Thomas Edison's Pearl Street power station in New York was a 19th-century version of combined heat and power, using the "waste" heat given off in the making of electricity to warm nearby buildings. And some of the systems in Maryland go back more than half a century.

Despite that long history, there aren't many in the state — fewer than 25, the Energy Department said. Users range from military bases to the Johns Hopkins Hospital.

Now their number seems poised to expand. Veolia Energy, which operates energy networks in Baltimore and across the world that supply heating and cooling to customers, is just one of the companies thinking of adding combined heat and power systems in the city. The company said it already is one of the largest operators of such systems worldwide.

"Combined heat and power is a proven technology," said John Gibson, vice president of Veolia Energy North America's South region. "It's been around for a long, long time."

Rob Thornton, president and CEO of the International District Energy Association, expects growth in all sorts of "microgrids" that allow users to operate independently from utilities when necessary.

"What I think is driving this notion of microgrid has been just the frequency and severity of grid interruptions," said Thornton, pointing to last year's derecho and superstorm Sandy as recent examples.

Connecticut launched a microgrid pilot project last year, an idea born of widespread power outages the year before. It's a shift from the long-standing assumption that bigger — and farther away — is better when it comes to power generation.

Cost alone drives some of the interest. Baltimore Mayor Stephanie Rawlings-Blake said in an interview earlier this year that she wants to push energy expenses down so that money can be spent on other priorities.

"I get excited when we talk about ways we can generate more energy ourselves," she said.

The city's nearly 5-year-old combined heat and power system at Back River, on Eastern Avenue in Baltimore County, runs on biogas created by the waste digesters on site. That means lower greenhouse gas emissions but also less power generation than the engines could otherwise produce — often 2 megawatts rather than 3 — because the amount of available biogas varies.

It's cost-effective even so, said Atwood of the city energy office. But he's planning to bring the system up to at least 4 megawatts — 40 percent of the facility's average usage — and supplement it with natural gas. He already knows what they'll use the extra heat for.

"There's a big tank of sludge that needs to be heated up before it goes into the digester," Atwood said.

The city is seeking BGE incentives for that project and for other wastewater and water treatment facilities and city buildings. Atwood said microgeneration has interested him for a while, but he got serious about it after the lower cost of natural gas made such projects more cost- effective.

The trio of engines that constitute the city's system at Back River are each about 6 feet wide and 22 feet long, not counting the bevy of pipes snaking out. Atwood is excited about the potential for more power plants this size in a country long accustomed to thinking big.

"Now it's like thinking small," he said.