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The project

I'm thinking about building a cheap DIY hydro energy storage (like a battery) system. I can build it but so far I can't really figure out the calculations to validate my idea and where to start mathematically.

I'm considering doing a small test with two equally sized water containers. To store, pump water up, to regain the energy, let it flow back down.

The main question is How to determine the efficacy and potential energy of hydro power energy storage? But below are some sub questions I came up with that I think need to be taken into account.

The tanks

For this I wanted to start small with two 220 liter tanks with the dimensions of height 108cm / diameter 57cm / opening diameter 37cm as shown below.

Tank option 1

Later I could consider using two IBC containers of 1000L instead of 116cm height / 120 cm width / 100 cm depth. as shown below.

Tank option 2

My plan is to stack them on top of each other (within some frame). Either directly on top of each other or by digging in the lowest tank and elevating the highest tank with a range between them of 0-3 meters in between. I suspect the further elevation contributes to a higher water pressure.

I would like to know what the ideal location would be to let the water flow out and if the angle/position of the tank matters.

The pump and generator

I did not choose the pump yet but considering a given (I guess) the flow rate of the pump, efficiency (loss) percentage and power usage. I would like to know: How much power (watt hour) and time would it take to pump all water from the lowest tank to the highest tank?

Same for the generator. Considering a certain volume, generator flowrate, maybe pressure, efficiency height and or pipe diameter sizes: How do I determine the right values for this calculation, what calculation do I use to determine how much power (electricity) in watt hours I could potentially regain from letting all water flow back down from the upper tank and how much time that would take?

With those two values, obviously I want to determine the approximate efficacy of the system, the potential loss of power, how much power to use to fully "charge" and how much power to regain to fully "drain".

The pipes

Perhaps the most important question. How do I determine the optimal pipe path (spiral down/straight or diagonally downwards), pipe diameter, pipe length, nozzle size (reduction size or ratio), distance from nozzle to generator blades, to increase pressure as much as possible? Assuming higher pressure results in higher RPM of the generator, results in higher output.

The liquid

I'm planning to use 23.3% saline water with a freezing point of -21.1C. I asked a related question about that here: Can saline water be used in a hydro storage system and would the ambient temperature or higher liquid viscosity affect performance?.

I guess above (sub questions) need to be taken into account for a realistic calculation.

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  • $\begingroup$ Too many questions in one post. Consider to break off each segment and build a series of questions based on the answers in sequence. Regarding efficiency, note that commercial/municipal systems using water storage depend on the difference in electricity rates costs day versus night. Pump up at night, spin generators during the day. $\endgroup$
    – fred_dot_u
    Jan 12, 2023 at 9:48
  • $\begingroup$ I guess you're right. But I'm not sure how to split these into multiple questions yet at the risk of losing the bigger picture and leaving out too many details. I would like to mainly focus here on the total Wh in, and total Wh out and the time needed for both. Running it all from solar power or using the grid prices smartly is of course interesting but I'm not sure that should be taken into account directly as higher technical performance is the main goal here. $\endgroup$
    – Bob Ortiz
    Jan 12, 2023 at 10:02
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    $\begingroup$ I would take the output just above the bottom so any sediment will not damage or block the turbine. Lots of sites show how to calculate the piwer available: Power = flow rate * gravity * height. $\endgroup$
    – Solar Mike
    Jan 12, 2023 at 10:19
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    $\begingroup$ So check out how the density and viscosity changes the power calculation. $\endgroup$
    – Solar Mike
    Jan 12, 2023 at 10:31
  • $\begingroup$ If your goal is to store energy, it doesn't make sense if you have to build the reservoir. You usually want pondage measured in cubic miles, heights measures in hundreds of feet, and a design life of a century or so. At present, minimum size is about 100MW. There is research being done to lower that. The goal is to reduce the capital expenditure from billions to a few millions of dollars. $\endgroup$
    – Phil Sweet
    Jan 13, 2023 at 14:41

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