Generation Technology Responsiveness and Flexibility

Introduction

In our last lesson, we explored the concepts of baseload, intermediate, and peaking generation. We learned that grid operators dispatch different types of power plants based on their economic and operational characteristics to meet the daily load curve. We saw that baseload plants like nuclear are slow and run continuously, while peaker plants like gas turbines are fast and used for short bursts.

This lesson builds directly on that foundation to address the learning outcome: Compare the response times and flexibility of different generation technologies.

We will formalize the idea of "flexibility," explore why it's becoming more important than ever, and then conduct a side-by-side comparison of different generation types. Finally, we'll look at how the UK's grid operator, National Grid ESO, quantifies these characteristics by procuring specific "flexibility services."

1. What is Power System Flexibility?

In simple terms, flexibility is the ability of a power system to manage changes and maintain the constant, critical balance between electricity supply and demand.

In the past, the main challenge was a predictable, but fluctuating, demand (the load curve). Today, the challenge is compounded by the rise of variable renewable energy sources like wind and solar. Their output changes with the weather, not according to the grid's needs.

This means the "conventional" part of the grid has to be even more agile to fill the gaps. The amount of demand that isn't met by variable renewables is called the net load, and it's this net load that flexible resources must follow.

Let's watch a short video to visualize why this balance is so crucial.

As the video explained, the system needs resources that can respond to changes in the net load. These resources are the sources of flexibility.

2. Comparing the Flexibility of Generation Technologies

Now, let's compare the key generation technologies based on their flexibility and typical response times. A technology's flexibility is determined by several factors:

• Start-up time: How long it takes to go from being offline to generating power.
• Ramp rate: How quickly it can increase or decrease its power output once online.
• Minimum stable generation: The lowest power level it can run at efficiently without shutting down.

Here is a summary of the main generation types, from least to most flexible.

Let's look at the reasons behind these characteristics.

Inflexible Baseload: Nuclear and Coal

As we saw in the last lesson, these large thermal plants are designed for steady, continuous operation. The process of heating massive boilers to create high-pressure steam is inherently slow and inefficient to start, stop, or change rapidly.

Flexible Generation: Gas, Hydro, and Storage

Gas Turbines are far more agile.

• Open Cycle Gas Turbines (OCGTs) are essentially jet engines. They can be fired up in minutes, making them perfect "peaker" plants to meet short, sharp increases in demand.
• Combined Cycle Gas Turbines (CCGTs) are more efficient because they use waste heat to power a second steam turbine. This makes them cheaper to run for longer periods, but the addition of the steam cycle makes them slightly less nimble than OCGTs. They are the workhorses of intermediate generation.

Hydropower and Pumped Storage offer even faster response.
The output can be increased almost instantly by simply opening a valve to allow more water to flow through the turbines. This makes them exceptionally valuable for responding to sudden changes on the grid.

Battery Storage is a newer technology that offers near-instantaneous response. Using power electronics, batteries can inject or absorb power from the grid in milliseconds, making them ideal for managing very fast fluctuations in frequency.

3. Flexibility in Practice: UK Ancillary Services

The National Grid Electricity System Operator (ESO) doesn't just hope for flexibility; it actively procures it from generators and other providers through a set of products called ancillary services.

These services are defined by their technical requirements, including how quickly they must respond ("Time to delivery") and for how long they must sustain that response ("Delivery duration"). This is where we can see the real-world application of response times.

The following document provides an overview of the need for flexibility in the GB system and details some of these services.

From that reading, you can see a clear hierarchy of response speeds, each designed to tackle a different type of problem:

• Dynamic Containment (DC):

  • Response Time: Under 1 second.
  • Purpose: A post-fault service to "catch" the frequency after a major, sudden event, like the unexpected loss of a large power station or interconnector.
  • Providers: Almost exclusively provided by batteries due to the incredible speed required.

• Fast Reserve:

  • Response Time: Within 2 minutes.
  • Purpose: To provide a rapid injection of power to handle large, but less critical, imbalances.
  • Providers: Can be provided by fast-acting gas engines, hydropower, or batteries.

• Short Term Operating Reserve (STOR):

  • Response Time: Within 20 minutes.
  • Purpose: A post-fault service to bring on more significant generation capacity to manage a sustained shortfall, for example, if a large power station is unavailable for a few hours.
  • Providers: Typically provided by OCGTs, diesel generators, and some flexible CCGTs.

This shows how the different response times of various technologies map directly onto the specific services needed to keep the grid stable. A nuclear plant can't provide Dynamic Containment, and it would be wasteful to use a battery to provide a 2-hour block of STOR. The right tool is needed for the right job.

Conclusion

In this lesson, we have quantified the operational characteristics of different power plants by comparing their flexibility and response times.

Key Takeaways:

• Flexibility is the ability of the power system to respond to changes in net load (demand minus variable renewable generation).
• Generation technologies have a wide range of flexibility:
  • Low Flexibility: Nuclear and coal plants have long start-up times and slow ramp rates.
  • Medium Flexibility: CCGTs can ramp up and down to follow daily demand patterns.
  • High Flexibility: OCGTs, hydropower, and pumped storage can start and ramp very quickly to meet peaks.
  • Very High Flexibility: Batteries can respond almost instantaneously.
• The UK's National Grid ESO procures flexibility through ancillary services with specific response times, such as Dynamic Containment (<1s), Fast Reserve (2 mins), and STOR (20 mins).

Preview of the next module:

This lesson concludes Module 2. We now have a solid understanding of how electricity is generated, transformed, and dispatched based on its characteristics.

The next module, "Renewable Generation in the UK," will dive deep into the technologies that are driving the need for more flexibility. We will start by looking at the current UK generation mix and the trends towards decarbonization, before examining the specific operational characteristics and grid integration challenges of wind and solar power.