I was looking at an interesting approach to micro-hydro power, which involves using solar-powered pumps to move water UP to a retention pond some distance up a hill, and then when you need electric power and it's cloudy or night, you run water from the up-pond to the down-pond. No, it's not terribly efficient, however, if you have the right terrain and the ability to cut a below-frost-line channel for the necessary piping, it's an interesting approach for micro-hydro.

It's also done on a HUGE scale for some hydro-electric generating stations, they take some of the excess grid energy capacity they have during low-need times to pump water back up into the reservoir and then during high-demand times, they just use the hydro power to supply the grid. It augments the natural fill rates for the systems.

it's nothing new. The electric demand swings, so such measures stores the energy so it can be used later when demands goes up. It also makes a more stable supply possible, because many of the alternative sources like solar and wind are not reliable enough for basic electricity. It takes to much time and energy to restart a conventional plant all the time.

Seems like you would lose a fair amount of energy along the way, but the one great thing is that you can eliminate the cost and maintenance of batteries as a storage medium.

Off-peak pumping to a retention pond, even one built above ground, to later get gravity flow, including through a mini-hydro generator is a common "battery" alternative on small farms and ranches. The pump is commonly powered by a windmill with elecrical power as a backup; solar is a new alternative. Often the goal is primarily to get the benefit of somewhat pressurized water without having to pump it.

I wonder if you could do something like this with compressed air ?

> The pump is commonly powered by a windmill

Pretty much every farm in America before WWII, and many during the Fifties in seriously rural areas. I lived on a farm with a windmill that pumped water up to a big tank on stilts, and that was our household water. We didn't use it to generate electricity, though - just drinking and washing.

Windmills and water pumps to holding tanks are a centuries old, known technology. I'd be interested in how efficient generating electricity from the flow of the water is.

Sounds very cool, and a bit more feasible than normal solar if you have the space but a lot of rain.

I hesitate to comment as I really have no facts/numbers or direct experience to offer, but rather an observation.

I don't think think you can assume such a system would be more cost-effective than simply charging a bank of 12 volt lead-acid batteries and then pulling the juice out after dark. I seems to me to be installation/scenario/design specific. At one end, if you just want to store enough power to get through the night, the pump up, drain down system seems very inefficient, but OTOH, if you can store a really large amount of water (big pond or lake size), you could have power for days or weeks, and maybe get to some sort of steady state of reliable power, daily power.

So I think the devil would be in the details of the specific installation/design/scenario, with the pump-up system having the ability to store larger amounts of potential power (water) cheaper if you have the space and water available.

Were you thinking of a specific scenario/application?

Instead of converting solar to electric to kinetic energy, with efficiency losses at each transition, you could use a home made hydraulic ram to raise the water to a holding pond. Kinetic to kinetic, gravity powered. These things were staples of the 1960's rural commune composting-toilet, solar-heating, organic farming crowd.

Like Nurse Mike I agree a ram pump Would be the way to go. After coming out of the turbine the energy that is left over could still be used to pump water up to the retention pond.