Renewable Curtailment
Since I’ve had the issues with renewable energy on my mind recently, I’m going to fill some of this space with some of my thoughts on the matter.
tl;dr
I think that storing excess electricity as hydrocarbons and using existing storage and infrastructure is the best way to deal with large amounts of intermittent generation.
Intermittent Renewable Energy
There are some well-known issues with renewable energy. If you read the comments section of just about any article covering renewable energy on a right-leaning site, you’ve seen the phrase “The sun doesn’t always shine and the wind doesn’t always blow”, referring to the intermittency of the solar and wind power generation. I’m going to mostly ignore hydroelectric, which is dispatch-able, and geothermal power, which is a constant source like nuclear, for the rest of this post. (Nuclear has a waste and WMD problem the other types of power don’t have)
And it is absolutely true, the wind doesn’t always blow and the sun doesn’t always shine. At the same time, the wind usually blows and the sun usually shines, which is nearly good enough. But not always.
Because of the way the electrical grid is currently built, electrical generation is matched to electrical load. If the grid has 15 GW of loads right now, there needs to be 15 GW of generating capacity. Electrical grid operators, assisted by computers, change the amount of electricity generated to match the load. If there is a mismatch, the voltage and frequency deviate from it’s nominal values. Which not so helpfully, are not the same everywhere in the world. We’ve got at least 110V, 115V, 120V 240V, 208V (shows up in three phase power systems). We also have both 50Hz and 60Hz, which doesn’t matter beyond clocks that use the predictable grid frequency to pace itself instead of an internal crystal oscillator.
Now, generators fall into a couple categories. Dispatch-able generators can change the amount of power they generate on command. They vary as to how much generation can be shed and how quickly. Large, relatively efficient generators driven by heat engines (coal, oil, gas, biomass, solar thermal). Relatively efficient, because heat engines have a thermodynamic efficiency limit, as described by Carnot, that is about 60% of the thermal energy can be converted to work at temperatures that melt the walls of you heat engine and the portion decreases with temperature. Since power companies don’t want their power plants to burn to the ground or melt into a puddle of glass, they operate at lower, less efficient temperatures. Heat engines all have energy storage includes (the fuel that is burned). Hydroelectric is also dispatch-able while also not being a heat engine (or rather, the heat engine is the hydro-logical cycle) and if a river is dammed, there is significant storage.
Baseload generators more or less can’t change the amount of power they generate much if at all. Nuclear fission power is this way. The nuclear fuel can be moderated to some extent with control rods, but the nuclear decay that powers fission reactors is not stopping. Geothermal is the same way, for much the same reason: nuclear fission and leftover heat from the formation of the planet are the main source of heat. Nuclear fusion is still a work-in-progress, like it has been for decades.
The last is the topic of this article: intermittent energy sources. Solar and wind. You get energy with wind turbines only when the wind blows and solar panels when the sun shines. Both of these are largely predictable, to the extent that your 7 day weather forecast is accurate. Take a large amount of intermittent renewable energy and combine this with the grid load following and you end up with three cases (bears?): too much, too little, and just right.
The solutions to too little is pretty straight forward: dispatch-able supply with storage. Fossil-fuel-powered generation is the majority of what is already built, but there are other possibilities. For renewables, pumping water into a hydroelectric reservoir counts as storage, as does biomass heat engines. Batteries store electricity regardless of source. My favorite is liquid air energy storage: liquefy air and store in an insulated tank, then boil it with ambient temperatures or waste heat and run it thru an expansion turbine. There are proposed solutions like load shedding that require either voluntary support by a large portion of the people using the electrical grid, or forcibly shedding loads with things like rolling blackouts (not preferred) or mandated smart grid controls which effectively takes control of your things and gives it to the grid operators, almost always a government organization. Personally, I won’t support the forced solutions. I also think you won’t get volunteers without incentives like time of day rates.
The just right case is not interesting, and also not the topic I have on my mind, which leaves us with…
Curtailment
Where there is too much renewable power generation and not enough load, or you can’t get the power from the generator to the load because you don’t have enough transmission capacity, you have to turn some off. This is curtailment. As renewables get greater adoption, more attention will need to be devoted to minimizing the amount of power curtailed. Storage for later use in batteries, compressed/liquefied air, pumped hydro, and similar electrical storage seems to get the most attention. I guess this is not surprising because the push seems to be “electrify everything!”, which I don’t agree with. There are resiliency issues with electric everything, and batteries are far too expensive and will continue to be for the foreseeable future.
I think a good way forward is to use energy that would otherwise be curtailed is to make hydrogen thru electrolysis, feed it into the natural gas grid, and convert it to methane with Sabatier reactors, methanol, and long-chain hydrocarbons via Fischer-Tropsh reactors, all of which can drop into existing energy infrastructure. If the carbon dioxide needed for these processes is obtained via direct air capture or gasification of biomass, the process is fossil-carbon neutral (for those that care about fossil carbon). Creating these hydrocarbons has the benefit that these can feed into industrial chemical processes like plastics and pharmaceuticals that currently use fossil fuels with only minor changes to already built, where pure electrical storage can’t do this.
Hydrogen seems to always be the fuel of the future, in large part because it’s always been a decade off for decades. Hydrogen has several issues, the main one in my mind is that hydrogen is so small that it leaks thru every container humans have devised to contain it. It also is not very dense volumetricly, so you need a larger gas tank compared to hydrocarbons.
Compared to hydrogen, the engineering challenges with hydrocarbons have for the most part already been solved. Tanks to contain them are cheap and widely available. Transportation and distribution infrastructure is already in place and well tested. Large amounts of storage already exists.
So, why do the people pushing hardest for renewables not bring this up? As I see it, they are overly fixated on a worldview where all carbon is evil, when in reality carbon cycling is where the solution to intermittent renewables likely lies. Electricity-to-hydrogen-to-methane allows for using existing natural gas peaking and baseload plants to provide electricity when the sun is not shining and the wind is not blowing. This can also be phased in gradually, starting with mixing hydrogen into the natural gas grid, then adding methanizers.
The main downside is the round-trip efficiency of around 34%. This efficiency can be increased if the waste heat can be put to use for building heating/cooling, heating greenhouses, industrial process heat and such. It is also more expensive than natural gas from fracking, though costs are coming down.