This article presents an overview of gas turbine combined cycle (CCGT) power plants. Modern CCGT power plants are producing electric power as high as half a gigawatt with thermal efficiencies approaching the 60% mark. In a CCGT power plant, the gas turbine is the key player, driving an electrical generator. Heat from the hot gas turbine exhaust is recovered in a heat recovery steam generator, to generate steam, which drives a steam turbine to generate more electrical power. Thus, it is a combined power plant burning one unit of fuel to supply two sources of electrical power. Most of these CCGT plants burn natural gas, which has the lowest carbon content of any other hydrocarbon fuel. Their near 60% thermal efficiencies lower fuel costs by almost half compared to other gas-fired power plants. Their installed capital cost is the lowest in the electric power industry. Moreover, environmental permits, necessary for new plant construction, are much easier to obtain for CCGT power plants.

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In the last 30 years, advances in gas turbine technology have almost doubted the thermal efficiency of new electric power plants. Modern gas turbine combined cycle (CCGT) power plants[1] are producing define power is high as half a gigawatt with thermal efficiencies approaching the 60% mark - almost twice the efficiency of power planes I learned about as an undergraduate ME student

In a CCGT power plant the gas turbine is the key player, driving an electrical generator. Heat from the hot gas turbine exhaust is recovered in a heat recovery steam generator (HRSC, a.k.a., a boiler), to generate steam, which drives a steam turbine to generate more electrical power. Thus it is a combined power plant (the gas turbine or Brayton cycle and the steam turbine or Rankine cycle) burning one unit of fuel to supply two sources of electrical power.

Most of these CCGT plants burn natural gas, which has the lowest carbon content (less CO2) of any other hydrocarbon fuel. Their near 60% thermal efficiencies lower fuel costs by almost half compared to other gas fired power plants. Their installed capital cost are the lowest in the electric power industry ($1000-1200/KW compared to a modern nuclear plant at$4500/KW (or higher) and lower by half than a fossil fueled steam power plant). Environmental permits, necessary for new plant construction, are much easier to obtain for CCGT power plants. Another environmental advantage is that less than half of the power plant output requires water cooling (to condense steam). This is an important consideration in many parts of the world, where cooling water may be in short supply, and air-cooled condensers may be needed.

And yet, it has been to my astonishment that I hear almost no reference to these superstar power plants in the general public, energy-commenting media or at two recent U.S. national energy forums I attended. The CCGT power plant is truly the “elephant in the room.”... ignored by many energy affectionatos, through perhaps ignorance or nearsightedness.

Two years ago I attended the US National Academes Summit on America's Energy Future, a conference held in Washington D.C., March 13-14, 2008. The two day summit featured some 26 presentations providing an overview of recent influential energy research studies and initiatives. Speakers included three secretaries of energy: James Schlesinger (he was the first under President Carter), Samuel Bodman (then in office under President Bush) and 1997 Nobel Prize winner and director of the Lawrence Berkeley National Laboratory, Steven Chu (subsequently appointed by President Obama).

This year I attended the US Department of Energy's March 2-3, 2010 Energy Innovation Summit, held just south of Washington, D.C. This conference, with about 1700 attendees, served to introduce and give a progress report on DOE's newly formed Advanced Research Projects Agency - Energy (ARPA-E), patterned after the US Department of Defense's well-renowned ARPA. Presentations given over the two days included those by DOE Secretary Steven Chu, General Electric CEO Jeff Immelt, New York Times columnist Tom Friedman, retired Lockheed Martin CEO Norm Augustine and Daniel Yergin (well-known energy consultant and author of The Prize, the story of oil).

However, in both of these major energy conferences, there was a complete lack of serious discussion of the important role of electric power gas turbines, in America's and the world's energy future. In discussions often centered around the “hydrogen economy” and “renewable energy” the actual role that gas turbine technology has and will play in the world's energy picture was benignly neglected or ignored. If mentioned, it was referred to as a transitional means to the employment of some future, immerging energy converter (e.g. the fuel cell, solar energy plants, or wind turbine farms) .

Consider the track record of some of the energy converters featured at these energy summits. Fuel cells have been around since 1839 and while research has greatly improved their performance, they always seem to be “the promise of things to come.” Wind turbine technology goes back 1000 years and, however efficient, suffers from the intermittent nature of wind.

Contrast this with the progress[2] made with gas turbines in less that four score years. The very first electrical power plant gas turbine was built and tested by Brown Boveri in 1939. It was installed at Neuchatel, Switzerland and had an output of 4MW, a thermal efficiency, η, of 18%, a firing temperature (turbine inlet temperature) of 998̊F (357̊C) and a relatively low exhaust temperature of about 530̊F (277̊C). Compare this very first power plant gas turbine to today's Mitsubishi Heavy Industries (MHI) M701G2 heavy frame “G Class” gas turbine, with an output of 334 MW, an η of 39.5%, a firing temperature of2732̊F (1500̊C) and a much higher exhaust temperature of 1089̊F (587̊C), eminently suited for combined cycle steam production.

With the much higher exhaust temperature, the MHI CCGT power plant has an overall on-site thermal efficiency of 59.1 %, yielding an aggregate output of 500MW Currently, three of these units with a combined output of 1500MW are replacing six conventional steam powered plants (43% efficiency) with a total output of 1050MW, in one third of the plant area, at the Tokyo Electric Power Company (TEPCO) Kawasaki Thermal Power Station in Japan.

The largest combined cycle gas turbines are the H class machines made by GE and Siemens. A new Siemens H class gas turbine, the SGT5-8000H, is rated at 340MW, making it the world's largest gas turbine. The turbine underwent testing in 2009 at Irsching, Germany, and is the heart of a new 530MW combined-cycle plant. Siemens expects to exceed 60% thermal efficiency for this new H machine.

Last February, in this column[3] I outlined the bright future for natural gas as a fuel for gas turbine power plants, based on new discoveries and the growth of liquefied natural gas facilities. It was also pointed out that the U.S. Energy Information Administration (EIA) projects that of an estimated 255 gigawatts of additional electric generating capacity to be added in the U.S. by 2030, 55% will be in the form of natural gas fueled gas turbine plants. This “elephant in the room” has a bright future- that could be made brighter by directing more gas turbine technology and research funding to push way beyond the unmatched 60% efficiency mark, now established.

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1992
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Combined Power Plants
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3