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.
That’s Jesse Jenkins’ wrapping up a provocative essay at Energy Collective. I disagree with the implication that ultra-low carbon energy systems must have huge amounts of classical storage and flexibility. Yes, if the system design followed the German Energiewende concept. No, if the system is designed to achieve the optimal nuclear-VRE contributions by exploiting productive variable demand or load-shedding to substitute for most of the storage and flexibility. The variable demand would be products with high value in the region of generation – e.g., syn-fuels, desalinated water, ammonia fertilizers, etc. Nathan Wilson explains the concept of using syn-fuels are the variable demand in the comments:
Should we over-build electrical generation and add fuel synthesis? It depends a lot on whether we need the liquid fuel for other purposes. There will always be some nations that can’t grow enough bio-fuel for their transportation system (maybe most nations). For these nations to get off fossil fuel, their syn-fuel industries will be roughly 1-2 times the size of their electricity industry.
Such a nation would need very little energy storage, since the syn-fuel plants would constitute enough dispatchable load. To put down some ball-park numbers, say the baseload power plant costs $6/Watt (plus fuel for nuclear at around 2¢/kWh) and the hydrogen plant costs an extra $1/Watt, including several days of storage. Then baseload electricity is around 8¢/kWh, and dispatch load at the syn-fuel plants adds 1¢/kWh to the cost of baseload for the idled equipment; this is really cheap peaking power (it does assume that as with today’s costs, per-Watt the chem plant is much cheaper than the nuke, and of course a cheap chem plant is crucial for applicability to low capacity factor off-peak wind and solar).
Thermal energy storage at nuclear plants or advanced batteries (for a few hours) might also fall to the $1/Watt point, but we would still have to pay depreciation for the storage, even on days we didn’t need it. When we don’t need the syn-fuel plant to load-shed, it makes product for its fuel customer, so it does not burden the electricity economics (in reality there would likely be a small payment).
I mentioned several days of hydrogen storage, but note that if liquid fuels (e.g. ammonia or DME) are produced, or if the local geology is suited to underground hydrogen storage, then seasonal energy storage is feasible.
Providing syn-fuel for the entire transportation market using plants configured for dispatchable load is such a powerful tool, that nuclear and renewables can almost be mixed freely on such a grid.
Note that the energy prices given (8¢/kWh for baseload electricity, $1/Watt for hydrogen plant) and 70% conversion efficiency suggest a hydrogen cost of $4.60 per gallon of gasoline equivalent. Conversion to ammonia fuel would add another 5-25%, depending on the technology (this improves storability/transportability and allows simpler ICE cars rather than expensive fuel cell vehicles).
This cost would not be attractive in the US unless the hydrogen/ammonia car got much better mileage than gasoline cars (20-50% better is likely). However, it is possible that the very high temperature nuclear plants in development coupled with thermo-chemical hydrogen production could reduce the cost substantially. Also in China and India nuclear power is only one third of what it costs in the US, so the retail price of ammonia syn-fuel would easily beat imported fuel.
I recommend going directly to the Jesse Jenkins essay Can Nuclear Power and Renewable Energy Learn to Get Along? which is generating a lot of well-informed discussion.
Update: later in the comments Nathan Wilson summarizes the specific case for ammonia instead of synthetic hydrocarbons.
Yes, fuel synthesis is a great way to utilize otherwise-curtailed sustainable energy. But instead of making methane or other hydrocarbons we should make carbon-free ammonia (NH3), to get several benefits:
- Capturing the needed nitrogen (which is 80% of air) is much cheaper than trying to capture CO2 from the air, sea, or biomass.
- Like diesel fuel, ammonia can be burned in high compression internal combustion engines (ICEs), which deliver higher energy efficiency than is possible with gasoline engines; ammonia burns cleaner than diesel, with zero-particulate emissions guaranteed.
- When leaked, ammonia is not a green-house gas (unlike methane), but it is a buoyant gas (unlike methanol and MTBE which can find their way into waterways).
- Ammonia use would not create another dependence on continued fossil fuel use to assure CO2 availability.
- Unlike hydrocarbons, ammonia does not release CO2 when burned, so ammonia (which is made today from fossil fuels for a price competitive with gasoline) can be used to allow fossil fuel with CC&S to join sustainable energy in providing non-CO2-emitting energy for transportation, construction, combined heat & power, or electrical peaking applications (so even countries and regions with deeply entrenched fossil fuel industries can achieve deep reductions in CO2 emissions).
- Ammonia can be economically stored in large above ground (refrigerated) tanks for seasonal energy storage (applicable to all locations, unlike underground methane storage which requires special geology).
see also: http://nh3fuelassociation.org/
Update: Jesse Jenkins inserted a comment that nicely illustrates the arithmetic of the curtailment impact on VRE of more or less nuclear contribution:
I grant that without lots of economic storage/sinks, you’d hit the cieling on renewables faster if you have a share of nuclear in the system as well. So if your goal is to increase renewables to their highest penetration before hitting their cieling before needing storage, then you’d want to back off of nuclear. But if your goal is to get to the lowest carbon power system as possible before needing storage, then I doubt that’s the best way to go.
Simple math here but I think this gets at the gist of it: if your system is say 20% nuclear, then you’d hit the renewables cieling roughly when their energy share = their capacity factor x (100% – nuclear’s share). So for solar at 10% CF, you’d hit the cieling at 10% x (100% – 20%) = about 8% of the energy mix from solar instead of 10% if you had no nuclear in the system. For wind at 33% CF, you’d hit the cieling at 33% x (100% – 20%) = about 26% of the energy mix from wind instead of about 33% if you had no nuclear in the system.
So yes, you lose a few percentage points of renewables share if you have 20% nuclear in your system versus if you don’t. But if carbon is your priority, it makes no sense to give up that 20% from zero-carbon nuclear in order to get 2% more solar or 7% more wind!
So again: if you want an ultra low-carbon energy system with high penetrations of solar or wind, you need massive amounts of economic storage and sinks and DR. And if you have those, nuclear and renewables seem to work just fine together. And if nuclear and renewables aren’t mutally exclusive, the lowest CO2 for the least money may very well be a hybrid system.