Radioisotope Thermophotovoltaic Research Articles

Influence of roughness and temperature on the emissivity of molybdenum in RTPV system

In this paper, the effect of different surface roughness and temperature on molybdenum's emissivity was studied. We measured and analyzed the emissivity of molybdenum with surface roughness ranging from 5 to 103 nm at 25, 600, 800, and 1030 °C. The emissivity of molybdenum increases with the temperature and surface roughness. For molybdenum with a flat surface, the emissivity is relatively low within the temperature range of 600–1000 ℃, with an average emissivity of less than 0.1. Whereas for samples with large roughness, the average emissivity is greater than 0.1 and changes significantly with temperature. Considering the cutoff wavelength of 2.2 μm for InGaAs cell, the proportion of the spectrum emitted by the samples at 600 ℃ in the 1–2.2 μm is around 10 %. At 1030 °C, the proportion reaches 44.1 % for the flat molybdenum and gradually declines with the surface roughness. Moreover, the effect of the surface roughness of molybdenum on the radioisotope thermophotovoltaic (RTPV) system was analyzed. The results show that a molybdenum emitter with larger roughness can improve the conversion efficiency of the RTPV system.

Optimized design and application performance analysis of heat recovery hybrid system for radioisotope thermophotovoltaic based on thermoelectric heat dissipation

Radioisotope thermophotovoltaic (RTPV) generators face a fundamental challenge, which is the significant output loss and energy wastage caused by the substantial thermal deposition resulting from the arrival of infrared radiation from the heat source at the energy conversion unit. This contradiction has gradually become a key limiting factor for its further development in recent years. In this work, a novel hybrid system based on the thermophotovoltaic-thermoelectric (TPV-TE) effect is proposed, in which the Bi2Te3-based I/W-type thermoelectric leg plays both heat dissipation and heat recovery for power supply. The heat recovery thermoelectric legs were prepared using a cold pressing mould and sintering method, and their Seebeck effect was used to convert the unwanted heat recover from InGaAs thermophotovoltaic cells into electrical power. The design preparation scheme of the heat recovery hybrid module and its potential application performance at 700–1000 K heat source temperature were investigated through experimental tests and simulations. The W-type TPV-TE heat recovery hybrid system exhibits good gain of 12.9% at 1000 K, and the output power can be increased by additional 21.6% in series mode. Simulations for environmental applications show better heat recovery in deep space than on Earth, with an output gain of 63.7% and an energy conversion efficiency of 11.23%. The heat recovery design enhances the competitiveness of the RTPV and provides new ideas for optimizing other heat recovery applications for thermal energy conversion systems.

A Review on Thermophotovoltaic Energy Conversions and its Space Power Applications

Thermophotovoltaic (TPV) system coverts heat radiations from various sources like Thermophotovoltaic (TPV) system coverts heat radiations from various sources like combustion of fuels, industrial waste heat and nuclear energy into electricity. To fulfil the demand of energy TPV is an alternate, can enable approaches to energy storage and conversion. The TPV model consists of multiple arrays of TPV cells, an emitter, a radiator and a filter. one of the important advantages of TPVs are the high efficiencies and direct conversion of DC power. This paper presents the research being conducted till date in the field of Thermophotovoltaic cell and space applications of TPV cells. We have Thermophotovoltaic has been regarded as an energy substitute in radioisotope deep space power system for thermoelectric. TPV provides outstanding potential improvement in mass specific power as well as in efficiency. TPV system also proposed for inner planetary solar system. This idea leads TPV capability to store energy in the form of heat energy rather than electrical energies which is common in photovoltaic system. The current effort to derive the demonstration of efficiency conversion up to 19% and it enhances the specific power W/kg at the system level. Next generation TPV concepts are also reviewed in order to explore the future space power application. The application of TPV that includes radioisotope Thermophotovoltaic (RTPV) and solar Thermophotovoltaic (STPV) plays a vital role in deep space powered systems.

Irradiation and temperature service stability of the radioisotope thermophotovoltaic generators based general purpose heat source

Highly stable and long-life power supply systems are crucial to the success of space exploration missions. The systematic analysis of the service lives and stability of radioisotope thermophotovoltaic (RTPV) generators with general purpose heat sources (GPHSs) is the cornerstone of future practical space applications. Its operational lifetime is mainly limited by the degradation caused by the irradiation of key transducer components by the isotope source, while its operational stability is mainly related to the temperature variation of the external environment. The irradiation and temperature stability as the main influencing factors of GPHS-RTPV were studied. Results indicate that the output performance of the GPHS–RTPV was maintained at 50.15% after 1 × 1013 n/cm2 neutron and 927.84 Gy γ irradiation. The service life was conservatively estimated to be 19.883 years according to equivalent irradiation experimental results. In addition, the output performance of the GPHS–RTPV fluctuates linearly with temperature, the gradient of Pmax with temperature was −6.7 μW/K·cm2 in a range of 276.5–354.5 K, and higher output at low temperatures. This study explored the lifetime and service stability of GPHS-RTPV under actual operating conditions in space, and lays the foundation for future practical applications.

An Efficient Photon Utilization Radioisotope Thermophotovoltaic Based on Curled Reflectors

For radioisotope thermophotovoltaic (RTPV) to produce higher output power, it is often required that the irradiance incident on the thermophotovoltaic (TPV) cell array be more uniform. However, the irradiance received by each cell of the TPV array is relatively different under the conventional cell array surround structure of RTPV, resulting in a serious overall electrical output mismatch loss and a lower output power utilization rate. Herein, an RTPV with curled reflectors optimized by finite‐element method is proposed. It is shown in the simulation results that the irradiation uniformity of the cell array can reach 87.5%, the total output power reaches 88.12 W, which is 39.8% higher than that prior to optimization, and the mismatch loss is reduced by 69.2%. In addition, the impact of the reflectivity of reflectors is also targeted from the perspective of practical applications, and it can still produce higher output power than that of the conventional structure even at reflectivity as low as 70%. Ultimately, the RTPV prototype with reflectors is developed, and the electrical performance is tested to verify the effect of irradiance improvement. Herein, reflectors with positive gain for RTPV are proposed, which provides a new idea for the development of high‐efficiency power supplies.

Structural Design and Thermal Performance Analysis of Heat Source Used in Radioisotope Thermophotovoltaic

Structural Design and Thermal Performance Analysis of Heat Source Used in Radioisotope Thermophotovoltaic

Irradiation and Temperature Service Stability of the Radioisotope Thermophotovoltaic Generators Based General Purpose Heat Source

Irradiation and Temperature Service Stability of the Radioisotope Thermophotovoltaic Generators Based General Purpose Heat Source

Irradiation and Temperature Effect of the Radioisotope Thermophotovoltaic Generators Based General Purpose Heat Source

Irradiation and Temperature Effect of the Radioisotope Thermophotovoltaic Generators Based General Purpose Heat Source

High-temperature and radiation-resistant spinel-type ferrite coating for thermo-optical conversion in radioisotope thermophotovoltaic generators

High-temperature radioisotope heat source surface coated with excellent performance of thermo-optical conversion coating can greatly improve thermophotovoltaic energy utilization. However, long-term high-temperature and radiation resistance of the thermo-optical conversion pose a great challenge. In this work, spinel-type ferrite thermo-optical conversion coating was proposed to improve the heat and irradiation stability and output power of a radioisotope thermophotovoltaic (RTPV) prototype. Under the continuous irradiation of 32.64 kGy for 24 h at 1400 K, the emissivity of the spinel-type ferrite thermo-optical conversion coating could maintain more than 90%. In addition, the surface morphology of the coating was changed under high temperature, which enhanced its surface emissivity. Spinel-type ferrite thermo-optical conversion coating was applied to the RTPV prototype, which achieved a maximum output power of 144.23 mW. Compared with the RTPV prototype without coating, the overall output increased to 181.51%. Spinel-type ferrite thermo-optical conversion coating showed great application potential in the fields of thermo-optical conversion and high-efficiency utilization of thermal energy.

High-Temperature and Radiation-Resistant Spinel-Type Ferrite Coating for Thermo-Optical Conversion in Radioisotope Thermophotovoltaic Generators

High-temperature radioisotope heat source surface coated with excellent performance of thermo-optical conversion coating can greatly improve thermophotovoltaic energy utilization. However, long-term high-temperature and radiation resistance of the thermo-optical conversion pose a great challenge. In this work, spinel-type ferrite thermo-optical conversion coating was proposed to improve the heat and irradiation stability and output power of a radioisotope thermophotovoltaic (RTPV) prototype. Under the continuous irradiation of 32.64 kGy for 24 h at 1400 K, the emissivity of the spinel-type ferrite thermo-optical conversion coating could maintain more than 90%. In addition, the surface morphology of the coating was changed under high temperature, which enhanced its surface emissivity. Spinel-type ferrite thermo-optical conversion coating was applied to the RTPV prototype, which achieved a maximum output power of 144.23 mW. Compared with the RTPV prototype without coating, the overall output increased to 181.51%. Spinel-type ferrite thermo-optical conversion coating showed great application potential in the fields of thermo-optical conversion and high-efficiency utilization of thermal energy.

Thermal Emission‐Enhanced and Optically Modulated Radioisotope Thermophotovoltaic Generators

Infrared radiation generated by high‐energy‐density radioisotope decay can be converted to electrical energy in radioisotope thermophotovoltaic (RTPV) generators. Thermal emission intensity and spectral properties have substantial implications in this thermal energy conversion process. To improve the performance of the RTPV generator, a silicone coating material is used as a thermal emission enhancer, and SiO2 is used as a filter. The silicone coating has excellent thermal emissivity at high temperatures. The SiO2 filter is used for optical modulation during the thermal energy conversion process. The heat transfer optimization problem caused by the internal temperature distribution of the system is discussed. Compared with the experimental model before optimization, the output power of the RTPV generator increased by 126% obtains an open‐circuit voltage of 2.64 V, an electric power of 89.88 mW, and an energy conversion efficiency of 5.62%. The RTPV generator is expected to be a potential candidate for energy supply in extreme environments.

Design and performance analysis of a 500-W heat source for radioisotope thermophotovoltaic converters

A radioisotope thermophotovoltaic (RTPV) system effectively converts the decay heat of radioisotopes into electricity via thermally radiated photons. In this work, a 500-W thermal heat source unit including 238PuO2 radioisotope fuel, shielding material, and selective emitter is designed from the viewpoint of radiation safety, thermal performance, and overall conversion efficiency by considering various shielding materials, fuel configurations, and packing factor (PF), defined as the ratio of fuel region volume to total heat source enclosure volume including fuel cladding and shield. The design study starts with a reference cubic configuration and extends to the more complicated configurations having separate cylindrical fuels. The results of the study showed that the heat source unit design suggested here can reduce the total radiation dose, peak neutron fluence, and maximum temperature using separate cylindrical fuel rods. For example, a design having a separated 3 × 3 cylindrical fuel rod array of 30% PF increases the overall efficiency by ~39% with similar maximum temperature and radiation doses in comparison with the reference heat source unit with a single cubic module and a 10% PF. This demonstrates the importance of the proper design of the RTPV heat source unit.

A review of radioisotope batteries

As an important kind of energy source, radioisotope batteries are attracting more and more academic researchers and people from industry due to the high power density, long lifetime (equal to half life of the radioisotope source), outstanding reliability, without maintenance, miniaturization and wide application compared with traditional dry batteries, chemical batteries, fuel batteries and solar batteries. Based on the optimization of functional materials and energy conversion types, radioisotope batteries have been developed for more than 15 species since the first β battery invented by Henry Mosley in 1913. This review describes historical background and development, limitations of key techniques in radioisotope batteries. The radioisotope source loading methods and thermocouple materials of radioisotope thermoelectric generator (RTG), the semiconductor materials and energy conversion units of radiation voltaic isotope batteries (RVIB) are analysed in detail. After an introduction of the basic principles and design requirements of radioisotope batteries, we discuss the technical proposals of different energy conversions for radioisotope batteries. Then, the most recent experimental results for several configurations and experimental set-ups of radioisotope batteries are introduced detailedly, including static thermoelectric type radioisotope batteries, radiation voltaic effect radioisotope batteries, dynamic energy transformation radioisotope batteries and piezoelectric energy transformation radioisotope batteries. The figure for PCEs (Power Conversion Efficiencies) in different radioisotope batteries from 1913 to 2015 is first demonstrated in the end, which illustrates the PCEs of RTG, RVIB are close to 10% and DIPS (Dynamic Isotope Power System) is higher than 23%. Thus, the PCEs of radioisotope batteries will increase effectively if some various high efficient energy conversion types like RTG, RTPV (Radioisotope thermophotovoltaic) and RTIGs (Radioisotope Thermionic emission Generators) are assembled. In the future, four main trends for radioisotope batteries include the better safety and reliability, the higher output power and power matching, the micro/nano integration and module combination of battery structure, the production of radioisotope source. We believe that the research and application of radioisotope batteries will be much attractive with the break through of these aspects made by academic and industrial world.

A radioisotope thermophotovoltaic converter with nanophotonic emitters and filters

Radioisotope thermophotovoltaic (RTPV) systems convert the heat of decay of radioisotopes into electricity via thermally radiated photons. In this work, we report the performance analysis of the RTPV system incorporating a cubic-shaped thermal cavity including 1.04kg of Plutonium-238 (238PuO2) and nanophotonic emitters facing low bandgap photovoltaic (PV) cells. The performance of RTPV was investigated by varying the nonidealities of nanophotonic emitters and filters and with introducing a weighting factor (W) that quantifies the benefit of each nanophotonic component. Unlike previous studies, we varied the packing factor (PF) defined by the volume ratio between the radioisotope and the overall thermal cavity, then investigated the effects of the PF on the conversion efficiency, optimal configuration, and radiation safety. Our results show that it is crucial to reduce the nonidealities of nanophotonics at low energy photon level (below the PV cell bandgap) to achieve a high conversion efficiency. With the current state-of-the art nanophotonics including 2D tantalum photonic crystal (PhC) emitter and tandem filter which using interference and plasma filter, we show that the RTPV has the potential to achieve ∼30% of conversion efficiency and surpass the conversion efficiency of conventional radioisotope thermoelectric generators (RTGs) with the same PF (∼10%). The increase in the PF from 10% to 70% provide approximately 23% of further increase in RTPV efficiency while limiting the increase in the total radiation dose only by ∼10%. This work will help the development of high efficiency nanophotonic RTPV converters.

A Radiation Analysis of 500W RTPV System having Cylindrical Sources

In this work, the radiation shielding analysis for a RTPV (Radioisotope Thermo-Photo-Voltaic) device using 238 PuO2 was performed to estimate the radiation dose distribution over source region and radiation doses in a specified tally zone. The radiation source intensities and spectra were evaluated with ORIGEN-S and the detailed shielding analyses were performed with MCNP6 and MONACO/MAVRIC (in SCALE 6.1). In particular, comparative shielding analysis using two different types of RTPV (cubic homogeneous source type and cylindrical heterogeneous source type) were performed to show their relative performances. The results show that the RTPV device using the heterogeneous cylindrical source type has lower dose values by 5~11% depending on the packing factor at the measurement cell than the one using the homogeneous cubic source type. The additional thermal analysis using COMSOL shows that the temperature distribution of the heterogeneous cylindrical source type is more flat than that of a homogeneous cubic source type.

Toward high performance radioisotope thermophotovoltaic systems using spectral control

This work describes RTPV-PhC-1, an initial prototype for a radioisotope thermophotovoltaic (RTPV) system using a two-dimensional photonic crystal emitter and low bandgap thermophotovoltaic (TPV) cell to realize spectral control. We validated a system simulation using the measurements of RTPV-PhC-1 and its comparison setup RTPV-FlatTa-1 with the same configuration except a polished tantalum emitter. The emitter of RTPV-PhC-1 powered by an electric heater providing energy equivalent to one plutonia fuel pellet reached 950°C with 52W of thermal input power and produced 208mW output power from 1cm2 TPV cell. We compared the system performance using a photonic crystal emitter to a polished flat tantalum emitter and found that spectral control with the photonic crystal was four times more efficient. Based on the simulation, with more cell areas, better TPV cells, and improved insulation design, the system powered by a fuel pellet equivalent heat source is expected to reach an efficiency of 7.8%.

Prototype of radioisotope thermophotovoltaic system using photonic crystal spectral control

This work describes a prototype of a small-size radioisotope thermophotovoltaic (RTPV) system with the two-dimensional metallic photonic crystal emitter and the low bandgap TPV cell. The project demonstrates the simulation and measurement of a system powered by an electrical heat source that mimics the radioisotope fuel pellet. The photonic crystal and the polished Ta3%W substrate are both used as the emitting surfaces to demonstrate the benefits of spectral control. The rest of the system is thermally insulated to increase the overall system efficiency. The photonic crystal emitter demonstrates four times more output power over a flat metal emitter from the 1 cm2 TPV cell. With more cell areas, better TPV cells and improved insulation design, the system is expected to reach an efficiency of 7.8%.

Analysis and optimization of In<sub>1-x</sub>GaxAsyBb<sub>1-y</sub> thermophotovoltaic cells under low radiator temperatures

In this paper,we investigated the heat to electricity conversion efficiency of In 1-x Ga x As y Sb 1-y radioisotope thermophotovoltaic (RTPV) converter with x=0.8 and y=0.18, taking account of the photons with energy below the cells bandgap using a comprehensive analytical process. This was done with a computer program designed for this reason, which allowed the computation of the cell performance under a variety of specified incident radiation spectra as well as a variety of material parameters. The results show that for an emissivity value of 0.78, a cell thickness of about 7µm with low front recombination velocity(700cm/s),a conversion efficiency greater than 29% can be obtained for radiator’s temperature of1300°k at ambient temperature. This efficiency will decrease as the cell temperature increase.

表面微结构辐射器几何结构对发射性能的影响

The physical dimension of mircostructural surface emitters is one of the key factors which determines the emission performance and system efficiency in radioisotope thermophotovoltaic( RTPV) systems. This paper preliminarily concluded the reasons for the emitters' characteristics of infrared radiation through the exploration of the effects that the width,height and walls' thickness of a single tungsten mircocavity played on the radiator's emission performance. Then the finite-differenttime-domain( FDTD) method was utilized to compare the emission performance of microcavities with different sizes. It is found that the emission performance matches well in the GaSb case taking into account of the efficiency curve of GaSb which the width,height and walls' thickness of a single mircocavity are set as 0. 8,1. 8,0. 1 μm,respectively.

Design and integration of small RTPV generators with new millennium spacecraft for outer solar system

The National Aeronautics and Space Administration’s recently inaugurated New Millennium program, with its emphasis on miniaturized spacecraft, has generated interest in a low-power (10–30 W), low-mass, high-efficiency RTPV (Radioisotope Thermophotovoltaic) power system. This led to a Department of Energy (DOE)-sponsored design study of such a system, which was assigned to OSC (formerly Fairchild) personnel, who have been conducting similar studies of a 75 W RTPV system for the Pluto Express Mission, with very encouraging results. The 75 W design employed two 250 W general purpose heat source (GPHS) modules that DOE had previously developed and safety-qualified for various space missions. These modules were too large for the small RTPVs described in this paper. To minimize the need for new development and safety verification studies, OSC generated derivative designs for 125 W and 62.5 W heat source modules containing identical fuel pellets, clads, impact shell and thermal insulation. OSC also generated a novel heat source support scheme to reduce the heat losses through the structural supports, and a new and much simpler radiator structure, eliminating the need for honeycombs and heat pipes.OSCs previous RTPV study had been based on the use of GaSb PV cells and spectrally selective IR filters that had been partially developed and characterized by Boeing (now EDTEK) personnel. They had supplied us with spectral data on filter reflectivities and cell quantum efficiencies. Two sets of data were furnished: one based on actual measurements made in 1993, and a more optimistic set based on projected performance improvements. Even the measured data set yielded significantly better system performance than present thermoelectric systems, but the projected data yielded much better system performance. Because of these encouraging results, OSC in the fall of 1994 initiated an experimental program at EDTEK to develop improved filters and cells, to demonstrate how much improvement can actually be achieved. OSC requested that first priority be given to filter improvements, because our system studies indicated that improved filters would have a much greater effect on system performance than cell improvements. By July 1995 EDTEK had achieved about 90% of the filter performance improvement projected in 1993. Work on further filter and cell improvements is continuing at EDTEK, as part of a joint effort with OSC and with DOE’s Mound Laboratory to develop and test a prototypic RTPV generator, with both an electrical heater and a radioisotope heat source.The improved filter performance data have been applied to the design of low-power (10–30 W) RTPV power systems, for possible application to new millennium spacecraft for missions to the outer solar system, where solar power generation is impractical. The results reported in this paper indicate that such systems can yield very attractive performance with the RTPV generator integrated with the miniaturized new millennium spacecraft.