Gas Turbine Combined Cycle (GTCC) Power Plants now provide about 30% of America’s electric power generation. The GTCC plants are the most efficient “Heat-Engines” ever developed. That is, they convert more heat energy into useful electric power than any other type of heat engine, with efficiencies of up to 64% under ideal test conditions being achieved. Did you ever wonder how GTCC plants came to be?

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Figure 1. A Typical Gas Turbine Combined Cycle Power Plant with over 60% Thermal Efficiency

Figure 2. The Heart of a Modern 500 -600 MW GTCC Power Plant, A Gas Turbine. From G-E posts on the internet

Two Reasons for the Growth of Natural Gas Power Generation

The growth of natural gas power generation from 2002 to the present day is largely due to two factors:

  1. Abundant and reasonably cost natural gas with the advent of Hydraulic Fracturing
  2. The development of highly efficient Aero-Derivative Gas Turbine engines coupled with efficient steam turbines that collectively achieve up to 64%  thermally efficient power generation.

The “Fracking Revolution” of natural gas production is a story unto itself, certainly a success story for American energy production. This is the focus of this brief article, to chronicle the development of the highly efficient gas turbine engine. However before starting our journey, let’s review a summary of the fuels used for power generation since about 2010.

Electricity generated from natural gas has grown over the last decade exceeding nuclear power generation, and even rivaling coal which has been the dominant fuel source for power generation for a century. The rise of natural gas fuel use can be directly attributed to Hydraulic Fracturing and the Shale Gas industry. The production of reasonably cost natural gas has driven both electric utilities and independent power generation companies to utilize an increasing percentage of natural gas, rather than other fuel sources. The natural gas power generation in the US during 2017 was approximately 33%. Figure 3 illustrates power generation in US from 2010- 2018 by fuel type.

Figure 3. Electricity Generation in the U.S.A.

The primary factor in producing low cost electricity is the energy cost in dollars per million BTU’s (British Thermal Units). The abundant Marcellus Shale Gas Formation in Pennsylvania, West Virginia, and Kentucky has made natural gas cost competitive with coal in energy available per dollar. Power generation cost in a Heat Engine whether from coal, oil, or natural gas is determined by the cost of the heat energy of the fuel source selected. The cost of electric power generation in a coal plant is approximately 80-85% of the fuel cost. In comparison, the fuel cost component for production of electricity in a gas turbine combined cycle power plant is approximately 90-95%. Hence, if the cost of natural gas rises sharply, so does the production cost, as illustrated in Figure 4.

For reference, the cost of coal in dollars per million BTU is between $1.50 and $2.50 per million BTUs. The competitiveness of GTCC plants even at a higher cost of fuel is the result of higher efficiency and less operational costs associated with personnel, FGD chemicals and less capital construction costs. GTCC plants are competitive with coal for power generation at gas costs of about $3.00 /million BTU.

Figure 4. Natural Gas Prices in the U.S.A. 2016-2018

Advantages and Disadvantage of GTCC (Gas Turbine Combined Cycle) Power Generation Units

In addition to abundant natural gas supplies in the US, there are other advantages that GTCC plants have over clean coal plants. These include:

  • Lower capital cost to construct the power plant
  • Approximately 50% greater Thermal Efficiency than Rankine Cycle Coal Plants (and approximately 50% less carbon emissions of a similar sized coal plant)
  • Faster and simpler government permitting
  • Greater acceptance by the general public as a clean energy source
  • Fast load response to back up wind and solar power sources
  • Fewer employees to operate and maintain the equipment and facilities

There is only one  one disadvantage that I can think of. That is creating an unbalanced portfolio of generation dependent on a fuel known for price volatility. Committing our future power generation to such a large percentage of natural gas fuel creates risk for future electricity price escalation.  This is because the cost component of fuel for production of electricity from natural gas plants is about 90-95% of the production cost. So, as long as natural gas remains below $3.00/ million Btu’s, that is competitive with clean coal plants. However, if natural gas prices rise then so will the production cost of electricity. The increase in production cost will pretty well track fuel cost. In other words, if natural gas prices double, so will the cost of electricity production. This is why a balanced portfolio of coal, nuclear, gas and renewable power generation is preferred.

This is just a quick overview of how natural gas plants have become so popular for electric generation. Now let’s get into the technical evolution of how these modern marvels came to be.

Aero-Derivative Gas Turbines

Gas Turbine power plants are the result of the continuous development and the evolution of modern jet engines as used for aircraft propulsion.

Figure 5. G-E Jet engine

Figure 6. An FA-18 using the most advanced jet engine technologies

The modern highly efficient Gas Turbines for power generation have their roots in the development of powerful, reliable and efficient jet engines. One of the first Aero-Derivative Gas Turbines for power generation in America was installed at the Belle Isle Station of Oklahoma Gas and Electric Company in 1949.

Figure 7. 3500 kW Capacity G-E 1949 Gas Turbine Installed at OG&E Belle Isle Station. This is now an ASME Historical Landmark.

A time-line covering the history of the development of jet engines and gas turbines as they developed for power generation:

  • 1930 English inventor Sir Frank Whittle obtains a patent for his version of a jet engine
  • 1936 German inventor Dr. Hans von Ohain obtains a patent
  • 1939 German Heinkel He-178 jet first flight
  • 1941 Whittle’s first English jet first flight
  • 1943 G-E begins work on J-35 GE
  • 1946 G-E J-35 powers the Republic XP-84 Thunderjet
  • 1947 G-E ALCO Gas Turbine is installed in a Locomotive
  • 1949 the first American installation of a Gas Turbine for power generation is installed at OG&E Belle Isle Station

Over the years since then, G-E, Alstom, Westinghouse, Pratt-Whitney, Siemens among other manufacturers have worked to develop the latest GTCC power plants. While Gas Turbines have grown to be very efficient, approaching 40% thermal efficiency, the huge breakthrough was in combining the gas turbine “Brayton Cycle” with the steam “Rankine Cycle”. By using the Gas turbine exhaust to generate steam and then pass the steam through a steam turbine, the total plant efficiency has been elevated to over 60%.

That my friends is why the latest gas turbines are referred to as “Aero-Derivatives”. The advancements of 3 dimensional machining, blade cooling, combustor technology and other refinements that were developed for aircraft engines have been applied to stationary gas turbines and scaled up in size to power modern 500-800 MW GTCC Power Plants.

Richard F. (Dick) Storm, PE, CEM
Williamson class of 6W2

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Appendix 1 / References