small panel output

conntaxman · April 2011

Would this be about the right output for a 36/3x6 cells in not really bright sun 17.5 volts and 2.5 amps.
Im looking go buy a MPPT solar charger, do these also take the voltage and turn it into amps. Say it is bright cloudy day and you get 15 vdc but only 1 amp. Will the MPPT solar charger use the voltage and change it into power. Thats how im thinking.
tks
John

mike95490 · April 2011

Re: small panel output

PV takes light and converts to electric.

The temperature of the panel is what sets the voltage (~0.55V)
The intensity of the light is what influences the amps (depends on size of cell)

So, possibly, a high voltage string might give more power via a MPPT controller, than the same panels paralled . I've not tired it.

MPPT only works in voltage down convert, in most controllers. I'm not sure any can work with only 15V and boost it to charge a battery

Cariboocoot · April 2011

Re: small panel output

To add to what Mike said, MPPT controllers are also very expensive. They're really only worth the investment on larger system (as in over 400 Watts).

conntaxman · April 2011

Re: small panel output

carib. I plan to have up to 1000 watts and that's why Im looking at a MPPT controller. I know that they are much better and you gain more power.
John

BB. · April 2011

Re: small panel output

For me, the best reason for MPPT is to run the Vmp-Array >> Vbatt (typically Vmp-array~100 VDC maximum) and still be ~95% efficient in down converting (PWM cannot do that)--It allows you to use much smaller gauge wire from the Array to Charge controller and/or send the same power much farther.

If your array is right on top of your battery bank, you can do with with PWM--but you still end up needing more wire and parallel connections (more combiner boxes/circuit breakers/fuses).

-Bill

conntaxman · April 2011

Re: small panel output

BB. I thought the "input" on the controllers what was the max for the amount of ELE. from the panel to the controller.Im looking at some that the input is around 30 something volts input. Maybe im reading it wrong.
John

Cariboocoot · April 2011

Re: small panel output

conntaxman wrote: »

BB. I thought the "input" on the controllers what was the max for the amount of ELE. from the panel to the controller.Im looking at some that the input is around 30 something volts input. Maybe im reading it wrong.
John

No, you're not reading it wrong. You're looking at (let me guess) BlueSky or Rogue?
If you look at the specs for Tristar, Outback, or Xantrex you'll see a maximum input Voltage of 150. Look at Midnight's line and it goes way up to 250+system Voltage! Xantrex also has one (or coming out) with 600 Volt input. Kind of specialized.

For the most part, maximum advantage is had with a nominal array Voltage that is 2X the nominal system Voltage. So the lower Voltage MPPT controllers work fine for running "24 Volt" panels to a "12 Volt" system, and will give some advantage on 24 Volt systems. But when you go way up in Watts - or to a 48 Volt system - you need the bigger controllers. The BlueSky I'm thinking of, for example, can only handle 30 Amps output @ 28.4 Volts (24 Volt system) which is under 1000 Watts.

BB. · April 2011

Re: small panel output

Sizing a solar array to the charge controller is a bit more complex that people first assume...

If you are doing a PWM controller... You need to pick Vmp-array voltage such that it matches the battery bank voltage... Typically:

Vmp ~ 17.5 volts for a 12 volt bank
Vmp ~ 35 volts for a 24 volt bank
Vmp ~ 70 volts for a 48 volt bank

Next, you need to look at the array maximum current (Imp/Isc)... You need to check with the instruction manual for the charge controller... Typically, Imp cannot exceed the controllers "output current rating". If you have too much array current, you can overheat/damage the PWM charge controller.

With MPPT charge controllers, the MINIMUM input voltage would be the same as above for the PWM controller...

The maximum voltage is the Voc-array-cold (Voltage open circuit array coldest temperature expected at installation). For many locations, a 150 Voc rated input MPPT controller would be around a Vmp-array of 100 VDC maximum (subfreezing weather increases Voc quite a bit).

For maximum array current (Imp-array)--The MPPT charge controller is actually a digitally controlled switching power supply. You can put more input power on the input than the MPPT controller can actually use--the MPPT controller will simply "throttle back" the output current to its maximum rating (safely and with long life).

So, assuming you have Voc/Vmp array properly sized, and you are working with, for example a 60 amp MPPT charge controller, and want the maximum cost effective solar array for your 60 amp controller (you will lose a little bit of power on clear / cold days around noon time):

60 amp * 14.5 volt battery charging * 1/0.77 panel+controller derating = 1,130 Watt Array @ 12 Volt bank
60 amp * 29 volt battery charging * 1/0.77 panel+controller derating = 2,260 Watt Array @ 24 Volt Bank
60 amp * 58 volt battery charging * 1/0.77 panel+controller derating = 4,520 Watt Array @ 48 Volt Bank

The above is how I would plan the system... You also need to check the installation manuals (before you purchase the equipment) to ensure your planned system will meet the requirements of the hardware.

-Bill

conntaxman · April 2011

Re: small panel output

Cariboocoot wrote: »

No, you're not reading it wrong. You're looking at (let me guess) BlueSky or Rogue?
If you look at the specs for Tristar, Outback, or Xantrex you'll see a maximum input Voltage of 150. Look at Midnight's line and it goes way up to 250+system Voltage! Xantrex also has one (or coming out) with 600 Volt input. Kind of specialized.

For the most part, maximum advantage is had with a nominal array Voltage that is 2X the nominal system Voltage. So the lower Voltage MPPT controllers work fine for running "24 Volt" panels to a "12 Volt" system, and will give some advantage on 24 Volt systems. But when you go way up in Watts - or to a 48 Volt system - you need the bigger controllers. The BlueSky I'm thinking of, for example, can only handle 30 Amps output @ 28.4 Volts (24 Volt system) which is under 1000 Watts.

....................
carib... yes it was the Blue Sky =solar Boost 2512i. and it was for 25 amp. at about $225.00 i think it was. For now that is all I can afford, and to start with this stuff to see if it works out like I thought. One thing solar isn't the way to go as some are thinking. Well I mean when they say going green, and the gov. trying to push solar, and trying to make most think that everyone will have a few panels on their roof and everything is Cheap and fine and dandy. ha ha ha . 99.9 percent of the people Won't have solar on their homes.I don't think, Unless the gov. gets another brain storm and GIVES it to them Free. [like everything else their giving away.]
John

Cariboocoot · April 2011

Re: small panel output

John;

We like to stay away from politics as much as possible around here but ...
The more people who invest in "over-priced" solar, the more money for R&D to make advances and improve efficiency. Eventually it may actually become cost competitive with grid power, at least in some locales. But that is somewhat dependent on utility prices going up (arguably to reflect the "true" cost). Until then, solar subsidies are necessary if installs are going to continue at any significant rate.

That's a big problem here in BC: Hydro electric is really cheap, and no incentives for grid tie at all. Right now BC hydro is having to justify a planned 8% rate increase. The per kW hour cost to consumers may go up a couple of cents.

In the mean time, we do what we can, eh?

Note to all: that's about as far down the political road as we want to go, okay?

conntaxman · April 2011

Re: small panel output

Hi carib. I didn't mean to get into politices' , and I didn't know you were from B.C. that dose mean British Columbia right. Never been their.
Back to the solar stuff. ha ha ,its alot better to talk about anyway.LOL.
If anyone is cutting glass, i just bought a glass cutter with a diamond point and it works Great, it only cost $4.95 off ebay.I used the other type of glass cutter with the metal wheel, and I didn't like it at all.It worked sometimes, This one it very good.and for the price you cant go wrong.
Back to the solar. Im just building panels and so far Im doing that just as though you were making a thermo pane window, also I put in silica in the sides, that takes away any moisture. And also seal them.I talked to a Glass company and he said it should hold up very long time.
Im useing 3x6 cells now, and was thinking of going to the ***** but price wise it comes out about the same.,also AMP'S s is about the same. you need 2 of the 3x6 to come up with about the same amps as a ***** oh well,
Reason im making the panels is Im retired and have limited income.
have a nice day all.
John

icarus · April 2011

Re: small panel output

Contaxman,

You should also realize that battery based solar comes at about twice the price per watt as grid tied solar, with about 1/2 the performance, leading to a net kwh cost of about 4 times as much for battery based as opposed to grid tie. Plus, you have to replace your batteries somewhat regularly.

If you are serious about "being green" or reducing your power costs, and you have the gird available, it is very much smarter to use it instead of batteries. If you are just playing/experimenting/getting your feet wet so to speak, then small battery based systems are great learning tools, but if you are really going to build a 1000 watt system you would do your self a favour to look into grid tie.

Tony

Cariboocoot · April 2011

Re: small panel output

As an addendum to what Tony just said, you can't use homemade panels with grid tie; it won't be allowed. Or shouldn't be, anyway; someone has reported that their inspector "didn't care". He should, as should the utility and the insurance company.

Homemade panels may not be as cheap as they first appear, long term. And they have a bad habit of failing despite best efforts of the builder. It can be a bit of a gamble.

small panel output

Comments

Categories