Kai Ryssdal: At the risk of telling you something you might already know, the renewable energy mantra usually goes something like this: 'We've gotta invest in green technologies -- solar, wind and biofuels -- or we're just going to keep being dependent on fossil fuels.'

It's entirely possible, though, that we're looking in the wrong direction, that there's a clean energy source with huge potential that isn't pie in the sky; and best of all, it's already here -- right under city streets.

From the Marketplace Sustainability Desk, Eve Troeh reports.

Eve Troeh: At Seattle's Pike Place Market, Beecher's Handmade Cheese lures customers by making its fromage right in front of them. Tourists press their faces to the glass as cheesemaker Adam Smith stirs a vat of curds and whey.

Adam Smith: It's doing what's called its cook. We heat it 10, 12 degrees over a period of 35 minutes, and then we hold it at that temperature for 40 minutes.

The vat's encased in a steel jacket. The lining leaves space for hot water. To get the water to the exact right temperature, Smith turns a valve that lets in steam. The steam comes through pipes run by a local company called Seattle Steam.

Stan Gent: We've been doing this for 120 years.

Stan Gent runs Seattle Steam. He shows me the old brick plant. One wall's painted with a mural of killer whales. Inside, workers prep the furnace for a giant fire that will boil water. They burn recycled wood. Gent says it's greener and cheaper than fossil fuel.

Gent: The wood's coming from the city itself, broken pallets and packaging. When the electric utility cuts the trees to keep 'em off the power lines, it'll eventually end up with us.

When the company started in the 1800s it made two things: steam and electricity. The electricity ran the downtown streetcar. Making the power for that created extra heat. The heat got converted to steam, and sold to buildings. When the streetcar went away, the company dropped electricity and stuck to steam. But from the late 1800s to the 1930s, most urban power plants sold both. And some city plants, like New York's, still do. But lots of cities grew cold to the idea of power plants downtown.

Tom Casten: They were originally coal, it wasn't very clean.

Tom Casten founded Recycled Energy Development. It's a thermal energy company in Chicago.

Casten: You had ashes and dust on the street, and people wanted them out of their neighborhood.

So Casten says we moved power plants further from cities. And lost steam in the process. You can't send steam more than a few miles, so power plants just let that extra heat float into the air.

Casten: We throw away an amount of heat from making electricity that is sufficient to heat almost all of the buildings in the United States.

Casten says power plants today are much cleaner. He wants to put them back in city centers, build more pipes, and use all that wasted heat.

Energy analyst Sam Jaffe says that idea's had a hard time getting attention.

Sam Jaffe: It's not considered a very sexy topic.

But Jaffe says it does have one sexy selling point.

Jaffe: Efficiency is the lowest-hanging fruit.

As energy prices rise, buildings and factories want to cut their energy bills. Jaffe says if they can use steam, they'll use less electricity or gas.

Jaffe: It's the easiest way to reduce greenhouse gases and also maybe the cheapest way of getting more power for less dollars.

So as business looks around for better ways to use energy, he says the hiss of a steam pipe could get more attention.

I'm Eve Troeh for Marketplace.