Solar panels for a DC solar pool pump

Question

Ok - total newbie question here. I'm trying to work out the size or number of solar panels required to run a DC pump for use as a solar heating system for a swimming pool.

I've located what looks like a suitable pump with the following specs

1. Voltage: 8V-24V DC (Standard:12V DC), 4.5amps
2. Max Flow Rate for 15W model : 11.5 L/Min - Max Water Head: 3M
3. Min start-up power consumption less than 2 Watt

The aim is to direct connect the pump to the solar panel allowing for it to operate whilst sufficient sunlight in order to pump water into the heating elements of the pool (think black plastic tubes on roof of house).

Now the question. Given 24v, 4.5amps I think it equates to approximately 100watts. The solar panels from a local supplier give specs such as:

• Maximum power: 120W
• Rated voltage: 12V
• Open circuit voltage: 21.8V
• Short circuit current: 7.14A

The other alternative is the 200w panel which has specs like:

• Maximum power: 200W
• Rated voltage: 24V
• Open circuit voltage: 45.3V
• Short circuit current: 6.15A
• Voltage at max power: 37.8V
• Current at max power: 5.31A

Thus which panel would work; 120W or 200W ? Also do I need to worry about the open circuit voltage and short circuit current ?

thanks for you help grant

Answer 1

First of all, are those numbers for the pump all from the same model? Because the range of the numbers is rather large: you say that it starts with 2 watts, nominally runs at 15 watts(?), might sink 4.5 amps at 12 volts, which gives 54 watts. And even larger numbers are calculated in your question, which probably aren't relevant.

Looks to me that even the 120 watt panel is overkill, but it can be connected directly to that pump if the pump can inded handle 24 volts, which is more than the open circuit voltage, which is the maximum voltage the panel can ever generate.

The thing to realize here is that it's the amps that turn the motor, not the volts. Volts are something that have the capability of giving amps to a load. And pushing amps through a load generates volts across the load. In your case, there's a solar panel, which is more like a constant current source, unlike, say, a battery, which is more like a constant voltage source.

When you connect the solar to the motor and there is sun, the panel pushes amperes to the load. The 120W panel looks like it might push something like 6 amps to the motor. That would perhaps generate something like 16 volts over the motor, although this number is only based on a rough feeling. That would mean that some 96 watts are dissipated somewhere in the pump - and that seems quite a lot for a pump that nominally takes 15 watts. I don't know; maybe the pump is electronically regulated somehow so that it never draws so many amps...?

All in all, I have a bit uneasy feeling about this. It would help if you could provide some more info about the pump. A link to a datasheet or a product page, maybe?

Answer 2

EDITED TO CLARIFY VOLTAGE LIMIT

It looks to me like you can probably direct connect the 120 W panel to that pump. If you have a link to the pump specifications, that would help.

You need to worry about the open circuit voltage only in the sense that you don't want the operating voltage to be higher than the pump maximum. For the 100 Watt panel, since the Voc is lower than pump max, it is not an issue. I don't think you need to worry about the short circuit current.

The open circuit voltage of the 200 W panel is quite a bit higher than the pump max. It is not clear that the pump would load it enough to bring the voltage down to a safe level. So I would not use the higher power panel in this application.