Panels in Series

System

I realize this is undoubtedly a basic question, but I couldn't hit the right keywords in the search engine to find a post that really answered the question, so...

The question is, what are the issues, if any, involved in hooking PV panels together in series? The system I'm looking at putting together is to run my house, off grid, in a very hot, sunny climate (north Kenya).

I'm looking at 8 Kyocera 135w panels, set up as 4 parallel sets of 2 panels in series to provide input to a 24v system. MPPT charge controller, 24v true sine wave inverter/charger (since I'll have a generator running on occasion).

Thanks!

stevend

Re: Panels in Series

Connecting panels in series increases the voltage while leaving the current unchanged. Using the Kyocera KD135GX-LPU as an example, if you connect two in series then the resulting voltage is double the Voc (22.1V) for a total of 44.2V. The current, the Isc (8.37A) stays the same.

Connecting in parallel does the opposite, the current adds and the voltage stays the same.

Multiply the current by 1.25 since the output can be higher than the rated values.

So 4 parallel sets of 2 in series is 44.2V, 41.85A.

The higher the current you have, the thicker the wire you need. For 8.37A, 14AWG wire will do. For 41.85A you need 8AWG wire. Alternatively you would run two sets of wires to your charge controller and divide your panels in two. So two sets of 2x2 panels, each set being 44.2V, 20.93A needing 10AWG wire.

However, the voltage drops depending on the length, thickness, temperature of the wire. For that you need a voltage drop calculator like this one:
http://www.nooutage.com/vdrop.htm
Assuming a 30 foot wire length, 44.2V, 41.85A, in PVC conduit, at 35C (you know better than me for Kenya) I get a 3.5% voltage drop, so 1.55V drop giving 42.7V.

If your charge controller is the Flexmax 60 MPPT then it can handle 42.7V and 41.85A easily. The output of the Flexmax 60 would be the number of panels x the watts per panel divided by Vmp, so (135x8/17.7) 61amps. This is too high for the Flexmax 60 so it'd be a Flexmax 80 instead.

-Steve

mike95490

Re: Panels in Series

stevend wrote: »

....... 61amps. This is too high for the Flexmax 60 so it'd be a Flexmax 80 instead.

-Steve

Actually, the only time it would be too high for a 60A MPPT controller, is in perfect conditions, in cold winter days with snow !! When the panels heat up on the roof, their voltage goes down some, and you can only expect 80% of nameplate STC power out of them. So, unless you have a tracker that aims the PV at the east for sunrise, on cold mornings, you generally won't see 61 amps produced.
Additionally, the MPPT controllers generally throttle back, as they heat up toward their max temperature, or full listed output.

But inspectors may want to see some extra overhead, so check local electrical codes.

System2

Re: Panels in Series

mike90045 wrote: »

Actually, the only time it would be too high for a 60A MPPT controller, is in perfect conditions, in cold winter days with snow !! ......
But inspectors may want to see some extra overhead, so check local electrical codes.

This is Africa....no inspectors or electrical codes to worry about. And Lokichogio is NEVER cold, more's the pity.

Thanks guys.