- Pumped-storage hydroelectricity - https://en.wikipedia.org/wiki/Pumped-storage_hydroelectricit.... Sure it's not suitable everywhere.
- There's always Molten Salt as well - http://www.solarreserve.com/en/technology/molten-salt-energy....
Also, solar isn't the be all and end all of renewable energy generation. Wind, Solar, Hydro, and Geothermal all have different strengths and weaknesses, and flexibility of generation is important.
But above all, I think security of supply is probably the number 1 requirement (unless you're a big fan of load shedding - https://en.wikipedia.org/wiki/Rolling_blackout), and therefore, we'll still need energy sources that are reliable.
I predict a move away from coal towards natural gas. Sure it's not perfect, but it's not the worst either.
A couple of additions and caveats.
Friend of mine at ORNL has been talking up compressed air energy storage (CAES). I'm somewhat dubious due to the Boyles law heating/cooling problem (gasses heat when compressed, cool when expanded, and the resulting heat flows affect both efficiency and equipment function -- gas turbines don't spin well when iced solid).
Pumped-hydro is the most efficient storage solution bar none, but is painfully restricted by siting opportunities. Being able to use oceans as a lower baisin might address this, though salt-water engineering is a significant concern (corrosion and other issues).
Interesting to see molten salt as at large scale that pencils out a viable for even, say, a two week total energy replacement solution for the US. You'd need tankage (insulated) roughly corresponding to current Oaklahoma oil transshipment storage facilities.
Electricity-to-fuel is another option, with Sabatier and Fischer-Tropsch cycles both proven. Carbon source from seawater appears somewhat promising, though my concern is that there's been 50 years of research without a large-scale demonstration project, making me suspect deeper issues.
I actually am a fan of load-shedding, and suspect that the concept of dispatchable load as opposed to supply may be part of the future way of thinking.
Coal => gas is a no-brainer, though also not a total solution, by a long shot.
Geothermal is highly underrated, especially if currently protected areas are opened for consideration. The Yellowstone caldera could possibly supply up to 15% of total US electrical demand.
There also are ideas to use the ocean as the higher basin. See https://trustyetverify.wordpress.com/2014/12/11/energy-from-..., https://www.technologyreview.com/s/510806/a-manmade-island-t...
I haven't seen anything about it recently, but the Navy was/is funding a demonstration plant in Hawaii. I really hope it works out as I'm skeptical that the developing world will shift to electric vehicles anytime soon.
Her Ted Talk[1] and their website[2] explains it much better than I do.
[1] https://www.youtube.com/watch?v=hZiaTV6uvFQ [2] http://www.lightsail.com/
When sizing an industrial air system a rule of thumb is to budget 7-8hp of compressor for 1 hp of air motor. That is 12% efficiency, or only worth it if you had no other source of power. I'll give them the benefit of the doubt and assume a tightly integrated system you could do 3x better, but it still doesn't come close to batteries.
If someone has plug-plug numbers that are better, let me know.
My understanding isn't particularly deep, and I'd like to run numbers on storage, volume, pressure, etc. But I suspect this'll have to be big enough that constructing cans really isn't viable.
If it was more efficient, than we would see air powered cars (not the scam that surfaces every other year).
Industry only uses it to power things because you can have simple & compact actuators and high rpm motors.
Pumped hydro's remarkably efficient, fast-responding (minutes), but extremely limited in scale. See Tom "Do the Math" Murphy, but I think you'd need ~1,500 or was it ~15,000 Hoover-dam sized project for US storage capacity.
Batteries are fairly efficient, but respond slowly (you can neither charge nor discharge them rapidly), and at scale virtually all have massive substrate / materials shortages -- lead and lithium particularly. Iron-based batteries should be abundant, and liquid metal or molten-salt batteries seem to be the most abundant. Storage densities aren't great, but if you're building stationary facilities, that's not a concern. The 600C - 800C temperatures may be though. Having a neighbourhood melt-out wouldn't be pretty.
Biomass might work for standby thermal generation.
Thermal salt storage also pencils out. But any thermal storage system, that is, hot stuff you use to boil a working fluid that runs through a gas turbine, suffers from Carnot efficiency losses, about 35-45 max efficiency. If in a desert area, you've got problems of cooling your working fluid without venting too much of it (or the coolant). Not insoluble problems, but an issue.
Heat also doesn't store indefinitely though it should be good for hours to a few days.
Fuel synthesis has the advantage of scaling fairly arbitrarily large, particularly with liquid hydrocarbons. Put them in liquid-proof tanks and they'll stay there. Storage stability is proven to 100s of millions of years, so there's that. They're also transportable and can be utilised on-site. You lose 50% in hydrogen electrolysis, plus 65% on Carnot, for a net return of about 17%, but if you've sufficient peak surplus, that's viable.
CAES has the hot/cold problem, but there's a lot of air, and there's a lot of underground reservoir. Again I'm not sold that it pencils out, but it's possible.
Other options include direct banking of heat for buildings and/or industrial processes -- these use a lot of energy, and direct application and storage avoids electric or electric-to-fuel conversion losses.
It'll be interesting.
Anythhing exposed to fluid isn't. Groundwater flws, tanks in air.
I'm not sure about possible ignition explosion risk from residual methaane or hyrogen, say.
Molten salt is efficient because of the phase change, but also much more difficult and expensive to handle than a big (well enormous) bunch of hot rocks. Last thing I heard, he's currently trying to get demonstration plant up and running - it sounded like they'd already done some experiments at Siemens before he left.
Anyway, nowhere near ready, but just to show that there are still low-tech options out there being examined.
Another low-tech option is biomass, either wood chips/pills or other stuff processed into methane.
Yeah, it's not as cheap as coal, but with good grid connections we need less storage than people seem to think, at least according to the research papers I've seen.
Some numbers: 100% renewable energy with a capacity of 340% would require 8% storage just for Germany, 3.8% for Europe (due to larger area which means more diverse weather). Also 8% for Europe using no solar and more wind instead.
The 340% capacity is needed to get to the 100% coverage since there is more downtime compared to old energy which had 180% in 2007.
If we go to 390% capacity (which corresponds to 170 GW additional production as calculated for Germany) we only need 0.9% storage.
Depending on the relative pricing of additional capacity and storage, the price for building the whole infrastructure can go down by half. (The upper limit given in numbers is of the same order of magnitude as Europe's GDP in 2009, 12·10^12 €)
[0] (German) http://onlinelibrary.wiley.com/doi/10.1002/piuz.201201301/ab...
Much better than water - higher energy capacity means smaller units and higher performing insulation making losses much lower.
Heat loss is a bit problem in stored water DHW scenarios and will get even more of a problem if we begin building houses properly (which surely has to happen at some point).
1) You can move them around, as in an electric car. That's useful if you want to do mobile stuff like driving. Or if you just flat-out want to move the facility somewhere it will be more useful.
2) They can be efficiently decentralized, which benefits reliability, and decreases transmission loss.
But anyway, the solution is to use any and all technologies, right? Batteries make sense some places, gravity storage elsewhere.
While an interesting theory, I can't think of a single scenario where this would be economically justified.
> 2) They can be efficiently decentralized, which benefits reliability, and decreases transmission loss.
Transmission losses are about 6%, so not really any economic gain.
I have no idea if that's realistic. Does someone know what the weight/distance to power output would be limited at...
Play with this and you'll immediately see it doesn't seem useful. e.g. storing a 50-tonne weight at 10m gives you less than 1.5 kWh of energy.
Flywheels that don't become fly-apart wheels is one problem.
Containment of fly-apart wheels another. Buried concrete pipes are usually what you're looking at. You need mass to contain these things.
The per-kg energy densities just aren't all that high. If you want reactive load to soak up very short-term demand / supply fluctuations, you can get that. And you've got crazy charge-discharge cycles.
Bearings are mostly solvable with magnets, as I understand. But you still lose a given percentage of storage over short time -- ~1% per hour or so, within a few powers of 2. Enough that long-term storage isn't viable.
And the thing that really screws you over is that the Earth moves. Geometric precession of your flywheel is something you've got to deal with.
There's a place, but I don't think it's large-scale, long-term grid storage.
There is a modern effort to use the technology in a massively scaled up fashion by Heindl Energy[1]. Their concept uses pumped water to raise a rock weight 100 meters or more in diameter.
[0] https://en.wikipedia.org/wiki/Hydraulic_accumulator [1] http://www.heindl-energy.com/hydraulic-rock-storage/overview...
Actually, it's not too bad, just slower. Batteries improve around 8% per year, if I remember right.
That's less than semiconductor based stuff improved when Moore's law held, but pretty impressive compared to lots of other technologies.
[1] http://www.candlepowerforums.com/vb/showthread.php?88468-Tri...
[2] http://www.candlepowerforums.com/vb/showthread.php?417736-Te...
Thanks for digging up the numbers!
In any case, a bit of Googling will give you more details about the improvements than my faulty memory..
It's generally used in homes with 'storage heaters' which heat up overnight and release heat during the day while you're out at work. https://en.wikipedia.org/wiki/Storage_heater
http://www.emotorwerks.com/index.php/component/rsform/form/1...
In areas where the utility is not set up to do this, you can programming it to monitor the grid and prefer charging when the grid is less carbon intensive.
See my comment further below https://news.ycombinator.com/item?id=11520789
It is in fact these very expensive gas plants that solar is destroying the market for first, both because they are the most expensive to run, but also because solar happens to align with the natural peaks in demand.
To be brief, I left it out, but my ballpark guesstimate is that, due to the cost of storage, for the main grid, solar for free is still not cheap enough. So, that is an answer to a question in the OP -- how cheap does solar have to be? My guesstimate is $0.00 per megawatt hour is still not cheap enough. In really simple terms, the grid doesn't want solar, even for free because of the need for storage and the fact that solar is unstable, e.g., due to just a sudden summer thunderstorm. The grid is super big on stability -- a small source of instability, and the whole grid of the US NE can go down, and at least once did.
Another, related point is that often the wholesale cost of power on the grid has been ballpark $0.005 per KWh, that is, half a penny per kilowatt hour. So, already, now, on the grid, intermittent power is nearly worthless, even when it is excess power from a rock solidly stable source.
And why might there be excess power? Because at the generators it's not so easy to adjust power levels quickly. So, when there's an excess, just sell it off where can, get half a cent for it, and go on.
Very much what the grid wants from the generators is rock solidly stable power, 24 x 7, no snap, crackle or pop, not even for two seconds, for years at a time. And we have that, now, that is, at least from the generators. Sure, for the lines and poles out to the burbs, during an ice storm, the reliability can be lower.
Solar? Fine, if it is cheap enough AND there is good storage to make it reliable enough AND the storage is not too expensive.
Again, the short answer, for solar, for the grid -- the storage costs too much.
Again, we're talking about the grid. There are other candidate uses for solar power -- taking salt out of water (get to store the clean water), pumping water uphill (if have a big dam handy), getting hydrogen from water (get to store the hydrogen or just pump it into a pipeline), making gasoline from coal and water (get to store the gasoline and pump it into a pipeline). So, these uses all have the feature that there is a good way to store the results of the solar power.
People have worked hard on storage for a long time, and that there is little or none installed on the grid indicates that so far no one has a good approach to storage.
Likely, get the solar panels with 100% efficiency for free, and the issue will remain -- cost of storage. A lot of this is just my opinion, but what's been deployed on the real grid makes me suspect I'm basically correct. We can be sure that lots of smart engineers have checked the figures very carefully, and so have the public utility commissions, so we don't really have to go over all the figures ourselves. It's pretty clear that the smart engineers that have done all the arithmetic carefully don't want solar for the grid.
But, sure, some people want to tax carbon, and tax it enough that solar with available storage will be cheap enough. That's another issue.
You'd still have the time the sun is not shining, which would likely be more expensive than before (a gas powered plant would only be running half the time - effectively doubling its capital/construction cost per kWh).
So you'd definitely have an improvement on some levels but I'm not sure that would really help that much.
The real big breakthrough would be somme big improvement on storage. I'm not convinced this will happen quickly (at least not in anyway similar to the declining cost of PV panels). The consumer electronics industry has been paying $$$ for decades for better batteries/battery r&d and has seen pretty poor gains.
I think Tesla and the electric car manufacturers will definitely rapidly get the price down by say 30% because of economies of scale but will then hit a brick wall as more of the cost becomes raw materials, transport, etc which will set a floor on the cost of batteries.
Happy to be proved wrong though :)