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The application of Appropriate Technology

Part 1: Photovoltaic (PV) Cells

1.1 The I-V Curve

Semiconductor solar cells convert sunlight into electricity using the photovoltaic effect. Incident light falls on the cells and creates mobile charged particles in the semiconductor which are then separated by the device structure to produce electrical current.

The vast majority of solar cells are made from crystalline silicon. Single crystal cells are the most efficient however, cheaper multicrystaline cells are also popular. Even cheaper amorphous silicon cells are also available and used widely for small consumer products but rarely used for power systems.

A single PV cell will produce between 1 and 1.5W at a voltage of 0.5 to 0.6V under standard test conditions. Standard test conditions are: an irradiance of 1kW/m2, standard reference AM1.5 spectrum[1], and a cell temperature of 25°C.

A characteristic I-V curve is shown in figure 1. The important points are:

  • Short Circuit Current (ISC) – This is the maximum current that the cell can provide and it occurs when the cells is short circuited. Unlike other small scale electricity generating systems PV cells are not harmed by being shorted out.
  • Open circuit Current (VOC) – This is the maximum voltage that exist between the cells terminals and is obtained when there is no load connected across them.
  • Maximum Power Point (PMax) – The point on the I-V curve at which maximum power is being produced by the cell. Note that since the graph is not a straight line, the power produced will vary depending on the operating voltage (figure 2); although the voltage at any point on the graph can still be calculated using P=IV. PMax occurs on the ‘knee’ of the I-V curve.
A typical I-V curve for a silicon photovoltaic cell.

Figure 1: A typical I-V curve for a silicon photovoltaic cell.

 A typical power output vrs. voltage curve for a silicon photovoltaic cell.

Figure 2: A typical power output vrs. voltage curve for a silicon photovoltaic cell.

In Practice PV cells do not operate under standard conditions. The two parameters that have the most bearing on their performance are temperature and irradiance.

1.2 The Affects Of Temperature

Figure 3 shows the effects of temperature on the I-V curve of a PV cell. ISC increases slightly with temperature by about 6µA per °C for 1cm2 of cell, this is so small that it is normally ignored. However, a more significant effect is the temperature dependence of voltage which decreases with increasing temperature. Typically the voltage will decrease by 2.3mV per °C per cell.

Temperature effects on the I-V curve of a PV cell.

Figure 3: Temperature effects on the I-V curve of a PV cell.

The effects of irradiance on the I-V curve of PV cells.

Figure 4: The effects of irradiance on the I-V curve of PV cells.

1.3 The Affects Of Irradiance

Solar irradiance is a measure of the sun’s energy, under standard conditions the amount of energy reaching the Earth’s surface on a clear day is taken to be 1kW/m2. The amount of irradiance reduces with the slightest amount of haze and becomes quite small on over cast days. ISC is directly proportional to the irradiance: so that if irradiance halves so does ISC. The voltage variation is very small and usually ignored

The power produced under different conditions, as a function of voltage, is shown in figure 5. Figures 4 and 5 clearly show that the voltage at which PMax occurs does not vary much with irradiance.

PV cell power output as a function of voltage.

Figure 5: PV cell power output as a function of voltage.

Irradiance values are normally given as an average per day, so that the average global irradiance may be 4.5kW/m2 per day. If we assume that most of this radiation falls on the solar panels in the six hours between 9am and 3pm we can estimate the average irradiance falling on the panels throughout the day: in this case it will be 0.75kW/m2. Irradiance is sometimes denoted by I however, to avoid confusion with current it is denoted here by G. When looking up values for G be careful to note whether they are for horizontal panels or panels inverted from the horizontal by an angle equal to the location’s latitude.

  1. [1]The AM1.5 spectrum is the spectrum (i.e. range of wavelengths) provided on a clear day by the sun when its rays have passed through an average depth of atmosphere to the Earth’s surface.