WP3: Power Conversion Module · 2018-10-02 · WP3 - Objectives Development of a hybrid thermionic-photovoltaic (TIPV) module Fabricate the thermionic (TI) emitter and transparent

Grant agreement 737054 Review Meeting – Bruxelles, 6th February 1

WP3: Power Conversion Module

electrons

photons

Thermionic Photovoltaic

emitter

TPV cellcollector

Micro-spacers

Participants

cooling

system

Hybrid thermionic-photovoltaic converter, by A.Datas. Appl. Phys. Lett. 108, 143503 (2016)



n- substrate

n

p

emitter / cathode

TPV cell IR PV generation

A (+)

B (-)

Microspacers vacuum micro-gap ~ 1-3 µm (avoid space charge)

Collector coating very low workfunction < 2 eV transparent

Emitter coating low workfunction ~ 3 eV

Cooling

Heat



2-terminal Inverted 3-terminal

p

n

n- substrate(transparent)

n- substrate

n

p

emitter / cathode emitter / cathode

TPV cell

electronhole

photon

holeelectron

photon

A (+) A (+)

B (-) C (+)B (-)

TI anode



2-terminal Inverted 3-terminal

A (+)

B (-)

A (+)

C (+)B (-)

thermionic

photovoltaic


WP3 - Objectives

Development of a hybrid thermionic-photovoltaic (TIPV) module

Fabricate the thermionic (TI) emitter and transparent collector with low work functionmaterials

Develop thermal and electrical insulating micro-spacers for the thermionic inter-electrode gap

Fabricate the thermo-photovoltaic (TPV) cell for efficiently converting the radiationemitted by a body operating in the range 1000-2000 °C

Design and optimize a cooling system for the TIPV module, in order to avoid possibledetrimental effects on the performance of the converter.

Provide a characterization system for quantifying the conversion efficiency of the TPVcell

Provide a characterization system for testing all the TIPV components up to thetargeted TI cathode temperature (2000 °C)

Demonstrate the feasibility of the TIPV proof-of-concept and fabricate TIPV devices, tobe further integrated within the overall AMADEUS technology in WP4


Gantt chart and activity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

T1.1 D1.2 D1.4/D1.5 D1.7 D1.10

T1.2 D1.1 D1.3 MS1 D1.6 D1.8 D1.9

T2.1 D2.1 - MS2 D2.2 - MS3

T2.2.1 D2.3

T2.2.2 D2.4

T2.3 D2.5 - MS4

T2.4 D2.6

T2.5 D2.7 - MS5

T3.1.1 D3.1

T3.1.2 D3.6 - MS7

T3.2 D3.5

T3.3.1 D3.7 - MS7

T3.3.2 D3.8

T3.3.3 D3.9 - MS8

T3.4 D3.3

T3.5.1 D3.2 - MS6

T3.5.2 D3.4 D3.12

T3.6 D3.10 - MS9/MS10 D3.11 - MS11

T4.1 D4.1 - MS12

T4.2 D4.2 - MS13

T4.3 D4.3 - MS14

YEAR 1 YEAR2 YEAR3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

T1.1 D1.2 D1.4/D1.5 D1.7 D1.10

T1.2 D1.1 D1.3 MS1 D1.6 D1.8 D1.9

T2.1 D2.1 - MS2 D2.2 - MS3

T2.2.1 D2.3

T2.2.2 D2.4

T2.3 D2.5 - MS4

T2.4 D2.6

T2.5 D2.7 - MS5

T3.1.1 D3.1

T3.1.2 D3.6 - MS7

T3.2 D3.5

T3.3.1 D3.7 - MS7

T3.3.2 D3.8

T3.3.3 D3.9 - MS8

T3.4 D3.3

T3.5.1 D3.2 - MS6

T3.5.2 D3.4 D3.12

T3.6 D3.10 - MS9/MS10 D3.11 - MS11

T4.1 D4.1 - MS12

T4.2 D4.2 - MS13

T4.3 D4.3 - MS14

YEAR 1 YEAR2 YEAR3

TPV cell development

Emitter development

Device characterization

Collector coating

Device cooling

Final PoC

✓ D3.1 - Report on emitter candidates

✓ D3.2 - Upgrade of VTEC

Micro-spacers

Collector + micro-spacers

Upgrade

✓ D3.3 - Optimal design of cooling system✓ D3.4 - First report on advance characterization of TIPV

o D3.5 – D3.8 Recipes for TPV cell, cathode, anode, microspacers


WP3 – Tasks/subtasks and their development

T3.1 T3.1 Development of the thermionic emitter

T3.2 Development of the TPV cell

T3.5.1 Upgrade of VTEC

T3.4 Cooling system design and optimization

T3.5.2 Characterization of thermionic and photovoltaic devices

T3.1 T3.3 Development of the thermionic collector coatings and microspacers


WP3 – Development: Task 3.1

Thermionic cathode

Substrate

Emitting layer

T3.1.1 – Selection of emitting materials/Emitter

Substrate: Refractory metals (W, Ta, Mo, Re)Emitting layer: many candidates

LaB6, CeB6, AlN:H, CNx, nanodiamond, perovskites

Characteristics of the substrate:1. high melting point (> 2000 °C)2. large thermal conductivity (> 50 W/mK)3. high thermal stability;4. very low electric resistivity (~10-4 - 10-5 Ω cm)5. coefficient of thermal expansion comparable to thatof the emitting layer

Emitting layers grown by Pulsed Laser Deposition

Characteristics of the emitting layer:1. work function ϕC as low as possible, but at thesame time larger than the anode one ϕA

2. spectrally selective emissivity matched with thePV cell, i.e. high (low) emissivity for wavelengthssmaller (larger) than the wavelength correspondingto the bandgap of the PV cell active material (870nm for GaAs and 1700 nm for InGaAs);3. high melting point4. thermal expansion coefficient comparable tothat of the substrate5. thickness in the range 10-20 nm for satisfyingsuitable optical and electric requirements



T3.1.2 Development of thermionic emitting layer

Work function determination by UPS

Samples Φ (eV)*

La-B 3.0

Ce-B 3.1

Al-N 2.8

C-N 4.0

*after 30 s of sputtering for removing surface contaminations

Work function determination by thermionic measurements

Richardson-Dushmann fit:J= ART2exp(-Φ/KBT)

Samples A* (A/cm2K2) Φ (eV)J @ 2000 °C

(A/cm2)

La-B 10.14 2.82 3.19

Ce-B 9.63 2.96 1.34

Al-N 77.30 3.19 2.83

C-N 5.47e-5 1.58 0.023

Main chemical-physical properties of the materialsinvestigated by XPS, SEM, and XRD



T3.3.1 – Development of the collector coatings

BaF2 and BaO deposited by magnetron sputtering:

• BaO films showed a too high workfunction (4.1 - 4.3 eV) due to

oxygen incorporation excess

• Better results with barium fluoride coatings:

Φ = 3.0±0.1 eV for a thickness of 0.15 nm on p-type GaAs

The lowest work function

First objective: to deposit a very thin layer of an alkali-metallic compounds for lowering the work function of GaAs (4.6-4.8 eV for p-type GaAs) down to 1.7 eV

Two possible alternative strategies:• Controlled deposition by e-beam technique for achieving a better composition.• In-situ deposition of Cs and Ba coatings by commercial alkali-metal dispenser is an

on-going activity: it should guarantee lower work function values.

Sub-monolayer(non continuous film)

Sub-monolayer(non continuous film)



T3.3.2 – Development of micro-spacers

The minimization of the inter-electrode gap leads to preventspace-charge conditions and tomaximize the TI energy conversionefficiency

Defined geometry for mechanical stability and reduction of the contact surface in order to limit heat transport by conductionShape: circular – Size: 5-10 µm-diameter – Pitch: 50-200 µm

Need of vacuum gap at least in therange 1-3 µm, but phenomena ofthermal expansion could occur:risks of electrodes’ contact

Dielectric ceramic microcolums for guaranteeing thermal and electric insulation

Selected materials: Aluminum oxide (Al2O3)and Zirconium oxide( ZrO2)

Optimal emitter-collector gap for thermionic energyconverters, Appl. Phys. Lett. 100, 173904 (2012)

AMADEUS solution: dielectric microspacers



GaAs

Fabrication of GaAs PV cells

I-V curves

1.4eV525 nm n-GaAs 2e18

5 nm p+ AlAs 8e18

5 nm p+ GaAs 8e18

N++ GaAs substrate

500 nm p++ GaAs 8e18-1e20

5000 nm n+ GaAs 2e17

GaAs (AA06)

30 nm p+ Al0.5Ga0.5As 4e18

900 nm p+ GaAs 2e18

200 nm n+ Al0.33Ga0.67As 1.33e18

Layer structure

2 mm



InP

Layer structure

I-V curves0.74eV

In0.53Ga0.47As

Fabrication of InGaAs TPV cells

2 mm

70 A/cm2 and 20 W/cm2



0

5

10

15

20

25

0.00 0.20 0.40 0.60

Cu

rre

nt

De

nsi

ty (

mA

/cm

2)

Voltage (V)

S1 (InGaAs)

S2 (InGaAs)

AA06 (GaAs)

Fabrication of IBC TPV cells

pn

n- substrate

emitter / cathode

10x10 mm2 5x5 mm2 n-contact

p-contact

IBC TPV cell



T3.5.1 – Upgrade of the characterization system

UHV DN 160 with full metal CF standard

Laser

source

Some fundamental characteristics: Vacuum level ≃10 -8 mbar Capacity to manage 2000 °C without any problem Cooling of the TIPV anode to avoid overheating Deposition of coatings in-situ Possibility to approach the electrodes with 1 µm resolution



Numerical simulation of a cold copper plate based cooling system of the TIPV anodeconverter – Input delivered by CNR, IONVAC, and UPM

Detailed steady-state 3D CFD model describing the conductive and convective heattransfer process occurring within the TIPV anode/cooling system

Test of two different cooling mediums (H2O+15% glycol ethylene and liquid N2) undera wide range of operating conditions in terms of input thermal heat flux and coolantproperties

Simulation of alternative scenarios to optimize the TIPV anode/cooling system

Test of additional conceptual cooling systems (heat sink, water pipe cooling)

Cooling medium: H2O&15% ethylene glycol

Realistic operating conditions

Extreme operatingconditions Qmax/4

ΔΤ≈18 oC

Tc=5 oC, mc=0.4 kg·s-1

Many results have been obtained from simulations: investigations as a function of several parameters are carefully reported in Deliverable 3.3



T3.5.2 – Characterization of thermionic and photovoltaic devices

10-10

10-9

10-8

10-7

10-6

10-5

10-4

-10 -8 -6 -4 -2 0 2

Case C

Case B

Case A

Curr

en

t d

ensity (

A/c

m2)

Voltage (V)

T =1260 ± 10 °C

Experiment Cathode / Emitting layer

Anode

case A 8 nm-thick LaB-O (ΦC = 3.26-3.34 eV from UPS)

Gold (Au) anode(ΦA = 5.0-5.1 eV fromUPS)

case B 15-nm-thick CeB-O emitting layer(ΦC = 3.6-3.9 eV from UPS)

p-type GaAs wafer / 40-nm-thick BaF2 coatinganode(ΦA = 3.7 eV from UPS)

case C 15-nm-thick CeB-O emitting layer(ΦC = 3.6-3.9 eV from UPS)

n-type GaAs wafer / 40 nm-thick BaF2 coatinganode(ΦA = 3.8-3.9 eV fromUPS)



Device efficiency measurement setup

T3.5.2 – Characterization of thermionic and photovoltaic devices

0.1 0.2 0.3 0.4 0.5 0.6 0.7

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

Ele

ctr

ic P

ow

er

(wa

tt)

Heat (Watt)

PowerVsH Efficiency

Heatsink

Heat flux sensor

TPV cell

Cu baseplate

Cu baseplate

DBC substrate

PELTIER COOLER

Emitter

Pcool(Tcell)

Q

Pel

T1

T2

V (Q, T1, T2)

Tcell

Pin Pout

el

el

outin

el

PQ

P

PP

P

eloutin PQPP

Energy balance in the TPV cell

Set point (TPV cell temperature)

Preliminary result

power

efficiency


WP3 – Deliverables

Del. 3.1: Report on the emitter material candidates

Definition of the thermionic cathode: substrate + very thin emitting layer (10-20 nm).

Description of the main characteristics of both substrate and emitting layer:

• Melting point

• Thermal expansion coefficient

• Thermal conductivity

• Thermal stability

• Electrical resistivity

• Work function

• Spectral emissivity

A list of possible candidates for acting as substrate and emitting layer has been established.

A guideline for the development of the thermionic cathode has been traced: first materials tobe investigated as best candidates, found through the discussion (W as substrate,nanostructured borides deposited by pulsed laser deposition as emitting layers), and possiblealternatives (Mo, Ta, and Re as substrates; nitrides, perovskites, and H:diamond as emittinglayer).



Del. 3.2: Upgrade of VTEC

Definition of the heating strategy for reaching a cathode temperature of 2000 °C: heatingsystem by a continous laser diode up to 250 W has been chosen.

Detailed description of the design of the upgrade of VTEC (Vacuum & Temperature ElectronicCharacterization) system: analysis of the materials involved and the geometrical solutionsadopted to respond to the scientific needs

Report of the fabricated components and the system installation in CNR labs.



Del. 3.3: Optimal design of cooling systemAn evaluation of the proposed design (Cold Copper Plate) in V-TEC upgrade has been performed using

defined boundary conditions and input thermal fluxes.

A steady-state 3D CFD model describing the conductive and convective heat transfer process within theTIPV anode/ cooling system has been applied.

In case of water/ethylene at low Tin (5 oC) and high mass flow rate (0.05 kg·s-1) the anode temperaturecan withstand up to 180 W·cm-2, without its upper part exceeding 100 oC – If the PV cell is placed nearthe anode bottom, the system can move towards higher thermal fluxes, up to 250 W·cm-2.

Increasing the heat transfer coefficient results in an increased heat removal from the TIPV anode.However, the velocities inside the tubes are extremely high in this case (almost 40 m/s).

Pressurized liquid nitrogen (at P>5 bar) can be used for input fluxes higher than 300 W·cm-2 –Unexpected boiling should be avoided in the closed tank.

Cooling medium: Liquid NitrogenCooling medium: H2O&15% ethylene glycol Tc=-196.15 oC, mc=0.074 kg·s-1

Tc=40 oC, mc=0.05 kg·s-1



Del. 3.3: Optimal design of cooling system

Use of materials with lower thermal resistance in the TIPV anode system can improve the coolingefficiency of the copper tank

The water pipe cooling system results in improved cooling efficiency compared to the standarddesign for the same coolant mass flow rate

The heat sink with straight fins cooling system has almost the same cooling efficiency with thecopper tank, but its efficiency can be further improved with a higher coolant mass flow rate

Both new conceptual designs should be further investigated in terms of mechanical stability

The possible optimization of the cooling system has been discussed, by highlighting how to improve the present system and by proposing different solutions.

Water pipe cooling system Heat sink with straight fins cooling system



Del. 3.4: First report on advanced characterization of photovoltaic and thermionic devices

Description of the first characterizations of the two single devices: thermionic by the VTECupgrade and TPV by an ad-hoc measurement system for evaluating the conversion efficiency.

Analysis of potential and possible bottlenecks (to be overcome) of the two systems.

Report of the preliminary performance of TI and TPV devices at M12.

Careful review of the future activities and improvements of the advanced characterizations, inthe meantime the final recipes of TI and TPV converters get a mature status.

0 1 2 3 4 5

0.000

0.002

0.004

0.006

0.008

0.010

Outp

ut pow

er

density (

W/c

m2)

Output voltage (V)

Black curve: experimental dataRed curve: hypothesis ΦA = 1 eV

0.1 0.2 0.3 0.4 0.5 0.6 0.7

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Heat (Watt)

PowerVsH Efficiency

power

efficiency

thermionic thermophotovoltaic


WP3 – Milestones

Milestone Brief Description Due date Comments

MS6 Critical analysis of the VTEC temperature upgrade

M6

MS7 Cathode & Anode work function (WF)

M15

MS8 Inter-electrode spacers fabrication on anode

M20

MS9 Convergence of thermionictechnologies

M24

MS10 TIPV proof-of-concept M24

MS11 TIPV device for WP4 experiments

M30

The verification was performed. A temperature close to 2000 °C can be

achieved.

At M12: cathode WF close to the target value (2.7 eV); anode WF is still higherthan the expected one (1.7 eV). Risk-

mitigation measures have been proposed.

Preliminary development is satisfying, but it is too early for a definite

evaluation.

Too early for an evaluation.




WP3 – Perspectives

Towards the PoC demonstration…

Low cathode and anode work function

Some µm gap required for TI

Transparent anode required for TIPVintegration

Low temperature anode required (for TPVoperations)

After the first year of project, some key-points have to be considered for reaching the goals of WP3and fabricating and demonstrating the TIPV device:

Ensure thermal stability and the suitable operating conditions of all the components for theapplication at 2000 °C

Obtain the condition ΦC› ΦA for a proper operation of the TIPV

Find the optimal design for InGaAs-based TPV anode

Evaluate the behaviour of the TIPV anode with respect to the cooling capacity of the system



Participants

Thank you for the attention


Backup slides?


T3.2 – TPV cell development

• Semiconductor material growth by MBE

InP

In0.56Ga0.44As

In0.53Ga0.47As

In0.53Ga0.47As In0.56Ga0.44As

Lattice matched Unstrained

Possible defects




RHEED calibration of In/Ga ratio growing InAs Quantum Dots on GaAs




Bad morphology due to the defects induced by lattice difference between layers



• XRD Measurements (very first devices)

Excess of Indium content(~ 3-6 %)

Desired

In0.53Ga0.47As

Obtained

In0.56-0.59Ga0.44-0.41As



• InGaAs TPV cell results (very first devices)

The First InGaAs TPV cell

J-V curve

Documents

WP3: Power Conversion Module · 2018-10-02 · WP3 - Objectives Development of a hybrid thermionic-photovoltaic (TIPV) module Fabricate the thermionic (TI) emitter and transparent