Photovoltaic Technology. Topics for this lecture

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1 Photovoltaic Technology How to produce electricity directly from the sun Lecture prepared with the able assistance of Chris Polashenski, TA Topics for this lecture How does PV work AC/DC and inverters Battery vs. grid-tied, or both Sizing a system Scalability to the global climate problem 1

A bit of history The photo-voltaic effect was first identified in 1839 by French

com) Process in a nutshell: - Photons hit semiconductor surface.

conduction level. - Electrons in conduction level travel and make electricity.

2 A bit of history The photo-voltaic effect was first identified in 1839 by French physicist Alexandre-Edmond Becquerel (not Henri, his dad). (allposters.com) Process in a nutshell: - Photons hit semiconductor surface. - If energy level is just right, it bumps electrons from valence position up to a conduction level. - Electrons in conduction level travel and make electricity. But we are engineers and we can explain this better than that. Semiconductors Four valence electrons 2

3 Silicon in a pure state Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si All electrons used in bonds Not very conductive Si Si Si Si Si Si Si Si Si Si Doping Three valence electrons Five valence electrons 3

4 Missing electron Extra, mobile electron Because some electrons are now mobile, we now have a conductive material! 4

5 Now what? Where is this going? Make a p-n Junction electrons holes 5

6 How the p-n junction works Now: - Each side started from an electrically neutral state. - As negative charges move left, a charge imbalance is generated. - This creates an electric field, which in turn erects an obstacle to further flow of electrons. - All the electrons and holes in the depletion layer have combined and annihilated each other. - This creates a voltage across the junction. The p-n junction 6

7 photon from sunlight Voila! Electricity! 7

8 Close up. Note the very fine wires and crystal grains A contemporary photovoltaic cell made of silicon crystal. Note the array of wires ( Photovoltaic Array connected in series additive voltage ( ( 8

9 Current Photovoltaic Cells - Silicon Crystals and Granular Silicon (most popular material for solar cells) - Wafers180 to 240 μm thick - Must first be refined to a highly pure form - Then doped" with other elements - Single layer of p-n diode (the n-type by Phosphorus and p-type by Gallium) - One excitation per photon - 85 % of solar cell market - Life expectancy of >30 years - Energy payback in 2-8 years (positive) So-called First-Generation How does PV work AC/DC and inverters Battery vs. grid Sizing a system. Scalability to the global climate problem 9

thatsolarguy.com/components/servicepanel.

10 Sun Energy flow Solar Panel Batteries Inverter Power Grid Electric Panel Household Loads / k7cJBm.jpg 10

significantly more dangerous power Significant inefficiencies (90-95%.

11 Inverters Link between panels and household power Convert between DC and AC current Panels are DC, houses are wired for AC DC = significantly more dangerous power Significant inefficiencies (90-95%... efficient) Provide link to batteries Only necessary when batteries are included. 11

12 How does PV work AC/DC and inverters Battery vs. grid Sizing a system Scalability to the global climate problem Stand-alone systems vs. Grid-tied systems Stand-alone System meets all electrical need for building No connection to conventional power grid Goal = Zero Energy Grid-tied System meets some or all of electrical demands Requires connection to power grid Goal = Net-zero Energy 12

13 Sun Stand-alone Solar Panel Batteries Inverter Household Loads Electric Panel / k7cJBm.jpg Advantages: Stand-alone Works in remote locations Protection against power failures Disadvantages: Requires much more powerful system Designed for worst-case scenario Must produce more power than average consumption Significantly more expensive Greater environmental impact, perhaps even than simply using grid power Could run out of power 13

14 Sun Grid-tied Solar Panel Inverter Power Grid Electric Panel Household Loads / k7cJBm.jpg Grid-tied Advantages: System does not have to cover all electrical needs at all times Requires less surface area for panels and no batteries Less expensive Disadvantages: Does not prevent grid power failures Can be dealt with by small battery bank 14

15 When would you use different systems Grid-tied Use it anytime possible Off-grid When connection to the grid is not possible Grid-tied with battery backup Anytime a backup power source is considered critical Should not be used unless backup is required How does PV work AC/DC and inverters Battery vs. grid Sizing a system Scalability to the global climate problem 15

Sizing a system What is your need? What are your goals? How much sun is available? www.

Size ought to be based on energy bill, BUT First reduce load Find which appliances are least

16 Sizing a system What is your need? What are your goals? How much sun is available? What is your need? Size ought to be based on energy bill, BUT First reduce load Find which appliances are least efficient. Typical consumption values available from US Dept. of Energy website, but can also use a wattmeter to check devices. Can get from Howe Library! 16

How much sun do you get? http://www.nrel.gov/gis/solar.

17 How much sun do you get? Solar System Sizing: Grid-tied Month Solar Radiation Panel Efficiency (%) Energy Demand for Hanover, NH Solar System Size kwh/m^2/day kwh/month m^2 January 3.8 February 4.5 March 4.8 April 5.1 May 5.4 June 5.4 July 5.5 August 5.4 September 4.9 October 4.0 November 3.1 December 2.8 AVERAGE % ~10 X30 area of panels 17

18 Solar System Sizing: Off-grid Month Solar Radiation Panel Efficiency (%) Energy Demand for Hanover, NH Solar System Size kwh/m^2/day kwh/month m^2 January % February % March % April % May % June % July % August % September % October % November % December % max ~10 X50 of panels Does not provide enough power for long cloudy periods Requires significant energy storage 21kWh per day stored (design for ~3 day storing) How does PV work AC/DC and inverters Battery vs. grid Sizing a system Scalability to the global climate problem 18

19 Global Scalability Predicted Global energy use in exajoules/year (exa=10 18 ) Theoretical Solar Potential 3,900,000 exajoules/year Plenty theoretically, but what limits technical potential? Background Grand energy challenge - double demand by triple demand by : TW 2050: TW World Energy Demand total TW industrial developing energy gap ~ 14 TW by 2050 ~ 33 TW by US ee/fsu (Source: Argonne National Lab) 19

20 Scalability Technical Limits Conversion Efficiency (a few percent to 20%) Land Use (still need to grow crops, have ecosystems) (still between 1,575 and 49,837 exajoules, depending on who s estimate you like) 20

21 Scalability Technical Limits Timing. Oooh. Technical Problem Transmission Storage CST (concentrated solar thermal) Heat up pressurized steam or liquid sodium, run through turbine at night 21

22 Smart Grid Trigger high energy loads at peak production Charge electric vehicles Electric Smart Car Complementary or Dispatchable Generation 22

Scalability Economic Limits Photovoltaic installations in most parts of the country will not produce a return on investment without government incentive.

23 Scalability Economic Limits Photovoltaic installations in most parts of the country will not produce a return on investment without government incentive. PV is one of the LEAST cost effective ways to reduce carbon emissions. What other ways are less costly? Cost of Solar Electricity 100 $0.10/W p $0.20/W p $0.50/W p Efficiency % $1.00/W p $3.50/W p Thermodynamic limit at 1 sun Shockley - Queisser limit: single junction Cost $/m 2 I: bulk Si II: thin film III: next generation $/W peak values based on peak sun of 1000 W/m 2 (Source: Argonne National Lab) 23

24 Carbon Free 24

GCEP RESEARCH SYMPOSIUM 2012 STANFORD, CA. Sally M. Benson. Director, Global Climate and Energy Project Stanford University OCTOBER 10, 2012

May 15-16, 2013 Research at GCEP in: Solar Energy and Photocatalysis GCEP RESEARCH SYMPOSIUM 2012 STANFORD, CA Sally M. Benson Director, Global Climate and Energy Project Stanford University Bruce Clemens