Radioisotope Thermophotovoltaic Generator Design and Performance Estimates for Terrestrial Applications

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Citation Wang, Xiawa, Walker Chan, Veronika Stelmakh, and Peter Fisher. “Radioisotope Thermophotovoltaic Generator Design and Performance Estimates for Terrestrial Applications.” 2017 25th International Conference on Nuclear Engineering July 2–6, 2017 Shanghai, China. Volume 3: Nuclear Fuel and Material, Reactor Physics and Transport Theory; Innovative Nuclear Power Plant Design and New Technology Application (July 2, 2017). doi:10.1115/ icone25-66607.

As Published http://dx.doi.org/10.1115/ICONE25-66607

Publisher ASME International

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Citable link https://hdl.handle.net/1721.1/124353

Terms of Use Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use. Proceedings of the 2017 25th International Conference on Nuclear Engineering ICONE25 July 2-6, 2017, Shanghai, China

ICONE25-66607

RADIOISOTOPE THERMOPHOTOVOLTAIC GENERATOR DESIGN AND PERFORMANCE ESTIMATES FOR TERRESTRIAL APPLICATIONS

Xiawa Wang∗ Walker Chan Peter Fisher Institute for Soldier Nanotechnologies Veronika Stelmakh Electrical Engineering Department Institute for Soldier Nanotechnologies Physics Department Massachusetts Institute of Technology Massachusetts Institute of Technology Massachusetts Institute of Technology Cambridge, Massachusetts 02139 Cambridge, Massachusetts, 02139 Cambridge, Massachusetts, 02139 Email: [email protected]

ABSTRACT radioisotope generators are mostly used in polar areas, floating This work provides the design methods and performance es- buoys on water, and underwater in deep ocean floor to power de- timates of the radioisotope thermophotovoltaic system (RTPV) vices like transmitters, sensors, and coast guard lights. In the for terrestrial applications. The modeling is based on an ex- 1960s, several modules were successfully launched in the polar perimentally tested prototype using two-dimensional high tem- regions and had demonstrated for as long as 11 years of continu- perature photonic crystal to realize spectral control. The design ous operation. In water applications, the power generators were efforts focus on the optimization of the system efficiency and con- tested and used in a wide range of projects, from sea surface to as tain the heat source number, the size of the energy conversion ele- deep as 2200 feet on the ocean floor [1]. Table 1 shows some of ments, the insulation configuration, and the heat sink design. An the terrestrial thermal-based radioisotope generators developed equivalent circuit model was developed for the thermal and elec- and used before [1, 2]. trical performances. Based on a specific output requirement, an In comparison to the space generators, most of the terrestri- optimized heat source number and energy conversion area can al ones are smaller, have shorter lifetime, and lower output level. be computed for a certain cell type and insulation design. The The most common output level ranges from several watts to tens selection and characterization of the low bandgap thermopho- of watts, with only 1 - 2 modules exceeding 100 Watts. In the- tovoltaic (TPV) cells applicable to the generator are compared ory, the power generator can be designed and implemented into and discussed. The generator’s heat sink design uses extended any output level. Practically, it is convenient to design the sys- fins and the performance is estimated based on the external op- tem as a standard power module with the output level around erating conditions. Finally, the work provides a design example ∼40 W that can satisfy the power budget in most cases. If of a terrestrial RTPV generator with an output level of ∼40 W e more power is needed, multiple modules can be used together electrical power (W ) using InGaAsSb cell, reaching an efficien- e to satisfy the output requirements. The system’s critical figure cy of 8.26%. of merit is the efficiency, which is the ratio of the electrical output power to the decay heat released by the fuels. Reducing the INTRODUCTION system weight becomes a lower priority in comparison to their Thermal-based nuclear power generators can provide elec- space counterparts because of the lower cost of deployment. On trical energy for interstellar probes and remote terrestrial sensors the other hand, terrestrial generators require a higher degree of for decades when solar energy is not available. In land missions, tamper-proof and shielding to prevent radiation contamination and vandalism. Even though the power modules are used in remote places, they are actually accessible for installation and oc-

∗Address all correspondence to this author.

Downloaded From: https://proceedings.asmedigitalcollection.asme.org on 03/21/2019 Terms of Use: http://www.asme.org/about-asme/terms-of-use Output power Model Year Application Location [W] NAP-100 1960 Prototyping 131 Unfueled Weather Station 1961 Transmitter 5 Arctic region Generator SNAP-7A 1961 Coast ﬂashing light 11.6 Buoy in Curtis Bay SNAP-7B 1963 Coast guard light 68 Navy ﬂoating buoy Underwater SNAP-7E 1962 6.5 Atlantic Ocean bottom acoustic beacon Sentinel 21A 1966 Oceanographic sensor 25 Island in Bering Strait SNAP-21 1976 Sensor and telemetry 10 Antarctica URIPS-8 1976 Transmitter 8 Antarctica Sentinel-25C1 1977 Transmitter 25 San Juan Seamount Sentinel-100F 1974 Quartz clock timer 125 Eleuthera Island, Bahamas

TABLE 1: RADIOISOTOPE GENERATORS FOR TERRESTRIAL SURFACE AND UNDERWATER APPLICATIONS.

casional maintenance. In thermal-based radioisotope power generators, plutonium- 238, which is an alpha emitter with a large decay heat, is used as the fuel source. The fuels are processed and pressed into a pellet containing 151 g radioactive materials and encapsulated in the iridium clads [3]. Four fuel pellets are further packaged into the rectangular shaped general purpose heat source (GPH- S) that releases ∼250 W thermal power (Wt) at the beginning of the mission. A radioisotope generator’s thermal source normally contains one or multiple GPHS units stacked together and oper- FIGURE 1: RTPV GENERATOR USING PHOTONIC ates at a temperature from 800 - 1200◦C. The thermal energy is CRYSTAL SPECTRAL CONTROL. converted to electricity by various ways, such as thermal-electric materials, Stirling engines, alkali-metal thermal to electrical con- verters (AMTEC), and thermophotovoltaics [3–6]. THERMAL AND ELECTRICAL DESIGN The radioisotope thermophotovoltaic system, abbreviated as Prototype Configuration (RTPV), uses the infrared emission from the high temperature The configuration and measurement results of the radioiso- emitter attached to the heat source to generate electricity by the tope thermophotovoltaic prototype system was detailed in [8]. low-bandgap thermophotovoltaic (TPV) cells. A high efficiency The system setup is shown in Figure 2. An electrical button system depends on high performance TPV cells and good ther- heater was enclosed in a cylindrical Inconel case equivalent to mal management to drive the heat to the emitter to get converted. the size of one plutonia fuel pellet. The heater was connected Spectral control is one approach to improve the system efficiency through a hollow Inconel tube, which was supported by a stain- by shaping the emission spectrum of the high temperature emitter less steel plate fixed on the vacuum flange. A 1 cm2 square stand patterned with periodic micro-fabricated cavities [7]. The emis- extrudes from the heater holding the emitter. An InGaAsSb cell sion spectrum is shaped so that more convertible photons above having a bandgap of 0.55 eV with an area of 1 cm2 was placed the cell bandgap are emitted while the radiation in the far in- 1.2 mm away from the emitter as the energy conversion elemen- frared is suppressed to reduce the waste heat as shown in Figure t. The heater was insulated by multilayer insulation (MLI) made 1. With spectral control and high performance cells, an RTPV with copper foil separated by zirconia powder to reduce heat loss. system is expected to reach higher efficiency than the current- The emitter reached 950◦C while the TPV cell was maintained ly deployed radioisotope thermoelectric generators (RTGs) us- at room temperature. The generated electrical power and the re- ing Seebeck materials. The modeling in this work is based on quired heat input were recorded to compute the system efficiency. an experimentally tested prototype that has resolved the materi- We observed that spectral control using the photonic crystal was al compatibility issues and demonstrated the benefits of spectral four times more efficient by comparing the measurement results control [8]. using the selective emitter and a flat tantalum emitter. The anal-

Downloaded From: https://proceedings.asmedigitalcollection.asme.org on 03/21/2019 Terms of Use: http://www.asme.org/about-asme/terms-of-use Power vs. temperature for source and loads 600 Source Power (2 GPHS) Emitter power A = 45 cm2 550 emitter Emitter power A = 55 cm2 emitter Emitter power A = 65 cm2 500 emitter

450

400 Power [W] 350

300 FIGURE 2: RTPV PROTOTYPE CONFIGURATION. After [8] 250

200 900 950 1000 1050 1100 1150 Heater temperature [°C]

FIGURE 4: IMPEDANCE MATCH OF THERMAL MODEL.

er the maximum amount of power to the selective emitter, the source and load impedance must be matched. Figure 4 shows the effective thermal power delivered to the emitter from the fuel source and the required emissive power from a given area emitter. For a fixed amount of effective heat flow at a given heater temperature, a matching impedance always exists by varying the emitter area. The next step is to find the optimal crossing point to FIGURE 3: EQUIVALENT CIRCUIT MODEL OF RTPV ensure that the TPV cell is operating at its maximum efficiency. GENERATOR.

TPV Cell Selection and Characterization ysis in this work is based on the system modeling, which was Choices of TPV Cells. TPV cell is the energy conver- constructed based on the prototype measurement. sion element and its performance is one of the most critical factors to determine a high efficiency system. For a typical TPV system, the heater normally operates from 800◦C - 1500◦C, which Equivalent Circuit Modeling corresponds to the desirable cell bandgap from 0.45 eV - 0.77 eV. The thermal and electrical performance of an RTPV genera- Cells with the proper bandgap include the germanium cell and tor can be modeled by the equivalent circuit as shown in Figure the ones from the III-V group, including their ternary and quar- 3. tery alloys. Figure 5 shows the blackbody radiation spectra at The GPHS units function as the thermal current sources with various temperatures and the corresponding cutoff wavelengths a fixed heat flow rate depending on the number of units used. for typical TPV cells. The parasitic heat loss Qparasitic describes the heat loss through Germanium cell has the proper bandgap and was used in the radiative barrier surrounding the ineffective surfaces of the early TPV research. However, pure germanium cells are limit- heat source. The radiative barrier is made of the multilayer insu- ed by the difficulty in passivating the cell surface and the poor lations (MLI) consisting of highly reflective metal foils separat- electrical performance caused by the high intrinsic carrier con- ed by low outgassing zirconia powder. The thermal impedance centration. The exceptionally high carrier concentration leads to Zcavity describes the heat loss through the optical cavity. In or- a reduced open-circuit voltage and a high level reverse saturation der to achieve a high view factor, the cell and the emitter are current [9]. Even though not commonly used, efforts to make placed as close as possible to ensure that most of the radiation high-performance and low-cost germanium cells suitable for T- is incident on the cell and gets converted. The heat flow Qemitter PV application are still ongoing [10]. is the effective radiation that’s incident on the cell to generate TPV cells made from the III-V compound semiconductors electricity. The thermal impedances Zparasitic and Zcavity are the have shown great performances in thermal energy conversion. source impedance and Zemitter is the load impedance. To deliv- GaSb cell is commercially produced and is the most widely used

Downloaded From: https://proceedings.asmedigitalcollection.asme.org on 03/21/2019 Terms of Use: http://www.asme.org/about-asme/terms-of-use Blackbody Radiation Temperature and TPV Cell Bandgap Spectral Radiance of Various Emitters 0.8 40

2D Photonic Crystal Selective Emitter

] 0.7 35 ]

-1 MgO Selective Emitter Ge -1 Blackbody Emitter nm · 0.6 nm 30 · Graybody Emitter -2 InGaAsSb InAs -2 m · Si m · -1 0.5 GaSb -1 25 sr sr · ·

0.4 T = 2000°C 20

0.3 15 T = 1500°C 0.2 10 Spectral radiance [W

Spectral Radiance [kW ° 0.1 T = 1200 C 5 T = 1000°C 0 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 1 2 3 4 5 6 7 8 9 Wavelength [µm] Wavelength [µm]

FIGURE 5: CELL BANDGAP AND CORRESPONDING FIGURE 6: EMISSION SPECTRA OF DIFFERENT TYPES BLACKBODY RADIATION SPECTRA. OF EMITTERS.

one. It has a bandgap of 0.726 eV, which corresponds to a heater region, and eventually to the contacts to form a current through temperature at 1400◦C. The cell is fabricated using zinc diffusion external load. The TPV cell efficiency ηcell is defined as the ratio in a GaSb wafer to create a built-in electric field that improves of electrical output power to the thermal power from the emitter, carrier collection and reduces junction reverse current. InGaAsS- which is dependent on the emitter’s spectrum, the cell’s quantum b cells have a even lower bandgap at 0.55 eV without sacrificing efficiency, and the cell’s intrinsic electrical properties. too much electrical performance [11]. The cell is a great candi- The TPV cells are often characterized by a emitter with a date for a TPV system since it reduces the heater temperature to known spectrum matched to the bandgap of the cell, such as a around 1000◦C. However, the fabrication of the cell is challeng- near-blackbody emitter, a graybody emitter, or a selective emit- ing and expensive. High performance InGaAsSb cells are only ter. The blackbody and graybody are broadband emitters that demonstrated in a few labs without any further plan for large- have equal emissivity over the entire spectrum. A selective emit- scale production. The In Ga As cell is a ternary from the III-V x 1-x ter’s spectrum, on the other hand, can be tailored for different group with a bandgap changeable between 0.6 eV - 0.74 eV by cells and generally exhibits higher emission for photons above varying the composition ratio x. InGaAs cells fabricated on In- the cell bandgap and high reflectance for the unconvertible ones. P wafers have demonstrated over 20% efficiency with a radiator Figure 6 shows the emission spectra of different types of emitter- temperature of 1039◦C [12]. With the semi-insulating properties s. of the InP substrate, the cells can be further developed into the The intrinsic factors that determine the cell efficiency in- monolithic interconnected module (MIM), enabling high-voltage clude the external quantum efficiency (EQE) and the parasitic and large-area devices [13]. resistances. The EQE is the cell’s wavelength dependent spectral Other TPV cells, such as the InAs or InAsSbP ones, have response to the incoming radiation and affects the generated pho- even lower bandgaps and extend the spectral response to longer tocurrent in the cell. A high EQE above the cell bandgap means wavelength. However, Auger recombination is a fundamental that most of the convertible photons are absorbed and contribute limit in these extreme low-bandgap cells so that the open-circuit to the external current. Practical approaches to improve the cell’s voltage is very low. As a result, these materials are mostly added EQE include coating the cell with anti-reflective layers, passivat- to the GaSb or InGaAs cells to create a tandem cell [14]. Table 2 ing the cell surface, and adding rare-earth elements to lead to lists the properties of commonly used TPV cells. impurity gettering effect within the cell bulk [15–17]. An EQE level in the range of 0.5 - 0.6 is very common, but 0.8 - 0.9 is Characterization of TPV Cells. The working mecha- possible for very high quality cells [18–20]. The series and shunt nism of a TPV cell is similar to a solar cell. The infrared photons resistances are the parasitic resistances depending on the cell’s from the emitter strike the p-n junction to generate electrical car- fabrication and packaging. The series resistance is the added re- riers. The minority carriers in each region drift over the depletion sistance that comes in the path of the current, and includes the region by the built-in potential, diffuse across the quasi-neutral cell’s emitter-base contacts, the busbar, and the wire bonding.

Downloaded From: https://proceedings.asmedigitalcollection.asme.org on 03/21/2019 Terms of Use: http://www.asme.org/about-asme/terms-of-use Bandgap @20 ◦C Center TPV Cell Types Challenges [eV] Wavelength [µm] Si 1.2 1.03 High bandgap Ge 0.66 1.88 Poor electrical performance GaSb 0.726 1.71 Fabrication InGaAs 0.6-0.75 2.07-1.65 Fabrication InGaAsSb 0.55 2.25 Fabrication InAs 0.354 3.50 Auger recombination InAsSbP 0.39 3.18 Auger recombination

TABLE 2: PROPERTIES OF COMMON TPV CELLS.

The shunt resistance is the leakage across the p-n junctions that TPV Cell Efficiency Chart can be caused by the impurities in the junction region or a short- 8%

age along the peripheral of the cell. A high series resistance or a 7% low shunt resistance need to be avoided for a high performance cell. 6% In TPV application, it is useful to convert different cells’ 5% performances to the same illumination condition to have a fair

comparison. Blackbody radiator is more suitable for this task be- 4% cause it has a fixed illumination condition at a given temperature and often can give enough information about the cell’s spectral Cell Efficiency 3% response and conversion efficiency. In Figure 7, several reported InGaAs data by Tan 2% InGaAs data by Wilt measured efficiencies under blackbody illumination are summa- GaSb by Tang GaSb by JX Crystals rized for the three types of most commonly used TPV cells: GaS- 1% GaSb cell by Fraunhofer b, InGaAs, and InGaAsSb [18,21–24]. Some of the data reported InGaAsSb cell by Lincoln Lab 0% in literature are converted so that the cell’s efficiency is calcu- 800 900 1000 1100 1200 1300 1400 1500 lated over the entire blackbody spectrum without any cutoff or Blackbody Radiator Temperature [°C] spectral filters. The data in dashed lines are plotted because the cells’ parameters (including EQE, series and shunt resistances, FIGURE 7: EFFICIENCY OF DIFFERENT TPV CELLS dark current level, etc) are completely specified [18]. The anal- ILLUMINATED BY BLACKBODY RADIATORS. ysis in the following work is also based on these two cells with detailed characterization. Most of the cells under blackbody illumination achieve an efficiency below 10%. With spectral control points highlighted. For an RTPV system with an output level using selective emitters, the cell efficiency can normally show around 40 We, 2 GPHS units are needed if InGaAsSb cells are 50% improvement over the blackbody [8]. used. The system is expected to produce 41.3 We with a system efficiency at 8.26%. If GaSb cells are used, 3 GPHS units will be necessary and system is expected to output 38.1 We at an Efficiency of RTPV Generator efficiency of 5.1%. In the terrestrial RTPV generator design, the number of GPHS unit is chosen from 1 - 4 based on the output requiremen- t. The emitter area was simulated to find the optimal efficiency HEAT SINK DESIGN point for systems based on GaSb cell and InGaAsSb cell. The For RTPV generators, the TPV cell needs to be maintained temperature of the heater as a function of the GPHS number and at or below room temperature for optimal performance. In ter- the emitter area is shown in Figure 8. As expected, the tempera- restrial applications, the generator is mostly used in cold air or ture increases monotonically with the number of GPHS units and water and the heat rejection mainly relies on external convective a reducing emitter area. cooling. Therefore, the heat sink is often equipped with short- By varying the size of the emitter, an optimal ηsystem can be er and more closely spaced fins to maximize the convective heat achieved for an RTPV system having any number of GPHS units transfer area with the external environment. Figure 11 shows the and any type of TPV cell. Figure 9 and 10 show the overall 3D high power SNAP-7B generator with its short fins for waste heat surface plots of the output power with the maximum efficiency rejection [2].

Downloaded From: https://proceedings.asmedigitalcollection.asme.org on 03/21/2019 Terms of Use: http://www.asme.org/about-asme/terms-of-use Heater temperature for terrestrial generator Output power performance based on GaSb cells

50 1400

1300 40 Maximum efficiency points C]

° 1200 30 1100

1000 20 Output power [W] 900

800 10

Heater temperature [ 4 700 120 3 600 Number of heat source 100 4 80 2 60 2 3.5 40 ] 250 3 20 Number of GPHS units 200 1 Emitter area [cm 2.5 150 2 ] 2 100 1.5 50 1 0 Selective emitter area [cm FIGURE 10: OUTPUT POWER BASED ON GaSb CELLS.

FIGURE 8: EMITTER TEMPERATURE AS A FUNCTION OF HEAT SOURCE NUMBER AND EMITTER AREA.

Output power performance based on InGaAsSb cells

60 Maximum efficiency points 50

30 Output power [W]

4 FIGURE 11: HEAT SINK FIN DESIGN FOR WASTE HEAT 250 3 Number of heat source 200 REJECTION. 150 2 2 100 ] 50 1 Emitter area [cm hA ≡ kP characterizes the dominant mode of heat transport through the fins. When Bi 1, which is realistic in most cases, the fin FIGURE 9: OUTPUT POWER BASED ON InGaAsSb CELLS. temperature is mostly uniform in the transverse direction and the problem reduces√ to one-dimensional. The heat transfer rate is In the cold polar regions, the average air temperature ranges solved as Q = hPAk(Tchassis − Texternal) ∗ tanh(mL). Here h is from -50◦C - 3◦C year round with an average wind speed of 4 - 6 the convective cooling coefficient, A is the cross section of the m/s corresponding to a convective heat transfer coefficient from fin, P is the cross section perimeter, k is the thermal conductivity 10 - 100 W/m2K. In water applications, the typical temperature of the fin material, Tchassis and Texternal are the temperature of the ◦ ◦ chassis and the external environment, and m is the dimension- of sea surface water ranges from 0 C - 26 C. As depth increases, q ◦ ◦ hP the water gets colder and eventually reaches 0 C - 3 C in deep less number m = kA . For the 250 W heat generated by one ocean. The convective coefficient of water is much higher than GPHS unit, figure 12 shows the fin’s heat dissipation capability air and generally has a value of 500 - 3000 W/m2K [25]. as a function of convective heat transfer coefficient and the rejec- The convective cooling through extended fins is a very well tion temperature assuming nickel alloys as the fin material. The studied heat transfer case. The dimensionless Biot number Bi generator is assumed to be equipped with 40 fins with each one

Downloaded From: https://proceedings.asmedigitalcollection.asme.org on 03/21/2019 Terms of Use: http://www.asme.org/about-asme/terms-of-use -5 Fin cooling effect as function of external environment is a vacuum chamber with a pressure of around 10 Torr made with nickel alloys. The material is selected because of its rigidity and seawater compatibility [26]. Fin radiators are added on the chassis sides and on the top surface containing the TPV cell to facilitate direct cell cooling. 400 The design parameters and the performance estimates are given in Table 3. The generator has enough cooling capability 300 assuming a rejection temperature of -10◦C and a convective cool- 2 200 ing coefficient of 25 W/m K. The system is expected to output 41.3 We at an efficiency of 8.26%. The major limitation comes 100 Dissipated heat [W] from the TPV cell efficiency, which is ∼10% with the selective emitter in the modeling in this work. Convective60 heat transfer coefficient [W/m 50 10 40 0 30 -10 °C] CONCLUSION 20 -20 10 -30 The work provides the design methods for the terrestrial 2 Rejection temperature [ K] radioisotope thermophotovoltaic system (RTPV) using photonic crystal spectral control. The system modeling is based on an FIGURE 12: FIN PERFORMANCE AS A FUNCTION OF experimentally tested prototype. The thermal and electrical per- REJECTION TEMPERATURE AND CONVECTIVE HEAT formances are designed based on an equivalent circuit model to TRANSFER COEFFICIENT. find the optimal conversion efficiency. The work further extend- s to the TPV cell selection and the heat sink configuration. In the end, we gave a design example of a generator outputting ∼40 extending L = 3 cm in length and having a thickness of 1 mm and We with an expected efficiency at 8.26% for water/air application a height of 20 cm. with a rejection temperature lower than -10◦C and a convective If the generator is used in water and cold air, the outer shel- cooling coefficient higher than 25 W/m2K. l plus the cooling fins are sufficient to keep the chassis at near room temperature. The cooling only becomes challenging when the generator is used in dry and hot air. For example, when the ACKNOWLEDGMENT rejection temperature is higher than -10◦C or when the heat trans- This work was partially supported by the Army Research fer coefficient is less than 20 W/m2K, additional cooling, such as Office through the Institute for Soldier Nanotechnologies under adding more fins or a forced convection fan , is necessary to help Contract No. W911NF-07-D0004, the MAST contract 892730 with the heat dissipation. In the very extreme case, the generator and by the S3TEC DE-SC0001299. can also be immersed in a circulating cooling liquid to help with heat removal. However, active cooling needs to be avoided as much as possible because of the power consumption.

RTPV SYSTEM DESIGN EXAMPLE This section provides a design example of a module RTPV generator with an output of ∼40 We based on InGaAsSb cells as shown in Figure 13. The system requires two GPHS units to provide 500 Wt decay heat. The heat source is housed inside a thin-shelled Inconel canister with an emitter stand extruded from one surface. The photonic crystal selective emitter is brazed onto the stand. A circular Inconel support is designed to be welded or brazed to the metal canister and the external chassis to sustain the weight of the heat source. Multilayer insulation made with highly reﬂective foils separated by zirconia powder is used to cover the ineffective areas of the heat source with cutouts for the emitters and the support. The TPV cell is mounted on a copper substrate directly attached to the generator chassis. The chassis

Downloaded From: https://proceedings.asmedigitalcollection.asme.org on 03/21/2019 Terms of Use: http://www.asme.org/about-asme/terms-of-use Label Description Material T-ES Extrusion stand Inconel T-PhC Photonic crystal Tantalum T-CM Cell mount Copper T-TPV TPV cell InGaAsSb T-TF Top ﬁn Nickel alloy T-HS Heat source canister Inconel T-OS Outer shell Nickel alloy T-IS Support Inconel T-RF Radial ﬁn Nickel alloy

FIGURE 13: RTPV MODULE POWERED BY 2 GPHS UNITS FOR TERRESTRIAL APPLICATION.

Design parameters Function Components Materials Critical dimension Parameters GPHS Pu-238 Number of units 2 Canister Inconel Thickness 1 mm Heat source Height 3 mm Emitter stand Inconel Area 87.9 cm2 Emitter TaW alloy Area 87.9 cm2 Energy conversion TPV cell InGaAsSb Area 87.9 cm2 elements TPV cell mount Copper Thickness 1 mm Layer number 60 MLI Layers Copper foil Insulation Foil thickness 0.001’ (0.025 mm) MLI spacer Zirconia powder Particle size 30 µm Thickness 2 mm Support Inconel Support and housing Perimeter 5 cm Chassis Nickel alloy Thickness 5 mm Length 3 cm Thickness 1 mm Heat sink Fin Nickel alloy Height 30 cm Fin number 40 Performance estimates Output power 41.3 W Heater temperature 945◦C Chassis temperature 20◦C System efﬁciency 8.26%

TABLE 3: RTPV TERRESTRIAL GENERATOR DESIGN PARAMETERS AND PERFORMANCE ESTIMATES.

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