## 421 Efficiency of Solar Cells

As indicated above, PV cells behave in this fashion:

1. Power output increases linearly with solar flux.

2. Power output increases linearly with decreasing cell temperature.

3. Power output increases linearly with cell size (follows from 1 above).

Output voltage, volts

FIGURE 4.10

Example current-voltage characteristics of a silicon cell showing the effects of solar irradiance level and load resistance.

Output voltage, volts

FIGURE 4.10

Example current-voltage characteristics of a silicon cell showing the effects of solar irradiance level and load resistance.

It is easy to show that efficiency of a PV cell or array is given by an equation of this type:

The reference efficiency is given by the manufacturer and is usually 12 to 18%. The temperature coefficient of efficiency f is 0.004 C1. If one knows the average solar radiation, I, the average cell, panel, or array output (in units of watts) can be found from e = n a I

The same approach can be used to find instantaneous output if the instantaneous solar radiation and cell temperature are used in the two expressions above.

## Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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