01
Jul
12

Efficiency by the Numbers

Lee Langston

The following is an article that was originally written as a “web exclusive” on the magazine’s website. Due to popular demand, we are running it again here.

When policy makers sketch out future energy scenarios, they too often overlook the best technology we have.
By Lee S. Langston

One would be hard pressed to come up with a more star-studded line-up than the attendees at the National Academies Summit on America’s Energy Future, which I attended in Washington in March 2008. The two-day summit featured some 26 presentations providing an overview of recent influential energy research studies and initiatives, and like Scrooge’s Christmas visitors, featured Energy Secretaries past (James Schlesinger), present (Samuel Bodman), and future (Steven Chu).

Yet another presenter was New Mexico Senator Jeff Bingaman, chair of the Senate’s Committee on Energy and National Resources. In his remarks, Bingaman illustrated just how long the U.S. has been grappling with energy-related issues with this quote:

“I am inaugurating a program to marshal both government and private research with the goal of producing an unconventionally powered, virtually pollution-free automobile within five years.”

That was President Richard Nixon, addressing Congress in 1970.

The talks centered around renewable energy, the oft-mooted hydrogen economy, and kicking the oil habit, but as with other recent general energy conferences I have attended, there was almost a complete lack of serious discussion on the contribution of gas turbines. Sometimes in such discussions, gas turbines are referred to as a transitional technology, on the way to the employment of some future, emerging energy converter (e.g. the fuel cell, solar energy plants, or wind turbine farms). But they are not presented as a key technology that could well play a central role in the nation’s—and the world’s—energy future.

Which, in my view, is what gas turbines are. No other technology currently available or likely to be deployed in the next decade has the potential to obtain so much power from so little energy. In a future where fuel supplies are likely to be constrained (due to geological, climatological, political, or economic factors) the inherent efficiency of the gas turbine will not be overlooked.

To illustrate the way the gas turbine changes the fundamental assumptions about energy converters, consider the superstar of electricity production, the combined cycle gas turbine power plant. These plants operate at thermal efficiencies approaching 60 percent, which makes them far and away the most efficient large energy converters we have.

Understanding why this is so impressive requires reflecting on the Second Law of Thermodynamics. Lord Kelvin presented the idea this way: “It is impossible to construct an engine that, operating continuously, will produce no effect other than the extraction of heat from a single reservoir and the performance of an equivalent amount of work.” For instance, if a power plant engine—say a Brayton cycle gas turbine—receives heat, Qin, from a reservoir (the combustion of a fossil fuel supply or a nuclear reactor), work W (the turning of a generator to produce electricity) is produced, but there must be part of Qin that is rejected as heat, Qout. The engine’s thermal efficiency, h, is then defined as

or in words, useful output divided by costly input, where engineers strive to makeh as large as possible.

The heat that must be rejected (Qout) as a consequence of the Second Law is contained in the gas turbine exhaust. But while that heat cannot be used by the first machine, it can be used to provide energy input to another engine, provided that the temperature of the rejected heat is high enough for the “bottoming” engine to produce more work, and in turn, reject heat as required by the Second Law. Two engines working together in this way are in what has become known as a “combined cycle.”

In the case of modern Brayton cycle gas turbine, its Qout (typically at 1,000 oF (538 oC) is sufficient to produce steam to run a Rankine cycle steam turbine to generate more electrical power. The combined thermal efficiency (hcc) of the two heat engines (Brayton gas turbine and Rankine steam turbine) can be derived fairly simply from the First Law of Thermodynamics and the definition of h (Equ. (1) to get an expression for the thermal efficiency of the combined cycle (CC) given by

hcc= hGT  + hST  – (hGT )(hST )

where hGT  and hST are the thermal efficiencies of the Brayton cycle gas turbine and the Rankine
cycle steam turbine respectively.

That simple equation gives insight as to why combine cycle gas turbine power plants are superstars. Suppose hGT  is 40 percent, which is a reasonable upper value for current high performance gas turbines. A reasonable value for a Rankine cycle operating at typical CC conditions would be 30 percent. The sum of those two individual efficiencies minus their product becomes:

hcc= 0.40 + 0.30 – (0.40)(0.30) = 58 percent.

The efficiency of the two turbines working in combined cycle is, in fact, greater than either of the two heat engines working separately.

 

Just how did this leap in power plant efficiency come about? Although the combined cycle power plant concept had been proposed in thermodynamic text books for many years, its widespread realization didn’t occur until the early 1990s, when the cumulative effects of long-term gas turbine research and technology ushered in high temperature gas turbine power plants.

We have come a long way from the very first electrical power plant gas turbine, which was built and tested by Brown Boveri in 1939. The 4 MW plant, installed at Neuchatel, Switzerland, had a thermal efficiency of 18 percent, a firing temperature (turbine inlet temperature) of 998 oF (537 oC) and a relatively low exhaust temperature of about 530 oF (277 oC).

Compare that very first power plant gas turbine with one today, the Mitsubishi Heavy Industries M701G2 heavy frame “G Class” gas turbine. It has an output of 334 MW, an h of 39.5 percent, a firing temperature of 2,732 oF (1,500 oC), and a much higher exhaust temperature of 1,089 oF (587 oC). Such a high outlet temperature is eminently suited for combined cycle steam production.

With the much higher exhaust temperature, the Mitsubishi Heavy Industries combined cycle power plant has an overall on-site thermal efficiency of 59.1 percent, yielding an aggregate output of 500 MW. Currently, three of these units with a combined output of 1,500 MW are replacing six conventional steam powered plants (which ran at 43 percent efficiency) with a total output of 1,050 MW, in one third of the plant area, at Tokyo Electric Power Co.’s Kawasaki Thermal Power Station in Japan.

What is the secret to a three-fold increase in efficiency over 70 years? Hard work. Probably more funding for research and development has been fruitfully devoted to the gas turbine—both in its jet engine aviation form and as a power plant—than for any other prime mover. In the last 50 years, for instance, ASME’s International Gas Turbine Institute has had over 15,000 refereed technical papers presented at its gas turbine conferences. And gas turbine research and development has involved a wide range of basic science and technology.

Advanced materials and heat transfer research, for example, has yielded long-lived superalloy turbine blades and vanes (some of them composed of single crystals; see “Crown Jewels,” February 2006) that are operated for tens of thousands of hours in gas path flows at temperatures greatly exceeding alloy melting points. Experimental work, analysis and computer modeling in fluid mechanics, heat transfer, and solid mechanics have led to continued advances in compressor and turbine component performance and life. Gas turbine combustion is constantly being improved through chemical and fluid mechanics research. And this list could continue almost indefinitely.

The gas turbine is every bit as high tech as a fuel cell or a wind turbine, but its low profile makes it easy to overlook. That’s a shame, because as policy makers grapple with the very real challenges facing the world’s energy system, the need for a clean, efficient energy conversion technology will be glaringly apparent. How fortunate it is that such a technology already exists, in the form of the combined cycle gas turbine, ready to be deployed on a much wider scale.

 

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The Editor

John G. Falcioni is Editor-in-Chief of Mechanical Engineering magazine, the flagship publication of the American Society of Mechanical Engineers.

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