I wonder if it would be possible to scale down pumped hydro to something the size of a tall water tower. Probably not worth it due to the small height difference and volume, and you'd also lose efficiency due to the smaller turbine, but I still do wonder what the actual figures are like for something like that. Especially in cost per kWh stored compared to lithium.
I saw a video like a month ago on YouTube where somebody pumped something like 250 liters of water on his roof (7 meters elevation, if I remember correctly) and let it run through a homebuild generator.
The power capacity of that setup was like that of an AA battery, if I remember correctly.
You need huge amounts of water and lots of height difference.
Yeah, pumped hydro is one of those power sources that doesn't scale down at all.
However, home scale conventional hydro is a thing. If you live on a hill with a creek that runs down it you can install a generator that produces a usable amount of consistent power for the cost of a few hundred meters of PVC pipe, a barrel, some connectors, and a small electric turbine. Search for micro-hydro if you want to know more.
Like all hydro solutions it's heavily dependent on the geography and only suitable for well under .1% of the population, but where it is feasible it can be a huge step towards off-grid living. Add in some batteries, maybe supplement with some solar panels, and you've got all of the electricity you need year round at a somewhat affordable price.
> Search for micro-hydro if you want to know more.
I've probably seen everything YT has to offer on that. :-D
While I really enjoy those videos and the "let's just do it" vibe with old washing machines as generators, I'm also somewhat happy that Germany has rather strict regulations on how to use water from streams and rivers. Some installations are quite disturbing for local ecosystems. But being off-grid in remote areas these solutions seem like a probable compromise.
Most micro-hydro setups seem to divert only part of the stream and only do it for a few hundred meters or so. These streams are typically much too small to be salmon spawning grounds or the like so the ecological impact seems pretty minimal. It's not like they're drinking all of the water, it gets returned to the stream right below the turbine.
Based on some extremely back of napkin math (e.g. doing one of those activities where you cycle to power a light bulb) that seems pretty far off. Was the flow rate extremely low? Have a link?
250 litres x 7 metres is 1750 kilogram-force-metres.
Wolfram Alpha [0] says this is about 5 Watt-hours, and gives some other handy comparisons:
> ≈ 0.45 × metabolic energy of one gram of fat ( ≈ 38000 J )
> ≈ 0.63 × energy released by burning 1 gram of ethanol ( ≈ 27000 J )
> ≈ metabolic energy of one gram of sugar or protein ( ≈ 17000 J )
> ≈ (0.02 to 0.09) × typical kinetic energy of a car at highway speeds ( 200000 to 900000 J )
> ≈ 0.5 × typical battery energy content of an alkaline long-life C battery ( ≈ 9.6 W h )
> ≈ 0.55 × typical battery energy content of a nonrechargeable Lithium-Thionyl Chloride AA battery ( ≈ 8.6 W h )
> ≈ 0.92 × typical battery energy content of a carbon-zinc D battery ( ≈ 5.2 W h )
Gravitational potential energy is just really, really low energy density. Fortunately it's reasonable to have pumped-storage reservoirs that contain trillions of litres of water.
I think a bit more than that, the article says 250 million gallons which is more like 960 million litres. That squares with the dimensions given: 15m x 15m x pi x 75m x 20 tanks = 1 million cubic metres = 1 billion litres.
Oahu uses about 225MW on average in residential electricity [0], so this could store an hour or so of excess capacity. That's not nothing, but you might want to 5x or 10x it to really smooth out demand and supply peaks for the island.
Note that you also need a set of tanks or a lake at the bottom of the hill. The tanks are 75m tall, so how are you accounting for that in your 120m of vertical distance?
I subtracted 100 from their position on red hill at 220-ish meters to account for the fact that the tanks are buried in the ground (I don't know precisely where).
>Oahu uses about 225MW on average in residential electricity [0], so this could store an hour or so of excess capacity. That's not nothing, but you might want to 5x or 10x it to really smooth out demand and supply peaks for the island.
This solar plant & battery system was completed last month. The battery is able to store 144 MWh. This shows me that planners and people thinking about this are willing to invest & commit to power solutions at this scale. I don't know if you are aware or have read up on the red hill controversy. These tanks were used to store fuel by the Navy. They leaked earlier this year and poisoned the Red Hill aquifer and have caused quite a bit of local controversy.
I'm struggling with the idea of writing a letter to the editor outlining this concept, but also emphasizing how converting these tanks to a water storage/ energy storage facility would improve local resilience to drought, bring down water costs, improve environmental conditions etc.
I've seen some people talk about adding a heavy weight on top of the water in a pumped hydro system - something like a big concrete lid on top of a cylinder type reservoir.
They do say it is about equivalent to a single AA alkaline. It is not exactly an optimized efficient setup, but still, it gets the point across. Pumped hydro is a grid scale option, in places that have the ability to store a lot of water.
250L x 7m is 17kN, or about 5W/h, or about 2 AA 2450mAh cells from IKEA.
Pumped storage scales well though, as the stored volume scales squarely to the required container material (e.g. storing 25,000L will only take 10x the material while storing 100x the energy).
It can work, it's just a cost equation. You need a large amount of mass to get meaningful amounts of energy stored. So pumped hydro is nice because you are basically using natural reservoirs and some cheap pumps and pipes.
A water tower needs to be built first and only stores a limited amount of potential energy. The problem is not the efficiencies but the cost of building a big enough tower. A tower with the same capacity of a pumped hydro reservoir would be insanely expensive. Or alternatively building many thousands of smaller towers that add up to the same capacity would also not be cheap.
There are a few alternative gravity systems being tested out in the form of e.g. big cranes with weights that drive generators, mine shafts with similar systems, etc. Proving they work efficiently is not the issue. It's just scaling their deployment cost effectively.
Why would you need a water tower? If you're in flat geography at sea level, you could probably access tidal energy to drive generators. If you're flat and far from water, couldn't you dig a shallow hole and a deeper hole, and put the generator and pumps in between? I mean, even in Kansas it's not that hard to find an elevation difference of 50 feet.
It's just an example from the comment I replied to was using; I agree it's not a very feasible thing to do. But I used it to illustrate the difference in cost between that and pumped hydro using natural reservoirs. The point being here that you'd need a stupidly large amount of water towers to come close to the amount of energy you can store in a pumped hydro reservoir
Whether it's a water tower, a big crane, or whatever that holds the mass that you then use to drive a generator. You have to build it or use nature to have some natural height difference. Building infrastructure like that is expensive. So nature is preferable and way cheaper.
I come from a relatively flat part of the world; so that's not really an option there. Gravity based batteries are nice where they can be done cheaply. But otherwise probably not ideal.
Say you have a 1100 watt microwave. A watt is 1 joule per second. That gives you enough energy to run that microwave for about 11 minutes.
A better example would be existing municipal water towers. A 500,000 gallon water tower that is 129 feet tall costs about 2 million dollars. This is based on a public notice I found for a city in the USA.
That is 1892700 liters and 39 meters. I am guessing that the average height the water actually falls is 25 meters.
The average American household uses about 30Kwh per day. Which is 108,000,000 joules.
1892700 * 9.81 * 25 * .80 / 108,000,000 = 3.4
So 2 million dollars for 3.5 households-days worth of energy.
I don't know if having 1 days worth of energy is too much or too little per household.
And beyond the cost of the tower itself you need the generators and lines and holding pond for the water. Or underground tank or whatever you want to use.
It's very expensive.
However it is probably the cheapest option right now as far as current technology goes.
It would probably be cost effective for a off-grid shack in the woods, though.
Hmm that is an interesting idea, but it would be even more expensive.
If you take that water tower, place it on top of a 80 m hill, then dig down for say another 40 m for the second tank you now have a total of around 160 m of height difference. That would bring it up to 22 households-days or if my conversions are correct, 660kWh, which is not insignificant.
Then again if google is correct the price for an equivalent lithium bank would be only like 100k so it's not even close.
That website is full of deranged ramblings which are largely disconnected from reality. Case in point:
> MY NOTE: well whoop-dee-doo. China generates 16.2 trillion terawatt-hours (TWh) a day. That’s 64 billion times more than all of the open-cast mines can provide. Better start digging more holes!
16.2 trillion terawatt hours a day is ~4 million times the total global insolation or 35 billion times total global energy usage. This is more wrong than asserting that a single man on a bicycle generator could produce our entire primary energy worldwide.
I find it's a useful barometer of what crazy disinformation is going to be spread next because it has been SEO'd so well.
It also often covers interesting topics, and many of the stories are actually about something important or useful.
Additionally many of the things it 'debunks' are actually terrible ideas, and some of the arguments it uses are valid.
For gravity storage the costs and inefficiencies are a bit prohibitive for the use case of an unsubsidized high capacity (more than 1 day) option, and it's not really competitive with batteries for less (and is highly power limited).
Joule for joule, pretty sure I'd rather be standing 10 feet from a failed water tower than a failed pressure vessel. 1/2 m*v^2. That squared gets you every time.
That's the problem with the US capitalist approach to renewable energy. "Can I downsize this to a scale that makes sense in our economic reality?" Dang, there dies another viable idea. Instead of changing the status quo we have some warm libraries and city halls.
