Charlotte-Mecklenburg Utilities plans to lease space to solar farms at four of its water and sewage treatment plants, and install a biogas-fueled power plant at a fifth plant.
As part of a city initiative, the department in November sent out a request for proposals for up to 8 megawatts (AC) of solar panels at its Franklin, McDowell, Sugar Creek and Irwin Creek plants.
Third-party developers would lease unused land at the plants and own the solar arrays, although the city would have an option to buy them later.
Charlotte Douglas International Airport issued a similar proposal this month for up to 53 megawatts of solar capacity. That would be among the state’s largest arrays and would be installed over parking areas and between runways.
Solar is hot as panel prices drop. North Carolina, boosted by a 2007 green-energy law, became the fifth-largest state for installed capacity in 2012, according to the Solar Energy Industries Association.
Apart from open space for solar, Charlotte-Mecklenburg’s sewage treatment plants also produce another energy source – biogas, a byproduct of heated “digesters” that break down solids. Biogas is mostly methane, which in turn is the main component of natural gas.
The utilities department recycles its biogas, using it to maintain the proper working temperature of the digesters. But McAlpine, its largest treatment plant, produces too much gas. The excess is burned off.
“We know we could take that flared gas and use that power and the heat,” said Jackie Jarrell, superintendent of CMU’s environmental management division.
The plan is to install a combined heat and power system. Biogas will fuel an engine and generator that produce about 1 megawatt of electricity. That energy, equivalent to 16 percent of McAlpine’s total electricity needs, will power equipment at the plant.
The heat from the engine’s exhaust, meanwhile, will heat water that will be piped back to the digesters.
The project is estimated to cost $3.7 million, to be repaid through energy savings over about a decade. A city study of a few years ago identified waste treatment plants as high priorities for saving energy and curbing greenhouse gases.
A 2011 Environmental Protection Agency study found that combined heat and power systems were fueled by biogas at 104 U.S. sewage treatment plants, generating up to 190 megawatts of electricity. The systems are technically feasible at 1,351 more sites, the study said, and would be “economically attractive,” with a payback of seven years or less, at up to 662 of those plants.
Most of CMU’s other waste treatment plants also flare excess methane.
“It’s a project that we’re looking at and then see how it works elsewhere,” Jarrell said. “You have to look at risk and whether you can make a business case for it.”
A request for proposals for McAlpine was issued in March, but the project is still in the early stages, Jarrell said. A proposed contract for the engine purchase is expected to go to Charlotte City Council by the end of the year.
Henderson: 704-358-5051; Twitter: @bhender
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