Help Needed in Solar Pool Heating Experiment

CaliSteveCaliSteve Registered Users Posts: 1
edited May 2019 in Solar Water Pumping #1

Hi all,

This is my first experience working with solar, so thanks in advance for your patience.  I have a lot to learn!

I’m trying to setup a pool pump that will be used to send pool water through coils of irrigation hoses that get heated by the sun to give a boost to the pool heating system along with solar rings.  I’m starting with one set of coils as a proof of concept and plan to scale it out so that it’ll actually make a difference.  Perhaps I’ll just limit the heating to the spa portion, which is much more manageable in terms of volume of water to heat.  There are many DIY projects out there that I’ve looked through that have a similar setup, but not many where the pump is powered by solar.  I thought I’d give it a try as a fun way to learn about solar systems and how they work, and if it helps heat the pool more efficiently, then all the better.

In terms of requirements, I just need the pump to run when the sun is strong – late morning to mid afternoon (I live in California) – so I didn’t think that there was a need to have a charge controller and a battery as part of the setup.  But I’m running into an unforeseen issue (at least to my amateur mind) that I need help resolving.  Let me first explain the setup I have so far:

100w solar panel -> low voltage disconnect -> buck/boost converter -> motor pump speed controller -> DC motor pump.  I also have a 3 pin switch hooked up that can turn off the system manually.  Specs of each component are listed at the bottom.

When the sun is bright, everything works well.  LVD is set to switch off at 11V and switches on when it hits 17V.  Buck boost converter outputs voltage at a steady 13V and output current is limited to 2A at the moment (don’t need full power of the pump for now).  I can adjust the pump speed through the pump speed controller by turning the dial. 

This breaks down however, when clouds pass by or late afternoon comes by when the panel isn’t providing enough juice.  I would be ok with the system just shutting down (no sun, no need to run the water through), but the LVD and/or buck boost converter keeps on switching on and off repeatedly every second under low light, and that cannot be good for the circuitry.  From my reading, it seems that under weak sun, the pump tries to pull more watts than can be supplied, which causes the panel voltage to crater and the whole system crashes.  As soon as the motor stops, the panel recovers and delivers enough watts for the LVD and buck boost to operate again a second later.  Then the motor starts up again which starts the whole crash cycle over again.  Did I get that right?  For any given sun intensity, I can dial down the speed controller to keep the system from crashing, but it’ll happen sooner or later as the sun weakens at the end of the day.

Is there a no-battery/no-charge controller solution to resolving this behavior?  Not sure if there’s an issue with my settings on the LVD or buck boost converter that is causing this.  I haven’t dabbled with capacitors yet, but not sure if those will do anything more than just delay the inevitable crashes for a few moments. 

If the battery+charge controller is the only way to fix this behavior, then what do you recommend for PWM vs MPPT charge controller for my case?  MPPT will get the most out of my panel, but do I really need that for a simple setup like mine and will a simple PWM controller suffice?  Something basic like this one from Renogy: For battery sizing, I think the standard answer to a 100W panel is something like 25-35ah, but provided that I don’t need this to be running in low/no light conditions, do I need that large of a capacity?  For this use case, I would have to hook up the motor pump to the load terminals on the controller (and not to the battery terminals) for the pump to shut down properly by the controller?  And the controller would basically replace the LVD and buck boost converter?  I think I can still have the pump speed controller connected so I can adjust the pump intensity.  Any recommendations?  

Thanks again for the learning opportunity.


Specs of the components:

Solar panel:

100w 12V monocrystalline, Maximum Power at STC:100 W, Optimum Operating Voltage (Vmp): 17.9 V, Optimum Operating Current (Imp): 5.72 A, Open Circuit Voltage (Voc): 21.6 V, Short Circuit Voltage (Isc) 6.24 A, Maximum System Voltage: 600 V, Maximum Series Fuse Rating: 15A


Low voltage disconnect:

12-36 input voltage, 20 max amps before breaker trips


Buck boost converter:

5.5-30V input voltage range, 0.5-30V output voltage range, 0-3A output current range, ability to adjust output voltage and current


Motor pump speed controller:

6-90V operating voltage, continuous operating current: 9A, overload and short circuit protecting current: 15A



1100GPH, 12VDC, 3A draw, 13ft head

I know a bilge pump might not last very long in these conditions but it’s relatively cheap and serves its purpose for this experiment.


  • EstragonEstragon Registered Users Posts: 4,451 ✭✭✭✭✭
    IMHO, there are a couple of options.

    The first is adding a battery & controller.  You may be able to hook the pump up to CC load terminals, but you'd want to double check max current in specs.  This would take some tweaking in voltage settings so as to avoid running the battery down at night, and also handling passing clouds reasonably well.  

    The second is adding a more sophisticated (and expensive) pump controller, eg.

    The first option is likely cheaper initially, but batteries wear out.  The second option is designed for what you want to do, but higher initial cost.

    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
  • BB.BB. Super Moderators, Administrators Posts: 30,051 admin
    Have you looked at standard solar pool heating systems and seen what they entail? And possibly start there, with a "solar friendly" pump?

    In general, solar pool heaters do not try to raise the a small amount of water to a high temperature (i.e., 80F in, 120F out) with a few gallon per minute flow rate.

    Instead, they move a lot of water with a low temperature rise (i.e., 80F to 82F)... This is helpful in both massively increasing solar efficiency (solar pool heaters are operating at near or slightly above ambient temperatures, so there is little need for cover glass/insulation). It is usually helpful too in keeping operating temperatures low and allowing you to use (for example) plastic collectors instead of metal tubing and fins. You end up with multiple short paths to heat the water (and lots of flow)... Vs (for example), a few long paths with high water temperature and low flow rates.

    And there are rules of thumbs (how much sun, ambient temperatures, 3 season, 4 season, etc.)... But, as I recall, a starting point is a solar collector that is about 50% to 100% of the surface area of the pool.

    The down side with high flow rates is that you usually need a good sized pump+motor to move that much water,,, Your pump is something like 3a*12v=36watts... A typical circulation pump in on the order of 1.5 HP (or >1,000 Watts). Even if you go with a very efficient pumping system--You may be looking at 300-500 Watts, which is still 10x larger that your first try. If you have an existing pool pump (cleaning, gas heater, etc.), then just using the existing pump may be more cost effective overall. And add a GT solar (Grid Tied) system to power your whole home...

    In any case, math will help you a lot here... What is the size of your pool, and how much energy do you need to feed it per day... Do you have an insulating pool cover?

    If you want to build a your version of a thermal pool heater collector (UV protected plastic pipe or tubing of some sort), you can certainly do some experiments and see how much energy you can collect with some simple measurements.

    Take some water source (pool pump, water tap, etc.) and set up water flow through your collector... Say 1 gallon per minute through your 10 sq foot collector. And around noon with your panel pointing roughly towards the sun), measure the input and output water temperatures and see what you get...

    Just to throw some math/guesstimates at the problem. Assume solar noon, clear weather, 1,000 Watts per sq meter from sun. 10 sq ft collector, guess 40% efficiency (pure guess), and 1 gpm flow rate:
    • 1,000 Watts per sq meter * 10 sq ft panel * 1/10.764 sq ft per meter (conversion of watts/sqmeter to watts per sqft) = 929 Watts of solar energy
    • 929 Watts * 0.40 estimated collector efficiency = 372 Watts of heating for 10 sqft panel
    • 1 gallon of water = 8.33 lbs per gallon
    • to heat 1 gallon of water per minute for 60 minutes => 8.33 lbs per minute * 60 minutes = 499.2 lbs of water per hour
    • 1,000 WH = 3,412 BTU
    • 372 Watts * 1 hour = 372 WH of heating
    • 372 WH * 3,412 BTU/1000WH = 1,269 BTU per hour
    • 1 BTU = 1 lb of water heated 1F
    • 1,269 BTU/Hour / 499.2 lbs of water per hour = 2.54 degree F rise for 1 gpm or 60 gph flow rate
    It is certainly possible I made one or more math errors--Feel free to review and correct any mistakes you may find.

    And then there is how much sun you get in your area (and the tilt of the panels). You can use a simple website like this to find out how many hours a day your flat panel (non-tracking) collector will harvest (in hours of sun per day--I.e, in winter around 2-3 hours of sun per day, in summer around 5-6 hours per day) based on 1,000 WH per "hour of sun" average radiation.

    Anyhow--You can experiment with your collector and see how well different configurations, flow rates, and temperatures will work for your application.

    There are lots of ways to do solar thermal systems... (solar thermal pool heater) and see what looks interesting to you...

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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