Comparing Aeroderivatives and Reciprocating Engines for Fluctuating Power Demand

By Reed Lengel, Christian Mieckowski and Bonnie Marini

For decades the technology chosen for a particular size power block was based on the largest size gas turbine that would fit. The installed cost of a power plant with one gas turbine costs less than a similar sized plant with two gas turbines, and larger engines are typically more efficient than smaller engines. Evaluations based on the cost of generation for base load operation supported this decision, but today this is changing. Owners are choosing to build large power blocks out of multiple small turbines and in some cases even smaller reciprocating engines. The growing portfolio of renewable generation and the need for flexible partnering generation is at the root of this change and has resulted in many discussions about how to appropriately evaluate the technology for these new needs.

When the implementation of renewable generation was in its infancy there was no data indicating how plants would be dispatched or behave with renewables.

Today many regions have renewables on the grid and many of the plants selected to support renewable integration are operating. One way to understand the results of these technology choices is to compare data from some of these operating flexible plants.

This approach removes the uncertainties of theoretical operating scenarios or ideal conditions, and provides a quantifiable comparison amongst real plants delivering power.

For this study two different technologies have been identified to compare for renewable integration:

Aeroderivative simple cycles — Based on their aerospace counterparts, these Brayton Cycle units are relatively compact and light weight, and designed for rapid continuous cycling and ramping.

Like an aircraft, they can be called upon to start and stop frequently throughout the day with relatively low O&M costs compared to other generation technologies.

Additionally, due to their high compression ratio, they are typically the most efficient simple cycle power plants in operation and are available in single-unit sizes from 4 MW to 70 MW or more.

Reciprocating internal combustion engines (RICE) — Similar to the engine under the hood of a car, these Otto Cycle machines are relatively large and heavy. These characteristics limit the maximum size (and output) of each machine to less than 20 MW, but typically 5 MW to 18 MW for power generation applications.

They are designed to ramp quickly, but must be kept warm with their auxiliary systems while idle to maintain a fast-start capability.

Since the machines have lower output, numerous individual engines are required to deliver energy equivalent to a single aeroderivative gas turbine. However, multiple units offer the flexibility to maintain a high part-load efficiency by dispatching the required units to base load and maintaining the rest in a ready to start condition.

To understand how plants with these technologies are really operating, the total operating & maintenance costs for several sites throughout the United States was collected. Available operational data for 2015 and 2016 was analyzed. Figure 1 shows the annual capacity factor for each of the units. This data offers a broad range of dispatch from under 5 percent to over 20 percent, and shows no correlation between dispatch and technology type when comparing reciprocating engines to aeroderivatives.

Figure 2 shows the total specific operating and maintenance costs for these plants in $/MWh.

To enable comparison, fuel costs for all facilities were normalized to $3.50/mmBTU. The eight years of reciprocating engine plant data ranked one through eight for highest O&M cost against the field of aeroderivative gas turbine plants.

To assess whether this cost is impacted by dispatch, these two figures are combined in Figure 3 where each year of operation was plotted by dispatch, with color indicating technology type. This data shows no correlation between capacity factor and O&M cost, and reveals a higher average, maximum and minimum O&M cost for the RICE plants.

Operational Efficiency

A key benefit of the RICE plant is that each unit can be operated independently. For a plant that consistently runs at very low load demands the RICE plant, in theory, will have a lower heat rate than the part loaded aeroderivative gas turbine. This is primarily due to the ability to shutdown small segments of output and maintain the rest at base load.

As shown in Figure 4, heat rates of the plants identified above were compared, and the actual plant efficiencies were seen to be similar between the aeroderivative plants and the RICE plants. The plant heat rate includes the auxiliary loads of the idle units.

Therefore in practice, the energy required to keep the RICE units ready to respond to higher demand by energizing idle/shutdown units may be counteracting the benefit of the lower engine fuel use (higher efficiency).

Contributors to O&M differences

Oil Usage

Regardless of the technology, an operating engine requires a lube oil system. For gas turbine plants this is a closed system that involves minimal loss of oil and has extremely low maintenance requirements.

Reciprocating engines have an entirely different need when it comes to lube oil consumption, due to the fact that the oil comes in contact with combustion parts. This contact will cause lube oil vaporization that will both increase PM emissions, and require more oil to be added. Oil consumption can be in the range of 0.8 to 1.2 g/kWhr, for an annual cost in the range of $400,000 to $600,000 per engine.

The consumed lube oil cost is one contributor to the higher O&M costs for these engines.

Standby Operation

Another O&M cost difference between technologies is the investment needed to maintain an engine in ready to ramp condition.

Aeroderivatives need minimal power to start up quickly. The SGT-A65 TR aeroderivative for example does not require a turning gear or barring sytem. Due to its three rotor design, a small starting motor (215kW at peak) geared to the HP rotor is all that is needed to bring the unit to ignition speed.

For reciprocating engines, extra power is needed to be ready to start fast. Like a steam turbine in a combined cycle plant, the cylinders of a reciprocating engine must be hot before starting up. The energy needed to maintain ready to start conditions can be over five times the standby power needed for an industrial gas turbine of the same size. This consumption, when not producing power, negatively impacts the overall economics of the plant.

For low dispatch plants, the impact can be significant.

Other Technology Differences

Emissions

Renewable generation is being installed to reduce the environmental footprint of power generation. When building plants to back up generation, environmental impact should also be a consideration.

The actual annual averaged plant heat rates are similar between aeroderivatives and reciprocating engines, and therefore so is the amount of CO2 generated per MW. An additional contributor to greenhouse gas generation is unburnt hydrocarbons (UHC).

The nature of the combustion process in a reciprocating engine results in a much higher level of UHC in the form of methane slip, which has a global warming potential of up to 36 times higher than CO2. Aeroderivatives produce about 98 percent less UHC, 70 percent less particulates and 90 percent less formaldehyde.

RICE engines also produce more NOx and CO than aeroderivative options. Using the Siemens SGT-A65 TR as an example, this aeroderivative produces <25 ppmV of NOx and <100 ppmV of CO for loads between 50 percent and 100 percent, allowing turn down to meet low load needs when they arise while remaining in emissions compliance.

Emissions for the low NOx versions of the Wartsila 20V34SG is reported to produce 45 ppmV NOx and 226 ppmV of CO. For both units, these affluents can be reduced by using an SCR, but the reciprocating engine will require roughly twice as much ammonia.

Plant Footprint

Building a large plant from multiple small units has pros and cons. To produce the same amount of power, an aeroderivative solution uses less than 1/3 the space of the reciprocating engine option.

As discussed above, in order to facilitate quick start, the reciprocating engine must be kept warm. To achieve this, RICE units are typically housed in a building adding additional capital costs and ancilliary loads.

Conclusion

Both reciprocating engines and aeroderivative simple cycles offer the ability to start fast and cycle frequently to support peaks in demand and the inherent intermittancy of renewable generation.

A study of operating plants in the US indicates that both of the solutions discussed are being dispatched to meet these needs.

While it was expected that the use of multiple small units would offer higher operational efficiency to overcome the higher capital costs, the opposite was seen to be the case.Looking at the installed and operating fleet, aeroderivatives show a lower cost of generation as well as a reduced environmental footprint.

This result may be driven by the high ancillary load of the RICE units to remain in a ready to start condition, as well as high use/loss of lubricating oil during operation. Aeroderivatives also show benefit in environmental signature with lower NOX, CO, and greenhouse gas generation. Further discussion is warranted on the comparison between these technologies and today’s fast start combined cycles.