Two years ago, Kalaeloa Partners L.P. decided to examine the plant’s system design to determine what capital upgrades could be implemented to increase plant output and efficiency. They found that an evaporative cooling system was one such upgrade that could do just that.

A close look at system design

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 (HRSGs), plus a balance of equipment that completes the combined cycle.

While low-sulfur fuel oil is the primary fuel, No. 2 diesel fuel is used as a backup source and for short-duration startup and shutdown of the gas turbines. Propane is used to ignite the turbines.

As part of the conversion process, air enters the compressor of the gas turbine via the air intake. After the air is compressed, it enters the combustor to be mixed with fuel and then burned.

As the fuel burns, chemical energy from the fuel is changed to heat (or thermal energy) in the form of hot gas. The hot gas then enters the turbine, where some of the thermal energy of the gas is converted into mechanical energy to drive the compressor as well as the generator via a common rotor shaft.

Approximately two-thirds of this mechanical energy is needed to drive the compressor; the remaining energy drives the generator, where the mechanical energy is converted into electrical energy that is sold to HECO.

The exhaust gas of the turbine contains a significant amount of remaining thermal energy, which is passed through the HRSGs to produce steam. This steam is produced at two pressure levels and transported downstream via the high- and low-pressure steam headers.

From there the steam is directed to the steam turbine, where the steam’s thermal energy is converted into mechanical energy to turn a rotor shaft connected to a generator, completing the combined cycle.

Normal operation is for all of the steam to enter the steam turbine and to extract the steam required by the adjacent Tesoro refinery, thus maximizing plant efficiency. Alternately, the steam can either be bypassed around the steam turbine directly to the main condenser, or to the process steam lines going to Tesoro Hawaii Corp.

Recover lost power

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 upon mass flow, quality, and ambient temperature of the air drawn into the combustion chamber.

“The cleaner and cooler the air taken into the turbine, the more efficient the turbines operate, resulting in higher power output,” said Randy Koncelik, project engineer at the Kalaeloa Cogeneration plant. “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. That’s when Kalaeloa contacted a few evaporative cooling manufacturers, including Munters Systems Division (Fort Myers, FL).

“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 L.P. decided to retrofit each of the 11N gas turbines with Turbidek®, a stand-alone evaporative cooling system designed and developed by Munters to increase output levels and improve thermal efficiency.

How evaporative cooling works

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.

According to Munters, its system cools the inlet air, creating denser air and giving gas turbines a higher mass flow rate and pressure ratio, resulting in an increase in power output and efficiency.

“By significantly ‘densifying’ the air, this evaporative cooling system optimizes the gas turbine combustion process by increasing oxygen levels,” said Larry Klekar, sales manager for Munters Systems Division. “Concurrently, the air scrubbing effects of ‘Glasdek’ evaporative cooling media removes many airborne contaminants and particulates before they enter the turbine.

“This decreases the maintenance required on filters and other equipment, reducing operating costs. It also extends the life of gas turbines which saves on capital expenditures.”

According to Koncelik, Kalaeloa projected an approximate 2.1-MW increase on each combustion turbine (CT), for a total plant 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 [1-]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 benefits

Other major benefits of Kalaeloa’s evaporative cooling system include:

• A reduced pressure drop in the inlet of the gas turbine filter house; “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 [evap cooling] 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 inches of water to 0.3 inches of water. The air encounters less pressure drop on the way into the CT compressor, improving mass flow and yielding higher efficiency and power output.”

• The new system is low maintenance; “The old ISF system has six 40-hp motors which had to be maintained routinely, as all six ran continuously,” Koncelik noted. “The [new] 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 five- to seven-year life expectancy, given the water conditions at our site.”

  According to Klekar, recovered turbine outputs of 15% have been reported when using evaporative cooling to cool inlet air where relative humidity is at its lowest and energy is in peak demand.

  “With the new system and an ambient wetbulb 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.