Strings

Re: Strings

booboo wrote: »

I was told that if I have one string of panels and want to add another one to the same inverter, I have to add the exact same brand, size and number of panels to the second string. Is there away around this? Like a box that combindes the power of unlike or uneven strings or even wind generators then feed them into the inverter?

In general (we are talking about Grid Tied Solar inverters--correct?), when adding panels to your system, adding identical panels (voltage/current ratings) is the "most efficient"... However, you can mix and match--assuming you do not go too far off the specifications.

For example, if you are adding parallel strings--you don't care about the current in each string (say one is 4 amps, and the other is 5 amps--they both add together for 9 amps)...

What you do care about for series panels is how far of the Vmp (Voltage maximum power) are off from each other... Ideally, you would probably want them within 10% or so... If they are far apart, you run the risk of the MPPT algorithm finding either the lowest panel Vmp (and get reduced voltage * current) or the highest panel voltage (and only get voltage times the current from the high voltage string)...

With current in series matching--pretty similar... Solar panels are more or less current sources... So if put two panels in series with large current differences, then the smaller panel will tend to limit the current flow through the parallel string. Again, if you can match them to within 10% or so--you should be OK.

In any case, the farther the "mismatch" the more power you are going to loose vs if everything was identical.

And when you have wildly differing devices (like solar PV and wind turbine)--you will need a separate Inverter (or charge controller in the case of batteries) because the needs are so different (solar panels, just open a switch if you don't need the power, opening a solar panel connect with a switch will not damage anything--with wind turbines you short them out if you don't need the power to prevent them freewheeling and self destructing in high winds).

Most (all?) Grid Tied inverters are MPPT type devices (Maximum Power Point Tracking) and will work to maximize the equation P=V*I (Power=Voltage*Current). If you paralleled Solar PV and Wind PV, the Pmax point (V,I) will almost certainly be different between the two types of devices. And whatever the MPPT controller chooses, at best it can only maximize the output of one of those attached devices (possibly even causing the other to add nothing to the overall power being generated).

The above would also apply to MPPT type Solar Charge controllers (like Outback and Xantrex, et. al.). Each class/size/output rating of device will need its own dedicated controller to maximize harvesting of power. And, possibly different configuration/programming (solar PV--just open a switch to stop charging; Wind--turn on a diversion/shunt load to prevent the turbine from over-speeding and the batteries from overcharging).

-Bill