Forcing coal and gas-fired power plants to ramp up and down more frequently to compensate for the growing number of wind farms connected to the power grid will reduce their efficiency and increase maintenance costs.
The adoption of renewable energy laws and renewable portfolio standards (RPS), requiring power purchasers to give priority to clean generation from intermittent sources such as wind, will require fossil fuel generators to operate more flexibly, according to a recent study funded by the U.S. Department of Energy.
Plants originally designed to provide baseload will in future have to operate on a load-following or cycling basis, ramping up and down at short notice. The actual costs of using baseload plants to follow load are poorly understood, but likely to be substantial.
“Utilities know that thermal cycling does damage plant components, but the total cost impact is rarely fully understood in terms of increased forced outages and increased operation and maintenance costs,” according to a report on the “Impact of load following on power plant cost and performance” published by the National Energy Technology Laboratory (NETL) in October 2012.
Every shutdown and cold start cycle at one small (340 megawatt) coal-fired power unit in Texas cost an extra $157,000 – including $120,000 for additional maintenance and almost $16,000 in wasted fuel – according to one engineering study conducted by consultants Intertek.
The problem is that fossil fuel plants, especially older varieties, were not designed to be repeatedly powered up and down.
Major components at Britain’s oldest coal-fired power plants, for example, were designed to last for 5,000 hot starts, 1,000 warm starts and 200 cold starts (“Renewable electricity and the grid: the challenge of variability” 2007).
As different parts of major components heat up and cool down faster than others, differential rates of expansion cause high stresses. The resulting metal fatigue and creep causes cracking, significantly increasingly the maintenance budget and raising the risk of failure and unplanned outages.
Pipework, boilers and blades on steam turbines in coal-fired power stations are all expensive components at significant risk from repeated restarts and shutdowns, and a source of extra cost.
Simple gas-fired turbines handle ramping up and ramping down better. But many modern gas-fired plants employ combined cycle systems (one gas turbine followed by a steam turbine in sequence) because it is more efficient. Unfortunately, the steam turbine component has all the same cycling problems as a coal-fired plant.
Some CCGT plants can de-couple the two turbines, which gives more flexibility but at the cost of reduced efficiency.
The secondary steam turbine can be decoupled from the gas turbine when the plant is operating in load-following mode, in which case the efficiency benefits of running a combined cycle plant are lost. Or the two turbines can continue to operate together, in which case the slow ramping up or down of the steam turbine limits the flexibility of the overall system.
Efforts to make fossil-fuel generation cleaner by building coal plants employing integrated gasification and combined cycle (IGCC) with carbon capture and storage (CCS) systems, or CCGT plants with CCS, would likely make matters even worse.
These plants have so many expensive and energy intensive processes that they would not be able to adjust power output up and down quickly without wasting enormous amounts of energy in the process, slashing their efficiency and raising emissions per unit of power massively.
It takes 150-250 minutes for a natural gas plant with combined cycle to start up from cold, 12-15 hours for a conventional coal-fired power plant, but an estimated 24 hours to start up an IGCC plant, according to NETL.
Most power plants have some flexibility to vary their power production in the short term by operating at less than their full output. But even here flexibility is limited by operating constraints, which may be getting worse for the newest and most energy efficient plants.
Conventional coal-fired plants can turn down their output by a maximum of about 50 percent. Any lower and their efficiency drops to the point where they violate air quality controls. Combined cycle gas plants can achieve turn-down rates of about 40 percent. IGCC plants can only achieve turn down of 20 percent.
But as power generation plants get more efficient, they also become more complex and costly to construct, and increasingly uneconomic to run at anything other than full output. CCGT and IGCC (if it were ever widely built) need to operate as base load.
Shutting down and restarting CCGT and IGCC systems also wastes enormous amounts of fuel, as systems have to be heated to operating temperatures. In the event of an unexpected ramp down, any unusable gases have to be flared off or run inefficiently through a single-cycle turbine.
NEW GRID TOPOLOGY
Relatively inefficient single cycle gas turbines are likely to be the only practical option for load-following on the grid. Demand for them seems set to rise as the grid becomes less flexible in other ways, with increased numbers of wind turbines, as well as CCGT units and perhaps IGCC and supercritical coal-fired plants.
In the meantime, the large fleet of coal-fired power stations that will continue to provide significant power in the United States (and other countries) through 2030 will operate in an increasingly inefficient and costly fashion, with increased carbon emissions per unit of power generated and significantly higher maintenance charges.
“Boiler and turbine manufacturers have recognised that the power plant of the future will be required to follow load and cycle much more than in the past,” according to NETL.
“Manufacturers are designing systems and components to better survive the cycling environment and developing controls and operating procedures to accommodate rapid load changes,” the study added.
But the cost of equipment and maintenance seems set to rise significantly as more durable equipment is rolled out and old plants are retrofitted.
While the case for more wind generation will remain compelling for policy makers, the extra emissions from less efficient operation of back-up fossil fuel plants must be set against the savings from wind power. (Editing by Anthony Barker)
John Kemp is a Reuters market analyst. The views expressed are his own
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