This article will cover various design aspects of OpenSolar, including inverter stringing, micro-inverters and power optimisers. It will also touch on each of these in relation to the solar modelling softwares used to calculate output in OpenSolar (PV Watts and System Advisor Model)
Stringing and determining the number of optimisers/microinverters
- There are two modes: Manual and Automatic
- Manual is generally the preferred method for stringing as it gives you full control over how you want the system to be strung. The default inverter modelling system can be changed in control > design & hardware > setbacks & design settings > simulation settings.
- Automatic stringing will select an inverter from your active list based on the system size and a DC/AC ratio of 1.3.
In the above screenshot, manual inverter modelling has been used, allowing you to design the string configuration yourself. Here, there are two strings on the same MPPT, one on the 12 panel array and the other on the 8 panel array. To design the stringing, simply click +string or +MPPT and then click the first panel in a row, and then the last panel, to string them.
- We currently assume that for micro inverters there will be one micro inverter per panel.
- Optimisers are added as other components with the type set to “DC Optimiser”.
- Once you have optimisers in your list of components in your account then you can click on the use DC optimiser button in the panels section in studio.
- The number of optimisers that are used in the system are determined by the panels that are used and the sizing constraints set on the optimiser.
- We can automatically determine whether the panel to optimiser ratio should be 1:1 or 2:1 etc. based on the specs of the panels and the optimiser (based on power, current, and voltage).
Effect of Optimisers/Micro-inverters using PVWatts
Using a micro-inverter or optimiser will change the mismatch loss to 0%
It will also apply a Shade mitigation factor (SMF) of 33%. So if there is any shading applied, the loss from that shading will be reduced by 33%.
Effect of Optimisers/Micros using System Advisor Model (SAM)
This will change the default losses:
If a string inverter is used with no optimisers then the losses will be:
Module mismatch = 2%
Diodes and connections = 0.5%
DC wiring = 2%
If an optimiser is used with a string inverter then the module mismatch will go to 0%, the DC wiring will go to 1% and the optimiser's efficiency will be used
If a micro-inverter is used the module mismatch will go to 0% and the DC wiring will go to 0.1%.
For shaded string inverter (no optimiser) systems, SAM will incorporate a validated data set to look up what the output will be based on the shading values calculated from OpenSolar 3D.
For shaded micro and optimiser systems, the shading values will have a direct relation with the output. So when the system is x% shaded, the output would also be reduced by x%.