We report the design, optimization, and experimental results of large area commercial silicon solar cell based thermophotovoltaic (TPV) energy conversion systems. Using global non-linear optimization tools, we demonstrate theoretically a maximum radiative heat-to-electricity efficiency of 6.4% and a corresponding output electrical power density of 0.39 W cm−2 at temperature T = 1660 K when implementing both the optimized two-dimensional (2D) tantalum photonic crystal (PhC) selective emitter, and the optimized 1D tantalum pentoxide – silicon dioxide PhC cold-side selective filter. In addition, we have developed an experimental large area TPV test setup that enables accurate measurement of radiative heat-to-electricity efficiency for any emitter-filter-TPV cell combination of interest. In fact, the experimental results match extremely well with predictions of our numerical models. Our experimental setup achieved a maximum output electrical power density of 0.10W cm−2 and radiative heat-to-electricity efficiency of 1.18% at T = 1380 K using commercial wafer size back-contacted silicon solar cells.
© 2015 Optical Society of America
In thermophotovoltaic (TPV) energy conversion systems, direct conversion of thermal radiation to electricity is achieved via the photovoltaic (PV) effect [1, 2]. TPV energy conversion offers many advantages, including the promise of highly versatile, modular, and compact high power density energy conversion systems that have no moving parts, leading to quiet and robust operation. In addition, virtually any high grade heat source can be used, including waste heat , fossil fuels [3,4], radioisotopes [5–7], and solar energy [8–10]. Compared to conventional solar PV conversion, the heat source is significantly closer to the PV cell, resulting in photon flux and power density that are orders of magnitude higher. However, due to the much lower temperatures achievable in practical TPV systems (T < 2000 K), the majority of emitted photons lie in the near- to mid-infrared (IR) spectrum, hence it is widely acknowledged that the path to higher TPV efficiencies lies in the development of low bandgap TPV cells. In fact, many TPV system experimental efforts have been reported for gallium antimonide (GaSb) [4,9,11,12], indium gallium arsenide (InGaAs) [5,6,13], and indium gallium arsenide antimonide (InGaAsSb) cells [10,14]. In contrast, efforts to utilize silicon (Si) cells for TPV energy conversion systems have been virtually non-existent as the larger energy gap of Si demands higher operating temperatures for efficient energy conversion. However, the application of Si PV cells is interesting as they are abundant, cheap, and commercially available in large sizes. In addition, the technology is significantly more mature and closer to theoretical limits compared to low bandgap TPV cells.
To the best of our knowledge, only three experimental efforts have been reported to-date for Si cell based TPV (Si-TPV): Bracewell and Swanson reported a TPV efficiency of 10% (does not include cavity losses) with specially designed p-i-n Si cells and a blackbody cavity emitter at T = 2350 K ; Qiu et al. reported an output power density of 0.2W cm−2 using Sunpower solar cells specially designed for low solar concentrations and an ytterbia (Yb2O3) rare-earth selective emitter ; Bitnar et al. reported an overall fuel-to-electricity efficiency of 3.96% and a corresponding output electrical power density of 0.104 W cm−2 using solar cells developed by the University of New South Wales (UNSW) and a Yb2O3 rare-earth selective emitter at T > 1700 K . However, in all of these investigations, direct measurements of radiative heat-to-electricity efficiency that include cavity losses were not performed; this is vital in order to understand the main loss mechanisms in TPV energy conversion systems. Additionally, lower temperatures (T ≈ 1500 K) are more practical given the difficulties of engineering systems that are reliable over long time scales at high temperatures. In this investigation, we propose a combination of optimized two-dimensional (2D) metallic PhC selective emitters and 1D PhC based cold-side dielectric filters that will enable reasonable performance at lower temperatures. We will first discuss the methods used to obtain optimized designs of both the selective emitter and the selective filter. Performance predictions using a realistic TPV system level numerical model will then be presented. Following that, the experimental setup used to validate the numerical models as well as the measurements obtained will be discussed, before presenting our concluding remarks.
2. Design and optimization
2.1. Selective emitter: 2D metallic photonic crystals
The primary challenge of designing Si-TPV systems for a lower T = 1500 K lies in the small fraction of energy that is potentially convertible due to Si possessing a high energy gap (E g = 1.12 eV). For a greybody with emittance ɛ = 0.9, only 2.8% of the radiative energy is convertible. The remaining non-convertible photons emitted result in parasitic heat losses, which would also lead to highly undesirable elevated PV cell operating temperatures. Efficient spectral control is thus a necessity. Spectral control can be achieved firstly via the use of selective emitters to preferentially emit convertible photons. To date, various selective emitters have been investigated; from rare-earth oxides [18,19], to 1D [20,21], 2D [22–24], and 3D PhCs [25–27]. Here, we select the 2D tantalum (Ta) PhC as the selective emitter as this design offers a sharp emittance cutoff that is easily shifted and optimized [28,29], is scalable to large areas , and has been proven to be thermally stable at high temperatures in high vacuum conditions . The 2D Ta PhC consists of a square array of cylindrical holes with radius r, depth d, and period a etched onto an optically smooth Ta surface as shown in the inset of Fig. 1.
Rigorous coupled wave analysis (RCWA) methods  were used to obtain the reflectance of the 2D Ta PhCs at all angles of incidence, which allows us to infer the hemispherical emittance via Kirchoff’s law. The Lorentz–Drude model of Ta fitted to elevated temperature (T ∼ 1500 K) emittance  was used to capture the optical dispersion of Ta in the simulations. To ensure accuracy, the number of Fourier expansion orders were doubled until the results converged. We have also confirmed that simulations using conventional finite-difference time-domain (FDTD) methods  agree very well with RCWA formulations based on both polarization decomposition and normalized vector basis when more than 320 Fourier expansion orders are used.
Optimized designs of 2D Ta PhCs can be easily obtained using the formalism outlined in Ref. 29. The global optimization routines were implemented via NLOpt, a free software packaged developed at MIT that allows comparison between various global optimization algorithms . The following figure of merit was used for the optimization:35]. Using this, the optimized 2D Ta PhC design for our target operating temperature of T = 1500 K was obtained. As can be seen in Fig. 1, the emittance shows excellent match with the external quantum efficiency (EQE) of the UNSW Si cells. Figure 1 also shows the emittance of flat Ta with an optimized 85 nm anti-reflection coating of hafnium oxide (HfO2 ARC Flat Ta), which is an easily fabricated alternative. η TPV and J elec for all three selective emitters in an Si-TPV system with T = 1500 K and view factor F = 0.99 (achievable with 100 mm × 100 mm flat plate geometry with s = 500 μm) are shown in Table 1. As can be seen, the optimized 2D Ta PhC selective emitter enables greater than 100% improvement in η TPV over the greybody emitter (ɛ = 0.9), while still maintaining high of 0.76.
2.2. Cold-side selective filter: 1D dielectric photonic crystals
Another spectral control approach relies on recuperating non-convertible photons using front surface reflectors [36–38] and back surface reflectors [39, 40] on the PV cell. In this investigation, we consider a simple, experimentally realizable solution based on a variant of the quarter-wave stack 1D PhC: the exponentially chirped distributed Bragg reflector (DBR) . It is essentially a periodic quarter-wave stack with an exponentially varying period, such that the effective stop band is broadened. The period of the l-th stack is given by:Eqs. (2) and (5), we can see that only two parameters (λ c and b) define the exponentially chirped DBR, and thus the optical properties. Again, we used NLOpt to determine the optimum values for maximum FOM defined as follows:
In this investigation, we choose tantalum pentoxide (Ta2O5) and silicon dioxide (SiO2) with n of approximately 2.1 and 1.5 respectively; they are both highly transparent in the wavelength range 0.25 μm < λ < 5.0 μm, and are industry standard optical coatings [42, 43]. To ensure practical realization, we consider only 30 bilayers for the filter design. Using NLOpt with x = 0.15, the optimum DBR filter was determined to have λ c = 1.397 μm and b = 0.211. When combined with the optimized 2D Ta PhC, this filter enables an improvement of approximately 511% and 75% in η spec compared to respectively the greybody (ɛ = 0.9) and optimized 2D Ta PhC based TPV system without the filter, albeit at a slightly reduced η emit. The improvement is clearly seen in Fig. 2, whereby the effective spectral irradiance incident on the PV cell for 1.1μm < λ < 2.3 μm is mostly suppressed, i.e. reflected back to the emitter.
The optimized exponentially chirped DBR filter can further be improved via the needle synthesis optimization method . This algorithm is, in essence, a perturbative method. In this case, the original optimized exponentially chirped DBR filter was repeatedly modified via the addition or subtraction of a thin layer of material (≈ 10 nm to 20 nm) at a location which results in the greatest improvement in the FOM. The process was repeated until the FOM ceases to improve. By applying this on the optimized exponentially chirped DBR filter, a further 77% improvement in η spec is seen when coupled with the optimized 2D Ta PhC selective emitter. As can be seen in Fig. 2, the needle synthesis method improves on the original optimized exponentially chirped DBR filter by further reducing effective spectral irradiance incident on the PV cell for λ > 2.3 μm, albeit at a smaller penalty of allowing a small number of photons through for 1.1μm < λ < 2.3 μm.
2.3. Thermophotovotaic system performance
Using the numerical model described in Ref. 29, estimates of η TPV using UNSW Si PV cells and various emitter–filter combinations in F = 0.99 TPV systems were obtained; the results are illustrated in Fig. 3. As T increases, η TPV decreases primarily due to series resistance under high current injection, which is ≳ 5 times of that under standard AM1.5 solar irradiance. Thus, specially designed Si cells for low solar concentrations (C = 5–20) would be more suitable for TPV applications.
For the greybody (ɛ = 0.9), maximum radiative heat-to-electricity efficiency η TPV, max of 2.35% is achieved at T = 1640 K. By replacing the greybody with the optimized HfO2 ARC Ta or optimized 2D Ta PhC selective emitter, η TPV, max can be increased to 4.58% (at T = 1680 K) and 4.97% (at T = 1650 K) respectively. The highest η TPV, max of 6.44% (at T = 1660 K) is achieved with the optimized 2D Ta PhC and needle synthesis refined DBR filter combination. In this investigation, the target operating T is 1500 K; η TPV and J elec at T = 1500 K for various emitter and cold-side filter combinations of interest are shown in Table 2.
3. Experimental method and results
3.1. Solar cell packaging
In this investigation, we used state-of-the-art Sunpower solar cells, which are to-date the most efficient commercially available Si PV cells boasting a solar-to-electricity efficiency of 25.6% . One of the key reasons for superior efficiencies of Sunpower Si cells is the back-contacted design, i.e. all electrical connections are located at the back of the solar cell, thus eliminating shading losses that plague conventional front-contacted designs. For the particular cell we were using, 6 terminals (3 +ve and 3 −ve) of the size of 7.5mm × 7.5 mm are provided for electrical connections. All other areas need to be free of electrical connections to ensure no electrical shorts exists. Concurrently, for efficient TPV energy conversion, it is important to ensure that the Sunpower Si cell is in good thermal contact with the heat sink. In order to achieve both precisely placed electrical connections and excellent thermal path elsewhere, the Sunpower Si cell was mounted on top of a Bergquist HPL Thermal Clad. It is, in essence, similar to a printed circuit board, of which electrical connections are provided by copper (Cu) foils, while the remaining area is covered with a thin layer (38 μm thick) of high thermal conductivity (κ = 3.0 W m−1 K−1) dielectric material, all on top of a 1 mm thick Cu substrate. To adhere the Sunpower Si cell to the Bergquist HPL Thermal Clad, a layer of 3M 8805 thermally conductive (κ = 0.60 W m−1 K−1) electrically insulating pressure sensitive tape (125 μm thick) was used at areas requiring electrical insulation, while a small amount of electrically conducting silver-filled grease (AREMCO Heat-Away 641-EV) was applied on electrical contacts. Since the Bergquist HPL Thermal Clad is not perfectly flat, it was bonded onto a thicker (10 mm) aluminum (Al) substrate with Epotek’s H74 thermally conductive epoxy (κ = 1.25 W m−1 K−1). A thin layer of thermal grease (AREMCO Heat-Away 641-EV, κ = 5.58 W m−1 K−1) was then applied between the Al substrate and the heat sink. The bolting force coupled with the thicker and thus stiffer Al substrate ensures good thermal contact with the heat sink. Using this, we were able to cool the Sunpower Si cells to room temperature.
3.2. Thermophotovoltaic cavity design
The key parameters that we desire to measure are η TPV and output electrical power P elec. In order to measure η TPV, a precise calorimetric approach to accurately measure the net radiant power emitted P rad is necessary. This was achieved in the experimental setup shown in Fig. 4, which was specifically designed to accurately account for all energy transfers in the system; input electrical power P in to the heater (100 mm diameter HeatWave Labs 1200 °C UHV Heater) and P elec of the PV cell were measured simply using accurate voltage and current meters (Fluke 289 True RMS Meter, Fluke i310s Current Clamp); measurement of the parasitic conductive heat loss P cond was performed by monitoring the temperature difference of the bottom Al posts using two high-accuracy 100Ω Class A DIN Platinum 3-wire resistance temperature detectors (RTD); parasitic radiative heat loss of the Cu radiation shields were neglected since they are at much lower temperatures of ≲ 500 K, thus contributing ≲ 3% of the overall energy balance (in fact, ignoring this results in a more conservative measurement of η TPV, albeit at an acceptably small error). η TPV is then given by:
3.3. Thermophotovoltaic system measurements
In this investigation, we are focused on obtaining experimental results using just the optimized HfO2 ARC flat Ta emitter. This serves as a vital preliminary experimental investigation, and more importantly allows us to verify the numerical models presented in Ref. 29, before deciding to undertake significant efforts toward realizing large area 2D Ta PhCs, and fabrication and implementation of the needle synthesis refined DBR filter, both of which require efforts beyond the scope of this publication.
Stock flat 2.5 mm thick Ta sputtering targets of 100 mm diameter and were sourced from Shanghai Jiangxi Metals Co. The flat Ta wafer was then polished to a mean surface roughness of ≈ 5.94Å and surface flatness of ≈ 5.0 μm. The polished Ta wafer was then deposited with 85 nm of HfO2 using atomic layer deposition (Cambridge Nanotech Savannah 200). The thickness and refractive index of the HfO2 coating was verified using an ellipsometer (J. A. Woollam Co. M2000). In addition, ɛ of the HfO2 ARC Flat Ta measured indirectly using the FTIR (Nexus 870) reflectance accessory (PIKE Technologies VeeMAX II) with a known standard Al mirror (Thorlabs) was found to match extremely well with numerical predictions.
Accurate measurements of the top surface temperature of the emitter are of extreme importance, without which inhibits comparison of experimental data to numerical models. Here, we spot welded type K thermocouples on top of the emitter. Since the area of the weld covers ≲ 0.01% of the total area of the emitter, it is safe to assume that the perturbation is small, and thus this method is a reasonably accurate measurement of the top surface temperature. Indeed, the experimentally measured P elec shown in Fig. 5(b) with emitter-PV cell separation s = 2.0 mm shows excellent agreement with simulations. Highest recorded P elec = 7.6 W and η TPV = 1.18% was obtained at T = 1380 K; at this point the heater internals were at T = 1500 K, which is the heater’s maximum temperature rating, and thus higher temperatures are unattainable with this setup.
However, note that measured η TPV’s are ≈ 50% of numerical model predictions denoted by Ideal Setup in Fig. 5(a). In the Ideal Setup numerical simulations, the heater is assumed to only comprise of the selective emitter with diameter of 100 mm. However, as can be seen in the inset of Fig. 5(a), there are heat shields that contribute to the effective size of the heater such that the measured diameter is 118 mm, and more significantly 28% of the effective area is covered by the heat shields. The inner and outer heat shields comprise of inconel and stainless steel respectively. In fact, by including the area of the heat shields in simulations assuming reasonable parameters of ɛ = 0.9 and T ≈ 250 K cooler than the selective emitter top surface for the heat shields, we obtained simulation results that match extremely well with the experimental results as indicated by Current Setup numerical predictions in Fig. 5. As seen in Fig. 5(b), the heat shields contribute no P elec due to lower temperatures, but result in an increase in net radiative power absorbed by the PV cell by ≈ 80% compared to just the selective emitter alone.
In summary, we have investigated Si PV cells for TPV applications as they are inexpensive and commercially available in large sizes, and the technology is significantly more mature and closer to theoretical limits compared to low bandgap TPV cells. Using global non-linear optimization tools, we have demonstrated theoretically a maximum radiant heat-to-electricity efficiency η TPV of 6.4% and output electrical power density J elec = 0.4 W cm−2 at T = 1660 K when implementing both the optimized 2D Ta PhC selective emitter, and the needle synthesis refined DBR cold-side selective filter.
In addition, we have developed an experimental large area TPV test setup that enables accurate measurement of η TPV for any emitter-filter-TPV cell combination of interest. Our experimental setup achieved a maximum electrical power output P elec of 7.6 W and η TPV of 1.18% at T = 1380 K using standard wafer size back-contacted Sunpower solar cells. The experimental results agree extremely well with numerical predictions.
This work is partially supported by the Army Research Office through the Institute for Soldier Nanotechnologies under Contract No. W911NF-13-D-0001. Y. X. Y., W. R. C., V. S., and M. S. are partially supported by the Solid-State Solar-Thermal Energy Conversion Center (S3TEC) Energy Research Frontier Center (EFRC) of the Department of Energy under Grant No. DE-SC0001299. V. R. gratefully acknowledges funding by the Austrian Science Fund (FWF): J3161-N20.
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