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standalone/ offgrid solar inverter without batteries? (slrpnk.net)

submitted 1 year ago by [email protected] to c/[email protected]

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Would it not be nice to have an inverter (2kw) which is off the grid that can work by having a very small buffer (1 kwh) to power applications that can work intermittent. So when the sun goes away, the applications (such as a pump or fan) work for one hour more and then stops. Like a UPS.

I work for a company that uses small solar pumps for small holder farmers and these pumps do not have batteries. So when the sun is away, the pump simply stops working and continues to operate when the sun is back.

I would like something like that for my shed. My question is this, does anyone knows the correct terminology for such an inverter and or do you know a brand as an example that can do this?

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[–] [email protected] 2 points 1 year ago* (last edited 1 year ago) (1 children)

An inverter just converts DC to AC. You are more in charge controller territory with this question. I understand what you are asking for, so I don't mention that to be pedantic, but to help your search. There are a lot of applications that could run this way, so it's a good question.

I've built a small scale version of what you are talking about to aerate a tiny aquarium intermittently. It uses a 5v/6watt panel, a blocking diode, a 4F super capacitor and a buck/boost converter driving a 5v 0.43 amp air pump.

Now the first version of the system was just a panel hard-wired directly to the pump. It worked ok, but it only started running just a little before solar noon because the pump has some internal friction to overcome before it will run. All that morning and afternoon sun was wasted in that setup.

With the new set-up, the pump starts running intermittently much earlier because the super capacitor accumulates the trickle of energy and then releases it at full current once at capacity. Once drained, the charge controller cuts out because it can't deliver the set voltage and the cycle begins again. During the brightest part of the day, the panel is delivering enough energy to run the pump constantly, so the capacitor just hangs out with a full charge ready to deliver if a cloud comes along to lower the panel output briefly.

It's hard to estimate, but my best guess is the pump runs about twice as frequently in a typical day than it would with just the naked panel setup I started with.

[–] [email protected] 2 points 1 year ago (1 children)

That is a nice small setup. The air pump is a small brushed pump? Why are you still using a buckbooster if the voltage is already 5v from panel and the airpump need 5v? Will the solar panel deliver in morning and evening not the 5v but 3v?. I am suprised that a 5v airpump exist and that it is able to pump air into water (as that cost quite some pressure to overcome)

[–] [email protected] 1 points 1 year ago

Yes, it's a DC brush motor, 0.43 amps. It puts out just a trickle of air and is used by small-boat fishermen to keep a live well aerated. The buck booster cuts out very cleanly when the voltage drops below a set point, that's one reason. The other reason is I found cool buck/boost converter that has some features I wanted to play with. That's honestly the real reason.

Panels put out pretty constant voltage, but variable current. That's the role of the super capacitors, to accumulate charge when the light is dim, still 5v but an amperage below the minimum needed for the pump. Did I understand that part correctly?