# Alternatives to batteries for "residential" power?

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.)

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.

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.

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.) • Another crazy idea: Nickai Tesla was working on an energy harvesting antenna which would pick up the sun's ionizing radiation swirling around Earth. He said (quoting a Youtube educational video I saw yesterday) that there would be enough energy not to need gasoline powered generators any more. We don't know what happened to his research as the government took everything from him as he died. I will spend some time finding out more, since there isn't that much info available anywhere. Maybe buy into John Searle's engine. I am sure that the patents can be bought for a fat sum. Unconnected homebre May 1, 2020 at 8:14 • Comments are not for extended discussion; this conversation has been moved to chat. – L.Dutch May 2, 2020 at 20:00 • worldbuilding.stackexchange.com/questions/23118/… May 3, 2020 at 0:41 • @Mazura, interesting, but not quite the same. That's asking about storing energy for centuries. I only need to store energy for days, maybe months. May 3, 2020 at 17:37 • Flywheels is currently answer #4 of 19 at the link. IMO, if 'energy storage' ("store energy") is in the question, Tim B's answer has you covered, because it('s short and to the point) has a link to Energy Storage, Wiki, in which you will find a link to FES, while the accepted answer here links to some random PDF. - The 'dupe' only offers an additional caveat; yours is a subset of it. May 6, 2020 at 0:19 ## 14 Answers 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. • Spiffy! I hadn't seen this. I imagine they might want 6-8, though; that would be 2+ days of theoretical capacity with zero input. (Obviously, in the event of an outage, they are going to try to limit consumption, e.g. turn off the pool...) Apr 30, 2020 at 17:05 • Sheesh... I kept scratching my head, trying to figure out the difference between "nameplate power" and "nameplate energy" and finally realized they are different units (KW vs KWH). So, okay, my guesstimate power consumption was 4 KW average; call it 8 KW peak. That means one of these suckers can supply the residence, but only for 4-12 hours. So, yeah, probably want 12 of them. According to Google that probably costs around$130k... "a lot", but not at all implausible. Apr 30, 2020 at 17:55
• Unless you story depends on having a super accurate representation of the energy infrastructure of the challet, your best bet is to not bore the audience with the details. Even then if your within an order of magnitude you'll probably be good enough. Apr 30, 2020 at 20:51
• Meh, I'm anal that way. Sure, it's the difference between a character on a house tour going "wow, there's six of those?" vs. "wow, there's a dozen of those?", i.e. this entire question will likely boil down to a single sentence, but... like I said; anal that way 😉. (To (mis)quote the shirt, 'I don't suffer from World Building Disease. I enjoy every minute of it!') Apr 30, 2020 at 21:02
• At less than 5 feet on a side, if you've room for a large pool and waterfall, fitting a few of these shouldn't be much of a problem. May 1, 2020 at 14:12

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.

• pumped storage hydropower is used for large power plants now, but would work just as well on a small scale, note the reservoir does not need to be natural, a series of tanks or man made pond works just as well. you can buy premade 300,000 gallon tanks. two of those supply everything you need and then some. it can also double as grey water storage for irrigation,bathing, ect. .
– John
Apr 30, 2020 at 16:23
• I have seen this done on a small scale in the mountains for a ranchette type home. A portion of the water higher on the mountain that would otherwise come down a stream instead comes down a pipe and turns a turbine at the bottom. Nice because flow is constant - excess just comes down the stream. And cheap: pipe, little turbine. Apr 30, 2020 at 16:25
• I'd also note that pumped storage (as with most other physical power storage schemes) takes time to ramp up and ramp down. When the family turns on their stove, the electricity demand instantly increases, but the pumped storage plant might need a couple minutes to open the sluice gates and let the turbines start generating. So, you'd still need some sort of battery bank to even out those second-to-second changes in demand. May 1, 2020 at 11:32
• Throw in an Archimedes screw instead of a pump for more coolness factor. The screw could potentially double as the turbine as well; it's driven by an electrical motor to pump up hill, but then when power generation from other sources is low, it is instead used as a generator.
– user43774
May 1, 2020 at 14:24

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".

• I saw an instance of that exact article (on a different site). I'd be looking for 100x their system, and the impression I got was that it may not scale well. I'd have to call this... "plausible", as compared to flywheels, which I'd consider "proven" (albeit with a definite limited lifespan that admittedly CAES doesn't seem to have). Apr 30, 2020 at 19:18
• The higher the pressure, the higher the amount of energy stored... and the lower the efficiency. Compression heats the air, which then cools (and you lose energy). The thermal insulation cost will be horrendous. May 1, 2020 at 15:37
• @AdrianColomitchi Right, the thermal energy cost rises with the energy density of the system, which hurts efficiency unless you can use that byproduct heating/cooling for something useful. That's why I suggested needing many of these compressed air "batteries", rather than just trying to cram more air into a single tank. May 1, 2020 at 15:54
• @AdrianColomitchi, which is why the system the cited article "recommends" operates at about 10 bar. May 1, 2020 at 17:23
• @Salda007 -- instead of a dedicated HX for pool heating, I would have the intercooler drive a hydronic source loop in a multi-source buffer-tanked hydronic system. Using the buffer-tank basically gives you an "any source mix - any load mix" flexibility that you just can't get if you're matchmaking sources and loads ahead of time in your system design. May 2, 2020 at 16:48

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. • @Adrian Scam or not, the idea is something that would work. It seems to be about as efficient as pumped-storage hydroelectricity, and it could be built on flat ground. May 1, 2020 at 10:52 • @TannerSwett there’s little evidence that it would work. Conceptually? Yes. Realistically? No chance. – Tim May 1, 2020 at 18:28 • @Tim I look at at from the perspective of "What's plausible enough for a sci-fi reader to accept in a story?" I think Energy Vault passes that test, regardless of whether or not the company becomes a commercial success. May 1, 2020 at 19:56 • it wouldn't work, the energy demand to move the crane eats all your recovered energy. their recovered energy ignores rewinding the cable, pivoting hte crane, and all the automation needed for it to work. it also assumes perfect weather conditions. – John May 2, 2020 at 19:11 • Flywheels have a horrible energy density problem, and a horrible bleed rate. NASA uses them in some situations because the dark time for a satellite is short and you can make them to have a longer life than batteries. (And when there is no repairman durability become quite important.) May 3, 2020 at 0:55 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. • this really depends on the OP being wrong about the energy needs, if they need heat, have a workshop, or have farmland, such energy demands are not unreasonable. – John May 2, 2020 at 19:07 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. • Solar must have storage if it's your only power source; it isn't sufficiently reliable (also, y'know, night). Pretty sure the same is true for wind. I'm not sure but what even generators have short-term storage to handle load fluctuations, and they're less efficient at low load if you aren't storing the excess. Even grids need storage to handle fluctuations. The nice, clean, reliable power that many of us enjoy (and that my chalet-owners certainly want) is because of storage. May 1, 2020 at 14:35 • Also, my current plan is that the generation is separated from the house; storage — which will be much closer to the house — is also backup in case something happens to the lines. May 1, 2020 at 14:35 • Mountain streams freeze in the winter. During the summer, you might be able to get away with just solar+hydro, because your big load (cooling) hits during the day. But in the winter, your big load (heating) hits at night, when you've got neither solar nor hydro to power things. – Mark May 1, 2020 at 23:01 • Heating wouldn't be an issue. What mountain chalet wouldn't have a fireplace? A well insulated house doesn't even need that big of a fireplace to keep warm. May 2, 2020 at 21:13 • @Mark Depends on where the place is and the size of the stream. The OP doesn't describe how cold the winters are or where about the house is. May 2, 2020 at 21:18 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. • D'oh! Why didn't I think of this? BTW, any info on storage capacity of such systems? Apr 30, 2020 at 16:56 • There are quite a few ways to store hydrogen: reader.elsevier.com/reader/sd/pii/…. A lot of safety issues are only present when you want to drive them around, that are not present for simple storing outside in a yard. People also have huge propane tanks in their back yard. May 1, 2020 at 12:02 • Yeah, I'm thinking they'll have a 1kG tank for propane. (Big house, big heated pool. I'd certainly cut that by half without the pool.) Actually, what I read for hydrogen is that it's the fuel cells that are problematic. May 1, 2020 at 14:39 • Fuel cells is a relatively new technique, but I don't know specific problems with them? May 1, 2020 at 14:40 • @Matthew, fuel cells using atmospheric oxygen wear out quickly, because of the other things in the air. If you're making your own hydrogen through electrolysis of water, you can store the oxygen as well and avoid that problem. – Mark May 1, 2020 at 22:57 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. • That's nice in theory, until you realize that 1850 tonnes is about the same as the weight of ten average size houses. Good luck devising a reliable system to haul that up and down 100m of mountain. May 1, 2020 at 1:36 • @alephzero: Pumps and Gravity? Seems to work pretty well for hydroelectric storage. May 1, 2020 at 8:27 ## 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. • This is a really cool idea! May 4, 2020 at 16:34 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. 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 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. 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. • Conveniently, "motor" and "generator" are synonyms... it almost goes without saying that a motor/generator is connected to a flywheel. The main issue is I'm not sure if there are systems with the available capacity (flywheels seem to be high output, but not very high density systems, which is the opposite of what I need), and/or if they would be prohibitively expensive. I can probably see spending$1M, but not \$100M. Apr 30, 2020 at 16:59

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.