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 Suns energy to electricity

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?

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. n 1 sin(angle of incidence)=n 2 sin(angle of refraction) n 1 sinθ 1 =n 2 sinθ 2

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

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

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

Fresnel Lens at work Fresnel lens melting brick International Automated Systems Fresnel system

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.

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.

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

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

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

Trough Pictures

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.

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

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.

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.

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.

Silicon Element 14 in the periodic table Very common element (sand, glass composed of it) 8 th 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.

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.

P-types If you dope with an element with only 3 valence electrons, there is a vacancy, or hole left where the 4 th 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

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.

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!

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.

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.

Generations of Solar cells Second generation-Thin Film Cells – made by depositing one or more thin layers (thin film) of photovoltaic material on a substrate. – thickness range of such a layer varies from a few nanometers to tens of micrometers. – Involve different methods of deposition: Chemical Vapor deposition the wafer (substrate) is exposed to one or more volatile precursors, which react and/or decompose on the substrate surface to produce the desired deposit. Frequently, volatile by-products are also produced, which are removed by gas flow through the reaction chamber.

Thin Film deposition techniques Electroplating – electrical current is used to reduce cations (positively charged ions) of a desired material from a solution and coat a conductive object with a thin layer of the material. Ultrasonic nozzle – spray nozzle that utilizes a high (20 kHz to 50 kHz) frequency vibration to produce a narrow drop size distribution and low velocity spray over the wafer These cells are low cost, but also low efficiency

The Third Generation Also called advanced thin-film photovoltaic cell range of novel alternatives to "first generation” and "second generation” cells. more advanced version of the thin-film cell.

Third generation alternatives non-semiconductor technologies (including polymer cells and biomimetics) quantum dot technologies – also known as nanocrystals, are a special class semiconductors. which are crystals composed of specific periodic table groups. Size is small, ranging from 2-10 nanometers (10-50 atoms) in diameter. tandem/multi-junction cells – multijunction device is a stack of individual single-junction cells hot-carrier cells – Reduce energy losses from the absorption of photons in the lattice upconversion and downconversion technologies – Put a substance in front of the cell that converts low energy photons to higher energy ones or higher energy photons to lower energy ones that the solar cells can convert to electricity. solar thermal technologies, such as thermophotonics(TPX) – A TPX system consists of a light-emitting diode (LED) (though other types of emitters are conceivable), a photovoltaic (PV) cell, an optical coupling between the two, and an electronic control circuit. The LED is heated to a temperature higher than the PV temperature by an external heat source. If power is applied to the LED,, an increased number of electron-hole pairs (EHPs) are created.These EHPs can then recombine radiatively so that the LED emits light at a rate higher than the thermal radiation rate ("superthermal" emission). This light is then delivered to the cooler PV cell over the optical coupling and converted to electricity.

Efficiency and cost factors Average cost per peak watt is $1.00-$3.00. Coal fired plant is $1.00/watt. Efficiency is not great. – Recall, 77% of the incident sunlight can be used by the cell. – 43% goes into heating the crystal. – Remaining efficiency is temperature dependent – Average efficiency of a silicon solar cell is 14-17% The second and third generation technologies discussed are designed to increase these efficiency numbers and reduce manufacturing costs

Novel approaches UA astronomer Roger Angel Uses cheap mirrors to focus sunlight on 3 rd generation solar cells (triple junction cells) which handle concentrated light $1.00 per watt achievable- competitive with coal plants Potential: 1 solar farm 100 miles on a side could provide electricity to the whole nation Does not have to be all in one place