Solar Power

Hot Hot Heat

Did you know?

• Covering just 4% of the Earth’s desert area with photovoltaic cells could supply all the world’s energy needs.

• The Gobi Desert alone could supply up to 20 times the world’s energy requirements.

Introduction

• http://www.youtube.com/watch?v=x2zjdtxrisc• Most widely used solar cells are p-n cells.• Emerging technologies use other methods of

electron transport, like diffusion of electrons following electrochemical potential.

How it Works

• A traditional solar cell consists of two layers of semiconducting material (‘doped’ to make n- and p-type semiconductors) sandwiched together and protected by a covering.

• Adding boron to silicon makes p-types• Adding phosphorous to silicon makes n-types

How it Works (contd.)

• Photons from the sun hit the semiconductors, which by virtue of their composition have an electrical field established between them.

• Electrons are knocked loose of the semiconductors and flow through an attached wire to produce electricity.

• The established electric field keeps the electrons moving in only one direction.

How it Works (contd.)

• The complimentary positive charge, called ‘holes’ travels in the opposite direction of the electrons down the atomic chain.

• When a photon excites an electron in the crystal lattice composition of the silicon, the electron leaves an atom in the valence band (held together by covalent bonds), and moves to the conduction band. This ‘excitement’ of the electron stems from a higher energy level gained by the addition of the photon. The covalent bond that the electron was previously a part of now has one fewer electron (hole), and electrons from other bonds flow in, creating other holes.

Photon Actions

• Photon can pass through silicon (low energy)• Photon can reflect off the surface• Photon can be absorbed, but only if the photon

energy is higher than the silicon band gap value.

• Band gap=the amount of energy required to free an outer shell electron from its orbit (in this case, from valence band to conduction band)

Here’s a Picture

Alternate Technologies

• Trough system= a parabolic trough is used to concentrate light on a tube positioned in the reflector’s focal line filled with working fluid. The fluid is superheated and used to make steam/power.

• A good compromise of price and efficiency.

Alternate Technologies

• Dish Stirling System= a parabolic reflector concentrates light on a focal point. A stirling heat engine (which generates mechanical power by alternately compressing and expanding a fixed quantity of air or working fluid) is located at the focal point. This device has the highest efficiency of any CSP technologies.

Alternate Technologies

• Solar Power Tower= An array of mirrors (called heliostats) focus light on a tower with a working fluid. The system is less advanced than trough systems but slightly more efficient and has a better capacity for energy storage.

• Very cheap.

History of Solar Power•“Passive Solar” (utilization of sunlight to heat homes in winter) has been used by Greeks, Romans Native Americans, and others•Archimedes and His Mirrors: The burning of Roman ships at Syracuse•Englishman Auguste Mouchout developed the first steam engine powered entirely by the sun in 1861•Albert Einstein receives the Nobel Prize in Physics for his discovery of the photoelectric effect in 1921•Scientists at Bell Laboratories developed the first silicon solar cell capable of generating a measureable electric current in 1953•“Space Race” of 1950s and 1960s sees solar power used in satellites and spacecraft•Arab Oil Embargo of 1973 produces increased interest in solar energy, much funding for research•Since 1970s, solar power has become much more cost-effective, but lower oil prices have hurt the marketability of solar energy

“Greenness”/Energy Payback• One recent study shows that solar panels

reduce carbon emissions by 89 percent compared with traditional fossil fuel production

• However, this doesn’t account for trace emissions of nitrogen trifluoride and hexafluoride , which are much more (like 17,000+) times as bad as CO2

• Energy Payback Time (EPBT): amount of time it takes for a product to produce as much energy as was needed to manufacture that product• EPBT for solar panels is improving, but currently is about 10 months-2 years• Although only used in a few types of solar panels, cadmium is toxic to many ecological food chains and should be recycled• Also, regulation in some countries (like China) is rather lax, and production of solar panels there emits more greenhouses gases than are produced in U.S.

$$$Cost$$$• As of September 2006,

solar power cost 20 cents per kWh. Coal and hydroelectric cost 4 cents per kWh, with nuclear at 7 cents

• Japan and Germany lead the world in solar panel usage, with Spain and the U.S. following behind

$$$Solar Costs$$$• http://www.costhelper.com/cost/home- garden/solar-panels.html• Typical costs for solar panels: a) 1 kW system: $10-15,000 b) 2 kW system: $16-20,000 c) 5 kW system: $35-45,000• The 2 kW system provides most of the energy for energy-efficient homes, and the 5 kW system provides all the energy needs of most conventional homes• Solar-powered swimming pool heaters cost from around $2,500-$7,500; swimming pool heaters are the most efficient form of solar for many climates and can have EPBT’s of around 1.5-7 years• State and federal tax credits and/or rebates can help mitigate the cost of installing solar panels or heaters• Some new marketing strategies and business models, such as company-owned solar panels with fixed rates and manufacturing simpler systems are attempting to make solar panels and solar energy more economical

Discussion Questions

• How should local, state, and federal governments be involved in solar panel production?

• Should the U.S. import solar panels from China, even though environmental restrictions on solar manufacturing are more lax there?

• What do you think about solar? Is it a hot commodity or a cosmic bust?