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Can Nuclear Power and Renewable Energy Learn to Get Along?

Jesse Jenkins's picture

Jesse is a researcher, consultant, and writer with ten years of experience in the energy sector and expertise in electric power systems, electricity regulation, energy and climate change policy...

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  • Apr 15, 2014
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Full Spectrum: Energy Analysis and Commentary with Jesse Jenkins

Nuclear power and variable renewable energy sources like wind and solar power “don’t play well together.”

That’s a commonly accepted nugget of wisdom these days. I heard the argument most recently during an interesting colloquy on Twitter this week with Fresh Energy CEO Michael Noble, reporter Matthias Krause, and author and editor of Renewables International Craig Morris.

If true, the idea that renewables and nuclear don’t mix has important implications. It would mean that if we want to build an ultra-low carbon electricity system to confront climate change, we may face two mutually exclusive paths: one path dominated by nuclear energy (call it the French paradigm) and the other dominated by variable renewables (call it the German paradigm).

(In fact, supporters of the German Energiewende use this argument that large penetrations of renewables are incompatible with nuclear as one of the justifications for the nuclear phase-out underway there now).

The more I think about this, however, the more I’m convinced that the accepted wisdom that renewables and nuclear mix like oil and water is true only up to a point.

In fact, if we want to build an ultra-low carbon system powered by variable renewables, we’re going to have to solve precisely the same technical challenges that will make a hybrid renewables and nuclear power system possible as well.

My thinking is as follows, and I present this as a hypothesis for discussion and with plans to analyze this in more detail in the future (i.e. using power systems modeling)…

I begin with this basic point: In rough terms, once a variable source of renewable energy, such as wind or solar power, reaches an energy penetration level (measured as the share of total energy supply) equal to that source’s average capacity factor, aggregate output from that variable renewable energy source will routinely fluctuate between 0 and 100 percent of total electricity demand.

For example, if the average capacity factor of solar photovoltaics is 10 percent (about what it is in Germany), once solar PV reaches about 10 percent of the system-wide energy mix, solar output will vary from 100 percent of demand when producing at full capacity on a bright mid-summers day and 0 percent when night falls. Wind turbines in the breezy American Midwest have a capacity factor closer to 35-45 percent, so wind would reach a ceiling at about 40 percent of energy share in that region. 

There are two important implications of reaching this point where a renewable energy source’s share equals its capacity factor.

First, without energy storage, high penetrations of renewables don’t leave much room in the power system for nuclear power plants (or any other “baseload” power plant).

While nuclear reactors can technically “ramp” or vary output up and down to follow loads (albeit less flexibly than gas turbines), “cycling” or shutting down entirely and start up again later is too challenging for a nuclear plant to do routinely. Yet at high penetrations of variable renewables, every other plant on the system would have to be capable of routinely cycling on and off.

  • Summary: it’s true then that absent energy storage and flexibility, high penetrations of variable renewable energy sources doesn’t play well with nuclear.

Second, increasing the penetration of renewables beyond the point where energy share equals capacity factor would mean the renewable source would begin to regularly produce more electricity than demanded. Without storage or energy sinks willing to buy up excess power, renewable generators would then have to curtail a growing share of their output and waste any associated revenues.

In practice, this ceiling could actually be reached before renewable energy penetration equals capacity factor, as production would begin to regularly exceed demand on high output/low demand days long before this point.

In addition, if renewables are exposed to wholesale prices (and not subsidized outside the wholesale market, i.e. with feed-in tariffs), the market prices earned by renewables would be negatively correlated with their output. Wholesale prices are lowest precisely when renewable generators are all cranking out power (again, this assumes no energy storage/sinks.) At some point, adding more renewables just wouldn’t be profitable any more. If renewables have to pay for the system balancing services and flexibility needs they contribute to, this economic limit is reached even earlier.

This point where energy share = capacity factor is probably a generous ceiling for renewable energy penetration absent storage then.

If solar capacity factors typically range from 10-20 percent and wind from 25-45 percent, that makes it awfully hard to reach an ultra low carbon energy system powered principally by renewables. Once these sources reach a combined share of maybe 30-40 percent of the energy mix, technical and economic constraints will make it very hard to increase their share further.

  • Summary: absent energy storage and sinks that can make profitable use of excess energy and massive system flexibility to handle variations in renewable output from 0 to 100 percent of load, penetration of variable renewables is effectively constrained below the point where their energy share equals their capacity factor.

If we want to increase renewable penetration beyond these levels and drive truly deep decarbonization of the power system, we therefore need massive amounts of new system flexibility to match demand with varying renewable energy output.

We’d need electric batteries and thermal energy storage to shift output to when its needed, dynamic load shifting and demand response to align demand with output, and ‘energy sinks’ to make productive use of excess output.

But here’s the kicker: if we have the massive amounts of storage and flexibility needed to achieve an ultra-low carbon electricity system dominated by variable renewables, we also have the storage and flexibility needed to make a hybrid nuclear-renewable power system feasible as well.

With that kind of system flexibility, we could store energy and shift loads to avoid having to cycle off and on nuclear plants and limit their ramping only to when it’s the most economical way to provide system flexibility.

  • Here’s my contention then: If you want an ultra-low carbon renewable energy system, you need storage and flexibility. And if you have storage and flexibility, then renewables play just fine with nuclear.

Maybe renewables and nuclear can learn to get along after all. Maybe they won’t offer competing visions for a low-carbon power system in the end.

Let’s discuss…

(A note for comments thread: this post isn’t asking whether you want a nuclear-renewable hybrid power system. It’s asking whether a renewable-hybrid system is technically and economically feasible if we did want one. This is a post about what options we have, not which ones we want to chose. So let’s save those discussions on which you’d choose for another day. Thanks! -Jesse)

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Grace Adams's picture
Grace Adams on Jun 2, 2015

Most developing nations are close enough to the equator to make solar a much better option for renewable energy than wind.  So I doubt that they will really want wind. Wind is good mostly more than 30 degrees away from the equator.  They will want energy storage of some sort. 

Grace Adams's picture
Grace Adams on Jun 2, 2015

Can the cost of flexibility be internalized into the cost of variable renewables by measuring net metering in $ rather than kWh with time of day pricing? You don’t want to drive suburbanites with rooftop solar off the grid into lower cost batteries completely, but you also don’t want to end up trying to balance utilities’ books on the backs of the poor.  Utilities have a hard enough time trying to collect utility bills from the poor already.

Grace Adams's picture
Grace Adams on Jun 2, 2015

Yes, synthetic fuel would be nice useful flexible load to follow variable renewables and variable but beyond power of utilities to control other loads.  I would hope some of the synthetic fuel could be sustainable liquid fuels for transportation, especially diesel, since rechargeable battery electric vehicles could largely substitute for ICE engine powered vehicles for light duty vehicles.

Grace Adams's picture
Grace Adams on Jun 2, 2015

Yeah!  California and its neighbors to the east can certainly use the desalinated seawater.  And some pumped storage hydro should also be welcome. Maybe the most politically feasible, if not really cost-competitive, way to cope with fossil fuel lobbyists would be a program to buy fossil fuel reserves especially coal as mineral rights at fairly generous prices to support income of too big to fail (politically powerful) fossil fuel firms (limit to keeping their incomes up to a fair amount). Also sell much of the synthetic fuel through too big to fail oil firms, splitting the income between the nuclear power plant operator and the too big to fail oil firms.

Grace Adams's picture
Grace Adams on Jun 2, 2015

With further improvement in cost and efficiency of energy storage, it might be feasible to get 80% of our electricity from some combination of solar and wind, a much improved smart electric grid with lots of storage, geothermal (both hydro and enhanced), some seasonal hydro as primary power, bio-gas (from sewage, livestock manure, municipal solid waste, any other wet organic waste material), some biomass from forestry waste, powdered dried algae from unwanted algae blooms, and as a last resort, some natural gas.

My two big objections to nuclear are huge cost over-runs on nuclear power plants and an indefinitely long wait for spent fuel rods sitting around on the nuclear power plant campus from which they come in dry casks that have to be replaced every 50 years with no guarantee that the world can remain civilized long enough to make new dry casks all that time. Maybe those two objections can be overcome with improved technology.

We are a lot of R&D away from having a sustainable liquid fuel for transportation. Maybe we will get lucky, and it will turn out to be a good sink for surplus electricity, that can be stored until it is wanted either to to make more electricity or to use as liquid fuel for transportation.

 

 

 

Grace Adams's picture
Grace Adams on Jun 2, 2015

I didn’t think the replacement life of VRE was all that short. Technology is improving rapidly for both solar and to some extent wind. Some parts, like batteries for storage and inverters need replacement after 15 or 20 years. solar panels come with at least 20, sometimes 25 year warranties, and we haven’t had the most recent new models long enough to see any die of old age, maybe some lost to breakage from something falling on them.  If you treat enhanced geothermal like a heat mine, running it as baseload, you need to redrill another kilometer down every six years, until the first hot rock reservoir manages to reheat itself after anywhere from 50 to 300 years so it can be reused, or until it seems more cost effective to move the whole geothermal installation a few miles further down the transmission lines to start over. If you can treat it more like dispatchable power with around 50% use ratio, maybe you can go 12 years between redrilling. I believe China must have some geothermal in the foothills of the Himalyas near its western border. Big hydro dams do tend to silt up and need to be either abandoned or dredged.  China will have to decide whether to keep with almost all VRE or start adding nuclear power to its mix. 

Nathan Wilson's picture
Nathan Wilson on Aug 15, 2015

Steve, of course variable renewables can be used to produce syn-fuel.  But you’ll also have the challenge of explaining to the public certain bad news (especially compared to the nuclear alternative).  

When dispatchable load is added to the grid, it provides a valuable service which deserves compensation: the synfuel plant will buy power at a discount (this discount must grow as the capacity factor drops).  This discount is funded by raising the price that everyone else pays for renewable electricity.

Also, fuel is easy to transport from place to place, which tends to make market prices converge.  People in many locations will find that synfuel for them is an import, since their local renewable resources won’t be able to compete with the low cost regions.  This will be unpopular in fossil fuel producing regions which face loss of energy jobs (long distance electricity transmission does the same thing).

Finally, with variable renewables, thermal generation will likely still be needed as backup, likely with subsidy support (unless you want to occasionally declare “load shedding grid emergencies” aka TOU price spikes, which people in developing nations are used to but hate).  Plus you’ll have the difficult choice of burning clean but expensive synfuel at transportation fuel prices in the backup plants, or cheap and dirty fuel (such as dirty but politically powerful German lignite coal).

Jesper Antonsson's picture
Jesper Antonsson on Aug 16, 2015

Very good points. However, it’s unclear to me why thermal generation would need subsidy support. I’d think that in developed nations, consumers would be able and willing to pay the very high spot prices necessary to support thermal backup plants and generation.

What does need support, though, is the renewable generation itself, since income from synfuel plants likely won’t pay for the marginal wind turbine and solar cell (which will produce mostly excess power that would be of no value if the synfuel plant doesn’t take it). Of course, this is just restating what you said about discount and “raising the price that everyone else pays”.

I think it is kindof premature to talk of thermal backup, btw. It’s ok in sci-fi such as this, I guess, but for the foreseeable future, thermal is the power source and intermittent power extends its fuel somewhat. And it seems the primary purpose to extend thermal fuel, for now, is to get rid of nuclear.

Grace Adams's picture
Grace Adams on Aug 17, 2015

Often resources that lots of persons want badly but are unwilling to pay for are often paid for through government in general revenue taxes.  In this case, most need energy storage, whether thermal energy storage, or electric storage or pumped hydro storage, whateverr will get the job done, and it makes sense to have utilities control storage, hopefully with computers doing the job automatically, rather with humans getting bored and dozing off.  It might make sense for federal government to buy energy storage of assorted types, have utilities maintain and operate them, have both firms producing extra energy and wanting to store surplus for later, pay half the cost of the storage, and have customers of electric or of heat from distric heating or whatever, pay for the energy and the other half of the cost of storage as they buy energy, of whatever form.

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