Three orange engines sit in a building on the sprawling grounds of the Back River wastewater treatment plant, producing a steady din. And about 20 percent of the electricity the facility needs.
"It's basically a mini power plant," said Ted Atwood, director of the Baltimore City Energy Office.
This is "microgeneration" — operations that look nothing like the sprawling complexes that most of the country relies on for light, heat and air conditioning. Making electricity on site isn't new, but advocates say interest is growing among significant energy users, driven by cheaper prices for natural gas to fuel systems and reliability worries in the aftermath of extreme weather.
The goal: keep electricity flowing even when nearby power lines go down — and save money to boot.
Baltimore's city government, which runs the Back River facility, plans to expand capacity there and build power-generation systems at other sites.
"We're moving forward aggressively," Atwood said. "I'm looking at one building where the energy costs are about $800,000 a year, and after we put this facility in, we will save about a quarter of a million dollars per year."
What Baltimore wants to build are "combined heat and power" systems, more efficient than conventional power plants because their owners can use both the electricity and heat they produce. The heat can be used to heat water or produce steam, which can in turn be used to power absorption chillers that help cool the air.
President Barack Obama called last August for a 50 percent increase in combined-heat-and-power capacity by 2020, directing federal agencies to assist in the effort.
"Meeting this goal would save energy users $10 billion per year, result in … in new capital investment in manufacturing and other facilities that would create American jobs, and would reduce emissions equivalent to 25 million cars," the White House said at the time.
Baltimore Gas and Electric Co. and two other Maryland utilities now offer incentives for building combined heat and power systems, part of an effort to meet state goals for reducing energy use. BGE, which launched its program last summer, said about a dozen projects have applied for the program in its territory.
In the Baltimore region, five were built in the past 10 years, according to the U.S. Department of Energy.
The new applicants include hospitals, industrial users and local government — Baltimore's planned projects are in the mix.
"Customers can save energy, not through behavior or conservation but through using high-efficiency equipment," said Bill Wolf, manager of energy-efficiency programs for BGE.
Because they're already on site, combined heat and power systems don't have to rely on overhead power lines to deliver what they produce. They can be engineered to disconnect from the larger electrical grid when disruptions hit and keep going.
That makes the systems more cost-effective than generators meant only for emergencies, Wolf said.
"If you're going to put out money for a backup generator, you could put a little bit more in and get a CHP unit," he said.
Upfront costs vary by system size and requirements but can run $2 million to $3 million for a 1-megawatt system, Wolf said. The incentives offered by BGE are designed to offset 30 percent or more of the installation costs.
Wolf said the proposed systems range from 75 kilowatts — enough power for a building the size of a small elementary school — to 2.5 megawatts, the equivalent of 33 of those schools.
Even 2.5 megawatts is small scale for a power plant, though. The nuclear reactors at Calvert Cliffs in Southern Maryland produce more than 1,700 megawatts of electricity.
Though the BGE incentives aren't for homeowners, companies do sell combined heat and power systems meant for individual houses, which really puts the micro in microgeneration. Still, experts warn that big and small users of electricity alike aren't necessarily good candidates for the systems because they need to have a continual use for the heat, too.