Media-type coolers remain the most common, capable of increasing turbine power output from 5 percent to 15 percent at an electric-generating plant. This system pulls air through a wetted honeycomb-like material, usually composed of cellulose fiber. Inside, the air evaporates off the convoluted surfaces of the wetted media, thereby cooling the inlet air. This technique does not require compressors, chiller coils or cooling towers. It's relatively inexpensive to install and functions over a range of conditions.
There is a downside, however. As relative humidity increases, so does the dollar cost per kilowatt. Additionally, retrofitting an evaporative cooler often requires enlargement of ducts and inlet-air housing, raising capital costs. Overall though, if installation modifications are minimal, evaporative coolers generate cost-effective cooling. Generally speaking, the effectiveness of an evaporative cooling system depends on the surface area of water exposed to the airstream and the water's residence time. That's why duct modification often is required in order to enlarge the evaporative surface and thereby the amount of cooling accomplished.
The Coyote Springs Combined Cycle Plant in Boardman, Ore., built by Portland General Electric Co. (PGE), Portland, Ore., considered media-type cooling when the plant was designed in 1997 but ruled it out. The company found that to create an evaporative surface large enough to adequately cool inlet air, media-type cooling would entail substantial duct enlargement and higher operating costs.
The facility also looked at mechanical chillers, which are not restricted by ambient conditions and are capable of giving a large power boost at high humidity levels. There are various types of chillers, including compressor, thermal storage and absorption chillers, with some increasing output by up to 20 percent.
Chillers do, however, have significant installation costs and can be energy intensive. In drier climates especially, chillers often compare poorly to evaporative coolers. But once relative humidity rises above 60 percent, chillers come into their own. Considering all the factors, though, PGE rejected the installation of chillers for the Coyote Springs facility.
Situated on the south bank of the Columbia River in eastern Oregon, the plant operates through hot dry summers, which would reduce performance of its 159 MW GE Frame 7-FA turbine by 15 percent or more. Accordingly, the plant conducted a comprehensive evaluation of inlet-air cooling, which led them to Mee, in Monrovia, Calif.
"We found that most cooling methods were expensive and required structural modifications to buildings and air-inlet housings," says PGE's Cheryl Bryant, the mechanical engineer in charge of specifying and implementing the cooling system. "Media-type evaporative cooling worked out to be 250 percent more costly to install than inlet fogging" at Coyote Springs.
"After we factored in maintenance and running costs, we decided to go with high-pressure fog," which resulted in a 16 MW output increase and a significant improvement in heat rate, Bryant says.
Instead of the complicated media arrangement associated with standard evaporative coolers, high pressure fog systems create a large evaporative surface area by atomizing the supply water into billions of super-small spherical droplets. The size of the droplet plays an important role in the amount of cooling that takes place. To a meteorologist, airborne water droplets less than 40 micron dia comprise a fog, while droplets larger than that comprise a mist. In inlet-air cooling, it is vital to make a true fog, not a mist. True fogs tend to remain airborne due to Brownian movement, which is the random collision of air molecules that slows the droplets' descent. Mists descend relatively quickly. In still air, for example, a 10 micron droplet falls at a rate of about 1 m in 5 min, while a 100 micron droplet falls 1 m in 3 sec.
"As well as increasing water-residence time in the airstream, small droplets speed up the evaporation process," says Thomas Mee III, CEO at Mee Industries.
Other factors being equal, the speed of evaporation of water depends on the surface area of water exposed to the air, he says, noting that the surface area is greater per unit of water in inverse proportion to droplet diameter. Water atomized into 10 micron droplets yields 10 times more surface area than the same volume atomized into 100 micron droplets. At Coyote Springs, for example, Mee Fog system droplets have an average diameter of 15 micron, providing maximum evaporative potential and high efficiency.
System Components
The Mee Fog system installed at Coyote Springs comprises a series of high-pressure pumps connected to a demineralized water supply. Computerized controls operate an array of tubes containing the fog nozzles. The individual components are:
"This is by far the most common location for high-pressure fog manifolds," says Mee, whose company has installed more than 100 fog systems in gas turbines throughout the world. "Installation usually requires one or two outage days, and it calls for only minor modifications to the turbine air-inlet structures."
Overall, PGE reported that the facility increased anticipated output by about 16 MW (2 MW per cooling stage), a 10 percent improvement. During hot summer days, the fog system achieves as much as 30oF (16oC) of cooling.
"We wanted increased megawatt output and lowered heat rate, and that's exactly what the Mee Fog system delivered," PGE's Bryant says. "For us, it proved far more economical than other cooling options." PCE
For more information from Mee Industries, Monrovia, Calif., call (800) 732-5364; e-mail info@meefog.com; or visit www.meefog.com.
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