Solar submersible: system is operational, now add battery or cistern?

A simple water tower storage will avoid all the ongoing maintenance and expense
of a battery system. Bruce Roe

Elevated water, but I don't think you need fancy check & pressure valves, just a float switch to cut off pump

As others have suggested, KISS.
Get an elevated tank large enough for 2-3 days of watering.
About as simple and as reliable as you'll get.
Also, a lot less complicated than screwing around with batteries. Maybe even less costly.

Thanks, guys. I am certainly trying to keep it simple, as per my last line about reliability. Don't get me wrong, I do not want batteries. I've been doing everything I can do to avoid batteries throughout the full year it's taken us research, plan and execute getting this thing built.

Here's the issue: 2 to 3 days worth of water is a range of 18,000 gallons (bare minimum) to 50,000 gallons. Even if these tanks were just on the ground we're looking at a minimum of about $20,000 for a plastic water tank at 20,000 gallons. That's approximately 4 times what we paid for the entire system as is once you figure all the incentives, etc. Then if you want to go to 50,000 gallons your looking at galvanized and I'm not even going to bother, you can easily visualize that this is not feasible. And to put it even more simply, how am I going to fill that tank? If I have enough run hours to fill a giant tank like that, why am I not watering the orchard with those solar hours? Yes, there are rainy periods where I could feasible dedicate time to filling a tank, but the rule of thumb here is that you're never more than 3 days away from a drought. And it doesn't matter if it rained 6 inches on Monday. You need to water on Thursday unless you are certain its going to rain Friday or Saturday.

Now, if you want gravity to supply 15 PSI out of a tank (of any size) as an independent system (without the output from the pump), you have to have it elevated to about 35 feet. And yes, I already priced that and its well in excess of $50,000 for a tower that's going to hold even a couple thousand gallons of water. It clearly has to be steel and I'm not even going to contemplate talking to the county about permitting that kind of thing.

And there's no wiggle room on the PSI requirement. There's fertigation involved, etc., many reasons.

Also, as far as simplicity goes, I do not see how these 6 components are not in the realm of simple: a tank, a platform, a check valve, a pressure switch, an automated valve, some water and gravity (that one doesn't even count, really its the same as "platform"). The most expensive part is the tank: $2,000, likely less even with freight. The platform is some railroad ties and some quick-crete and some other lumber: $300. Check valve: $200 (top end). Pressure switch $30. Oh, I forgot some 2" PVC, $20. DC latching valve of some sort: $150?

That's $2,700. I bet I can accomplish this for less than $2,000. And I'm guessing (without having done adequate research to know) that $2,000 is less than what I'm looking at for a quality battery system that would be hassle free and reliable.

The point is, A) what is the bare minimum PSI I need to generate from this source of potential energy that I am releasing as kinetic energy into a pressurized system? How do I figure that out in relationship to what the pump does during heavy cloud cover? Basically, if its hazy out the pump is still spinning at full speed. If a dark cloud comes over for 20 minutes I drop off to as low as 0.34kW call it roughly 25% output. That's the lowest I have observed between about 8:30 AM and 6 PM, this time of year. All I need this thing to do is release water for 2 minutes here and 5 minutes there and maybe 20 minutes sometimes. Then I fill it back up after watering when I see its getting down below half full and that just requires me to use the generator for 20 or 30 minutes after hours.

At a 2" diameter of pipe its dumping somewhere between 30 and 55 GPM into the system. Something like that (in theory). The check valve prevents any of that from working against the already struggling pump, and at this point the duality of pressure and volume (acceleration) is where I don't know how to do the math. Heck, maybe the check valve isn't even necessary, I really don't know.

Point B) What obstacles am I not accounting for above? Surely there is someone who has contemplated what I'm saying here, but maybe its a bad idea based on the lack of information I am finding. Why?

As I've mentioned, I do have a generator and it works fine in a pinch. But relying on that necessitates me being there, and the point of all of this was me not having to be there every day that it just might be cloudy.

I am thinking, you do not need to be watering 24 hours a day? So how many hours
do you need? if you could get 8-9 hours at max power in clear sun, and lose a lot
less under occasional clouds, would that be enough?

The above is what is happening here, getting this full power curve under sun, and
putting out double the power under partial clouds, with multiple groups of panels
facing rising, noon, and setting sun.
The length of useful day is not yet maximized here. Bruce Roe


NScurJn17.jpg


3Direction.JPG

WM.jpgMax 32 GPM for a total of 9 hours run time (1,920 GPH) is what I'm trying to maintain. Max total per day: 17,280 gallons. That is not possible (or needed) throughout the short-day months. Then the valve timers are all set for a 6 hour run period. I am not wanting to water at night. Indeed I specifically do not want to water at night (fungus).

What I do want is the most consistency in pressure (and volume) throughout the 6-9 hours of day-time run period. My only obstacle to that is intermittent cloud cover. If the cloud cover is severe, that likely means its raining, so who cares? But that's not always the case, especially this time of year here in Florida. Also, photosynthesis decreases under lower light, and therefore water uptake is decreased. The graph above demonstrates that in real time. That is actual soil moisture monitoring from the orchard I'm discussing here. You'll see that it rained yesterday. All the rest of that is irrigation. Before yesterday it had not rained in 38 days.

I also remind myself that anywhere north of where I stand here in North central Florida has a shorter shortest day and a longer longest day, so that has allowed me to feel confident that the 6 hour period is a safe minimum.

Here is another of my curves, note 3 months earlier, but looks almost the same. I am
at 42 deg Lat at 61084, way north of FL. You will have less seasonable variation than I do.
Bruce Roe

NScurve.jpg

There's a lot of surplus tankage around for less than that. If my memory serves, I once found a used 22,000 or so gal. tank in a boneyard for a client at ~ $8K + shipping for use for non potable H2O. Google surplus tanks. There's a lot of them around. Might be worth a snoop for future ref.

Thanks, I have looked for used tanks. The orchard is certified organic, so that factors. There is little out there that fits the bill, and volume is not the objective. Cost and manageability are the objectives. Well, the true objective is rate of flow in intermittent releases of water to allow the pump to catch up by only having to work against the volume and pressure of the bladder tank. That portion of the system is effectively "walled-off" from the cistern intake and the rest of the lines by the check-valve.

Solar panels > submersible pump > bladder tank (probably holds 60 gallons of water set at the low pressures being used, so 2 minutes worth of water or less per fill) > short run of 2" PVC > check valve

X inlet from cistern > valve with independent pressure switch (exactly how best to arrange that I do not know) > cistern X (this is the extent of the new "section")

...mainline down to submains > zoning subs > drip line > 1/2 gallon per hour emitters: 3,840 = 1,920 gallons per hour or 32 gallons per minute regulated at 15 PSI. Pump will yield in excess of 32 gpm in good sun against the full TDH of the furthest zone which includes the pressure requirement against the friction loss.

Really, if I thought a very tall tank at 1,000 gallons would suffice I would do that and then the tank itself is only $800 (new). There are tanks that are 4 feet in diameter and approximately 12 feet tall. That would also be nice because the weight of the tank empty is manageable with 2 guys by hand and even when its full the support platform could probably be built higher at a reduced cost as well. But that's only 30 minutes worth of dump time - it may or may not work. I suppose if I did it with a small tank and found out that it didn't work I could add a second tank on a second platform with a PVC connection between the two. That might be a decent idea since really the 2,000 gallon tank is pretty much exactly twice as much as the 1,000 gallon tank. That would only leave the minimal amount of plumbing and the second platform as additional costs. A more modular way of looking at it I guess.

I have posted on an engineering forum. If I have any luck there (or if I get properly shot down) I will post what I find here in case someone sometime in the future has the same idea, for better or for worse.

What I'm not getting is how that cistern is going to feed any water into a system that's at 15 psi. In your OP, you said you'd put it on a 5ft high platform but as you pointed out in a subsequent post, to get 15psi from just gravity the water needs to be 35ft high. At best, a 12ft tall tank on a 5ft platform will supply water at 7.4 psi and go down from there as water flows out of the tank.

It may help all of us if you posted a figure of your setup and how you are thinking of tying in a backup tank.

Thanks, NorthRick. I think you just said it far better than I have been saying it. I do realize that the relatively small elevation of the cistern I am stating here is well under 15 PSI. In order to get it above 15 PSI it would have to be 35 feet in the air, and that's not feasible. But I think you just touched on the essence of my question perfectly. Its really not 15 PSI, because the pump is still producing some (specifically unknown and variable) amount of pressure and I'm just trying to add to that by "cutting on" the cistern dump at some level of pressure under 15 PSI. Once that valve on the cistern opens, what's going to happen? Surely the water isn't just suspended and unable to flow out because of the pressure coming up from the submersible? It can't be. Especially not with the check valve placed after the bladder tank and before the cistern intake. It has to go somewhere and it can only go one place. And that displaces the water that would have been coming from the submersible, and that can also only go one place - down the line (which is all actually slightly down hill of ground level at the wellhead throughout the remainder of the drip system).

I guess that's what I mean by the duality of flow rate and pressure. I'm starting to suspect that this is why they use a measure of acceleration to describe this in the metric system: meters per second squared. Gravity is acceleration, after all. This is a hydrodynamics problem, its not really even a solar topic at all at this point. But I still appreciate everyone's time in considering it.

I will have to work on a drawing. I did run it by a rep from Grundfos and he thought I was on the right track but admitted that he had never seen such a thing. He is trying to get me to someone that would be able to help, they have pretty high level customer service in my experience so far.