Off grid system - School project

[pfgiroux]

Hi, I’m a electrical engineer student in Montreal.
I’m currently designing a solar panel system for a school project and will eventually install it. I need help with the choice of some equipment. I have 4x bifacial solar panels 370W and 4x battery lithium ion 12V (datasheets are in attach files). I will use the battery at 24V because I have equipment at this voltage and they will use around 1 kWh for a day of use. I want my system to be self sufficient so power my 24Vdc equipment and charge the batteries. The excess energy produced will be unload in a resistance bank. From my understanding, a mppt for 4 solar panel will cause problem with shading and a company from here told me that 4x microinverters (1 per pannel) don’t produce a voltage that is steady enough to supply a 120Vac/24Vdc converter/charger. So, if someone can enlighten me with this I would gladly appreciate it. Also, my first langage is french so don’t hesitate if you don’t understand something.
Thanks a lot !!

 

[12VoltInstalls]

The excess energy produced will be unload in a resistance bank

You mean running a heater?
Elsewise a SCC will just turn down panel loading - no excess energy dump needed.

From my understanding, a mppt for 4 solar panel will cause problem with shading

No, if shading is a problem do not run them in series, put them in parallel with an SCC that can process the amps. Or just don’t put them in a shading problem area.

and a company from here told me that 4x microinverters (1 per pannel) don’t produce a voltage that is steady enough to supply a 120Vac/24Vdc converter/charger.

You need a solar charge controller for a battery system, not microinverters.

What hardware are you planning? An all-in-one? You don’t need a converter-charger.
You need a standard solar charge controller. And perhaps an inverter. And it sounds like you want a grid-powered 120VAC 24V battery charger?

So, if someone can enlighten me with this I would gladly appreciate it. Also, my first langage is french so don’t hesitate if you don’t understand something.

Are you a first year student?
Or already along a ways?
Your English seems fine except for the accent…ha ha

Anyways- an off-grid system usually doesn’t have a 120VAC charger. Perhaps by off-grid you mean no grid feed/contribution, just self contained.

 

[pfgiroux]

To put you in context we want to do an agrivoltaic concept but on a building's roof. So at 24Vdc we have 4x stepper motors to rotate the panels, 2x water pumps and 2x valves for irrigation and a bunch of sensors.

There are a few trees where the system will be install so there will be some shade at some point.

We don't wanna use the grid unless there is no other way. The idea is to use the micoinverters to have 120Vac and the running heater would simulate a load like if it was connected to a building. Plus, we were hoping we could use the 120Vac to charge our batteries with a converter/charger and power our 24Vdc equipment.

Not a first year student but I don't have experience with solar system,

Thanks !

 

[timselectric]

Micro inverters need to see a grid source to turn on and produce.

 

[pfgiroux]

Micro inverters need to see a grid source to turn on and produce.

Ok and then I can use the 120Vac bus to connect a converter/charger for my 24Vdc ? Do I need additional equipment ?

 

[timselectric]

I would recommend an off grid AIO (All In One).
Growatt, EG4, MPP, to name a few.
It will give you everything you need. And simplify things.

 

[timselectric]

And read a lot of threads in this forum.

 

[pfgiroux]

And read a lot of threads in this forum.

I already have the solar panels and the batteries. And yes I plan to do it.

 

[Mattb4]

To put you in context we want to do an agrivoltaic concept but on a building's roof. So at 24Vdc we have 4x stepper motors to rotate the panels, 2x water pumps and 2x valves for irrigation and a bunch of sensors.

There are a few trees where the system will be install so there will be some shade at some point.

We don't wanna use the grid unless there is no other way. The idea is to use the micoinverters to have 120Vac and the running heater would simulate a load like if it was connected to a building. Plus, we were hoping we could use the 120Vac to charge our batteries with a converter/charger and power our 24Vdc equipment.

Not a first year student but I don't have experience with solar system,

Thanks !

You seem to be mixing grid tie and off grid setups. Microinverters are grid tie. This means they need a grid in order to parallel with. Most of the ones that are of any quality do not output 120vAC. To use a grid tie you must understand parallel power supplies.

If you are using an off grid but grid assist setup ( This is what I run at my home ) your power from the PV panels goes to an All in one (AIO) that incorporates a SCC (Solar charge controller), ATS (Automatic transfer switch) similar to a UPS in function, inverter/charger. It has connections for PV, battery, AC in (grid) and AC out. It functions either from PV/Battery to inverter then AC out. or it takes in Grid power and bypasses the inverter to supply AC out. While it is in bypass (or pass through) it can use grid power to charge the battery alongside PV power.

You may wish to research off grid solar inverters.

 

[timselectric]

Planning comes before purchases.
You have 12v batteries. But want a 24v system.
You have solar panels, without knowing what the SCC requirements are.
You're already starting off wrong.

 

[pfgiroux]

Planning comes before purchases.
You have 12v batteries. But want a 24v system.
You have solar panels, without knowing what the SCC requirements are.
You're already starting off wrong.

Panels and batteries are loaned by the school so no purchases. What do you mean 12V batteries but want a 24v system like if it wasn't possible to make a combination of series parallel .

 

[pfgiroux]

You seem to be mixing grid tie and off grid setups. Microinverters are grid tie. This means they need a grid in order to parallel with. Most of the ones that are of any quality do not output 120vAC. To use a grid tie you must understand parallel power supplies.

If you are using an off grid but grid assist setup ( This is what I run at my home ) your power from the PV panels goes to an All in one (AIO) that incorporates a SCC (Solar charge controller), ATS (Automatic transfer switch) similar to a UPS in function, inverter/charger. It has connections for PV, battery, AC in (grid) and AC out. It functions either from PV/Battery to inverter then AC out. or it takes in Grid power and bypasses the inverter to supply AC out. While it is in bypass (or pass through) it can use grid power to charge the battery alongside PV power.

You may wish to research off grid solar inverters.

Ok, so if I want to be off grid, is it a good option to use a similar system as this one ? Panels in parallel for the shade's problem and I would connect a heating resistor on the 120Vac to simulate a small house load (it's a school project so much simple than connecting to a building grid for us)

 

[timselectric]

Panels and batteries are loaned by the school so no purchases. What do you mean 12V batteries but want a 24v system like if it wasn't possible to make a combination of series parallel .

These aren't lead acid batteries.
Each battery has a built-in battery management system. (BMS)
If placed in series they won't stay even with each other. Unless you buy extra equipment. (A battery balancing system)

 

[Mattb4]

A few problems with the setup in the image. SCC that have a load output terminals are strictly limited to how much amperage is supplied versus the battery output terminals. If you want to supply 24vDC loads you will need to take it off the battery. PV panels that output 24vDC will not charge a 24vDC battery. Know the ratings of the panels for Voc, Vmp, Ioc, Imp. Match them to the SCC.

For loading from your inverter you can simply use a light bulb to show that you have AC power. Considering your loads are mainly 24vDC you will not want to use much AC power.

 

[pfgiroux]

A few problems with the setup in the image. SCC that have a load output terminals are strictly limited to how much amperage is supplied versus the battery output terminals. If you want to supply 24DC loads you will need to take it off the battery. PV panels that output 24vDC will not charge a 24vDC battery.

For loading from your inverter you can simply use a light bulb to show that you have AC power. Considering your load are mainly 24vDC you will not want to use much AC power.

Ok I understand. The vmp of my pannels is 40V

 

[Mattb4]

Ok I understand. The vmp of my pannels is 40V

Make sure your SCC can handle the Panel Voc.

 

[pfgiroux]

Make sure your SCC can handle the Panel Voc.

Ok perfect, very helpful ! What are your thoughts on placing the 4x 12V batteries in series-parallel to have the 24V system ?

 

[12VoltInstalls]

The idea is to use the micoinverters to have 120Vac and the running heater would simulate a load like if it was connected to a building. Plus, we were hoping we could use the 120Vac to charge our batteries with a converter/charger and power our 24Vdc equipment.

I would recommend an off grid AIO (All In One).
Growatt, EG4, MPP, to name a few.
It will give you everything you need. And simplify things.

Basically you need a 24V supply, limited to no 120VAC for grid assist on cloudy days, and something to handle your panel’s VOC.

An AIO will do all that with ~750Wh/day of self-consumption.
Making 120VAC to convert it to 24VDC is like walking counter-clockwise all the way around the house to go in the back door when the back door is already only 4’ away to your left.

 

[Mattb4]

Ok perfect, very helpful ! What are your thoughts on placing the 4x 12V batteries in series-parallel to have the 24V system ?

Not familiar with the Lithium battery that you posted the spec sheet on. You would want to check with the manufacturer about series connecting them. Might require a different BMS.

 

[12VoltInstalls]

Might require a different BMS.

Right.
Some batteries list ‘not for serial’ or state how many can be serialed, some say nothing.

You would want to check with the manufacturer about series connecting them