The historic breaking of the 60% combined cycle efficiency barrier by Siemens’ H class technology at Irsching 4 (see p 19) – apparently with plenty of margin to spare and the promise of 61.5% achievable within the next three years or so – seems to have unleashed a flurry of public relations activity by the other gas turbine OEMs. Siemens is unquestionably first over the line, with the groundbreaking plant due for commercial hand over to E.On in July. But it turns out all the other OEMs consider themselves capable of achieving 61% efficiencies and beyond, and, like Siemens, they are all emphasising that this high efficiency can be achieved in combination with very high operational flexibility.
GE has even coined a new term, “FlexEfficiency rating”, to capture the concept and measure it, as well as launching a new plant design, the FlexEfficiency 50 (see pp 16-18) – the first of which will be built in Turkey, integrated with a solar tower installation and a wind farm – with start-up in 2015 (p 4).
Not so long ago it was thought by some, not least GE, that steam cooling, of stationary gas turbine parts, and even of rotating components (as pioneered in GE’s Baglan Bay H class plant) would be needed to achieve combined cycle efficiencies over 60%. But others, including Siemens, have argued all along that it can be done with clever deployment of air cooling alone. Also, Siemens took a decision ten years ago (a very shrewd judgement, as it has turned out, considering that at the time the role of combined cycle was considered to be predominantly baseload) that their H class turbine should be highly flexible, ie capable of cycling and frequent and fast starts. This effectively ruled out steam cooling and required the new machine to be fully air cooled.
GE’s new offering, the FlexEfficiency platform, is also fully air cooled, being based on an upgraded version of the FB gas turbine, with advanced compressor technology (drawing on GE’s aviation experience) and an additional power turbine stage. Where this leaves steam cooling is an interesting question. Mitsubishi is retaining steam cooling of stationary components in its new J class series (p 4), but its application to rotating parts, as in GE’s H class machines, while a very impressive technological achievement, would appear to have been something of a technological cul de sac – perhaps the Concorde of the gas turbine world – the imperative of flexibility being its undoing.
In turn it is the rise of renewables that is driving this demand for flexibility. Launching the FlexEffiency 50 in Paris in May, GE’s Paul Browning, president and CEO, thermal products, rather strikingly described it as “a natural gas fired combined cycle plant whose purpose is to help enable greater uptake of renewable energy.”
Alstom is also citing the growth of renewables and the need for combined cycle plants to provide “an increasing role in grid load balancing support – with high cycling and part load operation being more the norm” – as a key consideration behind the launch of its new upgraded version of the GT26, with “evolutionary” modifications to LP turbine, SEV (2nd stage combustor) and compressor (again drawing on aviation experience, via Alstom’s tehnology sharing agreement with Rolls Royce).
Alstom, with its sequential combustion technology, regards itself as “the pioneer in operational flexibility” and believes that “efficiency under part-load operation is even more important than the efficiency at baseload in many cases.”
The new GT26 (currently under test at Birr) is also said to be capable of over 61% efficiency in combined cycle, with the combined cycle plant able to be “parked” on-line at around 20% load (about 100 MW), or even less, and ramped back to baseload, delivering 350 MW to the grid in less than 15 minutes, enabling “integration of intermittent renewable sources of energy.”
Remarkably, therefore, the accommodation of renewables has now become pretty much the key driver for combined cycle technology, and with all the combined cycle OEMs now saying their technologies are capable of 61% efficiency and more, flexibility is emerging as the new battleground where they are seeking to demonstrate competitive edge.
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