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Power Station Increases Gas Turbine Output

With the energy crisis in California and similar scenarios looming across the United States and even the world, industry continues to seek innovative solutions to the dilemma of ever-increasing fuel costs. The demand for energy worldwide continues to rise faster than generating capacity, and the deregulation of the electric power industry has created demand for products that will make power producers more efficient.

Modern gas turbine technology has made enormous progress toward the related goals of increasing plant capacity and improving efficiency, but even these modern plant designs may suffer heat losses unless proper cooling techniques are implemented.

For a gas turbine plant in Jamaica, upgrading its facility with two evaporative cooling units is improving operations and increasing output and efficiency. The result is recovered power that is generating significant revenue over time.

The Hunts Bay Power Station in Jamaica is a 668-megawatt (MW) combined-cycle cogeneration power plant located in Kingston Jamaica. The plant is owned by Jamaica Public Service Company Ltd. The Hunts Bay facility features Two GT Browns and one GE Frame 7 combustion turbines.

Nitrous oxides and carbon monoxide emissions are continuously monitored and controlled at the facility with minimal environmental impact, production and process requirements. The average annual growth in demand for electricity in Jamaica over the past 10 years was approximately 5 percent and the forecast for the next five years is 6 percent annum.

"Based on our current installed capacity of 668 megawatts (MW), it is expected that by the years 2003 and 2004 the demand will have surpassed our generating capacity," said Dave Stamp, Facility Engineer for Hunts Bay Power Station.

It was with this in mind that Jamaica Public Service Company Ltd. investigated ways to economically increase the capacity of its generating units. One such method utilizes evaporative cooling technology to cool the inlet air to the gas turbine.

Because large amounts of air are required to operate gas turbines, the power output and fuel consumption of a gas turbine generator are 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 is, the more efficient the turbines operate, resulting in a higher power output," said Stamp. "Conversely, as the air inlet temperature rises, power output falls and efficiency decreases."

In order to prove the suitability of the inlet air cooling technology to the Jamaican climatic conditions, a pilot project was conceived. Gas Turbine no. 4, a John Brown Engineering MS5001 (Frame 5) unit with an ISO rating of 25.5 MW [59 degrees F and 14.7 pounds per square inch absolute (psia) inlet air] and a site rating of 21.750 MW [88 degrees F and 14.7 psia], was selected.

"We became aware of inlet air cooling as an option after noticing a Munters advertisement on the subject," said Stamp. "We would later contact them for an economic analysis."

One type of cooling is evaporative cooling, which uses a cooling fluid, usually water, that is brought into contact with the incoming air. The water evaporates as it absorbs latent heat from the incoming air, thereby reducing the dry bulb temperature.

"Evaporative cooling is most effective in drier climates with high cooling potential," said Stamp. "As is the case with all evaporative cooling technology, the extent of cooling is limited to the wet bulb temperature."

Another type of inlet air cooling is refrigerative cooling. This type is used when cooling below the wet bulb temperature is required. More complex and expensive electromechanical equipment is needed as greater energy inputs are required to achieve temperatures lower than the current wet bulb temperature.

In the direct expansion type refrigerative system, the refrigerant is circulated through the cooling coils placed directly in the path of the inlet air, thus supplying the cooling effect. The indirect system utilizes a secondary refrigerant, usually water, which is cooled in a direct expansion heat exchanger. It is then circulated through cooling coils placed in the path of the inlet air.

According to Stamp, there were several reasons Hunts Bay chose to use an evaporative cooling system at the plant versus other cooling methods.

"We chose evaporative cooling because of its ease of retrofit installation, low operating cost and low inlet pressure drop," said Stamp. "In addition, it was economically the right choice for us and the unit was also easy to install."

Climatic conditions in Jamaica were ideal for installing evaporative cooling.

"A gradual reduction in capacity is expected with increase ambient temperature, hence in Kingston, with high ambient temperatures of 90-92 degrees F in the summer months, only approximately 85 percent of ISO MCR can be realized," said Stamp.

"While power companies in southern climates realize the impact evaporative cooling can have on turbine performance, we are also seeing opportunities developing as far north as Toronto," said Larry Klekar, sales manager for Munters Systems Division. "Inlet cooling returns the investment so rapidly that installations are a good decision throughout most of North and South America.

In evaporative cooling, intake air is passed through wetted media 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, resulting in an increase in power output and efficiency.

"By significantly densifying the air, Munters TURBIdek evaporative cooling system optimizes the gas turbine combustion process by increasing oxygen levels," said Klekar. "Concurrently, the air scrubbing effects of Munters' 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."

The pilot test was conducted for six months, from January through June 2000. The results of the test proved that Hunts Bay Power Station benefited from the installation of the Munters evaporative cooling system.

"We improved our efficiency with the addition of the evaporative cooling unit on Gas Turbine No. 4," said Stamp.

Other benefits of the evaporative cooling system include:

• The maximum load achieved during the test was 24.6 MW at 88 degrees F. This represents an increase of 2.4 MW and will provide for annual savings of $3,203.
• A reduced pressure drop in the inlet of the gas turbine filter house. The evaporative cooling system is allowing for a reduced pressure drop on the air-inlet side. The air encounters less pressure drop on the way into the combustion turbine (CT) compressor, improving mass flow and yielding higher efficiency and power output.
• An average reduction in heat rate of 1.6 percent with annual savings of $40,857.
• The evaporative cooling system is low in maintenance. "Installation of the Munters system has proved to be a cost-effective, low-maintenance way to increase output levels and improve thermal efficiency," Stamp noted.

"Cooling the combustion air for a gas turbine plant is a smart way of getting more power," said Klekar. "With Munters' Evaporative Cooling System and an ambient dry bulb temperature of 95 degrees F, it's possible to recover as much as 15 percent of the lost power just by cooling the intake air.

"We are extremely pleased with the performance of the Munters evaporative cooling units and we plan to install more units on our turbines in the near future, " Stamp noted.

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