ECE 333 Renewable Energy Systems Lecture 16 Photovoltaic

ECE 333 Renewable Energy Systems Lecture 16: Photovoltaic Materials and Electrical Characteristics Prof. Tom Overbye Dept. of Electrical and Computer Engineering University of Illinois at Urbana-Champaign overbye@illinois. edu

Announcements • • • HW 6 is should be turned in today HW 7 is 4. 2, 4. 5, 4. 9, 4. 13, 5. 2; it will be covered by an in-class quiz on April 2. Read Chapter 5 (Photovoltaic Materials and Electrical Characteristics) 1

Total Clear Sky Insolation • The total insolation is the sum of the direct beam, diffuse and reflected – Most is direct beam; models for diffuse and reflected are more approximate (and certainly site dependent) q is angle between sun and normal to collector Superscript h indicates values on surface in front of collector; r is estimates reflectivity of surface, ranging from 0. 8 for snow to 0. 1 for dark shingles 2

Ameren Solar Energy Production • • Many groups publish there actual solar output, so actual data from a similar facility is often available A local example is data from Ameren, including from a variety of different solar pv technologies on the roof of their headquarters in St. Louis; total for the mono and poly ones are 32. 56 and 32. 76 k. W https: //www. ameren. com/missouri/solar-energy-produced 3

California Solar Shade Control Act • • The shading of solar collectors has been an area of legal and legislative concern (e. g. , a neighbor’s tree is blocking a solar panel) California has the Solar Shade Control Act (1979) to address this issue – – – No new trees and shrubs can be placed on neighboring property that would cast a shadow greater than 10 percent of a collector absorption area between the hours of 10 am and 2 pm. Exceptions are made if the tree is on designated timberland, or the tree provides passive cooling with net energy savings exceeding that of the shaded collector Modified in 2008 to exclude systems designed to offset more than a building’s electricity demand 4

The Guilty Trees were Subject to Court Ordered Pruning This case resulted in 2008 modification to exclude existing trees (in this case the redwoods were planted before the panels were installed) For more information see: http: //www. josre. org/wp-content/uploads/2012/09/California_Solar_Shade_Control_Act-JOSRE_v 2 -161. pdf Image Source: NYTimes, 4/7/08 5

Solar PV can be Quite Intermittent Because of Clouds Intermittency can be reduced some when PV is distributed over a larger region; key issue is correlation across an area Image: http: //www. megawattsf. com/gridstorage. htm 6

Predicable, but Quite Large Dropoff Later as the Sun Sets • Known as the "Duck Curve" Image: http: //www. caiso. com/documents/flexibleresourceshelprenewables_fastfacts. pdf 7

Photovoltaics (PV) Photovoltaic definition- a material or device that is capable of converting the energy contained in photons of light into an electrical voltage and current University of Illinois Solar nd place Decathalon House – 2 "Sojourner" overall in 2009 exploring Mars, 1997 Rooftop PV modules on a village health center in West Bengal, India http: //www 1. eere. energy. gov/solar/pv_use. html http: //www. solardecathlon. uiuc. edu/gallery. html# 8

PV History • • • Edmund Becquerel (1839) Adams and Day (1876) Albert Einstein (1904) Czochralski (1940 s) Vanguard I satellite (1958) Costs have recently dropped quite substantially Image Source: http: //pubs. rsc. org/en/Content/Article. Html/2013/EE/c 3 ee 40701 b 9

US Solar Capacity • US solar capacity is growing at close to 100% yearly, with most of the growth in solar PV (as opposed to solar thermal) In 2004 we got 6 GWh from solar PV; in 2009 it was 735, in 2013 8121 and in 2014 it was 15, 874 (about 0. 4% of total electricity) Image: http: //cleantechnica. com/2014/04/24/us-solar-energy-capacity-grew-an-astounding-418 -from-2010 -2014/ 10

PV System Overview • Solar cell is a diode • Photopower coverted to DC Shadows • Shadows & defects convert generating areas to loads • DC is converted to AC by an inverter • Loads are unpredictable • Storage helps match generation to load 11

Some General Issues in PV • The device • Efficiency, cost, manufacturability automation, testing • Encapsulation • Cost, weight, strength, yellowing, etc. • Accelerated lifetime testing • 30 year outdoor test is difficult • Damp heat, light soak, etc. • Inverter & system design • Micro-inverters, blocking diodes, reliability 12

What are Solar Cells? • • Solar cells are diodes Light (photons) generate free carriers (electrons and holes) which are collected by the electric field of the diode junction The output current is a fraction of this photocurrent The output voltage is a fraction of the diode built-in voltage Short-circuit current + Open-circuit voltage Voltage Current • • n-type - p-type Load Maximum Power Point 13

Standard Equivalent Circuit Model Where does the power go? Load (maximize) Shunt resistance (minimize) Diode Photocurrent source Series resistance 14

Photons • Photons are characterized by their wavelength (frequency) and their energy Planck's constant (h) is 6. 626 10 -34 J-s Velocity of light (c) is 3 108 m/s • In this context energy is often expressed in electronvolts (e. V), which is defined as 1. 6 10 -19 J – This is the amount of energy gained by a single electron moving across a voltage difference of one volt Above equation for E can be rewritten with E in e. V and l in mm 15

Band-Gap Energy • • Electrical conduction is caused by free electrons (electrons in conduction band) At absolute zero temperature metals have free electrons available and hence are good conductors – • Metal conductivity decreases with increasing temperature Semi-conductors, such as silicon, have no free electrons at absolute zero and hence are good insulators 16

Band-Gap Energy, cont. • • Silicon conductivity increases with increasing temperature; at room temperature they only have about 1 in 1010 electrons in the conduction band Band gap energy is the energy an electron must acquire to jump into the conduction band Band Gap and Cut-off Wavelength Above Which Electron Excitation Doesn’t Occur Quantity Si Ga. As Cd. Te In. P Band gap (e. V) 1. 12 1. 42 1. 5 1. 35 Cut-off wavelength (μm) 1. 11 0. 87 0. 83 0. 92 17

Silicon Solar Cell Max Efficiency • • Wavelengths above cutoff have no impact This gives an upper bound on the efficiency of a silicon solar cell: – • • Band gap: 1. 12 e. V, Wavelength: 1. 11 μm This means that photons with wavelengths longer than 1. 11 μm cannot send an electron into the conduction band. Photons with a shorter wavelength but more energy than 1. 12 e. V dissipate the extra energy as heat 18

Silicon Solar Cell Max Efficiency • • For an Air Mass Ratio of 1. 5, 49. 6% is the maximum possible fraction of the sun’s energy that can be collected with a silicon solar cell Efficiencies of real cells are in the ~20 -25% range 19

Solar Cell Efficiency • Factors that add to losses – – • Smaller band gap - easier to excite electrons, so more photons have extra energy – • • Recombination of electrons/holes Internal resistance Photons might not get absorbed, or they may get reflected Heating Results in higher current, lower voltage High band gap – opposite problem There must be some middle ground since P=VI (DC), this is usually between 1. 2 and 1. 8 e. V 20

Maximum Efficiency for Cells • The maximum efficiency of single-junction PV cells for different materials was derived in 1961 by Shockley and Queisser 21

Review of Diodes • • Two regions: “n-type” which donate electrons and “p-type” which accept electrons p-n junction- diffusion of electrons and holes, current will flow readily in one direction (forward biased) but not in the other (reverse biased), this is the diode http: //en. wikipedia. org/wiki/File : Pn-junction-equilibrium. png 22

The p-n Junction Diode • Can apply a voltage Vd and get a current Id in one direction, but if you try to reverse the voltage polarity, you’ll get only a very small reverse saturation current, I 0 • Diode voltage drop is about 0. 6 V when conducting 23

The p-n Junction Diode Voltage-Current (VI) characteristics for a diode k = Boltzmann’s constant 1. 381 x 10 -23 [J/K] T = junction temperature [K] Vd = diode voltage Id = diode current q = electron charge 1. 602 x 10 -19 C I 0 = reverse saturation current 24

Circuit Models of PV Cells • The simplest model of PV cell is an ideal current source in parallel with a diode I I + PV cell • • VLoad ISC Id + VLoad - The current provided by the ideal source ISC is proportional to insolation received If insolation drops by 50%, ISC drops by 50% 25

Circuit Models of PV Cells ISC Id + Vd - + I VLoad The subscript SC is for short circuit - • From KCL, ISC = Id + I, and the current going to the load is the short-circuit current minus diode current • Setting I to zero, the open circuit voltage is 26

PV Cell I-V Characteristic • For any value of ISC , we can calculate the relationship between cell terminal voltage and current (the I-V characteristics) • Thus, the I-V characteristic for a PV cell is the diode I-V characteristic turned upside-down and shifted by ISC (because I = ISC – Id) The curve intersects the x-axis at VOC and the y-axis is ISC • 27

PV Cell I-V Curves dark curve • I-V curve for an illuminated cell = “dark” curve + ISC 28

PV Cell I-V Curves • • More light effectively shifts the curve up in I, but VOC does not change much By varying the insolation, we obtain not a single I-V curve, but a collection of them 29

Need for a More Accurate Model • • The previous circuit is not realistic for analyzing shading effects (we’ll talk more about shading later) Using this model, absolutely NO current can pass when one cell is shaded (I = 0) 30

PV Equivalent Circuit • • • It is true that shading has a big impact on solar cell power output, but it is not as dramatic as this suggests! Otherwise, a single shaded cell would make the entire module’s output zero. A more accurate model includes a leakage resistance RP in parallel with the current source and the diode RP is large, ~RP > 100 VOC/ISC A resistance RS in series accounts for the fact that the output voltage V is not exactly the diode voltage RS is small, ~RS < 0. 01 VOC/ISC 31

PV Equivalent with Parallel Resistor From KCL, I = ISC - Id – IRP • + Vd - I + (maximize) ISC Id RP Shunt resistance drops some current (reduces output current) Photocurrent source Parallel-Only V Load • - For any given voltage, the parallel leakage resistance causes load current for the ideal model to be decreased by V/RP 32

PV Equivalent with Series Resistor • Add impact of series resistance RS (we want RS to be small) due to contact between cell and wires and some from resistance of the semiconductor From KVL Series-Only Series resistance drops some voltage (reduces output voltage) • Photocurrent source I ISC Id + Rs + (minimize) Vd V - - Load • The output voltage V drops by IRS 33

General PV Cell Equivalent Circuit The general equivalent circuit model considers both parallel and series resistance Equivalent Circuit Rs Vd - (maximize) + RP ISC Id + V Load I (minimize) Photocurrent source • - 34

Series and Shunt Resistance Effects • Parallel (RP) – current drops by ΔI=V/RP • Series (RS) – voltage drops by ΔV=IRS 35

Fill Factor and Cell Efficiency Fill Factor (FF) = VR • IR Isc • Voc Fill factor is ratio at the maximum power point Cell Efficiency (h) = area = VOCISC area = VRIR Imax • Vmax Isc • Voc • FF Incident = Incident Power 36

Multijunction Cells - n-type + p-type n-type Problem: Single junction loses all of the photon energy above the gap energy. + p-type - 1. 1 1. 0 + p-type Load Energy (e. V) 180 4. 0 3. 0 2. 5 2. 0 1. 7 1. 5 1. 3 0. 9 160 140 Intensity (m. W/m 2 -mm) 80 60 40 20 0 300 500 700 900 1100 Wavelength (nm) Cell #4 (0. 6 e. V gap) Each cell captures the light transmitted from above. Cell #3 (1. 0 e. V gap) 100 Cell #2 (1. 4 e. V gap) 120 Top Cell (1. 9 e. V gap) Solution: Use a series of cells of different gaps. 1300 1500 37

Record laboratory thin film cell efficiencies 38