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

Any guidance out there?

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.

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.

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

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.

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 rate 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 increase will be greater than the rate of decrease 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.

I just found this forum, so that's why I'm a few weeks late to your questions. Here we go...

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

That's actually a good idea, especially considering the pump you're looking at.

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.)

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.

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.

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

Thanks so much J.P.M. and NochiLife . 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.
NochiLife 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

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 .

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.)

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.