Evaporative Cooling System Lightens Gas Turbine Plant’s Load
Kalaeloa Cogeneration Plant (KCP) is a combined-cycle combustion turbine facility located in Kapolei, HI. The plant converts chemical energy from fuel into electrical heat energy that is sold as electricity and steam. KCP generates approximately 20% of the electrical needs for the island of Oahu.
As a partnership between ABB Energy Ventures and Kalaeloa Investment Partners (KIP), the cogeneration plant provides a portion of the steam needs for Tesoro Hawaii Corporation, one of the two oil refineries in the state of Hawaii, as well as 180 MW of firm capacity net electrical power to Hawaiian Electric Company, Inc. (HECO).
In 1997, Kalaeloa Partners L.P. decided to examine the plant’s system design to determine what capital upgrades could be implemented to increase the plant’s output and efficiency. They found that an evaporative cooling system was one such upgrade that could do just that.
The Need For Air
The combined cycle plant design includes two ABB 74.6 MW type 11N gas turbines, one ABB 51.5 MW extraction/condensing steam turbine, and two Deltak heat recovery steam generators (HRSG), plus a balance of equipment that completes the combined cycle.Air enters the compressor of the gas turbine via the air intake, then the combustor to be mixed with fuel, and then burned. The resulting chemical energy is changed to heat in the form of hot gas, which enters the turbine. Some of this thermal energy is converted to mechanical energy to drive the compressor as well as the generator via a common rotor shaft.
The exhaust gas is passed through the HRSGs to produce steam. The steam is directed to the steam turbine and mechanical energy is used to turn a rotor shaft connected to a generator, completing the combined cycle.
Large amounts of air are required to operate gas turbines. Because of this, the power output and fuel consumption of a gas turbine generator is highly dependent on mass flow, quality, and ambient temperature of the air drawn into the combustion chamber.
“The cleaner and cooler the air taken into the turbine is, the more efficient the turbines operate, resulting in higher power output,” said Randy Koncelik, project engineer at KCP. “Conversely, as the air inlet temperature rises, power output falls, and efficiency decreases.”
Kalaeloa Partners knew it could recover lost power by cooling intake air before it entered the gas turbine. And that’s when Kalaeloa contacted a few evaporative cooling manufacturers, including Munters Systems Division (Fort Myers, FL).
How Evaporative Cooling Works
“We chose an evaporative system over the other types of cooling systems such as fogging and air chillers because of simplicity, reliability, and cost,” Koncelik said. “The fogging systems did not appear to have the track record of producing the reliable cooling effect we were looking for, and the air chillers are very costly to install and operate.”After careful analysis, Kalaeloa Partners decided to retrofit each of the 11N gas turbines with Munter’s TURBIdek®, a standalone evaporative cooling system as well as Munter’s GLASdek® evaporative cooling media, which is designed to remove many airborne contaminants and particulates before they enter the turbine.
In evaporative cooling, intake air is passed through one or more wet pads to simultaneously absorb humidity and cool the air. The cool, humid air is then directed to the area where it is needed. The TURBIdek system cools the inlet air, creating denser air and giving gas turbines a higher mass flow rate and pressure ratio.
Savings Above and Beyond Expectations
According to Koncelik, Kalaeloa projected an approximate 2.1 MW increase on each combustion turbine (CT) for a total output increase of 4.2 MW.“Actual power increases have been higher than anticipated, closer to a 5 MW total increase,” Koncelik said. “In addition to increasing the CT output, we’ve seen almost a full MW increase on the steam turbine as well. That’s because the heat energy in the exhaust gas has increased, allowing the HRSG to produce more steam for the combined cycle to take advantage of.”
Other major benefits of Kalaeloa’s evaporative cooling system include a reduced pressure drop in the inlet of the gas turbine filter house and a reduction in maintenance required by the system. “We originally had in place an inertial separator filter (ISF) which cleaned the incoming air of large particles as the first stage of filtration,” Koncelik said. “The design of the Munters system calls for the ISF to be removed and the evaporative cooler to take its place. This reduces the pressure drop on the air inlet side from 1.3 in. of water to .3 in. of water.”
The TURBIdek system is designed to be low maintenance. “The old ISF system has six 40-hp motors which had to be maintained routinely, as all six ran continuously,” Koncelik said. “The Munters system has only one 10-hp motor running at a time, so less overall maintenance is expected over the life of the equipment. We just make sure the water feed headers are continuously delivering water of proper quality and that the media is wetted evenly. The system has been in service since 1998 and the media is still in good condition. The media has a 5- to 7-year life expectancy given the water conditions are our site.”
According to Larry Klekar, sales manager for Munters Systems Division, recovered turbine outputs of 15% have been reported when using evaporative cooling to cool inlet air were relative humidity is at its lowest and energy is in peak demand.
“With Munters’ TURBIdek system and an ambient wet bulb temperature of 60˚F, it’s possible to recover as much as 15% of the lost power just by cooling the intake air,” Klekar said.
And that recovered power can generate significant revenue over time for a gas turbine operation.
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