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Nick Melosh
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New methods for solar energy
conversion: Combining heat and light
Prof. Nicholas A. Melosh Materials Science & Engineering, Stanford University
SIMES, SLAC
with ZX Shen (AP/Ph/SIMES), Roger Howe (EE)
Solar Power Conversion
A lot of high-quality energy is available from the sun… how can we
harvest it?
Solar Thermal (CSP) Photovoltaics (PV)
• Converts sunlight into heat
• Concentrated solar thermal
• Uses well-known thermal
conversion systems
• Efficiencies of 20-30%
• collects fraction of incident energy
• “high grade” photon energy
• direct photon to electricity
• efficiencies 19-24% (single junction Si)
How Efficient are fossil fuels?
Coal Plants are ~33% efficient.
Natural Gas Plants are > 53% efficient!
How come?
Combined Cycles.
A GE gas turbine
A Better Way: Combined Cycles
Premise: A high-temperature photovoltaic combined with a
thermal engine is the most effective way to maximize output
efficiency.
photo-electricity out
thermo-electricity out
waste heat
Combined HT-PV/ Thermo Cycle
5800º C
600º C
100º C
h ~ 25%
PV Stage
Thermal
Process
h ~ 25%
100%*(25%) = 25%
75%
75%*(25%) = 19%
total: 44%
Combined cycles can take two modest performance
devices to form a very high efficiency device.
High-Temperature PV
The key is to develop a PV cell that
can operate at high temperatures
• Ading 20% efficient High Temperature-PV (HT-PV)
increases a 25% thermal system to 40%
• Current ~12¢/kWh LCOE could decrease to
~7¢/kWh
• Possibly add on to existing designs
Can we make a High-Temperature Photovoltaic?
No.
Thermionic Emission
• Operates at very high temperatures;
generally >1200ºC
• Robust
Boiling electrons from a metal:
Hot metal
e- V
Boiling water:
• Input heat energy
to overcome
energy barrier to
change liquid into
gas
Overcome work-
function energy
barrier
Thermionic Devices
from the University of Illinois Chemistry
Learning Center, www.chem.uiuc.edu/clcwebsite/cathode.html
tube cathode
anode electrical
supply
Thermionic Energy Converison
Hot cathode
e-
V
Cold anode
vacuum
• Want large cathode fc for
large Vout
• … but, need small
cathode fc to keep
temperature reasonable
• Vout ~ 0.1- 0.2 eV
• Result: poor efficiency
fc
Improving thermionic emission
• How to increase the output voltage?
hν
Ec
Ev
semiconductor
Schwede et al, Nature Materials, 9 ,762–767, 2010
Photon Enhanced Thermionic Emission (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
• 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
Theoretical Efficiency
Schwede et al, Nature Materials, 9 ,762–767, 2010
• GaN with Cs coating
• Thermally Stable – Eg = 3.3 eV
– 0.1 μm Mg doped
– 5x1018 cm-3
Experimental Demonstration
3.3 eV
~0 to
0.5 eV
Gallium Nitride
Photos: Robert Laska
Experimental Apparatus:
Experimental Apparatus
removable sample mount
optical access
heater
not visible:
- anode
- Cs deposition
source
Photon-independent Emission Energy
Energy distribution for
different excitation energy
• Identical energy
distributions
• 0.5 eV thermal voltage
boost significant
• 400ºC = 0.056 eV
• efficiency ~10-4
• Photon energy should not matter
above band gap
• Very different from photoemission
• Green = just above gap
• Blue = well above gap, not above
Evac
3.3 eV
3.7 eV
Average energy:
3.8 eV
Heterostructured cathode performance
• Very strong temperature
dependence
• Yield increases 10x from 40ºC to
120ºC
• PETE current dominates
photoemission
• Limited by thermal stability of
CsO coating
Improved yield from 10-4
to 2.5%
Adding PETE onto Existing Equipment
• 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
Tessera 20 kW Stirling
concentrator dish
PETE device
Stirling engine
31.5% Thermal to electricity conversion [Mills, Morrison & Le Lieve 2004]
285°C Anode temperature [Mills, Le Lievre, & Morrison 2004]
Theoretical Tandem Efficiency
Making PETE Cost Competitive
PETE efficiency
• Original SunCatcher LCOE = 12¢/kWh
• Add PETE onto that system
Cost Estimates:
https://www.nrel.gov/analysis/sam/
http://www.energy.ca.gov/sitingcases/solartwo/documents/a
pplicant/afc/volume_02+03/MASTER_Appendix%20B.
• $10k for a 12” wafer (high)
• 25 kW unit
• ~0.33 A/cm2 current density
• ~18 ft diameter dish
• = 0.40 $/W
To compete with Natural Gas,
PETE must be 15-20% efficient
at ~$.40/W