BAND THEORY OF SOLIDSTopic 9: Solar Energy
Solar energy is one proven source that can eventually meet the world’s energy needs for the long term.
The amount of solar power reaching the Earth is about 170,000 TW. Approximately how many times greater is this power than the world’s energy consumption rate?
(a) 100 (b) 1,000 (c) 10,000
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Contribution of Solar Energy
What percentage of total world energy consumption (2008) was supplied by electrical power generation via wind-solar-biomass-geothermal? (a) < 1 % (b) 3 % (c) 13 %
Wind–Solar–Biomass–Geothermal
Electrical Power generation 0.7%
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For the breakdown of renewables, look at the top (purple) component for electrical power generation using wind/solar/biomass/geothermal.
This percentage does NOT include power generation using hydroelectric power plants.
Hopefully, this pie chart puts into perspective the very small contribution by wind/solar power generation at present, but it is growing fast!
Data Source: http://www.ren21.net/Portals/97/documents/GSR/REN21_GSR_2010_full_revised%20Sept2010.pdf
Sun’s Intensity at Earth
How does the sun's intensity at Mars compare to its intensity at Earth?
(a) higher (b) same (c) lower
Sun
Earth
Mars
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Note that the intensity is INVERSELY related to the distance r squared from the light source.
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Sun’s Intensity: Inner & Outer Planets
In a solar system far, far away the sun's intensity is 200 W/m2 for a planet located a distance R away. What is the sun's intensity for a planet located at a distance 5 R from the Sun? (Format = X)
5R
R
Sun
outer
sphere
inner
sphere
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The RATIO of Intensity (outer) / Intensity (inner) = r(inner)^2 / r(outer)^2 = R^2 / (5R)^2 = 1 / 25
The intensity at the outer planet is this ratio times the intensity at the inner planet:
Intensity (outer) = (1/25) (200 W/m^2) = ?? W/m^2
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Factor of 2 decrease because of Earth’s curved surface.
Factor of 2 decrease because 50% darkness.
Decreases due to clouds, etc.
IEarth = 1370 W/m2
IAverage = 235 W/m2
IMid-day ~ 1000 W/m2
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The actual intensity measures the power per unit area incident on an area oriented perpendicular to the incoming sunlight.
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Light Bulb's Intensity
What is the intensity (W/m2) of a 100 W light bulb at a distance 5 cm from the bulb’s center? (Format = XXXX)
The intensity of this 100 W light bulb at a distance 5 cm from the center is _________ than the sun's intensity at mid-day.
(a) higher (b) lower
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Intensity = Power / 4(pi)r^2 where r = 5 cm or 0.05 m
Intensity = (100 W) / [ (4) (3.14) (0.05 m) (0.05 m) ]
Intensity = ?? W/m^2
Do not forget to convert the distance from the light bulb in centimeters to meters by dividing by 100!
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Sun’s Intensity: Annual Average in U.S.
Rank the cities from highest to lowest annual average solar intensity.
(a) Denver (b) Detroit (c) Phoenix (d) Richmond
Is the annual average solar intensity of a city only determined by its latitude? (a) yes (b) no
150 W/m2
>500 W/m2
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The ranking of cities from highest to lowest solar intensity is: Phoenix, Denver, Richmond, Detroit
Look at the cities of Denver and Richmond. Denver is located at a latitude 2 degrees north of Richmond, but it has a higher annual average solar intensity. This is due to factors such as sunnier skies and a higher altitude with thinner atmosphere.
The picture shows the annual average sun intensity for surfaces perpendicular to the sunlight.
Adapted from National Renewable Energy Laboratory.
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Seasons
(a) Changing distance between the Earth and sun during year.
(b) Constant tilt angle of the Earth as it revolves around the sun.
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The seasons are caused by the constant tilt angle of the Earth with respect to the solar system as the Earth travels around the sun.
In the summer, the northern hemisphere is pointed toward the sun.
In the winter, the northern hemisphere is pointed away from the sun.
If the Earth had no tilt, then there would be no seasons.
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N
S
Earth
Sun
N
S
What is the tilt of the Earth away from the vertical?
(a) 10.5º (b) 23.5º (c) 45.5º
Summer Solstice
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The tilt of the Earth is more than 20 degrees and less than 30 degrees .
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Seasons: Earth's Tilt and Sun's Location
In the Northern hemisphere, what is the Earth's orientation during the winter solstice, spring or fall equinox, and summer solstice?
(3-digit answer)
(1) North pole is tilted toward the Sun, making the sun appear higher in the sky.
(2) North pole is tilted away from the Sun, making the sun appear lower in the sky.
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Winter Solstice (Northern Hemisphere)
During the winter solstice in Richmond (latitude = 37.5º), what is the angle (yellow angle in picture) of the sun at noon from the "vertical"?
(a) 14º (37.5º–23.5º) (b) 23.5º (c) 37.5º (d) 61º (23.5º+37.5º)
Richmond
Equator
23.5°
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Winter solstice in Richmond = Earth's tilt + latitude of Richmond (Sun lowest in sky)
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Spring or Fall Equinox
During the equinox in Richmond, what is the angle of the sun at noon from the "vertical"?
(a) 14º (37.5º–23.5º) (b) 23.5º (c) 37.5º (d) 61º (23.5º+37.5º)
Sun
Richmond
Equator
Equinox
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Summer Solstice (Northern Hemisphere)
North Pole is tilted 23.5º toward from Sun and Sun is "high" in sky.
In Richmond, what is the angle of the sun at noon from the "vertical"?
(a) 14º (37.5º–23.5º) (b) 23.5º (c) 37.5º (d) 61º (23.5º+37.5º)
37.5°
23.5°
Richmond
Equator
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Summer solstice in Richmond = atitude of Richmond minus Earth's tilt (Sun highest in sky)
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Passive solar homes have south-facing windows with overhang, good insulation, and a high thermal mass for heat storage.
During which seasons is the sun highest and lowest in the sky?
(1) summer solstice (2) equinox (3) winter solstice
Summer
Sunlight
Winter
Sunlight
South
North
overhang
Look at the picture and refer to the previous slides.
This is why a passive solar home has an "overhang" above the windows so that the sun only enters the windows in the winter when it is low in the sky.
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Which direction is south in this picture (taken at noon)?
(a) Left (b) Front (c) Right
LEFT
RIGHT
FRONT
Roof
Windows
Roof
Overhang
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In this picture, the sun is shining into the roof windows from the southern part of the sky.
This is the house that Dr. Baski's parents built in 1976 during the energy "crisis“ at that time.
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Solar THERMAL Power: Water Heaters
Hot-water heating is what percent of energy use in a typical U.S. home? (a) 1% (b) 20% (c) 50%
What country requires that solar water heaters be installed for all new homes and has the highest solar energy use per capita?
(a) U.S. (b) China (c) Israel
Rooftop hot-water heater.
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Hot-water heating is typically 20% of the energy use for an American home.
The country that uses the most solar energy PER CAPITA is Israel. (population of ~7.5 million people)
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VIDEO
Focus
sunlight
Emerged as a significant new power source during 2006 – 2010.
In early 2010, 0.7 GW of CSP in U.S. and Spain.
Main types: Dish, Power Tower, Linear Concentrator
Why are parabolic mirrors used in CSP systems?
(a) Mirrors spread sunlight over a larger region.
(b) Mirrors focus sunlight to a small region.
http://www.solarpaces.org/News/Projects/projects.htm
Parablic Dish Systems:
Parabolic dish systems  consist of a parabolic-shaped point focus concentrator in the form of a dish that reflects solar radiation onto a receiver mounted at the focal point. These concentrators are mounted on a structure with a two-axis tracking system to follow the sun. The collected heat is typically utilized directly by a heat engine mounted on the receiver moving with the dish structure. Stirling and Brayton cycle engines are currently favored for power conversion. Projects of modular systems have been realized with total capacities up to 5 MWe. The modules have maximum sizes of 50 kWe and have achieved peak efficiencies up to 30% net.
Power Tower Systems:
A power tower converts sunshine into clean electricity for the world’s electricity grids. The technology utilizes many large, sun-tracking mirrors (heliostats) to focus sunlight on a receiver at the top of a tower. A heat transfer fluid heated in the receiver is used to generate steam, which, in turn, is used in a conventional turbine-generator to produce electricity.
Early power towers (such as the Solar One plant) utilized steam as the heat transfer fluid; current US designs (including Solar Two, pictured) utilize molten nitrate salt because of its superior heat transfer and energy storage capabilities. Current European designs use air as heat transfer medium because of its high temperature and its good handability.
Individual commercial plants will be sized to produce anywhere from 50 to 200 MW of electricity.
Parabolic Trough Systems:
The sun's energy is concentrated by parabolically curved, trough-shaped reflectors onto a receiver pipe running along the inside of the curved surface. This energy heats oil flowing through the pipe, and the heat energy is then used to generate electricity in a conventional steam generator.
A collector field comprises many troughs in parallel rows aligned on a north-south axis. This configuration enables the single-axis troughs to track the sun from east to west during the day to ensure that the sun is continuously focused on the receiver pipes. Individual trough systems currently can generate about 80 megawatts of electricity.
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(a) 2 TW (b) 4 TW (c) 10 TW
What percentage of this total capacity was from solar PV?
(a) < 1 % (b) 5 % (c) 10 %
Data Source: IEA
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Q1: Add up the electricity capacity for the various sources (coal, gas, hydro, nuclear, renewables)
Q2: solar PV percentage = 100 x (8 GW Solar Capacity / Total Capacity in GW) very small!!
Data Source: http://www.eia.doe.gov/iea/contents.html
Note: The actual average electrical power generated in 2006 was ~2 TW.
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Photovoltaic cells use special materials called semiconductors to directly convert light energy into electrical energy. (NO thermal process!)
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The most common semiconductor material today used in solar (or photovoltaic = PV) cells is polycrystalline Si.
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Silicon Solar Panels
If your house required an average 3 kW electrical power during the day, then how many thousands of dollars would it cost to install silicon solar panels to meet this need? (1-digit answer)
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Installation cost = 3000 W x ($2/ 1 W) x (1/1000) ~ ?? thousands of dollars (does not include labor!)
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Semiconductors on the Periodic Table
On the periodic table, metals are blue and semiconductors are red.
The elements lithium, zinc, silicon, and tellurium are:
(1) metals or (2) semiconductors (4-digit answer)
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Find the color of the elements on the periodic table. All of the listed elements will eventually be discussed in this class!
Taken from www.periodni.com/en/
Semiconductors are partially or "semi" conducting.
Unlike in metals, electrons in a semiconductor can only occupy allowed energies, which are separated by unallowed energies known as the "Energy Gap."
What energies should most electrons in a semiconductor have?
(a) Allowed lower energies
(b) Unallowed medium energies
(c) Allowed higher energies
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Do electrons spontaneously drop down to lower energies or "get excited" to higher energies.
Analogously, does a ball spontaneously roll down an incline to a lower potential energy or roll up to a higher energy?
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Semiconductor Energy Diagram
Normally, electrons fill up the lower energy "valence band" and none are in the higher energy "conduction band".
Analogy: Cars in a two-level parking lot, where cars are electrons and the parking deck levels are the lower and upper energy bands.
Can electrons move in a full valence band? (a) yes (b) no
Analogy: Can cars move to new parking spots if all the spots are filled?
Egap = energy "gap"
(electrons not allowed!)
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Electrons cannot move if there are no "empty spaces" or holes for them to move into!
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"Negative" n-type Semiconductor
Dopants are impurity atoms that add electrons (or holes) to the semiconductor to make it conducting!
Donor atoms add electrons to the conduction band (n-type).
Can electrons move in a partially filled conduction band in an n-type semiconductor? (a) yes (b) no
negative electrons
n-type semiconductor
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Electrons can move (or conduct) in a partially filled conduction band where there are 'empty spaces'.
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"Positive" p-type Semiconductor
Acceptor atoms remove electrons from the valence band, leaving behind positive "holes" (p-type).
Can electrons move in a partially empty valence band in a p-type semiconductor? (a) yes (b) no
Lower
Energy
Higher
Energy
Conduction
band
Valence
band
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Electrons can move (or conduct) in a partially empty valence band where there are 'empty spaces'.
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Formation of p-n Junction
To form a solar cell, we need to make a junction between p-type and
n-type semiconductors.
At the p-n junction below, the free electrons initially "diffuse" or move _______ and the free holes move _________. (2-digit answer)
(1) toward left side (p-type) (2) toward right side (n-type)
"free" electrons
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When the p-n junction is initially formed, the 'free' electrons in the n-type material on the right diffuse to the left (where there initially are no electrons).
The 'free' holes in the p-type material on the left diffuse to the right (where there initially are no holes).
Eventually, the free electrons and holes stop moving (due to an opposing electric field that forms).
The junction then has a "voltage" like a battery.
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Formation of p-n Junction
Eventually, the free electrons and holes stop moving and the p-n junction has a "voltage" like a battery.
After the p-n junction forms, its left side is _______ and
its right side is ________. (2-digit answer)
(1) negative (2) positive
The left p-type side has extra negative charge (free electrons).
The right n-type side has extra positive charge (free 'holes').
Notice how the bands are now slanted downward at the p-n junction.
The left side is at a higher electron energy and the right side is at a lower electron energy.
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Add Light to Make Electricity!
If a photon of light has enough energy, it can then "kick" an electron into the conduction band and leave behind a hole in the valence band!
If this happens at the p-n junction, the electron moves ______ and
the hole moves _______, which creates electricity! (2-digit answer)
(1) toward left side (p-type) (2) toward right side (n-type)
photon in
p-n Junction
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Electrons roll "down" to the lower-energy, n-type side on the right.
Holes "bubble up" to the p-type side on the left.
Note that the motion of holes is opposite in direction to the motion of electrons, since they are opposite in charge.
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Light comes in energy packets called photons with energy Ephoton that is inversely related to the wavelength l of the light.
Energy of Light "Photons"
Remember that visible light has wavelengths of 400 to 750 nm.
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Materials for Solar Cells: Si and CdTe
Today, the efficiencies of the best silicon and CdTe solar cells are:
(1) less than 20% (2) more than 20% (2-digit answer)
Crystalline Si
Data from National Renewable Energy Laboratory located in Golden, CO
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Bandgap Energy of CdTe Solar Cell
Solar cells with a bandgap energy of 1.4 eV have optimum efficiency for sunlight. CdTe solar cells have a bandgap energy of 1.5 eV.
What is the wavelength l (nm) of light having the minimum energy to create ‘electron-hole’ pairs in CdTe? (Format = XXX)
This wavelength is: (a) infrared (b) visible (c) ultraviolet
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Wavelength = hc / Ephoton = (1240 eV nm) / (1.5 eV ) = ?? nm
Remember that visible light is from 400 to 750 nm. Shorter wavelengths are in the UV region and longer wavelengths are in the IR region.
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Cadmium Telluride (CdTe) Solar Panels
First Solar is a leading company that manufactures thin film CdTe solar panels (2012: $2.7 billion sales for 1.9 GW capacity). This annual capacity is approximately equivalent to the power generation of:
(a) two windmills (b) two nuclear power plant
Why are thin film CdTe PV panels becoming popular?
(a) more efficient than Si (b) cheaper than Si
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Information concerning the company First Solar is taken from their website at www.firstsolar.com.
http://en.wikipedia.org/wiki/Cadmium_telluride_solar_cell
Cadmium telluride (CdTe) photovoltaics describes a photovoltaic (PV) technology that is based on the use of cadmium telluride thin film, a semiconductor layer designed to absorb and convert sunlight into electricity.[1] Cadmium telluride PV is the first and only thin film photovoltaic technology to surpass crystalline silicon PV in cheapness for a significant portion of the PV market, namely in multi-kilowatt systems.[1][2][3]
Since inception, the dominant solar cell technology in the marketplace has been based on wafers of crystalline silicon. During the same period, the idea of developing alternative, lower cost PV technologies led to the consideration of thin films and concentrators. Thin films are based on using thinner semiconductor layers to absorb and convert sunlight; concentrators, on the idea of replacing expensive semiconductors with lenses or mirrors. Both reduce cost, in theory, by reducing the use of semiconductor material. However, both faced critical challenges.
The first thin film technology to be extensively developed and manufactured was amorphous silicon. However, this technology suffers from low efficiencies and slow deposition rates (leading to high capital costs) and has not become a market leader. Instead, the PV market has grown to almost 4 gigawatts with wafer-based crystalline silicon comprising almost 90% of sales.[4] Installation trails production by a slight time lag, and the same source estimates about 3 gigawatts were installed in 2007.
During this period, two other thin films continued in development (cadmium telluride, and copper indium diselenide or CIS-alloys). The latter is beginning to be produced in start-up volumes of 1–30 megawatts per year by individual companies and remains an unproven, but promising market competitor due to very high, small-area cell efficiencies approaching 20%.[5]
HISTORY
Research in CdTe dates back to the 1950s,[6][7][8][9][10][11] because it was quickly identified as having a band gap (about 1.5 eV) almost perfectly matched to the distribution of photons in the solar spectrum in terms of optimal conversion to electricity. A simple heterojunction design evolved in which p-type CdTe was matched with n-type cadmium sulfide (CdS). The cell was completed by adding top and bottom contacts. Early leaders in CdS/CdTe cell efficiencies were GE in the 1960s,[12] and then Kodak, Monosolar, Matsushita, and Ametek.
By 1981, Kodak used close spaced sublimation (CSS) and made the first 10% cells and first multi-cell devices (12 cells, 8% efficiency, 30 cm2).[13] Monosolar[14] and Ametek[15] used electrodeposition, a popular early method. Matsushita started with screen printing but shifted in the 1990s to CSS. Cells of about 10% sunlight-to-electricity efficiency were being made by the early 1980s at Kodak, Matsushita, Monosolar, and Ametek.[16]
An important step forward occurred when cells were being scaled-up in size to make larger area products called modules. These products require higher currents than small cells and it was found that an additional layer, called a transparent conductive oxide (TCO), could facilitate the movement of current across the top of the cell (instead of a metal grid). One such TCO, tin oxide, was already being applied to glass for other uses (thermally reflective windows). Made more conductive for PV, tin oxide became and remains the norm in CdTe PV modules.
Professor Ting L. Chu of Southern Methodist University and subsequently of University of South Florida, Tampa, made significant contributions to moving the efficiency of CdTe cells to above 15% in 1992, a critical level of success in terms of potential commercial competitiveness.[16] This was done when he added an intervening or buffer layer to the TCO/CdS/CdTe stack and then thinned the CdS to allow more light through. Chu used resistive tin oxide as the buffer layer and then thinned the CdS from several micrometres to under half a micrometre in thickness. Thick CdS, as it was used in prior devices, blocked about 5 mA/cm2 of light, or about 20% of the light usable by a CdTe device. By removing this loss while maintaining the other properties of the device, Chu reached 15% efficiency in 1991, the first thin film to do so, as verified at the National Renewable Energy Laboratory(NREL).[16] Chu used CSS for depositing the CdTe. For his achievements in taking CdTe from its status as “also-ran” to a primary candidate for commercialization, some think of Ting L. Chu as the key technologist in the history of CdTe development.
In the early 1990s, another set of entrants were active in CdTe commercial development, but with mixed results.[16] A short-lived company, Golden Photon replaced Photon Energy, when it was bought by the Coors Company in 1992. Golden Photon, led by Scot Albright and John Jordan, actually held the record for a short period for the best CdTe module measured at NREL at 7.7% using a spray deposition technique. Meanwhile Matsushita, BP Solar, and Solar Cells Inc. were active. Matsushita claimed an 11% module efficiency using CSS and then dropped out of the technology, perhaps due to internal corporate pressures over cadmium. A similar efficiency and fate eventually occurred at BP Solar. BP used electrodeposition inherited from Monosolar by a circuitous route when it purchased SOHIO. SOHIO had previously bought Monosolar. BP Solar however never made a complete commitment to their CdTe technology despite its achievements and dropped it in the early 2000s. Another ineffective corporate evolution occurred at a European entrant, Antec. Founded by CdTe pioneer Dieter Bonnet (who made cells in the 1960s), Antec was able to make about 7%-efficient modules, but went bankrupt when it started producing commercially during a short, sharp downturn in the market in 2002. Purchased from bankruptcy, it never regained the technical traction needed to make further progress. However, as of 2008 Antec does make and sell CdTe PV modules.
There are a number of start-ups in CdTe today: Q-Cells' Calyxo (Germany), GE’s PrimeStar Solar (Golden, Colorado), Arendi (Italy), and Abound Solar (Fort Collins, Colorado). Including Antec, their total production represents less than 70 megawatts per year.[17] In February 2009, Roth & Rau announced to develop turnkey CdTe production lines and launch the business before end of 2009.[18]
SCI and First Solar
The major commercial success to emerge from the turmoil of the 1990s was Solar Cells Incorporated (SCI). Founded in 1990 as an outgrowth of a prior company, Glasstech Solar (founded 1984), led by inventor/entrepreneur Harold McMaster,[19] it switched from amorphous silicon to CdTe as a better solution to the higher-cost crystalline silicon PV. McMaster championed CdTe for its high-rate, high-throughput processing. Technical leadership came from a team that included Jim Nolan, Rick Powell, Jim Foote, and Peter Meyers, with consulting help from Ting Chu and Al Compaan (U. Toledo). SCI started with an adaptation of the CSS method then shifted to a vapor transport approach, inspired by Powell.[20] In February 1999, McMaster sold the company to True North Partners, an investment arm of the Walton family, owners of Wal-Mart.[21] John T. Walton joined the Board of the new company, and Mike Ahearn of True North became the CEO of the newly minted First Solar.
In its early years First Solar suffered setbacks, and initial module efficiencies were modest, about 7%. Commercial product became available in 2002. But production did not reach 25 megawatts until 2005.[22] The company built an additional line in Perrysburg, Ohio, then four lines in Germany, supported by the then substantial German production incentives (about 50% of capital costs)[23]. In 2006 First Solar reached 75 MW of annual production[22] and announced a further 16 lines in Malaysia. The more recently announced lines have been operational ahead of schedule[24]. As of 2008, First Solar is producing at nearly half a gigawatt annual rate,[22] and in 2006 and 2007 was among the largest PV module manufacturers in the world.[25]
Issues
Solar Cell Efficiencies
Best cell efficiency has plateaued at 16.5% since 2001.[26] The opportunity to increase current has been almost fully exploited, but more difficult challenges associated with junction quality, with properties of CdTe and with contacting have not been as successful. However, until recently the number of active scientists in CdTe PV was small.[27] Improved doping of CdTe and increased understanding of key processing steps (e.g., cadmium chloride recrystallization and contacting) are key to progress. Since CdTe has the optimal band gap for single-junction devices, it may be expected that efficiencies close to exceeding 20% (such as already shown in CIS alloys) should be achievable in practical CdTe cells. Modules of 15% would then be possible.
Process optimization
Process optimization allows greater throughput at smaller cost. Typical improvements are broader substrates (since capital costs scale sublinearly, and installation costs can be reduced), thinner layers (to save material, electricity, and throughput time), and better material utilization (to save material and cleaning costs). Making components rather than buying them is also a traditional way for great manufacturers to shave costs. Today’s CdTe module costs are about $110/m2 (normalized to a square meter).[28] Costs are expected to reduce to $75/m2.
Thus a practical, long-term (10–20 year) goal for CdTe modules resulting from combining cost and efficiency goals would be $75 per 150 watts, or about $0.5 per watt.[29] With commodity-like margins and combined with balance-of-system (BOS) costs, installed systems near $1.5/W seem achievable. With Southern California sunlight, this would be in the 6 to 8 US cents per kWh range (e.g., based on economic and other assumptions used in algorithms such as in the United States Department of Energy and NREL's Solar Advisory Model).[30]
Tellurium supply
Perhaps the most subtle and least understood problem with CdTe PV is the supply of tellurium. Tellurium (Te) is an element not currently used for many applications. Only a small amount, estimated to be about 800 metric tons [31] per year, is available. According to USGS, global tellurium production in 2007 was 135 metric tons[32]. Most of it comes as a by-product of copper, with smaller byproduct amounts from lead and gold. One gigawatt (GW) of CdTe PV modules would require about 93 metric tons (at current efficiencies and thicknesses),[33] so this seems like a limiting factor. However, because tellurium has had so few uses, it has not been the focus of geologic exploration. In the last decade, new supplies of tellurium-rich ores have been located, e.g., in Xinju, China.[34] Since CdTe is now regarded as an important technology in terms of PV’s future impact on global energy and environment, the issue of tellurium availability is significant. Recently, researchers have added an unusual twist – astrophysicists identify tellurium as the most abundant element in the universe with an atomic number over 40.[35][36] This surpasses, e.g., heavier materials like tin, bismuth, and lead, which are common. Researchers have shown that well-known undersea ridges (which are now being evaluated for their economic recoverability) are rich in tellurium and by themselves could supply more tellurium than we could ever use for all of our global energy.[36][37] It is not yet known whether this undersea tellurium is recoverable, nor whether there is much more tellurium elsewhere that can be recovered.
Other issues
Cadmium
Another issue frequently mentioned, is the use and recycling of the extremely toxic metal cadmium, one of the six most toxic materials banned by European Union's RoHS regulation. According to First Solar's annual report[38], the CdTe solar panel is not in RoHS compliance, not listed in the exemption product list, but not currently listed in the restricted product list either. So the product's future RoHS compliance status is uncertain[39]. First Solar has a self-imposed recycling regimen that provides a deposited amount (<$0.05 a watt) that covers the costs of transport and recycling of the module at the end of its useful life.[40][41] Recycling has been fully demonstrated on scrap modules. In a validating test, Vasilis Fthenakis of the Brookhaven National Laboratory showed that the glass plates surrounding CdTe material sandwiched between them (as they are in all commercial modules) seal during a fire and do not allow any cadmium release.[42] All other uses and exposures related to cadmium are minor and similar in kind and magnitude to exposures from other materials in the broader PV value chain, e.g., to toxic gases, lead solder, or solvents (most of which are not used in CdTe manufacturing).[43]
Price vulnerability
A subtle issue with CdTe and with all thin films in relation to greater efficiency PV module technologies is the potential impact of commodity inflation. Greater efficiency modules incur a better balance of system commodity cost per unit output. Thus such inflation can have a greater percentage impact on system cost. This is another reason that continued efficiency improvements are important.
Solar tracking
Almost all thin film photovoltaic module systems to-date have been non-solar tracking, because the output of modules has been too low to offset tracker capital and operating costs. But relatively inexpensive single-axis tracking systems can add 25% output per installed watt.[30] This is climate-dependent. Tracking also produces a smoother output plateau around midday, allowing afternoon peaks to be met.
Market viability
Success of cadmium telluride PV has been due to the low cost achievable with the CdTe technology, made possible by combining adequate efficiency with lower module area costs.[25] Direct manufacturing cost for CdTe PV modules has reached $1.12 ea watt,[44] and capital cost per new watt of capacity is near $0.9 per watt (including land and buildings).[45] However, module cost alone is not enough to assure the lowest installed system price. Thin films, including CdTe, are less efficient than most wafer silicon modules. Typical wafer silicon modules are 13% to 20% efficient, while the best CdTe modules were about 10.7% efficient; recent modules produced at First Solar and measured by NREL have shown CdTe modules with efficiencies at 12.5% or greater. Many components of an installed PV system (e.g., support structures, installation labor, land) scale with system area; and less-efficient modules require more area to produce the same output (all other things being equal). The impact of area-related costs on CdTe systems is about $0.5 per watt of extra cost.
Notable systems
Recent installations of large CdTe PV systems by First Solar confirm the competitiveness of CdTe PV with other forms of solar energy and how close it is to being competitive with conventional natural gas peakers:
A 40MW system being installed by juwi group in Waldpolenz Solar Park, Germany: at the time of its announcement, it was both the largest planned and lowest cost PV system in the world. The price of 3.25 euros translated then (when the euro was equal to US$1.3) to $4.2/watt, much lower than any other known system.[46]
A 7.5-megawatt system to be installed in Blythe, CA, where the California Public Utilities Commission has accepted a 12 US cent per kWh power purchase agreement with First Solar (after the application of all incentives).[47] Defined in California as the "Market Referent Price," this is the price the PUC will pay for any daytime peaking power source, e.g., natural gas. Although PV systems are intermittent and not dispatchable the way natural gas is, natural gas generators have an ongoing fuel price risk that PV does not have.
A contract for two megawatts of rooftop installations with Southern California Edison, where the SCE program is designed to install 250 megawatts at a total cost of $875M (averaging $3.5/watt), after incentives.[48]
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Manufacturing of CdTe Solar Panels
CdTe solar panels are more cost effective compared to silicon because the CdTe is a thin film, vs. the bulk material used for silicon.
What is the thickness of a CdTe film compared to a piece of paper?
(a) 2% paper thickness (b) 20% (c) 100%
Glass
Evaporation
*
A piece of paper is about 100 microns thick and the CdTe film is about 2 microns thick.
http://en.wikipedia.org/wiki/Cadmium_telluride_solar_cell
Page *
Manufacturing of First Solar CdTe Solar Panels
A production ‘line’ produces ~1,000 panels (70 W each) per day. What is the PV power capacity produced by one ‘line’ in one year?
(a) 2.5 MW (b) 25 MW (c) 250 MW
What is equivalent power capacity in number of windmills?
*
(70 W / panel) x (1000 panels / day) x 376 days x (1 MW / 10^6 W) = ?? MW / year
All information concerning the company First Solar is taken from their website at www.firstsolar.com.
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Cost of First Solar CdTe Solar Panels
Assuming a cost of $1/Watt for CdTe solar panels, how much would it cost in thousands of dollars to buy the panels for a 3-kW home?
(1-digit answer)
*
A similar calculation was done on an earlier slide for silicon solar panels.
Current price (2011): $0.75 /Watt First Solar (CdTe).
Page *
0 GW
10 GW
20 GW
30 GW
40 GW
50 GW
60 GW
70 GW
The lower prices is due to a large increase in supply
How much solar manufacturing capacity is available in 2012?
(a) 10 GW (b) 30 GW
(c) 70 GW (d) 100 GW
The lower prices have put solar companies under pressure.
Stock Price for First Solar
08
09
10
11
12
$310
$24
*
Information concerning the company First Solar is taken from their website at www.firstsolar.com.
http://en.wikipedia.org/wiki/Cadmium_telluride_solar_cell
Cadmium telluride (CdTe) photovoltaics describes a photovoltaic (PV) technology that is based on the use of cadmium telluride thin film, a semiconductor layer designed to absorb and convert sunlight into electricity.[1] Cadmium telluride PV is the first and only thin film photovoltaic technology to surpass crystalline silicon PV in cheapness for a significant portion of the PV market, namely in multi-kilowatt systems.[1][2][3]
Since inception, the dominant solar cell technology in the marketplace has been based on wafers of crystalline silicon. During the same period, the idea of developing alternative, lower cost PV technologies led to the consideration of thin films and concentrators. Thin films are based on using thinner semiconductor layers to absorb and convert sunlight; concentrators, on the idea of replacing expensive semiconductors with lenses or mirrors. Both reduce cost, in theory, by reducing the use of semiconductor material. However, both faced critical challenges.
The first thin film technology to be extensively developed and manufactured was amorphous silicon. However, this technology suffers from low efficiencies and slow deposition rates (leading to high capital costs) and has not become a market leader. Instead, the PV market has grown to almost 4 gigawatts with wafer-based crystalline silicon comprising almost 90% of sales.[4] Installation trails production by a slight time lag, and the same source estimates about 3 gigawatts were installed in 2007.
During this period, two other thin films continued in development (cadmium telluride, and copper indium diselenide or CIS-alloys). The latter is beginning to be produced in start-up volumes of 1–30 megawatts per year by individual companies and remains an unproven, but promising market competitor due to very high, small-area cell efficiencies approaching 20%.[5]
HISTORY
Research in CdTe dates back to the 1950s,[6][7][8][9][10][11] because it was quickly identified as having a band gap (about 1.5 eV) almost perfectly matched to the distribution of photons in the solar spectrum in terms of optimal conversion to electricity. A simple heterojunction design evolved in which p-type CdTe was matched with n-type cadmium sulfide (CdS). The cell was completed by adding top and bottom contacts. Early leaders in CdS/CdTe cell efficiencies were GE in the 1960s,[12] and then Kodak, Monosolar, Matsushita, and Ametek.
By 1981, Kodak used close spaced sublimation (CSS) and made the first 10% cells and first multi-cell devices (12 cells, 8% efficiency, 30 cm2).[13] Monosolar[14] and Ametek[15] used electrodeposition, a popular early method. Matsushita started with screen printing but shifted in the 1990s to CSS. Cells of about 10% sunlight-to-electricity efficiency were being made by the early 1980s at Kodak, Matsushita, Monosolar, and Ametek.[16]
An important step forward occurred when cells were being scaled-up in size to make larger area products called modules. These products require higher currents than small cells and it was found that an additional layer, called a transparent conductive oxide (TCO), could facilitate the movement of current across the top of the cell (instead of a metal grid). One such TCO, tin oxide, was already being applied to glass for other uses (thermally reflective windows). Made more conductive for PV, tin oxide became and remains the norm in CdTe PV modules.
Professor Ting L. Chu of Southern Methodist University and subsequently of University of South Florida, Tampa, made significant contributions to moving the efficiency of CdTe cells to above 15% in 1992, a critical level of success in terms of potential commercial competitiveness.[16] This was done when he added an intervening or buffer layer to the TCO/CdS/CdTe stack and then thinned the CdS to allow more light through. Chu used resistive tin oxide as the buffer layer and then thinned the CdS from several micrometres to under half a micrometre in thickness. Thick CdS, as it was used in prior devices, blocked about 5 mA/cm2 of light, or about 20% of the light usable by a CdTe device. By removing this loss while maintaining the other properties of the device, Chu reached 15% efficiency in 1991, the first thin film to do so, as verified at the National Renewable Energy Laboratory(NREL).[16] Chu used CSS for depositing the CdTe. For his achievements in taking CdTe from its status as “also-ran” to a primary candidate for commercialization, some think of Ting L. Chu as the key technologist in the history of CdTe development.
In the early 1990s, another set of entrants were active in CdTe commercial development, but with mixed results.[16] A short-lived company, Golden Photon replaced Photon Energy, when it was bought by the Coors Company in 1992. Golden Photon, led by Scot Albright and John Jordan, actually held the record for a short period for the best CdTe module measured at NREL at 7.7% using a spray deposition technique. Meanwhile Matsushita, BP Solar, and Solar Cells Inc. were active. Matsushita claimed an 11% module efficiency using CSS and then dropped out of the technology, perhaps due to internal corporate pressures over cadmium. A similar efficiency and fate eventually occurred at BP Solar. BP used electrodeposition inherited from Monosolar by a circuitous route when it purchased SOHIO. SOHIO had previously bought Monosolar. BP Solar however never made a complete commitment to their CdTe technology despite its achievements and dropped it in the early 2000s. Another ineffective corporate evolution occurred at a European entrant, Antec. Founded by CdTe pioneer Dieter Bonnet (who made cells in the 1960s), Antec was able to make about 7%-efficient modules, but went bankrupt when it started producing commercially during a short, sharp downturn in the market in 2002. Purchased from bankruptcy, it never regained the technical traction needed to make further progress. However, as of 2008 Antec does make and sell CdTe PV modules.
There are a number of start-ups in CdTe today: Q-Cells' Calyxo (Germany), GE’s PrimeStar Solar (Golden, Colorado), Arendi (Italy), and Abound Solar (Fort Collins, Colorado). Including Antec, their total production represents less than 70 megawatts per year.[17] In February 2009, Roth & Rau announced to develop turnkey CdTe production lines and launch the business before end of 2009.[18]
SCI and First Solar
The major commercial success to emerge from the turmoil of the 1990s was Solar Cells Incorporated (SCI). Founded in 1990 as an outgrowth of a prior company, Glasstech Solar (founded 1984), led by inventor/entrepreneur Harold McMaster,[19] it switched from amorphous silicon to CdTe as a better solution to the higher-cost crystalline silicon PV. McMaster championed CdTe for its high-rate, high-throughput processing. Technical leadership came from a team that included Jim Nolan, Rick Powell, Jim Foote, and Peter Meyers, with consulting help from Ting Chu and Al Compaan (U. Toledo). SCI started with an adaptation of the CSS method then shifted to a vapor transport approach, inspired by Powell.[20] In February 1999, McMaster sold the company to True North Partners, an investment arm of the Walton family, owners of Wal-Mart.[21] John T. Walton joined the Board of the new company, and Mike Ahearn of True North became the CEO of the newly minted First Solar.
In its early years First Solar suffered setbacks, and initial module efficiencies were modest, about 7%. Commercial product became available in 2002. But production did not reach 25 megawatts until 2005.[22] The company built an additional line in Perrysburg, Ohio, then four lines in Germany, supported by the then substantial German production incentives (about 50% of capital costs)[23]. In 2006 First Solar reached 75 MW of annual production[22] and announced a further 16 lines in Malaysia. The more recently announced lines have been operational ahead of schedule[24]. As of 2008, First Solar is producing at nearly half a gigawatt annual rate,[22] and in 2006 and 2007 was among the largest PV module manufacturers in the world.[25]
Issues
Solar Cell Efficiencies
Best cell efficiency has plateaued at 16.5% since 2001.[26] The opportunity to increase current has been almost fully exploited, but more difficult challenges associated with junction quality, with properties of CdTe and with contacting have not been as successful. However, until recently the number of active scientists in CdTe PV was small.[27] Improved doping of CdTe and increased understanding of key processing steps (e.g., cadmium chloride recrystallization and contacting) are key to progress. Since CdTe has the optimal band gap for single-junction devices, it may be expected that efficiencies close to exceeding 20% (such as already shown in CIS alloys) should be achievable in practical CdTe cells. Modules of 15% would then be possible.
Process optimization
Process optimization allows greater throughput at smaller cost. Typical improvements are broader substrates (since capital costs scale sublinearly, and installation costs can be reduced), thinner layers (to save material, electricity, and throughput time), and better material utilization (to save material and cleaning costs). Making components rather than buying them is also a traditional way for great manufacturers to shave costs. Today’s CdTe module costs are about $110/m2 (normalized to a square meter).[28] Costs are expected to reduce to $75/m2.
Thus a practical, long-term (10–20 year) goal for CdTe modules resulting from combining cost and efficiency goals would be $75 per 150 watts, or about $0.5 per watt.[29] With commodity-like margins and combined with balance-of-system (BOS) costs, installed systems near $1.5/W seem achievable. With Southern California sunlight, this would be in the 6 to 8 US cents per kWh range (e.g., based on economic and other assumptions used in algorithms such as in the United States Department of Energy and NREL's Solar Advisory Model).[30]
Tellurium supply
Perhaps the most subtle and least understood problem with CdTe PV is the supply of tellurium. Tellurium (Te) is an element not currently used for many applications. Only a small amount, estimated to be about 800 metric tons [31] per year, is available. According to USGS, global tellurium production in 2007 was 135 metric tons[32]. Most of it comes as a by-product of copper, with smaller byproduct amounts from lead and gold. One gigawatt (GW) of CdTe PV modules would require about 93 metric tons (at current efficiencies and thicknesses),[33] so this seems like a limiting factor. However, because tellurium has had so few uses, it has not been the focus of geologic exploration. In the last decade, new supplies of tellurium-rich ores have been located, e.g., in Xinju, China.[34] Since CdTe is now regarded as an important technology in terms of PV’s future impact on global energy and environment, the issue of tellurium availability is significant. Recently, researchers have added an unusual twist – astrophysicists identify tellurium as the most abundant element in the universe with an atomic number over 40.[35][36] This surpasses, e.g., heavier materials like tin, bismuth, and lead, which are common. Researchers have shown that well-known undersea ridges (which are now being evaluated for their economic recoverability) are rich in tellurium and by themselves could supply more tellurium than we could ever use for all of our global energy.[36][37] It is not yet known whether this undersea tellurium is recoverable, nor whether there is much more tellurium elsewhere that can be recovered.
Other issues
Cadmium
Another issue frequently mentioned, is the use and recycling of the extremely toxic metal cadmium, one of the six most toxic materials banned by European Union's RoHS regulation. According to First Solar's annual report[38], the CdTe solar panel is not in RoHS compliance, not listed in the exemption product list, but not currently listed in the restricted product list either. So the product's future RoHS compliance status is uncertain[39]. First Solar has a self-imposed recycling regimen that provides a deposited amount (<$0.05 a watt) that covers the costs of transport and recycling of the module at the end of its useful life.[40][41] Recycling has been fully demonstrated on scrap modules. In a validating test, Vasilis Fthenakis of the Brookhaven National Laboratory showed that the glass plates surrounding CdTe material sandwiched between them (as they are in all commercial modules) seal during a fire and do not allow any cadmium release.[42] All other uses and exposures related to cadmium are minor and similar in kind and magnitude to exposures from other materials in the broader PV value chain, e.g., to toxic gases, lead solder, or solvents (most of which are not used in CdTe manufacturing).[43]
Price vulnerability
A subtle issue with CdTe and with all thin films in relation to greater efficiency PV module technologies is the potential impact of commodity inflation. Greater efficiency modules incur a better balance of system commodity cost per unit output. Thus such inflation can have a greater percentage impact on system cost. This is another reason that continued efficiency improvements are important.
Solar tracking
Almost all thin film photovoltaic module systems to-date have been non-solar tracking, because the output of modules has been too low to offset tracker capital and operating costs. But relatively inexpensive single-axis tracking systems can add 25% output per installed watt.[30] This is climate-dependent. Tracking also produces a smoother output plateau around midday, allowing afternoon peaks to be met.
Market viability
Success of cadmium telluride PV has been due to the low cost achievable with the CdTe technology, made possible by combining adequate efficiency with lower module area costs.[25] Direct manufacturing cost for CdTe PV modules has reached $1.12 ea watt,[44] and capital cost per new watt of capacity is near $0.9 per watt (including land and buildings).[45] However, module cost alone is not enough to assure the lowest installed system price. Thin films, including CdTe, are less efficient than most wafer silicon modules. Typical wafer silicon modules are 13% to 20% efficient, while the best CdTe modules were about 10.7% efficient; recent modules produced at First Solar and measured by NREL have shown CdTe modules with efficiencies at 12.5% or greater. Many components of an installed PV system (e.g., support structures, installation labor, land) scale with system area; and less-efficient modules require more area to produce the same output (all other things being equal). The impact of area-related costs on CdTe systems is about $0.5 per watt of extra cost.
Notable systems
Recent installations of large CdTe PV systems by First Solar confirm the competitiveness of CdTe PV with other forms of solar energy and how close it is to being competitive with conventional natural gas peakers:
A 40MW system being installed by juwi group in Waldpolenz Solar Park, Germany: at the time of its announcement, it was both the largest planned and lowest cost PV system in the world. The price of 3.25 euros translated then (when the euro was equal to US$1.3) to $4.2/watt, much lower than any other known system.[46]
A 7.5-megawatt system to be installed in Blythe, CA, where the California Public Utilities Commission has accepted a 12 US cent per kWh power purchase agreement with First Solar (after the application of all incentives).[47] Defined in California as the "Market Referent Price," this is the price the PUC will pay for any daytime peaking power source, e.g., natural gas. Although PV systems are intermittent and not dispatchable the way natural gas is, natural gas generators have an ongoing fuel price risk that PV does not have.
A contract for two megawatts of rooftop installations with Southern California Edison, where the SCE program is designed to install 250 megawatts at a total cost of $875M (averaging $3.5/watt), after incentives.[48]
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Total Price of Installing Solar
What percentage of the total cost of installing a solar array is due to the solar cells in the U.S, and Germany? (2-digit answer)
(a) 5% (b) 15% (c) 40% (d) 60%
4 kW installation.
Solar Photovoltaic Plant in Nevada
El Dorado Solar PV plant with 167,000 panels (~60 W each) on 80 acres for a maximum power output of 10 MW.
This PV plant is approximately equivalent to how many windmills?
(a) 10 (b) 100 (c) 1,000
The panels are tilted toward which part of the sky?
(a) Northern (b) Southern
*
Q1: Refer back to your notes for the typical power of a large windmill.
Q2: If you are in the Northern hemisphere, in what part of the sky is the sun located?
http://www.semprageneration.com/eds.htm
El Dorado Energy Solar
In December 2008, Sempra Generation completed the initial phase of construction on its first large-scale solar power development, a 10-megawatt (MW) thin-film, photovoltaic (PV) solar-cell installation using technology supplied by First Solar, the world's leading manufacturer of this technology.
El Dorado Solar is the largest thin-film solar technology operation in North America. It is located on 80-acres of a remote section of Boulder City, Nev., near the existing 480 MW El Dorado Energy natural gas-fired power plant. Sempra Generation is considering a future expansion of the solar power project.
It generates clean, emissions-free solar power during periods of peak electric demand throughout the region without the use of water. The project illustrates Sempra Generation's commitment to clean, renewable power development.
SAN DIEGO, CA--(Marketwire - December 22, 2008) - Sempra Generation, a subsidiary of Sempra Energy (NYSE: SRE ), today announced the completion of the company's first solar energy project, a 10-megawatt (MW) photovoltaic power-generation facility adjacent to the company's existing 480-megawatt El Dorado Energy power plant near Boulder City, Nev., about 40 miles southeast of Las Vegas.
The El Dorado Energy Solar project is the largest operational thin-film, solar-power project in North America. Construction began in July 2008, and involved the installation of more than 167,000 solar modules on 80 acres of desert property designated as a renewable energy zone and leased from Boulder City.
Sempra Generation also announced it has entered into a 20-year power purchase agreement for the new project's entire output with Pacific Gas and Electric (PG&E), the utility serving northern and central California. The contract is subject to approval by the California Public Utilities Commission.
At peak production El Dorado Energy Solar will generate enough electricity to power approximately 6,400 homes.
"This is a significant step in the development and deployment of renewable solar power," said Michael W. Allman, president and chief executive officer of Sempra Generation. "It reflects the commitment by Sempra Generation and western U.S. utilities to meet the challenges posed by climate change with reliable, renewable energy. The size and scope of this new solar generation facility clearly demonstrates that we can build projects on a scale that helps utilities meet their renewable energy goals."
The project's solar modules employ an advanced thin-film semiconductor technology to convert sunlight into electricity without air emissions or water use. These modules will generally produce more electricity under real-world conditions than conventional solar modules with similar power ratings.
"The El Dorado Energy Solar facility will be the first of our contracted solar projects to come online," said Jack Keenan, chief operating officer for PG&E. "We are pleased to partner with Sempra Generation as we add renewable resources to our power mix and continue to provide some of the cleanest energy in the nation."
Additional expansion phases of the project are under consideration.
Unlike some solar power projects, El Dorado Energy's solar power plant will not use water or other liquids in the power-generation process. This water conservation feature makes the project especially suitable to the arid U.S. Southwest. As with other solar projects, the new Sempra Generation facility will generate electricity during the day when customer demand peaks.
Arizona-based First Solar (NASDAQ: FSLR ) was the engineering, procurement and construction contractor for the project and is charged with monitoring and maintaining the plant.
Formed in 1999, First Solar is a worldwide industry leader in thin-film photovoltaic solar-module manufacturing with 2007 revenues of more than $500 million.
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3TW
"Footprint" for Solar Photovoltaic to Power U.S.
To supply 3 TW using solar photovoltaic power plants, the land "footprint" would be a substantial fraction of New Mexico.
What are some issues with solar energy?
see calculation in Appendix
Is this area "footprint" reasonable? What happens at night?
Where is the solar intensity highest in the U.S.? Is that close to major population centers?
(
)
gth
ometers