Wind Energy tp://cenlamar.files.wordpress.com/2008/07/plan_3tiermap.jpg The text gives (on page 407 in slightly different units) the formula: P = 0.3*D 2 V 3 (W.s 3 /m 5 ) D-turbine diameter V- wind velocity So a 9m/s wind provides 27 times the power that a 3m/s wind
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Wind Energy The text gives (on page 407 in slightly different units) the formula: P = 0.3*D.
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The text gives (on page 407 in slightly different units) the formula:
P = 0.3*D2 V3 (W.s3/m5)
D-turbine diameter
V- wind velocitySo a 9m/s wind provides 27 times the power that a 3m/s wind provides!!
Types of Windmills/turbines
According to wikipedia, as of 2006 installed world-wide capacity is 74 GW (same capacity as only 3.5 dams the size of the three-Gorges project in China).
Altogether, there are 150,000 windmills operating in the US alone (mainly for water extraction/distribution)
7% efficiency, but work at low wind speeds
Up to 56 % efficiency with 3 blades, do very little at low wind speeds
Nuclear and Coal have very large “load factors” (these plants tend to run most of the time, and provide “base load” capacity. Other types of plants, like Natural gas, can be “fired up” more quickly and tend to be used to accommodate peak loads (sometimes called “peaking plants”). Wind has a typical capacity factor of only about 20% (solar is probably a little more, but still much less than 50%).
COAL GASIFICATION CARBON-SEQEST. PLANT(Approved to be sited in Edwardsport, IN)
Projected to open in 2012. Cost now projected to be $2.35B (up from original estimate of $1.3B) for 650MW Capacity. (>$3600/kW; this is not cost-competitive with other technologies at present; it’s requiring big rate hikes and heavy gov’t subsidies.) At that, it is only designed to capture “some of the 4Mtons of CO2 expected to be produced per year.
Water wheels through the ages
ITAIPU (Brazil/Paraguay)
http://www.solar.coppe.ufrj.br/itaipu.html
ITAIPU (Brazil/Paraguay)
http://www.solar.coppe.ufrj.br/itaipu.html
Other approaches to Solarhttp://www.cnn.com/video/#/video/tech/2008/06/10/obrien.algae.oil.cnn
Vertigro algaeBiofuels system. Requires about 1000 gallons of waer for each gallon of bio-diesel. But this could be promising!Perfect sort of thing for term paper!
• Isolated atoms have electrons in shells” of well-defined (and distinct) energies.
• When the atoms come together to form a solid, they share electrons and the allowed energies get spread out into “bands”, sometimes with a “gap” in between
EnergyGap (no available states)
p- and n-type semiconductors
Conduction band
Valence band
Gap
Energy
Position
_ _ _ _
_ _ _ _
p-type n-type
•Separate p and n-type semiconductors. The lines in the gap represent extra states introduced by impurities in the material.• n-type semiconductor: extra states from impurities contain electrons at energies just below the conduction band•p-type has extra (empty) states at energies just above the valence band.
p-n junction and solar cells
Conduction band
Valence band
Gap
Energy
Position
_ _ _ _
_ _ _ _
p-type n-type
•When the junction is formed some electrons from the n-type material can “fall” down into the empty states in the p-type material, producing a net negative charge in the p-type and positive charge in the n-type
p-n junction
Conduction band
Valence band
Gap
Energy
Position
_ _ _ _
_ _ _ _
p-type n-type
•When the junction is formed some electrons from the n-type material can “fall” down into the empty states in the p-type material, producing a net negative charge in the p-type and positive charge in the n-type
_
+
p-n junction and solar cell action
Conduction band
Valence band
Gap
Energy
Position
_ _ _ _
_ _ _ _
p-type n-type
•When a light photon with energy greater than the gap is absorbed it creates an electron-hole pair (lifting the electron in energy up to the conduction band, and thereby providing the emf).•To be effective, you must avoid:
•avoid recombination (electron falling back in to the hole).•Avoid giving the electron energy too far above the gap•Minimize resistance in the cell itself•Maximize absorption
•All these factors amount to minimizing the disorder in the cell material
_
+
Basics of Photo-Voltaics• As with atoms, materials like
semiconductors have states of particular energy available to their electrons.
• Absorbing a photon of sufficiently short wavelength (i.e. high enough energy) can lift an electron from the filled “valence” band of states to the empty “conduction” band of states.
• If you can achieve a spatial separation between the “elevated electron” and the (positive) hole it left behind, you have used the photon as a source of EMF
• Blue light works, Red light doesn’t (to oversimplify it a little bit)
• Need to absorb the light– Anti-reflective coating + multiple layers
• Need to get the electrons out into the circuit (low resistance and recombination)– Low disorder helps, but that is expensive
• Record efficiency of 42.8% was announced in July 2007 (U. Delaware/Dupont).
• Crystalline Si: highest efficiency (typically 15-25%), poorer coverage, bulk material but only the surface contributes, expensive (NASA uses them).
•Not all light is short enough wavelength (previous slide; some panels now have multiple cells stacked with lower layers senstive to less-energetic photons)
•Electron-hole recombination (i.e. some of the electrons don’t get out into the circuit; Hence single crystal Si is higher efficiency than polycrys. Or amorphous).
•Some light goes right through the active layers (hence, sometimes you see a reflective layer at the bottom)
Solar Cell Efficiency
http://en.wikipedia.org/wiki/Solar_cell
Typical types
Single Crystal(highest efficiency) Poly-crystalline
http://en.wikipedia.org/wiki/Solar_cell
Essentials of PV design
Basics of Photo-Voltaics
A useful link demonstrating the design of a basic solar cell may be found at:
This house in Oxford produces more electricity than it uses (but only about 4kWh/yr!! According to the NREL, hardly worth selling)
Example of a retrofit
Typical products
15W systems for $100 (was $150 in 2007)
Flood light system for $390 (LED’s plus xtal. Cells; claimed “150 W”)
Battery charges (flexibleAmorphous cells)
http://www.siliconsolar.com/
Advanced designs-multilayers
http://www.nrel.gov/highperformancepv/
Possible future technology: CIGS (Cu-In-Ga-Se) thin film cells. Presently theyAre less than 20% efficient (in lab, much less from production lines), but they could be much less expensive and more durable than amorphous Si. E.G. Heliovolt claims to have thin-film CIGS cells and claims to be at a $1/W price point. ( http://www.heliovolt.net/ )