Efficient Solar Design Voltages+MPPT

Wanderman

Greetings all. I have the opportunity to design a system for RVs and small dwellings based around semi-flexible (3mm) panels.

Nominally it will revolve around 3 panels that produce ~410watts

***36V Specs
Pmax 170 w
Voc 41v
Vbm 36v
IPM 4.73a
ISC 5.10a
63” x 26.75” (1600mm x 680mm) 140 Watts

Pmax 140 w
Voc 41v
Vbm 36v
IPM 3.89a
ISC 4.20a
29” x 31.5” (990mm x 800mm) 100 Watts

Pmax 100 w
Voc 41v
Vbm 36v
IPM 2.78a
ISC 2.99a

***24V Specs
Pmax:170W
Voc:28V
Vpm:24V
Ipm:7.09A
Isc:7.65A


Pmax:140W
Voc:28V
Vpm:24V
Ipm:5.84A
Isc:6.30A


Pmax:100W
Voc:28V
Vpm:24V
Ipm:4.17A
Isc:4.50A

***18V Spec (ONLY 100w shown)
Pmax 100 w
Voc 21v
Vbm 18v
IPM 5.56a
ISC 5.99a


these will be charging 12volt battery bank(s) that start at 230a/hrs and expand depending upon usage and location.

We have the option of producing these panels in 18v 24v or 36v

We are attempting to decide on an MPPT controller to maximize efficiency of the limited panels. Right now the leaders are Blue Sky's 3024iL and the Tristar TS-MPPT-45.

In the RV application cable runs will be no greater than 14ft.

The question is what voltage to manufacture and which controller to buy to maximize efficiency (and reliability).

Thanks,

Richard

mike95490

Re: Efficient Solar Design Voltages+MPPT

The Tristar MPPT is more efficient, with several stages that disconnect, as the avaiable power decreases, so less internal losses. The 60A MPPT is same footprint, and has WEB HTML interface, so you might keep that as an "upgrade".
The tristars also have Battery Temp Sensor inputs, and Remote Voltage Sense. I don't know if the BS has that.

niel

Re: Efficient Solar Design Voltages+MPPT

b sky does have battery temperature sensor ability, but it is sold separate. b sky is older technology and has no ability for remote battery voltage sensing. b sky also has a low input voltage maximum making the tristar the better buy by far imho.

dmiller

Re: Efficient Solar Design Voltages+MPPT

I wonder how beneficial MPPT is on a 18v panel system on an RV.

niel

Re: Efficient Solar Design Voltages+MPPT

if that is an 18v vmp then you will have some gain and with a deeper battery discharge you may get a bit more out of mppt (do not discharge batteries farther than 50% dod just for a minute gain). heat may lower the pv voltage some and thusly lower the gains in summer, but even then you will still show gains over a pwm.

dmiller

Re: Efficient Solar Design Voltages+MPPT

Unless the controller is decided to always be MPPT, it seems the lower voltage panels should be used. That would allow the use of just about any charge controller with sufficient capacity. The panels should always be run in parallel. The number of watts per bypass diode needs to be part of the design.
With a MPPT price premium of $250, buying more panel watts instead of the more expensive controller may produce more power. Especially is it meant larger panels rather than more panels.
The best choice is only know by specific analysis. But if MPPT power improvement is 10%, the numbers may favor more panel in this application. If the MPPT improvements is >20% the numbers would likely favor the expensive controller.
Without real numbers, subjectively "up to 30% more charging" with MPPT means 10%. If the units could real world average 30% improvement the marketing claim from test conditions would be at least "up to 50%".
I suspect that in the very dynamic and sub-optimal world of mobile solar the active decision making of MPPT controllers is either helpful or makes a mess of things. The quality of the MPPT implementation may be more important than stationary solar.
If we're lucky we may get a bit of insight from someone who engineers these things.

BB.

Re: Efficient Solar Design Voltages+MPPT

Roughly, you can compare MPPT to PWM efficiency as:

• 14.5 volts battery charging / 18 Vmp panel = 0.80 eff PWM
• A MPPT controller is around 90-95% efficient

So, roughly you have a 10% loss for PWM on a "Typical day". But, remember that MPPT controllers have more losses (tare losses) than PWM--so if you have a 60 amp MPPT charge controller and 10 amps of solar panels (allowing for later expansion)--the MPPT can be less than 90-95% efficient (again, depends on particular controller design).

In general, systems less than 200 watts--PWM is probably more cost effective. Systems over 400 watts are a good candidate for MPPT. For systems inbetween, either will easily meet your needs.

Unless you have specific issues (very long wire runs from solar array to charge controller, very wide temperature ranges like a high desert/mountains, or want detailed statistics)--Just do a paper design of both types of system (PWM and MPPT). Look at the costs vs features and make your decision that best fits your needs/capabilities.

-Bill

mike95490

Re: Efficient Solar Design Voltages+MPPT

I've noticed on my Morningstar 60A MPPT, that it shows MPPT only in the BULK phase of charge, when it switches to ABSORB, it apparently does not need the extra amps, and maybe throttles back into some PWM mode. (no AM radio, so I can't tell whats happening).

SG, any insight on this?

BB.

Re: Efficient Solar Design Voltages+MPPT

Maximum Power Point Tracking (as I understand) is only "needed" when the output power is pretty much near maximum available from the solar panels.

Once the battery is nearing full charge, the power output requirements go down, so there is no need for MPPT--and the controller will just take power (more or less) like a PWM controller from the solar panels.

-Bill

niel

Re: Efficient Solar Design Voltages+MPPT

as bb said it's not needed in those modes and your observation is totally normal.

Dave Angelini

Re: Efficient Solar Design Voltages+MPPT

mike90045 wrote: »

I've noticed on my Morningstar 60A MPPT, that it shows MPPT only in the BULK phase of charge, when it switches to ABSORB, it apparently does not need the extra amps, and maybe throttles back into some PWM mode. (no AM radio, so I can't tell whats happening).

SG, any insight on this?

They all do this Mike! Outbacks, XW's ect. Think about it! I know you know the answer!

Dave Angelini

Re: Efficient Solar Design Voltages+MPPT

OK a clue! If the output voltage is being regulated at the set-point...

dmiller

Re: Efficient Solar Design Voltages+MPPT

The percent of system cost of an MPPT controller is so much greater on a 12v RV system compared to larger 48v home system that it needs to be more carefully considered. Also consider that most RV systems are used anywhere near 365 days a year.
There are clearly poorly designed RV systems. There are poorly installed RV systems (when the panels aren't connected to the cc the systems tends not to work well). But the only measure of success is if the user is happy.
There are no power bills. The economics can be measured, but a meaningful historical comparison would be difficult.

mike95490

Re: Efficient Solar Design Voltages+MPPT

Dave Sparks wrote: »

OK a clue! If the output voltage is being regulated at the set-point...

I've got it, I sort of answered it in my own post. "When the batteries don't need full amps......"

So the MPPT advantage is only in bulk, not the other stages.

Dave Angelini

Re: Efficient Solar Design Voltages+MPPT

If the output is regulated, and the load is fixed or decreasing, the converter can't be at the mppt unless the load increased. Assuming that there is enough source.

No ! The converter can be at mppt in float if there is enough load! Load can be more than the battery load roger?

Dave Angelini

Re: Efficient Solar Design Voltages+MPPT

For instance, tomorrow I am planning to keep my controller close to mppt in absorption by running an icemaker for a party. Are loads not wonderful?

mike95490

Re: Efficient Solar Design Voltages+MPPT

Oh yeah, that's why I've set my water pump on a timer, Noon - 2 pm, after the batteries are mostly out of bulk, and into absorb.

nigtomdaw

Re: Efficient Solar Design Voltages+MPPT

The Rogue MPPT charge controller would seem a economical fit as it comes with a LCD display, and a Battery Temp Sensor as Standard, at $295 it stacks well against the standard PWM Xantrex and Morningstar Units which work out very similar in cost when you add the display and BTS. Has the added vantage of data logging, 5 yr guarantee.

HTH Nigel

dmiller

Re: Efficient Solar Design Voltages+MPPT

I would like to go Rogue, but not enough amps. XW and control panel is going to be $700. Plus giving money to a company that couldn't care less about customer support.