Long post warning!
I have perhaps an unusual property here with 200’ elevation change up a very steep hillside. At some point I will be putting a nice water storage tank up the hill to provide a buffer, so that the household water can be gravity fed without pressure tanks. A pump in the cistern at the house, set to run occasionally, would refill the uphill tank which then gravity feeds the house.
It occurs to me that a much larger version of this could be used like a large battery for off-grid solar. I did a little figuring today, and wanted to share my conclusions. Obviously all these numbers are specific to my 200’ hillside, but anyone with an elevation like this could consider a similar setup.
Pumped hydro storage systems are nothing new, they exist at utility scale all over the world, using massive lakes and hydroelectric dams. The challenge here is getting a small scale setup to be large enough to be worth the trouble, and weighing the inefficiencies against the longevity of the battery (indefinite). Plus the cost, of course.
With stored water and a high hill, we want a high-head low-flow turbine style, which means a Pelton.
Some resources for anyone thinking about this.
- A New Zealand company is selling these DIY Pelton turbine kits for a reasonable cost, $1400 + shipping:
- A calculator on their site helps simplify the calculations tremendously:
Also a rule of thumb I found online, which matches up fairly well with the Powerspout calculator outputs:
Power = 1/10 x Head x Flow
This formula is based on imperial units: power in watts, head in feet, and flow in gallons per minute.
OK, let’s imagine I set up an above ground pool at the top of the hill, and another one at the bottom. a 28’ pool 54" deep holds roughly 20,000 gallons. I can get those all day long on FB Marketplace for around $300.
Playing with the calculator, the lowest flow rate that still maintains good efficiency is around 30 GPM. I sized the system for that, with the thought of steadily charging a small buffer battery bank. It could also be throttled up and down to match the household load, but you will lose some efficiency there, especially while throttled down very low.
At a flow rate of 30GPM, the turbine should generate about 522 watts. We can run this way for 667 minutes, or about 11 hours. That gives us 5.8KWh of usable storage.
A return pump must be added to get the water back up the hill again. It’s harder to find an off-the-shelf pump to do this, but after a lot of reading pump charts and efficiency curves, it looks like a 3HP electric motor with the correct style pump should put out 80GPM at 100PSI. The pressure is needed to get up the hill, and the flow is needed to refill the tank within 4 hours, which is our average full-sun hours. This motor will take around 2500w to run, after 4 hours it will have used about 10KWh. This is the downside to this setup, you will lose around 40-50% of your input energy to efficiency losses. More on that later. This pump will cost somewhere around $1200.
I would need around 500’ of 2" pipe to get down the hill, another $1200. Adding another thousand for valves, wiring, and misc. pieces, we arrive at $5400, for our 5.8 KWh storage battery. About $1K per KWh.
Expanding the capacity of the system would be cheap, two more pools for $600 to add another 5.8 KWh, bringing us to $517 per KWh. Or dig a really big pond, which I may want to do anyway. Nothing else need change, the turbine can flow well over 30GPM efficiently, if you needed higher output - but I think 500w continuously is enough for our no-solar usage periods, mostly overnight.
An interesting effect of this is all the ways you can move water around. I could set up a high-flow woodgas powered pump to run for a few hours and recharge the system on a cloudy day. And of course the stored water is useful for lots of other things, like gardens and animals - we wouldn’t drink it, I’m not sure about the quality of water that’s been pumped hundreds of times.
Now to the cost vs efficiency question. I can buy off the shelf LIFePO4 BattleBorn (very nice) batteries for $1000 per usable KWh, and have no moving parts, no maintenance, and have a 90% efficient round trip instead of 50%. They are warrantied for 10+ years.
Lead-acid is far cheaper, at about $300 per usable KWh. But the lifespan is short and they are high-maintenance. You have to over-size the batteries by twice, because they can only discharge to 50%. However with good maintenance, you can get started on a tight budget.
So I probably won’t build this system. It was a fun exercise, but lead-acid is better price-wise and more conventional. Then upgrade to lithium as budget allows.
But here’s the one big advantage, the reason it won’t quite go away in my mind… repairability.
I could build, repair and maintain the entire water storage system myself, with low-tech components, and if maintained it will last forever. In a long term SHTF scenario, this would be the system of choice. I cannot make or repair lead-acid or lithium batteries, and they will all eventually need replaced. With solar panels being so cheap now, I could easily oversize the system to compensate for the low storage efficiency. Is that the price of longevity? Just throw more panels at it?
I’m curious what you folks think. Has anyone played with pumped storage?