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 energy:Photo-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 Planck’s 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!