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Expansion of the Genesee Power Plant in Alberta, Canada is currently taking place using the MHPS M501J gas turbine in a single-shaft combined cycle configuration. This is said to result in a reduction in building size and cost savings

Five years ago, the pattern was clear. U.S. combined cycle power plant designs strongly favored a multi-shaft arrangement, whereby the steam turbine (ST) and gas turbine (GT) each drive their own generator. Europe and other 50 Hz. countries favored the single-shaft concept: The GT and ST operate on a single shaft, share the same generator and typically have a clutch included.

Today in the U.S., however, many new plants are opting for single shaft. These facilities are largely located close to shale gas plays. In Pennsylvania, for example, several new plants are taking advantage of abundant Marcellus shale gas resources, mature gas pipeline and transmission infrastructure, and proximity to major load centers.

The Marcellus shale region encompasses over 100,000 square miles. It is the largest source of natural gas in the U.S., capable of producing more than 16 billion cubic feet per day (Bcf/d). The U.S. Energy Information Administration (EIA) projects that natural gas production in the Marcellus Shale will reach close to 150 Tcf through 2040. Such a motherlode of gas is understandably attracting the interest of utilities and independent power producers (IPPs). They are arranging long-term gas supply contracts at favorable terms to gain a sound economic footing for new facilities. They believe that a single-shaft design gives them the best chance of profitability.

Single-shaft studies

The first single-shaft combined cycle plant with a clutch was installed in Saarbrucken, Germany in 1972. It used two Frame 5Ns. The GTs and STs were each connected through a gearbox to a clutch and then to the generator. From those humble beginnings, the single-shaft concept gained steady ground. There are now over 400 single-shaft units around the world, including 16 single-shaft plants in the U.S. harnessing Alstom GT24 GTs. The U.S., however, has traditionally held less than 10% of the total. But its number has risen fast in the last few years.

Part of the reason for this shift may lie in empirical results backed up by research. A study by Mahmoud Edris of the German engineering and consulting firm Lahmeyer International in 2009 (Comparison Between Single-Shaft and Multi-Shaft Gas Fired 800 MWel Combined Cycle Power Plant), modeled various configurations using Siemens SGT5-4000F turbines. and multi-shaft options in terms of performance, environmental impact, space limitations and other factors. As the Edris’ analysis showed, a single-shaft arrangement was more efficient for that particular GT model if only electricity was needed, but not if steam was being sold in addition.

How does it work? The basic function of the clutch in the case of single-shaft combined cycle power plants (CCPP) is to automatically engage and disengage the steam turbine from the GT/generator. The clutch has an input shaft with helical splines. When the input shaft and its sliding component (ST side) reach the same speed as the output (GT/generator side), the main torque transmitting gear teeth move smoothly into mesh, thanks to an internal pawl and ratchet mechanism. The entire action is cushioned by a dashpot and dampener. Conversely, when the ST is shut down, the torque reaction within the main gear-toothed mesh is reversed, initiating automatic disengagement. The clutch also makes use of centrifugal force and oil distribution to ensure that no metal-metal contact occurs during normal operation.

OEM support

Turbomachinery OEMs have taken different approaches to the single-shaft versus multi-shaft debate. Mitsubishi Hitachi Power Systems (MHPS) has been building single-shaft combined cycle plants for many years, particularly in Japan as well as Mexico, Taiwan, Ireland, Thailand and South Korea. But it is now bringing them to North America.

Capital Power, for example, is adding one at its Genesee Power Plant in Alberta, Canada. Each of the two power trains will be configured in an electric generator located on a common shaft between the M501J GT and the SRT-50 ST. MHPS will also supply the Heat Recovery Steam Generator (HRSG) for each train. This facility will add more than 500 MW and is likely to come online in 2018/2019. Capital Power said single shaft offered a significant reduction of the Genessee plant size, resulting in overall cost savings.

GE, on the other hand, was almost exclusively a multi-shaft provider in the U.S. until recently. Despite the fact that the company’s first single-shaft CCPP went on line in 1968 and that it has both single-shaft and multi-shaft designs in its portfolio, it had reserved the former largely for 50 Hz. territories. GE’s first FlexEfficiency 50 plant for EDF in France with a promised 61% net baseload efficiency is a good example. The 510 MW plant that recently came online uses a GE 9F 7-series GT, a D-14 ST and GE W28 generator in a single-shaft configuration. The company is now erecting single-shaft plants around the U.S.


Twinning up gas plants

The Lackawanna Energy Center is an example. This 1,500 MW natural gas-fired CCPP is being developed by Invenergy. It is currently being constructed in the Borough of Jessup, Lackawanna County, Pennsylvania and is due to open in 2018. GE is supplying three GE 7HA.02 air-cooled turbines, and related generators, CHP modules and ORC systems. Industrial Heat Transfer is providing the heat exchangers and Babcock Power is supplying three HRSGs with integrated duct burners, as well as auxiliary equipment, and an ST. The engineering, procurement and construction (EPC) contractor for the project is Kiewit Power Constructors. CMI Energy will supply the boiler for the power plant. The facility will be selling power into PJM Interconnection. Situated near Scranton, Pennsylvania in the Northeastern part of the state, the bulk of the natural gas supplies will be purchased from Marcellus producer Cabot Oil and Gas. A seven-mile gas pipeline expansion project is due to be completed later this year. The availability of large quantities of nearby gas eliminates fears of high fuel prices. Cabot Oil & Gas has secured a 10-year sales agreement to become the sole supplier to the Lackawanna Energy Center. With the EIA predicting that 45 GW of coal plants will go offline between 2014 and the end of 2021, and with many of these being in the heavily populated northeast, these new plants appear to be on a sound economic footing.

The 1,013 MW Moxie Freedom run by IPP Caithness Energy is another plant deploying GE turbines and using use natural gas exclusively from the Marcellus Shale. It is being built by Gemma Power Systems. The natural gas to power the plant will come from Cabot Oil & Gas wells in Marcellus with the pipeline and associated transmission lines close by. The power also goes to the PJM Interconnection grid. The plant’s two power trains will be equipped with a GE 7HA.02 turbine, a generator, a Babcock Power HRSG, duct burners, a steam turbine, and a control system. Howden will supply air- and gas-handling equipment, and Haldor Topsoe is supplying the SCR gear. Its single-shaft combined cycle configuration gives Moxie the flexibility to run one unit while the other is offline, or based on fluctuating demand. But this is just the start. Many more of these plants are under serious discussion. For example, GE is expected to supply the turbomachinery for what is likely to be a single-shaft facility in Renovo, PA. Bechtel plans to begin construction in 2018.

Siemens has also been establishing single-shaft plants in the region to harness Marcellus shale gas. The Liberty Power Project in Bradford County and the Patriot plant (both 829 MW) in Clinton Township, Pennsylvania, use Siemens SGT6-8000H gas turbines in a single-shaft configuration.

Each plant consists of an SGT6- 8000H, a Siemens SST6-5000 steam turbine, a Siemens hydrogen-cooled SGen6-2000H generator, an HRSG and a Siemens SPPA-T3000 control system. Siemens touts its single-shaft design as offering economic advantages as a result of low investment costs, smaller footprint, shorter startup times, higher efficiency and a greater flexibility during operation. These are only two of a total of seven single-shaft facilities Siemens is building in the U.S. along with Panda Power Funds (three in Pennsylvania, three in Texas and one in Virginia). When finished, these will amount to more than 5,800 MW.

Which configuration is best? Certainly, there has been a major change of heart in the U.S. as regards the deployment of natural gas-fueled CCPPs. The former dominance of multi-shaft facilities in North America seems to have swung back in favor of the single-shaft concept. That does not necessarily mean, however, that it is the ideal configuration for all cases. There are many factors to consider including initial equipment costs, efficiency, plant use, power demand, available space, transmission capacity, steam demand and facility expansion plans, to name a few.

A complex issue

The decision on which configuration to employ requires careful consideration. Here are a few points to ponder:

Initial costs — Single-shaft plants tend to be a little cheaper to build as they have fewer generators. A standardized design offers obvious design optimization advantages.

Efficiency — This is a complex issue. Single-shaft plants are more efficient when comparing the 1x1 configurations of each. But what about comparing multiple 1x1 single-shafts with 2x1 multi-shaft blocks? In this case, you must consider how much time the plant will operate at full load compared to partial load. Single-shaft configurations are more efficient when demand is 50% or lower, since one unit can be completely shut down. But if steam is in demand for cogeneration or district heating, multi shaft makes it easier to correctly size the ST and generator to match fluctuating steam availability. A curveball is then thrown when plant designers consider long-term cost of ownership, as flexibility trounces base load operation.

Space — 1X 1 single-shaft configurations are more compact than multi-shaft. But a 2x1 multi-shaft block may have a smaller footprint than two 1x1 single-shaft trains.

Transmission capacity — The plant should be sized so it does not exceed transmission capacity. This may determine whether single shaft or multi shaft is more efficient when dealing with transmission constraints.

Expansion plans — Single-shaft units can be added in smaller increments than multi-shaft blocks. This makes it simpler and easier to scale a plant gradually to meet rising demand, or incrementally redevelop an existing power plant site. Once redeveloped, the assets could be financially packaged and sold to investors. The emphasis placed on these factors will vary from site to site. Thorough analysis that takes into account the many facets and nuances of configuration selection is the only way to make an informed decision.

Morgan Hendry is President of SSS Clutch Company. SSS Clutch has installed clutches in more than 400 single-shaft gas turbine plants around the world. For more information, visit