Power Engineering

Flexible Gas Turbines Help to Integerate Renewables

By Bruce Rising, Strategic Business Management, and John M. Wilson, Vice President Product Sales, Siemens Energy

Renewable energy has rapidly increased in recent years, helping to provide cleaner power generation. But renewables only provide capacity when the wind is blowing or the sun is shining, and this capacity can fluctuate often and quickly. This dynamic has introduced the necessity to provide a complimentary generation that can mirror the fluctuations from renewables like wind and solar.

Siemens has pioneered one of the first gas turbines to offer shaping power. Shaping power is power that can more easily follow energy demands, ramping quickly, cleanly and efficiently, with no impact on the service interval of the engine.

The U.S. power generation, transmission and distribution system comprises one of the largest electrical distribution networks on earth, supporting the largest economy in the world. Colloquially described as "the grid," it is geographically disbursed over much of North America, encompassing all of the United States, Canada and parts of Northern Mexico. In 2007, the installed generating capacity of the U.S. system was approximately 1,000 GWe of electricity (net summer capacity), provided by a mix of fossil coal (sometimes referred to as "steam coal" or "thermal coal"), gas turbines (operating as either combustion turbines or combined cycles), nuclear (essentially a thermal system with nuclear fuel to produce steam instead of fossil fuel), and finally hydroelectric power (and pumped storage). The remaining generation sources (wind, photovoltaic, concentrated solar power) combine to provide the balance of the installed capacity. Reflecting the origins and locations of the first large load centers, the system is sub-divided into a series of loosely interwoven grids, some tightly interconnected at specific junctions, and others with only limited connections to other distribution networks (ERCOT/TRE, is one example of a fairly isolated grid). This subdivision also aids in supporting system reliability, through features such as voltage support and localized customer service.

Renewable Power Generation

The U.S. has a long history of utilizing renewable assets for energy conversion. Nearly 7 percent of the energy produced in 2009 came from hydroelectric power. With the recent push to promote the renewable power generation industry, more than half the states have established definitions of what is, and what is not, a renewable technology. When tabulating renewable generation, the large quantity of installed hydropower is usually excluded, except in a few cases where legislation has specified inclusion (for example, Wisconsin).

The next most prolific renewable energy technology is based on wind energy generation. With breakthroughs in turbine design, it has become possible to fabricate and deploy wind turbines averaging 2 to 3 MWe in scale (in contrast to the 300 to 500 kWe size limitations of the late 1980s and early 1990s).1 Technical innovations and financial incentives allowed the U.S. to grow wind capacity to over 40,000 MWe at market competitive prices, with thousands expected to be added annually. But unlike fossil generation or nuclear, wind (and comparable renewable energy sources) cannot always be dispatched, and in some cases; the renewable resource may become a "stranded" asset.

New Power Generation

The presence of a strong nuclear component in the U.S. has supplied a measure of energy security with the benefit of almost zero CO2 emissions. However, nuclear power is not easily adapted to complement the variability of renewable generation. And there may be significant risks, not always evident, perhaps most recently demonstrated in Japan.

Larger than nuclear power, coal fired generation makes up the bulk of the U.S. capacity, but with its own litany of complex regulatory obstacles facing the both the industry and the fuel source. Substituting gas-fired generation for coal or oil almost immediately solves many of the environmental problems related to emissions and quickly addresses regional air quality issues. Using a gas turbine as the prime mover produces electricity at a very high efficiency. In the U.S., gas turbine (or combustion turbine) power generation comprises about 35 percent of the installed base, providing about 24 percent of the total energy.2 Because of the flexibility inherent in the gas turbine design, it can be adapted to fill the role to meet peak demand, as well as base load and intermediate load. This flexibility is evident in the ability to cycle rapidly, allowing the output to more closely follow the demand requirements as well as to follow the less predictable output that might be associated with a large renewable supply base. While there are both winter peak and summer peak, typically the most stringent requirement in the U.S. is the summer peak, when load is greatest and the performance of the dispatchable generation is significantly degraded.

Design Features

A gas turbine is an air-cycle machine with air (and a small percentage of fuel and/or water) serving as the primary working fluid. Power (MW) is a function of the air flow, the pressure ratio and the turbine inlet temperature. The gas turbine is the central component of combined cycle plants, producing roughly 65 percent of the total power, and supplying the energy for the bottoming (steam) cycle. The thermal efficiency of the gas turbine is set primarily by the maximum temperature at the expansion turbine inlet and the overall compressor pressure ratio. An innovative design example is the SGT6-5000F (Siemens Gas Turbine), which includes an oversized compressor and an extensive inlet guide vane control system to modulate compressor flow to the turbine. This allows the turbine to operate across a broad band, where the operator can choose to run in the high output mode, the high efficiency mode or anywhere in between.

Figure 1 shows the operational band for the F(5) combined cycle, revealing the flexibility of the generating plant across the ambient temperature range. This operating flexibility can be employed in conjunction with duct firing to extend the range even further.

With relative ease, the shaping power capability of the SGT6-5000F can convert from a maximum output, which might be required to meet peak load demands during hot days, to more efficient operation, which would favor the economics by maximizing the number of operating hours. This is achieved nearly instantaneously with no performance degradation or impact on parts life.

Combustion System Design

Virtually all recent gas turbine combustion systems manufactured and sold for the U.S. market are based on a pre-mixed combustor design. This design approach reduces the emissions of NOx from the gas turbine, but does so without using a diluent to reduce flame temperature. Referred to as Dry Low NOx (DLN), or Ultra-low No x (ULN) combustion systems, they have typically been more expensive than non-DLN combustor designs, although the O&M costs associated with water injection are almost always higher.

The combustion system design, in particular the DLN and ULN, presents a unique set of conditions. Optimally, it is designed to minimize emissions of NOx, carbon monoxide and voltaile organic compounds, although nearly all of that effort has been concentrated on emissions control with natural gas. The DLN or ULN performance capability is affected by changes in fuel quality and composition more so than the conventional diffusion combustor designs it replaced.

A gas turbine (DLN or ULN) designed to operate on natural gas will likely require a completely different combustion system compared to that operating on a low Btu fuel gas or liquid fuel. However, recent technical developments may allow the use of a broader range of fuels, including liquid fuels, in a DLN or ULN system that is designed for gas-only operation.3,4

Resource Supply: Natural Gas

The widespread availability of new natural gas supplies, as well as the influences of tight regulatory standards, are expected to reform the power generation landscape. Natural gas-fueled power generation, the bulk of that being either a gas turbine or a combined cycle, is in a position to play an even larger role. Complicating this picture is the need for expanded flexibility for any new capacity additions. Nearly all gas turbines operating in the U.S. are natural gas-fueled only, as multi-fuel-capable gas turbines are rare in the U.S. Technology dependence on a single fuel —natural gas —will clearly benefit if the supply base is extremely robust. And all indications are that this is the case.

In terms of gas production, shale gas in 2009 made up 14 percent of total U.S. natural gas supply. Shale gas production gas is expected to continue to increase and constitute perhaps 45 percent of U.S. total natural gas supply in 2035, as projected by the Energy Information Administration.5 Supplies also appear to be available at competitive prices. The long-term price forecast for natural gas is less than $6/MMBtu, with current prices less than $4/MMBtu in some regions.

Environmental and Regulatory Drivers

The U.S. has among the most stringent environmental requirements in the world. Environmental considerations are a dominant factor in product design, development and selection of any power generation system. The lack of emissions from a renewable energy technology such as a wind turbine is a key factor in their selection and siting.

From the OEM's perspective, the most immediate regulatory impact is with engine-specific emissions; the project owner or site developer must typically address more complex issues associated with how those emissions affect existing air quality where the project is built. The OEM will typically design the product to meet a unique set of environmental requirements. The most all-encompassing at the first stage is the New Source Performance Standard (NSPS), the most recent update of the NSPS set a benchmark of 15 ppm NOx (15 percent O2 corrected) from the gas turbine6. After selection by the developer, the next phase is to determine its environmental impact at the location where the project is to be constructed.

But not all pollutants are the same. Unlike the trace pollutants (NOx, SO2, CO, and so on), there is no viable, large-scale control technology to deploy for controlling CO2 emissions, with the possible exception of switching to a lower carbon content fuel. The relative small number of facilities in the world that do capture CO2 extract only a small portion of the total exhaust content and usually sell the extracted gas as a product to another industry. This is quite different from having a marketable emission control product for carbon capture.

The potential influx of non-dispatchable resources into the power grid is expected to have a substantial impact on the design and operation of the electrical power generation and distribution system. Not only must the system architecture be considered (geography, transmission losses, voltage support), but also the components that are the key building blocks (gas turbines, nuclear, smart grid and so on). From the power generation perspective, equipment manufacturers have responded by developing packages with a combination of rapid cycling and fast starting.

The current installed base of assets, in particular the thermal fleet, is likely to be at risk with regard to the required dispatch features needed to integrate a growing footprint of renewables. There have been few in-depth analyses of the potential outcomes, but anecdotal evidence exists that the older thermal units are experiencing component failures related to deep and rapid cycling.

From this, we conclude that the gas turbine with its rapid start capability, using natural gas fuel (which is forecasted to remain as an abundant supply) will play an increasingly expanded role both in increased capacity (MW) and energy supply (MWh) to the U.S. economy. The current lull in the economy, coupled with high reserve margins regional, is expected to reverse, producing a robust need for these assets to support the growth of the renewable generation.

References

1 G. M. Joselin Herbert, S. Iniyan, E. Sreevalsan, and S. Rajapandia,A review of Wind Energy Technologies, Renewable & Sustainable Energy Reviews, 11(2007) 1117-1146.

2 US EIA Electric Power Monthly, March 2011.

3 Gokulakrishnan, P., Ramotowski, M. J., Gaines, G., Fuller, C., Joklik, R., Eskin, L. D., Klassen, M. S., and Roby, R. J., A Novel Low NOx Lean, Premixed, and Prevaporized Combustion System for Liquid Fuels, J. Eng for Gas Turbines and Power 130, pp. 0510501-1 - 0510501-7, 2008.

4 Patents: US7089745, US7322198; US 7770396.

5 EIA Annual Energy Outlook 2011.

6 Title 40 CFR Table, Code of Federal Regulations , Part 60 - STANDARDS OF PERFORMANCE FOR NEW STATIONARY SOURCES, Subpart KKKK - Standards of Performance for Stationary Combustion Turbines- Nitrogen Oxide Emission Limits, from the U.S. Government Printing Office.

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