Solar Energy: Conversion into Electricity
How Solar Energy is Generally Used:
What about the Direct Conversion into Electricity?
Water Into Steam:
- Heat Water into Steam
- Turn photons into electrons
Photons into Electrons: PhotoVoltaic Devices
- Charge Generation --> Photoelectric Effect
- When photons stike a metal, their energy is used to liberate loosely
bound electrons and therefore induce a current. This will be demonstrated
in class on Wednesday.
- Efficiency of this process depends upon the material
- This is the principle behind many digital cameras, otherwise
known as CCD cameras .
These kinds of cameras are used in astronomy to take digital pictures . Incoming photons are converted into units
of electric charge and stored at individual pixel locations. Different
amounts of charge represent different intensity levels. This encoded
information is a digital image.
To make use of the photoelectric effect, we need material that is a
goo conductor of electricity and which can be manufactured in bulk
at reasonable cost. This conditions strongly constrain the available
choices. For most practical aspects, Silicon is the material of choice.
Schematic structure of energy bands in Silicon:
- is abundant on the earth and readily found in the crust.
It is a direct product of fusion inside stars. It can be easily
recovered from the crust and mass-produced.
are cheap because silicon works well for circuit boards and is
an easily recoverable material from the earth's crust.
The result is a world-wide enconomy centered around semi-conductor technology.
- Has four outer (valence) electrons to bond silicon atoms together
in a crystal
- Under normal circumstances, there are no free electrons available
in silicon to conduct electricity. All the electrons are used to
bind the atoms in place to form the crystal.
- The conduction band is empty and therefore no current can be
carried by the material.
Hence, if a silicon atom receives at least 1.11 Electron
Volts from some source, a valence electron will move
to the conduction band. Once an electron is in the conduction
band, the material can carry a current and the material is
now a conductor.
The Electronic Universe Project