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  #51  
Old 06-07-2019, 11:11 AM
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XT, you absolutely have the right idea, though you are 1 step back from the full answer.

First, the majority of the electric load/energy load (since most of the cars and trucks will be electric), you have to handle with either immediate renewable production or short-term batteries.

That is, very large distributed wind grids are optimal when the batteries are expensive, when they are a bit cheaper then solar works.

But you realize that you still need a substantial backup reserve for that 3% edge case where the solar + wind just isn't cutting it, even with overcapacity and lots of batteries.

It's a "3%" case in that this form of backup power is a small fraction of the energy you need but you do need a long term, stable storage of the energy.

And the simplest answer to this is either you just keep using fossil fuels, or you electrolyze water to hydrogen...and reform it to methane.

Why methane? It's much more stable and energy dense than hydrogen, storing far more energy (orders of magnitude more) in the same volume. It also doesn't leak out of tanks and connections nearly as readily. You then store the methane underground, in, ironically, depleted natural gas wells.

You get the CO2 either by atmospheric carbon capture, or the easiest thing to do is to recycle the output of your power plant, like so : https://en.wikipedia.org/wiki/Allam_power_cycle

So you store CO2 in underground formations or large surface tanks. When renewable energy is plentiful, you electrolyze water, in large efficient plants, and combine it with the CO2, storing it up over months as methane.

During those rare 3% cases when the weather is against you or grid interconnects have failed or many other causes, you burn the methane, keeping the CO2.
  #52  
Old 06-07-2019, 11:19 AM
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Note: efficiency doesn't really matter for this form of long-term energy storage. (short term would be efficient batteries). That's because, since it's handling an edge case that will be less than 10% of the time, it's just fine if you need to produce 2-3 times the renewable energy that you get back after the storage. During a sunny day in California, there is excess energy on the grid during today's buildouts, with the market price dropping to fractions of a cent.

Note that while my idea above of using Allam cycle plants is technologically interesting, the actual backup solution for the 10% case will have to be whatever is actually cheapest.

Last edited by SamuelA; 06-07-2019 at 11:21 AM.
  #53  
Old 06-07-2019, 12:05 PM
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Originally Posted by zimaane View Post
Would it be practical to store excess energy as thermal energy? Say, during the day, the electricity could be used to heat up something inert (salt?) then at night the hot material could be used to boil water to drive a turbine?
Sure, but it's best if we can store it as heat (or cold) and use it as heat/cold later, rather than converting it back to electricity (which is very inefficient).

And there are already many systems that take advantage of off-peak electricity to heat water or make ice for later use. And it's nothing new - 40 years ago my grandmother in Japan had a tank water heater that ran off night-time electricity. I know someone in Phoenix AZ whose condo building has an ice storage air conditioning.

I really think this is the ultimate solution to renewable energy storage. Don't store it as electricity, but instead use energy when it's available. Do you really care if your freezer goes down to -30F at night (when electricity is available/cheap) and drift up to 0F during the day? Do you care if your car starts charging immediately when you get home, or wait till 3am? (Assuming you can override it if you really do need it charged now.) Would't you switch to a slightly larger capacity water heater if it saved you money (by refilling only when electricity is cheap)?

Obviously we need a smart power grid with smart meters to incentivize people to make these changes. But that's probably easier than building massive energy storage plants.

Last edited by scr4; 06-07-2019 at 12:08 PM.
  #54  
Old 06-07-2019, 12:41 PM
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If you want to work with hydrogen and store it for reserve energy, consider adding a bit of N. 3 parts H to one part N produces a substance that is far easier to handle than raw hydrogen, can be catalyzed fairly easily to extract the H (probably for fuel cells), offers a tad more than 3x the storage density and can be manufactured from renewable electricity sources. And, well, we have no shortage of N.

Note that fuel cells produce a large amount of waste heat: during polar vortex type events, that waste heat could actually be useful.
  #55  
Old 06-07-2019, 02:45 PM
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Originally Posted by eschereal View Post
If you want to work with hydrogen and store it for reserve energy, consider adding a bit of N. 3 parts H to one part N produces a substance that is far easier to handle than raw hydrogen, can be catalyzed fairly easily to extract the H (probably for fuel cells), offers a tad more than 3x the storage density and can be manufactured from renewable electricity sources. And, well, we have no shortage of N.
Absolutely. Anhydrous ammonia is also a deadly poison gas, so you need to produce it/store it/burn it in large facilities far from places where people live.

But yes, I was thinking of methane because it's safer and you can fuel vehicles, including rockets and airliners, with it. But for large storage of backup energy for electric power, NH3 might be a good choice.

Note you have to store it at -33C for it to remain liquid - it expands 850 times in the gas phase, so that's a negative. You'd have to store it in very large refrigerated tanks.
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Old 06-07-2019, 03:51 PM
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Note you have to store it at -33C for it to remain liquid - it expands 850 times in the gas phase, so that's a negative. You'd have to store it in very large refrigerated tanks.
Why are you storing it at standard pressure? Do you have it in big open vats?
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Old 06-07-2019, 05:43 PM
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Why are you storing it at standard pressure? Do you have it in big open vats?
Shrug. Maybe pressure vessels are cheaper. They are for natural gas - lng is only used when the volume matters, such as for moving it by ship.
  #58  
Old 06-10-2019, 02:27 PM
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A hydrogen filling station in Sandvika, just outside of Oslo, Norway, just had a massive explosion and is still on fire.

It will be interesting to see what went wrong, if it should have been preventable and what it will do to the progress of hydrogen vehicles in Norway.

The station was producing hydrogen by electrolysis with solar power from a nearby building and serviced a rather small number of fuel cell cars.
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