technology deals fell 29 percent last year to $7 billion, from a record $9.9 billion in 2011, according to the Cleantech Group. But the portion of that sum focused on conventional fossil fuels nearly tripled, to a record $556 million.

A few clean technology investors have stayed away from oil and gas despite the temptation. Khosla Ventures, for example, has been a major backer of advanced biofuels even as most of these companies have failed to live up to their promise.

The firm does not invest in companies that support fossil fuels.

But the drilling boom has led to countless investments in this gray area between clean and dirty.

Axine: Backed by Chrysalix and Royal Dutch Shell, Axine wants to make the drilling process known as fracking less dangerous by treating wastewater produced during drilling without chemicals.

Picarro: One of the cleantech investments of Greylock Partners, it is attempting to make natural gas production less harmful to the climate with its leak-detection system.

Neos Geosciences: It is backed by a "greentech" fund at Kleiner Perkins Caufield and Byers, where Al Gore is a partner. The company helps oil and gas companies find the most promising places to drill for oil and gas - and avoid drilling in ecologically sensitive locations - using sensors mounted on helicopters.

In 2007, a Sunnyvale, Calif.-based company called Liquid Robotics, Inc. began developing a seagoing drone that required no fuel. Its original purpose was to transmit live songs of humpback whales over the Internet.

VantagePoint's Alan Salzman thought Liquid Robotics' technology was great, and he had a good idea who might agree. He set up a meeting with Schlumberger, the giant oil and gas drilling services company, and the two companies have since formed a joint venture.

Liquid Robotics' drones will likely be used to help detect leaks from drilling operations and make sure the water is free of whales when oil companies search for oil. But the robots will also help locate new deposits of oil and gas.

"We are not philosophical purists," Salzman says. "We're investors."

Key terms

NEW YORK (AP) -- Here's a look at some key energy terms and technologies.

Source rock: Wide, thin layers of sedimentary rock, like frosting in the middle of a layer cake, that are interspersed with oil and gas. In the past, drillers had to look for places where oil and gas had seeped out of this rock and into large pools that were easy to tap. Now drillers can extract oil and gas directly from source rock, opening up vast new resources.

Fracking: The colloquial name for hydraulic fracturing. This is the practice of injecting water, sand and chemicals into source rock to crack it and create escape routes for oil and gas. Its increased use has raised concerns that the chemicals used could seep into groundwater, either through faulty wells or if it is not disposed of properly.

Horizontal drilling: Companies used to drill wells straight down into the earth to tap pools of oil and gas. Now they can drill down and then change the angle to follow thin layers of source rock, reaching more oil and gas with each well.

Down-hole sensors: Engineers have developed increasingly sophisticated sensors that follow drill bits and measure physical characteristics of the rocks and fluids underground. The information can be sent via fiber optic cable to engineers at the surface.

Remote drilling: Using computers, engineers can direct drill bits from command centers thousands of miles away. This reduces drilling costs.

Walking rigs: In the past, when rigs were finished drilling a well, they had to be disassembled and trucked to the next location, a process that took several days. Now some rigs can "walk" hundreds of feet on hydraulic shoes to the next drilling spot. This reduces drilling costs.

Biofuels: Liquid fuels such as ethanol and biodiesel made from plants. Biofuels made from corn are common. "Advanced biofuels" made from plant waste or algae are more environmentally beneficial, but companies have not been able to produce them at reasonable costs despite big subsidies.

Renewable energy: Energy derived from renewable sources such as wind, solar, geothermal or plant matter. This type of energy is growing fast in the U.S., but it remains a small contributor to the nation's energy mix. Ten percent of U.S. gasoline demand was met with biofuels and 4.8 percent of the nation's electricity was generated by wind, solar and geothermal last year.

Clean energy: Energy that pollutes less than coal and oil, the dominant sources of fuel for electricity and transportation. Natural gas is considered by some, including the Obama administration, to be clean because it emits far fewer pollutants than coal or oil. Nuclear power is also considered clean by some because nuclear plants emit no greenhouse gas or other pollutants into the air. Others consider only renewable energy truly clean.

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