High Efficiency Integrated Solar Energy Converter

Transcription

1 High Efficiency Integrated Solar Energy Converter Research findings on photon enhanced thermionic emission SES2050 seminar , Oslo, Norway Aapo Varpula, Jyrki Tervo VTT Technical Research Centre of Finland

2 Click Project to participants edit Master title style VTT: Jouni Ahopelto, Aapo Varpula, Kirsi Tappura, Mika Click to edit Master text styles Prunnila, Jyrki Tervo Second level DTU: Ole Hansen, Kasper Reck KTH: Sebastian Lourdudos, Yanting Sun VU: Tadas Malinauskas Fortum Ltd: Eero Vartiainen Picosun Ltd: Minna Toivola 2

3 Click Outline to edit Master title style Introduction Click to edit Master text styles Modelling Second work level and results Experimental work Design of PETE demonstrator SWOT analysis of PETE Publications Conclusions 3

4 Click Inroduction to edit Master title style In 2010 Schwede et al. [Schwede2010] proposed the photon-enhanced thermionic emission (PETE) device which is a photovoltaic device operating at Second level high temperatures. This technology can be combined with the existing thermal Fourth solar level energy systems, thereby allowing the solar-energy-to-electricity conversion efficiencies above 50% to be potentially reached. Its main characteristics include high temperature operation, use of high illumination intensity (i.e., the intensity is 100 W/cm 2 at 1000 suns) and promise of efficiencies much higher than conventional solar cells. 4

5 Goals of the work Click to edit Master title style To develop theoretical PETE solar cell model To develop specific PETE solar cell designs To evaluate materials and structures for PETE solar cells. To evaluate the efficiency and commercial potential of PETE solar cells. Second level 5

6 Modelling work Click to edit Master title style The first detailed model of PETE devices developed Takes the relevant effects in semiconductors into account Both numerical model for all calculations and analytical formulas for special cases and approximate calculations Model published in Journal of Applied Physics in 2012 [1] Second Modelled level cathode material: Silicon Analysis Third of space levelcharge effects in emission current measurements Equivalent circuit Fifth model level of PETE devices developed Extension of the model New materials Czochralski and magnetic Czochralski silicon [2] GaAs and InP [3,4] Electron density dependence of the electron diffusion constant (a minor effect) [4] [1] A. Varpula, M. Prunnila, Journal of Applied Physics 112, (2012). [2] A. Varpula, K. Reck, M. Prunnila, O. Hansen, PETE-2014, Tel Aviv, June [3] A. Varpula and M. Prunnila, EU PVSEC Proceedings, p. 331, EU PVSEC 2014, Amsterdam, Sep [4] A. Varpula, K. Tappura, M. Prunnila, Si, GaAs, and InP as cathode materials for photonenhanced thermionic emission solar cells, Solar Energy Materials & Solar Cells, revised manuscript submitted. 6

7 Click Modelling to edit results Master title style Surface recombination and (effective) Click electron to affinity edit on Master cathode text surfaces styles must be controlled in order to reach high efficiencies Second level [1 4] Silicon is feasible Third level cathode material for PETE devices Fourth [1,2] level Bulk recombination Fifth seems levelto have lower effect on the efficiency of PETE devices Standard Czochralski silicon should be adequate [2] GaAs and especially InP seem to be very promising for PETE because of their strong photon absorption characteristics High efficiency [3,4] Effect of recombination on emitting surface [2] Efficiency Efficiency 14% 10% 5% 30% 25% 20% 15% Cathode temperature (K) Comparison of cathode materials [4] Under 1000 suns 10% InP 5% GaAs Si 0% Cathode temperature (K) 0 cm/s 10 2 cm/s 10 3 cm/s 10 4 cm/s 10 5 cm/s Apparent efficiency from thermally generated electrons [1] A. Varpula, M. Prunnila, Journal of Applied Physics 112, (2012). [2] A. Varpula, K. Reck, M. Prunnila, O. Hansen, PETE-2014, Tel Aviv, June [3] A. Varpula and M. Prunnila, EU PVSEC Proceedings, p. 331, EU PVSEC 2014, Amsterdam, Sep [4] A. Varpula, K. Tappura, M. Prunnila, Si, GaAs, and InP as cathode materials for photonenhanced thermionic emission solar cells, Solar Energy Materials & Solar Cells, revised manuscript submitted. 7

8 Click Experimental to edit Master title style Materials testing and PETE measurements were carried out in an ultra-high Second vacuum (UHV) level chamber at DTU. The chamber was fitted with a 4-axis sample manipulator, Fourth loadlock, levelion gun for sputter cleaning, caesium Fifth evaporator, level a copper anode, a 30 W laser source for sample illumination, an X-ray source and a hemispherical energy analyser for XPS and work function measurements. Other measurements: HR-XRD, AFM, Raman spectroscopy, STS/STM, LITG 8

9 Click Studied to materials edit Master title style - highly doped p-type and n-type Click silicon to edit Master text styles - surface Second structured level silicon - caesiated silicon - cesiated Third and level oxidized silicon - GaAs - GaAs with InP nanodots - C12A7 - Graphene - cesiated graphene - InAs/GaAs - In x Ga 1-x N Distance between pyramid tips 3 20 µm Distance between 50 nm dots: ~50 nm 9

10 Click Demonstrator to edit Master design title style Experimental setup designed for Click demonstration to edit of operation Master of text PETE styles devices Features Second Illumination level through transparent vacuum chamber or via an optical feedthrough (no restrictions Third level on the type of external photon source) Vacuum with Fourth standard level vacuum pumps Replaceable emitter Fifth and level collector sample chips Additional heater for temperature control of emitter Cs-sources for work-function and space charge reduction Mostly commercial components used Present design with KF flanges Chamber can be replaced by CF-flange version for higher vacuum ( 10 9 mbar) Spacers for accurate control of the gap between emitter and collector 10

11 Click Operating to edit principle Master title style Second level Cs vapour Photons Electrons Cs vapour Emitter/absorber Cs-source Collector Cs-source 11

12 Exploded view Electrical feedthroughs for Click to edit Master title style emitter (1), emitter heater (2), and Cs-sources (2) Alignment pins Standard KF flanges Glass cross working as vacuum chamber Second level Optional port Sample holder Emitter heater Emitter Spacer Collector Spacer Standard KF flanges Copper block for electrical and thermal contact Spacer Water-cooled highcurrent copper electrical feedthrough for collector Overview Sample holder 12

13 Assembled sample holder Contact wire for emitter Click to edit Master title style (acts also as mechanical support) Power wire 1/2 for emitter heater emitter heater Power wire 1/2 for Cs-sources Second level Alignment pin Power wire 2/2 for Power wire 2/2 for Cs-sources Power wire 1/2 for Cs-sources Alignment pin Emitter heater Emitter Alignment pin Spacer Emitter heater Copper block for electrical and thermal contact Perspective view Collector Side view (cross section) 13

14 Click Illumination to edit possibilities Master title style Second level Light source Light source Through transparent chamber wall Through port with optical feedthrough 14

15 Click PETE to publications edit Master title style Number of articles citing the original PETE paper [5] Second level Scopus 13 th Nov, Our scientific article (1 st detailed model) published in August 2012 Original paper published in September HEISEC begins Our publications: 2 conference presentations 1 conference paper 1 scientific article [5] J.W. Schwede et al., Photon-enhanced thermionic emission for solar concentrator systems, Nature Materials 9, 762 (2010). 15

16 SWOT analysis of PETE Click to edit Master title style Strengths - Potential for high efficiency - No moving parts - No emissions Click (greenhouse to edit effect Master and pollution) text styles - Allows cogeneration of solar electric power and heat Second level - Can be incorporated into existing solar concentrator systems Low amount Fourth of materials level needed Use of exotic materials Fifth levelpossible Device cost not issue Opportunities - New IPR (materials and structures) - Markets available Device manufacturing Material deposition tools Power generation Weaknesses - Based on non-existing materials (problems with work function, heat emissivity, charge-carrier recombination) - Vacuum and high temperature differences required Complex system Illumination more difficult Tight material requirements - Maximum efficiency requires Light concentration Secondary heat engine - Direct measurement of phenomenon difficult - Solution of space charge problems requires small gap, neutralizing plasma or other means Threats - Realization difficulties - No stable low work function materials can be found - Competition 16