Quick-response gas turbines to prosper as renewables grow stronger

Flexibility to dispatch power is a key element for developers to consider when choosing technologies for natural-gas power plants in the United States, the vice-chair of the Gas Turbine Association (GTA) and a risk analyst have told FC Gas Intelligence.

As more intermittent renewable energy sources are added to the system, new natural-gas power plants should be able to dispatch according to the demands of a fluctuating market. There is an increase in the need for natural-gas technologies able to start and peak quickly to supply power when there is no sun shining or wind blowing.

“The advice I would give [developers] is to build a plant that can do everything well,” said Kenneth Hall, the vice-chair at the GTA, who spoke to FC Gas Intelligence in his capacity as Siemens Power & Gas’ principal expert in gas-turbine engineering.

“What history has taught us is that predicting the future is a pretty risky business. Many of our customers built plants thinking they would dispatch one way and then the market changed and the plant needed to do something else.”

The increase of renewable energy sources in the electricity market is causing volatility in real-time electricity prices, according to Gary Dorris, chief executive of consultancy and software developer Ascend Analytics.

"That extreme volatility poses opportunity for very flexible resources, but it also poses severe risk in the resources that are less flexible in their ability to shut down, or very inefficient at minimum load," he said.

Quick-response technologies to benefit

Power plants that can ramp up quickly to full load, be online a few hours, and them drop offline are best positioned to respond effectively to prices in the short-term electricity markets and to benefit from the opportunities presented by volatility, according to Dorris.

"The bottom line for developers is flexibility yields substantial value as we look forward," he said. "And we can only expect more of the renewables and greater volatility."

Renewable energy sources accounted for 13% of US total electricity generation in 2015, and this share is projected to increase to 18% by 2040, according to the Energy Information Administration (EIA).

Dorris said internal combustion engines are able to peak faster and adapt to volatility in the real-time energy markets. Combined-cycle with duct firing can also have an increase in picking flexibility, and monetize by selling ancillary services in the electricity market.

“Ancillary services are becoming more valuable as the market prices get more volatile,” he said.

Ancillary services help guarantee reliability and stability in the transmission grid, supplying load regulation, voltage support and frequency control, among other services. They become more important to stabilizing the system and managing increased variability as more intermittent renewable power plants are integrated.

Improvement in conventional technologies

Simple-cycle power plants have been considered the ideal complement to the fast increase of renewable sources in the US for a long time, because of their ability to be dispatched and reach full load within a few minutes, faster than combined cycle.

But Siemens' studies on the matter show that modern combined-cycle plants have the flexibility to work well with renewables, and both simple and combined cycles are viable options, according to Hall.

“The best choice depends on the size of the plant and the expected dispatch,” he said.

Flexible high-efficiency combined cycles for large facilities, for example, have the ability to start fast, quickly change load, ramp up and down while maintaining low emissions, at a low cost of generation.

“In a conventional combined cycle, the gas turbine goes through a series of load-hold points as the engines increase in power, enabling the steam cycle to be gently warmed up,” Hall said. “Today, with a Siemens Flex-Plant combined cycle the gas turbine can be started the same way it is in simple cycle – and just as quickly.”

Many Siemens combined-cycle plant configurations are able to go from shutdown to baseload in about 30 minutes, with more than 60% efficiency in power generation. In January, Fortuna’s combined-cycle power plant in Lausward, Germany, using Siemens' SGT5-8000H gas turbine, broke a record by achieving 61.5% net combined-cycle efficiency.

For lower-dispatch plants, Hall said there is a full range of simple-cycle technologies, including large and small frame units with selective catalytic reductions, a technology able to reduce emissions, and aeroderivative gas turbines.

Simple-cycle turbine technologies have been improved in order to start and ramp up faster as well.

“In an older unit there was a wait time to purge before the engine started,” Hall explained. “We now offer an option to eliminate this pre-start purge.”

Some Siemens' technologies can reach base load in less than 10 minutes, like the SGT6-500F turbine, which can deliver 200 MW in that timeframe.

Hall explained that with this broad variety of solutions, customers are choosing different flexible technologies to complement renewables, and not just one type of turbine.

“Instead of choosing a plant specified for a specific dispatch need, now you can build a plant that can change as the world changes around it,” he said.

By Anna Flávia Rochas