Retrofit “turbocharger” boosts GT output on hot days to increase peaking capability, backs up intermittent renewables

New-technology solution. The discussion on emerging technologies for fast start/fast ramp during the Reliability and Strategic Planning session was the perfect segue for Bob Kraft, Founder/CEO of PowerPHASE LLC, to introduce to the CCJ editors, during the ensuing coffee break, the company’s latest product—TurboPHASE™. He said it can boost the output of a combined-cycle plant by up to 15%, and a simple-cycle gas turbine by up to 20%. This is achieved by addition of a skid-mounted assembly, consisting of an efficient reciprocating natural-gas or diesel engine and an intercooled compressor, which injects hot high-pressure air directly into the combustion section of the GT, thereby increasing mass flow through the turbine.

The additional power is available within a minute or so of starting the auxiliary engine, the former PSM founder and president said. For a 7FA.04-equipped 2 x 1 combined cycle, a 5% TurboPHASE injection into each gas turbine means more than 40 MW is available (18 MW from each GT, plus 5 MW from the steamer) almost instantly. It can be used to (1) compensate for the sharp drop in power output experienced by some intermittent renewable resources during sudden changes in weather, and/or (2) provide a dispatchable source of peaking power.

The amount of air injected into a given gas turbine is determined by an engineering study that considers specific limits of plant equipment—such as the GT, generator, transformer—as well as the ambient range for which the extra power is desired. Typically, the equivalent of from 5% to 10% of compressor inlet air can be added in the combustion section.

TurboPHASE evolved from the company’s work aimed at developing efficient, grid-scale modular compressed-air storage packages that Kraft believes will have international application where and when energy storage is better appreciated. But the value of the system to generating companies today goes well beyond its inherent ability to back up intermittent renewables and to enhance the value of existing gas turbines for peaking duty.

Specifically, it is a less costly and more efficient alternative than steam injection and air inlet chillers for increasing combined-cycle output and performance on hot days. Kraft said that the use of steam injection and inlet chilling can match the GT power boost of his “turbocharger” but those alternatives reduce steamer output and increase parasitic power consumption, penalizing both total output and heat rate. TurboPHASE has no effect on base-load combined-cycle efficiency, running or not. The heat rate of the TurboPHASE incremental power on such a unit is substantially better than any simple-cycle gas turbine available—and it actually improves the efficiency of simple-cycle engines.

When asked about other options for quickly boosting output, Kraft shared his thinking on fogging and duct burners. Fogging, he admitted, was an efficient way to increase power, but the experienced jet-engine designer said the amount of additional output is limited. Likewise, duct burners can add peak power but there’s an efficiency penalty.

Regarding emissions, the engine can be equipped with an SCR and an oxidation catalyst to hold NOx and CO emissions, respectively, within permit limits. Alternatively, the recip’s exhaust can be injected into the GT exhaust stream to use emissions control solutions provided with the gas turbine.

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