It’s one of the world’s most ubiquitous sources of energy and it’s barely been tapped.

And there’s probably a source of it a few feet away.

I speak of waste heat, the warmth that comes off of industrial equipment, household appliances and notebook power bricks. Waste heat is essentially electricity or petroleum that you paid for but likely didn’t get to use in a productive manner. Whenever AC current gets converted to DC, or the engine in your car kicks into action, the result is waste heat.

While a little heat from a notebook or a cell phone doesn’t seem like a maelstrom, the cumulative effect of waste heat is staggering. Back in 2008, researchers at UC Berkeley estimated that the U.S. consumes 100 quadrillion BTUs, or quads, of energy a year and that 55 to 60 of those quads get dissipated as waste heat. The Wartsila-Sulzer two-stroke diesel engine for ships is considered one of the most efficient in the world: half of the energy gets converted to waste heat.

Half, and in many cases more, of the energy in quite a large number of data centers gets consumed by air conditioners to get rid of waste heat. Think of it: half of the power bill goes to getting rid of another form of energy that cost you money but you couldn’t use.

Echogen, a startup, says it can exploit a fluid called supercritical carbon dioxide (ScCO2) to convert heat into power for less than four cents per kilowatt-hour. The average retail price of electricity in the U.S. hovers above 9.5 cents.

On the total opposite end of the spectrum, companies such as Komatsu, Alphabet Energy and Phononic Devices are devising chips that can convert heat to power directly. No mechanical compression needed. Wrapping these around components in a car or truck could provide power to run the air conditioner, which in turn would boost mileage. It could be a cheap, quick way to meet CAFE standards.

Like in many green fields, the challenge lies in the details. Heat from servers and computer is equipment is “low grade,” i.e., not enough of it exists to make a recovery system worthwhile. Factories are complex environments, too, and the chips from companies like Phononic are just coming out of the lab.

Will it go mainstream or stay on the fringes? If anything, you have to admit that once you think about waste heat, you start to see it everywhere.

Read more on this topic in a joint effort by General Electric Ecomagination and Greentech Media, and join in on the conversation here.

Tags: ac dc, ac power, alphabet energy, bernouli principle, bismuth telluride, cars, china recycling energy corporation, data centers, dc power, energy efficiency, gas mileage, komatsu, phononic devices, recycled energy development, silicon nanowires