Renewable electricity generation has many environmental advantages, but adding large amounts of far-flung renewable resources to a grid requires increased operating flexibility from dispatchable generators when the wind doesn’t blow or the sun doesn’t shine. One promising option: A combined-cycle plant based on Alstom’s GT24/GT26 combustion turbine can be “parked” at approximately 20% plant load while producing emissions comparable to those during baseload operation—with little loss in thermal efficiency. When demand returns, the combined cycle can return to baseload within minutes.
Renewable generation capacity, mainly solar and wind, has grown rapidly over the past couple of years in the U.S. and several other countries, outpacing all other forms of generation except natural gas. Utilities are developing these alternative generation resources for reasons that range from altruism (it’s good for their customers and the environment) to compliance (a renewable portfolio standard that must be met). Regardless of the reason, adding nondispatchable renewables to an electricity transmission and distribution grid designed for instantaneous demand response adds an additional layer of complexity for power generators and grid operators.
Large generators, such as coal and nuclear plants, were traditionally designed for baseload power supply. As renewable resources have entered service, coal-fired plants in some regions of the U.S. have transitioned into intermediate load plants, and many combined-cycle plants, also originally designed as baseload, are now daily peaking plants. In some locations, combined-cycle plants are “two-cycled” daily: The plant is brought online for the morning peak, shut down or reduced to minimum load, and then started again to meet an evening peak.
To be sure, the fate of gas-fired combined-cycle plants is intimately tied to the price of natural gas. As a result, many are operated only during peak hours of the summer months. The point is that the dispatch order of plants is in flux, as every utility or grid operator uses its unique economic rules and predictive methods to determine how much spinning reserve and dispatchable backup power is required to handle potential grid demand excursions.
The options for providing spinning reserve or backup power for renewable generation are few, and they are all expensive. Sometimes, purchased power can fill the deficit, assuming that sufficient reserve capacity is available. Some electricity suppliers have built small, reciprocating engine or simple-cycle gas turbine plants for the sole purpose of grid stabilization and renewable backup power. Many regions rely on gas-fired simple-cycle or combined-cycle plants as renewable generation backup and live with the practical limits of that decision: The turndown of industrial gas turbines is limited, part-load efficiency is poor, start-up from cold iron is slow, emissions at part-load operation usually exceed permit limits, and adding more start-up hours shortens equipment life.
Usually, the decision about what type of backup power to provide entails making do with current assets rather than installing new ones to optimize evolving spinning reserve and backup power demands on the grid.
An Elegant Solution
If you ask a grid operator or dispatcher responsible for meshing renewable and conventional generation to describe the characteristics of the ideal dispatchable plant, the likely response will be “a gas-fired plant that has quick start-up capability to respond to system emergencies, offers exceptional turndown with rapid load response, stays close to design point efficiency, and meets emissions limits at all loads.” Multiple small gas-fired reciprocating engine plants come close but are not economic when considering a power block of several hundred megawatts. Existing combustion turbines can turn down to about 50% load, but their thermal efficiency drops and emissions skyrocket. Unlikely as it seems, there is a combined-cycle design that has the ideal specifications.
Alstom has developed a Low Load Operation Capability (LLOC) for its KA24 and KA26 combined-cycle power plants, which are based on the advanced GT24/GT26 sequential combustion turbine (CT). The LLOC allows the plant to operate at loads less than 25% while maintaining operation of the steam side of the plant with respectable plant efficiency. The result is a plant design that not only avoids the need for plant shutdown in the evening and start-up in the morning but also remains ready to provide renewable generation spinning reserve. (See the online feature “What Utility Executives Think About the Smart Grid” for evidence that North American resource planners are recognizing the need for this sort of flexible generation.) As an added benefit, this plant’s ability to maintain the highest efficiencies at part-load conditions provides an operator with a competitive advantage in a merchant power market (see sidebar).
There are a number of operational and maintenance implications of Alstom’s LLOC that will be explored in the remainder of this article. The key design features that should be of great interest to both grid operators and plant owners are these:
- The plant can be operated at very low load during periods of low spark spreads (typical overnight operation), and the grid operator can make immediate use of its spinning reserve once spark spreads increase or if emergency power is required by the grid.
- Avoiding unneeded CT start-ups eliminates the CT lifetime penalty incurred for each CT stop/start.
- Plant emissions levels are similar to those at baseload and well within typical permit requirements. Cumulative emissions are reduced compared to parking a power block at a higher minimum load.
- Energy consumption saving during low spark spread periods compared with operating at higher minimum loads.
- Full online spinning reserve, thereby avoiding start failure risks and possible associated grid dispatch penalties.
Email
Comments
Print
Reuse
