PV Water Sytem Design Help

Jardin del Alma · March 2011

Hello Folks,

I am working on finally putting in an off grid water system after years of saving and I still have some important details to work out. I am going to try and give all the basic info to get started and then pose my questions where things are still fuzzy.

Since we are in the desert of the southwest, a reliable system is imperative.

Well water will be pumped
53' to static water and 125' to bottom of the well
Well yields 16 GPM at 90 feet & 30 GPM At 114 feet
Tank elevation is 39 feet above the ground level of the wellhead
Tank elevation is 15 foot 8 inches above house
We also use water for crops at 64 feet below Tank elevation

I am hoping to not use any batteries yet still have better pressure at the house than the 6.3 PSI I calculate the 15' 8" elevation would give us (At 1 PSI / 2.3 Feet).

Also because of loss in wires etc. The solar array for the pump will be located at the well and NOT near the tank (at least 1000' of linear distance from wellhead to tank).

I figure that we will need a pressure tank or booster pump, or maybe I need both to get to reasonable pressure but this is where my plan goes fuzzy and I need help.

So, is there a way that I can boost the pressure, and NOT have to use batteries to have pressure at night?

What components would I need to achieve this goal?

I had wondered about Hooking the booster pump or pressure tank (or both) up to a separate PV array that would build up enough pressure in a large enough tank to allow us enough pressurized water for nighttime use (just some washing dishes and such). I am just not sure if this will work or if I would be damaging equipment by doing it.

I am open to here all suggestions as we would love to be "independent" and off grid with our water system.

Thank you for your time!

westbranch · March 2011

Re: PV Water Sytem Design Help

A little more info needed... how much water do you use overnight that needs t be pressurized? toilet tanks do not need to fill instantly for instance. Is the pressure needed for showers etc?

cheers
Eric

Cariboocoot · March 2011

Re: PV Water Sytem Design Help

Welcome to the forum.

Can you get pressure without a booster pump? Possibly. That elevated tank could provide significant pressure. Possibly too much, depending on its size, shape, and piping. If 6.3 psi is all it will deliver you will not be happy. You may need a small pressure pump like one of these: http://www.solar-electric.com/shacdcwapu.html

Otherwise, if you have pressure from the tank running 30-40 psi and need to maintain a sort of minimum pressure in the house you can install a large pressure tank with a check valve on the input: whenever the "system" pressure is higher than the "house" pressure, the tank will be recharged. The check valve will keep the "house" water from backfeeding into the rest of the system, resulting in a pressure drop.

I'm sure at least Tony will have something valuable to add to this.

SCharles · March 2011

Re: PV Water Sytem Design Help

Well, you will have maybe a hundred feet of head, more or less, it sounds like. Not bad. The 1000 ft run will have some friction for the flow, but again, not a deal-killer. Good idea to have the panels at the well head area.

We have a 328 ft deep well static near the top of that, then about a hundred foot head like you have, plus around 350 linear distance to the cistern we fill. We only run the well pump to periodically top off the cistern. We have a Grunfos soft-start 110 v. pump and it works fine, 5 gpm.

What is your rough elevation [above sea level]?

If you want to boost the pressure from the tank, you will definitely need a boost pump [um, unless you want to relocate the tank higher up the hill]. Real good ones come in 12 v. and 24 v. models; we have the 12 v. However, you will also need the pressure tank or you'll be running that boost pump every time you turn on the tap. Hard on the pump.

I don't know enough about the variety of available pressure tanks to answer your specific question about them. Ours is about 10 gallons. But we don't have a flush toilet, the only water use after sunset is for washing dishes and showers and filling the dog water dish. We don't do any other water-using activities after sunset [clothes washing, etc.] There may well be large pressure tanks that would do what you want [avoid batteries], but someone else will have to address that.

mike95490 · March 2011

Re: PV Water Sytem Design Help

I'd put the main pump panels at the well

Use at LEAST 1.5" pipe for the water, that long of a run will induce a lot of friction loss, and if you can keep it low, you may not need a booster. Each foot of elevation gives about 0.4 psi of water pressure.

Check my .sig for the 1,000 feet of trenches and lines and conduits we installed.

Jardin del Alma · March 2011

Re: PV Water Sytem Design Help

Thanks for the quick replies....

As far as the questions go here is what I have.

We are at 5,400 feet elevation

We need enough water to do dishes (maybe 5 or 6 gallons is my estimate as to what we use on a larger set). Then maybe a shower every now and then (usually these are taken at the end of the day before the sun goes down). Lets say another 6 gallons.
The trick is that our population will grow at times here on the farm and we will go from a family of 4 to a group of 8 - 10 at peak. So I am going to guess at using a max of 30 - 50 gallons at night when we are at peak population. Since not everyone is water / energy conscious lets go with 50 gallons to be safe and if we need to we can go down to 30 gal if that's a deal breaker.

So what I am starting to understand (please correct me if I am wrong) is that the pressure tank provides the pressure to the water, but you need a booster pump to push against the pressure that the tank holds. If this is the case I would assume that you need more pressure than the tank provides to fill the tank. Right?

Does the pressure tank use electricity (if so how much?) or is it that the air is just put in at the beginning and continues to create the pressure without electricity?

I love learning

Jardin del Alma · March 2011

Re: PV Water Sytem Design Help

Oh and if it matters we are going to have a 7,000 - 10,000 gal poly tank.

We do sometimes have to irrigate from this same well.

Cariboocoot · March 2011

Re: PV Water Sytem Design Help

Couple of observations/comments before I'm off to work.

A ten minute shower can use 30 gallons easy (low-flow shower head; 2.9 GPM). You may want to up your water usage estimates.

Pressure tanks need no electric. They're basically a big rubber bladder full of air pressure pushing back against a reservoir of water. You can set the min pressure by adjusting the air inside. 28 psi is typical - 2 psi below pump turn-on pressure.

Whether or not you need the pump is determined by how much pressure you actually get from your reservoir. A full tank will produce more pressure than a near empty one, so the output pressure will vary. With a check valve in place, every time the reservoir is above the PT pressure it will fill and re-energize the PT. The valve will prevent higher pressure in the tank from back-feeding into the reservoir. If there is never 30+ psi (for example) from the reservoir you will need a pump to boost pressure. It would be controlled by a pressure switch at the PT - usually 'on' at 30 and 'off' at 40.

BB. · March 2011

Re: PV Water Sytem Design Help

The typical pressure tank is just an Air Chamber charged to ~20-40 PSI (whatever your minimum required line pressure is). Good ones have a plastic or similar bladder to prevent Air/Water from mixing... Otherwise the air dissolves into the water and looses air pressure (i.e., a water logged tank).

What is your irrigation water usage? Pressure and volume? Do you need the same pressure as in your home?

For a home with small/reasonable pressure requirements like yours, a small battery powered DC Pressurization pump + pressure tank is a pretty nice solution. Does not take much in the way of solar panels and small battery bank to provide the needed power.

Throw large volumes of water for irrigation with a pressurization requirement--Takes much more energy to satisfy those needs.

-Bill

mike95490 · March 2011

Re: PV Water Sytem Design Help

Jardin del Alma wrote: »

Does the pressure tank use electricity (if so how much?) or is it that the air is just put in at the beginning and continues to create the pressure without electricity?

Just the pump that pressurises it. A 60gal pressure tank has 2 chambers, one is a 30 gallon Air Balloon, filled from the factory. The other side is where your water is pumped in. As water is pumped in, the balloon is compressed, so it's maybe 40 gal water, and 20 of air. As water is used, when you reach 20gal water, and 40 of air, you loose pressure, the bladder can't manage 100% volume of the tank.

Thats why I like elevated tank and low restriction (wide) pipe.

Jardin del Alma · March 2011

Re: PV Water Sytem Design Help

I am definitely getting clearer on how this all works (beyond just assumptions)...Thank you all

There is no way to move the tank higher without a platform which is also not an option.

The pressure needed for irrigation is not huge. Most of this relies on higher flow than it does on pressure as it is a drip system. I have even thought about just bypassing the pressure system and using the gravity pressure (approx. 27PSI) in a large diameter pipe and this is probably where we will go with it.

I do not see how we are going to get enough pressure to fill the pressure tank even if it was at our house level without a booster pump. However, depending on loss due to having to go back uphill to the house I may be able to get enough by going downhill to the fields.

Approx. 27 PSI gained from gravity on 64' of drop from tank to field
However, then we have to push water uphill 50' to the house

I do not think this will work but I thought I would put it out there.

So, unless the above idea works I think we need to go from the tank to a booster pump which then fills our pressure tank that holds enough water to get us through the nighttime use. Then in the morning when sun is shining the booster pumps non-battery array kicks in and refills the pressure tank.

Any problems with this?

mike95490 · March 2011

Re: PV Water Sytem Design Help

you should also be able to have some nighttime water from gravty flow, and you will need a check valve to keep the booster pump from backf-feeding the tank & irrigation lines

westbranch · March 2011

Re: PV Water Sytem Design Help

Jaridn, can you give us a bit of information about the elevations of eaach of the parts of your system?
If you start with the well (ground level as a reference point with an elevation of 'zero' );
house elevation;
tower elevation;
tower height (bottom of tank);
tank height;
water pressure at tank, and any other information you may have that you may not already have supplied.

Yes, you pump water into the pressure tank with a second pump and then use it as needed.

For planning purposes we use 50 gals per person per day, in a conventional Canadian house, for all uses. (our personal use is much lower)

I would have a dedicated pressurized supply line for the shower(s) only and use gravity feed to provide the rest of your water needs other than the drip irrigation. This will reduce the amount of time the pressure pump has to run.

If you only pressurize the shower you could plan for a much lower expected usage and pick a tank size to fit. Pressure tanks come in sizes up to 100 us gallons , or larger if you search for them... weigh the cost of power to run the pump against tank cost.

PS, when the pressure pump is placed at a lower elevation than the storage tank, the gravity pressure at that point is added to the pump pressure, thus the pump does not have to work as hard to reach its cut-off point.

Hope this helps,
Eric

Jardin del Alma · March 2011

Re: PV Water Sytem Design Help

If the wellhead, at ground level were 0 then:

house elevation; 23 feet
tower elevation; No tower, tank will be on a hill, see bottom of tank
bottom of tank; 39 feet
tank height; 7 feet (5,000 gal tank)
tank width; 12 feet diameter

I have recalculated our water needs and figured the following:

using 50 gal / day / person at a population max of 10 = 500 gal / day

Though we have unbelievable sunshine here in the southwest and it is usually sunny (note: we have 7.5 hrs of sunlight at well area on Winter Solstice) I figured on a week of storage....which equals 3,500 gal for our max population.

I then just bumped up to a 5,000 gal tank so that the rest can be for irrigation purposes.

My thought is that we want to have the capability of a good supply and we can adjust fill levels in winter if we felt it necessary to lower them to ensure a regular use and cycling of water.

Does this sound reasonable?

Also, it seems that I will be putting the booster pump and pressure tank near the house to take advantage of that 6 PSI of gravity flow to ease the booster pumps workload.

Now some of these questions make me wonder if there is PSI gain due to the weight of the water in the tank. When I asked someone about this before they said there was no gain there and though I was surprised I figured that given their background they would have known.

Is that true or is there PSI Gain?

Cariboocoot · March 2011

Re: PV Water Sytem Design Help

Jardin del Alma wrote: »

Now some of these questions make me wonder if there is PSI gain due to the weight of the water in the tank. When I asked someone about this before they said there was no gain there and though I was surprised I figured that given their background they would have known.

Is that true or is there PSI Gain?

There will be some pressure gain. How much depends on the level of water in the tank, friction losses through the piping, and the size reduction. With zero restrictions, water will seek its own level and produce no pressure. Reduce the size of the pipe and the weight of the water will push the pressure up, with a loss in flow rate. From the looks of your elevation differences you won't experience much pressure gain even with a full tank.

Now someone will tell me I'm wrong, which means 50 years of experience was spent in some alternate reality where physics is different. How do I get back there?

westbranch · March 2011

Re: PV Water Sytem Design Help

Hi Jardin, there is a rule of thumb for water pressure per foot of elevation: you get a bit less than 1/2 PSI per foot of elevation drop, or conversely it takes ~1/2 PSI to lift water 1 foot... (the exact number is .43)

so there is 16 foot difference between the bottom of the tank,
so the max pressure would be < 8 PSI, with line loss etc probably ~= 7 PSI

the 7 feet of water in the tank would yield ~ 3psi, not worth worrying about as it will vary constantly... just know that if the tank is full, your pressure will be a tad higher.
HTH

Something that has not been discussed yet is freezing temperatures since you are at high elevation . How have you addressed this issue?

Eric

Blackcherry04 · March 2011

Re: PV Water System Design Help

Here is where I'd start and solve one issue at a time. A lot depends on your skills, pocketbook. The house pressure is the easy part. The well and storage is what you have to get right first.

Look at this pump. Two PV panels will support it and it'll give you 500-600 GPD with 6 hours of sun. I'd run PEX tubing and put a float valve in the storage tank and make a pressure tap at the well, once you know the back pressure on valve closure, you'll know what kind of controller you need to shut the pump off or you could let it overflow and use the extra some other way ( if there is extra ).

Setting the pump depth in the well, you may only need 70-80 feet, I don't know the replenish rate. Thats a good thing, better water and a lot cleaner and easier on the pump.

http://www.solar-electric.com/sh93susowapu.html

Edit : here is the Installation manual.

http://www.thesolarstore.com/manuals/9300usermanual.pdf

BB. · March 2011

Re: PV Water Sytem Design Help

Just remember to keep the PEX tubing in the dark... I don't believe it will stand up to sun/uv (do not store in sun, etc.).

-Bill

Blackcherry04 · March 2011

Re: PV Water Sytem Design Help

BB. wrote: »

Just remember to keep the PEX tubing in the dark... I don't believe it will stand up to sun/uv (do not store in sun, etc.).

-Bill

Good Point, The UV resistance of PEX is limited, I like the push/lock fittings and have a distaste for Polyethylene , though it may be best here. There so many details giving advice in a system like this. I'd probable set up two pumps in the well if I was dependant on it for supply, but that's me. Redundancy is good, if you can afford it. He has a big storage capacity, so he should be ok.

russ · March 2011

Re: PV Water Sytem Design Help

Blackcherry04 wrote: »

Good Point, The UV resistance of PEX is limited, I like the push/lock fittings and have a distaste for Polyethylene , though it may be best here. There so many details giving advice in a system like this. I'd probable set up two pumps in the well if I was dependant on it for supply, but that's me. Redundancy is good, if you can afford it. He has a big storage capacity, so he should be ok.

Distaste for polyethylene? Why?

Blackcherry04 · March 2011

Re: PV Water Sytem Design Help

russ wrote: »

Distaste for polyethylene? Why?

To many leaks, thin wall, rock holes and if it bends it'll leak. I don't like the fittings, not a hose barb fan, just not my favorite.

Peter_V · March 2011

Re: PV Water Sytem Design Help

Is the house already plumbed? You can (somewhat) compensate for low pressure by using larger pipes.
Any chance of raising the storage tank? Perhaps a second, smaller tank, on a tower, maybe just 50-100 gallons.

One idea that will need batteries, but perhaps not a lot of energy, would be to use a pressure pump like they use in RVs. These automatically sense when the water is running and pressurize it, so they only draw power when the water is running.

westbranch · March 2011

Re: PV Water Sytem Design Help

Blackcherry 4... thin wall? Thin wall poly pipe is not suitable in this application! It only has a max of 75 psi rating for 3/4" and that is not what is appropriate for well to cistern use. it is meant for low pressure drip irrigation and such.
regular well poly piping has to be heated with a torch or ?? to soften it enough to get the barbed fitting to go into the pipe. It would take a pipe wrench or 2 to get it to 'bend' and collapse... and a lot of work.

Well grade poly piping will be in the range of 175 PSI and is thick wall, +- 3/16" x 1 1/2 " poly.

Never had a collapsed pipe in over 40 years... poly reins in this application, and it is also fairly resistant to UV degradation.
Alternative is Iron Pipe.... or

As a guide, the pump should be suspended about 10 feet above the floor of the bore hole is it is lined (cased), if not, place the pump at about the level of the bottom of the casing. This allows it to be 'pulled' for service or when the bore hole collapses... other wise the only solution is to re-drill the holes and pulverize the pump...

Any water that is pumped from whatever level does not 'drag' on the pump as it has no weight until it is pushed above the surface of the wells static level.

You are right about getting things right and Jardin should get the tank as high as possible to max out on gravity pressure as it might suffice. A friend with 5 PSI is amazed how much pressure he has at the taps.

Pex in the house is great as it eliminates corners which cause pressure loss.

HTH
Eric

PV Water Sytem Design Help

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