ECE 695 Numerical Simulations Lecture 35: Solar Hybrid Energy Conversion Systems. Prof. Peter Bermel April 12, 2017

ECE 695 Numerical Simulations Lecture 35: Solar Hybrid Energy Conversion Systems Prof. Peter Bermel April 12, 2017

Ideal Selective Solar Absorber Efficiency Limits Ideal cut-off wavelength for a selective solar absorber at 100 suns and 1000 K is 1,110 nm Maximum thermal transfer efficiency is 71.94% Bermel, Peter, et al. "Selective solar absorbers." Annual Review of Heat Transfer 15.15 (2012). 4/12/2017 ECE 695, Prof. Bermel 2

Selective Solar Absorbers at T=1000 K (100 suns) ECE 695, Prof. Bermel 4/12/2017 3

Outperformance of Tunable Ideal Selective Absorbers Rugate filter T=2120 K, C=2000: 33% increase in <h t > 16.5% increase in energy generation/day 4/12/2017 ECE 695, Prof. Bermel 4

Solar PV/Thermal Hybrid Conversion Spectral splitting advantages: Puts each photon to its best use Adds cheap storage Reduces PV heating P. Bermel et al., Energy & Environmental Science (2016). ECE 695, Prof. Bermel 4/12/2017 5

Efficient Solar PV/Thermal Hybrid Conversion P. Bermel et al., Energy & Environmental Science (2016). ECE 695, Prof. Bermel 4/12/2017 6

Thermoelectric Topping Cycles Stabilize Efficiency Above 550 C P. Bermel et al., Energy & Environmental Science (2016). ECE 695, Prof. Bermel 4/12/2017 7

Optimal PV Bandgap Around 2.1 ev for 500 suns P. Bermel et al., Energy & Environmental Science (2016). ECE 695, Prof. Bermel 4/12/2017 8

Solar PV/Thermal Hybrid Conversion Efficiency P. Bermel et al., Energy & Environmental Science (2016). ECE 695, Prof. Bermel 4/12/2017 9

Solar PV/Thermal Hybrid Conversion Dispatchability P. Bermel et al., Energy & Environmental Science (2016). ECE 695, Prof. Bermel 4/12/2017 10

Efficient Solar PV/Thermal Hybrid Conversion ECE 695, Prof. Bermel 4/12/2017 11

Selective Solar Absorbers 215 nm 300 um Si 3 N 4 Si 300 nm Ag Schematic of the structure for selective absorber based on Si substrate with 215nm Si 3 N 4 front antireflection coating (ARC) and 300nm Ag back reflection layer. Heights are not to scale. H. Tian et al., Appl. Phys. Lett. (2017) 4/12/2017 ECE 695, Prof. Bermel 12

Method - Fabrication Si3N4: PVD Sputtering: a magnetron sputtering system with unheated stage (custom built by PVD Products). The condition of the sputtering is 100w, 5mTorr, 15sccm Ar. 7rpm rotation of the stage. The deposition time is 86min around 200-215nm, as determined by spectroscopic ellipsometry (Filmetrics). Ag: CHA evaporation: Deposition rate is 1.5 Å/s (as determined by a quartz crystal monitor) for around half an hour until 300 nm. Si substrate: N type (Phos), 2 inch, <100>, 254-304um thickness, double side polished, with resistivity 10-20 W cm (Pure Wafer). 4/12/2017 ECE 695, Prof. Bermel 13

Direct Thermal Emission Measurement System Heater/Emitter Chamber H. Tian et al., Appl. Phys. Lett. (2017) Cu Tubing PM 3: D=1.5 ; EFL=2 FTIR PM 1: D=3 ; EFL=4 PM 2: D=4 ; EFL=4 The sample is heated by the heater, and the emitted light is collected and guided by the Cu tube, transmitted through a CaF 2 window, reflected by three off-axis parabolic mirrors (PM 1, 2, and 3, Edmund Optics) to a Fourier Transform InfraRed (FTIR) spectrometer with a mercury cadmium telluride detector and KBr beam splitter (Thermo Fisher Nicolet 670). 4/12/2017 ECE 695, Prof. Bermel 14

300 m m Si Experiment & Simulation at Room Temperature Measurement (solid lines) and simulation (dashed lines) of the emissivity of selective absorbers with (red lines) and without (black lines) front coating at room temperature. Measurements performed by a Lambda 950 spectrophotometer with an integrating sphere (Labsphere). The thicknesses of Si 3 N 4, Si and Ag are 215nm, 300 m m and 300nm respectively. 215 nm 300 um 300 nm Si 3 N 4 Si Ag H. Tian et al., Appl. Phys. Lett. (2017) 4/12/2017 ECE 695, Prof. Bermel 15

300 m m Si Experiment & Simulation at High Temperatures 0.8 without Si 3 N 4 AR coating 1.0 with Si 3 N 4 AR coating 0.6 0.8 Emittance 0.4 0.2 515+5C 468+15C 415+20C Emittance 0.6 0.4 0.2 519+10C 468+10C 410+15C 331+15C 0.0 1 10 Wavelength [m m] 353+15C 1 10 High spectral selectivity is observed at 468 ºC in both samples, with a cutoff wavelength of approximately 1.3 m m. Higher short-wavelength emittance is both predicted and observed for the structure with a Si 3 N 4 AR coating 0.0 Wavelength [m m] H. Tian et al., Appl. Phys. Lett. (2017) 4/12/2017 16

Thin Si film optimization targeted @ 550 C Emissivity for selective absorbers with different Si thicknesses. Optimal Si 3 N 4 thickness is used for each curve which is 80 nm. The temperature is set at 550 and the F-P interference around the Mid-IR is smoothed out for more clear comparison. Less MWIR absorption is experienced for thinner layers of silicon because all samples are in the intrinsic regime, and free carrier absorption dominates. 300 um Si 20 um Si H. Tian et al., Appl. Phys. Lett. (2017) 5 um Si 4/12/2017 ECE 695, Prof. Bermel 17

Optimization Summary for 550 C Dependence of solar thermal transfer efficiency for different Si thicknesses on the concentration. The Si 3 N 4 thickness is fixed at 80nm, and the temperature is 550C. Thinner layers of silicon experience less reradiation; however layers which are too thin have less absorption, which puts an upper bound on. H. Tian et al., Appl. Phys. Lett. (2017) 4/12/2017 ECE 695, Prof. Bermel 18

Thermoelectric Figure of Merit ZT: Silicon and Silicon Germanium ZT vs. T ZT 1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 N-GPHS RTG Si80Ge20 N-nanostructured Si80Ge20 n-type nanostructured Si p-gphs-rtg Si80Ge20 p-nanostructured Si80Ge20 p-nanostructyred Si 0.0 200 300 400 500 600 700 800 900 1000 1100 1200 1300 Temperature (K) 4/12/2017 ECE 695, Prof. Bermel 19

Solar PV/Thermal Conversion Efficiency with Improved TE Calculated system exergy as a function of the selective solar surface (SSS) temperature differential (75-350 C) and the ZT value of the thermoelectrics. We assumed 500x solar concentration. As one goes from no thermoelectrics to ZT=4, an overall improvement of 8% can be achieved. These calculations take into account reradiation losses at high SSS temperatures P. Bermel et al., Energy & Environmental Science (2016). 4/12/2017 ECE 695, Prof. Bermel 20

Cost per unit power output [$/W] 100 10 1 0.1 0.01 q=1[w/cm 2 ] q=10[w/cm 2 ] q=100[w/cm 2 ] ZT=1 ZT=4 ZT=1 ZT=4 ZT=1 ZT=4 0.001 0% 1% 10% 100% Fill factor Cost per power output $/W as a function of fill factor for ZT=1 and 4, and heat fluxes which reflect the heat sink performances. This graph assumes a maximum temperature of 2300K and ambient temperature of 310K, with the TE material price $500/kg, and substrate $26/kg. 4/12/2017 ECE 695, Prof. Bermel 21

30% R - Resistance ratio 25% 20% 15% 10% 5% r c =1x10-6 r c =5x10-6 r c =1x10-7 0% 1% 10% 100% Fill factor Ratio of parasitic electrical resistance to entire resistance as a function of Fill factor, with a variation of specific contact resistivity [Ohm.cm 2 ]. Conditions are for 1000 elements per 1cm 2 module. 4/12/2017 ECE 695, Prof. Bermel 22

1000 Initial $/kw = Energy cost $/kwh at 0h Energy cost [$/kwh] 100 10 1 (a) (b) 100h 1000h 10000h 0.1 0.01 0.1 1 10 100 Normalized TE leg length d =d/d opt Energy cost vs. normalized leg length, assuming the same parameters as before. Thicker TE legs cause higher efficiency but reduced power output. The economical optimum exists in between the design for maximum power output and maximum efficiency. 4/12/2017 ECE 695, Prof. Bermel 23

Six full element array chips were fabricated on the quarter wafer. Each element array chip contains 200 elements, and area size of each element is 120 m m 120 m m. 4/12/2017 ECE 695, Prof. Bermel 24

400 elements of 200 n-eras:ingaalas and 200 p- ErAs:InGaAs elements, 10 m m thick, were bonded on the upper and lower AlN plates via wafer scale approach and flip-chip bonding technique. 4/12/2017 ECE 695, Prof. Bermel 25

NOV Mo Cu Solder Z Y X SiGe 500m m 10m m 50m m 100m m Temperature ( C) Thermomechanical 2D simulation for a 5-leg TE module in ANSYS. The TE material is SiGe, and the substrates are Molybdenum. The largest stress at interfaces is on the order of 56 MPa (DT = 170 o C) in share stress (horizontal-direction) 4/12/2017 ECE 695, Prof. Bermel 26

Next Class Next time, we will discuss engineering grand challenge problems, and what role simulations can play in addressing them 4/12/2017 ECE 695, Prof. Bermel 27