Max panel output current

[SpecialK]

Hello all,

 

I'm planning a self build solar array, looking to use some of the new Canadian solar 655w panels in series.

Looking at the max output current on these, it says 17.2A on the data sheet.

Looking to run these in to a 4kwh inverter. Looking at the inverter data sheets, I figured you need to look at the max input current that it's rated at, however I notice most of these are around 12A...only the Sunny Boys seem to be around 15A.

I asked a supplier what he thought and he came back and quoted me the short circuit current rating for the inverter, a Solis, which was 22A.

I'm puzzled why he would use the short circuit parameter, surely you have to go by the max input current?

Second point...if you connected these 655w panels in series to an inverter string whose max input was 12A, would the inverter merely clip the current input if it went above 12A?

Any advice would be greatly appreciated...

Thank you

 

[Radian]

The inverter MPP tracker will adjust to the maximum input current the inverter can handle. Note that this may cause the input voltage to the inverter to rise towards the sum of the maximum OC voltage ratings for the modules - this is where the actual danger arises. Exceeding the maximum input voltage will definitely do damage. You should be aiming at no more than 75% of this figure IMO.

 

As well as ensuring the maximum OC voltage is within limits you will also have to consider the minimum voltage that will be presented by your series string. If the aim of using these high power modules is to reduce their number, you may be providing insufficient Voltage to start the inverter.

[SteamyTea]

21 minutes ago, SpecialK said:

run these in to a 4kwh inverter

It is 4 kW.

kW is power, kWh is energy.  kwh is nonsense.

[Radian]

1 minute ago, SteamyTea said:

It is 4 kW.

kW is power, kWh is energy.  kwh is nonsense.

 

I know you want to educate and that's very noble. I'm currently so depressed by world events, I can't bear to lay any more misery on anyone else. Let's be as gentle to each other as we can 😔

[SpecialK]

Thank you @Radian...your point about max OC ratings for the module noted.

In terms of what you mentioned for the min inverter start voltage, I was aware of this, but thanks for that too

@SteamyTea yes of course, appreciated, I was not paying attention to what I was typing as usual, but great you pointed it out... thanks

 

I'm going to put five of these panels on a south facing pergola 

Thinking of getting a hybrid inverter so I can add batteries later.

thanks

 

 

[Radian]

17 minutes ago, SpecialK said:

I'm going to put five of these panels on a south facing pergola 

 

Damn that's a good idea. I've chickened out of fixing them to our roof but we once had a pergola against our SE wall and I could always rebuild it as an array support. The grapevine would thank me for reinstating it too 🍇

[SpecialK]

@Radian yes, I don't want them on my roof either, which is four sided, IMO it's structure is not great for bearing extra load. 

The decking underneath the Pergola needs shade in the summer for it to be comfortable in the sun, so I thought this would be a good idea. Will tilt the panels above 10 degrees and just overlap them on to the summer house, so that the rain from the panels flows in to the summer house gutter system. 

Pergola needs strengthening a bit.

Might add additional panels on the summer house roof later.

[Radian]

This has opened a number of opportunities I hadn't really thought about. I have an 8m wide, 3m deep, South facing deck that gets too hot to use in the summer. Unfortunately its around 30m from the house. I don't fancy trenching in a SWA but it sure would make a fine spot for an PV array.

[SpecialK]

@Radian same here, have another 20 to 30 m of cable run to the inverter in the garage, would use ducting and a good good SWG to cut down losses 

Edited March 5, 2022 by SpecialK

[Temp]

I'm no expert on PV but many years ago I spoke with people building a solar powered plane. They explained that the key for them was to operate the cell close to its maximum power point which did not occur at maximum current. Apparently changing the current drawn from the cells changes the voltage in a non-linear way so it was necessary to adjust the current drawn to find the maximum power point. To do that they used a variable pitch propellor. Presumably modern inverters do something similar then convert whatever voltage and current that turns out to be to the grid voltage.

[Radian]

Yes @Temp I would expect all modern inverters to have MPPT (Maximum Power Point Tracking) as significant conversion efficiency gains are available by optimising the load presented to the PV modules. It's also the process that allows us great flexibility in choosing our PV modules and the number we use.

 

It's quite a fascinating design challenge - measuring the power being transferred is as simple as measuring the voltage (V) at the input of the inverter along with the current (I) flowing in. But then what do you do with the measurements?

 

Power is V*I and, in the case of the inverter, can be changed by controlling the load impedance presented by the conversion stage. If the load presented by the DC to AC converter is started off at a high impedance and gradually reduced while monitoring V*I the power should increase up to some maximum point then begin to fall off again as current continues to increase but the PV voltage falls. The MPP has therefore been found. The trickier bit lies in the algorithm that adapts this process to the dynamics of numerous variables like the radiant energy being absorbed and other factors that affect this such as shading and temperature.

 

I had a go at designing one for the PV/battery combo I use to power my garden lighting and the best I could come up with at the time was to hunt up and down, constantly going past the MPP in each direction to achieve a good average but then all kinds of finer tuning has to come into play to manage situations such as sudden loss of power due to passing clouds. All good fun though.

[SteamyTea]

Isn't part of the problem that PV cells are basically current source devices, and the current they produce is proportional to the light they receive.

 

[Radian]

6 minutes ago, SteamyTea said:

Isn't part of the problem that PV cells are basically current source devices, and the current they produce is proportional to the light they receive.

 

Yes, the only distinction being that they're a 'practical' current source not an 'ideal' one.

[SteamyTea]

6 minutes ago, Radian said:

Yes, the only distinction being that they're a 'practical' current source not an 'ideal one

I asked a question, a year or so ago, about the best way to control the power of a resistance heater. Control the current or the voltage delivered?

Not sure if it was a good question in reality, but no one answered it anyway.

Really a matter of what is the best way to control power when there are several variables. 

 

(While I was the best in my class at university, better than the lecturer, my knowledge is very limited in reality)

 

[Radian]

Practical considerations aside, there can't be any theoretical distinction between current vs voltage control of a non-reactive load. The commutative property of multiplication makes the power product the same.

[SteamyTea]

1 minute ago, Radian said:

Practical considerations aside, there can't be any theoretical distinction between current vs voltage control of a non-reactive load. The commutative property of multiplication makes the power product the same.

Yes, but it was more from a practical viewpoint.

What is the fundamental differences in circuit design.

Though these days it seems you buy a complete chip that can do the lot.

My gut feeling is, if you run things at a higher voltage, you can get away with 'lighter' cabling and controls.

[Radian]

Well, it's really a matter of semantics and how you 'dispose' of the unwanted power if your supply is fixed i.e. the mains supply.

 

Semantics because the end result will be always involve a reduction in both the current and the voltage to the load. In order to reduce the current you have to reduce the voltage and vice versa.

 

Assuming, however, that you want lossless power control (i.e. you don't want to dispose of the unwanted power in another resistance) then you will need to introduce a component to buffer some energy at certain times. This switched-mode system can be as simple as pulse width modulation where the buffering comes from the time constant of the heating device. Or it could be that you choose to monitor either current or voltage and feed this measurement back to the switch to adjust the power accordingly. Voltage measurement is more convenient than current (there are no intrinsic current measurement devices, they all measure a voltage drop which is proportional to current) so this seems to me to be the only practical distinction.

 

Ultimately though, if open-loop PWM is used with a predetermined duty factor then there is no voltage or current control - just power.

Edited March 6, 2022 by Radian