Solar Panels

[noelex 77]

Originally Posted by hellosailor

Some of the MPPT controllers claim a slightly higher efficiency at higher voltages, i.e. 4% loss at 12v, 2% loss at 24v, so it might buy you something--IF all the panel inputs were at that voltage and IF the MPPT controller was rated for higher efficiency.
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Generally the opposite is true with better efficiency as the panel voltage comes close to the battery voltage, but the effect is only small

[mikefossl]

I replaced 4 x 55 watt Siemans 16v panels with 2 x 185 watt Kyocera 28v panels last summer. I also changed the charge controller from a Morningstar Prostar PWM to a Tristar MPPT charge controller. I'm charging 6 x 6V AGM and am running an Isotherm 12v refrigeration system. The fridge draws about 7 amps and in the summer heat runs about 50% of the time, so about 80 ah. The old panels couldn't keep up and the new ones have power to spare even on cloudy days in the tropics.

The advantage of using solar to run the fridge is that the cold cloudy days at higher latitudes reduce the power required as well as the power produced.

[Jon Hacking]

Originally Posted by s/vPainkiller

Is there an advantage to using 24V panels in a 12V system? assuming that I'm going to use an MPPT controller that can handle this... is there a benefit to using 24V panels - either in efficiency, reliability or cost?

Yes there is. Besides the points mentioned above, running your panels in series can (sometimes) help with shading. A lit cell produces ~0.5v, & a shaded cell consumes ~0.7v. Your MPPT needs at least ~15v to charge your batteries. Running your panels in series will halve the current to the MPPT, but the MPPT only cares about Watts (Amps*Volts) & its conversion efficiency is pretty good (95%+).

If your panel placement means that much of 1 whole panel will be shaded while the other is unshaded, then the panels should be separated & run in parallel (the shaded panel will not take anything from the other panel). But if your shading is more typically a cell or 2 (or even 5) shaded by a rope or windcharger or something, then the extra voltage you get from running the panels in series will allow you to overcome the voltage drop from the shaded cells & your MPPT will still have plenty of soup to charge your batteries.

A hot panel will produce less voltage, a fact glossed over by most mfgs. See Kyocera's site for more realistic numbers than 1KW/sq-m & 20°C. At temp in the tropics a panel will put out ~16v at max power, or ~32v for 2 in series. So if more than ~15 cells are shaded (half of 1 panel) the whole series array will shut down. But if the panels are in parallel, shading only 1-2 cells in each will shut down that whole panel. So the series/parallel decision really depends on how your panels will be shaded.

[Jon Hacking]

Originally Posted by mikefossl

The advantage of using solar to run the fridge ...

... Is that the hotter the sun, the colder the Beer!

[Jon Hacking]

Originally Posted by noelex 77

Generally the opposite is true with better efficiency as the panel voltage comes close to the battery voltage, but the effect is only small

Noelex, your graphic is too small for me to read, but I must take issue with your claim. If efficiency improved as panel voltage approached battery voltage, then we should see ultimate efficiency when they're the same, which is silly. The MPPT needs some voltage to work its DC/DC conversion, & there are some fixed transistor losses. Those losses become less relevant as the input voltage becomes higher & the MPPT has more voltage to work with.

[goboatingnow]

Originally Posted by noelex 77

Generally the opposite is true with better efficiency as the panel voltage comes close to the battery voltage, but the effect is only small

What are we talking about here, open circuit voltage, nominal voltage. What. Remember the VI curved of a panel isn't linear. Equally they have reasonable output impedance. I don't think there's any advantage in having the nominal voltage at 12, better higher voltage and regulate it, this gives you more operating " envelope"

Dave

[2nd Dog Watch]

How many here have programmed their charge controllers for automatic equalization charge? Assuming of course your charge controller has such a feature and I would guess most if not all MPPT controllers do, my Blue Sky does. My solar system was new 6 years ago, consisting of two 130 watt Kyocera panels, a Blue Sky 3024i MPPT, and 3 Rolls 185 ah wet cell, 550 amp hour total.........I never programmed my MPPT to do the auto equalization and now I wish that I did because my batteries are now heavily sulfided and well beyond the stage to bring them back. I did put a NanoPulser on last year but it's hard to say. My specific gravity tester shows overall poor readings. I'll need to replace them now. I should have got at least 10 years and I know people who have gotten 15 and 20 years out of Surrett/Rolls. I lost on this one!

BATTERIES ARE NOT CHEAP....My advise to anyone interested in utilizing solar for their boats is to also understand batteries and that they need proper care, otherwise just go and install a generator.

[noelex 77]

Originally Posted by Jon Hacking

Noelex, your graphic is too small for me to read, but I must take issue with your claim. If efficiency improved as panel voltage approached battery voltage, then we should see ultimate efficiency when they're the same, which is silly. The MPPT needs some voltage to work its DC/DC conversion, & there are some fixed transistor losses. Those losses become less relevant as the input voltage becomes higher & the MPPT has more voltage to work with.

Sorry about the graph. The server will not let me download it in more detailed format. The top line with the greatest efficiency is for a panel voltage of 17v. The next line down is 34V
It is from the Outback FX60 manual, which you can find online.

Buck converters are generally least efficient when the voltage needs changing the most. They do need some voltage difference as you say, but a nominal 12v panel with a typical MPP voltage of around 17v (which is virtually all 12v panels) provides the greatest efficiency. When you go to a 24v panel with a MPP voltage of around 34v the effciency is less.
This is exactly what the graph shows.

The differences however is small and there are usually other more important factors influencing the decision..

[noelex 77]

Originally Posted by goboatingnow

What are we talking about here, open circuit voltage, nominal voltage.

Dave

The controller will try and keep the panels at the MPP voltage so this is the voltage to use when looking at efficiency.

Some controllers will display this voltage.

The MPP voltage will vary with conditions but you can get some idea from the solar panel specification sheet this will list the voltage at load (Vpm).
For most of the Kyocera 12v panels this is 17.9 v. This needs to be de-rated for temperature, so about 17v is a typical operating voltage for a 12v panel with a MPPT regulator.

[Jon Hacking]

Originally Posted by noelex 77

...The MPP voltage will vary with conditions but you can get some idea from the solar panel specification sheet this will list the voltage at load (Vpm). For most Kyocera 12v panels this is 17.9v. This needs to be de-rated for temperature, so about 17v is a typical operating voltage for a 12v panel with a MPPT regulator.

I'm a bit disappointed in Kyocera, as they used to publish performance specs at the sort of cell temps we see in the tropics, but at least they still give temperature coefficients. The current Kyocera specs for their 135W (nominal) panel show a Max Power voltage of Vmp=17.7 @25°C (77°F) cell temp, with a temp derating of -.52%/°C. In the tropics our panels are in free air above the davits but they still get up to ~50°C (122°F), which brings Vmp=17.7-(13%)=15.4V. If you're trying to charge wet cells at 14.4V, there's not much voltage for an MPPT controller to work with. I still can't find your graph & I don't know what you mean by a "buck" converter, but I'd be surprised if an MPPT controller wasn't more efficient with 16V to work with instead of 1V.

[cfarrar]

I never programmed my MPPT to do the auto equalization and now I wish that I did because my batteries are now heavily sulfided

Solar panels should reduce sulfation by lessening the total discharge and topping off the batteries after each discharge. However, I would equalize on shore power (or maybe I a generator if I had one), so that I'm guaranteed of a constant source of power until the equalization is complete.

Colin

[cfarrar]

For the OP, we have 3 Kyocera panels (216W) wired in parallel to an MMPT controller. This means 12.5-13.0 amps at the charger, so hear in New England this easily runs the fridge (which draws 50-60 amp-hrs a day) unless we get multiple days of thick overcast.

Before you worry too much about number of panels, sizes, voltages, and wiring, realize that your choices for mounting the panels will simplify these decisions. And once you decide the setup the installation really isn't that complicated.

[noelex 77]

Originally Posted by Jon Hacking

I'm a bit disappointed in Kyocera, as they used to publish performance specs at the sort of cell temps we see in the tropics, but at least they still give temperature coefficients. The current Kyocera specs for their 135W (nominal) panel show a Max Power voltage of Vmp=17.7 @25°C (77°F) cell temp, with a temp derating of -.52%/°C. In the tropics our panels are in free air above the davits but they still get up to ~50°C (122°F), which brings Vmp=17.7-(13%)=15.4V. If you're trying to charge wet cells at 14.4V, there's not much voltage for an MPPT controller to work with. I still can't find your graph & I don't know what you mean by a "buck" converter, but I'd be surprised if an MPPT controller wasn't more efficient with 16V to work with instead of 1V.

15.4v is lower than you will generally see in practice, but is still fine for the converter.
converting 15.4v to 14.4v is more efficient than converting 30.8v to 14.4v .


A typical calculation would be at 40 deg c which on my Koyocera panels gives 17.9 -0.92x15=16.52v (the temp coefficient is -092v/ deg C ) My regulator display the panel voltage and it is very rarely below 16 v.

A “buck” converter is an electronic term for the type of converter used by 95% of MPPT solar regulators


The handbook book for the outback controller can be downloaded here and the efficiency graphs are on Page 62

OutBack Power / Products / Charge Controllers / Flexmax