Alternative to batteries for storing PV energy

[Robert Pugh]

Pumping water uphill has been a long standing practice for storing excess energy produced in hydroelectric applications. Off grid PV systems for aquaponics gets expensive fast when you add in batteries for overnight/cloudy times. Has anybody given thought to how pumping water with PV to a higher point than the aquaponics system might be harnessed for later use?

RAM? (seems pretty inefficient)

What other pumping mechanisms might be used?

While it might be less efficient than storing energy in batteries, the long term costs are bound to be lower. Thoughts?

Does the volume of water needed just make this completely impractical? Might be an interesting to do the math.

Anyway, just thought this would be an interesting thought experiment...

[Bradly]

A downside to this neat idea might be the fact that the water has to pump uphill...meaning that the storage location for this pumped water would have to be elevated considerable...whether on higher ground or raised tank.  Converting the water stored back to energy would probably cost way more than simple batteries to store regular sun power I'm guessing...although I have no idea how one would do that. 

 

[Chris Carr]

I agree. 

Just from pondering on the idea I think the volume of water is just too much to be efficient. Lets say you had a 300 gallon IBC tote system, and you wanted to turn over half the system volume an hour and run it 36hours "off grid" using stored water as an energy source which is 100% efficient. That is still 5400 gallons of water you need stored and in reality you its likely more like 50% efficient at best, so now you're looking at 10k gallons to pump a small system for a day and a half.

I think if the goal is to energy efficient it is best to tackle the problem with system design and require much less energy as an input in the first place. 

This is more the purpose of this thread http://community.theaquaponicsource.com/forum/topics/aquaponics-wit...

[George]

I'll stick with batteries for now and hope to buy them only once.  Mine haven't fallen below 12.5V lately so they should last many years.  We shall see.

[Robert Pugh]

Chris, that thread is interesting, and it goes to show we're thinking along the same lines! Your specific comments are just what I'm trying to work out. That post is very broad and I want to explore just one specific area.

I'm not trying to solve problems in the third world in which AP's dependence on electricity makes it less feasible. I'm thinking of things I can do here in the US with my resources. When looking at an off-grid system the batteries are expensive and have to be replaced every 3-5 years. I'm happy using PV, it'll last 25+ years without replacement, but I don't like the cost of batteries. Since significant potential energy can be stored by pumping water uphill, it seems worth doing some back of napkin calculations.

So just to keep this thought experiment going...

Let's go with Chris' assumptions:

50% efficiency

300 gal tote system

36 hr pumping reserve

10K gal storage tank.

One implicit assumption Chris made in this post (but mentions it in the "no pump" post) was that the ratio of water pumped to water stored was 1:1.  However, the ratio also depends on the height at which the reservoir resides. Let's also assume that we can place the reservoir higher and get the ratio to 2:1. This will shrink the reservoir size back down to 5K gal.

So what's it take to move 5K gallons....500gph pump for 10 hours. Here in So. Cal we get 5 hours of good insolation, so 2x200watt PV panels can run two 500gph pumps for 5 hours each. You could fill the reservoir in a single sunny day. This is based on a very old 180W 500gph aquarium pump I had in the attic. I'm sure there is more efficient equipment out there. I'm sure we could get more precise and calculate the losses at the inverter etc., but good enough for now.

My thoughts are to keep the water reservoir isolated from the AP water. Then we don't have to worry about keeping the reservoir aerated and circulating while not in use.

Lunch break is over, so I'm going to give the pumping mechanism some more thought and post again later.

[Chris Carr]

Glad I see we are on the same page anyway :) The pumping mechanism is where I put the idea on hold. Essentially what is needed is a double closed loop system, where movement from loop A will pump B.

"Charging" the energy source, ie pumping uphill i think is the easy part. A sterling is a great use for this. They also make water pumps specifically designed for this and run directly off of the PV panels for solar, or integrated directly by a windmill.

[Robert Pugh]

Agreed, pumping up is easy and off the shelf. Utilizing the potential energy, not so much. I'm thinking of a mechanical metering device, like a pendulum, to control the downward flow and draw energy from it. I'm looking into old clocks for inspiration...

[Chris Carr]

Wikipedia says a common waterwheel can reach about 60% efficiency so 50% is a good way to ballpark it. Looks like some power plants use this method to store large amounts of energy during off hours for load balancing. A  well made water turbine can get >70% efficiency doing this.

http://en.wikipedia.org/wiki/Pumped_storage_hydroelectricity

Still seems pretty impractical to me, but interesting to think about anyway.