Solar Photovoltaic Systems

This section covers the conversion of incident solar radiation at a panel to electricity.

Photoelectric Effect

  • When a photon (from the sun) strikes an atom it can liberate an electron.
  • Normally this electron will rejoin an atoms valence shell and be lost

Electrons and Holes

  • In semiconductors, the vacancy left by a liberated electron behaves like a positive charge carrier.
  • We call this vacancy a hole.

PV Cell Operation

  • PV cells use a PN junction to create a region where this liberated electron and hole will be swept apart
  • These charges can then create a current.

PN Junction

  • By manipulating the materials in the layers, an electric field is created
  • This electric field separates the electron from the hole

Cross Section of Cell

Equivalent Circuit

The simplest model of a solar cell is

  • a current source that is proportional to the incident radiation
  • a diode that models the PN junction

Current Source

An ideal current source provides a set amount of current to any voltage

Diode

An ideal diode only allows current to flow in one direction.

Solar Panel Model and IV Curve

The solar panel is modeled as a diode.

Diodes

  • Diodes allow one-way current flow
  • At a high enough voltage, the diode begins to conduct in one-direction

Diodes

Simple Diode Model

PV IV Curve

Together, these two elements create this IV curve. This curve shows the real (non-ideal) behavior of the diode.

Photovoltaic IV Curve Plot

  • note the short circuit current
  • note the open circuit voltage
  • these are important parameters

Resistor

PV graphical determination

Recall that for two elements, the current and voltage are given by the intersection of two IV curves.

Power Determination

The blue curve is the product of the current and voltage at each point on the curve. This represents the power.

This means different resistor values will draw different amounts of power.

Variation with Insolation

Since the current provided by the current source varies with the amount of solar insolation, the IV curves vary as well.

More complete model

To account for internal resistance and shading effects, we have this model of the cell.