Gen Sets Offer Total Solution
Staff -- Consulting-Specifying Engineer, 12/1/2001 1:00:00 AM
When Tampa Electric in Florida needed to add generating capacity, the utility's officials discovered that the city's wastewater treatment plant needed to improve the efficiency of gas-fired drum dryers at its sludge-drying facility—and that the plant also needed to increase emergency backup power.
A pair of natural-gas engine/generator units, using cogeneration, were installed to meet the needs of both utility and waste water plant. The engine/generator sets produce both electricity and exhaust heat that provides process heating. At the same time, the units will serve the 96-million-gallons-per-day treatment plant with emergency standby power.
"The fact that the exhaust heat from the engines is being captured in the sludge-drying operation means that every time natural gas prices rise, the project will make even more sense economically," says John Kelly, project manager for the utility.
Location of the engine/generators at the wastewater plant also results in a much lower transmission-line loss for Tampa Electric in delivering power to the plant.
"Finally, it provides the city with another source of emergency power," says Kelly, "because we can run the engine/generators in 'island mode' if the facility were to lose grid power."
With the retrofit of two engines capable of 2.9 mW of power each, the wastewater plant's power now comes from two fuel types: 5.8 mW from natural gas and 7.2 mW of diesel power.
Engines will operate in a combined-cycle mode—producing heat as well as power—during weekdays, when the sludge-drying process is in operation. The engines' 700°F exhaust heat will be used in turning-drums that dry the sludge. On weekends, the units will operate in a single-cycle mode.
"The city is projecting savings of $1.4 million, by avoiding the purchase of two additional 2 mW backup generators," says Kelly.
Adds John Drapp, the wastewater utility's operations manager: "I don't know of any other distributed generation agreement that's quite as involved as this one. It's a first, in that Tampa Electric has placed equipment on a City of Tampa site, for the express purpose of sharing power and heat, along with providing emergency power.
"We already had permits to add extra diesel-powered standby generators when the utility proposed the natural-gas/cogeneration idea."
From Pure Power, Winter 2001.
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