Alternatives to batteries for "residential" power?

Question

So... I have a family that lives in a remote mountain chalet. They need utilities. I think I'm set for gas (propane), water, septic and communication, but I have a question regarding electricity. Now, I plan to give them super-sufficient generating capability (combination of wind and solar) and may even hand-wave them being "on grid" (so they can sell their extra capacity). However, they would like to be able to supply themselves without the grid and without resorting too much to propane-powered generators (though I expect they'll have one for backup).

Now, this means they need a fairly capacious ability to store energy in order to meet demand in the face of a fluctuating supply (especially for solar). The "obvious" answer is a really big battery bank, but batteries are so... pedestrian.

What could they use instead of batteries? I'm guesstimating the household's energy use to be about 4KW average (big house, big pool), though figure in a pinch they could cut this by at least a third. Based on that, I figure they would want a minimum 100 KWH storage capacity, though ~500 KWH would be much better. Bonus points for long lifespan and low maintenance.

I am leaning toward 'flywheel', but I'm not sure how feasible this is, and I'm open to other ideas. (Am I completely insane to not just use batteries?)

(This is an alternate reality, and the family has considerable financial resources and political clout. They can probably get away with something that ordinary people would have trouble getting permitted.)

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Postmortem:

First off, thanks, as always, for all the interesting answers! Lots of interesting stuff here that hopefully will help others as well.

For my purposes, however, my characters would rather not engage in a massive geological engineering project when batteries could do the same job at a price that is comparably negligible. I'm also looking for something where the storage can be very close to the chalet; my objective (someone else's may differ, which is why I love to see these answers even if I don't use them!) is to have backup in case of e.g. a delivery line breakage. Also, since they're sitting on a mountain, geothermal seems "iffy"; there's some topsoil, but they're a lot closer to sitting on solid granite than your average house.

That all steers me toward something that can fit in a (large) shed; batteries, HFCs, CAES, or my original idea, flywheels. Both HFCs and CAES seem "fiddly" by comparison, and it's less clear if the technology is well-proven. Batteries, of course, are known to work, but are somewhat limited in lifespan, wear out a bit more from charge/discharge cycles, and as stated, they're boring 😉. That leaves flywheels, which are well-proven, fairly efficient, and absolutely meet the desired cool-factor.

Why I accepted sphennings' answer:

Mazura says:

Flywheels is currently answer #4 of 19 at the link. IMO, if 'energy storage' ("store energy") is in the question, Tim B's answer¹ has you covered, because it('s short and to the point and) has a link to Energy Storage, Wikipedia, in which you will find a link to FES, while the accepted answer here links to some random PDF.

(¹ I assume Mazura is talking about this answer to a different question?)

So, there's the "minor" issue that the answer I assume Mazura is referencing isn't even posted here, which makes it hard to accept. However, even it it was, I would still accept sphennings' answer.

First, I'm not sure the order in that article has any significance, and anyway the first two are impractical for a "family residence" as noted above. Second and more pertinently, because Wikipedia is, at least for my purposes, shallow. The "rule" for accepting an answer is to accept the one that is most helpful, and that was sphenning's, hands down.

Wikipedia provides generalities, which is nice if you're trying to understand how something works. It's less helpful for making concrete decisions. That "random PDF" that sphennings turned up (and which I hadn't managed to find on my own, so thank you again!) pointed me at, not a vague technological overview, but a specific, existing system that is pretty much exactly what I had in mind. There's a significant difference between some vague notion of "maybe something using this technology could work" (Wikipedia) and "this specific, existing product meets my needs" (sphennings' answer). Unlike Wikipedia, that not only gave me hard numbers on what sort of system I would need, but it gave me a price tag... which is not trivial (around $130k²), but for my purposes, totally within reason.

Again, the criteria for acceptance is the answer that "solved your problem or was the most helpful in finding your solution". While I'm thankful for all the answers (and have upvoted accordingly), IMNSHO sphennings deserves (and has received) the credit for the answer that was most directly helpful to my specific problem.

(² Not an actual quote from the company, but based on a Google result that mentioned what price point the company is "trying" to hit. However, since this is for a story that doesn't take place in the real world, I don't mind hand-waving and assuming that someone hit that goal, if not better.)

Answer 1

A quick google search uncovered a pamphlet (or at this page) from a company offering a flywheel storage system with a capacity of 32kWh. A bank of 4 of these units would exceed your 100kWh capacity requirements.

These units have a 30 year design life, and are designed for continuous functioning with no limit on discharge cycles per day.

Judging by this it's entirely plausible for an eccentric chalet owner to have flywheels for their energy storage needs.

Answer 2

Pumped Storage

If your family has significant resources, and access to two water reservoirs (One at a high altitude up the mountain, the other lower down), then they could have a custom pumped storage system installed. When they are running an electricity surplus, they pump water from the lower reservoir to the upper reservoir. When they have an electricity deficit, they allow water to run through a piping system from the upper reservoir, driving a turbine, and generating electricity.

As a rough calculation, the energy stored is equal to $mgh$, where $m$ is the working mass of the water in the reservoir, $h$ is the height difference between the reservoirs and $g$ is the acceleration due to gravity ($9.81 ms^{-2}$). A back of the envelope calculation suggests that a pair of reservoir with 2000 tonnes of water, separated by a 120m drop, has an energy storage capacity of about 654kwh. One of the nice things about water is that it's pretty dense, so if we approximate a lake as roughly hemispherical, then a 2000 tonne lake is only about 21m across, which is not egregiously oversized. Even allowing for inefficiencies, such a system could plausibly meet your requirements.

Additional bonuses are that if the upper reservoir collects sufficient rainwater water, then it may be a net generator over time. It may also be a source of drinking water, depending upon cleanliness.

A big downside is that it would probably be expensive to set up, as it would require a fair amount of civil engineering to install, when compared to a battery system.

Answer 3

Compressed air energy storage

Use excess energy to run an air compressor, which fills a storage tank with high-pressure gas. When you you need to draw energy, simply release some of the pressurized gas to generate electricity. The system has some losses due to adiabatic heating/cooling, but that thermal energy can also be put to use heating/cooling the home.

One advantage of this system over batteries is that its storage capacity does not degrade over time, and the storage medium does not normally need to be replaced - it will last for a very, very large number of discharge cycles, while an electrical battery's capacity diminishes over time. In the long term, it could be a more cost effective solution than batteries, although the upfront cost is typically higher. Compared to other potential energy storage systems, it does not require large masses or a means of vertical displacement, and compared to kinetic energy storage systems, it's not as prone to frictional degradation over time. Portability doesn't seem like a very big selling point for a home energy system, but I'll also point that you could easily take this "battery" from place to place, unlike any mass-based system like a flywheel or pumped storage.

This site goes into much greater detail about the mechanics and efficiencies of residential compressed air energy storage. For a research-grade system, they quote a capacity of 410Wh for a system that takes up 0.6 cubic meters of space. You'd need a lot of these to get to the KWh-level storage required, but with plenty of space and money, and some additional optimization of the technology, one might be able to make it work. Compressed air energy storage has also been used at a much larger scale, in cities like Paris, Dresden, and Buenos Aires, although personal residential applications seem rare. You might need to do a little handwaving to take this from "plausible" to "practical".

Answer 4

Flywheel, water tower, or stack of blocks

If you want something with a little more flair than a battery pack, you're in luck. There are several options for your "cost is no object" energy storage.

Flywheel

You could use a flywheel. According to Scientific American, there's one proposal that proponents say could "deliver distributed and highly scalable storage for around $1,333 a kilowatt."

Water tower

You can use electricity to store weight above and then use the power of gravity to turn motion back into electricity. Pumping water is one option. Excess electricity runs a water pump that pushes water uphill into storage. When you need more power, the valves holding the water in storage open up and the water runs through a turbine to generate electricity. You want something stylish, so check out these spiffy water towers for inspiration. The family might enjoy a very large reminder of their status and off-grid preparedness. You'd need a pretty big tower (or an uphill lake) but hey, they're rich, right? Since this is a mountainous area, they should build the tower on the tallest point in the area, both for storage efficiency as well as showmanship. Perhaps they'll build their own Peachoid or maybe they'll go with a classier look like this tower from Austria.

Stack of blocks

It sounds odd, but you can store energy by stacking blocks like you're a megarich toddler. The company Energy Vault has a neat concept for a system that would give your family a very visible representation of its wealth and preparedness. When the system detects excess electricity, an electric crane starts stacking heavy blocks. When the system needs more electricity, the crane grabs a block and lowers it to the ground, generating electricity. According to Quartz, "The round-trip efficiency of the system... is about 85%—comparable to lithium-ion batteries which offer up to 90%." The conceptual rendering below would certainly give the family something to show off to the neighbors.

Answer 5

I think you're drastically overestimating their energy needs:

The pool can be heated via direct solar heat. Using electricity from solar just to run a heater is not efficient, instead they can circulate water into what is basically a radiator on a sun-exposed surface. There is no need for heating to run 24x7, as water retains a lot of heat. (Especially if the pool is insulated, and in a greenhouse-type environment.)

The pool can also be used as a heat "battery". Because water retains a lot of heat, they can heat the pool during the day, then use that heat during the night to keep the house warm. Heating and cooling is the most energy-intensive part of a house, and not having to worry about battery capacity for that is very useful. (I'm going to ignore cooling, because if they need cooling, the solar panels are probably working just fine.) Combined with decent insulation and proper architecture, there may not be any heating needed most nights, with the only active component being a small pump.

The remaining electrical usage is basically lights, appliances, and electronics. If they want to be prepared for off-grid use, they would probably have LED lights, and laptops have a battery built in already. The only real appliances to worry about at this point are:

• Fridge/freezer (surprisingly efficient, especially if they bought smart.)
• Stove (They could have an emergency propane stove, or simply not cook at night.)
• Washer/dryer/dishwasher (There's no need to wash clothes/dishes at night.)
• Hot water heater (The big load is from daily morning showers.)

Basically, as long as they're willing to forego heavy usage of power at night when off-grid, they can get away with a surprisingly light battery system. You can't just take the energy usage of an on-grid house, and assume that an off-grid house will be the same; they have different priorities.

That being said, if you still want an excuse for something bigger, they could be using arbitrage on the power grid: They have some kind of energy storage, they charge the system when power is cheap (at night, usually), then discharge it into the grid when power is expensive (mornings and evenings, usually). This also lets them sell their renewable energy when it's the most expensive.

Answer 6

Why have storage?

A remote mountain chalet, I'd have a residential hydro electric plant with wind and solar

You could also have biogas production from the family's waste and green material and run the generators from the methane instead of propane. This is doubly viable if the family has cattle and/or horses. The biogas production could be set in the barn to use the animals' waste with a side effect of heating the barn in winter.

Also a super efficient house means a super insulated house. Triple glazed gas filled windows. Fully insulated. You could even bury an Earthship style house into the side of the mountain so it's protected from the cold, avalanches and forest fires

An Earthship uses the front section as a greenhouse to grow food even in the middle of winter and requires very little heating to stay warm even in the coldest locations.

The pool and spa would be indoor and could be heated from waste heat from log fire or waste heat from a biogas plant.

You can do a lot without needing storage.

Answer 7

Hydrogen from solar panels and Fuel Cells

So you generate energy, you just need to convert it into a long term energy source. Belgian scientist have tested making hydrogen fuel directly with solar cells: News articles and link from KU Leuven itself.

With a "simple" fuel cell and the hydrogen you can create electricity any time you want.

Your only limitation is the amount of hydrogen you can store. But with some precaution you can store quite a lot of hydrogen quite safely.

Answer 8

Weight. Either rocks or water. It’s inefficient but it’s simple (which is pretty critical if you’re in a remote part of the world), and doesn’t rely heavily on exotic electronics to work well.

When you have spare energy use it to do work pushing the weight uphill. Pump water into rooftop reservoirs. Use big motors to lift heavy weights from the bottom of a deep well to the top. When you need energy reclaim the potential energy you’ve stored (mass times height travelled times g) by letting the water flow down through a turbine or turning that motor into a generator and slowly dropping the weights.

If you can move 1850 metric tonnes of stuff up 100m then in an ideal world you’d have 500kWh stores. In reality it will probably be closer to your 100kWh lower limit (and you’d have spent more energy getting it up there). It’s stored energy though: once it’s up it’s up. If you can move it higher you need less weight, if you can lift more weight you need less elevation.

Now, obviously this method of energy storage works much, much better if you can do it at large scales and in appropriate locales. There’s a reason hydroelectric dams and reservoirs aren’t on every hillside in the world. A mountainside chalet though? Sounds like you have steep mountains on your side. And for an isolated chalet as part of a comprehensive power storage system a few big holes and heavy rocks might just cover you for a short time.

Answer 9

Rail Storage

Besides pumped storage, the other option that uses the mountain terrain would be rail storage. As a bonus, it can also be used as the means of transport.

The gist of it is a locomotive with heavy concrete blocks attached going up and down a hill: https://www.vox.com/2016/4/28/11524958/energy-storage-rail Getting 100 KWH is fairly reasonable by moving 100 tons down 500 meter elevation assuming 70% efficiency. Turns out the weight of a train car is around 100 tons, so you don't need a massive train for it.

Basically you can have a very eccentric family that get to their home by an electrified rail. All you need to turn it into energy storage is fill a spare train car with concrete and park it at the top. When they're home and want the energy, they attach the concrete car and let the train slowly roll downhill. If they want to leave the house, they won't need the energy, so detach the concrete car and just use the lighter locomotive for transport. Hopefully they get back by sun-down so they have the energy generation to get back to their house! Or they can have 2 trains running on parallel tracks, but that would double the cost.

Answer 10

Electrochemical generation of cellulosic ethanol.

https://en.wikipedia.org/wiki/Cellulosic_ethanol

Celluosic ethanol is a real thing, but currently uneconomical because yield is not worth the energy inputs. But in your fiction you could have something like a biofermenter for your people in which excess solar or wind is plowed into hydrolysis of cellulose, generating ethanol electrochemically.

They could call it the still, and the product "moonshine". It would be flavored by whatever feedstock they used to produce it.

The moonshine they make (out of hay and leaves) is used like propane for a generator and also directly to power vehicles.

This is chosen because it is not from our world, but neither it is bizarre science fiction - a near future or close parallel dimension technology appropriate for wealthy people in the country.

Answer 11

There are many good answers here, so I shall add some oddball.
Super Springs. You wind up a fancy high density spring while you have excess power and tap into that energy when you are low. There are some works by Paolo Bacigalupi exploring this, such as the windup girl

Answer 12

A mill run with storage pond, and using a water driven generator when the wind and solar energy do not bring in enough energy.

You only need a small stream to top up the mill pond, as long as you need only relatively little energy.

If the stream is not giving enough power with just one generator, you can make a series of ponds, with generators between the levels. The same water can be used many times, even without getting it back up the slope.
But you could pump it back up in times where the solar and/or wind power give more energy than you need, making it into a kind of power storage.

This would of course be in addition to solar and wind energy, and the power saving tips given in the other answers.

Answer 13

Any sort of non-battery storage feasible at the technological capacity of your characters would almost certainly involve storing energy kinetically, and so I would agree with your assessment that flywheels are the best system. Now, referencing you asking whether not using batteries is insane: to an extent, yes. Without a vacuum chamber in which to spin the wheel, frictional losses to air would seep away stored energy, however the flywheel remains the most efficient kinetic storage system regardless. Implementing such a system would not be difficult at all given the circumstances of your characters. You need to attach an electric motor to your power generation source and use that to spin up the wheel, and then use a rotating magnet mounted on the wheel’s axis to induce a current in a wire that you can move or remove depending on whether or not you want to take out energy or continue storing it.

Answer 14

A fuel cell would probably be the most convenient way of doing that.

Alternately, if a fuel cell is too close to a "battery", use a water tower.

Excess power is used to pump water up to a container at the top of the tower. When power is needed, a valve is opened and water falls down, powering a generator.