Question about Stand Alone System simulation model

[miguelpdv]

Good day.

I'm running a simulation of a 24V standalone system, powered by 2 x 60 cell modules wired in parallel. The ambient temperature of the design site is typically 28°C at noon, so the PV panels working temperature should be well above 50°C on full power hours.

I was wondering if the model takes in account losses caused by the battery charging voltage not being in the maximum current area of the IV curve of solar panel. I think that the temperature is forcing the panel to work at the right part of the MPP knee of the IV curve, hence not being able to deliver full Imp. This effect should be stronger with low insolations.

I've run a reference simulation with the same power but using 36 cell panels wired in series, and surprisingly I got less power (about 11%) with the 36 cell panels when expecting the opposite behaviour.

For your reference I have the newer version of PVsyst (6.39).

Please tell me if the model is taking this parameters in account, or I should modify some parameters to get a better estimate.

Thanks, and regards from Mexico!

[André Mermoud]

Yes of course the simulation takes the voltages (battery and PV module) into account. The corresponding loss is named "Pmpp loss" in the loss diagram.

Now a 60 cells module is really not suited for the direct charging a 24V battery bank. You should use at least 66 cells modules, the usual design is to use 72 cells.

For better results, the MPPT controllers are now not much more expensive than usual switch-off controllers. And you get rid of the Module's voltage constraint.

[miguelpdv]

Hi André,

Thank you for your kind response.

I work for a PV company here in Mexico, and I'm actually running this simulation because a client wanted a proof that the systems he purchased (and designed) will not work correctly. I've accepted to run the simulation and thought it would be very obvious that the results will be much more favorable for a system with 72 cell system working alongside a 24V battery bank, however for my surprise, I got the exact opposite results.

Just to give some background, on real life operation, the systems stopped working after about a year because dead batteries. This is consistent with the info of the manufacturer of the battery, stating about 340 cycles @ 0% DOD. The system has a 100 W charge running all night, and at the end the system was only capable of turning the load for 3 hours, so I think the real energy flowing to the batteries is about 25% of the design energy.

When running the same model in PVsyst, but changing modules, I get 1,039 kWh/yr for 2S2P 125 Wp modules and 1,166 kWh/yr for 1S2P 250 Wp modules. Just for reference, I have 2S2P 115 Ah @ 12V batteries on both models. As you can see I'm getting 11% higher production on 60 cell system. Should I introduce another parameter to get more accurate results? Is there a not so obvious reason for the model to output higher energy yields on 60 cell model?

Good day!