This article is a memoir of William Emmet, a General Electric engineer in the field of combined-cycle gas turbine power plants. Despite the odds against the idea, several combined mercury and steam plants were built and achieved the promised high efficiency. This improbable achievement can be credited to a General Electric engineer named William Emmet. While Emmet’s early experience had been with direct current, he recognized the benefits and challenges of alternating current. The fuel efficiency of Emmet’s mercury dual cycle was eventually made obsolete by increased steam plant efficiencies from higher pressures and reheating the steam. Emmet’s contributions today are mostly hidden improvements in rotating electric machinery and apparatus. In contrast, his success in developing the impulse turbine helped create a technology base of engineers and manufacturing. It positioned General Electric to take the lead in turbochargers for piston aircraft engines, and later global leadership in aircraft jet engines and land-based gas turbines for electricity and industry.
They might not seem as futuristic as hydrogenpowered fuel cells or concentrated solar thermal facilities, but combined-cycle gas turbine power plants are perhaps even more remarkable. These plants take one unit of fuel (generally natural gas) and use it twice, running it first through a gas turbine connected to an electric generator, and then using the still-hot exhaust to make steam that turns a conventional steam turbine. If you want to squeeze every last bit of work out of fuel, this is the way to do it.
The first combined-cycle gas turbine plants were small demonstrations cobbled together in the 1950s, but by the late 1990s, they had become very large and very efficient. Today, the most efficient heat engine ever built is a combined-cycle gas turbine in Irsching, Germany, which has a measured efficiency of 60.75 percent. In an era where reducing power industry carbon emissions is of great importance, a natural gas-burning combined-cycle plant like the one in Irsching can generate electricity while producing 70 percent less carbon dioxide than a conventional coal-fired boiler.
But the combined cycle—using the heat of a working fluid to generate electricity and then to generate steam to make more electricity—dates much earlier, long before gas turbines became practical.
The year was 1914 when a patent was issued for a high-efficiency combined-cycle power plant. In this case, in a process generally called a dual or binary cycle, mercury was the high-temperature working fluid and drove mercury vapor turbines in the top cycle. The bottom cycle would use steam.
The mercury turbine exhaust would flow to a condenser and transfer its heat to boil water. The system could reduce coal use by 25 percent.
Steam turbines were a relatively young technology at the time. The original motivation was that a turbine had just one moving part. Thus, it could be a cheaper and compact replacement for the cumbersome low-speed piston and cylinder steam engines that had powered the industrial revolution for two centuries.
The somewhat unexpected advantage of a turbine was a higher efficiency. It could harness the huge low-temperature volume expansion of the steam down to ambient temperatures and a near perfect vacuum. This would require prohibitively large pistons, cylinders, and valves in traditional engines.
Mercury had a thermodynamic efficiency advantage. It condensed at temperatures higher than the boiling point of water.
While the design promised a higher efficiency, it required a large inventory of toxic mercury of uncertain availability and cost, along with challenges of developing mercury turbines, boilers, heat exchangers, pumps and piping, and the complexities of controlling multiple turbines and generators.
Despite the odds against the idea, several combined mercury and steam plants were built and achieved the promised high efficiency. This improbable achievement can be credited to a General Electric engineer named William Emmet. He was already credited with commercializing steam turbines for his company, and also turbo-electric ship propulsion. He would devote much of the next 25 years developing cycles with mercury as the working fluid.
Emmet came to engineering by an unlikely, roundabout route. He entered the United States Naval Academy in 1877, a few years after steam engineering had been added to the curriculum under Civil War naval veteran Robert Thurston, who in a few years would become the founding president of ASME. Thurston also pioneered mechanical engineering education at Stevens Institute of Technology and Cornell University.
William Emmet's academic performance was undistinguished. When he graduated in 1881, the peace-time Navy needed a limited number of officers, so commissions went to the highest-ranked cadets. Emmet served in the Navy as a post-graduate cadet midshipman.
When he was mustered out in 1883, Emmett had no career prospects. So he took a low-paying job repairing carbon arc street lamps in lower Manhattan.
It was a dead-end job: Thomas Edison had recently invented a sealed incandescent lamp. Dead-end maybe, but it provided valuable experience. Emmet observed how electric equipment failed, and began to study the limited amount of useful information published on the subject of electricity. He identified better designs, and started to consider himself an engineer.
At the same time, city transportation by horses was about to be replaced by electric trolleys. In Richmond, Va., another Naval Academy graduate, Frank Sprague, built the country's first successful system. Sprague hired Emmet to oversee systems in Chicago, Cleveland, and Pittsburgh.
Trolley motors were sealed against the harsh, wet conditions of the streets, and so they were prone to overheating and burnout. Emmet concluded it would be better to let the motors breathe. He implemented changes on failed motors, and then on new motors. He patented an improved commutator and better insulation.
Later, after a brief period working for Westinghouse in Pittsburgh, Emmet joined the Buffalo Railway Co., where he designed an improved motor. It got the attention of the Edison Electric Co. in Schenectady, N.Y.
He joined Edison Electric's Chicago office in time for 1892 World's Fair. It is remembered for displaying marvels of the new Electric Age, along with a gigantic Ferris wheel, powered by steam.
Meanwhile the General Electric Co. was being formed by the financier John Pierpont Morgan, who considered competition wasteful. Morgan was acquiring Edison Electric and other companies to create a monopoly on electric equipment. The headquarters and much manufacturing and research would be consolidated in Schenectady.
The merged company recruited engineering talent, including William Emmet and a German immigrant, Charles Steinmetz, who would achieve fame as the Electric Wizard of Schenectady. Emmet and Steinmetz would spend the duration of their careers in Schenectady. They received honorary degrees and hundreds of patents. Steinmetz introduced the study of electrical engineering to Union College.
Together, they took General Electric in new directions.
While Emmet's early experience had been with direct current, he recognized the benefits and challenges of alternating current. Voltage could be stepped up by transformer for efficient transmission and stepped down for safe use. It was being demonstrated with the transmission of hydroelectric power from Niagara Falls to Buffalo, a distance of 25 miles. Once again, Emmet educated himself, and in 1894 published a book, Alternating Current Wiring and Distribution, which remains useful today as a text for students and handbook for practitioners. Steinmetz observed Emmet was one of the few engineers who understood alternating current principles.
Meanwhile, the rapidly growing electric power system was defining the potential for steam turbines.
Westinghouse was winning what was known as the Battle of the Currents over Edison's direct current systems. The lower speed of reciprocating engines was better for dc generators, because a continuous switching action is required to extract the power from the rotor. In contrast, a much higher speed was possible and desirable with ac generators, because the primary power is produced in the stationary windings, and there is no need for continual switching action to extract the electricity.
Charles Curtis, a Columbia-educated engineer and lawyer, held a patent for a multiple stage impulse turbine. The idea was to use nozzles to convert pressure into kinetic energy before it enters the moving blades, which allows a reduction in casing or shell pressure. The potential was enhanced by the invention of a supersonic nozzle by the Swedish engineer Gustaf de Laval.
General Electric bought Curtis's patent rights—for $1.5 million, according to one published source, The Design of High-Efficiency Turbomachinery and Gas Turbines by MIT professor emeritus David Gordon Wilson. GE put Emmet in charge of developing the turbine. Drawing on his experience with vertical shaft hydro turbine generators at Niagara Falls, he designed a configuration with the ac generator mounted above the steam turbine. Commercial success was realized in 1903 with a 500 kW machine, which was installed at the Fall River Co. in Newport, R.I.
A paper, “The Curtis Vertical Turbine,” presented at the 1988 ASME Winter Annual Meeting by an ASME Fellow, Euan Somerscales, lists more than 100 vertical machines built by 1913, when larger sizes dictated horizontal shafts. A 5,000 kW unit displayed at the General Electric plant in Schenectady is an ASME Historic Mechanical Engineering Landmark.
Emmet commercialized the first combined mercury and steam plant in 1923. General Electric built the plant for Hartford Electric Light Co.
Whereas the original introduction of the steam turbine utilized the existing boiler and balance of the plant technology, the combined mercury and steam cycle also required development of a mercury boiler, a mercury condenser, a firebox that boiled mercury, preheated combustion air, and super-heated steam, along with mercury turbines.
The unique properties of mercury relative to water required a search for compatible materials. The mercury turbine shaft should allow no leakage from the system, whereas a steam turbine could allow limited leakage.
A 1929 issue of Time magazine included an enthusiastic article under the headline “Mercury into Power.” According to Time, “More successful and profitable than attempts to create gold from mercury is the actual creation of electricity with mercury in Hartford, Connecticut.” It explained that a second plant was starting up, and that “William LeRoy Emmet of General Electric has invented and developed the machine.”
Other plants using mercury turbines were built in New York, New Jersey, Connecticut, Massachusetts, and New Hampshire. A chapter entitled “Binary Vapor Cycles” in the 1946 edition of Steam Power Stations by Gustaf Gaffert presents the design, operating conditions, and performance of these plants.
Up until the time of his death in 1941 at the age of 82, Emmet remained convinced that all electric power plants should use mercury. He was also designing mercury cycles for railroad and ship propulsion.
The last dual cycle plant to be built was Schiller Station, which started operating in Portsmouth, N. H., in 1950. The mercury boiled at 200 psia and 1020 ̊F and condensed at 505 ̊F. Heat from the condensing mercury boiled water at 540 psia and 475 ̊F, which was then super heated to 800 ̊F by the coal-fueled fire. The nominal description was 23,000 kW from the mercury and 30,000 kW from the steam turbine. The overall combined heat rate was 8760 Btu/kwh, which corresponds to a plant efficiency of 39 percent.
The fuel efficiency of Emmet's mercury dual cycle was eventually made obsolete by increased steam plant efficiencies from higher pressures and reheating the steam.
The mercury cycle at Schiller Station was phased out, and replaced with a coal-fueled high-pressure steam plant, and more recently with woodfueled boilers. The present day environmental concern is not the mercury cycle that once existed, but the airborne mercury from burning coal. Thus, the time for mercury cycles has gone with the efficiency improvements with high-pressure steam and reheat, the advent of gas-fueled combined cycles, and the environmental protection and safety agencies, which did not exist when the mercury cycles were constructed.
Emmet's contributions today are mostly hidden improvements in rotating electric machinery and apparatus. Many of his achievements have become part of history. Just as his mercury dual cycle became obsolete, so his work in turbo-electric ship propulsion has been mostly replaced with direct-drive diesel engines.
On the other hand, his success in developing the impulse turbine helped create a technology base of engineers and manufacturing. It positioned General Electric to take the lead in turbochargers for piston aircraft engines, and later global leadership in aircraft jet engines and land-based gas turbines for electricity and industry.
In other words, William Emmet has left an enduring legacy.
Editor's note: ASME published William Emmet's An Autobiography of an Engineer in 1940.