The Physics of Photovoltaics: An Analysis of Solar Panel Cost, E ffectiveness and Efficiency

1. The Physics of Photovoltaics: An Analysis of Solar Panel Cost, Effectiveness and Efficiency Group 16: Angel Needham-Gilles, NicoMongillo and Nick Pauley

2. Project Overview As a group we decided to go the analytical route. The purpose of our project was to investigate the physics of solar panels including their production and how they convert light to energy. Also investigated are their efficiency and efficacy as long-term “green” solutions as well as some of the political implications of their use.

3. The Raw Materials Required for Solar Panels • There are several varieties of cells today that can be used to absorb sunlight and convert those photons into electrical energy. These include: • CIS (Copper Indium Diselenide) Cells • CdTe cells (Cadmium Telluride) Cells • Organic Cells • Multi-junction Cells • Although multi-junction cells have the highest efficiency among solar cells achieved in a laboratory setting (demonstrated by the following graph), Silicon Cells (monocrystaline, polycrystaline and amorphous) due to their low cost and decent efficiency are the most feasible for wide production and will thus be the main interest of our study.

4. What’s in a Polycrystalline Si Solar Cell? -Polycrystalline cells are slightly less efficient thanmonocrystallinesolar cells, but are cheaper to manufacture in wafer form. -The diagram below from How Stuff Works illustrates the order of the raw ingredients needed to create a generic polycrystalline silicon cell. They include: A layer of glass for protection Antireflective coating Contact grid Highly purified N-Type Silicon P-Type Silicon Back Contact.

5. Raw Materials Continued • There is no threat to the global supply of any of the raw materials used to manufacture silicon solar cells, even if their production dramatically accelerated. • Siliconis an abundant element in the earth’s crust, and is in no way potentially threatened with shortage. • There is some concern about the supply of materials needed for non-silicon based cells: The development of CIGS cells “might be slightly constrained by shortages of gallium and selenium,” while, mass production of CdTe cells may behamperedby tellurium availability (Lynn, 208, 2010).

6. Generic Manufacturing of Silicon Panels: • Starting point for a polycrystalline cell is in molten form, “cast in substantial blocks,” and then cut down to smaller bricks and eventually into a thin wafer (Lynn, 2010). • “As the molten silicon cools, crystallization occurs simultaneously,” and these cells are soldered to a diode that conducts electricity (Lynn, 2010). • For a helpfully illustrative video on how a generic silicon cell is produced click on the following link: • Discovery Channel: Solar Panel Manufacturing

7. Forms of Solar Energy Gathering • A single unit is referred to as a cell • A collection of cells is a module • A collection of modules is an array http://science.nasa.gov/science-news/science-at-nasa/2002/solarcells/ Note that while they may all be made up of the same type of cell a module and an array will have different efficiencies due to the empty spaces between each cell

8. How Solar Cells Work: The Photoelectric Effect • The basic principle of photovoltaics – the branch of solid state physics which has a focus of turning light into electrical energy • It is a property of certain materials where photons of light are absorbed and the energy causes electrons to be “knocked loose” • Those materials are referred to as semiconductors

9. Semiconductors http://www.cpushack.com/MakingWafers.html • Semiconductors can be natural or created through “doping” where impurities are added to give more or less electrons • Two kinds of semiconductors used for a single solar cells * A Positive P-type (Electron Poor) * A Negative N-type (Electron Rich) Pure Silicon – while it is the most commonly used element for solar cells, it is usually doped with other materials to create the specific type of semiconductor needed

10. P-N Junction • The electric potential barrier between the two semiconductors of a solar cell • Creates a low resistance path for excited electrons to flow through • “Loose” electrons flow from the rich end to the poor one creating a direct current *This is called the photovoltaic effect and explains why the true name for solar cells are PV cells http://express.howstuffworks.com/exp-solar-power1.htm

11. The Band Gap http://mousely.com/encyclopedia/Band_gap/ • A property of the atoms of the semiconductor • This is the energy gap between two “bands” of energy between tow electron states in a solid • Only photon energy which matches the band gap energy of the material can free an electron from that state. Energy of a photon

12. Solar Cell Efficiency • Considers how much energy available and compares it to how much energy is used productively Solar cell efficiency ( ) depends on - The total power light power density (JV) on the cell - The actual potential difference of the system - The actual current density of the system - The “fill factor” constant which is the ratio between the actual values and the maximum values

13. Efficiency – The Band Gap Problem • Get image from book Certain photon energy levels which are created by the sun get absorbed or reflected by the atmosphere, which prevents solar cells from accessing that particular level for its electrons. (Note: AM means Air Mass which is equivalent to the thickness of the atmosphere) (Picture Credit – Lynn 2010)

14. The Multijunction Solution • Multijunction cells are the most efficient solar cells when tested • Overcomes the issue of a single band gap by incorporating many different materials into a single cell, thus adding more band gaps • Multiple band gaps allow for more of the available light energy to be used http://science.nasa.gov/science-news/science-at-nasa/2002/solarcells

15. The Best Solar Cell Research Efficiencies as Recorded by the NREL (National Renewable Energy Lab) (Picture Credit - http://www.nrel.gov/pv/thin_film/docs/kaz_best_research_cells.ppt) Spectrolab Multijunction ConcentratorsThree-junction (2-terminal, monolithic)Two-junction (2-terminal, monolithic) Crystalline Si CellsSingle crystalMulticrystallineThin Si Thin Film TechnologiesCu(In,Ga)Se2CdTeAmorphous Si:H (stabilized) Emerging PVOrganic cells 36 Spectrolab Japan Energy 32 NREL/ Spectrolab NREL 28 NREL UNSW UNSW 24 UNSW Spire UNSW NREL Cu(In,Ga)Se2 14x concentration UNSW Stanford Spire UNSW Georgia Tech ARCO 20 NREL Efficiency (%) Sharp Georgia Tech Westing- house Varian NREL NREL NREL 16 UniversitySo. Florida NREL No. Carolina StateUniversity NREL AstroPower Euro-CIS Boeing Solarex ARCO Boeing 12 Kodak Boeing UnitedSolar AMETEK Masushita United Solar AstroPower Kodak Boeing 8 Monosolar Photon Energy RCA Solarex University California Berkeley Boeing Princeton University of Maine 4 RCA RCA UniversityKonstanz RCA RCA RCA NREL RCA 0 Year 1975 1980 1985 1990 1995 2000

16. Other forms of Efficiency • Two kinds of paybacks * Energy Payback – how long does it take for the solar cell to make the energy it took to make the panel itself * Cost Effectiveness – how long does it take for the solar cell to generate the energy equivalent to its cost

17. Current Statistics • Energy Payback: *It takes the average silicon solar cell in the United States between two (for the lower half) to four years (for the upper half) • Average life of a silicon solar cell: 20-25 years • Cost Effectiveness: * It currently costs about $7.00 per Watt but can go as low as $4.30 per Watt (This is a significant decrease from the $300 per Watt cost during the 1970’s) * Note that once installed unless it has a sun tracking system installed to it, its only needed fuel is sunlight Data from Lynn 2010

18. Money Matters • Solar cells now cost $3.50 per watt of production capacity vs. $70 in the 1970s. • However, the finished product of the actual panel itself will run for at least $215. Highly efficient ones will typically go for around $1000. • Panels made from scrap solar cells (those broken in manufacturing) can be purchased for a cheaper price but are highly inefficient which seemingly defeats the purpose.

19. The Vassar Switch:Vassar Dorms Using Solar Energy • 50% of Dorm Electricity * Total Cost: $1,921,330 – $3,537,151 * Total Area: 40,460 – 73,436 sq m * Average Monthly Savings: $8000 * Cost Payback: 15.00 – 23.09 years • 100% of Dorm Electricity * Total Cost: $3,903,911 - $7,135,553 * Total Area: 80,921 – 146,873 sq m * Average Monthly Savings: $ 16,000 * Cost Payback: 15.4 – 23.22 years As of 2008, Vassar was paying $16,000 a month for dorm electricity alone. Based on some of the leading New York electric companies and New York’s average solar irradiance of 4.47 kWh/sq m per day… Statistics Credit goes to Cooler Planet.com

20. Solar panels only generate electricity during daylight hours. In most cases this means that it will only provide energy for half of the day. Obviously, electricity used at night is gathered from energy stored during daylight collection hours. Weather obviously also obstructs the absorption of photons. Temperature, however, has little effect on the efficiency of solar panels. Pre-existing pollution can interfere as well which will prove difficult in industrial areas and in cities. For these reasons, solar panels cannot, unfortunately, be the sole provider of energy for the United States Will Solar Energy REALLY reduce dependency on other resources?

21. Will solar power REALLY affect importation of foreign fuels? • 40% of the energy supply in the US comes from petroleum • 60% of this oil is imported. Because those who control these foreign resources hold so much power, there is little political will to develop solar technology.

22. Government Incentives Explored • In some instances the federal government offers benefits to those who invest in solar technology such as theResidential Federal Tax Credit or Commercial Federal Grant. • However, regulations are set on the state, NOT federal levels. Thiscan make it more difficult to be eligible for programs that typically do not reimburse consumers for more than 30% of the total cost of their solar panel projects.

23. Developing Technology: Thin Film Solar Energy • A company in California called Nanosolar (2007) has recently been mass-producing a thin film capable of producing high levels of energy from sunlight at a cost of one dollar per watt, competitive with coal. Compared to usual solar cells that require glass, aluminum, copper and silicon, these cells are a thin film consisting of five layers: • 1. Aluminum foil for stability. • 2. Molybdenum Electrode • 3. CIGS absorber / semi-conductor: an ink made of a mix of copper, indium, gallium, and selenium. • 4. As in the old solar model a P/N junction, a semi-conductor that doesn’t absorb light. • 5. Lastly, a clear zinc oxide semi-conductor. • Check out the cool animation at the bottom of the page! http://www.popsci.com/popsci/flat/bown/2007/green/item_59.html

24. Environmental impacts associated with photovoltaic production • Coniff (2010) sites a report from the Scripps Institution of Oceanograpahy that found the greenhouse gas NF3 (nitrogen tri-flouride), a common by product of production of thin-film solar cells (an “economical and increasingly popular solar power format”) has “17,000 times the warming potential of carbon dioxide,” potentially undermining any environmental gain in the widespread use of the developing technology (Yale e360, November, 2008). Additionally, Cadmium is a heavy metal known used in CdTe solar cells It is known to cause harmful long term effects when ingested or inhaled such as kidney failure, lung damage and brittle bones While dangerous, it is a by product of zinc mining and its application in solar cells would be a much more productive use When recycled and carefully disposed of it there is hardly any risk using it

25. Future Applications • Constant trend of increasing efficiencies across all forms of solar cells • Inventive methods currently being considered include *solar panels on sattlelites which beam the energy back to earth in the form of microwaves *desert spanning solar farms *laser sunlight collectors to focus sun rays right at the solar cells http://www.maximumpc.com/article/news/solaren_quench_pges_energy_thirst_with_spacebased_solar_power http://pneumaticaddict.wordpress.com/page/25/

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