Solar and wind emission-effect on fossil fuel plants ‘negligible’

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High penetrations of wind and solar on the grid in the western United States has a negligible impact on the emissions associated with cycling thermal power plants, new research from the National Renewable Energy Research Laboratory published yesterday reveals.

To accommodate higher amounts of wind and solar power on the electric grid, utilities must ramp down and ramp up or stop and start conventional generators more frequently to provide reliable power for their customers – a practice called cycling.

Last year, research paper from another US government laboratory, the Argonne National Laboratory indicated that the carbon reduction benefits might not be as high as expected because of this cycling which makes gas and coal fired power stations less efficient.

But the new NREL study finds that the carbon emissions induced by more frequent cycling are negligible (<0.2%) compared with the carbon reductions achieved through the wind and solar power generation evaluated in the study.

Debra Lew, NREL project manager for the study, spent two years investigating the impact of cycling and the impact of wear and tear on the additional demand on thermal plants that have to ramp up and down in response to renewable generation.

“The fact that wind and solar do decrease overall emissions across the system across the west and that these impacts of cycling on emissions are relatively small, that I think we expected to see,” she told PV Tech.

Phase 2 of the Western Wind and Solar Integration Study follows on from the phase 1 study in 2010 into the viability, benefits and challenges of integrating high penetrations of wind and solar power into the western grid. The first report found that integration of 35% wind and solar energy into the electric power system will not require extensive infrastructure if changes are made to operational practices.

Wind and solar can increase annual operating costs for fossil-fuelled generators by $35 million to $157 million, while reducing fuel costs by $7 billion, the study found.

The study also revealed some unexpected differences in the impacts on coal and gas generation and in high wind or high solar penetration scenarios.

Wear and tear costs of cycling in high wind scenarios were more favourable than for high solar penetrations.

“High wind scenarios tend to turn off coal plants over time on a weekly cycle when there is a lot of wind,” said Lew. “Whereas solar tends to keep the thermal plants on but turned down to minimum generation levels on an almost daily basis. In order to meet the evening peak, the high solar case tends to turn on the thermal plants in the evening to meet that peak. If there's a lot of wind in the evening, that helps to deal with the evening peak.”

But Lew added that the predictability of sunrise, sunset and the direction the sun moves throughout the day should reduce the variability issues associated with solar.

“This was the first time we'd really been able to dive deep into high penetrations of solar,” she said. “We find with solar that the sunrise and sunset events dominate our variability issues. The good thing is that you know when the sun is going to rise or set. We find that when you take out that known solar variability, cloud variability is smoothed out by the fact that we're aggregating solar over many plants.

“Solar dominates our variability issues and wind dominates our uncertainty issues but both we find are manageable we just have to make sure we do these things to manage them. The most important thing is flexibility in the system.”

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