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Looking to add a solar powered water pump to heat my hot tub water

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  • NochiLife
    replied
    I also had the thought that maybe go even simpler and use a fish tank heater connected to the panel. I may try that a bit while I order the pump. All my heaters are 110 ac. I'm thinking the regulation circuitry wont function but it may just heat anyway.
    TECHNICALLY, yes, they will heat. (I actually am doing something like that for my LiFePo4 bank: I have a cheap digital thermostat switching a waterbed heating mat directly to my solar array to warm the bank up. My controller automatically disables the MPPT if the LiFePo4 bank is too cold to charge in wintertime.) However, the best solar panels (that I know of) only reach 20% efficiency at converting sunlight into electricity--and believe me, 100W isn't going to heat very much water! Worse, the panel is only producing 1/5 the voltage the heaters are rated for (22v vs 120v)...it won't do much of anything.
    HOWEVER, if you fancy some entertaining ideas (especially if your solar panel has a waterproof junction box), you could try using the solar panel itself as a water heater (they sure get hot in full sunshine!), and the power from the panel to run a $15 pump. Solar panels actually are more efficient at low temperatures, so you'd (try to) kill two birds with one stone. Just a wild harebrained thought, at your own risk .

    Is there anyway to use one of my inductive ac pumps I already have? Most are around 100 watts just that they are also 110ac.
    Probably, though you'll need a small charge controller, battery, and inverter. There are several problems, though:
    - Inductive AC motors are terribly inefficient (50% efficiency or less, compared to ~75-90% for DC motors)
    - Most cheapo Chinese inverters struggle with inductive loads. (Believe me, I have one: it may have only been $280 for a 3KW pure sine inverter--but it's a royal pain with any inductive loads! It is a true pure sine inverter--verified with an oscilloscope--but it's pathetic at starting anything inductive!) There is a rather hilarious YouTube video of someone (wajaeboseben) testing eleven (11) different cheap 12v inverters, rated from 350-1500W, to see if they could start a 1/2 HP synchronous (inductive) AC water pump. Most of them failed to start it--and two of them just produced a cloud of smoke.
    - You might have a 100W solar panel, BUT an inverter will lose at least 10% of that power. AND if you have a PWM-style charge controller, there goes another chunk of power. In short, you'd need a bigger solar panel. Plus a timer to shut it off at night, or else the battery will end up getting fully cycled every day, and won't last very long

    I'd just recommend getting a small BLDC pump or two, and wiring them directly to the panel. It'll be a lot cheaper, simpler, and efficient. (And in case you're wondering, no, it won't work to directly wire an AC induction motor to the solar panel. You can try it, but nothing will happen.)

    [USER="55402"]NochiLife[/USER] I am wondering how I can only use 20 or 30 watts out of a 100 w panel without a problem? Dont I need something to either use or store that energy? Like how I have to connect my battery first to the charge controller so the panel doesn't blow it out.
    thanks for the great help
    I understand your concern; however, an MPPT is quite a bit different from BLDC pumps. (I don't think I could adequately explain here why that's an issue for an MPPT, but not for the pumps.) EDIT: recalled a conversation I'd had with Morningstar tech support. At least in the case of the Morningstar, it handles a maximum panel voltage of 150vDC--but the maximum voltage on the battery side is 75vDC. The problem is that with no battery connected, the MPPT can't regulate down far enough to keep the battery terminals below the maximum 75vDC. This may or may not be the case with other chargers; better safe than sorry.
    Maybe a bit of an analogy could help: a battery that isn't connected to anything is still providing power--it just isn't being used. If you put a solar panel in the sun without connecting anything to it, it's producing power--just the resulting power isn't being used. Like a car--it may have a 220HP engine, but it probably only takes 30HP to cruise down the freeway. For that matter, the panel will only produce 100W in full sun--in the mornings and evenings (or cloudy days), it'll produce far less.
    The only possible issue would be if the Voc (open-circuit voltage, i.e. the voltage the panel will produce in full sun with no load) exceeds your BLDC pump's maximum voltage; if the specs you posted match the panel, you'll be just fine (22.4Voc at the panel vs. 24v max on the pump).

    It's a lot of fun to see solar working for free.
    Last edited by NochiLife; 06-29-2019, 12:57 PM. Reason: give reason for why no battery can damage an MPPT

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  • nomadh
    replied
    Thanks so much [USER="16965"]J.P.M.[/USER] and [USER="55402"]NochiLife[/USER] . Vacation and a few things put the project on hold for awhile. I was just thinking about this and checked back in. I also had the thought that maybe go even simpler and use a fish tank heater connected to the panel. I may try that a bit while I order the pump. All my heaters are 110 ac. I'm thinking the regulation circuitry wont function but it may just heat anyway. Is there anyway to use one of my inductive ac pumps I already have? Most are around 100 watts just that they are also 110ac.
    [USER="55402"]NochiLife[/USER] I am wondering how I can only use 20 or 30 watts out of a 100 w panel without a problem? Dont I need something to either use or store that energy? Like how I have to connect my battery first to the charge controller so the panel doesn't blow it out.
    thanks for the great help
    Last edited by nomadh; 06-28-2019, 04:19 PM.

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  • NochiLife
    replied
    Originally posted by nomadh View Post
    I'm tired of using my expensive household pv system to cook water for my hot tub. But its a nice hot tub and I'm tired of unplugging it most of the time.
    I'm hoping to do this on the cheap using as many parts as I already have to some extent. I currently have a few induction type ac pond pumps, a 100 watt mono solar panel. I use the solar panel with a charge controller and a 12v gel battery. I use the panel, charge controller for my camping setup and I don't want to wear out the battery using it day to day otherwise I suppose I could use that setup, a 12v dc to 110 ac car adapter and a pump. Running it all through a coil of black hose during the day. Problem here is it certainly wont need to run at night and I dont want to keep discharging my battery. I might have a timer setting on that controller.
    I just found this forum, so that's why I'm a few weeks late to your questions. Here we go...

    I have to check but I was wondering if I made sure the pump was always on could I run the C C without the battery? I suspect if I did this the weakest link would be the cheapo car dc to ac converter. I'm worried if I were to burn it out then the CC wouldn't have a load and it would also fry. Is that right?
    While that might be dependent on the charge controller, it would be extremely risky regardless. My Morningstar TriStar MPPT will burn out if solar is applied before the battery is connected; the entire MPPT actually is designed to run off of the battery. If solar is available, it charges the battery--but it is using the battery to hold the output voltage within range. Basically, while there's a slim chance it MIGHT work, I can't recommend it, unless you want to experiment and let the rest of us know how it turns out

    So although I think this could work I don't see it as highly reliable or long lived. At the simplest I think I could hook the 100w panel to a simple DC pump. Is there any problem with this?
    That's actually a good idea, especially considering the pump you're looking at.

    I believe it has to use about 100 watts, correct? A 20watt pump set up direct would just burn up the pump, right? So either a 100 watt pump or 2 50 watters in parallel?
    No, no, and no again. Of primary importance in this situation is the voltage: The pump you linked is rated for 12-24vDC, and the Voc (highest output voltage) of your panel is 22.4v, so you'll be just fine. A BLDC (brushless DC) pump will only get damaged if overvoltaged (given over 24vDC), or reverse biased (+ and - leads reversed between the panel and motor).
    If the pump uses less power than the panel is generating, the excess power simply doesn't get used. It will not hurt the pump in any way. Similar to an off-grid solar system with the battery at "float"--the panels might have 3KW of generating capacity, but only 200W is actually being used to run the 'fridge, for example. You only have to "use up all the power" with a wind turbine, otherwise it could get damaged by overspeeding (if I understand correctly.)

    Then I need to know how pumps will work as they are starved during dawn/dusk. Do they just slow down? Lose head? Burn up or wear out? Also I'm not seeing a lot of selection in dc pumps especially at the budget end.
    With the BLDC pumps I have, they slow down and lose head as the voltage falls--actually providing a bit of a way to control the flow rate! While I have not tested my pumps at very low voltages, I would say that a BLDC pump should handle low voltages at sunrise/sundown just fine. (If not, or if you just want to be on the safe side, a simple relay can provide a low voltage cutout.) If the pump used a regular brushed motor, it could potentially get damaged, but a BLDC pump should be fine--ESPECIALLY if they're selling it for use with an itty bitty solar panel.

    There was this for $60
    https://www.amazon.com/dp/B07NMNFTGP...Cb9QRY320?th=1 [h=1]AEO Dry-Run Protection 12V-24V DC Brushless Submersible Water Pump, 410GPH, for Solar Fountain, Fish Pond, and Aquarium[/h] but i show Panel, recommended 18V 20W. Im thinking this pump couldnt handle my wattage? Can I add a dummy load in parallel like a 60 watt bulb?
    Pump looks good. Similar in price to the DC50E-24150 (same flow rate, but 15 meters of head; you don't need that for circulating water!) There's another variant of that DC pump that's rated at 2400LPH (634GPH), also 24v--if you need more flow rate. Costs about the same. https://www.ebay.com/itm/NEW-24VDC-M...sAAOSwaB5Xnbp3
    There's cheaper 24v BLDC pumps for circulating water, depending on how much flow rate you need. I mean, if you're on half a shoestring budget, there's one for $5 (free shipping) on eBay--it'll only pump 36GPH, though! https://www.ebay.com/itm/DC-24V-150L...kAAOSwnRpcZ342
    Here's one for $15 that runs 211GPH: https://www.ebay.com/itm/DC-24V-800L...4AAOSwrERch2uo

    And no, you don't need a dummy load.

    I dont have the exact panel specs but I think its similar to these

    Specifications
    Maximum Power: 100W
    Optimum Operating Voltage (Vmp): 17.8V
    Optimum Operating Current (Imp): 5.62A
    Maximum System Voltage: 600V DC (UL)
    Open-Circuit Voltage (Voc): 22.4V
    Short-Circuit Current (Isc): 5.92A
    Dimensions: 39.7 x 26.7 x 1.4 inches
    Weight: 16.5 lbs
    Just check your panel in full sun with a multimeter, and as long as the output voltage is 24vDC or less, you'll be fine connecting it directly to a BLDC pump. That panel should have more than enough power for even the 86W pump I linked. Just make sure the polarity is correct

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  • J.P.M.
    replied
    Originally posted by nomadh View Post
    I was figuring during the mid day it could get near 100. At least 95. Are you saying it will only pick up 1 deg per pass. I was figuring on restricting it some if it was passing 5hrough the pipe too fast but it seems 100 or 200 ft of 1/2" pipe would already do that.
    Do not confuse total amount of heat transferred per time period (say an hour) with the temperature of the water, or heat being heat transferred per area per time period. Generally, for applications such as yours, the greater the velocity of the water as it flows through the tubes, the greater will be the heat transfer [U]rate[/U] per ft.^2 of tubing exposed to the sun.

    Also, do not confuse quantity of heat (calories, BTUs) with quality of heat (temperature). You want to maximize the quantity of heat. You only want enough temperature in the water in the tubing to get the job of keeping the hot tub water hot.

    The general equation: Q = U*A*Delta T.

    Q = Total heat transferred across a surface - in your case that portion of the tubing that's exposed to the sun.
    U = Heat transfer flux in units of heat transfer per unit area per unit temp. difference between the sunlight tubing wall and the water in the tubing. In S.I. units: calories/(sec.*m^2* deg. C). In Customary units: BTU/(hr.*ft.^2*deg. F).
    A = the area of heated heat transfer surface (the portion of the tubing exposed to the sun(m^2 of ft.^2.)
    Delta T = For your application, the temperature difference between the tube wall and the water going through the tube.

    And, Q also = m*(T,out - T,in)

    m = mass flow rate of water in lbm/hr. or kg./sec
    T,out = water temp. at irrigation hose outlet.
    T,in = water temp. at irrigation hose inlet.

    You want to maximize Q, not necessarily T,out.

    The higher the mass flow rate (lbm/hr, or kg./sec.) can be kept, the lower the Delta T will be between inlet and outlet. But, U will increase as flowrate increases, and for most situations that rate of [U]in[/U]crease will be greater than the rate of [U]de[/U]crease of Delta T resulting in more Q for faster fluid velocities and lower temp. increases.

    You don't want high temp. or a lot of temp. rise of the water as it passes through the tubing. You do want a lot of heat transfer (more Q) tp maintain the tub water temp. at the desired use temp.The lower the tube temp. can be kept while still meeting the duty (maintaining the tub water temp. at the desired level), the more efficient the application will be. That means keeping the flow rates and fluid velocities as high as possible.

    Unfortunately, high fluid flow rates mean more pumping power. There is always a tradeoff in heat exchanger design between more heat transfer and required pumping power. usually it's a compromise. One trick to help with keeping pressure drop low(er) while keeping mass flow rate and heat transfer rate high(er) and also keeping Delta T low(er), is to manifold one single stream into several (maybe 5 to 10) parallel streams. One stream by itself will kill your application because of the induced pressure drop from a single stream that will take pumping power that's proportional to the square of the fluid velocity and also directly proportional to the flow length. Get a 10 stream manifold of 20 ft./coils (for example) and knock the required pumping power to ~ 1.0% of the of the single, 200 ft. coil. It'll work better.

    It'll work better yet with collectors made for the purpose. I'd get a 4 X 10 Fafco sheet, an AC pump and run it off house current and call it done, but that's another topic.

    Without getting into solar thermal collector or pool/spa heater design, 200ft. of 1/2" irrigation hose will provide ((12*1/2)/2)*200/144 = 4.16 ft.^2 of heat transfer surface. That'll provide something like maybe 4,000 to maybe 5,000 BTU of heat to a hot tub over a sunny day that's pretty warm. If your hot tub is anything like mine was (~ 7 X 7 ft. ~ 3, lbm water covered except when in use.) , it'll probably require something like 8,000 to 10,000 BTU/day to maintain ~ 100 - 105 F. temp. over a "warm" 24 hr. period.

    Take what you want of the above. Scrap the rest.

    Edit/add: see "builditsolar.com" for some ideas. Not much that's new, but still valid.
    Last edited by J.P.M.; 05-25-2019, 11:06 PM.

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  • nomadh
    replied
    I realized that later. Thanks for the info. 1 deg a pass would be fine but it still seems I should shoot for more.
    . I figure I can experiment with that once I got a pump running reliably. I really want to run it on solar but as a test maybe i will run it with a pump off of ac through heating coils. Even that should be much less power that the 1500 watt heater that's in there.
    still need info on the solar circuit.

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  • Mike90250
    replied
    What I intended to convey, was that it more desirable to raise the water temp only 1 degree per pass.

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  • nomadh
    replied
    I was figuring during the mid day it could get near 100. At least 95. Are you saying it will only pick up 1 deg per pass. I was figuring on restricting it some if it was passing 5hrough the pipe too fast but it seems 100 or 200 ft of 1/2" pipe would already do that.

    Leave a comment:


  • Mike90250
    replied
    You only want the water to pick up 1 degree of heat each pass, so if it's going in at 85, you only want 86 coming out. hotter the output, the more heat is "lost" in the too hot coils.

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  • nomadh
    replied
    Any guidance out there?

    Leave a comment:


  • Looking to add a solar powered water pump to heat my hot tub water

    I'm tired of using my expensive household pv system to cook water for my hot tub. But its a nice hot tub and I'm tired of unplugging it most of the time.
    I'm hoping to do this on the cheap using as many parts as I already have to some extent. I currently have a few induction type ac pond pumps, a 100 watt mono solar panel. I use the solar panel with a charge controller and a 12v gel battery. I use the panel, charge controller for my camping setup and I don't want to wear out the battery using it day to day otherwise I suppose I could use that setup, a 12v dc to 110 ac car adapter and a pump. Running it all through a coil of black hose during the day. Problem here is it certainly wont need to run at night and I dont want to keep discharging my battery. I might have a timer setting on that controller. I have to check but I was wondering if I made sure the pump was always on could I run the C C without the battery? I suspect if I did this the weakest link would be the cheapo car dc to ac converter. I'm worried if I were to burn it out then the CC wouldn't have a load and it would also fry. Is that right?
    So although I think this could work I don't see it as highly reliable or long lived. At the simplest I think I could hook the 100w panel to a simple DC pump. Is there any problem with this? I believe it has to use about 100 watts, correct? A 20watt pump set up direct would just burn up the pump, right? So either a 100 watt pump or 2 50 watters in parallel? Then I need to know how pumps will work as they are starved during dawn/dusk. Do they just slow down? Lose head? Burn up or wear out? Also I'm not seeing a lot of selection in dc pumps especially at the budget end.
    There was this for $60
    https://www.amazon.com/dp/B07NMNFTGP...Cb9QRY320?th=1 [h=1]AEO Dry-Run Protection 12V-24V DC Brushless Submersible Water Pump, 410GPH, for Solar Fountain, Fish Pond, and Aquarium[/h] but i show Panel, recommended 18V 20W. Im thinking this pump couldnt handle my wattage? Can I add a dummy load in parallel like a 60 watt bulb?


    I dont have the exact panel specs but I think its similar to these

    Specifications
    Maximum Power: 100W
    Optimum Operating Voltage (Vmp): 17.8V
    Optimum Operating Current (Imp): 5.62A
    Maximum System Voltage: 600V DC (UL)
    Open-Circuit Voltage (Voc): 22.4V
    Short-Circuit Current (Isc): 5.92A
    Dimensions: 39.7 x 26.7 x 1.4 inches
    Weight: 16.5 lbs

    The pump spec is the other issue. I think I have a lot of play in this. If I use my ac pump My bigger pumps still only use 120 to 140 watts, should push enough and have enough head plus I can move the coils from on top of the canopy 5 ft above to water level in front of the tub and if the coil is too long I could run the water in parallel hoses if needed. Possibly the water could move too fast and not pickup much heat


    Just wondering what direction I should be heading on this or any other ideas/gotchas?

    Thanks
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