PhotonPhoton EnhancedEnhanced ThermionicThermionic EmissionEmission Prof. Nicholas A. Melosh Materials Science & Engineering, Stanford University SIMES, SLAC
with ZX Shen (AP/Ph/SIMES), Roger Howe (EE) TheThe MeloshMelosh GroupGroup
NanomaterialsNanomaterials BehaviorBehavior atat InterfacesInterfaces
• Fabricating, interrogating, and controlling nanoscale materials at interfaces • Applications for Energy, Biology, and Electronics Log
0.1 1 10 Log 0.12 Intensity (Arb.Unit) 0.10 X 50 0.08 0.06 0 10 20 30 40 50 0.04 Kinetic Energy (eV) 0.02 0.00
Diamondoids Bio-Electronic Molecular Plasmonics and Energy Interfaces TodayToday’’ss OverviewOverview
1. Solar harvesting methods
2. How can we dramatically increase efficiency?
3. The PETE process
4. Improving PETE performance SolarSolar EnergyEnergy isis goinggoing toto bebe ImportantImportant
wikipedia; data from Energy Information Administration SolarSolar PowerPower ConversionConversion
A lot of high-quality energy is available from the sun… how can we harvest it?
Solar Thermal Photovoltaics
• Converts sunlight into heat • collects fraction of incident energy • Uses well-known thermal • “high grade” photon energy conversion systems • distributed power • Efficiencies of 20-30% • efficiencies of 24% (single Si- junction), 43% multijunction PhotovoltaicsPhotovoltaics
Thin Film Mono/ Multi crystalline Silicon
First solar Sunpower
Thin layers of Si, CdTe, CIS are cheaply coated Thick wafers of highly pure, single crystalline Si onto large area substrates. Lower performance are mounted on a flat panel. High quality, but but also lower cost than monocrystalline Si. more expensive than thin films. Typical Typical efficiencies vary according to materials; efficiencies 14-21% range CIGS ~20%; CdTe ~14%, -Si ~4-8%. Companies: Sun Power, Yingli, Q-cells, Companies: First Solar (CdTe), United Solar (a- Suntech, Sharp Si), Sharp SemiconductorSemiconductor MaterialsMaterials
Energy Level Diagrams Simplified Energy Level Diagram “allowed” electron energies
“conduction band” Ec
“forbidden” Eg Eg energies EF EF
Ev - - - e e- e e “valance band” - e- e e-
Energy x
jetro.go.jp
Silicon PhotovoltaicPhotovoltaic OperationOperation
e-
hν
usable p- photo- type n-type voltage (qV) Energy Level Diagram Energy Level h +
photon
Real Space Diagram Real Space V IssueIssue withwith singlesingle--junctionjunction PVPV’’ss
Solar power • Lose ~50% of incident energy immediately
thermalization loss, ~25% • Given off as heat • Further fundamental harvestable power, ~50% limits reduce maximum possible efficiency to ~34% absorption loss, ~25% • Monocystalline Si collects almost all the electrons created
• -Si and polycystalline Band gap energy Si have much higher recombination SolarSolar ThermalThermal ConversionConversion
Parabolic Dish/ Stirling Engine Parabolic Troughs/ Fresnel reflectors
Dishes focus light onto individual thermal engine, such as a Stirling engine. Companies: Tessera Solar,
Power Tower Trough reflectors focusing light onto pipe, Flat panel heating working fluid, which heats water for reflectors standard steam cycle. focusing onto single tower. Companies: Ausra, Siemens, Solar Millennium Conventional steam/liquid salt operation.
Companies: Brightsource, Abengoa, eSolar AA simplisticsimplistic comparisoncomparison
Thermal Collectors Photovoltaics
5800º C 5800º C
Solar power
T = 5200º; un-used Harvested power
600º C • absorption losses T = 500º 100º C • thermalization losses real ~ 25% 100º C real ~ 25%
• Full use of low grade heat • Partial use of high grade heat (high per-quanta energy) EnergyEnergy CostsCosts
Solar PV: 12-35¢/kWhr • High variability in Solar Thermal: 11-25¢/kWhr estimates of renewable energy
Cost • Want ~30-40% decrease in renewables cost Nuclear: 9-10¢/kWhr
Natural Gas: 7-9¢/kWhr
Coal: 4-6¢/kWhr
data: GCEP, IEA, “World Energy Outlook 2004”, EIA, California Energy Commission Report, 2007 HowHow EfficientEfficient areare fossilfossil fuels?fuels?
Coal Plants are ~33% efficient.
Natural Gas Plants are > 53% efficient!
How come?
Combined Cycles!
A GE gas turbine AA BetterBetter Way:Way: CombinedCombined CyclesCycles
photo-electricity out
waste heat
thermo-electricity out
Premise: A high-temperature photovoltaic combined with a thermal engine is the most effective way to maximize output efficiency. CombinedCombined HTHT--PV/PV/ ThermoThermo CycleCycle
5800º C
600º C ~ 25% PV Stage 100%*(25%) = 25%
“Quantum PV conversion process” 75%
~ 25% 75%*(25%) = 19% Thermal thermal conversion Process total: 44% 100º C Combined cycles can take two modest performance devices to form a very high efficiency device. HighHigh--TemperatureTemperature PVPV
The key is to develop a PV cell that can operate at high temperatures
• A 20% efficient High Temperature-PV (HT-PV) would increase current thermal performance by 60%
• Current ~12¢/kWh LCOE could decrease to ~7¢/kWh
• Possibly add on to existing designs
Can we make a High-Temperature Photovoltaic? NotNot yet!yet! PVPV atat HighHigh TemperatureTemperature
PV does not operate well at high temperatures! e- Energy e- dark current Vbi hν
p-type usable photo- n-type voltage (qV)
h+ Ideal Diode equation: eV eJJJ kT 1 SC 0 Carrier generation Dark current We need a device that can use large per quanta
photon energy (like PV), and combine it with the
high temperature operation and thermal
harvesting.
What processes can operate at high temperatures? ThermionicThermionic Emission:Emission: HistoryHistory
• Thomas Edison in 1880 describes “the flow of charged particles called thermions from a charged metal or a charged metal oxide surface, caused by thermal vibrational energy” • Sir J.J. Thompson discovers electrons in his cathode-ray tubes experiments done in 1897, receives 1906 Nobel Prize • Sir J.A. Fleming develops a thermionic vacuum tube and patents the first diode in 1904 • O. Richardson develops thermionic emission model (1902), winning the 1928 Nobel Prize “for his work on the thermionic phenomenon and especially for the law named after him.”
2 / kT J AT e E (Richardson-Dushman law)
-2 J = current density (Acm ), E = emitter work function (eV) ThermionicThermionic EmissionEmission
Boiling electrons from the surface: • Operates at very high heat in e- temperatures; generally >1200ºC
• Robust; long service lifetimes
• Low output voltages
V
J = AT2 e-φC/kT P = J (φC – φA ) ThermionicThermionic DevicesDevices
Satellite Power (circa 1980’s) Solar Testing (circa 2000)
• Work in the US and USSR space programs culminated in the Soviet flights of 6 KW TOPAZ thermionic converters in 1987 • Source of heat: nuclear pile • Basic technology: vacuum tubes • Machined metal with cathode-anode distance (>100 μm) and required cesium plasma PhotonPhoton EnhancedEnhanced ThermionicThermionic EmissionEmission (PETE)(PETE)
high-T
• Photo-excite carriers into conduction band
• Thermionically emit these excited carriers
• Overcome electron affinity barrier (not full work-function)
• Collected at low work-function anode
Schwede et al, Nature Materials, 9 ,762–767, 2010 PhotonPhoton EnhancedEnhanced ThermionicThermionic EmissionEmission (PETE)(PETE)
Thermionic Current: PETE Current: E ATJ 2 exp c ATJ 2 exp c exp F kT kT kT
2 n c ATJ exp i kTn
If Eg = 1.4 eV, then at 300 C:
ΔEf/kT 9 Jpete /Jtherm = e =1.4x10 PETEPETE vsvs PVPV
Thermal energy Solar power
Harvested power photon energy
n , FnF kTEE ln ni
• PETE harvests thermal and photon energy
• sub-band gap, thermalization, and recombination “losses” collected
• Can operate at higher voltages
• Balance photon absorption and heat generation
• High temperatures beneficial- doesn’t degrade performance
Seems good… but how do the actual numbers compare? FullFull PETEPETE SimulationSimulation
T C TA
PETE calculations:
• Each photon h>Eg is absorbed and excites one electron • Sub-bandgap photons are absorbed as heat • BB radiation losses and transfer included • Radiative and Auger recombination losses (Si params) included • ‘dark current’ from anode treated as thermionic emission
• Variables: solar concentration, TC , TA, , Eg, A, TheoreticalTheoretical EfficiencyEfficiency
• To adjust: Eg , χ ,TC • φA = 0.9 eV – [Koeck, Nemanich, Lazea, & Haenen 2009]
• TA ≤ 300°C • Other parameters similar to Si – 1e19 Boron doped Schwede et al, Nature Materials, 9 ,762–767, 2010 MaximizingMaximizing PETEPETE performanceperformance
n , FnF kTEE ln ni Maximize EF,n: • High solar concentration • Low recombination rates; electrons emit before relaxing
Maximize V: • Highest possible while still getting good electron yield at given T • Minimize anode work function
Maximize performance by controlling with surface coatings ExperimentalExperimental DemonstrationDemonstration
• [Cs]GaN thermally stable – Resistant to poisoning Experimental Apparatus: – Eg = 3.3 eV – 0.1 μm Mg doped – 5x1018 cm-3
~0 to 0.25 eV
3.3 eV ExperimentalExperimental ApparatusApparatus
optical access
removable sample mount
not visible: - anode - Cs deposition heater source Temperature Dependent Yield decreases with increasing T is higher with higher energy photons electron energy is determined by photo- • • • excitation PETE or photoemission? Direct Photoemission yield: • • PETE or Photoemission? PETE or Photoemission? PETE: PETE:
Direct Photoemission Direct Photoemission TT--DependentDependent EmissionEmission EnergyEnergy
• PETE electrons should show thermal distribution • Distribution should broaden with temperature • Measure emitted electron energy increasing T
Thermal distribution
Energy Distribution Width
Schwede et al, Nature Materials, 9 ,762–767, 2010 PhotonPhoton--independentindependent EmissionEmission EnergyEnergy
Energy distribution for • Photon energy should not matter above band gap different excitation energy • Very different from photoemission • Green = just above gap • Blue = well above gap, not above
Evac
Average energy: 3.8 eV 3.7 eV
3.3 eV • Identical energy distributions • 0.5 eV thermal voltage boost significant • 400ºC = 0.056 eV EvidenceEvidence forfor PETEPETE
• Yield dependence on temperature – decreases for direct photoemission – increases for above gap, but sub-ionization energies • Emitted electron energy increases with temperature
>0.1-0.2 eV higher than photon energy • Electron energy follows thermal distribution • 330nm and 375nm illumination produce same electron energy distributions at elevated temperature
– electrons harvested up to 0.5 eV additional energy from thermal reservoir WhatWhat wouldwould PETEPETE looklook like?like?
MostSimplest: efficient: Two Combineplanar wafers MEMs separated and nanotechnology by vacuum gap
Y. Cui P. Khuri-Yakub
Roger Howe
Shen/Melosh P. McIntyre
with Prof. Roger Howe, Dr. Igor Bargatin AddingAdding PETEPETE ontoonto ExistingExisting EquipmentEquipment
PETE device
Tessera 20 kW Stirling concentrator dish Stirling engine
• Several companies already operate Stirling-based CSP
• Record: 32% efficiency; annual efficiency ~23% to grid
• Add PETE front stage, thermally connect anode, cathode or both
• Use nanostructured PETE cathode to absorb light and emit electrons TheoreticalTheoretical TandemTandem EfficiencyEfficiency
31.5% Thermal to electricity conversion [Mills, Morrison & Le Lieve 2004] 285°C Anode temperature [Mills, Le Lievre, & Morrison 2004] WhatWhat efficiencyefficiency dodo wewe need?need?
Based on NREL LCOE cost analysis of SunCatcher parabolic Dish array with additional PETE cost and efficiencies.
https://www.nrel.gov/analysis/sam/ http://www.energy.ca.gov/sitingcases/solartwo/documents/applicant/a fc/volume_02+03/MASTER_Appendix%20B.pdf
Cost Estimates:
• Original SunCatcher LCOE = 12¢/kWh
• An 20% PETE module estimated cost Additional PETE efficiency $25,000 for a 50kW plant • ~$0.50/kW additional cost to dish • 160% thermal efficiency • LCOE = ¢7/kWh
To compete with Natural Gas, PETE must be 15-20% efficient at ~$.50/W ImprovingImproving ElectronElectron EmissionEmission YieldYield
Spicer 3-step photoemission model: 1) absorption 2) diffusion 3) surface emission
Means of increasing electron emission efficiency:
• Increase light absorption
• Maximize electron collisions with surface
• Minimize recombination NanostructuringNanostructuring IncreasesIncreases AbsorptionAbsorption
• Increasing Absorption
• Increasing Emission Ref: J. Zhu et. al., Nanoletters, 2008
Ref: R. Teki et. al., Nanoletters, 2006 GeGe NanowireNanowire samplessamples
thin film NW film
with Prof. Paul McIntyre, Irene Goldthorpe manuscript in preparation TaperedTapered GratingsGratings
350 nm width, 250 nm height, 700 nm pitch GaAs p‐doped, 5 x 1018 cm‐3 NEA Cs‐O, ‐0.2 eV TakeTake HomeHome MessagesMessages
• Combined Cycles may be practical means to greatly increase efficiency
• PETE is a new way to combine thermal and quantum processes
• ~15% efficiency could make significant impact on economics of energy production
• Nanostructuring could be an important way to increase efficiency AcknowledgementsAcknowledgements
Mike Preiner Ian Wong Dr. Ken Shimizu Piyush Verma Jason Fabbri Ben Almquist Nazi Devani Dr. Chenhao Ge Jules VanDersarl Lizzie Hagar-Barnard Kunal Sahasrabuddhe Jared Schwede Sam Rosenthal Dan Riley Vijay Narasimhan
Collaborators: Prof. ZX Shen (Stanford Physics), Wanli Yang, Will Clay Prof. Roger Howe, Igor Bargatin, J Provine Prof. Peter Schreiner, Prof. Andreas Fokin (Univ. Giessen) Dr. Trevor Willey (LLNL) Jeremy Dahl, Bob Carlson (Chevron)
Funding from GCEP and Moore Foundation