Problems with Solar Panel/Bilge Pump

[teachingdude]

Sorry if it's in the wrong spot, feel free to move.

The specific setup: 12v Bilge Pump wired to a 24v solar panel with a resistor (I think).

The situation: I'm a teacher at a school and we have 2 stream tables that have historically been run with a bilge pump connected to a small solar panel. It wasn't operating for a couple years, but we just got it up and running this last week and I was left in charge of it and I have already burned up 2 motors and have no idea whats going on. One theory was because at first I had them wired straight into it, then saw a little rectangular piece that i am assuming is a resistor. Burned up 1 motor before seeing that, then started adding that to the circuit. My other theory, maybe because it's a 12v pump hooked up to a 24v panel. But to my knowledge this was always the way it was, so I would assume it used to work so it should still work. One more detail, a third (brand new) pump seemed to die today, except it runs fine when I hook it up to a car battery. But when I hook it up to the solar panel in full sun producing 23v, nothing.

So...what do I need to change about my setup to make this work? The goal is to keep it as simple as possible. One of the great parts of this is that the students are able to set it all up themselves by getting the solar panels out and just alligator clipping the wires together.

 

[Lucien Nunes]

The output voltage of a solar panel is extremely variable, from zero to max rated volts, depending obviously on the incident light but also the connected load. The initial setup might have pulled the panel voltage down to around 12V,, but that doesn't guarantee that a different pump, or one working against a different head of water, would do the same. You didn't mention what the voltage was at the motor when running so it's hard to know exactly what occurred.

What is needed is a regulator, to ensure the pump receives a defined voltage. Most practical DC applications of solar power require one. Match the size of the regulator to the maximum wattage output of the panel. You don't technically have to use one intended for solar applications but doing so would give you predictable results. Many are intended for battery charging and are called charge controllers, but the principle is the same. They used to be just shunt regulators that dumped the excess power, but now they are PWM voltage converters that step down whatever voltage the panel is producing to what the user wants.

To know why the pump will not now work from the panel, we need to know what the voltage is at the motor terminals when connected. If the panel has developed a high internal resistance, it might read 23V off-load but the voltage collapses when the pump is connected. Most likely, the panel is now dead.

Small DC bilge pumps are not known for very long service lives. They are designed to run for a few minutes at a time, once an hour, not really all the sunny hours of the day. So I don't know how long I would expect it to last overall even at 12V. The AC motors used in line-powered fountain pumps do not have brushes and do not wear so fast. There are now brushless versions but only from obscure brands so I don't think they are likely to outlast a decent Rule or Jabsco pump. Finally, don't forget that dry-running can destroy a submersible bilge pump in minutes.

 

[teachingdude]

The output voltage of a solar panel is extremely variable, from zero to max rated volts, depending obviously on the incident light but also the connected load. The initial setup might have pulled the panel voltage down to around 12V,, but that doesn't guarantee that a different pump, or one working against a different head of water, would do the same. You didn't mention what the voltage was at the motor when running so it's hard to know exactly what occurred.

What is needed is a regulator, to ensure the pump receives a defined voltage. Most practical DC applications of solar power require one. Match the size of the regulator to the maximum wattage output of the panel. You don't technically have to use one intended for solar applications but doing so would give you predictable results. Many are intended for battery charging and are called charge controllers, but the principle is the same. They used to be just shunt regulators that dumped the excess power, but now they are PWM voltage converters that step down whatever voltage the panel is producing to what the user wants.

To know why the pump will not now work from the panel, we need to know what the voltage is at the motor terminals when connected. If the panel has developed a high internal resistance, it might read 23V off-load but the voltage collapses when the pump is connected. Most likely, the panel is now dead.

Small DC bilge pumps are not known for very long service lives. They are designed to run for a few minutes at a time, once an hour, not really all the sunny hours of the day. So I don't know how long I would expect it to last overall even at 12V. The AC motors used in line-powered fountain pumps do not have brushes and do not wear so fast. There are now brushless versions but only from obscure brands so I don't think they are likely to outlast a decent Rule or Jabsco pump. Finally, don't forget that dry-running can destroy a submersible bilge pump in minutes.

Thanks for the reply. I played around some more this morning and lets see if any of this helps narrow anything down.

I'm using Rule 12V 360GPH pumps. No idea how reliable these are. I also have 2 solar panels, so I can test between the 2 of them.

Like I said, with no load they produce 23-24v. When I hook up the motor mentioned above, they go down to 1 (tested on both panels). I also have a few brand new motors, so I tested one and it works. Voltage under that load was 6-7.

Is this drop showing that the panels are both dead?

So for now I can just hook up new motors and get everything running again, but if something needs to change to keep me from burning up motors, I want to change it before wasting another motor.

 

[James]

It sounds like the pumps are drawing a larger current than the solar panels can produce.
what rating are the panels? If known, if not, what size are they for a rough estimate.

 

[Lucien Nunes]

So do I understand that the old pump works OK on a car battery but drags the solar panel down to 1V, but the new pump pulls the solar panel to 7V and runs fast enough for your application? Are you testing them under the same conditions?

Try each pump on the battery and measure the current draw, under identical conditions. The head of water the pump is lifting against affects the current, which affects the voltage on the solar panel although not the car battery. The less backpressure on the pump usually the more current it will draw (due to higher flow rate and therefore higher torque). A pump sitting in a bucket of water with no hose connected to the outlet would pull the solar panel voltage down lower than one plumbed in and lifting a column of water, but clearly not down to the point where it doesn't run.

As for what is causing damage, is it possible that the pump under load holds the solar voltage at 7V, but when left unattended, sometimes it's not properly submerged in water so it occasionally runs dry and the loss of pumping load causes the voltage to shoot up. Then it's a double whammy, too high a voltage and running dry at the same time, which rapidly kills the motor.

 

[teachingdude]

It sounds like the pumps are drawing a larger current than the solar panels can produce.
what rating are the panels? If known, if not, what size are they for a rough estimate.

Ok. Now that I read everything and understand a little more, they are not 24v panels. I was reading the VOC thinking that was the same. The VMPP is 17.5 and they are advertised as 12v panels. Max power is 30watt. Here is a link to the panels

So do I understand that the old pump works OK on a car battery but drags the solar panel down to 1V, but the new pump pulls the solar panel to 7V and runs fast enough for your application? Are you testing them under the same conditions?

Try each pump on the battery and measure the current draw, under identical conditions. The head of water the pump is lifting against affects the current, which affects the voltage on the solar panel although not the car battery. The less backpressure on the pump usually the more current it will draw (due to higher flow rate and therefore higher torque). A pump sitting in a bucket of water with no hose connected to the outlet would pull the solar panel voltage down lower than one plumbed in and lifting a column of water, but clearly not down to the point where it doesn't run.

As for what is causing damage, is it possible that the pump under load holds the solar voltage at 7V, but when left unattended, sometimes it's not properly submerged in water so it occasionally runs dry and the loss of pumping load causes the voltage to shoot up. Then it's a double whammy, too high a voltage and running dry at the same time, which rapidly kills the motor.

Yes you understand correct and that the old pump worked ok on a car battery but pulled the panel down to 1v and did not pump. I only connected it to the car battery for a fraction of a second though because I was just doing a quick dry test and didnt want to run it dry. So I did not test the voltage when this was happening. I'll have to try your test you mentioned a little later and see what it does.

But...the test done that dropped the voltage to 7v was done in a bucket of water without any hose attached (like you mentioned). If I'm understanding correct, it makes sense in this case for the voltage to drop lower (is 6-7 within a "normal" range).

But you mentioned what might be causing damage. I could see this being the case. The hope is students connect them during recess, then disconnect at the end of recess. But they are elementary students, wouldn't be surprising if this gets forgotten and they run for the 2 hours between recesses. They "shouldn't" go dry because water should always be circulating, but its still possible that it happens.

 

[mainline]

Ok. Now that I read everything and understand a little more, they are not 24v panels. I was reading the VOC thinking that was the same. The VMPP is 17.5 and they are advertised as 12v panels. Max power is 30watt. Here is a link to the panels

Yes you understand correct and that the old pump worked ok on a car battery but pulled the panel down to 1v and did not pump. I only connected it to the car battery for a fraction of a second though because I was just doing a quick dry test and didnt want to run it dry. So I did not test the voltage when this was happening. I'll have to try your test you mentioned a little later and see what it does.

But...the test done that dropped the voltage to 7v was done in a bucket of water without any hose attached (like you mentioned). If I'm understanding correct, it makes sense in this case for the voltage to drop lower (is 6-7 within a "normal" range).

But you mentioned what might be causing damage. I could see this being the case. The hope is students connect them during recess, then disconnect at the end of recess. But they are elementary students, wouldn't be surprising if this gets forgotten and they run for the 2 hours between recesses. They "shouldn't" go dry because water should always be circulating, but its still possible that it happens.

Why not just use the panel/s to charge the battery via a charge controller, which would be the normal way to do things.

 

[teachingdude]

Why not just use the panel/s to charge the battery via a charge controller, which would be the normal way to do things.

Couple reasons. First, we don't currently have batteries, so saving the money (although you could argue I might end up spending more if I keep burning up motors). But also trying to keep it super simple. Our campus is a shared space, so we have to put the solar panels (and if we had batteries, those too) away every day. And it's all led by 4th graders. So getting a battery out every day and hooking it up just adds complication. But we also use this set up to reinforce our teachings on solar energy so that they can actually see solar working. Stand in front of the panel and it stops working, move away and it works again.

 

[teachingdude]

So do I understand that the old pump works OK on a car battery but drags the solar panel down to 1V, but the new pump pulls the solar panel to 7V and runs fast enough for your application? Are you testing them under the same conditions?

Try each pump on the battery and measure the current draw, under identical conditions. The head of water the pump is lifting against affects the current, which affects the voltage on the solar panel although not the car battery. The less backpressure on the pump usually the more current it will draw (due to higher flow rate and therefore higher torque). A pump sitting in a bucket of water with no hose connected to the outlet would pull the solar panel voltage down lower than one plumbed in and lifting a column of water, but clearly not down to the point where it doesn't run.

As for what is causing damage, is it possible that the pump under load holds the solar voltage at 7V, but when left unattended, sometimes it's not properly submerged in water so it occasionally runs dry and the loss of pumping load causes the voltage to shoot up. Then it's a double whammy, too high a voltage and running dry at the same time, which rapidly kills the motor.

Ok. Did the test on the car battery. 12.6 no load. When I touch the pump a momentary jump to 12.7, then sits back at 12.6. Both pumps exactly the same except the "old" pump is noticeably louder.

 

[mainline]

Couple reasons. First, we don't currently have batteries, so saving the money (although you could argue I might end up spending more if I keep burning up motors). But also trying to keep it super simple. Our campus is a shared space, so we have to put the solar panels (and if we had batteries, those too) away every day. And it's all led by 4th graders. So getting a battery out every day and hooking it up just adds complication. But we also use this set up to reinforce our teachings on solar energy so that they can actually see solar working. Stand in front of the panel and it stops working, move away and it works again.

Without a voltage/current regulator, the setup will never work correctly with max sun on the panels the voltage is going to be around 17v driving a 12v motor which is then going to draw a lot of current.
When there is less sun, the motor may not even start at all

If there isn't enough current, then you could try two panels via a regulator to drive the pump.