Boilers, Gas, Gas Turbines, O&M, Plant Optimization

Dispatch Efficiency

Issue 6 and Volume 121.

A New Way to Look at the Economics of Modern Flexible Combined Cycles

by Bonnie Marini, Phd

Power plant technology has come a long way in the last decade. New demands in the market, combined with the increased capabilities of today’s fossil fired power plants make comparing and choosing the right technology much more complicated than in the past. There is a lot of discussion on how new technology differs from conventional technology – but the real challenge is determining how today’s technologies compare with each other.

As we look to a future with more weather dependent renewable generation, many of the new fossil fired power plants will be built primarily as firming power – dispatching when renewables are not available and turning down or off when more renewable generation is available. One significant change in market options is the speed and responsiveness of combined cycle power plants. There are several fast-start combined cycle options now available, offering different operational styles and benefits. One key differentiator is how these various plants start.

The first fast-start combined cycles in the U.S. were commissioned in California. These plants introduced the concept of enabling a gas turbine to ramp to base load without a low load hold point. The first advantage to be seen was the tremendous reduction in start-up emissions. NOx and CO production dropped about of 90 percent. Features and capabilities have since been added to this product line so the rest of the cycle can start fast as well. One of the additions is the Co-Start option. With Co-Start the steam turbine starts very soon after the gas turbines, ramping to base load quickly, and depending on plant configuration, enables full plant power in about 30 minutes .

Other providers have also started to offer flexible, fast-start combined cycles, but with different start up scenarios. In these plants the gas turbine is ramped to minimum emissions compliance load(MECL) before reaching the first hold point. The subsequent start procedure is much like a conventional combined cycle, combining a series of holds and ramps to gently warm up the steam cycle. This start offers the same advantage in NOx and CO emissions reduction as the Flex-Plant, but it does not produce as much power as a Flex-Plant in early operation.

The high dispatch efficiency of the Irsching Combined Cycle Power Plant in Germany leads to lower CO2 emissions and the flexibility required to firm renewables. Photo courtesy: Siemens Energy

If the goal is purely reduction in start-up NOx and CO, these options are comparable. However, start-up emissions is only a very small part of the overall advantage fast-start combined cycles can offer.

When plants start and stop often, the cost of the start-up transient can become a significant lever in a plant’s profitability. Automakers are ahead of the power industry when it comes to providing simple, understandable information which clearly explains the benefit of automobiles designed to start and stop efficiently versus those that aren’t. Suppliers publish city gas mileage and highway gas mileage. It doesn’t take a technology expert to understand that if you use your car for short trips, city mileage is more important.

When power industry owners evaluate base load efficiency or heat rate, it is like evaluating you car’s mileage expectation from the highway gas mileage number alone. In today’s markets, where stopping and starting regularly is expected, the power generation industry should also be looking at a version of automobile city mileage, and this can be analyzed by looking at a plant’s dispatch heat rate instead of the conventional base load heat rate.

Heat rate is the result of a calculation of the amount of fuel needed to create a kilowatt-hour of electricity, typically at baseload. Heat rate is a practical expression of plant efficiency, using the units common to fuel purchase for the numerator, and the units common for electricity purchase for the denominator. (Efficiency is the dimensionless inverse of heat rate.)

Dispatch heat rate looks at these same parameters over the full dispatch time. To calculate the dispatch efficiency, the total amount of fuel needed for a given dispatch window is divided by the amount of electricity generated. The key difference between dispatch heat rate and a conventional heat rate is the fuel consumed to start and stop, and the reduction in efficiency during the entire transient are included in the dispatch heat rate calculation. The longer the plant is at part load, and the further it is from the design load, the lower the cycle efficiency and the more fuel the plant will use over the dispatch window. For example, if the plant is going to dispatch for 8000 hours continuously, one annual start, ramp and stop sequence has a negligible impact. However, if the plant is going to dispatch for 8 hours at a time, the dispatch heat rate can substantially impact the profitability of the facility.

To demonstrate the difference between conventional heat rate and dispatch heat rate, the chart on this page compares the dispatch heat rate for an H-class combined cycle to a Flex-Plant with Co-Start. The fast-start combined cycle which ramps to MECL reaches emissions compliance fast, but does include load hold points to warm the cycle up slowly. The Flex-Plant with Co-Start ramps the gas turbine to base load at its full ramp rate and the steam turbine follows shortly after.

As shown in Figure 1 on page 26, the difference in dispatch heat rate and conventional heat rate is dramatic. For a 12-hour operation window the dispatch heat rate for the Co-Start plant is about 100 BTU/kWh better than the alternative. For an 8 hour dispatch the benefit goes up to 150 BTU/kWh. This results in a very significant impact on the actual cost of generation for plants that respond to variable demand.

Dispatch Heat Rate vs. Conventional Heat Rate

A financial analysis was done for an H class 2×1 combined cycle plant to quantify the benefit of Co-Start capability. This analysis showed a 51-percent reduction is hot start cost, a 49-percent reduction in warm start cost, and a 35-percent reduction in cold start cost. For a plant with a duty cycle of a daily starter with 200 hot starts, 50 warm starts, and 10 cold starts per year, the estimated impact on NPV is a savings of more than $28,500,000.

The reason some plants are not designed for Co-Start is that this requires changes in the design of the balance of plant equipment and thoughtful integration of the entire cycle. The elimination of the series of holds enabling early dispatch of the steam turbine requires downstream equipment to have a more flexible design – and that starts with the boiler.

One way to eliminate the cyclic limitations in a combined cycle is to use a Benson once through boiler. In a conventional boiler, there is a high pressure drum which is a thick walled structure that expands and contracts with changing temperature. This drum is attached to thin walled tubes which are expanding and contracting at a different rate than the drum. The result is limited cyclic life. If those subcomponents change temperature too quickly or too frequently, low cycle fatigue induced cracking will occur.

In a Benson boiler the high pressure drum is eliminated. The critical life-limiting design element does not exist and the cycle can be ramped quickly without exceeding the design life of the boiler.

Benson boilers offer significant benefits in flexibility and have been proven for more than 18 years. There are more than 50 Benson boilers successfully operating in combined cycles around the world, but the flexibility of the Benson is only part of its advantage. The second benefit is the capability to enable high part-load combined cycle performance.

When operating a combined cycle at part load, the exhaust temperature out of the gas turbine typically rises. In a conventional boiler this higher temperature can result in overheating of superheater tubes in the boiler. The ability to control flow in the superheater is limited in these boilers, so to avoid overtemperature conditions, the steam is attemperated by injecting cool water into the steam. This reduces the temperature of the steam, mitigating the risk of tube overtemperature. However an undesirable consequence of this attemperation that some higher grade energy is transferred into lower temperature parts of the bottoming cycle. This results in lower part load efficiency.

Lodi Energy Center in California is another example of fast start Benson technology. Photo courtesy: Siemens Energy
Lodi Energy Center in California is another example of fast start Benson technology. Photo courtesy: Siemens Energy

In a Benson boiler, the need for attemperation is eliminated. There is no high pressure drum, so temperature in the superheaters can be managed by adjusting the flow into the evaporator. If gas turbine exhaust temperature rises, superheater flow increases and an overtemperature condition is avoided.

With a Benson boiler, eliminating the need for attemperation results in higher part load efficiencies, further enhancing the dispatch heat rate for these configurations.

Co-Start capability reduces start up NOx and CO, and improves dispatch efficiency. Co-Start also provides the added benefit of reducing greenhouse gas production. Lower fuel usage means less CO2 production, so these plants are cleaner when they start, cleaner when they run, and more economical to operate.

Plants like the Ullrich Hartmann combined cycle in Germany demonstrate this. This power plant broke the world record for combined cycle base load performance and was the first combined cycle in the world to exceed 60 percent net combined cycle efficiency. This same power plant can put 500 MWs on the grid in under 30 minutes. The high efficiency and high dispatch efficiency result in a low CO2 technology with the flexibility to firm renewables. This enabling benefit for renewable generation is also a contributor to a greener grid with high reliability.

Start Up Costs

The combined cycle solution offers the benefit of being a good fit for fluctuating demand and a good fit for base load.

Lodi Energy Center in California is another successful example of fast start Benson technology. Operating since 2012 this plant can ramp the gas turbine to base load with no hold points, enabling a dispatch heat rate much lower than either simple cycles or combined cycles without this capability. This highly efficient, flexible plant is also used to firm California’s expanding renewables portfolio.

The exciting fact about the latest generation of combined cycles is that you don’t have to choose between high base load efficiency and high dispatch efficiency. Today’s plants can start fast, load change quickly, hold emissions compliance at low load, and operate at high efficiency through a broad power range.

While the examples discussed look at a large combined cycle, this fast-start technology is available in a broad range of combined cycle plants, with power ranges from under 150 MW to plants well over 1000 MWs. The capability is primarily enabled by equipment outside the gas turbine, with technology like the Benson boiler which is in use in large and small plants around the globe.

Boiler Comparison

While there is a high level of interest in flexible technologies, determining the economic value of flexibility has been a challenge in the past. Introducing the simple concept of dispatch efficiency, like city gas mileage, to the power industry may be a simple way to help financial analyst make better choices for the bottom line and the environment of the future.


Bonnie Marini is director of Product Line Management at Siemens Energy