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

What would PETE look like?

Parallel-Plate Design:

high-T

Standard materials:

Si, GaAs

• 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.

pdf

• $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

Take Home Messages

• Combined Cycles may be practical means to greatly increase

efficiency

• PETE is a new way to combine thermal and photon energy

• Modest PETE efficiency could make economic impact