Described as the "turbine of the 21 century", GE's 7H single shaft, 60 per cent thermal efficiency combined cycle unit will make its commercial debut in Sithe Energy's 800 MW Heritage Station at Scriba in New York State.

The GE Frame 7H gas turbine, with steam cooled turbine blades, to be installed at the new Scriba combined cycle plant will be the first commercial application of US Department of Energy ATS technology. It is claimed to be the first system to exceed the psychological barrier of 60 per cent thermal efficiency.

Sited alongside Sithe Energy’s showpiece 1040 MW Frame 7F based Independence Station, the new $400 million project which Sithe and GE have joined forces to develop will be known as the Heritage Station.

The technology is a culmination of the partnership between the US gas turbine industry and the DoE that began in 1992 and has now come to industrial fruition with support from the Energy Department’s Federal Energy Technology Centre, Oak Ridge National Laboratory in Tennessee, and a consortium of the top US engineering universities.

Secretary of Energy Bill Richardson said, “Innovations from this programme have already improved today’s fleet of turbines. Now, this new agreement between GE and Sithe means that we are on the verge of moving an entirely new generation of technology into the market with the promise of even greater environmental and efficiency benefits.”

Planning permits for the project are due to be submitted in December 1999, and since the NY State permit applications process undertakes completion within 12 months, final permits are expected in December 2000, at the eleventh hour of the millennium. The first of the two 400 MWe single shaft combined cycle units is scheduled to be on-line at the end of 2002 and the second in 2003. No contracts had been placed at the time of writing.

As with the 500 MWe 9H system, the GE 7H has been introduced to the market as a single shaft combined cycle power unit with a purpose built exhaust heat recovery boiler.

The major advances in turbine output and efficiency mainly derive from the use of closed cycle steam cooling of the turbine blades, very high combustion temperature, incorporation of GE’s advanced aircraft engine technology including optimized compressor aerodynamics, single crystal turbine blades and advanced thermal barrier coating processes.

The exhaust heat recovery steam generator will be similar to a typical three pressure level combined cycle boiler, except that a substantial proportion of the cold reheat steam from the HP exhaust system will be diverted into the turbine steam cooling system. The cooling steam, which may amount to as much as 25 per cent of the cold reheat steam, will be returned into the IP section of the condensing steam turbine.

The gas turbine

GE’s MS7001H gas turbine contains an 18-stage compressor, a can-annular dry low NOx (DLN) combustion system, and a four-stage turbine. Closed circuit steam cooling supports the very high combustion temperature of 1427 °C (2600°F). The Stage 1 and 2 nozzles and buckets plus the Stage 1 shroud are steam cooled. Air cooling is used for the Stage 3 nozzle and bucket with the fourth stage being uncooled.

The rotor system is similar to earlier GE gas turbines, being supported by two bearings with the first rotor bending critical above the operating range. Through-bolt rotor construction is continued in both compressor and turbine rotors.

The MS7001H compressor provides a 23:1 pressure ratio with 1230 pps (558 kg/s) mass flow. The H System compressors are derived from GE’s high-pressure compressor used in the CF6-80C2 aircraft engine and the LM6000 aeroderivative gas turbine. The CF6-80C2 compressor is scaled up to 2.6:1 for the MS7001H with four stages added to achieve the desired combination of airflow and pressure ratio. On the MS7001H, the last stage from the MS9001H compressor is eliminated and a zero stage added at the front.

The nominal output for the first of the US Department of Energy’s ATS (Advanced Turbine System) specification machines will be 400 MWe. All enabling technology for the ATS has been built into the 500 MWe Baglan Bay 9H 50 Hz system, in Wales, UK (see MPS May 1999) but the ATS designation applies exclusively to the 60 Hz version. Detailed characteristics of the H System machines were first published in MPS June 1995.

H System combined cycle plant performance characteristics