Coal, Gas

Enhancements for Small CTs Could Help Big Turbines Too

Issue 7 and Volume 110.

Developers of a non-refrigeration technology that can enhance the performance of small combustion turbines – effectively creating greater output a cost below adding additional capacity – contend it will work with much larger turbines, too.

The Everest Cycle is a non-refrigeration method that cools air almost to the dew point – the point at which moisture condenses into water. Developed by Les Schlom, chief engineer for Everest Sciences, and Everest president, Andy Becwar, the indirect evaporative cooling technology’s primary application is improving gas turbine performance in power applications.

Efficiency gains on small GE, Rolls-Royce and Solar turbines have been dramatic – ranging from 10 to 30 percent. Heat rate is reduced 5 to 10 percent, fuel efficiency is increased, emissions are reduced and parasitic load is negligible. The amount of energy consumed by the indirect evaporative system is less than 10 percent of the energy consumed in a conventional refrigeration system and is equally effective at all temperatures. These systems, which also provide significant sound attenuation, are typically placed on a platform above the turbine to conserve ground space and minimize duct losses.

Everest sold its first two systems to cogeneration facilities in Toronto and Bakersfield, Calif., to be added to two 5 MW Rolls-Royce-Allison 501 turbines. In the Canadian installation, Everest is installing a hybrid package that uses a small amount of mechanical refrigeration contained within the proprietary package. This provides a constant 45 F inlet air temperature during the summer and increases power output more than 30 percent. The reason for the small amount of mechanical refrigeration needed in this particular application is the customer’s need to maintain a constant cogeneration output of power and heat under all weather conditions.

In California, the Everest Cycle is replacing an existing direct evaporative inlet air system and will allow the owner to stop buying power from the grid in the summer months. The user has calculated that this improvement over their traditional direct-evaporative system will pay back in less than two years.

The company’s initial market is for small turbines typically used in cogeneration.

A typical Everest Cycle cooling unit measures 8 feet wide, 8 feet tall and 12 feet long.
Click here to enlarge image

“We are presently marketing the Everest Cycle to turbines no bigger than 15 MW,” says Schlom. “We have successfully modeled the technology up to a GE LM 2500 (25 MW), but our immediate market is the 3.5 MW to 10 MW range. These small turbines respond particularly well to intake cooling.”

Becwar cites an example of the kind of bang-for-the-buck capacity enhancements the system can offer in a study conducted for an electric co-op in the western United States. It suggests the addition of the Everest Cycle to three exiting gas turbines will provide the same power increase as adding and additional gas turbine. The system will cost about $1.5 million compared to almost $7 million for another turbine. It will use only a slight amount of additional fuel for the increased power and it provides benefit at all times above 45 F.

“Another study for an oil company in Kern County, Calif., has shown that the addition of the Everest Cycle to their gas turbine fleet in just one location would produce almost 20 MW of additional power during peak periods,” he says. “It would have a payback of less than two years and supply greater mass flow for the customer’s down-hole recovery.”

The system works using a combination of multi-stage indirect evaporative cooling with an air washer as the final stage of cooling and filtration. This can offer a number of benefits. The air washer cleans the air of almost all particulate down to about 1.5 microns. The heat exchanger design provides some sound attenuation. And the indirect evaporative process produces air of much higher density than air cooled by direct evaporative or fogging processes and further increases the life of a turbine’s compressor and hot sections.

Another advantage is the ability to use low quality water. Studies have shown that even in dry locations, the increased power more than pays for trucking water to the site.

If the technology can be scaled up such enhancements could be significant for larger power producers operating big combustion turbines in simple or combined cycle applications.

“On a typical hot day in the southwestern United States when the temperature is 100 F or higher, Everest will bring the gas turbine back to ISO conditions, which means a 25 to 30 percent net power gain,” says Becwar. “Industry experts from most major companies in the gas turbine industry have witnessed and confirmed the performance of the full-scale prototype.”

GE, Solar and Rolls-Royce simulations show paybacks of less than two years in dry climates and three years or less in humid areas such as Houston and Biloxi, Miss. He says the system provides two to three times more power improvement than conventional direct evaporative technologies. It provides more than twice the power improvement of fogging and does not require the often high-cost and aggressive de-ionized water that fogging does. And there’s less maintenance needed than for conventional mechanical refrigeration systems.

“It’s much easier to budget for small applications initially, so we’re starting with small turbines,” says Schlom. The module Everest currently provides produces 9,000 cubic feet per minute (CFM) of air. That means a Rolls-Royce 501 takes three of the units totaling a nominal 27,000 CFM placed on a platform on top of the machine with the intake fed to the gas turbine. “Sure the technology can be scaled up to provide the same cost-benefit advantage to Frame-size turbines, says Becwar. “But there’s no way we can address that yet. The next step could be for something like the LM 2500. The bottom line is that the cost of adding this technology is a very small fraction of the price of adding a new turbine.”