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Clearing Up / Bearing Down

[April 7, 2017 / No. 1794]

A Tale of Deep Decarbonization and Resource Diversity

Summary: A 2016 Washington State University study found methane emissions from reservoirs are 25percent higher than previously thought, and more than 1 percent of human global greenhouse gas emissions. But the Northwest Power and Conservation Council has concluded that regional reservoirs are likely negligible sources of methane. This, and another study on deep decarbonization, also point to the value of resource diversity.

Hydropower is considered a non-carbon resource. One could quibble over ancillary and life-cycle CO² emissions from hydro, but fossil-fuel combustion is absent.

Then came a well-publicized study published last fall, led by two Washington State University researchers, which concluded reservoirs are "an underappreciated source of greenhouse gases, producing ... 1.3 percent of all greenhouse gases produced by humans," according to a WSU news release. The big culprit is methane -- 34 times a more-potent GHG than CO² over a century -- emanating from decomposing organic material.

Now comes the Northwest Power and Conservation Council with an analysis released in March that indicates the Federal Columbia River Power System -- encompassing 31 dams and associated reservoirs on the Columbia and Snake rivers -- is a negligible source of methane emissions.

The report published in BioScience drew on many other studies, and found that on a per-acre basis, reservoirs emit 25 percent more methane than previously thought. Reservoirs generally flood large areas with lots of organic matter that generate CO², methane and nitrous oxide in decomposition; these human-created water bodies also take in organic matter from upstream rivers.

"The strongest predictor of methane emissions from reservoirs was how biologically productive a reservoir was, with highly productive systems emitting the most methane," co-author and WSU associate professor John Harrison told Clearing Up in an email.

The study estimated reservoir-based methane emissions are similar to those from rice paddies and biomass burning, both of which are part of emissions estimates by the Intergovernmental Panel on Climate Change, WSU said.

The WSU report generated a flurry of caveats and rebuttals from the hydro world.

The WSU report generated a flurry of caveats and rebuttals from the hydro world. Regionally, Snohomish County PUD and the U.S. Army Corps of Engineers Walla Walla District offered distinctions for the FCRPS. SnoPUD said the federal system "does not possess the characteristics or conditions for methane production. The reservoirs do not have large amounts of plant or biomass present or flowing into them which creates methane, the waters are not stratified or eutrophic and circulate regularly; and they contain high levels of dissolved oxygen."

Meanwhile, the National Hydropower Association cited a 2016 DOE report finding "large uncertainties" in reservoir GHG-emissions studies. NHA said the science on this is "far from settled" and that some research has indicated reservoirs can be carbon sinks. It also emphasized multiple uses of reservoirs; only 3 percent of U.S. dams have hydropower facilities. And, NHA touted hydro's role as a solution to reduce GHG emissions in particular and pollutants in general.

The Council paid attention to this issue. "When we saw the initial press stories about the [WSU-led] research, that reservoirs behind power dams were generating methane, it was a concern," Council Member and Power Committee Chair Tom Karier told me. "One of the advantages of our hydro system, we always felt, was no carbon emissions. So we wanted our staff to look into this, to see how much of an issue it was for Northwest reservoirs."

Not much, is the latest thinking.

In a March 14 presentation to the Power Committee, Council staff reported an "initial conclusion" that comprehensive data for the region is unavailable, but the "(b)est available information" suggests the FCRPS "likely does not have characteristics (e.g. high nutrient enrichment) most strongly associated with high emissions."

On a GHG emissions per-kilowatt-hour basis, Council staff offered a rough estimate that an "example regional hydropower reservoir" produces 0.5 to 3 percent relative to a natural gas-fired plant, and 1 to 6 percent relative to a coal-fired plant.

"There could be additional analysis that would refine this, but the expectation is it would probably reduce what looks like a small number to something even smaller," said Karier, adding later, "At this point it looks like it's negligible."

My takeaway: Let the science of reservoir methane emissions continue. But for now, at least for the Columbia/Snake hydro system, it doesn't warrant policy or operational response.

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The main sources of energy-related Northwest CO² emissions -- according to Tomas Morrissey of the Pacific Northwest Utilities Conference Committee, speaking at our low-carbon power conference last October -- are transportation (48 percent) and electric power (34 percent). The latter figure is virtually certain to decline with the planned closure of 2,000-plus MW of coal-fired capacity serving the region.

Clearly, any serious approach to Northwest energy carbon reduction has to address the major issue of fossil-fueled travel.

On another note related to the above, I came across a March paper titled "Deep Decarbonization of the Electric Power Sector: Insights from Recent Literature," by Jesse Jenkins and Samuel Thernstrom, published by the Energy Innovation Reform Project.

They reviewed 30 international studies published since 2014 on big electric decarbonization, and reached some conclusions I found relevant to our region, most notably the value of resource diversity.

They wrote that a 100-percent-or-close-to-it variable-renewables power system is "theoretically possible."

So, in honor of this season of eternal hope, is a Seattle Mariners World Series title.

But such a system "would be significantly more challenging and costly than pathways that employ a diverse portfolio of resources," the authors wrote.

They cited many studies concluding that more-modest CO²-reduction scenarios -- one-half to two-thirds -- "can be achieved with a mix of commercially available technologies," specifically combined-cycle naturalgas-fired plants displacing coal, more wind and solar, and keeping nuclear and hydropower capacity. Getting to near-zero emissions "will require virtually all unabated coal and gas-fired power plants to be replaced by zero-emissions sources."

The report also listed numerous challenges for a variable renewables-dominant system, including a need for total capacity to, in some cases, exceed peak demand by a factor of three or four; land-use consequences; huge transmission buildouts; more and more curtailments; a need for long-duration seasonal energy storage far beyond current battery capabilities; and, not least, cost: onestudy posited that a completely renewables U.S. power system would cost at least twice as much as 80-percent renewables.

The authors concluded that there is "strong agreement in the recent literature that deep decarbonization ... is best achieved by harnessing a diverse portfolio of low-carbon resources. In particular," they continued, "low-carbon dispatchable baseload resources such as nuclear, biomass, hydropower, or [carbon capture and storage] are an indispensable part of any least-cost pathway to deep decarbonization."

The dream of an all-renewables power system is beguiling, but it is just that -- a dream. Better to focus on achievable, affordable carbon reduction, including an ongoing role for hydropower, which is a primary reason our regional electric system is already low-carbon. [Mark Ohrenschall]

Bearing Down is excerpted from Energy NewsData's Clearing Up publication. If you aren't a current subscriber, see for yourself how NewsData reporters put events in an accurate and meaningful context -- request a sample of Clearing Up.

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