Combined cycles: Looking back, looking ahead - I

The first gas turbine ever introduced in Neuchatel Switzerland in 1939 had only 17% efficiency. Its simplicity was a challenge to the more complicated steam cycle to be later married into a combined cycle (CC) as both the Brayton and Rankine cycles improved. The earliest CC units achieved 32% efficiency in1961. 50 years later, they had reached 61%. 

In 1939, Dr. Adolf Meyer of 

BBC

 already foresaw the future in combining steam and gas. He based his predictions of GT performance increases on higher GT temperatures, material improvement and blade cooling. Steam had already reached a 1,000°F and critics did not see gas turbines beating the steam cycle. However, GT reheat and intercooled concepts demonstrated gas turbine engineers could resolve this challenge. 

The prospects for improved performance were based on raising the turbine inlet temperature in the near future to 1,200°F, and raising the cycle efficiency (of the Neuchatel machine) from 18% to 23% (Table 1). Further combustion turbine development raised turbine inlet temperature in modern units to in excess of 2,500°F.

Turbine Inlet             Cycle Efficiences

Temp. °F                    Percent

1,000                          15 - 18%

1,200                          19 - 23%

1,800                          22 - 26%

Table 1: Cycle efficiency is closely tied to inlet air temperature

In June of 1949, GE harnessed a 3.5 MW gas turbine (1,346°F) to do the feed-water heating of a 35 MW conventional steam plant (Figure 1). While designated a combined cycle plant, it was not so in the true sense. Many plants in the 50s and 60s used the gas turbine as forced draft fan and preheater exhausting into a conventionally fired boiler.

The search for higher efficiency and output initiated by the early intercooled GT developments also triggered the first 75 MW gas-fueled/steam combined cycle electric power generating plant in 1961 at Korneuburg ‘A’ Austria, with two 25 MW Intercooled machines and one 25 MW steam turbine. The GT’s were quite efficient at 26% with a low exhaust temperature of 590°F. The addition of a fired super-heater increased steam temperature to 824°F overall plant efficiency to 32%. Combined Cycle plus waste heat recovery (today the norm) was now established as the most efficient and low cost route to electricity production.

This plant ran successfully until 1974 when higher fuel prices made it un-economic. It was replaced by Korneuburg ‘B’ power plant, consisting  of a single shaft 81 MW GT, a heat recovery steam generator (HRSG ) and a single steam turbine (ST) in combined cycle mode. Plant output of 125 MW was combined with a world record efficiency rate of 47%. The race was on for improved combined cycle efficiency with ever increasing single shaft GT sizes and improved Brayton and Rankine cycle efficiency.

GE and Westinghouse in the USA were quicker to push firing temperatures higher and we saw the early STAG (steam turbine and gas) single shaft CC systems emerge as true CC systems applied to power generation. Wolverine Electric’s Burnips unit, according to GE records, was the first of 2 STAG units of 21 and 11 MW CC output that went operational in 1967. Both are still running.

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