The Rankine Cycle described in the second part of this series is working hand in glove with the gas turbine Brayton Cycle and has given us remarkable energy savings. With an improved part load heat rate, 65% combined cycle efficiency is targeted by 2016, which does not seem to be a distant dream.
The Brayton Cycle
The key player in the projected energy plan burning the new shale gas is the internal combustion gas turbine using the Brayton Cycle, invented by George Brayton in 1870. In his invention, air was compressed and delivered to a combustion chamber where liquid fuel was injected and burned directly with the air. The hot gases then went to an expander.
The cycle is similar to the diesel cycle but uses a continuous air flow compressor, a continuous flow combustor and turbine for an uninterrupted flow instead of the “batch” type four-stroke piston arrangement. In actual practice, the adiabatic compression efficiency of the modern day large gas turbine is about 87% (polytropic efficiencies over 90%) and the adiabatic expansion efficiency ranges from about 90 to 92%. The simple cycle efficiency of the open cycle gas turbine has climbed from about 16 % to over 40 % today. Though John Barber was given a patent on the basic gas turbine in 1791, Bryton has been given credit for the cycle.
Brown Boveri industrial gas turbine
Brown Boveri built a large, heavy and bulky axial flow compressor and turbine design with a huge single combustor. It drove a 3000 RPM generator (50 cycle), and the output was 4000 KW. In 1939, it was manufactured and installed in Neuchatel, Switzerland, and the plant had an efficiency of 18 %. This was the first gas turbine to be installed in a central power station.
Invention of jet engines
UK Royal Air Force engineer Frank Whittle designed a jet engine using the Brayton cycle, and was able to develop the first workable jet engine potentially light enough for an airplane. It incorporated a back-to-back outflow double centrifugal compressor, multiple combustors (flame tubes) and a single stage axial flow turbine to drive the compressor. A more workable design was developed for an airplane; the jet engine called the Whittle W.1.X. was faster than a prop plane but had high fuel consumption and lacked durability and reliability.
In Germany, Hans von Ohain led the way developing his axial flow compressor design. Ohain used the work that Brown Boveri had pioneered, but was able to reduce the size and weight of the monstrous Brown Boveri design to make it suitable for a plane. These jet engines later formed the basis for airplane jet engines, power plant gas turbines and combined cycle power plants.
Early industrial gas turbine effort
The early industrial gas turbines used steam turbine manufacturing methods and jet engine technology taking advantage of the huge federal funding to come up with the so-called “heavy duty” designs. These units lagged behind the aircraft jets, but helped improve output and cycle efficiency. Soon, all commercial planes switched over from piston/props to gas turbine jets, and later to the more efficient large fan designs.
Performance by OEMs
GE and Westinghouse developed larger units again, hoping to sell them to the utility companies. GE came out with their Frame 7 and 9 models of about 40 and 60 MW respectively. Westinghouse took a bolder move and offered larger units, the 301 and the 501 models of 50 and 80 MW, both at 60 Hz. Westinghouse sold some to Dow Chemical Company at Freeport, Texas. Finally, the 60 Hz Westinghouse 501 – D was introduced with an output of 100 MW. At last, this size had more appeal to the utility companies. Westinghouse sold 19 of these 100 MW units to Aramco in Saudi Arabia in 1975 in one giant sweep. They called them their “big hummers”. The design was immature and required considerable field modifications before it was acceptable. Problems were solved and a reliable unit became available.
Nothing has been said about Mitsubishi Heavy Industry (MHI) of Japan so far, but this company has been and still is a major player for large gas turbines. MHI has been an active participant in the industrial gas turbine business internationally over the years, and has sold a number of units to Aramco in Saudi Arabia as an example. MHI linked up with Westinghouse way back in the 1960s but has branched off on its own since Siemens bought out Westinghouse’s US turbine business a few years ago.
Everything that has been said about Westinghouse and the 301 and 501 turbines applies to MHI. Westinghouse licensed their designs to MHI whereas GE only had foreign companies as joint manufacturers. MHI is now ready to put into operation its 2912 O F (1600 O C) “J” class gas turbine. Future plans call for an inlet temperature of 3092 O F (1700 O C). Improved TBCs and cooling are reported to be used.
ABB’s bold move
Asea Brown Boveri (ABB), head quartered in Baden, Switzerland, formerly Brown Boveri and now Alstom, took a bold move in 1994 and came out with a single shaft reheat gas turbine (RHGT) called the GT24, “Sequential Combustion”, 60 Hz 3600 RPM turbine. It had an ISO rating of 165 MW and over the years has been uprated through higher air flow, higher firing temperatures and better sealing and cooling to today’s rating of 200 MW, a 20 % gain. Its original efficiency was about 38 % which has increased to 40 %. At the same time, ABB also offered the GT26, a 50 Hz, 3000 RPM version, originally at an ISO rating of 240 MW, and uprated to today’s output of 290 MW with similar efficiency levels.
A redesigned GT26, now called the KA26 for combined cycles, is now undergoing tests in Birr, Switzerland. Improvements have been made through a technology sharing agreement with Rolls Royce. The compressor air flow and pressure ratio have been increased to 35. The reheat SEV combustor has been modified and the LP turbine now includes 3-D blading through R-R’s help. The new rating will be about 300 MW.
Both RHGTs had exclusive all-welded extended stiff shaft rotors. They had new 30 pressure ratio compressors, standard EV first combustors and new unique patented SEV reheat combustors with diffusers, ramp vortex generators where fuel was injected and then dumps for mixing. The RHC was positioned between a single stage high pressure turbine and a four stage LP power turbine. Cooling air was extracted and externally cooled. The exhaust temperature was originally about 1100 o F, later increased to 1140 o F for both units.
The first unit, the GT24, was installed in the US at the Jersey Central Power and Light Gilbert Power Station, N.J. Today there are over 275 GT24 and GT26 units in operation around the world. Brown Boveri introduced many innovations, and GE took the developments further, which will be discussed in the next part of this series.
Ivan G. Rice was past chairman of the South Texas Section of ASME (1974 – 75), past chairman of the ASME Gas Turbine Division (now IGTI) (1975 – 76). A Life Fellow Member of ASME and Life Member of NSPE/TSPE, he has authored many articles and ASME papers on gas turbines, inter-cooling, reheat, HRSGs, steam cooling and steam injection.