Radioisotope Thermophotovoltaic Generator Design and Performance Estimates for Terrestrial Applications The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. 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 Version Final published version 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. 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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 out- put 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 re- mote places, they are actually accessible for installation and oc- ∗Address all correspondence to this author. 1 Copyright © 2017 ASME 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- 2 Copyright © 2017 ASME 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 emit- ter. 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.