Nominal Voltage

Re: Nominal Voltage

firefly wrote: »

Is there a way to determine the Nominal voltage of a solar panel?

A solar cell is an illumination based current source that is clamped in maximum voltage by the forward biasing of its inherent diode. A current source is a high impedance source, not like a battery or other voltage sources that have low driving impedance.

Voc, or V Open Circuit is where the cell is unloaded and all illumination generated current is shunted back down the cell's inherent diode. For silicon this is about 0.65 volts and has a negative temp coefficient of -2 mVdc per degree C.

Vmp, or V Maximum Power point is a point of loading of the cell such that the inherent diode just barely starts to conduct. Most of the illumination generated current goes to the external load. Only 1-3% of the illumination current is shunted down the cell's inherent diode. This point has an operating voltage about 0.15 vdc per cell lower then the full Voc voltage. It produces the maximum V x I available from the cell. Again, it has a -2 mV/degC temp coefficient so as the cell gets hot this voltage point is lower. Normally, heated by the sun, the cell Vmp will be between 0.45v and 0.5v depending on ambient surrounding temperature.

Beyond this MPP loading point the cell acts pretty much like a current source. If loaded to 0.4 vdc it will produce the same current output as when it is loaded down to 0.2 vdc, for the same illumination level.

So 'nominal' has a loose definition. The 'desired' voltage is Vmp which yields the most power from the cell. An MPPT controller adjusts the load on the PV panel to maintain a voltage close to Vmp. As sun goes behind a cloud, the MPPT controller must lighten the load on the panel to maintain its Vmp voltage.

You may ask why 36 series connected cells in a PV panel for a 12v lead acid battery. It's because in the full sun, in a hot climate, the Vmpp will drop from 18v at room temp to about 15 vdc which, with some wiring and controller voltage drop, just makes the 14.5 vdc absorb voltage required to fully charge the battery.

A 'PWM' controller is just a switch that connects and disconnects the panel to the battery to regulate battery voltage. The PV panel will be operating in its current source mode. The PV panel will drop close to whatever the battery voltage happens to be. It is very important that under hottest temp the panel's cells stack inherent diodes conduction point voltage not drop below battery voltage. If this happens the battery will discharge high current into PV panel. This is why the 36 cells in a 12v PV panel. A blocking diode or switch is usually put in series with the battery just to be sure, and avoid the panel leakage resistance during darkness. Because the panel does not achieve Vmpp voltage the PWM controller is less efficient then a MPPT controller.