The right combination? Power plant operations

In line with today's soaring global energy demand, never before has flexible grid operation felt so critical. Speak to any power plant operator or supplier, and they will espouse the theory that capacity agility is the key to keeping the lights on in the 21st century.

In some ways, this hardly seems like a searing insight. Flexible generation - effectively, plants with efficient ramp rates - has long been identified as a means to counter the traditional uncertainties, such as load vicissitudes, that come with power generation.

What is novel, however, is the task of incorporating variable renewable energy sources, such as solar and wind, into the grid in a bid to help operators play their part in reducing emissions.

This grid trend, which has gained particular traction in the likes of China, India, Brazil and parts of Western Europe, has formed a core area of research for GE Energy since the early part of the past decade.

The group's initial studies revealed that the combined-cycle plants of the future - using a combination of gas and steam turbines together - needed to be much more flexible in dispatch, and ramp up and down more quickly and efficiently to meet renewable demands.

Subsequently, GE forked out more than $500 million in R&D to patent and develop the combined-cycle 510MW FlexEfficiency 50. The 50Hz single-shaft plant was subsequently given its official launch in 2011.

"We realised that the market for simple base-load gas combined cycles was no longer the future," recalls Marcus Scholz, director of advanced combined cycles at GE Power & Water. "That led us to revise and review the cycle and turbine design concept, and create the 9HA heavy-duty gas turbine, which is used with the 510MW simple cycle of the FlexEfficiency 50 plant."

Tandem cycle

The 9HA turbine serves as the central cog of the FlexEffiency 50 plant. Billed as the world's largest and most efficient gas turbine in its class, it promises to deliver the lowest life-cycle cost per megawatt due to the deployment of air cooling - as opposed to steam cooling, which was particularly prevalent in the early noughties.

It is also able to help the FlexEfficiency 50 achieve a ramp rate of 50MW a minute - more than double the rate found in most combined-cycle plants in operation today. Meanwhile, it has a net efficiency of 61% and can reach a turndown of as low as 40% of its base-load output, all while meeting air emissions mandates.

According to Scholz, what makes FlexEfficiency 50 so attractive is its all-inclusive design. Potential buyers can effectively expect to purchase an all-in-one product that naturally lends itself to flexible operations.

"It's an all-plant concept, which is really important to understand," he says. "It's not just a gas turbine feature: it includes the heat-recovery steam generator, the steam turbine and the plant control system. These come together to create a much more flexible plant, with an overarching strategy."

This, adds Scholz, was no small achievement. So what were the main challenges in achieving the final design?

"Several constraints had to be mounted to avoid designing a plant just for base-load duty," he says. "On the steam cycle, for example, you need additional bypass capabilities in order to part-load the plant and help it change much faster. So, rather than blowing off steam, you want to reuse that steam in a condenser.

"In addition, you need to foresee the material capabilities for thermal cycles. That's another reason why you need to take a close look at the steam cycle before you can even talk about having a flexible combined-cycle power plant."

Rapid response unit

The FlexEfficiency 50, however, was designed firmly with renewable energy in mind. As governments across the world pledge to wean themselves off fossil fuels in the name of having a positive impact on climate change, adding renewable sources to the grid has become a major goal - but one that has traditionally been hard to realise.

The main, and most obvious, issue is that of the variability and unreliability innate to generating power from the elements. Inadequate sunshine or winds that amount to no more than a swish of a cat's paw need to be compensated for.

GE's answer, Scholz admits, is based on using "the cleanest fossil-fired generation technologies out there" - a flexibility to use gas and liquid fuels, such as high-ethane gas and liquefied natural gas.

"Certainly, the renewables issue has created some volatile discussion in main markets, such as Europe, due to the intermittent nature of solar and wind," says Scholz. "But if you have, let's say, five responding combined-cycle capabilities, which are very efficient and respond quickly, but are also based on clean technology, you can compensate for any drop in a very short time. That is another way in which the FlexEfficiency 50 can enhance flexibility."

A full display of green credentials is therefore paramount for any power plant operator. With this in mind, the FlexEfficiency 50's carbon footprint is said to be just a tenth of that found in other combined-cycle plants with a similar output.

It is also able to achieve a carbon emissions reduction of roughly 12,700t a year - equivalent to ridding the EU of 6,000 cars from its roads, according to GE. Furthermore, the technology has been certified by Ecomagination, a GE forum dedicated to clean technology and sustainable infrastructure.

In fiscally straitened times, operators are also looking to get more bang for their buck. According to Scholz, they can expect to achieve savings of approximately $2.6 million a year under a standard operating profile of 4,500 hours a year. In addition, he claims the FlexEfficiency 50 can confer annual fuel savings of 6.4 million cubic metres of natural gas - equivalent to more than 4,000 EU households.

"There are a number of cost advantages," states Scholz. "For instance, in the past, risk-averse operators used to turn down or part-load a combined-cycle plant because of lengthier start-up times. With the FlexEfficiency 50, you have a highly reliable plant that can start up in under 30 minutes, which can take place after an overnight shutdown. Operators know that they are not losing money anymore through part-loading or leaving the plant at a low-load level, where it's less efficient."

Off the grid - for now

Despite there being no plant yet in commercial operation, FlexEfficiency 50 and the 9HA turbine have certainly generated considerable interest across the industry.

The flurry of tests that have taken place, some of which have been carried out in front of the world's media and regulators, at GE's state-of-the-art full-load testing facility in Greenville, South Carolina, US, are likely to have assuaged any doubts that may have arisen concerning the group's much-vaunted product.

As well as trialling the turbine at full capacity in a range of real conditions, the plant - born out of an investment worth $170 million - can also be put through its paces beyond normal operating parameters due to the testing facility not being connected to the grid.

"The key feature of the facility is that it's not grid-connected," says Scholz. "In fact, we don't even use a generator; we use an air compressor instead. The beauty of using this test cell is that it's very responsive and also very stable. This means you can operate it to explore the most aggressive movements of gas turbine acceptance in case of frequency deviations.

"Regulators have witnessed this and have seen, perhaps for the first time in their lives, what a gas turbine of this size can actually do."

A lofty claim, perhaps, but one we won't have to wait too long to validate. France's state-owned utility group, EDF, together with GE, is currently in the process of building the world's first commercialised FlexEfficiency 50 plant at its Bouchain site in northern France. Operations are set to begin next year.

Elsewhere, GE is purported to have penned a memorandum of understanding with Toshiba for a partnership to develop 50 and 60Hz combined-cycle power projects across the globe, which will be powered by the 9HA turbine technology. However, at the time of writing, details of this agreement remain scant.

What is certain, however, is that the FlexEfficiency 50 taps into the two aspects of power operation from which the product's portmanteau name is derived - flexibility and efficiency. This undoubtedly makes it appealing to operators keen to drive down emissions and costs, while, for the energy market at large, it could reignite a new age of competition in which the players that strategically invest in new-generation combined-cycle technology will reap the most lucrative awards.