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Sizing MPPT


JohanG

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I have made the following calculation regarding solar panel setup and MPPT feed 

9 x Renewsys Deserv 260W Solar Panels

tied to 

Victron SmartSolar 100V/30A MPPT Charge Controller

Series                            
No    Voltage    Amp    Number    Voltage    Amp    Hours    Total KW
3        30.72        8.47                         92.16    8.47        
Parallel                            
3        92.16        8.47                          92.16    25.41                     2.3418
                            
Panels                                    9        Per Day                  4    9.3671
                            
        Output        92.16 V    25.41A        
For detail look at the image

               
            image.thumb.png.a674905a8b7add0bcc08d2568b67ab8c.png  
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Those are 60-cell modules, even at very cold temperatures they should remain below 100V (three series). So the voltage is fine.

I don't know what your Isc (short circuit current) is, but it is probably around 8.5A. For three string, that is 26A, which is below the rating for the MPPT (35A), so also fine.

Total wattage is 2340W, at 48V that is 49 Ampere. Controller is only rated for 30A. So you're oversizing by about 60%. That is a tad too much.

I'd go with a 100/50 instead.

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Hi plonkster

Where does the of 48V that gives 49A come from?

The MPPT is 12/24 autoselect and my batteries are 12V connected parallel ( the inverter, Cotek: 2000W Inverter SK2000-Pure Sinewave, I have is 12V because at the time I did know better)

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2 hours ago, JohanG said:

Where does the of 48V that gives 49A come from?

I guessed. But I made a massive mistake, as you point out the controller is max 24V, the 100/50 too. The 150/35 is the smallest unit that is 48V capable.

2 hours ago, JohanG said:

The MPPT is 12/24 autoselect and my batteries are 12V connected parallel ( the inverter, Cotek: 2000W Inverter SK2000-Pure Sinewave, I have is 12V because at the time I did know better)

At 12V you're even worse off. The 30 in "100/30" is the maximum charge current on the battery side. So your controller is capable of max 14*30 = 420W of the 2340W provided by the 9 panels, about a sixth. You need a MUCH larger controller to put down that much power.

(why 14V? Best case scenario, rounded so I don't have to use a calculator. Lead acids charge at around 14.5V. At 12V nominal it's even worse, a mere 360W).

2340W... let's round up to 2400 to make the math easy. Divide by 12V. That's 200 ampere. You're going to need two 150/100 controllers (almost the most expensive ones) to put down that kind of current. In addition, the kind of battery you need to accept 200A would have to be at least 1000Ah lead acid (at 12V), or 400Ah LiFePO4. And then we start talking cabling... thick expensive things.

Basically, what I am saying, is you really don't want to do this with 12V. 12V is really no good beyond 1kw or so. Everything simply becomes too big.

What you can also do is scale it down to just two of those 260W modules. Put them in series, that's around 520W. Use a 100/50 charge controller. Again, you need a battery of at least 200Ah.

Edited by plonkster
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Plonkster

Thank you for the write up

I am slowly getting a grasp of this very complicated subject

How many "salesmen" out there really know what they are selling?

 

I have 4 x 100 Ah batteries at present as said previously

If I hook them in parallel then I have 48V and I replace the cotec inverter with  a Victron Phoenix 12V/24V/48V 800VA-700W Inverter

Victron Phoenix TECHNICAL

Cont. AC power at 25 °C (VA): 800

Cont. power at 25 °C / 40 °C (W): 700 / 650

Peak power (W): 1600

As I have a rainwater 0.75kW pump the Vicron will have to be able to manage the inrush current on startup.

You have previously covered this question of inrush ....  will the  Victron Phoenix do the job?

 

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1 hour ago, JohanG said:

If I hook them in parallel then I have 48V and I replace the cotec inverter with  a Victron Phoenix 12V/24V/48V 800VA-700W Inverter

If you hook the up in parallel, you'd have 12V at 400Ah. If you hook them up in series you'd have 48V at 100Ah. The total amount of energy would be the same (12V*400Ah = 48V*100Ah = 4800Wh 😃 4.8kWh).

1 hour ago, JohanG said:

Victron Phoenix 12V/24V/48V 800VA-700W Inverter

All Victron inverters can handle 200% peak for about one second. A typical induction motor has a startup peak of as much as 500% nominal. So I have a feeling this is going to be a bit too tight. From experience I can tell you that starting an 850W well point pump with a 1.6KVA worked  perfectly, so this is what I would advise (it allows some margin for other appliances too). I do know there is quite a jump in price though, at least go with a 1.2KVA (it will probably be fine, I just won't put my nuts on the chopping block if I was selling it to a customer 🙂 ).

Edited by plonkster
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6 minutes ago, JohanG said:

Would an inrush current limiter be helpfull ?

What you would probably look at is either a soft-start device (something that ramps the voltage to the motor and lowers inrush a bit), or a VSD (variable speed drive). A VSD will be more expensive than just buying a larger inverter, and I simply don't know about a soft-starter. Your google is as good as mine. If I remember correctly, it only really lower the startup current by maybe 30%, and it is highly dependent on the torque requirement of the load (the pump in this case). There is a certain amount of torque the motor has to exceed to overpower the stationary load and start moving it. Once you overpowered the stationary load, it is usually easier to get it up to speed. This means that a VSD or soft-starter must preferably jump to a power level that is sufficient to start moving the motor, and then ramp up from there. Whatever this power level is, that will become your new "inrush" current.

It really all depends on whether this is going to be cheaper than a larger inverter.

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To suit the 48V setup I will have to change the MPPT to 

Victron SmartSolar 150V/45A  

according to the spec sheet 

Nominal PV power, 48V: 2600W.

Battery voltage: 12/24/48V Auto Select 

So now it seems as if everything ties up?????

 

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