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Energy Express

  • Editor: Bernard Kippelen
  • Vol. 18, Iss. S3 — Sep. 13, 2010
  • pp: A314–A334
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Design and global optimization of high-efficiency thermophotovoltaic systems

Peter Bermel, Michael Ghebrebrhan, Walker Chan, Yi Xiang Yeng, Mohammad Araghchini, Rafif Hamam, Christopher H. Marton, Klavs F. Jensen, Marin Soljačić, John D. Joannopoulos, Steven G. Johnson, and Ivan Celanovic  »View Author Affiliations


Optics Express, Vol. 18, Issue S3, pp. A314-A334 (2010)
http://dx.doi.org/10.1364/OE.18.00A314


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Abstract

Despite their great promise, small experimental thermophotovoltaic (TPV) systems at 1000 K generally exhibit extremely low power conversion efficiencies (approximately 1%), due to heat losses such as thermal emission of undesirable mid-wavelength infrared radiation. Photonic crystals (PhC) have the potential to strongly suppress such losses. However, PhC-based designs present a set of non-convex optimization problems requiring efficient objective function evaluation and global optimization algorithms. Both are applied to two example systems: improved micro-TPV generators and solar thermal TPV systems. Micro-TPV reactors experience up to a 27-fold increase in their efficiency and power output; solar thermal TPV systems see an even greater 45-fold increase in their efficiency (exceeding the Shockley–Quiesser limit for a single-junction photovoltaic cell).

© 2010 Optical Society of America

1. Introduction

Thermophotovoltaic (TPV) systems convert heat into electricity by thermally radiating photons, which are subsequently converted into electron-hole pairs via a low-bandgap photovoltaic (PV) medium; these electron-hole pairs are then conducted to the leads to produce a current [1

1. H. H. Kolm, “Solar-battery power source,” Tech. Rep., MIT Lincoln Laboratory (1956). Quarterly Progress Report, Group 35, p. 13.

4

4. F. O’Sullivan, I. Celanovic, N. Jovanovic, J. Kassakian, S. Akiyama, and K. Wada, “Optical characteristics of 1D Si/SiO2 photonic crystals for thermophotovoltaic applications,” J. Appl. Phys. 97, 033529 (2005). [CrossRef]

]. As solid-state devices, they have the potential for higher reliability, vastly smaller form factors (meso- and micro-scales), and higher energy densities than traditional mechanical engines. However, most systems emit the vast majority of thermal photons with energies below the electronic bandgap of the TPV cell, and are instead absorbed as waste heat. This phenomenon tends to reduce TPV system efficiencies well below those of their mechanical counterparts operating at similar temperatures, as shown in Fig. 1(a) [5

5. H. Xue, W. Yang, S. Chou, C. Shu, and Z. Li, “Microthermophotovoltaics power system for portable MEMS devices,” Nanoscale Microscale Thermophys. Eng. 9, 85–97 (2005). [CrossRef]

]. Photon recycling via reflection of low-energy photons with a 1D reflector is a concept that significantly reduces radiative heat transfer [3

3. R. Black, P. Baldasaro, and G. Charache, “Thermophotovoltaics - development status and parametric considerations for power applications,” in International Conference on Thermoelectrics , 18, pp. 639–644 (1999).

, 4

4. F. O’Sullivan, I. Celanovic, N. Jovanovic, J. Kassakian, S. Akiyama, and K. Wada, “Optical characteristics of 1D Si/SiO2 photonic crystals for thermophotovoltaic applications,” J. Appl. Phys. 97, 033529 (2005). [CrossRef]

]. This approach can also be extended to encompass the more general concept of spectral shaping: directly suppressing emission of undesirable (below bandgap) photons as well as enhancing emission of desirable (above bandgap) photons. Such control is provided by complex 1D, 2D, and 3D periodic dielectric structures, generally known as photonic crystals (PhCs) [6

6. J. D. Joannopoulos, S. G. Johnson, J. N. Winn, and R. D. Meade, Photonic Crystals: Molding the Flow of Light, 2nd ed. (Princeton, Princeton, NJ, 2008).

]. Spectral shaping has been proposed and predicted to be an effective approach for high-efficiency TPV power generation [7

7. A. Heinzel, V. Boerner, A. Gombert, B. Blasi, V. Wittwer, and J. Luther, “Radiation filters and emitters for the NIR based on periodically structured metal surfaces,” J. Mod. Opt. 47 (2000).

15

15. S. John and R. Wang, “Metallic photonic band-gap filament architectures for optimized incandescent lighting,” Phys. Rev. A 78, 043809 (2008). [CrossRef]

]. This approach is illustrated in Fig. 1(b).

Fig. 1 Approaches to TPV conversion of heat to electricity. The traditional design is depicted in (a), and a novel approach based on manipulation of the photonic density of states is depicted in (b). The anticipated increase in efficiency associated with the latter approach can exceed 100%.

Two specific classes of designs have already been studied in depth: narrow-band thermal emitters exhibiting wavelength, directional, and polarization selectivity [11

11. I. Celanovic, D. Perreault, and J. Kassakian, “Resonant-cavity enhanced thermal emission,” Phys. Rev. B 72, 075127 (2005). [CrossRef]

,12

12. D. L. Chan, I. Celanovic, J. D. Joannopoulos, and M. Soljacic, “Emulating one-dimensional resonant Q-matching behavior in a two-dimensional system via Fano resonances,” Phys. Rev. A 74, 064901 (2006). [CrossRef]

], and wide-band thermal emitters with emissivity close to that of a blackbody within the design range but much lower outside the design range [7

7. A. Heinzel, V. Boerner, A. Gombert, B. Blasi, V. Wittwer, and J. Luther, “Radiation filters and emitters for the NIR based on periodically structured metal surfaces,” J. Mod. Opt. 47 (2000).

, 9

9. H. Sai, Y. Kanamori, and H. Yugami, “High-temperature resistive surface grating for spectral control of thermal radiation,” Appl. Phys. Lett. 82, 1685–1687 (2003). [CrossRef]

, 13

13. I. Celanovic, N. Jovanovic, and J. Kassakian, “Two-dimensional tungsten photonic crystals as selective thermal emitters,” Appl. Phys. Lett. 92, 193101 (2008). [CrossRef]

, 15

15. S. John and R. Wang, “Metallic photonic band-gap filament architectures for optimized incandescent lighting,” Phys. Rev. A 78, 043809 (2008). [CrossRef]

, 16

16. J. Gee, “Optically enhanced absorption in thin silicon layers using photonic crystals,” in Twenty-Ninth IEEE Photovolt. Spec. Conf. , pp. 150–153 (2002). [CrossRef]

]. Intermediate-band designs combining features of each are also possible.

The remainder of this manuscript is structured as follows: in section 2, we discuss our computational approach to simulating the performance of a single TPV design, as well as globally optimizing performance for entire TPV design classes. In section 3, we apply this technique to the μTPV generator, which uses a hydrocarbon fuel micro-combustor to heat our selective emitter. In section 4, we apply our computational approach to the solar thermal TPV system, which poses the additional problem of optimizing a selective absorber for sunlight. We conclude by summarizing our findings in section 5.

2. Computational Approach

The performance of the structures discussed in this paper are studied via a combination of optical and thermal models. Two tools are used to compute their absorptivity spectra. For layered 1D and 2D structures, we use the transfer matrix method [20

20. L. Li, “Formulation and comparison of two recursive matrix algorithms for modeling layered diffraction gratings,” J. Opt. Soc. Am. A 13, 1024–1035 (1996). [CrossRef]

, 21

21. D. Whittaker and I. Culshaw, “Scattering-matrix treatment of patterned multilayer photonic structures,” Phys. Rev. B 60, 2610–2618 (1999). [CrossRef]

] implemented by a freely available software package developed at the University of Ghent called CAMFR [22

22. P. Bienstman, “Rigorous and efficient modelling of wavelength scale photonic components,” Ph.D. thesis, University of Ghent, Belgium (2001).

]. Plane wave radiation is applied from air at normal incidence, and fields are propagated through each layer to yield reflectance, transmittance, and absorptivity. Note that although in principle radiation should be integrated over all angles, normal incidence is an excellent approximation for our structures up to angles of ±π/3: see Fig. 12. For more complex 3D structures, we employ a finite difference time-domain (FDTD) simulation [23

23. A. Taflove and S. C. Hagness, Computational Electrodynamics, 2nd ed. (Artech House, Norwood, MA, 2000).

] implemented via a freely available software package developed at MIT, known as Meep [24

24. A. F. Oskooi, D. Roundy, M. Ibanescu, P. Bermel, J. D. Joannopoulos, and S. G. Johnson, “MEEP: A flexible free-software package for electromagnetic simulations by the FDTD method,” Comp. Phys. Comm. 181, 687–702 (2010). [CrossRef]

]. Again, a plane wave is sent from the normal direction and propagated through space. On each grid point of a flux plane defined at the front and back of the computational cell, the electric and magnetic fields are Fourier-transformed via integration with respect to preset frequencies at each time-step. At the end of the simulation, the Poynting vector is calculated for each frequency and integrated across each plane, which yields the total transmitted and reflected power (first subtracting the incident-field Fourier transforms for the latter) at each frequency [24

24. A. F. Oskooi, D. Roundy, M. Ibanescu, P. Bermel, J. D. Joannopoulos, and S. G. Johnson, “MEEP: A flexible free-software package for electromagnetic simulations by the FDTD method,” Comp. Phys. Comm. 181, 687–702 (2010). [CrossRef]

]. To capture material dispersion, the c-Si regions are modeled with a complex dielectric constant that depends on wavelength, as in Ref. 25

25. C. Herzinger, B. Johs, W. McGahan, J. Woollam, and W. Paulson, “Ellipsometric determination of optical constants for silicon and thermally grown silicon dioxide via a multi-sample, multi-wavelength, multi-angle investigation,” J. Appl. Phys. 83, 3323–3336 (1998). [CrossRef]

. The lower-index dielectric materials considered in this work generally have very large band gaps; thus, their absorption and dispersion can generally be neglected over the range of wavelengths considered in this work [26

26. J. Zhao and M. Green, “Optimized Antireflection Coatings for High-Efficiency Silicon Solar Cells,” IEEE Trans. Electron Dev. 38, 1925 (1991). [CrossRef]

]. Errors can also arise due to discretization, which can be reduced at higher resolutions. Apart from these approximations, both of our calculation methods for the optical properties are exact. Our two methods agree well when applied to sample 1D and 2D problems, even in the presence of dispersion.

Fig. 12 Optimized emittance spectra of semiconductor selective absorbers depicted in Fig. 10, as a function of angle. Note that optimized designs with one or more front coating layers see fairly constant performance up to angles of ±60°.

The emissivity of each structure can be calculated from the absorptivity computed above via Kirchhoff’s law of thermal radiation, which states that the two quantities must be equal at every wavelength for a body in thermal equilibrium [27

27. G. Rybicki and A. Lightman, Radiative processes in astrophysics (John Wiley and Sons, 1979).

].

3. Micro-TPV Generator

3.1. System description

Our μTPV generator is a system designed to convert chemical energy stored in hydrocarbon fuel into electrical power within a form factor comparable to a matchbox [31

31. R. C. Pilawa-Podgurski, N. A. Pallo, W. R. Chan, D. J. Perreault, and I. L. Celanovic, “Low-power maximum power point tracker with digital control for thermophotovoltaic generators,” 25th IEEE Applied Power Electronics Conference, 961–967 (2010).

]. The basic design is shown in Fig. 2. The μTPV generator operates as follows: hydrocarbon fuel (e.g., propane or butane) is fed with oxygen into a microchannel defined within a silicon structure. Oxygen is supplied at a rate 50% higher than the stoichiometric ratio, to ensure the fuel is fully consumed. The inner surfaces of the microchannel are wash-coated with a 5% platinum (by weight) catalyst supported on γ-alumina (Sigma Aldrich). The hydrocarbon is catalytically combusted on the channel surface, releasing energy as heat. Catalytic combustion is more stable at small scales than homogeneous combustion, with the latter being constrained by increased radical and thermal quenching at the walls [32

32. C. Miesse, R. Masel, C. Jensen, M. Shannon, and M. Short, “Submillimeter-scale combustion,” AIChE J . 50, 3206–3214 (2004). [CrossRef]

, 33

33. S. Deshmukh and D. Vlachos, “A reduced mechanism for methane and one-step rate expressions for fuel-lean catalytic combustion of small alkanes on noble metals,” Combust. Flame 149, 366–383 (2007). [CrossRef]

]. The micro-combustor is designed such that the heat loss to the environment through conduction and convection is small [34

34. B. Blackwell, “Design, fabrication, and characterization of a micro fuel processor,” Ph.D. thesis, Massachusetts Institute of Technology (2008).

]. Thus, most of the heat is released as radiation, primarily in the infrared. Because of the external dimensions of the micro-combustor (1 cm × 1 cm × 1.3 mm), most of the radiation falls on the TPV cells positioned opposite the two large faces to directly convert the radiation into electrical power. Excess heat in the TPV cells is dissipated by air-cooled radiators on the external faces to surrounding heat sinks. Exhaust gases from the micro-combustor could be used to pre-heat the inlet stream in a recuperator to improve the energy efficiency of the system. The electrical output is optimized in real time under changing conditions via low-power maximum power point tracking technology, as discussed in Ref. 31

31. R. C. Pilawa-Podgurski, N. A. Pallo, W. R. Chan, D. J. Perreault, and I. L. Celanovic, “Low-power maximum power point tracker with digital control for thermophotovoltaic generators,” 25th IEEE Applied Power Electronics Conference, 961–967 (2010).

.

Fig. 2 Design of the μTPV generator. Hydrocarbon fuel flows from a storage tank to the interior of the selective emitter and back out. The heated selective emitter then radiatively couples to the nearby TPV module to generate electricity (adapted from Ref. 31).

This system has been demonstrated experimentally by the present authors, albeit at low efficiencies and with modest power output. Several factors account for this suboptimal performance. First, the thermal emission spectrum is poorly matched with the bandgap of the TPV cell. The one used in this experiment was based on the quaternary compound InxGa1–x As1–y Sby (x = 0.15, y = 0.12) with a bandgap of 0.547 eV. It is constructed with a 1 μm n-InGaAsSb base, 4 μm p-InGaAsSb emitter, an AlGaAsSb window layer, and a GaSb contact layer on an n-GaSb substrate, as described in Refs. 35 and 36. Details of the performance, such as external quantum efficiency, diode ideality factor [37

37. S. Sze, Physics of Semiconductor Devices (Wiley and Sons, New York, 1981).

], series and shunt resistance, and dark current, were extracted from experimental data [38

38. W. Chan, R. Huang, C. A. Wang, J. Kassakian, J. D. Joannopoulos, and I. Celanovic, “Modeling low-bandgap thermophotovoltaic diodes for high-efficiency portable power generators,” Sol. Energy Mater. Sol. Cells 94, 509–514 (2010). [CrossRef]

]. The experimental micro-combustor design was based on a plain silicon wafer as depicted in Fig. 3(a), which has high and uniform emissivity (∼70% of a blackbody’s) throughout the infrared spectrum. Operation of such a structure at T=1000 K results in high thermal emittance of low energy photons, peaking at 0.24 eV, well below the TPV bandgap energy. The net result is that 91% of the emitted thermal radiation is unavailable for conversion into electricity. This wasted thermal power can be worse than useless, as it could overheat a TPV cell with an inadequate heat sink, thus leading to substantial performance degradation [38

38. W. Chan, R. Huang, C. A. Wang, J. Kassakian, J. D. Joannopoulos, and I. Celanovic, “Modeling low-bandgap thermophotovoltaic diodes for high-efficiency portable power generators,” Sol. Energy Mater. Sol. Cells 94, 509–514 (2010). [CrossRef]

].

Fig. 3 Three 1D structures examined as selective emitters in this work: (a) a polished Si wafer (b) a polished Si wafer with a 4-bilayer 1D PhC, and (c) a polished Si wafer with a metal layer (tungsten or platinum) and a 4-bilayer 1D PhC. Their optimized emittance spectra are shown in Fig. 4; the resulting efficiency, power (per unit area), and overall figure of merit for each structure is listed in Table 2.

Another important variable affecting our results is the view factor, defined as the fraction of emitted photons received by the TPV cell. Of course, ideally its value would be 1, but in our experiments, view factor only reached a value of approximately 0.4, due to packaging challenges. The power obtained in a configuration with only one InGaAsSb TPV module below the emitter (of 0.5 cm2 area with 10% shadowing) measured at peak efficiency was 54.5 mW per cell. Adding three more TPV cells would quadruple the power output to 218 mW, for an electric power density of 121 mW/cm2 and power conversion efficiency of 0.81% (where efficiency is computed by dividing the electrical power output by the fuel heating flux). See Table 1 for more details. A simulation designed to take these issues into account found a close match to the experiment, with an electric power density of 120 mW/cm2 and a power conversion efficiency of 0.98% at normal incidence. This discrepancy comes from heat losses not included in the simulation, most notably, radiative emission on the sides of the selective emitter (which are not received by the TPV cell), as well as small amounts of conductive and convective heat transport. The reason that the latter two effects are excluded is that they can be reduced to very small values.

Table 1. Experimental measurements of the TPV micro-combustor system depicted in Fig. 2, with one TPV cell of area 0.5 cm2, when fueled by butane and oxygen, as a function of butane flow rate (note that all measurements yielded an open-circuit voltage Voc = 247 mV per cell). Note that Isc is the short circuit current of the cell, and FF is the fill factor, defined as the ratio of the maximum power output to the product of Isc and Voc

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The optimization problem considered in this section is how to design the micro-combustor so as to maximize the product of the electrical power (per unit area) P and power conversion efficiency η of the system – the figure of merit FOM=ηP. This FOM is chosen since space-constrained systems need both high efficiencies and high volumetric power densities.

The power (per unit area) can be calculated by starting with the current density,
J(V)=0dλ[2qcλ4ɛ(λ)EQE(λ)exp(hc/λkT)1][q(n2+1)Eg2kTd4π2h¯3c2eEg/mkTd+Jnr](eqV/mkTd1),
(1)
where q is the elementary charge of a proton, k is Boltzmann’s constant, h = 2πh̄ is Planck’s constant, c is the speed of light, λ is the wavelength, EQE(λ) is the external quantum efficiency of the TPV device (experimentally determined to be approximately 82% above the bandgap), ɛ(λ) is the emissivity of the selective emitter, T is the temperature of the emitter, Eg is the bandgap of the TPV device, m is the device ideality factor [37

37. S. Sze, Physics of Semiconductor Devices (Wiley and Sons, New York, 1981).

] (experimentally determined to be 1.171), Td is the device temperature, n is the refractive index of the TPV semiconductor region, Jnr is the dark current density induced by nonradiative recombination (experimentally determined to be 18 μA/cm2), and V is the applied voltage. The output power is obtained by maximizing the electrical output power (per unit area) P = JV (i.e., by setting d(JV)/dV = 0 and back-substituting V). The efficiency η is obtained by dividing P by the integrated radiative thermal emission Pemit=2hc20dλɛ(λ)/{λ5[exp(hc/λkT)1]}.

3.2. 1D selective dielectric and metallodielectric emitters

The structure we seek to optimize is depicted in Fig. 3(b). It consists of b sub-micron bilayers of silicon and silicon dioxide added on top of the silicon wafer of Fig. 3(a), with variable period a and chirping r (the ratio of the shortest to longest period is given by (1 –r)/(1 + r)). The chirping is introduced in order to broaden the range of reflected wavelengths, and is implemented via an exponential increase of the period from its lowest to highest value [39

39. P. Wilkinson, “Photonic Bloch oscillations and Wannier-Stark ladders in exponentially chirped Bragg gratings,” Phys. Rev. E 65, 056,616 (2002). [CrossRef]

]. We constrain the number of bilayers b to integer values between zero and five, to simplify fabrication. An extra cap layer of silicon dioxide is also introduced with a freely varying thickness t suitable for adjusting the phase of the emissivity spectrum. This gives rise to a total of four independent parameters (a, r, t, and b) for the initial optimization.

As shown in Fig. 4, it is found that substantial suppression of silicon emission can be achieved in the photonic bandgap region that extends approximately from 2.5 μm to 4.5 μm. At the same time, enhancement of the spectral emittance can take place for shorter wavelengths (λ < 2.5 μm). After optimization, it is found that projected power generation of the optimal layered structure jumps above 83.91 mW per cell, and the power generation efficiency approximately doubles to 2.042% (compared to a bare silicon wafer), representing an improvement in the overall figure of merit of 159%.

Fig. 4 Spectral emittance of four structures at 1000 K: a polished Si wafer (Fig. 3(a)), a polished Si wafer with a 4-bilayer 1D PhC (Fig. 3(b)), a polished Si wafer with tungsten and a 4-bilayer 1D PhC (Fig. 3(c)), and a platinum wafer with a 3-bilayer 1D PhC (similar to Fig. 3(c)). The efficiency, power, and overall figure of merit for each structure is listed in Table 2.

Adding in a thin layer of tungsten (W) with variable thickness w immediately above the silicon substrate, as depicted in Fig. 3(c), is projected to yield further performance enhancements. In particular, the projected power generation of the same TPV cell from before falls slightly to 69.01 mW per cell, but the power generation efficiency jumps dramatically to 2.912%, representing a cumulative improvement in the overall figure of merit of 204%.

Adding in an optically thick layer of platinum in lieu of tungsten (cf. Fig. 3(c)) actually yields the greatest performance enhancement, because by decreasing the radiated power to 48.65 mW per cell, it is also capable of achieving a dramatic efficiency improvement to 5.289%, for a 291% cumulative increase in the overall figure of merit relative to a plain silicon wafer. This data is summarized in Table 2.

Table 2. Predicted efficiency, power generation, and overall product figure of merit values for multiple μTPV emitter designs at 1000 K (view factor F = 0.4)

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Further improvements are projected to be possible via improvements in the temperature of operation and the view factor. For example, improving the view factor from 0.4 to the maximum value of 1 raises the projected efficiency of the optimized platinum-based structures to 13.22%. Furthermore, raising the temperature from 1000 K to 1200 K further increases efficiency to 21.7%. Note, however, that this efficiency neglects possible increases in the relative contributions of other losses such as convection, conduction, and enthalpic losses. Nonetheless, this represents a 20-fold improvement in efficiency over the initial silicon wafer design, and compares reasonably well with the theoretical maximum efficiency of 53.0% calculated for an idealized step-function emitter and single-junction PV material with identical cutoff wavelengths of λ = 2230 nm, which is only subject to radiative recombination [40

40. C. Henry, “Limiting efficiencies of ideal single and multiple energy gap terrestrial solar cells,” J. Appl. Phys. 51, 4494–4500 (1980). [CrossRef]

]. The remaining differences in efficiencies therefore come primarily from remaining wasted emission in the near-infrared in the platinum-based design, as well as slightly lower open-circuit voltages and fill factors caused by nonradiative recombination (primarily from bulk defects). While improving on the second issue is a demanding materials science problem, easier solutions are in principle available for the first problem of wasted infrared emission.

3.3. Rugate filters and selective emitters

The structures in the above section suffer from the common defect of emission in the mid-wavelength infrared (viz., 5–20 μm). One obvious approach to suppressing these wavelengths consists of combining a long-wavelength plasma filter in series with the aforementioned quarter-wave stack design [14

14. T. D. Rahmlow, D. M. DePoy, P. M. Fourspring, H. Ehsani, J. E. Lazo-Wasem, and E. J. Gratrix, “Development of front surface, spectral control filters with greater temperature stability for thermophotovoltaic energy conversion,” AIP Conf. Proc. 890, 59–67 (2007). [CrossRef]

]. However, these filters partially transmit wavelengths greater than 6 μm, and can fail at temperatures of 363 K and above. If one instead chirps the period of the quarter-wave stack, higher-order reflections can prevent emission at the short wavelengths needed for TPV power generation. However, the introduction of rugate filters can help suppress these higher-order reflections in a robust fashion. The simple principle behind them is to create a refractive index profile in optical thickness space that varies sinusoidally, so as to create a single pure Fourier component to which incoming light can couple to; the lack of any higher-order Fourier modes prevents reflection at higher frequencies [41

41. B. G. Bovard, “Rugate filter theory: an overview,” Appl. Opt. 32, 5427–5442 (1993). [CrossRef] [PubMed]

]. Thus, the introduction of rugate filters has the potential to increase efficiencies toward their theoretical single-junction limits [10

10. U. Ortabasi and B. Bovard, “Rugate technology for thermophotovoltaic applications: a new approach to near perfect filter performance,” AIP Conf. Proc. 653, 249–258 (2003). [CrossRef]

]. Because continuously varying refractive indices are challenging to fabricate (although possible in principle with nanoporous materials [42

42. J.-Q. Xi, M. F. Schubert, J. K. Kim, E. F. Schubert, M. Chen, S.-Y. Lin, W. Liu, and J. A. Smart, “Optical thin-film materials with low refractive index for broadband elimination of Fresnel reflection,” Nat. Photon. 1, 176–179 (2007).

]), we instead discretize each half-period a/2 of the sinusoid into m equal-thickness layers = 0,..., m – 1 with piecewise constant index n = (nmin + nmax)/2 + [(nminnmax)/2] sin[πℓ/(m – 1)].

Our optimization procedure is employed to optimize the efficiency of an emitter operating with a view factor of 1 at 1200 K with our realistic model of a TPV cell (with bandgap energy Eg = 0.547 eV, corresponding to a wavelength λ = 2230 nm). The independent parameters are the same four as for the first silicon/silicon dioxide chirped 1D PhC, with the number of materials in the rugate filter held constant at m = 6 and refractive indices ranging from 1.5 to 3.5. However, for this problem, the maximum number of periods is increased up to 40. We now find an optimal efficiency of 26.2%, representing a 21% improvement in relative efficiency compared to the optimized platinum structure. However, the spectrally averaged emittance for wavelengths below the bandgap remains relatively modest, at only 45.6% (corresponding to a power density of 319 mW/cm2).

3.4. Tungsten photonic crystal selective emitter

Fig. 6 (a) Side view of the tungsten 2D PhC selective emitter, consisting of partially open cylindrical cavities supporting multiple resonant modes with a low-frequency cutoff, arranged in a 2D square array. (b) The structure depicted in (a) plus a rugate filter (depicted here with 6 distinct materials and 6 periods of periodicity p) on top, separated by an air gap.

Because of its promising generic features, the combination of a rugate filter placed on top of a tungsten 2D PhC, separated by a small air gap (of at least 10 μm), as depicted in Fig. 6(b), was computationally optimized. This procedure includes all the independent parameters of the earlier rugate filter, plus three additional independent parameters for the 2D tungsten geometry (the radius, depth, and period of the cylindrical holes), for a total of seven independent parameters. In Fig. 7, its calculated emissivity is compared with the experimentally measured spectra of two non-optimized structures: a flat single-crystal tungsten wafer, and a 2D PhC with period a = 1.26 μm and radius r = 0.4 μm. The optimized structure has a larger period and radius than the latter structure, specifically a = 1.38 μm and r = 0.645 μm. This acts to red-shift the cutoff wavelength for the structure to a value appropriate for use in conjunction with a high-performing rugate filter and InGaAsSb TPV cell. Not surprisingly, the new cutoff of 2.3 μm is quite close to the bandgap wavelength for the TPV material.

Fig. 7 Emissivity spectrum of three tungsten structures: two experimentally measured (flat and a 2D PhC) and one computer-optimized (a 2D PhC with larger a and r).

Combining the optimized 2D tungsten PhC with an optimized rugate filter yields the spectral emittance displayed in Fig. 8 (assuming F = 1 and T = 1200 K). It is found that the power conversion efficiency stays approximately constant at 26.9%, while the average emittance for useful photons increases substantially, to 59.2%. This amounts to a 29.8% increase in power (per unit area) relative to the plain rugate filter by itself.

Fig. 8 Spectral emittance for combined tungsten 2D PhC and rugate filter. Emitted photons with wavelengths λ < 2.23 μm (depicted in blue) are capable of being absorbed by the InGaAsSb TPV device.

4. Solar Thermal TPV System

4.1. System design

A solar thermal TPV system is a variation on the standard TPV system, illustrated in Fig. 9, in which optical concentrators, such as parabolic mirrors or Fresnel lenses, are used to concentrate sunlight onto a selective absorber and emitter structure [45

45. W. Spirkl and H. Ries, “Solar thermophotovoltaics: an assessment,” J. Appl. Phys. 57, 4409–4414 (1985). [CrossRef]

49

49. E. Rephaeli and S. Fan, “Absorber and emitter for solar thermophotovoltaic systems to achieve efficiency exceeding the Shockley-Queisser limit,” Opt. Express 17, 15,145–15,159 (2009). [CrossRef]

]. The selective absorber is a structure designed to absorb solar radiation (as measured by the AM1.5 solar spectrum [50

50. ASTMG173-03, Standard Tables for Reference Solar Spectral Irradiances: Direct Normal and Hemispherical on 37 degree Tilted Surface (ASTM International, West Conshohocken, Pennsylvania, 2005). [PubMed]

]), but suppress thermal radiation induced by heating of the same structure. They are an integral part of various systems used to convert solar power into heat and/or electricity, such as solar water heaters, solar thermal power, and solar TPV power. In the case of solar TPV, the selective absorber is thermally coupled to the selective emitter, which allows the latter to reach the temperature necessary for most thermally radiated photons to match or exceed the semiconductor bandgap energy in the target TPV cell. The radiation subsequently passes through a filter, which recycles any low-energy photons, and then to the TPV cell, where electricity is generated. In short, solar thermal TPV uses sunlight as a heat source to perform the same basic physical conversion process as in Section 3. From that perspective, it is clear that the two halves of the overall solar thermal TPV system – the optical concentrator and selective absorber subsystem and the selective emitter and TPV cell subsystem – can be decoupled, with the output of the first half serving as input to the second half. In the following two subsections, each half is independently examined and optimized, starting with the optical concentrator and selective absorber subsystem, and concluding with the selective emitter and TPV cell subsystem.

Fig. 9 Diagram of a solar TPV system. Sunlight is collected via optical concentrators and sent to a selectively absorbing surface. That structure is thermally coupled to a selective emitter, which in conjunction with a filter, thermally emits photons with energies matched to the semiconductor bandgap of the TPV cell receiving them.

4.2. Semiconductor selective absorber

Several types of material structures are particularly suitable for selective absorption, such as intrinsic materials, semiconductor-metal tandems, multi-layer absorbers, metal-dielectric composite coatings, surface texturing, and coated blackbody-like absorbers [51

51. T. Sathiaraj, R. Thangarj, A. Sharbaty, M. Bhatnagar, and O. Agnihotri, “Ni-Al2O3 selective cermet coatings for photochemical conversion up to 500° C,” Thin Solid Films 190, 241 (1990). [CrossRef]

55

55. N. Sergeant, M. Agrawal, and P. Peumans, “High performance solar-selective absorbers using sub-wavelength gratings,” Opt. Express 18, 5525–5540 (2010). [CrossRef] [PubMed]

]. Among these, metal-dielectric composites are generally considered to have the greatest promise for high temperature applications (i.e., over 400 °C), with spectrally averaged absorbance of 0.94 and emittance of 0.07 for a single layer of graded Ni-Al2O3 cermet on stainless steel with an SiO2 AR coating at 500 °C [51

51. T. Sathiaraj, R. Thangarj, A. Sharbaty, M. Bhatnagar, and O. Agnihotri, “Ni-Al2O3 selective cermet coatings for photochemical conversion up to 500° C,” Thin Solid Films 190, 241 (1990). [CrossRef]

]. In second place are semiconductor-metal tandem structures, such as 0.5 μm germanium (Ge), 2.0 μm silicon, and an Si3N4 layer, which yields a weighted absorbance of 0.89 and emittance of 0.0545 at 500 °C.

In this section, we explore improvements to the semiconductor-metal tandems. The best way to combine solar absorbance and thermal emittance at a given temperature into a single figure of merit is to measure the thermal transfer efficiency ηt, given by the following expression [52

52. Q.-C. Zhang, “High efficiency Al-N cermet solar coatings with double cermet layer film structures,” J. Phys. D: Appl. Phys. 32, 1938–1944 (1999). [CrossRef]

]:
ηt=α¯ɛ¯σT4CI
(2)
where σ is the Stefan–Boltzmann constant, T is the operating temperature, C is the solar concentration ratio, i.e., the ratio of observed intensity to the solar intensity I (generally considered to be 1 kW/m2 under standard testing conditions [50

50. ASTMG173-03, Standard Tables for Reference Solar Spectral Irradiances: Direct Normal and Hemispherical on 37 degree Tilted Surface (ASTM International, West Conshohocken, Pennsylvania, 2005). [PubMed]

]), the spectrally averaged absorptivity of the selective surface is given by α¯=(1/I)0dλɛ(λ)dI/dλ, where dI/ is the spectral light intensity of the sun per unit wavelength under standard test conditions [50

50. ASTMG173-03, Standard Tables for Reference Solar Spectral Irradiances: Direct Normal and Hemispherical on 37 degree Tilted Surface (ASTM International, West Conshohocken, Pennsylvania, 2005). [PubMed]

], and its emissivity is given by:
ɛ¯=0dλɛ(λ)/{λ5[exp(hc/λkT)1]}0dλ/{λ5[exp(hc/λkT)1]}.
(3)

With the objective function defined above, we can then examine the performance of a perfect blackbody under certain conditions, then compare it to a semiconductor-metal tandem structure such as germanium and silver, then add an optimized single front-coating layer, then finally introduce a total of three dielectric layers in front and one behind. These latter three structures are displayed sequentially in Fig. 10.

Fig. 10 Three related semiconductor selective absorbers (a) germanium wafer on a silver substrate (b) previous with a single front coating layer (c) germanium on silver with a single dielectric back coating and three front coating layers in front.

Table 3. Selective absorber data for operation under unconcentrated light at 400 K

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To explore high-temperature applications, we follow the procedure outlined in Appendix B to compute the properties of silicon at 1000 K. With that data in hand, one can then employ structures based on those of Fig. 10 by substituting silicon for germanium. Silicon and silver alone at 1000 K (cf. Fig. 10(a)) yield a good match between the absorption cutoff and the solar spectrum, as shown in Fig. 13. As reported in Table 4, they offer performance 54% superior to that of an idealized blackbody when C = 100. Stronger short-wavelength absorption can be achieved by adding a single front coating (cf. Fig. 10(b)), as shown in Fig. 13. This addition yields 70.96% overall thermal transfer efficiency, 95% higher than a blackbody. Finally, using four gradually increasing index materials in front and one low index material in back (cf. Fig. 10(c)), yields 82.20% overall efficiency, 125% greater than a blackbody, and comparable to earlier efficiency numbers achieved for germanium at 400 K. The slightly lower performance can be attributed to the much greater overlap between the emission curves of the sun and a blackbody at 1000 K (compared to a blackbody at 400 K), as well as slightly weaker absorption from 1–2 μm than for the analogous structure in germanium at 400 K, which can be seen by comparing Fig. 13 with Fig. 11.

Fig. 13 Optimized emittance spectra of the semiconductor selective absorbers depicted in Fig. 10, with silicon substituted for germanium, designed for operation under concentrated AM1.5 sunlight at 1000 K and C = 100.
Fig. 11 Optimized emittance spectra of the semiconductor selective absorbers depicted in Fig. 10, designed for operation in unconcentrated AM1.5 sunlight at 400 K.

Table 4. Selective absorber data for operation under 100x concentrated light at 1000 K

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4.3. Selective emitter optical and TPV material joint design

Once the problem of generating heat is solved via a selective solar absorber, the remaining requirements are the same as before: to create a selective emitter system with thermal emission at desirable wavelengths. However, we extend the original work on the μTPV generator in the earlier section by allowing additional degrees of freedom for the energies of the TPV bandgap(s), and consider the system efficiency η defined previously to be our figure of merit this time, due to the fact that any such system will be much smaller than the system of concentrating mirrors – thus, space constraints can be removed. Performance characteristics can be projected for the idealized, physically reasonable case in which recombination is primarily radiative in nature (i.e., no surface or bulk non-radiative recombination is included). Mathematically, this corresponds to applying Eq. (1) with Jnr = 0 separately to each junction j with bandgap Egj; the total power is thus the sum of the power generated at each junction.

Fig. 14 Optimized emittance spectra for emitters at 2360 K (left) and 1300 K (right). The corresponding efficiencies are 54.2% and 44.7%, respectively.

It is also found that a tandem junction configuration has the ability to further improve performance. This corresponds to the optimization of before with an added bandgap parameter, subject to the constraint that the bandgap in front must have a higher energy bandgap than the one in back (otherwise, no useful photons would reach the junction in back). For an emitter at 2360 K, a dual bandgap structure with bandgaps of 1.01 eV and 0.82 eV yield a power conversion efficiency of 66.3% (22.3% higher than a single junction configuration), as is illustrated in Fig. 15. Even for an emitter of only 1000 K, the original regime in which efficiencies of 1% were observed (in Section 3), it is found that efficiencies can be maintained at a quite respectable level of 44.0% with a tandem-junction, thus representing a 45-fold improvement over the previously observed conversion efficiency of a plain silicon wafer with an InGaAsSb TPV cell. This substantially exceeds the Shockley–Quiesser limit for a single-junction PV cell of 31% without concentration (C = 1) or 37% under full concentration (C = 46200) [40

40. C. Henry, “Limiting efficiencies of ideal single and multiple energy gap terrestrial solar cells,” J. Appl. Phys. 51, 4494–4500 (1980). [CrossRef]

].

Fig. 15 Optimized emittance spectra for emitters at 2360 K (left) and 1000 K (right). The corresponding efficiencies are 66.3% and 44.0%, respectively.

5. Conclusions

By using two key examples, this manuscript has demonstrated that changing the photonic and electronic design of standard TPV systems can substantially enhance their performance. In particular, it was found that a μTPV generator with a relatively simple optical design can see its power conversion efficiency enhanced by up to a factor of 27 (to 26.2%) via changes in the selective emitter, adding a rugate filter, and retaining more heat (thus allowing the system to burn hotter than before – 1200 K instead of 1000 K – with the same fuel flow rate). Also, it was found that a solar TPV power system can concentrate and convert sunlight into electricity with an efficiency 45 times higher than previously found in experiment (44.7%) for a tandem junction TPV cell operating at 1000 K, through changes in both the photonic and electronic design parameters; this performance exceeds the Shockley-Quiesser limit for a single-junction solar cell under concentration. In short, TPV systems with properly chosen (i.e., optimized) photonic and electronic design elements offer extremely high theoretical efficiencies, as well as further unique advantages in reliability, portability, and power density.

Appendix A: Optimization Data

In this section, all of the fixed parameters, free variables, and figure of merit for every optimization is reported. Table 5 reports data for our selective emitter and TPV joint systems (note that the bandgap energies are fixed in the first 4 optimizations by the experimental InGaAsSb cell, and only allowed to vary in the last 4). Finally, Table 6 reports data for our solar selective absorbers, assumed to operate at various fixed temperatures T and AM1.5 solar concentrations C. Note that all designs are chosen to exhibit robustness in the presence of small disorder, i.e., changing any one optimization parameter by 1% should change the figure of merit less than that fractional amount.

Table 5. Selective emitter optimization results. Symbols are defined in the text; those with dimensions of length are quoted in nm, those with units of energy are quoted in eV, and those with dimensions of temperature are quoted in K. Note that different FOM values are not necessarily comparable

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Fig. 5 (Inset) Chirped rugate filter index as a function of position (using 6 materials) and (Main image) its emittance as a function of wavelength. Emitted photons with wavelengths λ < 2.23 μm (depicted in blue) are capable of being absorbed by the InGaAsSb TPV device.

Table 6. Selective absorber optimization results. Symbols are defined in the text; those with dimensions of length are quoted in nm and those with dimensions of temperature are quoted in K

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The operating temperatures projected for these systems range from 1000–1300 K. Proposed experimental future work includes structures made from silicon, silicon dioxide (quartz), tungsten, and platinum, which have melting points of 1687 K, 1923 K, 3695 K, and 2041 K, respectively. The calculations at 2360 K are only presented for informational purposes, and are not expected to be experimentally accessible in the near future.

Appendix B: High-temperature modeling

To calculate bandgap as a function of temperature, we use Varshni’s formula for electronic bandgaps, which is [56

56. Y. Varshni, “Temperature dependence of the energy gap in semiconductors,” Physica 34, 149–154 (1967). [CrossRef]

]:
Eg(T)=Eg(0)αT2T+β,
(4)
where Eg(0) is the bandgap at zero temperature, and α and β are empirical constants determined by experiment. For crystalline silicon, Eg(0) = 1.166 eV, α = 0.473 meV/K and β = 636 K; thus, the bandgap at 1000 K is expected to be approximately 0.88 eV, with significant absorption extending down to 0.7 eV, a value appropriate for selective solar absorption.

The specific form of the dispersion of the complex dielectric function of silicon as a function of temperature was studied by Ref. 57

57. C. Grein and S. John, “Polaronic band tails in disordered solids: combined effects of static randomness and electron-phonon interactions,” Phys. Rev. B 39, 1140 (1989). [CrossRef]

. The key insights of that work are that optical absorption can be modeled based on ab initio principles, and that there is an important connection between temperature and disorder. In particular, it is predicted that high temperatures will tend to smear out certain features over a broader frequency range. This approach can be used to predict the full dispersion relation at most temperatures below the melting point of the relevant material. The key prediction is that the imaginary part of the index will behave according to:
k(ω)={koexp[(h¯ωEf)/Eo],h¯ω<Efkoexp[(h¯ωEf)/αEo],Efh¯ω<Ef+2αEok1exp[β(h¯ωEg2αEo],Ef+2αEoh¯ω<Exk2h¯ωEx,h¯ωEx,
(5)
where ko, k1, k2, α, and β are temperature-independent material parameters determined by experiment, and Eo, Eg, Ef, and Ex are energies in the system displaying known empirically-determined temperature dependencies.

In Fig. 16, the dispersion of the imaginary part of the refractive index of crystalline silicon is modeled for room temperature (300 K) and shown to compare closely to experimental data reported in Ref. 25

25. C. Herzinger, B. Johs, W. McGahan, J. Woollam, and W. Paulson, “Ellipsometric determination of optical constants for silicon and thermally grown silicon dioxide via a multi-sample, multi-wavelength, multi-angle investigation,” J. Appl. Phys. 83, 3323–3336 (1998). [CrossRef]

. This model is then used to extrapolate the dispersion relation to a much higher temperature of 1000 K, and should hold for mono-, multi-, and poly-crystalline forms of silicon (but not amorphous silicon). That data can in turn can be employed in optimization of a crystalline silicon-based high-temperature selective absorber design.

Fig. 16 Model of the dispersion of the imaginary part of the refractive index for both T = 300 K, along with a comparison to experiment [25], and projected values for T = 1000 K.

Acknowledgments

The authors thank Nenad Miljkovic, Ananthanarayanan Veeraragavan, and Bo Zhen for valuable discussions. This work was supported in part by the MRSEC Program of the National Science Foundation under award number DMR-0819762, the MIT S3TEC Energy Research Frontier Center of the Department of Energy under Grant No. DE-SC0001299, and the Army Research Office through the Institute for Soldier Nanotechnologies under Contract Nos. DAAD-19-02-D0002 and W911NF-07-D0004.

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OCIS Codes
(350.6050) Other areas of optics : Solar energy
(230.5298) Optical devices : Photonic crystals

ToC Category:
Thermophotovoltaic

History
Original Manuscript: May 14, 2010
Revised Manuscript: July 15, 2010
Manuscript Accepted: July 16, 2010
Published: August 2, 2010

Citation
Peter Bermel, Michael Ghebrebrhan, Walker Chan, Yi Xiang Yeng, Mohammad Araghchini, Rafif Hamam, Christopher H. Marton, Klavs F. Jensen, Marin Soljačić, John D. Joannopoulos, Steven G. Johnson, and Ivan Celanovic, "Design and global optimization of high-efficiency thermophotovoltaic systems," Opt. Express 18, A314-A334 (2010)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-S3-A314


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References

  1. H. H. Kolm, “Solar-battery power source,” Tech. Rep., MIT Lincoln Laboratory, “Quarterly Progress Report,” Group 35, 13 (1956).
  2. B. Wedlock, “Thermo-photo-voltaic conversion,” Proc. IEEE 51, 694–698 (1963). [CrossRef]
  3. R. Black, P. Baldasaro, and G. Charache, “Thermophotovoltaics - development status and parametric considerations for power applications,” in International Conference on Thermoelectrics, 18, pp. 639–644 (1999).
  4. F. O’Sullivan, I. Celanovic, N. Jovanovic, J. Kassakian, S. Akiyama, and K. Wada, “Optical characteristics of 1D Si/SiO2 photonic crystals for thermophotovoltaic applications,” J. Appl. Phys. 97, 033529 (2005). [CrossRef]
  5. H. Xue, W. Yang, S. Chou, C. Shu, and Z. Li, “Microthermophotovoltaics power system for portable MEMS devices,” Nanoscale Microscale Thermophys. Eng. 9, 85–97 (2005). [CrossRef]
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