Fresnel Lens

1
Fresnel Lens

• Seen in lighthouses-used to form a concentrated
  beam of light.

2
Fresnel Lens at work

• Fresnel lens melting brick

• International Automated Systems Fresnel system

3
Reflection

• When light is incident on a surface, it can be
  reflected
• An interesting result is that the angle of
  incidence (incoming angle) equals the angle of
  reflection (outgoing angle.

4
Reflection from a curved surface

• When the surface doing the reflecting is curved,
  the light can be brought to a focus.
• The curved surface can be parabolic or spherical.
  
• Spherical surfaces are cheaper and easier to
  construct.

5
Power towers

• Use many collectors and focus the light to a
  central point.
• Achieves high temperatures and high power
  density.
• Each individual collector is called a heliostat
• Must be able to track the sun and focus light on
  the main tower

6
How they work

• Light is collected at the central tower, which is
  about 300 feet tall. There are on the order of
  2000 heliostats.
• Used to heat water and generate steam
• Steam drives a turbine which generates
  electricity
• Often include auxiliary energy storage to
  continue to produce electricity in the absence of
  sunlight
• More costly to construct and operate than coal
  fired plants.
• Good candidates for cogeneration-waste steam
  could be used for space heating

7
Solar troughs

• A parabolic shaped trough collects the light and
  focuses it onto a receiver.
• The receiver has a fluid running through it which
  carries the heat to a central location where it
  drives a steam turbine
• May have more than a hundred separate troughs at
  such a facility

8
Trough Pictures
9
Direct Conversion of sunlight to
energyPhoto-voltaics

• Photoelectric effect
• When electromagnetic energy impinges upon a
  metal surface, electrons are emitted from the
  surface.
• Hertz is often credited with
• first noticing it (because he
• published his findings) in 1887
• but it was seen by Becquerel
• In 1839 and Smith in 1873.

10
Photoelectric effect

• The effect was a puzzle
• The theory of light as a wave did not explain the
  photoelectric effect
• Great example of the scientific method in action.
• Up until this point, all the observations of
  light were consistent with the hypothesis that
  light was a wave.
• Now there were new observations could not be
  explained by this hypothesis
• The challenge became how to refine the existing
  theory of light as a wave to account for the
  photoelectric effect

11
Photoelectric effect explained

• Einstein in 1905 explained the photoelectric
  effect by assuming light was made of discrete
  packets of energy, called photons.
• Not a new idea, he was building upon an idea
  proposed by Planck, that light came in discrete
  packets. (in fact, Newton proposed a particle
  like explanation of light centuries earlier).
  The problem for Planck was his discrete packets
  were in conflict with the wave like behavior of
  light.

12
Photoelectric effect explained

• But now, a behavior of light was observed that
  fit Plancks energy packet idea.
• So electromagnetic radiation appears to behave as
  if it is both a wave and a particle.
• In fact, you can think of light as discrete wave
  packets-packets of waves which, depending upon
  the measurement you make, sometimes exhibit
  particle behavior and sometimes exhibit wave
  behavior.
• Einstein won the Nobel prize for his explanation
  of the photoelectric effect.

13
Semi conductors

• Devices which have conductive properties in
  between a conductor and an insulator.
• Normally, the outer (valence) electrons are
  tightly bound to the nucleus and cannot move.
• If one or all of them could be freed up, then the
  material can conduct electricity
• Silicon is an example of a semi-conductor.

14
Silicon

• Element 14 in the periodic table
• Very common element (sand, glass composed of it)
• 8th most common element in the universe
• Its 4 outer valence electrons are normal tightly
  bound in the crystal structure.
• However, when exposed to light, the outer
  electrons can break free via the photoelectric
  effect and conduct electricity.
• For silicon, the maximum wavelength to produce
  the photoelectric effect is 1.12 microns. 77 of
  sunlight is at wavelengths lower than this.

15
But its not quite this simple

• You also need to produce a voltage within the
  silicon to drive the current.
• So the silicon must be combined with another
  material. This process is called doping.
• 2 types of doping P and N
• If you replace one of the silicon atoms in the
  crystal lattice with a material that has 5
  valence electrons, only 4 are need to bond to the
  lattice structure, so one remains free. The doped
  semi conductor has an excess of electrons and is
  called an N type semiconductor.
• Doping elements can be arsenic, antimony or
  phosphorus.

16
P-types

• If you dope with an element with only 3 valence
  electrons, there is a vacancy, or hole left where
  the 4th electron should be.
• If the hole becomes occupied by an electron from
  a neighbor atom, the hole moves through the
  semiconductor. This acts like a current with
  positive charge flowing through the semi
  conductor, so it appears to have a net positive
  charge
• Called a P-type semiconductor.
• Doping elements could be boron, aluminum, or
  indium

17
Creating the solar cell

• To create the solar cell, bring a p-type silicon
  into contact with an n-type silicon.
• The interface is called a p-n junction.
• Electrons will diffuse from the n material to the
  p material to fill the holes in the p material.
  This leaves a hole in the n material.
• So the n-material ends up with an excess positive
  charge and the p material ends up with an excess
  negative charge.
• This creates an electric field across the
  junction.

18
Current in the solar cell

• Any free electrons in the junction will move
  towards the n type material and any holes will
  move toward the p -type material .
• Now sunlight will cause the photoelectric effect
  to occur in the junction. Thus free electrons and
  holes are created in the junction and will move
  as described above.
• Current flows!