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possibly naïve question on integrated MPPT solar inverters (off grid, DC coupled)

JAndle

Mad Scientist
Joined
Mar 1, 2021
Messages
43
Location
Belize, off grid on Corozal Bay
I have been working out the basics but I am a little baffled by this: 9600Wp system (for example) to a 48V battery must be providing 200A to the DC coupled node but these systems all have max charging current well under 200A. Should I assume there is a 3-way FET or IGBT coupling so that the inverter preferentially draws (e.g. PV) from the MPPT (leg one) independent of the battery but draws the balance of load from the battery (leg 2) and the battery charge current leg is current limited in leg 3? IE a delta connection of MPPT, inverter, and battery?

Otherwise it seems the charge current is also the PV source limit?

The goal is a 10-12 string, 500-600Voc system with up to 6x 200Ah LFP 48V cells(shy of 50kWh) and a 240V single phase inverter (MPPT and inverter integrated?) with an autotransformer to load balance split phase 120Vac.

I think I dismissed a lot of good inverters over the battery charge limits of the MPPT thinking that was the total PV voltage/current output.
 
I am not familiar with any "9600Wp" system.

In most cases, the MPPT charge limit is the PV source limit as well. If a unit has an 80A charge controller, you will never get more than 80A coming out of the MPPT regardless of whether it goes to the battery or the loads. You will find most units also list a maximum PV power.

I am not familiar with any MPP Solar or Growat type inverters that do not adhere to the above.

Many charge controllers can tolerate being overpaneled well beyond what they can handle to enable them to output peak power for longer periods of time with the excess PV power clipped, e.g, my Victron 250/100 MPPT has a 5,800W limit, but it can handle a PV array at 210Vmp and 70A meaning it can handle a 14,700W array.
 
One way to avoid charging battery at excessive rate is to have no more than that much PV panel or that much charge controller.

If you want to install more DC coupled power than the battery can handle (to support use by inverter), you need a system which measures battery current and regulates it.

I think some hybrid inverters with PV terminals will do this. That is what you describe, MPPT and inverter integrated. Have you identified one that supports the amount of PV you want?

Victron charge controller with battery shunt (and supporting hardware) can also do it.

My approach is AC coupled, a battery inverter with 80A max charger current configured (0.2C for my 400 Ah battery). PV goes to AC coupled grid-tie inverters, and delivers enough power to read 0.5C, but the extra is either used for AC loads or output is curtailed.
 
Was looking at 600W panels with 50Voc and 40Vmpp so 10-12 panels is 6000-7200W, but 9600W divided by 48 so much easier... In Belize in June I also have 1350W/m2 peak insolation, so I suppose I scale Ipp and Isc proportional to the 1000W/m2 data (and peak power)?

So the takeaway is, if I want to charge 500Ah of batteries in a 10 hour useful day (50A, C/10) *and* run a 120V/15A average load (so ~48V/40+A) I want to be looking at an 100A or better MPPT to be safe and reliable? 48V*100A is only 4800VA (ignoring efficiencies for a quick discussion)

I would be well over-paneled. Goal is to mount horizontal inside a shallow roof recess for hurricane immunity and allow peak efficiency May - July when heat and humidity are worst (small AC) and not adjust to 18 deg spring/fall or 36 deg winter since my load needs and days of rain are also lower...

for expansion sake I was going to look for a 9600~10KW system with dual MPPT tolerating 600Voc to allow efficient series panel wiring - no shade except clouds - and 48V/1000Ah total battery. But, there were some 96V systems and I am starting to see 96V LFP...
 
One way to avoid charging battery at excessive rate is to have no more than that much PV panel or that much charge controller.

If you want to install more DC coupled power than the battery can handle (to support use by inverter), you need a system which measures battery current and regulates it.

I think some hybrid inverters with PV terminals will do this. That is what you describe, MPPT and inverter integrated. Have you identified one that supports the amount of PV you want?

Victron charge controller with battery shunt (and supporting hardware) can also do it.

My approach is AC coupled, a battery inverter with 80A max charger current configured (0.2C for my 400 Ah battery). PV goes to AC coupled grid-tie inverters, and delivers enough power to read 0.5C, but the extra is either used for AC loads or output is curtailed.
I was not finding a good split phase all in one with high PV voltage. Just 'discovered' Victron autotransformer so now the better German and Taiwanese 240V systems are back on the menu.
 
My system is about that size. It is SMA, Sunny Island and Sunny Boy AC coupled.
I use 48V AGM which is managed by Sunny Island. If you use Lithium it has to be one with a compatible BMS.

A different option from same company is Sunny Boy Storage with 400V lithium. That would be only 6kW from battery and less surge to start motors.

The Victron autotransformer I read about is either 32A or 100A pass-through, but only 28A continuous, 32A peak transferred by transformer to other phase.

You can get any old utility transformer. Efficiency may be lower.

Another hybrid inverter some people here use is Solark. It is about 8kW or 9kW, but you can connect multiple.

 
My loads will be balanced enough and average 15A total. If I get 240V air conditioners that will help a lot. Rain water system has a sub-HP pump. Also not starting ground work till November and Belize is a 2 year process to build anything so 18-20 months out from buying and installing. Plan well, work smart. By then there may be some awesome 96Vdc battery systems ;) [edit: but I should stay with 48V to be under the typical 60Vdc SELV (safe extra low voltage) thresholds in the home utility closet ... just so I don't have to educate an inspector.]
 
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Was looking at 600W panels with 50Voc and 40Vmpp so 10-12 panels is 6000-7200W, but 9600W divided by 48 so much easier... In Belize in June I also have 1350W/m2 peak insolation, so I suppose I scale Ipp and Isc proportional to the 1000W/m2 data (and peak power)?

So the takeaway is, if I want to charge 500Ah of batteries in a 10 hour useful day (50A, C/10) *and* run a 120V/15A average load (so ~48V/40+A) I want to be looking at an 100A or better MPPT to be safe and reliable? 48V*100A is only 4800VA (ignoring efficiencies for a quick discussion)

I would be well over-paneled. Goal is to mount horizontal inside a shallow roof recess for hurricane immunity and allow peak efficiency May - July when heat and humidity are worst (small AC) and not adjust to 18 deg spring/fall or 36 deg winter since my load needs and days of rain are also lower...

for expansion sake I was going to look for a 9600~10KW system with dual MPPT tolerating 600Voc to allow efficient series panel wiring - no shade except clouds - and 48V/1000Ah total battery. But, there were some 96V systems and I am starting to see 96V LFP...

"10 hour useful day"

It's better to thinl of equivalent "high noon" hours of charging.

For Belize City, a 6kW solar system would perform as follows:

1614700673074.png

The AC Energy column is your monthly kWh and considers a 15% conversion inefficiency. The data also considers average weather impact on solar production.

It also assumes that you have sunrise to sunset panel exposure. Panels facing 180° and tilted at 15° year round.

At 15A * 48V = 720W average load = 17.3kWh per day.

Your lowest month is December. 669/31 =21.6kWh per day.

Batteries never spend their time at nominal voltage, so peak power tends to be at and above typical float voltages. A 100A charge controller will typically handle closer to 5800W @ 58V.

There is no inherent advantage to very high string series voltage UNLESS your panels are located far from the controller. There is actually a DECREASE in efficiency with higher string voltages, i.e., there is more loss converting 600V to 48V than there is converting 150V to 48V. However, this is also offset by reduced wiring losses. Multiple panels in series are more sensitive to partial shading - A single panel shaded can have a major effect on the entire string of panels. A blend of series and parallel may be more beneficial.
 
"10 hour useful day"

It's better to thinl of equivalent "high noon" hours of charging.

For Belize City, a 6kW solar system would perform as follows:
I have that chart/site - good to know it's the right source data.
The AC Energy column is your monthly kWh and considers a 15% conversion inefficiency. The data also considers average weather impact on solar production.

It also assumes that you have sunrise to sunset panel exposure. Panels facing 180° and tilted at 15° year round.
Yes, trees will be far enough removed to not be an issue. Due south but I am electing 0 degree for two reasons - it is optimum in the summer when I need the most load and it can lay spaced off a flat roof just under a course of bricks so near zero wind load. I might not be there to lower and lash them for a hurricane... Also laying flat maximizes my roof usage as built, but that is not too critical compared to seasonality of load.
At 15A * 48V = 720W average load = 17.3kWh per day.

Your lowest month is December. 669/31 =21.6kWh per day.

Batteries never spend their time at nominal voltage, so peak power tends to be at and above typical float voltages. A 100A charge controller will typically handle closer to 5800W @ 58V.

There is no inherent advantage to very high string series voltage UNLESS your panels are located far from the controller. There is actually a DECREASE in efficiency with higher string voltages, i.e., there is more loss converting 600V to 48V than there is converting 150V to 48V. However, this is also offset by reduced wiring losses. Multiple panels in series are more sensitive to partial shading - A single panel shaded can have a major effect on the entire string of panels. A blend of series and parallel may be more beneficial.
I was going for minimum copper loss and costs, but the DC/DC buck ratio losses do matter. I'll refine that.

Again, thanks!
 
There is no inherent advantage to very high string series voltage UNLESS your panels are located far from the controller. There is actually a DECREASE in efficiency with higher string voltages, i.e., there is more loss converting 600V to 48V than there is converting 150V to 48V. However, this is also offset by reduced wiring losses. Multiple panels in series are more sensitive to partial shading - A single panel shaded can have a major effect on the entire string of panels. A blend of series and parallel may be more beneficial.

Ahh, but that is where AC coupling comes in. :)

IF much of the AC consumption is to occur as the power is produced, while the sun is shining ...
240 Vrms AC is 340 Vpeak. Having a string of PV panels with Vmp slightly above that means the buck converter in a GT inverter does minimal conversion. Efficiency of 98% to 99% is available.

PV --> GT inverter --> AC --> battery inverter --> battery, only power stored for later goes to battery and later battery --> battery inverter --> AC

Due south but I am electing 0 degree for two reasons - it is optimum in the summer when I need the most load and it can lay spaced off a flat roof just under a course of bricks so near zero wind load.

Two arrays aimed at AM sun and PM sun would flatten power production throughout the day, lower peak but more hours. There is a slight reduction in total watt-hours due to longer path of light through atmosphere.
Flat production means less cycling of battery, more power going straight to AC loads. With production sustained later in the day, discharge of battery starts later.

My system has PV production sized to a bit more than 0.5C of battery; in one hour the production is about the entire usable capacity of my AGM battery. During the day, A/C and other loads run directly off PV production and battery is gradually charged.

Designed for < 600V on a cold day, typically 480 Voc and 380 Vmp.
 
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