Laser Enhanced Thermionic Energy Converter: IMatt R. Mowers, Patrick T. Sullivan, Tsuyohito Ito, Mark A. Cappelli

Mechanical Engineering Department, Thermosciences Division, Stanford University

Introduction and Motivation

TSD

Type:

Close-spaced vacuum diode

Cesium vapor-filled high pressure arc-ignited diode

Cesium vapor-filled low pressure diode

Characteristics:

Very small interelectrode spacing reduces space charge effect

Electron-cesium collisions ionize cesium, creating plasma that neutralizes space charge

Cesium ionizes on emitter surface (contact ionization); electron-cesium collisions in gap are negligible

Problems:

Hard to maintain interelectrode spacing at high temperature

collisions produce substantial resistance to current flow, resulting in energy loss

contact ionization requires high work function, which results in low current densities and efficiency

Potential for laser enhancement

Laser pumped Cs ionization process

A similar experiment by Inaguma et. al. [1] employed a pulsed laser at a wavelength of 852 nm to enhance TEC performance. The laser enhanced TEC was able to achieve near ideal short circuit current density, though at laser powers much higher than are expected from the cw 601 nm laser.

In high pressure (arc-ignited) diodes, laser improves space charge neutralization and thus pushes current-voltage characteristic closer to ideal. Parameters:TE = 1700 KTC = 800 KφΕ = 2.6 eVφC = 1.81 eVCs reservoir temperature = 560 Kgap distance = 0.25 mm

Parameters:TE = 2000 KTC = 1000 KφC = 1.81 eVV = φΕ - φCCs reservoir temperature = 510 Kgap distance = .08 mm

In low pressure diodes, laser fully neutralizes space charge at lower emitter work functions, improving efficiency and power density.A laser power of 16 W results in 3800 W more TEC power and 5.5% higher efficiency.

Thermionic Energy Converter Applications

Current TEChnologies Proposed Concept: Laser Enhanced TEC

Laser Enhanced TEC

851.4 851.6 851.8 852.0 852.2 852.4 852.6

0

200

400

600

800

1000

Sho

rt ci

rcui

t cur

rent

den

sity

[A/m

2 ]

Wavelength [nm]

2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.0000

0.0025

0.0050

0.0075

0.0100

0.0125

0.0150 Efficiency

Emitter work function [eV]

Power density (kW/cm2) Contact ionization fraction

Needed laser ionization fraction

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.60

1

2

3

4

5

6

7

Cur

rent

den

sity

[A/c

m2 ]

Output voltage [V]

Ideal Laser enhanced

Vi = 2eV Ignited mode

Vi = 3eV

Performance enhancement by pulsed 852 nm laser

Comparison of ideal, ignited, and laser enhanced modes of operation

Low pressure laser enhanced TEC performance and required ionization fraction

e

Heat In Heat Out

Electron Current

Emitter Collector

ee

Load

Power Out

Load

Heat In

V

Dense

601 nm Diode Laser

Load

Heat In Heat Out

Electron CurrentV

DenseCs Plasma

601 nm Diode Laser

J = current density (A/cm2)A = 120 A/cm2-K2 = emission constantTE= emitter temperatureφE= emitter work functionφC= collector work functionV = output voltagek = boltzmann constant

TECs are high temperature heat engines that are heat source agnostic, allowing incorporation into various energy systems as topping cycles to increase overall efficiency.

The main limitation of TEC performance arises from the fact that emitted electrons, while traveling through the interelectrode region, create a negative space charge that repels further electrons from entering the gap. The main solutions to this space charge limitation are outlined in the following table:

Efficiency = 46%

compared to 30% efficiency of Solar-Stirling acting without TEC

Exergy efficiency = 57%

A low power continuous wave laser tuned to 601 nm is an efficient source of ionization, reducing the need for contact ionization in low pressure converters or collisional ionization in high pressure converters.

Solar-Stirling parabolic dish with TEC topping cycle Natural gas combined cycle with TEC topping cycle

J = ATE2 exp(-[φC + V]/kTE) V > φE − φC

J = ATE2 exp(-φE /kTE) V < φE − φC

A Thermionic energy converter (TEC) directly converts heat into electricity via temperature-driven electron emission from a metal surface (emitter).

TECs are high temperature devices, (TE = ~2000 K andTC = ~1000 K) with no moving parts

TECs have potentially high efficiencies (~25%) andhigh power densities (~100 W/cm2).

Presented in GCEP Research Symposium, Stanford, September 2006 M. A. Cappelli: cap@stanford.edu

Analysis and Experiments/Results

TSD

2300 2350 2400 2450 25000

1000

2000

3000

4000

5000

6000

7000

Cur

rent

[A/m

2 ]

Emitter temperature [K]

DischargeCurrent EmissionCurrent

20 V, 5 mm, Ar: 0.2 TorrCollector temperature: 300 K

Model of the Cs Laser-Ionization Process

Preliminary Results Future Work

Particle-in-cell - Monte Carlo Simulation Experimental TEC Design

Summary

Acknowledgments

References

Bi-porous Ni Wick

Tube/Wick Heaters Conflat thermocouple

Feedthrough

Vacuum Val ve

Conflat Tee

AluminaTube CF to

Compression Fitting Adapter

Emitter and Collector

QuartzTube

Laser

Bi-porous Ni Wick

Tube/Wick Heaters Conflat thermocouple

Feedthrough

Vacuum Val ve

Conflat Tee

AluminaTube CF to

Compression Fitting Adapter

Emitter and Collector

QuartzTube

Laser

10-3 10-2 10-1 100 101102

103

Our experimental data (Mo) Data reviewed in PSST (Mo)

Simulation results reviewed in PSST (γi=0.01-0.02)

Vol

tage

[V]

Jp-2: normalized current density [mA cm-2 Torr-2]

0 1 2 3 4 50

5

10

15

20

25

30

Den

sity

[x10

17 m

-3]

Distance from the emitter [mm]

Electron Ion

Note: Left side is 0 V, 2400 K and right side is 20 V, 300 KP = 0.2 Torr

Pressure = 5 torrElectrode seperation = 2 mmApplied voltage = 210 V

Emitter Temperature = 1025 KElectrode seperation = 2 mmApplied voltage = 230 V

Emitter IR Heater

Collector IR Heater

Laser

ChromelLeads

Alumel

Tungsten Emitter

Nickel Collector

Alumina tube

Emitter IR Heater

Collector IR Heater

Laser

ChromelLeads

AlumelLeads

Tungsten Emitter

Nickel Collector

Alumina tube

0 2 4 6 8 10 12 14 16 18 20105

107

109

1011

1013

1015

1017

Cs(6S) Cs2

Cs(6P) Cs(8D) Cs+ Electron Fast electron

Den

sity

[cm

-3]

Time [µs]

Taken from El Genk [3]

Cs

Heater

Heater

Thermocouple

Thermocouple

Load

Heater

Sap

phire

Alu

min

a

Cold finger

Emitter

Collector

An experimental apparatus that allows a first evaluation of the laser enhanced TEC has been designed and fabricated. It is based on a design of Rasor [2].

The experimental TEC has been operated on argon, and results have been compared to published data. The infrared emitter heating method has also been tested successfully.

A model that returns ionization fraction of cesium for a given cesium density and laser power has been formulated. Its output is shown in the figure on the right.

A simple particle-in-cell - Monte Carlo (PIC-MC) model of a thermionic discharge has been developed and tested with argon kinetics.

(i) Operate experimental TEC on cesium, at higher temperatures, and with ionizing 601 nm laser to determine its impact on converter performance.

(ii) Adapt PIC-MC simulation for cesium thermionic discharges.

(iii) Improve Cs laser-ionization model and add this to PIC-MC simulation to model laser enhanced TEC.

(iv) Design, fabricate, and test a second-generation laser enhanced TEC for model validation.

Laser pumped Cs ionization process Computed evoloution of laser

enhanced Cs ionization

Emission and discharge current for Ar thermionic discharge

Second Generation Laser

Enhanced TEC

Density distribution for Ar thermionic discharge

Laser enhanced TEC Design

Argon discharge results compared to published data and simulations

Heating of Emitter with IR laser

TEC operating on argon

Electrode configuration and heating and ionizing methods

This research was supported by a grant from the Global Climate and Energy Project at Stanford University. We thank Nalu Kaahaaina for providing the initial vision for this project and Robin Bell for her contributions to the systems analysis.

TECs are high tempeature heat engines that show much promise as topping cycles capable of raising efficiencies of conventional energy systems. The laser enhanced TEC proposed here has the potential to improve TEC performance. Models have been constructed to simulate TEC performance and improvement from laser enhancement. They will become full simulations of a laser enhanced TEC.

An experimental apparatus has been constructed and tested on argon. It will be used to verify laser enhancement predictions. A second apparatus will be fabricated for further experimental proof-of-concept and model validation.

[1] T. Inaguma, N. Tsuda, J. Yamada, Trans. IEE Japan, 122-A, No.11 (2002).[2] N. S. Rasor, 33rd Intersociety Energy Conversion Engineering Conference August 2-6, IECEC-98-211 (1998).[3] Y. Momozaki and M.S. El-Genk, “An experimental investigation of the performance of a thermionic converter with planar molybdenum electrodes for low temperature applications”, in Proceedings of the Space Technology and Applications International Forum (STAIF-2001), M.S. El-Genk ed. (AIP, New York) 1142-1151 (2001).[4] N.S. Rasor, IEEE Trans. Plasma Sci. 19, 1191-1208 (1991).[5] Tam, A.C., and W. Happer, Optics Communications 21, 403 - 406 (1977).

Laser Enhanced Thermionic Energy Converter: IIPatrick T. Sullivan, Matt R. Mowers, Tsuyohito Ito, Mark A. Cappelli

Mechanical Engineering Department, Thermosciences Division, Stanford University