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)


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


Which Solar Technology?

Break even between PV and Thermal at ca. 1300 kWh.m-2.yr-1



Concentrated Solar Thermal Power not so new…


Solar Radiation

,Absorbed

,Transmitted

,Reflected 1Incident

1


Solar Collector• Flat Plate, T max ~70˚C

– Hot water, space heating– 30-50% heat loss

ambientT

1R

U

usefulq collectorT

I


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


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



Concentration Ratio /a rCR A A Aa, Ar =Area of aperture, receiver [m2]

Aa

Ar




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)



(Tiwari, 2004)

R

r

AaAr

max

maxmax

2 :sin

nD CR

max 2max

3 :sin

nD CR


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


Temperature has mixed effect


Troughs ~ 300˚C• CR~10-50


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


Dish ~ 700˚C• CR~200-500


Heliostat/Central Receiver ~ 800-1000˚C

• CR~500-3000


Central Receiver: Solar Two

http://upload.wikimedia.org/wikipedia/commons/b/b8/Fish_eye_Solar_Two.jpg


Central Receiver: Sandia CRTF

5 MW power

Flux to 280 W/cm2

Each heliostat is separately driven to focus its beam on the receiver

http://www.sandia.gov/Renewable_Energy/solarthermal/images/nsttf-e.jpg

http://www.sandia.gov/Renewable_Energy/solarthermal/images/tower-n.gif



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


Solar Thermal Energy Storage• Latent Heat/PCM

– Wax– Salts– Eutectics


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


Economics

• Current: Glass ~$65 /m2

• Emerging: Polymer rolls + Al substrate ~$30/m2

• NREL Targets:>90% reflectance

10-30 yr lifespan

$10/m2


Combined Cycle


Working Fluid Choice• Temperature Stability• Safe, non-toxic• Cheap

• Wetting vs. Drying Fluid


Organic Rankine• Lower quality (temperature) heat

• Drying fluid (fluid still superheated after turbine expansion)– CFCs: R-1XX– Hydrocarbons

• Isobutane• Methanol• Pentane• Many others



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


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


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


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


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



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”


p-n junction diagram

http://en.wikipedia.org/wiki/File:Pn-junction-equilibrium.png

http://upload.wikimedia.org/wikipedia/commons/f/fa/Pn-junction-equilibrium-graphs.png


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


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


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


0

Function of:

absorption coefficient ( ( ))

quantum efficiency ( ( ))

( ) ( ) ( )

where

( ) is incident photon flux

L

Q

g G Q dhc

G

Photogenerated Charge Carrier Generation Efficiency


Sizes important to PV: Absorption coefficient

Thicker is better.

You need at least 2 absorption lengths even with a back surface reflector.


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


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


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


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


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…


Single crystal vs. Polycrystalline Si


“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)


Collecting carriers out of the cell

Charge carriers need to be collected out of the cell

Some resistance appears at the metal/semiconductor interface


“Series Resistance”, RS

Minimize RS as possible– Ensure good contact with semiconductor– Ensure good conductivity within metal collector


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


I-V Curve


“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


Shunt and Series Resistance Effects

Parallel or Shunt (RP or Rsh) current drops by ΔI=V/RP

Series (RS) voltage drops by ΔV=IRS


Impact of TemperatureVOC decreases by ~0.37% per ˚C for crystalline silicon cells

ISC increases by about 0.05% per ˚C


PV system cost for 10 year payback


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?


PV History

http://www.nrel.gov/pv/pv_manufacturing/cost_capacity.html

Cost/Capacity Analysis

(Wp

is p

eak

Watt

)


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


Figure . Efficiency-cost trade-offs for three generations of PV technology (from reference ).

http://en.wikipedia.org/wiki/Image:Solar_cell.png


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


“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


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


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


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!)


Solution to Shadowing: Bypass Diodes


Module construction


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


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


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


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


Capacity Factor of PV

h/day of "peak sun"CF

24 h/day


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


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”


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


Inverting Power to AC• “Inverter” converts DC to AC


Si Substrate: Single Crystal (Czochralski)

http://upload.wikimedia.org/wikipedia/commons/6/6e/Czochralski_Process.svg

http://en.wikipedia.org/wiki/File:Monokristalines_Silizium_f%C3%BCr_die_Waferherstellung.jpg


Si Substrate: Polycrystalline Si


Si substrate: Wafering



PV module costs


Ribbon Si

Dendridic Web“WEB”

Ribbon Growth on Substrate“RGS”

String Ribbon “STR”


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


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


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


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


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


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”


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


Inverting Power to AC• “Inverter” converts DC to AC


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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Chemical, Biological and Environmental Engineering Solar Energy overview