Making the case for aeroderivatives11 March 2020
A recent white paper* from GE sets out the advantages of aeroderivative gas turbines relative to reciprocating engines**.
A key benefit of aeroderivatives is high efficiency in combined cycle configurations. The additional cost of a combined cycle plant, yielding higher efficiencies, pays off. When a GE LM2500 generator set is used in combined cycle mode, the annual fuel savings can be up to $16 million, thanks to the much lower combined cycle heat rate when compared to reciprocating engines. For power plants that operate most of the year in regions where fuel is above $6/MMBTU, the extra cost of the combined cycle is quickly paid for.
Another characteristic of aeroderivatives is that they do not have a minimum turndown and can operate indefinitely with no load.
Furthermore, an inherent feature of aeroderivatives is high efficiency at part load (with aircraft normally using their engines at partial load during cruising). The GE LMS100, for instance, can still provide 40% simple cycle efficiency at 50% load.
As well as fuel consumption, another factor that becomes important for reciprocating engines is lubricating oil consumption, which must be considered a significant consumable cost for the piston engine. Reciprocating engines may use up to 400 mL/MWh of lubricating oil, while an LM2500 will only consume about 2 mL/ MWh, some 200 times less. This can amount to savings of over 1 million US$ per year for a 100 MW facility.
Also to be borne in mind is that aeroderivative gas turbines have the highest availability of any thermal power technology. When a major inspection is required an aeroderivative gas turbine can be replaced within a few days, resulting in a unit availability of over 98% (source: ORAP data). This derives from maintenance practices in the aviation industry, where an aircraft never stops for engine maintenance; the engine is simply swapped out overnight, leaving the aircraft to resume profitable operation.
Due to lower availability, recips have an N+1 requirement if high performance is to be maintained, increasing CAPEX.
Annual maintenance events are about 50 times more frequent for with high-speed reciprocating engines than with aeroderivative turbines (see Figure 1). After just 500 hours, each unit in a multi reciprocating engine installation could require up to 36 OEM specified maintenance interventions; by comparison, aeroderivatives have their first, and only, maintenance event of the year at 4000 hours.
In a large multi-unit reciprocating engine plant, this typically means much higher person-hours for operations and maintenance versus an aeroderivative installation. An aeroderivative facility might only require 1800 person-hours of maintenance during a 3-year cycle, compared with over 15000 for a reciprocating engine based plant of equivalent size.
A typical 115 MW plant using three LM2500s in combined cycle may have greater capital costs than a similar sized reciprocating engine installation – but can save over $10 million dollars per year when compared to an HFO fuelled recip based facility when all expenses, including lube oil and maintenance, are taken into account. See table above.
Aeroderivative gas turbines are potentially more reliable and may have a higher availability than reciprocating engines (98.2% vs. 93% availability, according to ORAP), meaning they could be a more sensible investment for grid firming.
In fact, over half of GE’s global aeroderivative fleet (3000+ units) have demonstrated a reliability rate of almost 100% and over 98% availability rate, under certain specific conditions and with appropriate fuel.
GE aeroderivative gas turbines have at least two separate shafts, one shaft providing the necessary air flows and the other driving the generator. What this means, in practice, is that in the event of a grid frequency drop, even while the generator is being slowed down, the other shaft can quickly increase its speed, and consequently the unit’s power. This results in a much faster response to frequency fluctuations, helping to ensure a much more stable and reliable grid.
Another important aspect of grid stability is inertia. Not only are aeroderivatives larger machines in themselves, providing high system inertia, but they also all operate at 3600 rpm (for 60 Hz), further increasing system inertia when compared to medium speed reciprocating engines, typically running at 900 rpm – with four times the speed resulting in 16 times the inertia, for the same size generator. The extra inertia provided by aeroderivatives ensures less of the grid frequency fluctuations that can lead to blackouts.
Typical medium speed reciprocating gas engines have a ramp rate of about 5 MW/min, limited by engine dynamics and the nature of the combustion system. GE’s aeroderivative gas turbines, on the other hand, all have a nominal ramp rate of 50 MW/min, providing a much faster frequency control response in small grids. Real ramp rates, however, can be much higher than those for smaller load steps.
An aeroderivative main gas metering valve, for instance, has a rated opening time, from idle to full load of only 200 milliseconds, providing an instant response when needed. Another important feature when considering ramp rates is that aeroderivatives have no under-frequency trips, assuring they will remain online, even during the most intense frequency fluctuations.
Fuel flexibility is another major advantage of aeroderivatives. They can operate on gas or liquid fuels without significant power derate or pilot fuel requirements. Gas turbines can operate on natural gas, LNG, LPG, or diesel, among many other fuels, switching between these fuels without stopping and without a power reduction. They use the same combustion system for a wide fuel spectrum, starting with lean gas, high hydrogen content, coke oven gas, butane, LPG, light liquid distillates up to diesel fuel, or aviation kerosene. This flexibility gives operators the option to switch between fuels, depending on the economics, and to improve supply security by sourcing fuel from a diversity of sources.
While high-speed diesel reciprocating engines could potentially be run on fuels such as ethane, butane and propane, the result is often a significant performance drop and output derating.
Aeroderivatives can operate using such fuels without suffering any performance and output derating. They can easily switch from running on 100% diesel to 100% natural gas, whereas a high- speed diesel reciprocating machine can burn no more than around 70% natural gas in a mix with 30% diesel.
The table above compares fuel flexibility.
Aeroderivatives use best-in-class combustion systems and can achieve 15-25 ppm NOx emissions (depending on the specific model, fuel, and configuration) without the need for SCR (selective catalytic reduction) or the use of ammonia.
Reciprocating engines produce almost ten times more NOx and 10–17 times more CO (see Figure 2), as well as six times more particulate matter, and six times more VOC – even when operating with the same fuels and under the same conditions.
Furthermore, when operating on certain HFOs, reciprocating engines also emit a high volume of sulphur gases, while aeroderivatives operating on natural gas or LPG produce no sulphur emissions.
Another benefit of aeroderivative gas turbines is their ability to work hand-in-hand with renewables, facilitating their integration by providing a more stable grid.
Due to the nature of renewables — especially solar PV — frequency inverters must be used to convert DC into AC. These frequency inverters demand a stable grid frequency, otherwise, they cannot stay connected. There are many examples around the world where poor frequency control has inhibited the introduction of renewables. Solar panels are installed but cannot deliver power to the grid effectively as they continuously disconnect.
Aeroderivatives provide better frequency control than recips, and thus a more stable and reliable grid.
Furthermore, aeroderivative generator sets have much higher power density—with about 22 times more power output per unit— than high-speed diesel reciprocating engines, with a footprint that is significantly smaller than a typical reciprocating engine plant.
Not only are aeroderivative units smaller, but they are also much lighter per MW than their high- speed diesel reciprocating engine counterparts.
The lower weight means the machines can be more easily transported, and this is a great complement to fuel flexibility. In Ecuador, for instance, six GE TM2500 mobile generator sets were installed to run on diesel for immediate relief of a drought-related electricity crisis. A few years later, they were easily relocated 200 miles away to burn natural gas. Picking up and moving a high-speed diesel reciprocating engine farm in the same way—or building a whole new power plant—would have been considerably more cumbersome, time-consuming, and expensive.
In another application, two TM2500 units, rated at 34 MW each, were transported, installed, and started producing power in Puerto Rico, following Hurricane Maria, in only four weeks.
The TM2500 is a version of the LM2500 gas turbine, built on a movable trailer, the “TM” standing for trailer mounted.
Compared to the aeroderivative LM2500, reciprocating engine transport costs are at least three times higher due to the significantly increased weight, a factor of about 42x greater for a 100 MW plant. Air freight is not possible for major reciprocating engine components, while some aeroderivative gas turbines can be delivered rapidly by air anywhere in the world.
Reciprocating engines require pre-warming, so start time estimates need to specify whether they assume a “hot” or a “cold” engine. For GE’s aeroderivative gas turbines, the start-up time from cold to maximum power delivered to the grid is 5 minutes.
A further consideration is that during downtime, aeroderivative gas turbines consume no auxiliary loads, while reciprocating engines require a considerable parasitic load to maintain start readiness, keeping the engine and lube oil warm.
Aeroderivatives also have no minimum operating run time or stop time, so they can always be immediately re-started after a shutdown if required. Furthermore, they incur no maintenance cost penalties for daily starts, which is advantageous for peaking applications or for balancing renewables.
Voronezh warms to aeroderivatives
A recent significant project for GE’s aeroderivative technology is Quadra’s Voronezh 223 MW combined cycle CHP plant in Russia, which employs four GE SPRINT LM6000 gas turbines. GE Gas Power announced that the power plant achieved commercial operation on 1 February 2020.
Quadra – Power Generation is one of Russia’s largest regional (aka ‘territorial’) energy companies, providing power and heat, and the Voronezh plant is the key piece of energy infrastructure for the city, which has a population of over 1 million. The first unit at the Voronezh site was commissioned in 1933.
Start up of the new power plant will allow “gradual decommissioning” of obsolete equipment present at the site.
Evgeny Zhadovets, chief engineer of Quadra – Power Generation, said: “This power plant provides heat and power to more than half of Voronezh city’s residential buildings, as well as industrial and community facilities. It is extremely important that the new combined cycle facility will allow us to greatly increase the reliability of the city’s heat supply as well as increasing electric power output by 50% and improving energy efficiency.”
LM6000 aeroderivative technology provides great flexibility through its fast-start capability (as little as five minutes from
cold iron), daily/multiple startups with no impact on equipment maintenance cost, and low turndown capability, GE notes. Turbine life is extended, the company says, by an advanced cooling system that lowers the high-pressure compressor inlet temperature, and in turn effectively lowers the compressor discharge temperature. The SPRINT module is also equipped with a DLE combustion system providing fuel flexibility and helping to meet emissions limits. In addition, as part of the project, GE supplied a modular, multi-stage static air filter unit equipped with an anti-freeze protection system, which will help to achieve reliable operation of the gas turbines in winter conditions.
“GE’s aeroderivative gas turbine technology has already demonstrated its reliability and efficiency — up to 42% in simple cycle and up to 56% in combined cycle — at energy facilities in Russia,” said Michael Rechsteiner, CEO of GE Gas Power in Europe.
GE and Quadra have already successfully co-operated on gas turbine service projects. In 2017, the companies signed an agreement to service and upgrade four SGT-800 gas turbine units manufactured by Siemens and installed at Quadra’s Dyagilev and Alexin power plants. GE describes this as its first long-term “cross-fleet” contract in the Russia/CIS region, ie, involving the servicing of “other-OEM” gas turbines.
Above Image: The new Voronezh combined cycle CHP plant, employing four GE SPRINT LM6000 gas turbines
* Aeroderivative gas turbines, John Ingham and Monamee Adhikari, GEA34130
** These include high speed diesel engines (1000 rpm or more), typically up to about 4 MW per unit, medium speed diesels (400–1000 rpm), up to about 17 MW, and low speed diesels (60 to 275 rpm), up to about 80 MW. Depending on fuel type, reciprocating engines may be spark ignited lean burn, rich burn, or compression ignited (typically for liquid fuels)