Fickle winds, intermittent sunshine start to stress U.S. power system

The growth of U.S. wind power has begun to create operating challenges for nuclear and coal plants that must be ramped up and down as wind speeds vary, panelists at a Massachusetts Institute of Technology energy conference reported last week.

The MIT Energy Initiative symposium on integrating large-scale wind and solar power attracted executives of utility and transmission companies, senior government officials and academic researchers, whose comments were off the record. Some papers prepared for the conference were made public by their authors, and they define a growing challenge of matching the current U.S. mix of power plants with new requirements to respond quickly to changes in wind and solar resources.

"The power system needs more flexibility to handle the short-term effects of increasing levels of wind," said Ignacio Pérez-Arriaga, a professor at Spain's Comillas University and a visiting professor at MIT.

He and other speakers predicted the expansion of renewable power will continue as a clear option for reducing power plant carbon emissions. Nearly half of global electricity supply will have to come from renewable sources if world carbon dioxide emissions are to be cut to half of current levels by 2050, according to the International Energy Agency, he noted.

But utility regulation has not adapted to a future of high renewables, he warned. And a high penetration of wind and solar generation is likely to make wholesale electricity prices more volatile. These and other potentially disruptive issues "raise concerns about attracting sufficient investment in ... flexible plants" in competitive power markets, he said.

A paper by the Brattle Group says the expansion of renewable energy requires "more generation ... that can quickly ramp up and down, possibly with short start-up times and minimal cool-down times." Whether those needs for more cycling and peaking energy can be met by existing generators is not clear and must be given detailed study, the Brattle Group paper says.

Regulation, finance and operational changes needed

In the United States, the drop in demand for power that began with the recession in 2008 has left spare generation capacity that can be used to balance power supply to demand in this decade. But regulation, capital investment policies and operating practices all must change to maximize that potential, speakers said.

And right now, the difference in the peak demand for daytime power is growing in the United States, adding to the need for a more flexible system. Grid operators must plan for a future worst-case scenario of several consecutive days with very low wind and solar power coinciding with very high summer power demand, Pérez-Arriaga said. This is a key challenge in designing the long-term generation mix.


A major focus of the April 20 symposium was the impact of more frequent start-stop cycling of coal-fired generators, as they are called on to balance peaks and valleys in wind output.

Putting coal plants on a more rapid cycling schedule exposes valves, piping and other components to more extreme temperature shifts and potentially damaging changes in steam operation chemistry, said Steve Hesler, a program manager at the Electric Power Research Institute, in a conference paper. These can accelerate wear and tear and induce corrosion and stress, raising the risks of cracking and failure of metals and welds, he noted.

Hesler said that increased cycling of coal plants is already evident, resulting from the recession-caused drop in power demand, lower natural gas prices, and expansion of renewable generation. The bulk of the balancing services from U.S. coal plants is being met by smaller units built before 1970 that are typically run at relatively low capacities, rather than newer and larger coal plants whose owners run them as much possible to supply baseload power, Hesler wrote.

The smaller, older plants are most at risk from U.S. EPA regulation and competition from natural gas generation that currently benefits from low gas prices. As these older coal plants are retired, current flexibility of the generation fleet is likely to suffer, speakers said. "We're running out of flexible coal units," one participant said. "The newer plants aren't built well enough to do load following" in response to variations in renewable energy output.

"I can't imagine owners of coal plants ... making significant capital investments around ramping," said another participant.

Who pays for needed adjustments?

Both large coal units and nuclear plants are intended to be run full time, and the utility industry has spent decades training operators to do that. If they now are required to run the plants at varying outputs to respond to ups and downs in renewable energy, the risk of human error may rise, one conference participant said. "I'm sure they'll get there, but the human factor is not be underestimated."

Another participant said that manufacturers have designs for faster-responding gas-fired generators that would be better suited to handle the temperature and pressure stresses of ramping operations. But the industry has not seen utilities "rushing to the door" to purchase more adaptable but more expensive generators. "It's an economic decision."

The regulation of the U.S. electric power industry is still aimed at securing power at the lowest cost. But the changes in store for the power sector won't come for free, one speaker said. "There is no way we can accomplish this at a lower cost. So the question is, who pays?"

Officials of the American Wind Energy Association sparred with a representative of the Bentek Energy consulting firm, who presented a new analysis, "The Wind Energy Paradox." It asserts that increased wind energy output forces coal generation into inefficient start-stop operations that increase emissions of nitrogen- and sulfur-oxide pollutants.

To the extent that wind power will be backed up by gas-fired generators rather than coal, the gains in carbon emission reductions from wind are diminished, the Bentek report says.

The paper says that the increase in pollutant emissions caused by frequent cycling of coal- and gas-fired generation undermines the wind industry's claims about the emission reduction benefits from renewables. "It's not a very cost effective way" of saving carbon, SOx and NOx, the sulfur and nitrogen oxide pollutants, the report says.

AWEA responded via email, "There are more than two dozen different peer-reviewed wind integration studies from the U.S. and Europe, mostly by utilities. They show that the U.S. can accommodate a lot more renewable generation than we have today, at relatively modest integration costs and with significant emissions reductions."

"Similarly, their [Bentek's] model only looks at hourly snapshots and would therefore exclude the vast majority of the emissions savings caused when wind energy causes emitting sources to turn off for an extended period of time," AWEA said.

"The Bentek report overstates coal cycling costs and impacts by extrapolating from an extreme case of ramping a generator down from 100 to 40 percent of capacity in an hour which almost never happens. While it is fair to incorporate coal ramping costs and impacts, the study greatly overstates those impacts and does not reflect the way generators are committed and dispatched by grid operators," AWEA said.

'Fractured decisions' expected from states and regions

The conference concluded with the question of whether the patchwork of federal and state regulation and the stalemate over national climate and transmission policies in Congress would help or hinder a transition to more renewable power.

"Considerable progress" is being made by the Federal Energy Regulatory Commission, state regulators, regional transmission organizations and utilities in planning to accommodate more renewable power, tuning market incentives to create a more flexible system, and fairly allocating costs for this transition, the Brattle Group report says.

The reality is that states and regions will have the most to say about this process. "We do not see any grand, unifying theory of cost allocation for the costs of renewable variability, nor do the institutional differences, legacy generation, or indigenous resources across regions of the U.S. ... lend themselves to uniform solutions," the report says.

"While the road ahead may be contentious and laborious, there seems to be no technical or economic reason why a well-functioning regulatory system cannot find its way to a sustainable, reliable and economical destination."

Other conference participants were far less optimistic that a divided Congress and White House can rationalize climate and energy policies.

A larger, more sophisticated transmission network, including long-haul high-voltage direct-current lines, would expand the footprint for solar and wind generation, smoothing the daily and hourly variations in renewable energy output, speakers noted.

One industry executive said that many papers submitted to the conference assume that a stronger inter-regional transmission network would ease the integration of renewable power into the grid. With some exceptions, notably in Texas, that goal faces huge political and industry opposition, the speaker said.

"We have no right to that assumption in the U.S., and we shouldn't make it. We should assume instead we will be making fractured decisions."

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