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Thread: Gravity-fed pressure assist on solar submersible well pumping system-crop irrigation

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    Associate Engineer
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    Oct 2019
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    Gravity-fed pressure assist on solar submersible well pumping system-crop irrigation

    I had previously posted this question on a solar PV related forums site, but I am hoping this engineering forum will be a more appropriate place to ask my question. Most of the replies I received previously were off-topic and I realized that what I'm asking is an engineering question, so why not go to the source? The previous thread can be found at: https://www.*******************/forum...ery-or-cistern

    If I can add any other details or clarify in any way, please let me know.

    Question: I am trying to perfect a relatively large, solar-powered orchard irrigation system and I would like to add either battery assist or a gravity pressure assist from a large cistern. I would prefer to stay away from a battery installation. Then again, if you look at the word "battery" as storage for energy rather than storage for only electricity, specifically, then I feel like the cistern idea is a form of battery in itself. In fact, reading about a large, man-made reservoir that is used as a battery is part of where this idea came from: https://******************/the-inside...y-8984e81283c/

    First, here are the complete specs and some additional background.

    No utility power present on property
    Irrigation system is engineered to irrigate 6 zones, each zone is an acre. Irrigation system is regulated to 15 PSI. A range of 32-24 GPM at 80' feet TDH is how it all worked out on the full system design. Actual water level in the 8" well shaft is 19' and the pump is at 40' on 1-1/2" drop pipe / mainline is 2" / sub-mains are 2" / driplines are 3/4" poly. No appreciable elevation change across watering zones.

    Pump: Grundfos 40-SQF-4

    Controllers: Grundfos CU-200 (enabling the use of an 81-gallon bladder pressure tank set at 17 / 37 PSI), Grundfos IO-101 (enabling use of gas-powered generator for emergency back up)

    Array: 8, 325 watt, 8.7 amp, 38 volt panels. Wired in series, mounted on 2 poles so that both direction and tilt are adjustable in sets of 4 panels. Pump is rated for up to 300 volts (which I have) and 8.4 amps (which I have) and runs full out at 1.3kW (it cannot run faster). Solar array is 2.6kW max, and this has allowed me to extend my daily run period. System has been running for 1 month and my only issue is variability from passing cloud cover.

    Solution: either a battery assist set up or a elevated cistern, which I will explain. Problem: I do not know how to engineer either of these. Can I use a battery system to somehow "even-out" system performance on intermittent cloud cover scenarios? What kind of controller(s) and what kind of overall cost would this entail?

    Cistern idea: a 1,900 gallon tank which measures 64" X 155" tall. This will be set on an elevated platform 60 - 72" high. 2" discharge. 2" PVC connection into the existing 2" mainline with a check valve positioned immediately up stream of the cistern intake. A separate pressure switch which controls a valve on the tank outlet (independent of the pressure tank switch). Set the switch to cut on at 15 PSI (maybe lower) and cut off at approx 30 PSI (maybe significantly lower). The idea being that if I lose sunlight to the point that the pump is unable to pressure up to 17 PSI, then the cistern will start dumping water at ~15 PSI and will effectively "boost" the pump output until it is able to catch on and fill the pressure tank. At the max water elevation level of 18' and the minimum of 5' the available pressure assist is in a range of 7.8 - 2.6 PSI. 1900 gallons is the max volume of water used in 1 hour, but I only intend for the cistern to cut on intermittently for a few minutes at a time. At the head needed to fill the tank I should be able to run the pump on the generator and fill the tank completely in approx 40 minutes. That will require separate plumbing not mentioned above and I would want to fill the tank during non-peak solar hours.

    Or, that whole idea may be completely stupid for some reason I am not yet understanding, and the battery idea would be better and lower cost. I am also sensitive to reliability in the long term.

    Any advice is appreciated. I'm looking over all the resources available on this site and there may be something someone here can direct me to that would either keep me going or illustrate why this idea doesn't have much merit. Thanks in advance!
    Last edited by david_jarnagin; 10-09-2019 at 08:32 AM.

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