Step by Step to Decarbonization: The Importance of Operational Flexibility for Combined Cycle Power Plants

With energy systems in transition, the power industry needs to find answers to fast-changing market conditions. The increased influx of renewables in many countries around the world means that the supply side of the market is becoming more volatile, as wind and solar are among the most widely used renewable energy sources. This not only has dramatic effects on price curves but also creates challenges in terms of grid stability, as previously discussed in one of my articles. The logical implication is that fossil-fired power plants continue to play an important role in stabilizing both the grid and the market, albeit no longer used in baseload in many cases, but rather as a flexible backbone of the energy transition. Consequently, they need to adapt in order to remain competitive. One way to do that is by enhancing the operational flexibility of the existing assets.

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The framework becomes more volatile

Fact: Spot power prices are changing much faster than in the past. Fast startup capabilities and high ramp rates are crucial at times when renewable energy generation decreases, leading to sudden price spikes. There are instances where a gas turbine power plant has earned more money in 3 days than it did throughout the entire year. Conversely, an unexpected oversupply in the market may lead to negative spark spreads or even negative prices, resulting in power producers actually losing money for every minute their plant keeps producing electricity. What they need now are optimized ramp-down and shutdown processes.

And then there is the question of the energy source. Both gas and oil prices have experienced severe shocks over the past few years, highly influenced by geopolitical events. In the case of oil, we have witnessed prices exceeding USD 100 a barrel, as well as WTI future contracts with negative prices for the first time in history, in April 2020.

In addition to all this, there are changing environmental regulations at regional and national levels, the evolving role of CO2 certificates, regulated tariffs, shifts in regional requirements for frequency control, and numerous other variables.

For power producers with a fossil-fired fleet, these fast-paced developments can challenge the underlying business case of their operations, and operational flexibility can tip the scales between stranded assets and profitable business. Now, more than ever, the market dictates precisely how to run your plant.

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Looking into the future

Taking into account global, regional, and national plans for the energy transition, we can develop multiple scenarios for the future energy mix. In many regions, the role of fossil fuels will change dramatically. However, gas turbine power plants will remain an important part of the energy landscape in many countries around the world, providing fast power as well as the much-needed inertia from rotating equipment to stabilize the grid. Many scenarios also indicate that coal-fired power plants will be phased out first, due to environmental considerations and the decreasing need for fossil-fired baseload plants. This implies that the remaining gas turbine plants will have to generate even more power during the hours when renewables are not available. It is only logical that plant operators who can anticipate a shift from baseload to cycling operations and enhance their plant’s capabilities accordingly will benefit from upcoming changes in the system. For gas turbine operators, it is vital to prepare their plants for the future by maximizing operational flexibility to remain competitive.

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How to enhance operational flexibility of combined cycle power plants

There are several options to enhance operational flexibility, including extending low-load capabilities, optimizing startup and shutdown capabilities under various scenarios (hot, warm, and cold starts), and improving ramp-up and ramp-down load gradients. Many upgrades can be implemented through software updates for the control system, while others may necessitate the replacement of turbine or auxiliary system components. The specific measures for enhancing capabilities and performance can only be determined in collaboration with the plant operator following a comprehensive analysis of the entire plant, as they depend on multiple site- and equipment-specific factors.

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Fast startup

When moving away from baseload operation, startup time and reliability become crucial factors. Depending on whether we look at a hot start (within 8 hours after shutdown), a warm start (up to 48 hours after shutdown), or a cold start, we have several options for improving time and reliability of the process.

The usual way to start a combined cycle power plant is the sequential startup of the gas turbine and the steam turbine. Depending on the plant design, the gas turbine will already produce power a good time before the steam turbine which is dependent on the heat recovery steam generator (HRSG). In case of a hot start after an overnight shutdown, we can modify and enable the steam turbine to do a startup on the fly, starting in parallel to the gas turbine, using the steam produced in the HRSG from the beginning. For a combined cycle power plant, this modification can result in a startup time below 30 minutes.

Looking at a warm start, let’s say a day after shutdown, startup becomes a bit more difficult. But even here some modifications can cut startup times by as much as 50% - in some cases to under 60 minutes - while at the same time reducing fuel consumption and cutting emissions during the warm start.

For a cold startup, a big challenge is that the steam cycle – specifically in the high and intermediate pressure part of the steam turbine – will have cooled down, extending the startup time. One solution to avoid the turbine cooling down completely can be to apply electrical heating blankets, which can help to reduce startup time for cold starts up to two weeks after shutdown. Additional concepts aim to seal the steam turbine and create a vacuum in the condenser, resulting in improved startup efficiency and potentially reducing startup times by up to 50%.

Fast shutdown

Enhancing the shutdown process is vital for power plant operators to swiftly exit the market during periods of low or even negative energy prices, thereby minimizing financial losses and maximizing profitability. Moreover, optimizing the shutdown procedure reduces fuel consumption, resulting in cost savings and environmental advantages. With a combined cycle power plant, the shutdown process will usually use a sequential shutdown concept, involving a gas turbine hold time while the shutdown of the steam turbine is being initiated. With our Fast Plant Shutdown Concept, we can optimize the process to reduce shutdown process times significantly, in some cases to below 30 minutes.

Operational flexibility: Crucial for power plant operators, but also an enabler for the energy transition

As we continue the journey towards a more decarbonized energy mix, the pivotal role of gas turbine and combined cycle power plants in enabling the increased integration of renewables cannot be overstated. These fossil-fired power plants play a crucial role in stabilizing the market, thereby facilitating the extensive ramp-up of renewables. However, to fulfill this essential role, power plant operators must prioritize enhancing operational flexibility to effectively respond to fast-changing market conditions and evolving load requirements. Operational flexibility is not only a necessity for power plant operators but also stands as a key enabler for a more sustainable future, ensuring the seamless integration of renewable energy sources into the grid.

Should you wish to explore the topic of operational flexibility further, feel free to access the whitepaper "Maximizing power plant flexibility" and visit our Flex Power Services website.