Wind Energy 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 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

GE 2.5MW generator Blade diameter: 100m Wind range: 3.5m/s to 25m/s Rated wind speed: 11.5 m/s

Types of Windmills (cont.)

Altamont Pass (CA) turbines, built 1980’s

San Gorgonio Pass (CA) turbines, built 1980’s

Costs of generating electricity ( $US quoted ) Coal (Avg of 27 plants) $1K-$1.5K/kWe capital –$45-60/MW.h (Inv. 50%, O&M 15%, Fuel 35%) Gas (23) $ K/kWe –$40-63/MWh (Inv. 20%, O&M 7%, Fuel 73%) Nuclear (13) $1-$2K/kWe (DVB: probably more, esp. in USA) –$30-50/MWh (Inv. 70%, O&M 13%, Fuel 10%) Wind (19) $1-2K/kWe –$45-140/MWh (O&M 12-40%) –Load factor variability is a major factor in setting the costs of running a wind plant (similar problems would hold true for solar as well). Solar (6) approaches $300/MWh Cogeneration (24) estimated $30-70/MWh –Note the three separate cost categories and the different mix for these. Compare all of these to gasoline ($2/gal => $55/MW.h)

Load (or capacity) factors 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 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)

ITAIPU (Brazil/Paraguay)

Other approaches to Solar Vertigro algae Biofuels 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!

Basics of atoms and materials 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 Energy Gap (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). Amorphous Si: lower efficiency (5-13%) CIGS (5 -20 %) Synopsis of Solar Cells

Crude picture inside a solar cell Limitations on efficiency: Reflection of light from front surface 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

Typical types Single Crystal (highest efficiency) Poly-crystalline

Essentials of PV design

Basics of Photo-Voltaics A useful link demonstrating the design of a basic solar cell may be found at: There are several different types of solar cells: –Single crystal Si (NASA): most efficient (42.8% is the record, as of July 2007) and most expensive (have been $100’s/W, now much lower) –Amorphous Si: not so efficient (5-10% or so) degrade with use (but improvements have been made), cheap ($2.5/W) –Recycled/polycrystalline Si (may be important in the future)

Engineering work-around # 2: Martin Green’s record cell. The grid deflects light into a light trapping structure

Power characteristics (Si) cm 2 silicon Cell under different Illumination conidtions Material Level of efficiency in % Lab Level of efficiency in % Production Monocrystalline Silicon approx to17 Polycrystalline Silicon approx to15 Amorphous Silicon approx. 135 to7

Solar Cell Costs Costs have dropped from about $5.89/pk Watt output in 1992 to $2.73/pW in 2005

Solar House 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 (flexible Amorphous cells)

Advanced designs-multilayers Possible future technology: CIGS (Cu-In-Ga-Se) thin film cells. Presently they Are 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. ( )