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)

Recommended resources