Hybrid Energy Systems – Why We Might Need Them

by Will Davis

Today’s panel at the ANS 2016 Annual Meeting on hybrid energy systems might seem at first like somewhat of a misplaced effort, but it’s certain judging by the varied content provided at this and other sessions this week as well as by comments of varied experts that grid flexibility will be the “name of the game” as time goes on.  Hybrid energy systems, which combine more than one “prime mover” or energy generating source, may be one key to making it possible for nuclear energy to stay in the US game in the long term.

Shannon Bragg-Sitton, Nuclear Hybrid Energy Systems Lead at the Nuclear Science and Technology Directorate, Idaho National Laboratory led off with a basic presentation on why we might need hybrid generating systems.  Bragg-Sitton told the audience that studies on the increasing penetration of renewable assets onto the grid have led to shorter power peaks (in duration,) steeper ramp rates for fossil generators (as renewables rapidly come on or off the grid,) and lower turn-down overall (the difference between the maximum and minimum grid demand levels.)  This has the effect of making the grid less stable; the renewables are not flexible, and so force everything else to respond to their input to the grid.  To make up for this, in a general sense, we can employ either supply side flexibility (ability of dispatchable generators to respond to renewables’ variable input) or demand side flexibility (a number of ways) or both, she said.

According to Bragg-Sitton, there are a number of ways that hybridizing power generation could be accomplished to make up for the basic variability of renewable sources.  For example, she pointed out that it’s possible to continue operating nuclear plants at full power, without responding to the grid demand when renewables come on, by diverting their excess output to some other useful form which could earn revenue.  As one example, the energy could simply be stored for when it’s needed.  Or, the energy could be used right at that moment – in one way, as process heat for the manufacturing of chemicals or other materials.  In such a system the nuclear plants don’t need to ramp their output; they operate essentially in the “base load” mode, but they do NOT supply “base load” power to the grid.

Bragg-Sitton also described various ways to hybridize nuclear and renewable generation on a single site, essentially having one grid connection from a combination of nuclear and renewable sources.  The ultimate expression of this would be the “Energy Park” concept, wherein nuclear and renewable sources both provide power or heat to varied manufacturing, processing or desalination facilities on or near the site and also have a single, unified grid connection from which any combination of sources in the energy park can deliver the required energy to the grid as needed by the grid operator.

•See an earlier ANS Nuclear Cafe piece on “Agro-Industrial Parks” here.

Andrew Sowder of the Electric Power Research Institute followed with a presentation on a study performed by EPRI which was intended to show the effects on the grid of the early retirement of coal fired plants along with continued renewables penetration; he pointed out very clearly though that while the study was aimed at coal plants, the effect is exactly the same for nuclear plants in the US since with only a few exceptions the plants are operated in the base load mode.

Sowder pointed out that the “grid of yesterday,” with a large number of big, base load plants providing high quality energy to a number of consumers with a highly predictable demand curve is not at all the “grid of today,” wherein nondispatchable (variable) generation sources like renewables come on and off the grid, and where consumer demand patterns continue to become less predictable.   “The dynamics are completely different,” he said, noting that overall the grid operators have less and less control over their generating assets.

According to Sowder, it’s vital that the grid provide not only around-the-clock power, but that it continue to produce it at a high quality (stable frequency, stable sine wave curve) to prevent not only instability but damage to many various types of loads large and small.  Sowder also noted that with renewables, the peak power supply generally available is not matched with the peak power demands; this means that dispatchable generating sources (gas, coal and more frequently nuclear) have to adjust their power outputs to make up the difference.

The estimates by EPRI are that by 2050 or so over 100 GWe of coal plants will retire.  “Large, dispatchable or base load plants are being driven off the grid,” Sowder observed, as coal plants are forced to retire – largely due to economics driven by environmental regulations.  Sowder told the audience that nuclear plants could theoretically help make up for this loss, but that they’re not optimized to ramp power to be that necessary bulwark against renewables.  “Maintenance procedures at nuclear plants are optimized for base load, steady power, not for load following,” he remarked, and noted that a number of maintenance problems could crop up at well-run reliable nuclear plants (making them less reliable) if they were required to ramp power up and down very frequently.  Among the maintenance problems mentioned, the key issue, Sowder said, was fuel issues – noting that the industry has been very proud of its amazing reduction in nuclear fuel element failure rates and speculating that this might creep back up without new fuels being incorporated that are better able to tolerate frequent power changes.

Sowder finished up by pointing out that the EPRI study shows pretty plainly that, by 2050, many regions in the nation will have marginal (at best) grid stability due to the retirement of dispatchable plants and the increase in renewables penetration.

It seems clear, given the information above, that while there will continue in some regions to be a solid place for Gigawatt-class nuclear plants to run continuously in base load fashion, there will also be a need (and, seen the proper way, a real market opportunity) for perhaps smaller but certainly more flexible nuclear plants that can participate in either simply flexible, or very highly flexible hybridized power systems such as mentioned above.  Given the momentum of renewables in all quarters, this may be more of a necessity than an opportunity.

Will Davis is reporting from the site of the ANS 2016 Annual Meeting in New Orleans.


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