Direct to PV pump, variable resistors, lions, tigers...

[3 Acre]

Hi,
I'm working on a offgrid system of direct to PV components.
In this case, a 35W water pump to circulate nutrients to tomato plants in our greenhouse.
The project does not justify the cost of batteries, expensive controllers etc. and I'm allergic to unnecessary complexity!

Most 'redneck' direct to PV pump systems match panel output to pump input (35w PV to 35w Pump etc.) to avoid burnout.
The problem here is that my panels will normally output less than half their capacity - leaving the pump underpowered.

Continuous pumping is not vital as the header tank provides a buffer by gravity feed in the greenhouse, but I feel the pump might eventually fail under a low stuttering supply.

My thought was to over panel (100watts) and then limit the current (somehow) to 35 watts. That way the panel would only need to work at 30% efficiency to properly power the pump.

Would a simple 100watt variable resistor achieve this? Or am I missing something?

.

 

[3 Acre]

The more I think, the more it seems a variable resistor would not work as a current limiter for a variable supply!!
There must be some sort of solution to limit the supply current to 35 watts (without also resisting the current when it is below 35watts)?

 

[JaySolar]

I think your best bet is to get a panel with a voc that more or less matches the pumps voltage limit so you don't have add a variable resistor.

 

[yodamota]

you could also cover part of the panel. might work for you in your case

 

[3 Acre]

I think your best bet is to get a panel with a voc that more or less matches the pumps voltage limit so you don't have add a variable resistor.

That does seems the standard way. Concern is underpowering during low sunlight. Hoping for a way to link a larger panel to avoid that and limit the output somehow.

 

[3 Acre]

I think your best bet is to get a panel with a voc that more or less matches the pumps voltage limit so you don't have add a variable resistor.

That could give me a watt-matching panel, but I'm really after a larger panel (to compensate for low light conditions) with some sort of simple limiter to avoid burning out the pump when the sun really shines.

There are direct solar pump controllers for this task - or specialised pumps but they are lithium battery expensive!

 

[rhino]

Have you considered just using something like a Victron 100|30 which can work without a battery and connected directly to a DC load but you would actually have to test it with your pump to see if it actually works which would be risky since it might not work at all.. but this would solve your problems if it did.

 

[3 Acre]

Have you considered just using something like a Victron 100|30 which can work without a battery and connected directly to a DC load but you would actually have to test it with your pump to see if it actually works which would be risky since it might not work at all.. but this would solve your problems if it did.

It might work - if I understand correctly this should limit volts/watts - but an expensive* unit to experiment with
(*for this project at least - an adequate pump is only $25)
I wonder if the variable load of a motor (startup 'spike') might destroy it eventually.

 

[peakbagger]

FWIW I have about 100 watts of "antique" Siemens panels hooked directly to a submersible pump that pumps up to an elevated tank with a upside down bilge switch mounted in the top of the tank to shut the power off to the pump. I water my garden manually but could expect I could automate that aspect fairly easily using sprinkler components.

I have been doing it for years but it took me awhile to find the right pump, I bought a lot of $50 pumps and they usually worked for awhile but eventually bought a Nemo Pump. https://www.nemopumps.com/nemo-pump/ . Its homebrew design that works.

 

[marionw]

There are a number of DC-DC Buck Volt Converters that can handle a wide range of input voltage to a specific output voltage and can handle over 3 amps.

 

[3 Acre]

As a real world example over a period of years, that's worth a lot, thankyou.
This is the direct connection pump I found - but not cheap. Very low start up current draw.

https://www.motralec.com/public/fichiers/docs/Lowara_ECOCIRC_D5_VARIO.pdf

FWIW I have about 100 watts of "antique" Siemens panels hooked directly to a submersible pump that pumps up to an elevated tank with a upside down bilge switch mounted in the top of the tank to shut the power off to the pump. I water my garden manually but could expect I could automate that aspect fairly easily using sprinkler components.

I have been doing it for years but it took me awhile to find the right pump, I bought a lot of $50 pumps and they usually worked for awhile but eventually bought a Nemo Pump. https://www.nemopumps.com/nemo-pump/ . Its homebrew design that works.

 

[3 Acre]

There are a number of DC-DC Buck Volt Converters that can handle a wide range of input voltage to a specific output voltage and can handle over 3 amps.

Something like below? Would that cure the issue of excess from the 100w panel burning out the 35w pump on very sunny days?
I lack some basic watt/volt knowledge here!

 

[peakbagger]

As a real world example over a period of years, that's worth a lot, thankyou.
This is the direct connection pump I found - but not cheap. Very low start up current draw.

https://www.motralec.com/public/fichiers/docs/Lowara_ECOCIRC_D5_VARIO.pdf

I needed a bit more pump head on my setup. My pump is set about 15 feet (4.5 meters) below ground. The tank is a plastic IBC tank that sits on top of a stack of firewood. The top inlet is about 10 feet (3 meters) from ground level. The static water level in the well is a big variable that will change over the course of a season but to cover low water level later in the summer when I need water the most, that means I need a pump rated at 25 feet (7.6 meters). Most of the cheap DC pumps out there just do not have the pressure rating I need.

In general. many folks do not have practical experience sizing pumps and just buy one an hope. I used to deal with them as part of my job so I got more hands on experience than many folks have had.

 

[efficientPV]

I found this to be a pretty nice pump https://www.amazon.com/dp/B097L2TNB9

It will operate down to 5V. Below that the rotor locks and just draws current. A 24 to 12V buck converter with a 12V panel and you are set to go. The pump can be submerged.

 

[wpns]

I’ve had mixed results with this pump (see my other thread) but it is available with solar panel and controller: https://www.amazon.com/dp/B09D7QMRJK so might be an option for COTS solution.

 

[efficientPV]

I did find the little pump was very prone to cavitation and stopping if there were small air bubbles in the water. Head is only about a third of what they say.

 

[wpns]

I did find the little pump was very prone to cavitation and stopping if there were small air bubbles in the water. Head is only about a third of what they say.

Yeah, I’ve tried suction pumps but they tend to stop working and need re-priming when they get the smallest air bubble, where the submersible pump can run dry and pick back up when the water level rises with the tide.

 

[efficientPV]

Yeah, I’ve tried suction pumps but they tend to stop working and need re-priming when they get the smallest air bubble, where the submersible pump can run dry and pick back up when the water level rises with the tide.

Actually, this pump was totally submerged when it would stop working.

 

[3 Acre]

I found this to be a pretty nice pump https://www.amazon.com/dp/B097L2TNB9

It will operate down to 5V. Below that the rotor locks and just draws current. A 24 to 12V buck converter with a 12V panel and you are set to go. The pump can be submerged.

Thank you that's great!

I think I'll try a pump like this. Cheap enough to experiment with and likely enough head for this little system.

I'm struggling to fully understand the buck converter element (still learning my fundamentals).

Presumably 24v-12v because the 'open voltage' of a 12v panel is higher?

Does the converter ampage/wattage need to match the panel here ...or the pump?

Do I still need to use match panel and pump wattages with a system like this?

 

[3 Acre]

I’ve had mixed results with this pump (see my other thread) but it is available with solar panel and controller: https://www.amazon.com/dp/B09D7QMRJK so might be an option for COTS solution.

I spotted this - it's attractive as a complete system and would be useful for other (bigger) applications on our small farm - but I was a little unnerved by some of the feedback.
Thanks, I'll check out your thread.

 

[wpns]

I was a little unnerved by some of the feedback

Agreed, you get what you pay for.

 

[efficientPV]

The buck converter will take a higher panel voltage (16-21V) a convert that to higher current at 12V. That works fine till the pump is asking for more power than the panels can produce. At that point the panel voltage takes a nose dive. I believe the pump can take this situation as long as it is submerged to dissipate heat. The good part is pump voltage will never go above 12V.

 

[3 Acre]

That's helpful thankyou - I think I'm almost there on the rudimentals of this.

With the buck converter in place, is it the case that the pump motor (or other load) will only draw the current it needs (at 12v) and no more?

I think this is still the big gap in my knowledge here - can I use a panel with a higher wattage in such a circuit?
...or will it overpower the pump motor?

 

[wpns]

With the buck converter in place, is it the case that the pump motor (or other load) will only draw the current it needs (at 12v) and no more?

That’s correct.

 

[Quattrohead]

You need to stop thinking about watts and instead think about the volts and amps that the motor actually requires. Then we can start recommending some kind of voltage regulation. The pump will only take as many amps as it needs but too many volts will blow it.