Lithium ion can't store electricity across seasons, though, which is important anywhere outside equatorial areas. Gas can be left idle during the Summer, and used during the Winter. If the cost of solar power went below the marginal cost for burning gas (i.e. lower than the per unit fuel costs for gas generation), the most cost-effective solution might actually be to use solar during the Summer, and maintain a fleet of gas stations solely for the Winter months.

Still one of the most efficient energy storage systems is to pump water up a hill. There are losses in evaporation and conversion but these losses are low when compared to batteries. A system where we used solar power to pump sea water up into a man made reservoir would be very interesting.

Round trip efficiency of pumped hydro is 70%-80%, maybe up to 87% according to some people:

https://en.wikipedia.org/wiki/Pumped-storage_hydroelectricit...

A standard lithium ion batter is at 87% in the worst case, when it's AC-DC-storage-DC-AC. Going direct from solar panels to storage would save one trip:

http://www.catalyticengineering.com/top-ten-facts-about-tesl...

In any case, the efficiency of storage is only one aspect; it limits how cheap the storage can potentially be, but electricity lost due to inefficiency is not the primary driver of storage cost with any of the currently used technologies.

The efficiency of pumped heat electrical storage is a little worse than pumped hydro. Short description you use use a compressor to compress and heat gas. Use the hot compressed gas to heat rocks. Later recover the energy using a turbine.

The advantages over pumped hydro are,

The power density is much higher, 100000 joules/kg vs 300-500 joules/kg.

You can locate storage facilities pretty much anywhere that's out of shrapnel range. In case the big tank of gas and rocks blows up.

Doesn't have the environmental impact of dams. And rocks are significantly less precious than huge amounts of water.

What's fascinating is some of the research around combined solar & battery designs which to merge the two functions on a materials level.

I haven't heard of this, do you have any suggested reading material?

I was mostly thinking of this research at Ohio State

https://www.sciencedaily.com/releases/2015/08/150801082647.h...

But I think there may be other areas that I'm not finding specifically right now... ("solar battery" is a poor search term for this research...)

This sounds fantastic:

- a redox flow battery coupled directly to solar charging

- super high specific energy, ~0.33kWh/kg which is right on par with traditional solid lithium ion batteries

My remaining questions are:

- Compared to PV solar + lithium ion battery, how much insolation is required to store a kWh?

- What is the cost/kWh of the lithium iodide?

- What is the cost/kW of the solar charger, discharger?

- Can the anode and cathode be used to charge the battery from standard electricity, in addition to the (more efficient) solar method?

Thanks for this pointer, this gives me even greater optimism about the future of storage!

You might find this video interesting:

http://youtu.be/rsuB-6-n-MM

A system that uses solar to desalinate seawater and pump it to a reservoir by day, then used for hydro power at night and again for crop irrigation, would certainly be one of the most exciting green engineering projects out there

Yes. Unfortunately water desalination requires a ton of energy but you can use the sun directly.

Pumped hydro is always brought up, which is bizarre to me because it's highly dependent on your geography, unlike battery storage.

Why is it bizarre? It is a dead-simple, relatively efficient storage mechanism that could potentially coexist with municipal-scale reservoir systems (thus gaining cost efficiencies). Sure, Kansas, US is not the first place one would look to build one. But most of the world is not Kansas.

To flip that around, there exist applications where battery storage isn't reasonable. Is it thus also bizarre to bring batteries?

I can drop cargo containers full of Tesla Powerpacks anywhere in the world. I cannot build hydro pumped storage just anywhere.

...And?

You're not answering:

> To flip that around, there exist applications where battery storage isn't reasonable. Is it thus also bizarre to bring batteries?

The post I was replying to called talking about hydro storage "bizarre". I was pointing out that it isn't. The logic here seems analogous to saying that compact cars suck because you can't use them as a dump truck.

My argument is that battery storage will slowly become better than all other energy storage methods as the price comes down. Its also helpful that it can be transported and colocated anywhere necessary.

I'm curious - can you name a scenario in which battery storage isn't reasonable? It seems to me that once we hit the cost/price/weight/density points, they are a solution to pretty much any problem of power storage.

Seasonal imbalances, as stated otherwise in thread. Image you have no sun for a month each year. Good luck building enough batteries to handle that.

The strong part of pumped hydropower is that it scales really really well even if the baseline efficiency isn't stellar.

This is a roughly 12 hour reservoir:

https://www.google.com/maps/@43.9207263,-86.3978435,18314m/d...

It's a big power station, 1.8 GW, but is only providing service to portion of consumers in Michigan.

Making it 10 times bigger would probably be doable but it wouldn't be trivial.

It's also one of the better places on the planet to build a single reservoir system (otherwise you need 2 enormous reservoirs).

I wonder if the answer to seasonal variation will be to make fuel in the summer.

A larger connected world grid could solve some of the problems 10 years or so ago I read some articles of producing solar power in Africa and then using it in Europe.

Is pumped hydro dependent on geography? I'm no expert and I'm skipping over the research and calculation part, but I'd be somewhat surprised if solar panels on my house could generate enough electricity to pump more water up to a tank on my roof than I could run through for power generation.

It's highly dependent on geography, you need two separate, very large, reservoirs at very different elevations. It takes a lot of water across a big height differential to store electricity.

It kind of reminds me of people that think that we should use the exercise bikes in gyms to power the lighting; yes, brilliant, but actually run the numbers and you realize that humans output very very little energy even at peak exercise. The amount of waste on generators would be phenomenal.

There are some cases where pumped hydro makes sense, but there's limited scalability.

Wikipedia has a nice illustration of the amount of power stored

For example, 1000 kilograms of water (1 cubic meter) at the top of a 100 meter tower has a potential energy of about 0.272 kW·h (capable of raising the temperature of the same amount of water by only 0.23 Celsius = 0.42 Fahrenheit).

https://en.wikipedia.org/wiki/Pumped-storage_hydroelectricit...

Given the much smaller height of a rooftop tank, every 1000 kg of water pumped would support a number of minutes of normal household usage (a decent refrigerator would need several thousand kg per hour).

You can water pump uphill, or, as they're experimenting with here, you can pump air underwater: https://www.torontohydro.com/sites/electricsystem/GridInvest...

I'm sure compressed air in other forms would be a good store. You could also raise and lower weights into a mine, it doesn't matter so long as you can convert between electric energy and kinetic potential of some kind.

Quick calculation:

1000L tank on a roof which is ~4m high.

E = m * g * h = 1000 * 9.8 * 4 = ~40,000J

Which is about 12Wh not taking into account any losses.

You could maybe improve this by having the water fall into a well but still nowhere near what a battery can provide for a fraction of the cost/complexity.

A standard 100Ah battery will provide 1200Wh.

Does it work with seawater? Some countries have a frightful shortage of freshwater, so rather than risking loss due to evaporation, could you pump seawater uphill?

In theory seawater would work but it's important to consider the increased expenses and operating troubles from seawater. Corrosion, bio-fouling, increased need for filters and filter maintenance, etc...

Outside equatorial areas, lack of sun coincides with demand for heating. Combined cycle (correction: I meant cogeneration) heating plants are that winter months fleet.

Yes, if you could combine gas generation with district heating, that + solar generation really would fit together neatly.

Also, there are projects to do the thermodynamic equivalent of district heating over a single home! i.e. you burn gas to produce electricity, and use the waste heat to heat your house or office. A year or two back British Gas was doing a pilot using a gas boiler with a Stirling Engine to generate electricity (called the Ecogen IIRC). It seems to have been difficult/expensive to maintain, but it's an exciting principle.

If you had battery storage for solar during the Summer, you could use that equally to buffer that kind of gas generation in the Winter.

It seems to me sufficient solar and battery cost reductions effectively solve electricity generation for everywhere close to the equator. Add in that kind of waste heat boiler, generating electricity at a reasonable cost, and you have a solution for almost the whole planet.

composite thin cylindrical flywheels have great energy storage characteristics "greater than 400 Wh/kg can be achieved by certain composite materials" https://en.m.wikipedia.org/wiki/Flywheel_energy_storage -----

You could store the electricity through H2O electrolysis, directly burn for heat or reclaim through a fuel cell for electricity.

Electrolysis is only about 60% efficient. This is why a lot of effort is being put into Concentrating Solar Power which stores heat in molten salt.

https://en.wikipedia.org/wiki/Electrolysis_of_water#Efficien...

If solar becomes 3x cheaper than coal, a 40% loss isn't going to matter.

Better yet, those fuel cells can be "charged" wherever energy is plentiful then shipped as needed.

Sneakergrid anyone?

Is there an alternative to platinum yet?

Fuel cells sound really neat, and I had no idea you could actually by your own until a couple days ago. But the estimate of a small continuous use unit needing to have the catalyst replaced up to annually seems to make them a novelty for now (the existing units that is).

Interested in knowing more if you keep up with it though.

This would of course be incredibly inefficient. Maybe you were joking.

Electrolysis is too inefficient for in-place energy storage, but economies of scale could in fact make this acceptably efficient for long-distance energy transport; available solar resource varies by more than a factor of three even within the contiguous US. LNG tankers are already a worldwide business, and LNG equipment can be used to transport hydrogen, albeit at much lower energy density. (Generating methane or ammonia from the hydrogen to improve its transportability might be worthwhile.)

Transmission losses in the electric power grid are around 6.5%, but that's because the energy is typically only transmitted a few hundred kilometers; about 4000 km is the longest cost-effective distance for AC power transmission. LNG tankers can travel further than that.

Could also use the Hydrogen and CO2 to make methane and utilize existing infrastructure.

The Graduate, 2016: "I just want to say one word to you. Flywheels."

I can't really see widespread use of flywheels. They are good for high power, low total amount of energy situations, namely frequency regulation.

Bearings and motors/generators are very well understood, unlike lithium ion batteries, so I don't see them getting much cheaper as a flywheel industry expands.

Currently, the most expensive lithium ion battery is cheaper than the cheapest flywheel:

https://www.lazard.com/media/2391/lazards-levelized-cost-of-...

Flywheels are really cool. Especially when they explode sending out high velocity shrapnel and/or leaving a molten mass.

Still a flywheel powered car using three flywheels set at opposing angles (so their gyroscopic effects cancel) composed of tightly wrapped layers (so they cannot explode) would be awesome.

It isn't the shrapnel from flywheel failure that's hard to deal with. Frag containment, particularly of composite rotors that have high volume fractions of fiber and low volume fractions of binder, is actually pretty easy.

What's really hard to deal with is the angular momentum. Regardless of how the rotor fragments, angular momentum is conserved. What this means is that the failing rotor will produce amazingly high torques as it encounters stationary surroundings.

For typical rotor failures, you can figure that most of the angular momentum will get dumped into the environment in a few seconds as the fragments dig into whatever they hit and come to rest. So as a napkin exercise, if your flywheel spins up to full charge over a period of an hour, and then dumps all that momentum during ten seconds when it fails, you can kind of approximate the torque drama by multiplying the charging torque times the ratio of the charge duration to the failure duration. (I know the torque varies with the flywheel speed, etc. - this is just a disaster estimation exercise.)

The practical effect is that you have to secure the containment really well to make sure it doesn't break loose and roll through your facility on its way out into the street.

The only two ways I've seen of dealing with the angular momentum problem are (1) have two adjacent counter-rotating wheels that jam into each other when there's a failure or (2) have a containment shell that is free to rotate on one-time use mechanical bearings so it can take the failed rotor momentum and dissipate it over a reasonable time with a braking mechanism. Neither solution is cheap.

You could have a two wheel car if have a large flywheel.