chemical, biological and environmental engineering solar energy overview
TRANSCRIPT
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Chemical, Biological and Environmental Engineering
Solar Energy overview
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Advanced Materials and Sustainable Energy LabCBEE
SolarPrinciple: Lots of sunlight incident on Earth’s surface:
1.3x1017 kWh/yr insolation; total human energy use (estimated for *all* history) 2.7x1012 kWh
Largest potential source– Diffuse
• Needs lots of land• Could use “free surface” (as roofs of built areas)
– Variable (like wind, but less so)• Sun only shines half of day…• Weather/year cycle?
Harness through:– Thermal conversion (including passive solar
heating/cooling)– Photovoltaics (direct conversion to electricity)
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Advanced Materials and Sustainable Energy LabCBEE
Solar energy uses
“hot water” solar thermal not discussed here– Low grade heat can be used as industrial process heat
Neither is heating/cooling/daylighting – But daylighting is cheapest way to displace electrical use
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Advanced Materials and Sustainable Energy LabCBEE
Which Solar Technology?
Break even between PV and Thermal at ca. 1300 kWh.m-2.yr-1
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Advanced Materials and Sustainable Energy LabCBEE
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Advanced Materials and Sustainable Energy LabCBEE
Concentrated Solar Thermal Power not so new…
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Advanced Materials and Sustainable Energy LabCBEE
Solar Radiation
,Absorbed
,Transmitted
,Reflected 1Incident
1
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Advanced Materials and Sustainable Energy LabCBEE
Solar Collector• Flat Plate, T max ~70˚C
– Hot water, space heating– 30-50% heat loss
ambientT
1R
U
usefulq collectorT
I
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Advanced Materials and Sustainable Energy LabCBEE
Solar Collector2 -2 =area of collector [ ], =solar flux [ ]totalq IA A m I Wm
( )useful useful collector ambientcollector
total
q q T TU
q IA I
-1 -2
( )
=thermal conductivity of collector [ ]
useful absorbed loss
absorbed loss collector ambient
q q q
q I A q AU T T
U WK m
( )useful collector ambientq A I U T T ( )P out inmc T T
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Advanced Materials and Sustainable Energy LabCBEE
Concentrating Collectors
• Motivation– Increase intensity at collector– Less heat loss over a smaller area– Higher maximum temperatures– Smaller area = less material = lower cost
• Types– Trough– Dish– Heliostat/Central Receiver
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Advanced Materials and Sustainable Energy LabCBEE
Concentrating Collectors
Concentration Ratio /a rCR A A Aa, Ar =Area of aperture, receiver [m2]
Aa
Ar
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Advanced Materials and Sustainable Energy LabCBEE
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Advanced Materials and Sustainable Energy LabCBEE
Concentrating Collectors
Larger CR means a more efficient collector
( )
( ) 1
useful o a collector ambient r
useful collector ambientcollector o
a
q IA U T T A
q U T T
IA I CR
ηo= optical efficiency (includes absorbed fraction at collector and reflectivity of concentrating optics)
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Advanced Materials and Sustainable Energy LabCBEE
Concentrating Collectors
(Tiwari, 2004)
R
r
AaAr
max
maxmax
2 :sin
nD CR
max 2max
3 :sin
nD CR
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Advanced Materials and Sustainable Energy LabCBEE
Radiation from a body
• Bodies at above 0K emit radiation• Emissivity: ratio of emissive power of a
surface to that of a black body (ε=1.0).– For a blackbody: Q=AσT4
– For generic (“gray”) body: Q=AεσT4
• Higher temperatures lead to more energy lost by emitted radiation
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Advanced Materials and Sustainable Energy LabCBEE
Temperature has mixed effect
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Troughs ~ 300˚C• CR~10-50
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Advanced Materials and Sustainable Energy LabCBEE
Parabolic Trough Schematic
Focuses parallel rays to a line
A black pipe is placed with its center at the focus
Pipe can be in a vacuum or could have a glass cover tube to reduce convection
Cylindrical reflector can be on one half of the vacuum tube and approximates the parabolic shape
040208
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Advanced Materials and Sustainable Energy LabCBEE
Dish ~ 700˚C• CR~200-500
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Advanced Materials and Sustainable Energy LabCBEE
Heliostat/Central Receiver ~ 800-1000˚C
• CR~500-3000
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Advanced Materials and Sustainable Energy LabCBEE
Central Receiver: Solar Two
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Advanced Materials and Sustainable Energy LabCBEE
Central Receiver: Sandia CRTF
5 MW power
Flux to 280 W/cm2
Each heliostat is separately driven to focus its beam on the receiver
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Advanced Materials and Sustainable Energy LabCBEE
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Advanced Materials and Sustainable Energy LabCBEE
Central Receiver and Energy Storage: Sandia CRTF
• The large tank stores energy to use during cloud passage or at dusk
• The output power is extracted at a constant rate
090211
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Solar Thermal Energy Storage• Latent Heat/PCM
– Wax– Salts– Eutectics
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Advanced Materials and Sustainable Energy LabCBEE
What are concentrators made of?• Have to withstand extreme conditions (heat,
wind, temperature variation)• Silvered Glass, with low Fe content
– Thick Glass– Thin Glass
• Polished Alumina• Silvered Polymer
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Economics
• Current: Glass ~$65 /m2
• Emerging: Polymer rolls + Al substrate ~$30/m2
• NREL Targets:>90% reflectance
10-30 yr lifespan
$10/m2
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Advanced Materials and Sustainable Energy LabCBEE
Combined Cycle
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Advanced Materials and Sustainable Energy LabCBEE
Working Fluid Choice• Temperature Stability• Safe, non-toxic• Cheap
• Wetting vs. Drying Fluid
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Organic Rankine• Lower quality (temperature) heat
• Drying fluid (fluid still superheated after turbine expansion)– CFCs: R-1XX– Hydrocarbons
• Isobutane• Methanol• Pentane• Many others
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Advanced Materials and Sustainable Energy LabCBEE
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Advanced Materials and Sustainable Energy LabCBEE
What are Solar Cells?
Cu
rre
nt
Voltage
Open-circuit voltage
Short-circuit current
Maximum Power Point
n-t
ype
p-t
ype
-+
Load
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
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Advanced Materials and Sustainable Energy LabCBEE
Energy-band DiagramsElectrons in solids fill states until you run out
– Conduction band – top band, electrons are the charge carriers (support current flow)
– Valence band – bottom band, electrons normally live here unless excited to conduction band (by heat or light)
• An electron must acquire the band gap energy to jump across to the conduction band, measured in electron-volts eV– Silicon band gap energy is 1.12 eV– Also remember energy and wavelenght are related
hcE h
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Advanced Materials and Sustainable Energy LabCBEE
Energy-band Diagrams
http://upload.wikimedia.org/wikipedia/commons/c/c7/Isolator-metal.svg
The probability of finding an electron in a state is the Fermi distributionFermi level is the energy at which the probability of finding an electron is 0.5
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Advanced Materials and Sustainable Energy LabCBEE
Charge carriers: Electrons and HolesElectrons (which are, um…, electrons)
– Electrons move in the conduction band– Force is “electric field”
Holes (the “absence of an electron” in that state)– Holes move in valence band
Electrons create holes when they jump to the conduction band– Photons with enough energy move electron to CB– Create hole-electron pairs in a semiconductor
For a specific material, the charge carrier density is a constant
2 10 30 0 0
0
is intrinsic carrier density
where is free electron density For Si, 10
is hole density
i
i i
n
n n p n n cm
p
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Advanced Materials and Sustainable Energy LabCBEE
Solar Cell Max EfficiencyPhotons need to have at least bandgap energy (Egap)
Photons with a shorter wavelength but more energy than Egap dissipate the extra energy as heat
This limits effectively the maximum efficiency of a single junction cell to 30%– Multiple junction cells limit is 68% for infinite number of layers
Concept known as the Shockley-Queisser Limit
Quantity Si GaAs CdTe InP
Band gap (eV) 1.12 1.42 1.5 1.35
Cut-off wavelength (μm) 1.11 0.87 0.83 0.92
gapgap
hc hch E
E
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Advanced Materials and Sustainable Energy LabCBEE
The “p-n junction”“n-type” has excess electrons (can donate electrons)
“p-type” has electron deficit (can accept electrons)
Connecting an n-type semiconductor (doped to have extra electrons) to a p-type material (extra holes) creates “p-n junction” – n-type carriers diffuse into p-type material (fill available
energy states) – Result is excess positive charge at surface of n-type,
excess negative charge at surface of p-type– Creates a “built in electric field” at p-n junction– Region where carriers have diffused is “depletion width”
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p-n junction diagram
http://en.wikipedia.org/wiki/File:Pn-junction-equilibrium.png
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p-n junctions in the dark
Electrons diffuse from high to low concentration region
Electric field at junction pushes electrons away from junction
(same for holes)
Under no applied external potential, these are in equilibrium
No current
http://en.wikipedia.org/wiki/File:Pn-junction-equilibrium.png
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The p-n Junction Diode/ 38.9
0 0
0
( =1 is ideal diode, 2 is non-ideal)
For a Diode,
( -1) (at 25 C, ( -1) )
is reverse saturation current is "ideality factor"
is applied voltage is Boltzmann con
d dqV akT Vd d
d
a a
I I e I I e
Ia
Vk
stant is temperature (in K)T
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p-n junctions in PV devices
Photons generate hole electron pairs in p-n junction
E-field at junction pulls electrons to n-type (similar for holes)
Flow of holes and electrons creates a current
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Advanced Materials and Sustainable Energy LabCBEE
0
Function of:
absorption coefficient ( ( ))
quantum efficiency ( ( ))
( ) ( ) ( )
where
( ) is incident photon flux
L
Q
g G Q dhc
G
Photogenerated Charge Carrier Generation Efficiency
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Sizes important to PV: Absorption coefficient
Thicker is better.
You need at least 2 absorption lengths even with a back surface reflector.
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Advanced Materials and Sustainable Energy LabCBEE
Cell current under illumination (photocurrent)
( ) where , are the carrier lengths
is the carrier charge
L L p n p nI qg L L L L
q
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Advanced Materials and Sustainable Energy LabCBEE
PV cell model
/0
Net current:
( -1) -qV akTLI I e qg Lp Ln
/0
Diode dark current
( -1)qV akTdI I e
Photocurrent:
( )L L p nI qg L L
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Advanced Materials and Sustainable Energy LabCBEE
Maximizing voltage produced in cell
-Vmax has log dependence on light intensity
-You would like to use materials with large Lp, Ln (light doping)
max /0
max
max
In open circuit conditions then:
0 ( -1) -
Solve for V and get
ln 1
qV akTop p n
p nop
p n n p
p n
I I e qg L L
L LkTV g
q L p L n
Lp, Ln: Minority carrier diffusion lengths
tp, tn: Minority carrier lifetimes
pn, np: Minority carrier concentrations
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Advanced Materials and Sustainable Energy LabCBEE
Sizes important to PV: carrier diffusion
Thinner is better (Need to be able to diffuse to the contacts!)Optimal performance:
10 nm for organics
1-2 microns for CdTe, CIS, a-Si:H
2-10 microns for GaAs
20-100 microns for Si, Ge
Material Lifetime (msec) Mobility (cm2/V-sec) Ln Lp (mm)
x-Si ~ 100 1350 480 590 340
CdTe ~ 0.001 3 500 0.12 1.6
GaAs ~ 0.1 8500 400 50 10
CuInSe2 ~ 0.01 800 200 3 1
a-Si ~ 0.001 1 0.05
organics ~ 0.001 10-3 0.002
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Advanced Materials and Sustainable Energy LabCBEE
Recombination and losses within cell
Generated hole/electron pairs can recombine at defects– Impurities– Grain boundaries
Looks like further current loss within the cell– Use very pure single crystal material…
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Single crystal vs. Polycrystalline Si
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Advanced Materials and Sustainable Energy LabCBEE
“Shunt Resistance”, RP
IP is the current loss due to carrier recombination within the cell
Do we want to maximize or minimize RP?(RP and IP because they are “in Parallel” with solar cell)
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Advanced Materials and Sustainable Energy LabCBEE
Collecting carriers out of the cell
Charge carriers need to be collected out of the cell
Some resistance appears at the metal/semiconductor interface
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Advanced Materials and Sustainable Energy LabCBEE
“Series Resistance”, RS
Minimize RS as possible– Ensure good contact with semiconductor– Ensure good conductivity within metal collector
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Advanced Materials and Sustainable Energy LabCBEE
Short Circuit Current (ISC) & Open Circuit Voltage (VOC)
Short cell terminals together – no voltage drop outside of cell– no V at diode or RP to drive current through
– Short Circuit Current flow (ISC) is same as IL
• Leaving terminals open (setting I to zero)• Open circuit voltage (VOC) is
/0 ( -1)qV kT
SCI I I e 0
ln 1SCOC
IkTV
q I
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Advanced Materials and Sustainable Energy LabCBEE
I-V Curve
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Advanced Materials and Sustainable Energy LabCBEE
“Maximum Power Point” (MPP) and “Fill Factor” (FF)
MPP: point at which you get max power out (optimal operation)IMP, VMP Current and Voltage at Max. Power
Fill Factor (FF): Max performance of theoretical performanceGood cell, FF=0.7+; cheap cell, FF=0.4-0.6
P=VI
MPP
MP MP
SC OC
I VFF
I V
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Advanced Materials and Sustainable Energy LabCBEE
Shunt and Series Resistance Effects
Parallel or Shunt (RP or Rsh) current drops by ΔI=V/RP
Series (RS) voltage drops by ΔV=IRS
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Advanced Materials and Sustainable Energy LabCBEE
Impact of TemperatureVOC decreases by ~0.37% per ˚C for crystalline silicon cells
ISC increases by about 0.05% per ˚C
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Advanced Materials and Sustainable Energy LabCBEE
PV system cost for 10 year payback
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Advanced Materials and Sustainable Energy LabCBEE
Price relation w/volume shipped
Competitive at $1.x/Wp ($0.5/Wp has cost advantage over coal)From data, at 20-30 GWp installed (~10 Years More…)Technical breakthroughs?
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Advanced Materials and Sustainable Energy LabCBEE
PV History
http://www.nrel.gov/pv/pv_manufacturing/cost_capacity.html
Cost/Capacity Analysis
(Wp
is p
eak
Watt
)
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Advanced Materials and Sustainable Energy LabCBEE
What’s the deal with Germany?
PV has huge market penetration in Germany– Price incentives: government gives $0.56/kWh
price guarantee– Power from utilities only costs $0.20/kWh…
– Farmers converting fields to PV production
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Advanced Materials and Sustainable Energy LabCBEE
Figure . Efficiency-cost trade-offs for three generations of PV technology (from reference ).
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Advanced Materials and Sustainable Energy LabCBEE
Photovoltaics Solar Cells (PV)
semiconductor device that converts sunlight to electricity– Conventional
• Crystalline silicon – expensive, 10-15% efficiency• Amorphous Si – less expensive, 5-10% eff.
– High Efficiency: GaAs, InGaAs, CuInSe2, GaInP, etc..• Really expensive, 35+% efficient for multilayer devices (Boeing/NASA)
– Thin Films • Technology under development• Inexpensive (?)• Easy to fabricate/install
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Advanced Materials and Sustainable Energy LabCBEE
“energy payback”Q: How long until energy used for production is returned• Multicrystalline: 4 years current; 2 years anticipated• Thin-film: 3 years current; 1 year anticipated
• Assuming 30 year life, 90% - 95% excess
![Page 66: Chemical, Biological and Environmental Engineering Solar Energy overview](https://reader034.vdocuments.site/reader034/viewer/2022050714/56649e015503460f94aea876/html5/thumbnails/66.jpg)
Advanced Materials and Sustainable Energy LabCBEE
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
![Page 67: Chemical, Biological and Environmental Engineering Solar Energy overview](https://reader034.vdocuments.site/reader034/viewer/2022050714/56649e015503460f94aea876/html5/thumbnails/67.jpg)
Advanced Materials and Sustainable Energy LabCBEE
PV Modules
Multiple cells combined
• In series– Higher voltage output– Lowest current (ISC) cell
dominates string output
• In parallel– Higher current output– Lowest shunt resistance
(RP) dominates string output
![Page 68: Chemical, Biological and Environmental Engineering Solar Energy overview](https://reader034.vdocuments.site/reader034/viewer/2022050714/56649e015503460f94aea876/html5/thumbnails/68.jpg)
Advanced Materials and Sustainable Energy LabCBEE
ShadowingBecause of local shading (or failure), a cell will yield smaller ISC
When cell is forced to pass current higher than its ISC it becomes reverse biased– sinks power instead of sourcing it– enter the breakdown regime? (bad – permanent damage!)
![Page 69: Chemical, Biological and Environmental Engineering Solar Energy overview](https://reader034.vdocuments.site/reader034/viewer/2022050714/56649e015503460f94aea876/html5/thumbnails/69.jpg)
Advanced Materials and Sustainable Energy LabCBEE
Solution to Shadowing: Bypass Diodes
![Page 70: Chemical, Biological and Environmental Engineering Solar Energy overview](https://reader034.vdocuments.site/reader034/viewer/2022050714/56649e015503460f94aea876/html5/thumbnails/70.jpg)
Advanced Materials and Sustainable Energy LabCBEE
Module construction
![Page 71: Chemical, Biological and Environmental Engineering Solar Energy overview](https://reader034.vdocuments.site/reader034/viewer/2022050714/56649e015503460f94aea876/html5/thumbnails/71.jpg)
Advanced Materials and Sustainable Energy LabCBEE
Load I-V Curve and Operating Point
PV panels have I-V curves and so do loads
Use a combination of the two curves to tell where the system is actually operating
Operating point – the intersection point at which the PV and the load I-V curves are satisfied
![Page 72: Chemical, Biological and Environmental Engineering Solar Energy overview](https://reader034.vdocuments.site/reader034/viewer/2022050714/56649e015503460f94aea876/html5/thumbnails/72.jpg)
Advanced Materials and Sustainable Energy LabCBEE
Resistive Load I-V CurveStraight line with slope 1/R
– As R increases, operating point moves to the right
Optimal: use resistance that results in maximum power transfer
V IR MPPm
MPP
VR
I
![Page 73: Chemical, Biological and Environmental Engineering Solar Energy overview](https://reader034.vdocuments.site/reader034/viewer/2022050714/56649e015503460f94aea876/html5/thumbnails/73.jpg)
Advanced Materials and Sustainable Energy LabCBEE
Maximum power transferMatch of power transfer to resistive load changes with
insolation…
MPP tracker maintains PV system’s highest efficiency as the amount of insolation changes
![Page 74: Chemical, Biological and Environmental Engineering Solar Energy overview](https://reader034.vdocuments.site/reader034/viewer/2022050714/56649e015503460f94aea876/html5/thumbnails/74.jpg)
Advanced Materials and Sustainable Energy LabCBEE
Maximum Power Point TrackersMaximum Power Point Trackers (MPPTs) are often a
standard part of PV systems, especially grid-connected– Idea is to keep the operating point near the knee of the
PV system’s I-V curve
Buck-boost converter (DC to DC converter) can either “buck” (lower) or “boost” (raise) the voltage– Varying the duty cycle of a buck-boost converter enables
PV system to deliver the maximum power to the load
![Page 75: Chemical, Biological and Environmental Engineering Solar Energy overview](https://reader034.vdocuments.site/reader034/viewer/2022050714/56649e015503460f94aea876/html5/thumbnails/75.jpg)
Advanced Materials and Sustainable Energy LabCBEE
Capacity Factor of PV
h/day of "peak sun"CF
24 h/day
![Page 76: Chemical, Biological and Environmental Engineering Solar Energy overview](https://reader034.vdocuments.site/reader034/viewer/2022050714/56649e015503460f94aea876/html5/thumbnails/76.jpg)
Advanced Materials and Sustainable Energy LabCBEE
DC and AC Rated PowerEstimate the AC output power under varying
conditions
Pdc,STC - DC power of array under standard test conditions (STC) (1-sun, AM 1.5, 25˚C)
Conversion efficiency– Losses from inverter, dirty collectors, mismatched
modules, differences in ambient conditions, etc.– These losses can derate power output by 20-40%, even in
full sun
, (Conversion Efficiency)ac dc STCP P
![Page 77: Chemical, Biological and Environmental Engineering Solar Energy overview](https://reader034.vdocuments.site/reader034/viewer/2022050714/56649e015503460f94aea876/html5/thumbnails/77.jpg)
Advanced Materials and Sustainable Energy LabCBEE
DC-DC ConverterStep Up Voltage (Boost converter)
Step Down Voltage (Buck converter)
Example: “Buck” converter (step down)– During “ON” state current flows through load and inductor– Energy stored magnetically in inductor “L”
![Page 78: Chemical, Biological and Environmental Engineering Solar Energy overview](https://reader034.vdocuments.site/reader034/viewer/2022050714/56649e015503460f94aea876/html5/thumbnails/78.jpg)
Advanced Materials and Sustainable Energy LabCBEE
DC-DC convertersExample continued: “Buck” converter (step down)
– During “OFF” state, current flows through load, inductor and diode “D”
– Energy stored magnetically in inductor “L” is now released
![Page 79: Chemical, Biological and Environmental Engineering Solar Energy overview](https://reader034.vdocuments.site/reader034/viewer/2022050714/56649e015503460f94aea876/html5/thumbnails/79.jpg)
Advanced Materials and Sustainable Energy LabCBEE
Inverting Power to AC• “Inverter” converts DC to AC
![Page 80: Chemical, Biological and Environmental Engineering Solar Energy overview](https://reader034.vdocuments.site/reader034/viewer/2022050714/56649e015503460f94aea876/html5/thumbnails/80.jpg)
Advanced Materials and Sustainable Energy LabCBEE
Si Substrate: Single Crystal (Czochralski)
![Page 81: Chemical, Biological and Environmental Engineering Solar Energy overview](https://reader034.vdocuments.site/reader034/viewer/2022050714/56649e015503460f94aea876/html5/thumbnails/81.jpg)
Advanced Materials and Sustainable Energy LabCBEE
Si Substrate: Polycrystalline Si
![Page 82: Chemical, Biological and Environmental Engineering Solar Energy overview](https://reader034.vdocuments.site/reader034/viewer/2022050714/56649e015503460f94aea876/html5/thumbnails/82.jpg)
Advanced Materials and Sustainable Energy LabCBEE
Si substrate: Wafering
![Page 83: Chemical, Biological and Environmental Engineering Solar Energy overview](https://reader034.vdocuments.site/reader034/viewer/2022050714/56649e015503460f94aea876/html5/thumbnails/83.jpg)
Advanced Materials and Sustainable Energy LabCBEE
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Advanced Materials and Sustainable Energy LabCBEE
PV module costs
![Page 85: Chemical, Biological and Environmental Engineering Solar Energy overview](https://reader034.vdocuments.site/reader034/viewer/2022050714/56649e015503460f94aea876/html5/thumbnails/85.jpg)
Advanced Materials and Sustainable Energy LabCBEE
Ribbon Si
Dendridic Web“WEB”
Ribbon Growth on Substrate“RGS”
String Ribbon “STR”
![Page 86: Chemical, Biological and Environmental Engineering Solar Energy overview](https://reader034.vdocuments.site/reader034/viewer/2022050714/56649e015503460f94aea876/html5/thumbnails/86.jpg)
Advanced Materials and Sustainable Energy LabCBEE
Morphology Max Efficiency Productivity (cm2.min-1)
Single Crystal Single crystal 27.5% 6” boule pull speed ≈ 1mm.min-1
gives about 175(needs wafering)
Polycrystalline Columnar through thickness
22% 50x50cm2x0.1mm.min-1 gives about 1250(needs wafering)
WEB “Single Crystal” (111) Twinned material
17.3% 6-16
STR Fine columnar through thickness
15% 5-16
RGS Fine columnar through thickness
12% 7,500-12,500
![Page 87: Chemical, Biological and Environmental Engineering Solar Energy overview](https://reader034.vdocuments.site/reader034/viewer/2022050714/56649e015503460f94aea876/html5/thumbnails/87.jpg)
Advanced Materials and Sustainable Energy LabCBEE
Load I-V Curve and Operating Point
PV panels have I-V curves and so do loads
Use a combination of the two curves to tell where the system is actually operating
Operating point – the intersection point at which the PV and the load I-V curves are satisfied
![Page 88: Chemical, Biological and Environmental Engineering Solar Energy overview](https://reader034.vdocuments.site/reader034/viewer/2022050714/56649e015503460f94aea876/html5/thumbnails/88.jpg)
Advanced Materials and Sustainable Energy LabCBEE
Resistive Load I-V CurveStraight line with slope 1/R
– As R increases, operating point moves to the right
Optimal: use resistance that results in maximum power transfer
V IR MPPm
MPP
VR
I
![Page 89: Chemical, Biological and Environmental Engineering Solar Energy overview](https://reader034.vdocuments.site/reader034/viewer/2022050714/56649e015503460f94aea876/html5/thumbnails/89.jpg)
Advanced Materials and Sustainable Energy LabCBEE
Maximum power transferMatch of power transfer to resistive load changes with
insolation…
MPP tracker maintains PV system’s highest efficiency as the amount of insolation changes
![Page 90: Chemical, Biological and Environmental Engineering Solar Energy overview](https://reader034.vdocuments.site/reader034/viewer/2022050714/56649e015503460f94aea876/html5/thumbnails/90.jpg)
Advanced Materials and Sustainable Energy LabCBEE
Maximum Power Point TrackersMaximum Power Point Trackers (MPPTs) should be
part of PV systems– Idea is to keep the operating point near the MPP of the
PV system’s I-V curve
Buck-boost converter (DC to DC converter) can either “buck” (lower) or “boost” (raise) the voltage– Varying the duty cycle of a buck-boost converter enables
PV system to deliver the maximum power to the load
![Page 91: Chemical, Biological and Environmental Engineering Solar Energy overview](https://reader034.vdocuments.site/reader034/viewer/2022050714/56649e015503460f94aea876/html5/thumbnails/91.jpg)
Advanced Materials and Sustainable Energy LabCBEE
DC-DC ConverterStep Up Voltage (Boost converter)
Step Down Voltage (Buck converter)
Example: “Buck” converter (step down)– During “ON” state current flows through load and inductor– Energy stored magnetically in inductor “L”
![Page 92: Chemical, Biological and Environmental Engineering Solar Energy overview](https://reader034.vdocuments.site/reader034/viewer/2022050714/56649e015503460f94aea876/html5/thumbnails/92.jpg)
Advanced Materials and Sustainable Energy LabCBEE
DC-DC convertersExample continued: “Buck” converter (step down)
– During “OFF” state, current flows through load, inductor and diode “D”
– Energy stored magnetically in inductor “L” is now released
![Page 93: Chemical, Biological and Environmental Engineering Solar Energy overview](https://reader034.vdocuments.site/reader034/viewer/2022050714/56649e015503460f94aea876/html5/thumbnails/93.jpg)
Advanced Materials and Sustainable Energy LabCBEE
Inverting Power to AC• “Inverter” converts DC to AC
![Page 94: Chemical, Biological and Environmental Engineering Solar Energy overview](https://reader034.vdocuments.site/reader034/viewer/2022050714/56649e015503460f94aea876/html5/thumbnails/94.jpg)
Advanced Materials and Sustainable Energy LabCBEE
DC and AC Rated PowerEstimate the AC output power under varying
conditions
Pdc,STC - DC power of array under standard test conditions (STC) (1-sun, AM 1.5, 25˚C)
Conversion efficiency– Losses from inverter, dirty collectors, mismatched
modules, differences in ambient conditions, etc.– These losses can derate power output by 20-40%, even in
full sun
, (Conversion Efficiency)ac dc STCP P