PV water heating

sawmill
sawmill Solar Expert Posts: 93 ✭✭✭
in Caravan, Recreational Vehicle, and Marine Power Systems #1
I have 2- (unused) 290w panels, 39.4voc, 9.5 isc that I want to use on a17 gal preheat tank for Rennai.  Is there any advantage to wiring them in series or parallel or does it make any difference?   Also would there be any difference to what watt element is used since all available PV wattage is being converted to heat? 
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  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    edited August 2018 #2
    What's the voltage on the water heater element?
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
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  • sawmill
    sawmill Solar Expert Posts: 93 ✭✭✭
    #3
    1 have been experimenting with a 120v element like used in campers.  I don't know if an (cheap) ac element is more or less efficient than the ( expensive) dc ones.  I suspect the element doesn't care whether it's ac or dc.
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  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    #4
    You're correct, the element doesn't care if it's ac or dc. It's a purely resistive load.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
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  • Photowhit
    Photowhit Solar Expert Posts: 6,006 ✭✭✭✭✭
    #5
    sawmill said:
    I suspect the element doesn't care whether it's ac or dc.
    Pretty much it's a pure resistance load. It works out that half the voltage (resistance) will max out at 1/4 the power, so a 120v 1500 watt element will max out at about 375 watts at 60 volts. You would use vmp to calculate, and I suspect your panels would be close to 30 volts VMP, and likely to have a bout enough power at NOCT to run at 375 watts. Should be a good match.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
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  • mcgivor
    mcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    #6
    @Photowhit is correct 
    Here is a formula to calculate what wattage an element will be rated at for a given voltage, for interest purposes on h

    Say for example you had a 1500W  element rated at 120V and you were to apply 60V to it, this is the calculation you would use to determine the wattage.

    1500×(60/120)squared = 375 watts

    This demonstrates why it is best to match voltage ratings, now 1000W element 24V rating using 24v
    1000×(24/24)squared=1000W

    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS 
    Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.  
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  • sawmill
    sawmill Solar Expert Posts: 93 ✭✭✭
    #7
    Thanks for the insight guys.  Several years ago it would have been considered a sacrilege to use pv this way but with the current price of panels it makes sense in certain situations. 
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #8
    Estragon said:
    You're correct, the element doesn't care if it's ac or dc. It's a purely resistive load.
    What does care, is the Thermostat.  Getting a thermostat rated for high watts DC is not easy. 
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
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  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    #9
    Yup. Might be easier to run it at 120vac off inverter as an opportunity load.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
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  • sawmill
    sawmill Solar Expert Posts: 93 ✭✭✭
    #10
    My 3/4 mile gravity water system runs 60psi so the boiling point would be over >250F.  Even if one could boil the water once 60psi was reached it would start pushing water back to the storage tank.  The Rennai also  has a popoff.  Since these are 2 un used panels I thought this would be a good use.  
     I am still unclear about PV parallel or series connections or does it make any difference?
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  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    #11
    In series, for ~60v into a 120v element.

    In parallel (see @mcgivor post #6) 1500 (30/120)^2 = ~94w
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
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  • sawmill
    sawmill Solar Expert Posts: 93 ✭✭✭
    #12
    Thanks Estragon 
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  • mountainman
    mountainman Registered Users Posts: 439 ✭✭
    edited August 2018 #13

    Fwiw I have been experimenting with a 10 gal rv hwh. The element voltage is 120 unknown watts.   ohms are roughly 35 at 80f.  With 4 100 watt panels 18.9 vmp. Connected to the element  in series. Amps read 2.25 at 75.8 volts 170 watts    With 2s2p  1.09 38.5 v 42 watts. All in parallel 0.63 amps and 21.1 volts.13 watts.
    2kw array 6 345 q cells  make sky blue 60 cc
     6 230ah GC @36 volts 
    18 amp accusense charger. 3650 champion 
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  • sawmill
    sawmill Solar Expert Posts: 93 ✭✭✭
    #14
    Mountainman, your experiment sounds very similar to mine.  I have filled a 17 gal barrel and set the 2 panels(series) beside it for a real world experiment.  I am also located in the NC blue ridge mtns.  Last several days very little solar so nothing of value yet. 
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  • NANOcontrol
    NANOcontrol Registered Users Posts: 273 ✭✭✭
    #15
    since all available PV wattage is being converted to heat? 

    Not really.  If the panel voltage is not held to the power point a LOT of POTENTIAL power is lost.  A heater element will only be a good match for two hours a day. The rest of the day you will get 1/4 of the panels potential power.  Is there something magical about 17 gallons or is this tank already there?  A 6 gallon tank would be a batter match and probably provide hot water with out being a preheater.  I have a 9 gallon tank with a power point control and that provides all my hot water at camp, even enough extra for a dishwasher if done mid day. I've been experimenting with PV water heating for years. There are designs on youtube, 99% of them have no idea of what they are doing.
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  • mountainman
    mountainman Registered Users Posts: 439 ✭✭
    #16
    I've been doing some reading on pv water heating and it seem seems midday good sun needs lower ohms and low light early morn and afternoon requires higher ohms. 2  of the same watt elements are needed.  1 for good sun and  2 in series for low light. Panel Vmp/imp=ohms of elements needed. 
    2kw array 6 345 q cells  make sky blue 60 cc
     6 230ah GC @36 volts 
    18 amp accusense charger. 3650 champion 
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  • sawmill
    sawmill Solar Expert Posts: 93 ✭✭✭
    #17
    Results of my real world experiment-two 290w panels (series) will bring 17gal to 105f in a non isulated outside barrel.  Yes Nano,  the 17 gal tank is pre-existing for a Rennai on demand heater.  
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  • NANOcontrol
    NANOcontrol Registered Users Posts: 273 ✭✭✭
    #18
    I had to look back and see what the original question was. Looks like problem solved.  What element are you using? With the right controller and insulated tank, that much power could supply real hot water for a house. I sure like not buying propane for that. Just for amusement, this was my prototype water heater controller I used for a year.  The bubble wrap is IP-67 environment protection.  Not so good for spiders.
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