Forum Discussion
412 Replies
- A single 12v panel? I can't see going MPPT with it at all. You don't reduce the amps down the wire and there just isn't much to be done with the excess voltage.
Two panels and you might get better early/ late and reduced light output, will it be enough to justify the cost, especially vs adding a third panel if you can is up to you. I don't have an answer and the general consensus is different everywhere you look. Glean what you can from all of it, learn to understand why the choices were made and apply it all you your unique situation. It's only taken me three years to get to the point I think I know what my end system will be, now the fun starts. - CaTraveler,
I thought you use Voc when talking series and mppt? NinerBikes wrote:
jrnymn7 Also it's not clear where the 44V came from. You need to divide by Vmp. Voc means 0A and hence 0W output from the panel.jrnymn7 wrote:
"I'm seeing two 150w panels in series only producing a few amps more than my 245w panel. On a better controller I know of a 960w system using 160w panels in pairs for mid 40's producing 48a while I would expect 60a from four 250w 24v panels."
That sure seems counter-intuative. One would think the 12v's in series, having a higher combined Voc, would result in even more wattage being used by the controller. I can see if the mppt was rated for 50v, and you threw 46 volts at it, but most buck converters boast ~98% efficiency, when operated within their duty cycle.
Does it perhaps have to do with the 12v 160w panels being in series, and thus their output would become 160w / 44v = 3.6a, as opposed to ~9a Isc?
320w/44v = 7.27a series 320w/22v =15.1a parallel.- Niner,
I was focusing on the output of an individual panel, while in series, (~3.6 x 2 = ~7.2) but point taken. And yes, I shouldn't have switched to Isc, with regards to mppt. Thanks for the correction.
So, to compare two 12v in series with a single 24v, using mppt;
320w / 44v = ~7.3 vs. 320w / 36v = ~8.9a ???
Would this explain the lower output Jim referred to? jrnymn7 wrote:
"I'm seeing two 150w panels in series only producing a few amps more than my 245w panel. On a better controller I know of a 960w system using 160w panels in pairs for mid 40's producing 48a while I would expect 60a from four 250w 24v panels."
That sure seems counter-intuative. One would think the 12v's in series, having a higher combined Voc, would result in even more wattage being used by the controller. I can see if the mppt was rated for 50v, and you threw 46 volts at it, but most buck converters boast ~98% efficiency, when operated within their duty cycle.
Does it perhaps have to do with the 12v 160w panels being in series, and thus their output would become 160w / 44v = 3.6a, as opposed to ~9a Isc?
320w/44v = 7.27a series 320w/22v =15.1a parallel.- "But the questions still remain, at what point does mppt become justified, or even necessary, and how much solar is too much?"
When array voltage is FAR above the battery voltage
I do not believe MPPT is justified with a 17Voc array and a 12 Vdc system
Each and every installation is different
Each and every ones wants, needs, desires and finances are different
Some are concerned with long wire runs aka power losses - Some with array shading - Some with keeping the electronics out of sight - Some with keeping the DW happy - Some just like to play with the technology - Some with $$$$$ - Some ......
A proper Buck conversion stage power loss on the systems I've been involved has never been of any concern. Too many other power losses that far exceed the loss from a quality buck conversion stage
"Some say you cannot have too much, but I disagree."
To each their own - Thanks Jim,
Yes, that's my goal... to see what works best, given any individual situation. In my particular situation, 24v wasn't really an option, seeing as the local supplier I dealt with doesn't even carry 24v panels. And with my small system, 12v and pwm just made sense.
But the questions still remain, at what point does mppt become justified, or even necessary, and how much solar is too much? Some say you cannot have too much, but I disagree. - Hi,
I don't mind drift. In fact, lots of times I learn more from drift than from staying strictly on topic.
I do know that the next iteration of the Blue Sky 3024 had 20 amps of diversion load. However they wanted $79 to upgrade the chip with zero warranty if the installer broke off a pin. Mine only does 2 amps.
I seem to remember the Tristar pwm does diversion loads but the MPPT version does not.
For me it is a non issue unless I triple or quadruple the wattage of the system. jrnymn7 wrote:
"I'm seeing two 150w panels in series only producing a few amps more than my 245w panel. On a better controller I know of a 960w system using 160w panels in pairs for mid 40's producing 48a while I would expect 60a from four 250w 24v panels."
That sure seems counter-intuative. One would think the 12v's in series, having a higher combined Voc, would result in even more wattage being used by the controller. I can see if the mppt was rated for 50v, and you threw 46 volts at it, but most buck converters boast ~98% efficiency, when operated within their duty cycle.
Does it perhaps have to do with the 12v 160w panels being in series, and thus their output would become 160w / 44v = 3.6a, as opposed to ~9a Isc?
A few times now I have seen mentioned on the solar forums that converting down from higher Voc to lower voltage is inefficient and I see the results in what people are getting out of their systems. Basically the more balanced the Voc and Isc is, the better. It takes both and having one go up without the other doesn't produce the results you would expect. My 220w panel shot to over 40v on a cold day but there was no current. When it did settle down but was still at a higher Voc it only added 3/4a to the system. Taking it up in altitude would increase both Isc and Voc for a better result.
In photography the sweet spot of a lens is the aperture that it is it's sharpest. In Rc airplanes it is a combination of airframe, motor and prop. Find those sweet spots and the difference can be night and day. I believe that as more of us use and report on our solar that we will find that there are different outcomes depending on our choices. Not that anyone is right or wrong as long as their needs are met but rather that a person looking to build can see the differences in the choices and build to their needs.- Hi Don
When I realized how often we wouldn't be using the systems potential, I considered installing one of these. They come in a variety of wattage and temperature settings. I couldn't justify the cost to set it up vs what we would save in propane at the time, maybe if we went FT.
My controller has load terminals and some settings but not an automatic diversified load. I may have been able to do something with the timer and voltage settings but then there is the load limit. The manual for the Eco-w isn't real clear there. Certainly not the 600w available on a good day. Maybe a single large controller has bigger limits.
So it would have required a heavy run of wire to the back of the trailer with a switch and a fuse. It would be nice if there were a switch that I could turn on and it would turn off if the voltage dropped. For all that the best I could hope for is to help use less propane late morning as we heat the water for showers. After that we just don't use much hot water.
