24v panels to 12v inverter

[flaco]

Hi guys, I

[Sunking]

Hi guys, I’m new in the forum..
I want to connect an off grid system, having 24v panels and 24v wind generator, it is possible to connect to 12v PSW charger inverter using a 24v t0 12v transformer, how big should be the transformer..
Thanks a lot for the answers
Flaco

There is no such thing as a DC transformer. However there are some PWM controllers that can use 24 volt panels on 12 volt battery systems but it would be very foolish to do so because it will take your 100 watt panels and turn them into 30 watt panels.

[flaco]

There is no such thing as a DC transformer. However there are some PWM controllers that can use 24 volt panels on 12 volt battery systems but it would be very foolish to do so because it will take your 100 watt panels and turn them into 30 watt panels.

Thanks very much Sunking, may be you know about the DC-DC converters used in the trucks to step down from 24 to 12v can be used with solar panels?

[Sunking]

you know about the DC-DC converters used in the trucks to step down from 24 to 12v can be used with solar panels?

Yes I do know, but they are not transformers, they are DC to DC converters. But that does not change things, just compounds the losses.

With that said there is a way to use 24 volt panels on 12 volt systems. Heck you can use 100 volt panels on 12 volt systems by using a MPPT controller. Essentially a MPPT controller is a true DC-to-DC converter working in Boost-Buck configurations

[ripjohnnyc]

Yes I do know, but they are not transformers, they are DC to DC converters. But that does not change things, just compounds the losses.

With that said there is a way to use 24 volt panels on 12 volt systems. Heck you can use 100 volt panels on 12 volt systems by using a MPPT controller. Essentially a MPPT controller is a true DC-to-DC converter working in Boost-Buck configurations

So if I have 24V panels, I should get a 24V inverter and wire my battery bank to be 24V?

[inetdog]

Yes I do know, but they are not transformers, they are DC to DC converters. But that does not change things, just compounds the losses.

With that said there is a way to use 24 volt panels on 12 volt systems. Heck you can use 100 volt panels on 12 volt systems by using a MPPT controller. Essentially a MPPT controller is a true DC-to-DC converter working in Boost-Buck configurations

As I understand it, some work in Buck only, and cannot deliver an output to the panel which is higher than input voltage.

[billvon]

As I understand it, some work in Buck only, and cannot deliver an output to the panel which is higher than input voltage.

I think they all work in buck only.

[Sunking]

There are a couple of odd ball companies out there that offer Boost controller so you can charge 48 volt batteries with 12 volt panels, but it is a really dumb idea to do that. This POS is suppose to do it. Midnight Solar has limited Boost capability, or at least suppose to offer it soon, but again Boost is just a bad idea.

[inetdog]

again Boost is just a bad idea.

Do you see any merit in their example of a situation in which panel space is limited and so it is necessary to parallel a number of lower voltage panels pointing in different directions? That seems pretty specialized, but if a higher battery voltage is needed and it does not make sense to custom craft or order a higher voltage small panel using smaller cells, it might make sense. Assuming that the design is good, of course.

[Sunking]

Do you see any merit in their example of a situation in which panel space is limited and so it is necessary to parallel a number of lower voltage panels pointing in different directions?

In small Boost ratios like 1:1.1 for situations like a have a 12 volt panel connected to a 12 volt battery on blistering hot days, but up at say 1:2 all advantages are lost due to the high currents on the low side. The whole point of Switch Mode is to use high voltage input to minimize wire losses, and high conversion efficiency. If you loose the high voltage advantage, you loose half of the advantage plus you loose conversion efficiency.

[billvon]

Do you see any merit in their example of a situation in which panel space is limited and so it is necessary to parallel a number of lower voltage panels pointing in different directions? That seems pretty specialized, but if a higher battery voltage is needed and it does not make sense to custom craft or order a higher voltage small panel using smaller cells, it might make sense. Assuming that the design is good, of course.

I think the only case it makes sense is if you need to float a battery and are very close to the pack's float voltage as it is. That way the boost would run only on hot days where panel voltage is drooping. If you relied on it running it full time you'd lose a fair amount of efficiency.

In general bucks and boosts of all types are most efficient when the input voltage is close to the output voltage. In the case of the buck (i.e. standard MPPT) the savings comes not from the converter itself being more efficient at high voltages (it isn't; it's worse) - but because you can run thinner wires and get the same loss, or alternatively run the same wires and reduce your loss. Thus if you are willing to put in the heavier wiring you might see an advantage to boosting very occasionally, but it's a lot of complexity to add for a pretty minor benefit.

[Sunking]

In general bucks and boosts of all types are most efficient when the input voltage is close to the output voltage. In the case of the buck (i.e. standard MPPT) the savings comes not from the converter itself being more efficient at high voltages (it isn't; it's worse)

Not sure i can agree with that statement. What you say is true but is relevant. For example lets use Outback FM80 (page 108) on a 24 volt setup. Maximum efficiency is when the panel Vmp is at 34 VDC for 98% efficiency which supports your statement. However operating at 68 volts yields 96%, and at 110 volts at 95.5 %.