Are Simple Cycles or Combined Cycles Better for Renewable Power Integration?

It's been called "filling the duck pond," and it's the increasingly common challenge worldwide of balancing supply and demand when variable renewables are not feeding power to the grid. Gas-fired generation is often filling the pond, but the technology mix matters.

The growing portfolio of renewable power generation around the world has made the selection of the appropriate partnering technology a topic of great interest. With solar and wind power dependent on the whims of Mother Nature, reliable, dispatchable, and flexible power generation needs to be available to ensure a robust and reliable supply of electricity. Making the right technology selection requires balancing concerns about grid stability, the environment, and economics to ensure demand is met.

Providing reliable electricity means succeeding in instantaneously meeting demand with supply. Though electricity storage continues to show promise, the amount of practical, economic storage is limited, so the vast majority of electricity has to be produced as it is consumed.

Because electricity demand is constantly changing, the system has always needed to be able to increase and decrease the amount of power generated throughout the day. The weather dependency of renewables has introduced a new variable to power dispatchers-a fluctuating supply of energy from renewable resources. A concern has been raised by some power producers that these two drivers could combine in a way that might require many facilities to start and stop multiple times a day to fill the gap. Some have concluded that the solution is to add more simple cycles, but recent research with advanced dispatch modeling suggests that a balanced portfolio of flexible gas-fired combined cycles and simple cycles-very similar to a mix one might have chosen before renewables-is a better option for the future.

Dispatching with Renewables in the Mix

Understanding what will happen when more renewables are on a grid is not simple. The reaction of an individual solar or wind plant is a localized, time- and weather-dependent event. One attempt to explain what might happen was presented by the California Independent System Operator (CAISO) in 2013.

Figure 1 shows a curve called the California duck. The lines plotted are net load-the amount of generation that needs to be provided by nonrenewables sources-over the course of a 24-hour day with each line representing a different year. The top line, or the back of the duck, is 2013 and the bottom, or belly of the duck, is 2020, when California is set to produce 33% of its power from renewable resources. It is worth noting that California is currently considering a 50% renewable portfolio, which would result in an even lower belly.

1. The California duck.The lines represent net load (the difference between forecasted load and expected electricity produced by variable renewables) over the course of a 24-hour day with each line representing a different year. The top line, or the back of the duck, is 2013 and the bottom, or belly of the duck, is 2020, when California is set to produce 33% of its power from renewable resources. Source: California Independent System Operator (CAISO)

What can be seen from the duck is that the solar power generated during the day will result in a need for nonrenewable resources to ramp down in the morning and ramp up in the evening. A glance at this curve can result in an impression that most of the other generation on the grid will need to ramp on and off in order to support the renewables; however, the y-axis is cut off at 11 GW. Figure 2 shows the same information with the graph expanded down to zero. The blue area below the duck, the nonrenewable generation, is the largest share of generation. If environmental protection is a goal of the renewable initiatives, then it is clear that making the right choice for this blue "duck pond" is even more important than the renewable duck itself.

2. The duck pond of nonrenewable generation in CAISO.The blue area represents nonrenewable generation. Source: CAISO/Siemens

The driving concern for filling the duck pond is the large ramp at the belly of the duck going down and at the neck of the duck going up. Figure 3 shows an example of total and nonrenewable load over the course of a day. The blue line depicts total demand and the lower red line is the net load, which in this case is defined as the nonrenewable load.

3. Daily demand.This chart of load and net load on Feb. 24, 2013, shows that even without renewables, power has to ramp up and down in order to meet demand. Source: CAISO

This figure shows that even without renewables, power had to ramp up and down in order to meet demand. The difference between the past and the future is not the existence of the ramp but simply the amount of energy that needs to ramp. Changes in weather, demand, and plant availability will cause this curve to shift every day to meet the power demand.

Economics Drive Dispatch

To gain a broader understanding of how future dispatchable resources will need to behave in order to accommodate increased renewable generation, data developed in two recent studies of future dispatch behavior were evaluated with a specific focus on what types of plants will be needed to accommodate increased renewables. One study was conducted by CAISO and the other was conducted by the Ventyx Corp.

The results of both studies indicate that the majority of demand fluctuations will be supported by combined cycles. Even in a future grid, with an increase in highly fluctuating renewables, simple cycles will still play a roll but will be used primarily for low-dispatch, peaking demands.

The driver is economics. Driven by the levelized cost of electricity, simple cycles are typically dispatched <10% and combined cycles are typically more advantageous over 20% dispatch. The grey area in between depends on the actual plant configuration and conditions.

Transferring these criteria to the duck pond, you can see in Figure 4 the area showing where simple cycles make the best economic choice. Combined cycles are a better choice for the rest of the pond if they can meet the fluctuating demand-and analysis and history shows that they can. (Ed.: For studies suggesting other potential solutions that may be part of the picture, see "California Plans for Even More Renewable Power in Its Future".)

4. Best fits. This modification of the duck pond chart shows where simple cycle plants (red) and combined cycle generation (blue) are best fits. In the area between, the best technology depends upon plant-specific circumstances. Source: Siemens

Flexible Combined Cycles for Load Following

Combined cycles, especially modern flexible combined cycles like Siemens Flex-Plant are capable of meeting large, fast load changes. Modern combined cycles are getting so fast that they can exceed simple cycles in flexibility and benefit to the grid.

Siemens Flex-Plants, for example, can start fast and ramp up quickly, offering full gas turbine power in less than 15 minutes and full combined cycle power in about 30 minutes. They also offer high-efficiency operation over a wide range of available output and can quickly load follow over this available range. The higher efficiency of combined cycles also means they are dispatched and operating more often, and are therefore able to provide frequency support for the grid.

A good example of a flexible combined cycle is the Siemens H-Class power plant that has been operating since 2012 in Irsching, Germany. Figure 5 shows an example of the plant operation as it starts quickly in the morning, follows demand during the day, shuts down in the evening, and repeats this pattern the next day.

5. Typical daily operation at SCC5-8000H in Irsching, Germany.This plant starts quickly in the morning, follows demand during the day, shuts down in the evening, and repeats the pattern the next day. Source: Siemens

With Siemens Flex-Plants there is no need to trade efficiency for flexibility. This plant exceeds 60% net combined cycle efficiency and can add 500 MW of generation to the grid in 30 minutes. Operating Flex-Plants in the U.S. include Lodi Energy Center in Lodi, Calif. (see "Are Flexible Generation Plants Performing as Expected?" in the March issue of POWER) and the Temple and Sherman plants in Texas. Unlike most simple cycles, combined cycles often have a very large load range, enabling them to ramp up and down without having to shut down and restart.

Conventional Combined Cycles

It isn't only the new flexible combined cycles that can meet changes in load demand. Even conventional combined cycle technology has been used to meet changing loads.

An example of dispatch on a grid with renewables is shown in Figure 6. This is a simulation of a winter day on the Huntington Beach grid in California. Many of the plants modeled are not advanced Flex-Plant combined cycles but are conventional cycles.

6. Winter renewables scenario.This chart simulates a winter day on the Huntington Beach grid. Source: Ventyx Inc.

Figure 7 focuses on the energy provided by combined cycles and shows that they are providing the majority of the ramping support. The power from combined cycles ramps up and down to cover two peaks during the day. The magnitude of the energy supplied makes it practical to use large combined cycle plants to support this need. The red line toward the top of Figure 6 represents the simple cycles. In this case they are dispatched; however, they are not used to cover the changes in demand. Their dispatch is rather flat, and the amount of energy dispatched is minimal. It is less costly and more environmentally friendly to use the combined cycles to cover large demand changes.

7. Combined cycle contribution. This chart is a simulation of the ramping support from combined cycle power plants on the Huntington Beach grid. Source: Ventyx Inc.

Figures 8 and 9 show a projected summer day in Huntington Beach. Again, the dispatch of the simple and combined cycles show that the larger share of demand change is supported by combined cycles. In this case, the overall demand is high, and the simple cycles are dispatched to meet the peak in demand. This dispatch order on a high-renewable grid is similar to the dispatch order on a conventional grid. Combined cycles dispatch first because they offer a lower cost of generation and are followed by simple cycles to meet peaks in demand. There is no indication of a need for more simple cycles to support load changes.

8. Summer scenario. This simulation projects generation for Aug 1, 2023, on the same grid shown in Figure 6.Source: Ventyx Inc.

9. Summer cycles.In summer as well, combined cycles can respond to the majority of the demand change in this Aug. 1, 2023, simulation.Source: Ventyx Inc.

Similar data was extracted for a node in Texas, which has the largest supply of wind power in the U.S. Figure 10 illustrates that the same phenomenon can be observed there as well. The vast majority of load changes are supported by combined cycles first. Simple cycles are used primarily for peak demand and are not critical for supporting the large ramps in load that were seen in the past, or the even larger ramps in load that are expected in the future. Combined cycles are able to change load quickly and ultimately dispatch first due to the lower cost of generation.

10. Texas two cycles. As in California's solar scenario, combined cycles are an important technology for responding to variable wind generation in Texas. Source: Ventyx Inc.

While conventional combined cycles offer advantages over simple cycles for renewable integration, modern Flex-Plant combined cycles offer significantly more capability. Instead of only leveraging the benefits in ramping capability, these newer, more flexible plants can start as fast as a simple cycle, making multiple restarts viable.

Combined Cycles Win, Win, and Win

It seems rare when a choice is better in functionality, cost, and environmental footprint, but for high-dispatch plants, combined cycles win in all three areas. Flexible combined cycle power plants support renewables by being more efficient, cleaner for the environment, and flexible to meet the change in demand.

Higher efficiency results in a lower cost of generation. Electricity from a combined cycle can be on the order of one-third the cost for electricity from a simple cycle. Higher efficiency, offered in combined cycles, also means less greenhouse gas generation. Simple cycle efficiencies are in the range of 35% to 40%, while combined cycle efficiencies are in the range of 55% to 61%. For every MW generated, a combined cycles burns about 35% less fuel than a simple cycle and, consequently, produces 35% less carbon dioxide.

Fewer starts means less CO generation. Gas turbines typically produce more CO in a start than they do in 10 hours of operation, so leaving the plant running at low load is actually better for the environment than turning it off for a few hours.

A running engine also supports grid frequency. Most renewable generation does not provide rotating inertia, which is needed to maintain the grid at 60 Hz; however, gas turbines, help enforce speed stability. Grids without enough units producing rotating inertia need to add equipment, like synchronous condensers, to help stabilize the grid frequency.

Starting and Stopping Considerations

With all of these advantages, one may wonder why simple cycles are getting any attention at all. The reason often brought up is the economics of starting.

Historically, heavy duty gas turbines were designed to run for very long periods of time without stopping and to require service after a certain number of starts. Aeroderivative gas turbines, based on aircraft engines, are designed to start and stop frequently and don't require service after a set number of starts. If an engine is started frequently, the service-related costs can change the economics of the plant, resulting in an advantage for an aeroderivative engine. This is why at first glance, plants expecting to start and stop multiple times may look at an aeroderivative simple cycle as a good solution. However, even in the case of plants expecting multiple starts on a given day, this costly simple cycle approach is not optimum.

The worst day doesn't happen every day. A plant may actually start three times in a given day, but dispatch modeling shows that even at very high cycling plants, this happens infrequently. When looking at service costs, the number of starts a year is the critical factor. Even high-starting plants don't start an average of more than once a day.

Different plants will dispatch differently. If a simple cycle peaker is compared with a combined cycle, the duty cycles will not be the same. The lower levelized cost of electricity typically drives simple cycles to dispatch <10% of the time. A combined cycle will have an overall higher dispatch, have much longer runs, and will therefore stop and start less often than the simple cycle. The same duty cycle shouldn't be used for both plants.

For those simple cycle plants that are needed, the plants that fill that 10% dispatch window, economic evaluation shows that typically, frame units in simple cycle are a better economic choice over an aeroderivative. The benefit of the higher efficiency of a simple cycle aeroderivative rarely outweighs the lower capital cost of a frame unit for low-dispatch applications. With today's proven dilution selective catalytic reduction options, frame units can now meet low NO_x and CO requirements and often use much less water than some aeroderivative options.

The Cure Needs to Be Better Than the Disease

The primary driver for supporting wind and solar power generation is protecting the environment. Solar and wind plants don't create the pollutants that fossil-fired generation creates. Yet, while rules and legislation are popping up around the globe supporting or demanding renewable generation, there is little or no discussion about the partnering technology that may be needed to support renewable resources to firm the power supply.

As this article shows, the "duck pond" of nonrenewable generation is much bigger than the "renewable duck" itself. Given the information and experience available today, it may be the right time to revisit the requirements for fossil generation. With the right approach, we can ensure that the clean renewable portfolio is supported by an environmentally conscious mix of generation that can ensure reliable power for the future. a-

-Bonnie Marini, PhD (bonnie.marini@siemens.com) is director, 60 Hz Solutions Product Line for Siemens Power and Gas.

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