Solar Electric Power generation

1. Solar Electric Power generation • Two types: • Thermal -use sun’s ability to heat (usually water) to create electricity • Photovoltaic devices- a device which directly converts the sun’s energy to electricity

2. Solar Thermal • Obvious idea would be to use sunlight to boil water and provide steam to drive a turbine • But what happens when you place a container of water in the sun-it typically does not boil! • Need to concentrate or focus the sun’s energy to achieve this goal • How do we focus sunlight?

3. Basic properties of light • To answer this question, lets look at some basic properties of light in the wave description of light. • Refraction-light is bent at the interface between two media. • Snell’s law relates the angle of incidence and the index of refraction of medium 1 to the angle of refraction and index of refraction of medium 2. • n1sin(angle of incidence)=n2sin(angle of refraction) • n1sinθ1=n2sinθ2

4. Focusing light • If the interface is flat, the light is not focused. • Example-pencil in a glass of water • If it is curved in the correct fashion, i.e. the surface of a convex lens, the light can be brought to a focus convex concave

5. Fresnel Lens • For the most part, lens are very heavy, suffer from reflection at the surfaces, and are expensive to construct to the sizes needed to achieve the desired heating. • There is one type of lens, a Fresnel lens that can be inexpensively constructed from plastic

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

7. Fresnel Lens at work • Fresnel lens melting brick • International Automated Systems Fresnel system

8. 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.

9. 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.

10. 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

11. 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

12. 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

13. Trough Pictures

14. 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.

15. 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

16. 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.

17. 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.

18. 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.

19. 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.

20. 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.

21. 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

22. 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.

23. 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!

24. Solar Cells • Typically 2 inches in diameter and 1/16 of an inch thick • Produces 0.5 volts, so they are grouped together to produce higher voltages. These groups can then be connected to produce even more output. • In 1883 the first solar cell was built by Charles Fritts. He coated the semiconductor selenium with an extremely thin layer of gold to form the junctions. The device was only around 1% efficient.

25. Generations of Solar cells • First generation • large-area, high quality and single junction devices. • involve high energy and labor inputs which prevent any significant progress in reducing production costs. • They are approaching the theoretical limiting efficiency of 33% • achieve cost parity with fossil fuel energy generation after a payback period of 5-7 years. • Cost is not likely to get lower than $1/W.