Portland General Electric Company’s (PGE) Coyote Springs plant is located alongside the Columbia River in Boardman, Oregon, a town 160 miles east of Portland. Unlike the cool and rainy Portland area, where the summer highs average in the upper 70s and the relative humidity barely drops below 50%, in Boardman the temps reach into the 90s with relative humidity in the 20s. With afternoons being the peak hour for electricity demand and when prices are highest, the turbines output typically drops by 15%, forcing PGE to look for cost effective ways to recover that lost power and increase their bottom line.
Easy, Low-Cost Installation and Operation
During the summer months, when temp cause the turbine output to drop is when PGE started investigating inlet cooling as a way to extract more power from the unit. Installing a media-type evaporative cooler would require a significant enlargement to the inlet ducts as well as raising operating costs. A mechanical chiller could provide the required power boost, but with high installation and operating costs. Those costs would have been justified if the humidity in Boardman was significantly higher, but PGE instead found that an inlet fogging system would provide the cooling it needed at a much lower cost.
In 1997, PGE added a MeeFog inlet cooling system to extract extra output from the unit. Mee designed an eight-stage fogging system for the plant, giving it fine control over the exact amount of cooling required depending on ambient conditions. The pump skid contains eight CAT high pressure pumps, along with a weather sensor and programmable logic controller. Based on the demand for humidification, the controller will activate the right amount of pumps, which supplies the pressurized water to an array of 1120 impaction pin nozzles with a .006″ orifice.
Unlike chillers or media evaporative coolers, fog systems require little or no modification of the inlet-air duct or housing. PGE did, however, add access doors and drains to ease servicing and drainage of excess water. Like many plants, Coyote Springs installed its MeeFog system downstream of air filters, and upstream of silencers and trash screens.
Recovering Lost Output
At maximum capacity, the fogging system injects 50.4 gallons of water per minute into the airflow, broken down into droplets in the 3-30 micron range. Given the small droplet size and the fogging array placement, there is plenty of time for the droplets to fully evaporate before entering the bell mouth. Each of the eight stages provides 3.75 degrees of cooling, for about 30 degrees total when all stages are operating. This results in 2MW power boost per stage, or about a 10% overall improvement, when considering both the GT and the HRSG. In addition to the power increase, fogging lowers the amount of energy required to compress the air, resulting in an improved heat rate and lower fuel costs.
When PGE purchased a second Frame 7FA, the company ordered a pre-installed fogging unit, however it soon found that not all fogging systems are the same. The GE unit used a larger nozzle size (.008″) which produced much larger droplets than the Mee system. Additionally, GE installed the array closer to the compressor inlet, leaving less time for the droplets to evaporate.
“On unit 2, we had nothing but problems; we could see huge droplets coming off the nozzles,” says PGE Lead Engineer at Coyote Springs. “We took out their nozzle array and put in the Mee nozzle system and have been fogging the machine ever since.”
A Profitable Choice
PGE uses inlet fogging whenever the temperature rises above 60 degrees. During the summer the fogger might run around the clock.
“Fogging is an integral part of our economics; in a normal year, the fogger will generate $350,000 in net profit”, says PGE Lead Engineer. “Having properly designed and functioning fogging systems on both units has made us a tremendous amount of money compared to the cost of the system.”