Can Nuclear Power And Renewable Energy Learn To Get Along?

MUMBAI, India - May 23, 2014 - PRLog -- That is 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 do not 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 am convinced that the accepted wisdom that renewables and nuclear mix like oil and water is true only up to a point.

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 photovoltaic’s 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:

First, without energy storage, high penetrations of renewables do not leave much room in the power system for nuclear power plants (or any other “base load” 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.

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 subsidised 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 would not 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.

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

We would 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 is 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.

Jesse Jenkins is a Graduate student and Researcher at the Massachusetts Institute of Technology.

Recommended resources

Joint Institute for Strategic Energy Analysis workshop reports: "Nuclear and Renewable Energy: Potential Synergies"

Ruth et al. "Nuclear-Renewable Hybrid Energy Systems: Opportunities, Interconnections, and Needs" Energy Conversion and Management 78 (2014): 684–694

MIT Energy Initiative symposium report: "Managing Large-scale Penetration of Intermittent Renewables"

Data on daily operations of French nuclear reactors (by reactor and in aggregate) showing load-following and flexible operation.

Electricite de France (EDF) technical presentation on flexible operation of nuclear reactors.

OECD/Nuclear Energy Agency white paper: "Nuclear Energy and Renewables: System Interaction Effects in Low-carbon Energy Systems"

Denholm et al. "Decarbonizing the electric sector: Combining renewable and nuclear energy using thermal storage," Energy Policy 44 (2012): 301-311

California Science and Technology Council report: "California's Energy Future: Portraits of Energy Systems for Meeting Greenhouse Gas Reduction Requirements"

National Renewable Energy Laboratory: "Renewable Energy Futures Study"
Fosberg, "Hybrid systems to address seasonal mismatches between electricity production and demand in nuclear renewable electrical grids," Energy Policy 62 (2013): 333–341.

Mills & Wiser: "Changes in the Economic Value of Variable Generation at High Penetration Levels: A Pilot Case Study of California," LBNL (2012).

International Electro technical Commission, "Grid-integration of large capacity Renewable Energy sources and use of large-capacity Electrical Energy Storage," IEC (2012).