Digivation Hack Central

Optimal String Sizing for PV Systems

by on Apr.23, 2010, under Solar

Optimal string sizing for a given inverter can improve your return on investment.

As an example, I will use my system as an example. The PVI-5000-OUTD-AU efficiency plot is given below.

Aurora Efficiency Plot


The important panel parameters are:

Panel: TH175M24
Pmax: 175watts
Vmp: 36.2V
Imp: 4.85A
Voc: 43.9V
Isc: 5.30A

It can be seen from the inverter efficiency diagram that the sweet spot is at 50% full power and 345V string voltage. Note that it has 2 MPPTs. These may be independently specified.

Now the 28 panels for 4.9kW need to be split between the MPPTs such that the Pmax string voltages are near the optimal 345V. This gives us 2 optimal and a symterical choices:

  1. 2 x 9 Panels in parallel + 1 x 10 panels. This gives string voltages of 325.8V and 362V.
  2. 2 x 10 panels in parallel + 1 x 8 panels. This gives string voltages of 362V and 289.6V.
  3. 2 x 7 panels in parallel + 2 x 7 panels in parallel. This gives string voltages of 253.4V

The open circuit string voltage for this inverter must be less than 600V. This is 439V for 10 panels, so this parameter is ok.

By inspection of the efficiency plot, configuration 1 is the best. In fact is it 0.3% better than the symmetrical configuration, which would result in about $10 per year more income/power at 68c/kWh.

The MPPT which has 2 strings in parallel has a greater weight attributed when choosing panel configuration as it supplies more power.

Refs:
Aurora PVI-5000-OUTD-AU Datasheet
TH175M24 specs

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