|The lightweight SGT-750’s first application in North America was to supply combined heat and power to textile manufacturer Grupo Kaltex in Mexico Credit: Siemens|
With their origins in the earliest days of gas turbine use for power generation, aeroderivative turbines are now popping up in all sorts of new and unexpected roles. In response, manufacturers are focusing on new technology and new markets. It means that these relatively small and light machines are set to wield significantly more weight in the market, as Paul Breeze reports.
Aeroderivative gas turbines, the race-horses of the gas turbine stable in comparison with the workhorses of industrial gas turbines, have found niche applications within power generation ever since the engines were first adapted for stationary use. Today, however, the range of applications is expanding.
Certainly, aeroderivative gas turbines already occupy an important place in the history of power generation. They were the first gas turbines to be used for the production of electricity when aircraft engines were initially adapted for stationary power use. The primary role for gas turbines during this early phase was for peaking power. But, as the advantages of these machines began to be recognized, a new type of dedicated industrial or heavy duty gas turbine was developed, aimed directly at the main baseload generation market. With the launch of this cheaper gas turbine, two markets developed and diverged. At the heart of one was a heavy duty machine, often found in a large combined cycle power plant. The other market was based around the light, responsive and very efficient aeroderivative gas turbine, usually but not always in a simple cycle configuration. Today, while the role of the industrial gas turbine remains broadly the same as in the past, that of the aeroderivative is changing.
|Vineland Municipal Electric Utility operates the only municipally-owned generating facility in the US state of New Jersey, and is powered by a Trent 60 aeroderivative gas turbine unit Credit: Rolls-Royce|
The market for aeroderivative gas turbines within the power sector falls into a number of key categories. Traditionally, the power application has been for peak power generation, taking advantage of the fast start-up and good load-following abilities of aeroderivative units. These same characteristics make the turbines valuable for helping to maintain grid stability when a system has to accommodate large volumes of variable output wind or solar power capacity. Then there is the cogeneration and district heating market, with applications that exploit both the power and the heat output from a gas turbine. This category is expanding to encompass combined cycle applications, an area that would previously have been considered the sole preserve of industrial machines, while trigeneration is a potential future market.
More recently a third market category has grown in importance too, mobile power, where ease of use, small size, fuel flexibility and weight are all important. A packaged aeroderivative unit designed for this market might be used to supply emergency power in the US tornado belt during the storm season or it could be installed to provide a more reliable energy source for an isolated or island system in Africa or Asia.
According to Dan Kempf, the general manager for LM2500 and small gas turbines at GE Power & Water, the breakdown of the market will depend on unit size too. For units like GE’s LM 2500 (with a power output in the 20 MW – 40 MW range) around 65% of the market is in the oil and gas industry, where aeroderivative turbines are popular for gas compression and mechanical power duties such as pumping, while the power sector will take the remaining 35%. However, when you move up to the 50 MW and 100 MW power ranges the ‘powergen space’ becomes the main market. At the top end of the range, these units are starting to compete directly with industrial machines, Kempf observes. This represents a significant change in the use and appeal of aeroderivative gas turbine technology.
Advancing turbine technology
Aeroderivative gas turbines have always formed the vanguard of gas turbine development and that remains an important part of their role within the industry, as Harald Thaler, a director at Frost and Sullivan, notes. Advanced material or design developments are likely to be tested first in military engines before transferring to commercial aircraft engines if appropriate. Then after some years they may trickle down to the industrial ranges if they offer an economic advantage. History has taught heavy duty turbine manufacturers that to introduce revolutionary changes at the industrial turbine level can have dire economic consequences in the event of problems. Consequently, the pace of change at this level tends to be very conservative and therefore any technological advances will typically be found in aeroderivative turbines first.
The primary characteristics of an aeroderivative gas turbine are low mass and high efficiency. Weight is an obvious consideration. These engines are designed to be airborne and excess mass presents a penalty in terms of aircraft performance, efficiency and load carrying capacity. While for many stationary applications low mass is not particularly advantageous, there are key areas such as mobile power where it is valuable. In order to keep weight down, aeroderivative engines tend to use more advanced materials than industrial engines. These advanced materials will often allow them to operate at higher temperatures too and this means that they can potentially be more thermodynamically efficient than a similar industrial engine.
Add to this the fact that an aeroderivative gas turbine will have a compression ratio of up to 40:1, significantly higher than a standard industrial turbine, and it becomes clear why aeroderivative gas turbines are the most efficient gas turbines available. Typical performance is in the 41% to 42% range in simple cycle while the best units can achieve 46% and more when utilising additional techniques, such as intercooling. Intercooling, reheating and recuperation are adaptations of the simple gas turbine cycle that can be used on aeroderivative turbines to improve efficiency in stationary applications.
However, the technically advanced nature of the aeroderivative gas turbine comes with a penalty, cost. They are generally more expensive than the industrial equivalent. Against this they offer the advantages already noted of low weight, high efficiency, fast start-up, good part-load efficiency and the ability to load-follow with ease. Where these characteristics are required, the aeroderivative often makes an ideal choice but there are many applications where not all these attributes are desirable or required. Recognizing this, some manufacturers are now building hybrid machines that are part aeroderivative, part industrial gas turbine, trying to select the best properties of both types of machine.
A new breed of aeroderivative turbine
Typical of this new breed of gas turbine is the Siemens SGT-750, described by the company as a “low-weight industrial gas turbine designed and developed to incorporate size and weight advantages of the aeroderivative gas turbine whilst maintaining the robustness, flexibility and longevity of the traditional heavy-duty industrial gas turbine.” The first application of this machine in North America was to supply combined heat and power to textile manufacturer Grupo Kaltex in Mexico. The unit will supply process steam and around 33% of its electrical output to the plant at which it is installed. The remainder of the power output will be wheeled across the Mexican grid to other Grupo Kaltex production plants.
|The Trent 60 can provide up to 66 MW in simple cycle, with 42.5% efficiency Credit: Rolls-Royce|
Another new entry is the FT4000 Swiftpac power generation package from PW Power Systems. This is a modular system offering 60 MW in a single turbine configuration and 120 MW in a dual turbine configuration driving a single generator. Charles Levey, vice president, sales and marketing, at PW Power Systems says the package has a simple cycle efficiency of 41% to 42% and he expects the 120 MW version to start competing with industrial machines. “I think this will be a significant game changer,” he told Cogeneration and Onsite Power Production. US utility Exelon Generation purchased the first 120 MW version in mid-2013.
Rolls-Royce’s entry at this power level remains a pure aeroderivative turbine, the Trent 60 which provides up to 66 MW in simple cycle with 42.5% efficiency. GE, meanwhile has its LMS100 gas turbine, a 100 MW machine that was built by combining components from GE’s heavy duty gas turbine range with others from the company’s aeroderivative range to produce a hybrid. In this case the aim was to achieve extremely high simple cycle efficiency. GE claims that the machine can reach 46% efficiency, with start-up in 10 minutes and a part load efficiency of 40% at half the rated output. This high efficiency has made the machine appealing for peaking and load-following, GE says, particularly in California where 23 have already been installed. Elsewhere, two of the most recent units to enter service are installed in Russia at Inter RAO’s Dzubginskaya thermal power plant where they will provide up to 180 MW of peak power, 90 MW per unit, during the Sochi Winter Olympic Games.
The use of these high efficiency aeroderivative gas turbines in simple cycle for peak power and load following has a long history. What has been much less common is their use in combined cycle configurations. However, the growing volume of power from intermittent renewable sources such as wind turbines and solar photovoltaic generating units in some regions of the world is changing the role of gas turbines on the grid from one of base or intermediate-load service to that of grid support. This requires power plants that can start up and shut down quickly, that have high efficiency when operating at less than full output and that can ramp output rapidly, up or down, to meet changing grid conditions. Manufacturers are clearly positioning their large aeroderivative or hybrid aeroderivative-industrial gas turbines to compete in this market which today is still primarily the preserve of industrial machines. Much smaller machines are also now being deployed in a combined cycle configuration too.
New markets emerge
Another market sector of growing interest to aeroderivative gas turbine manufacturers is the mobile power market. This includes a number of different types of application. At GE, for example, the mobile market is about customers that need power very quickly or need a modular power plant that can be moved from place to place as needs change. A typical application of this type arose after the devastating earthquake and tsunami in Japan in 2011, when GE provided units to help re-establish the power grid.
The company has a truck-mounted version of the LM2500, the TM2500, that is designed specifically for this type of application. It is supplied on two trailers and has an efficiency of 37% at 60 Hz or 35% at 50 Hz. Opportunities in this market sector arise very quickly, Kempf said and are often near term, perhaps a customer looking for a block of 200 MW of power in six months. In order to achieve the necessary speed and simplicity, gas turbines for this type of application tend to be used in simple cycle.
Meanwhile, PW Power Systems has seen a growing market for distributed generation based on aeroderivative gas turbines in developing countries. “Many [of these countries] see distributed generation as an answer to a significant part of electrification,” said Levey. Provided there is an adequate fuel source, then installing a distributed generation unit can often provide a better solution than trying to extend the existing transmission and distribution system.
In developing countries where the grid may serve only the largest urban centres there can be a large number of smaller population centres, typically with some manufacturing, that have a primitive and inefficient island supply, based probably on old diesel units. The network will often be strung from factory to factory, Levey said, but a mobile aeroderivative power unit can help provide a more robust and stable distributed supply. An aeroderivative unit can offer good load following capability, a feature that can let down the older diesel units and ultimately be responsible for poor island system stability, he explains.
The aeroderivative engine is more environmentally friendly than a diesel engine, too. This can give them an advantage when the funds for the new power supply are being provided by international donor organisations, which increasingly demand high environmental standards.
|The LMS100 gas turbine is a 100 MW hybrid machine that combines components from heavy-duty and aeroderivative gas turbines Credit: GE|
For this type of market the mobile unit offers many advantages. The rapid delivery and simple installation, with all the main components pre-checked prior to delivery, reduces the need for skilled installers, it removes the need for local procurement and it removes the problem of commissioning errors. The bottom line is that it facilitates speed to market. Moreover because the units are mobile, they can be disconnected and moved elsewhere, six months or six years down the line, perhaps when national grid infrastructure has finally reached the island community.
Fuel flexibility is a further feature of aeroderivative gas turbines that is important for this market, as for others. Most aeroderivative gas turbines can operate on natural gas, biogas, and a variety of liquid fuels. In China, for example, GE has supplied aeroderivative gas turbines that burn the waste gases from a steel plant to provide electric power.
There are other markets where aeroderivative gas turbines are showing strongly. At Frost and Sullivan, Thaler and analyst Pritil Gunjan identified cogeneration as an area in which aeroderivative machines offer an efficiency advantage that places them in a strong position. In Asia, for example, where the cost of natural gas is high compared to the USA or even Europe, efficient use of fuel is particularly important. There are strong markets in China and India, the two countries that dominate the region economically, but also in Thailand and neighbouring Myanmar, which as it opens up to the world offers a promising future market.
Thaler also highlighted another advantage of the cross-fertilisation between industrial and aeroderivative gas turbines, service costs. This has always been an issue with aeroderivative units, he noted, because service and maintenance must normally be carried out by the turbine manufacturer. With the partial merging of product lines, more third-party service companies are likely to be available and this could bring running costs down further.
Meanwhile, technical advance continues but it is relatively slow today. Gas turbines are technically advanced machines and pushing them to every greater performance has become increasingly expensive. One of the main avenues of development is aimed at achieving higher turbine inlet temperature in order to raise efficiency. With temperatures already up to 1700ºC this requires exotic materials and extremely advanced blade design. Inlet temperatures will rise, but advances are likely to be incremental. There is one development, however, that promises to extend the market for aeroderivative gas turbines and that is trigeneration.
Trigeneration, or combined heating, cooling and power, is an extension of the more common cogeneration configuration to produce chilled water for refrigeration or air conditioning in addition to electrical power and heat. The three processes can be linked because an absorption chiller requires a source of heat, such as that found in a gas turbine exhaust, in order to drive the cooling cycle. A trigeneration plant can therefore offer a wider range of services than a simple cogeneration plant.
By integrating power production, heat capture and refrigeration, a trigeneration unit can provide year-round high efficiency operation, adapting its output to meet differing seasonal requirements. Aeroderivative gas turbines, with of their high simple cycle efficiency, can provide a good prime mover in such a system.
According to Frost and Sullivan, trigeneration is not seen widely as a commercial aeroderivative application yet and when units do become available they will face competition from gas engines. Some companies already marketing reciprocating gas engines for trigeneration. Nevertheless, this provides one more option from an increasingly large repertoire that aeroderivative gas turbines can perform and fits neatly into the global drive for greater energy efficiency.