Chemical, Biological and Environmental Engineering Solar Energy overview.
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Transcript of Chemical, Biological and Environmental Engineering Solar Energy overview.
Chemical, Biological and Environmental Engineering
Solar Energy overview
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)
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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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
Concentrated Solar Thermal Power not so new…
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Solar Radiation
,Absorbed
,Transmitted
,Reflected 1Incident
1
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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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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
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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Concentrating Collectors
Concentration Ratio /a rCR A A Aa, Ar =Area of aperture, receiver [m2]
Aa
Ar
Advanced Materials and Sustainable Energy LabCBEE
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)
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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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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Temperature has mixed effect
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Troughs ~ 300˚C• CR~10-50
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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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Dish ~ 700˚C• CR~200-500
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Heliostat/Central Receiver ~ 800-1000˚C
• CR~500-3000
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Central Receiver: Solar Two
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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
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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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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Combined Cycle
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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
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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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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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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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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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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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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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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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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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
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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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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“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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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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“Series Resistance”, RS
Minimize RS as possible– Ensure good contact with semiconductor– Ensure good conductivity within metal collector
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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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I-V Curve
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“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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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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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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PV system cost for 10 year payback
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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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PV History
http://www.nrel.gov/pv/pv_manufacturing/cost_capacity.html
Cost/Capacity Analysis
(Wp
is p
eak
Watt
)
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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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Figure . Efficiency-cost trade-offs for three generations of PV technology (from reference ).
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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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“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
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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
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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
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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!)
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Solution to Shadowing: Bypass Diodes
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Module construction
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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
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
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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
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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
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Capacity Factor of PV
h/day of "peak sun"CF
24 h/day
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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
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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”
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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
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Inverting Power to AC• “Inverter” converts DC to AC
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Si Substrate: Single Crystal (Czochralski)
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Si Substrate: Polycrystalline Si
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Si substrate: Wafering
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PV module costs
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Ribbon Si
Dendridic Web“WEB”
Ribbon Growth on Substrate“RGS”
String Ribbon “STR”
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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
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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
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
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
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
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”
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
Advanced Materials and Sustainable Energy LabCBEE
Inverting Power to AC• “Inverter” converts DC to AC
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