By Steve Blankinship, Associate Editor
Various technologies can increase the output and fuel efficiency of gas-fired turbines by lowering temperatures on the inlet side of the turbine. Inlet fogging, direct evaporative cooling, and refrigeration are the three most common means.
But indirect evaporative cooling takes inlet cooling a step further, producing lower inlet air temperatures than direct evaporative cooling or fogging, and equal inlet air temperatures to refrigeration but with substantially lower parasitic loads. Indirect evaporative cooling achieves greater air density than direct evaporation or fogging, and greater density means more mass flow through the turbine, meaning greater energy output.
The reason indirect evaporative cooling is more effective than conventional evaporative cooling is that it does not add moisture to the turbine’s inlet air as happens with direct evaporative cooling or fogging. A technology developed by Everest Sciences combines indirect evaporative cooling with a small amount of refrigeration in a hybrid design to achieve inlet air temperatures equivalent to standard full refrigeration techniques, but with much lower parasitic power loads. With today’s power requirements, rising fuel costs and increased awareness of CO2 emissions, indirect evaporative cooling could become increasingly attractive.
“Indirect evaporative cooling puts the moisture into the secondary air stream that does not enter the gas turbine,” said Dave Voeller, CEO of Everest Sciences. The company has two commercial indirect evaporative cooling systems operating successfully, one at a food processing plant in southern California, and another at a packaging products manufacturing plant in Ontario, Canada.
“The technology allows the gas turbine to produce more power at a lower heat rate than commonly used refrigeration, direct evaporation, or fogging-based inlet cooling methods,” said Voeller. After a rigorous development program, Everest is bringing its patented technology to the industrial gas turbine marketplace, initially for small gas turbines typically employed in distributed generation applications. The system also produces a cool secondary air stream suitable for other plant cooling applications or the cooling end of an organic rankine cycle.
Everest Sciences’ first production model, rated at 14 thousand cubic feet per minute (cfm) or 18 pounds per second (PPS), and designed for turbines up to10 MW, provides inlet air at temperatures as low as 40 F over a wide range of ambient atmospheric conditions ranging up to 110 F and up to 85 percent relative humidity. The system also provides filtration in a fully integrated package designed as a direct replacement for the turbine’s existing filter house. De-ionized water is not required, and the integrated design keeps pressure losses extremely low—another important factor in helping the turbine operate as efficiently as possible.
“The Everest Sciences indirect evaporative cooling technology can increase gas turbine hot day available power by up to 25 percent while reducing heat rate by up to 10 percent along with associated reduced CO2 emissions when compared to an uncooled gas turbine,” said Voeller.
California and Canada
Two Everest Sciences installations have been operating since 2006—one in Brantford, Ontario at a major packaging products manufacturer and the other in Bakersfield, Calif. at a food products manufacturing plant. Both are installed on 5 MW Allison 501K turbines used in combined cycle operations. Southeastern Canada and southern California are very different weather-wise; they were chosen by Everest to demonstrate its technology’s performance in different climates..
In Bakersfield, with its relatively dry climate, the Everest system uses the company’s two stage indirect technology. The California food production facility’s turbine produces all the electricity and process heat for the plant. The plant uses steam and heat exchangers to heat its frying oils. “I’m always looking for efficiency improvements and was having problems with our old evaporative cooler,” said Terry Bartz, utilities manager for the plant. “Because we were looking at overhauling it, I was looking at other alternatives. This has been an improvement over evaporative cooling.”
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Bartz said the new system produces consistently lower turbine temperatures and that the plant uses steam injection to boost power output. “With the cooler, denser air that the indirect evaporative system produces, we use less steam. And that’s where we’re saving money, on the steam injection.” He said the plant gets in the range of 8 F to 10 F. cooler temperatures supplied to the turbine than with an evaporative cooler.
In Brantford, with higher humidity, the Everest system uses the company’s hybrid indirect technology. Ron Harten, chief engineer for Sonoco’s packaging products plant near Toronto, said that although his facility’s electric load is fairly steady, hot, humid conditions in the summer, combined with increased demand for their products had reduced the turbine’s ability to meet all of the factory’s needs.
As a result, he was forced to buy power from the grid for a couple of months during the most expensive time of the year. And Ontario’s open electric market means that summer power prices get extremely high—in the range of 15 cents. With the new system in place, the factory is once again completely self-sufficient.
“I needed to be able to get more power out of our turbine and also flatten my ability to make power summer to winter. The obvious way to do that is to change the environment the turbine takes its air from.” He said that the indirect system also provides a better heat rate for the turbine, meaning less fuel gas is needed to make an equal amount of useful energy. “The change is not dramatic, but at the cost of fuel these days, an improvement of 0.5 to two percent is considerable.”
As an added benefit, when grid prices soar in the summertime, Harten can sometimes even sell a small amount of power (perhaps 500 kW) to the grid, producing revenue for the factory. “We’re an industrial producer of packaging materials and paperboard products. We have always made all of our paper from 100 percent recycled materials. We are very keen on efficiency and pride ourselves of being ahead of the competition on that. So this was a good choice for us.”
Voeller said the indirect evaporative cooling system is an integrated filtration and cooling unit, so it typically replaces the filter house.
“Both installations to date were simple,” he said. “The old filter house was replaced and the ductwork leading to the gas turbine hooked up to it with very slight alterations.”
Everest Sciences has designs that show how the indirect evaporative cooling process can fit on units up to 60 MW. “The very large frame machines were initially designed for ground applications, whereas many of these small ones were originally designed for aircraft applications,” said Voeller. Generally speaking, the aircraft derived machines are more sensitive to air density, and our technology produces the best results on this machines.
Everest Sciences’ units can be clustered in groupings of multiple units to serve different sized gas turbines. The company plans to have a 14,000 cfm building block unit this year and a 30,000 cfm unit next year.
“Those are the two basic sizes we can use multiples of to serve gas turbines with different air flows,” he said. A 5 MW unit would typically require 25,000 to 28,000 cfm of inlet air flow, so the 14,000 CFM units work well. A larger unit, such as an LM 6000 would typically require around 240,000 cfm of inlet air flow.
Voeller also said that while the physics work well for the larger combustion turbines, the company is working on the packaging.
“The large units have so much air flowing through them, we’re refining the overall footprint. We’ve got it well figured out for up to 60 MW, where we just replace the filter house. Above that size, we haven’t looked at it yet. Because there’s a huge market for 60 MW and down, that’s where we’re focusing.”
That strategy may make sense. There are more than ten thousand 5 MW combustion turbines worldwide, few of which (if any) have any form of inlet cooling. Most are used in combined heat and power configurations.
“Although these machines are generally aeroderivative, we also expect to go after the smaller microturbines as well,” said Voeller.