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.


Introduction
Thermophotovoltaic (TPV) transfer primarily infrared wavelength light into electrical energy by the phenomenon of photovoltaic effect.Thermophotovoltaic system coverts heat radiations from various sources like burning of fuels, nuclear and waste heat energy into electricity.To fulfil the demand of energy TPV is an alternate which can facilitates approaches to energy transfer and storage.This can entitle approaches to energy conversion and storage which can use very high temperature thermal sources than the turbines which are present in electrical energy generation now days [1,2,3].TPV construction and efficacy have improved since the early exhibition of 29% efficient TPV that used a tungsten emitter and a back scattered screen at a temperature of 2,000°C [4,5].However, sometimes at much lesser temperatures below 1,300°C, the observed efficiencies remain only as high as 32%, in comparison to expectations that TPV efficiencies can approach more than 32% [6,7].The cells employ band-edge spectroscopic filtering to achieve maximum efficiency, rejecting radiation of extraneous sub bandgap material and return to the source with highly polished back surface reflectors [3,5,8].In comparison with a photovoltaic cell or solar cell, a TPV device can store and ultimately transform the energy contained inside sub-bandgap photons.Efficiency of TPV cell is therefore assessed differently than that of a solar cell [9,10,11].This is because, under the conditions where it is anticipated to be utilized, the view factor of the TPV cell to emitter is significant [12].This demonstrates how the TPV cell, which differs from a solar cell, can return sub-bandgap radiation back to the source.The emitted energy of the sub-bandgap photon is maintained by reabsorption by the source by returning unreacted photons [13].The emitter gets heated from the light that is reflected and then rapidly absorbed, which lowers the amount of energy required to heat it [1,4,14].TPV devices to be an appropriate choice for producing electricity commercially, high emitter temperature, higher bandgap materials and higher performance multijunction configurations with bandgap processability made possible by the use of very high-quality metamorphic cell epitaxy and for a band-edge filtration a high reflexivity must be added.Larger bandgaps have almost steadfast effect on voltage of about 0.3-0.4V due to the thermal constraints on the rate of radiative recombination, which improves efficiency [15,16].Cells having lower bandgap are suffered more loss than cells with larger bandgaps because this loss accounts for a lower portion of the voltage in those materials [17].To keep a high-power density that scales with the emitter while using larger bandgap materials and working at extremely high temperatures [2,5,6].

Thermophotovoltaic cells
Radiant photons from a hot emitter are converted into energy in TPV, a branch of photovoltaics.It had earlier been considered as a technique for transforming thermal energy from nuclear sources into electricity, but efficacy problems prohibited it from being a realistic choice [4,5,6,18].But current revelations inside the photovoltaic and hybrid semiconductor wafer fields have improved significantly cell efficiency, renewing attention in TPV as one of the successful passive energy transfer technologies.TPV has become effectively utilized in a number of space applications.A type of photonic-based energy generation technology is known as thermophotovoltaics [19].A nearby narrow bandgap photovoltaic cell (commonly constructed using binary, tertiary, or quaternary semiconductor materials like InGaAs, GaSb, InAs, or InGaAsSb) transforms infrared (IR) radiation photons from the hot surface (the emitter) into electricity.Photovoltaic elements in the particular manner of a p-n diode are utilized in solid-state TPV energy transfer to turn radiated thermal photons into energy [1,6].

TPV system overview
The functioning of emitter at extreme temperature provides two big challenges to STPV power conversion, preserving spectral selectivity at high temperatures while adequately collecting sunlight to achieve temperature previous STPV implementations depended on the innate characteristics of elements like tungsten [2,3,4,20].Using macro cavity designs in the absorber is one method to effectively increase the inherent solar absorption rate of materials.This method usually requires high levels of optical concentration to achieve due to the high aspect ratio of the aperture required to increase absorption.In turn, such high optical concentration demands complex systems with only a low 65% optical efficiency [21].Finally, the previously reported STPVs have been restricted to conversion efficiencies of around due to the dependence on the inherent spectral characteristics for the absorber emitter [1,4,22].

Current State of TPV Cell Techniques
This review compares each energy source to decades worth of experimental TPV literature to identify key design elements and technical gaps [23,24].A lot of studies have been done on TPV cells recently, and many of them have been published in scholarly publications.The following are some of the main TPV cell research areas.TPV cells need materials that can both absorb and emit infrared light [25,26,27].To increase the effectiveness of TPV cells, researchers are conducting experiments with a range of materials, including semiconductors, plasmonic materials, and carbon nanotubes [28,29].To increase the effectiveness and performance of TPV cells, researchers are looking at several device designs.Multijunction cells, nanostructured cells and microcavity cells are a few examples of these materials [6,30].Modern TPV conversion uses plane narrow band gap semiconductor materials with P-n junctions.The Infrared radiation from the source of heat goes into the TPV device and generates electron-hole pairs [31].The thermodynamical efficiency of current compound semiconductor TPV devices is just more than 20% due to the size of the planar p-n junction [5,6,32].Identifying the elements of present TPV system designs that are advantageous and potential due to experimental heterogeneity in the TPV literature which involves emitter and cell temperatures, shape of the cavity, and system size, progress is difficult to achieve [1,2,32,33].By comparing each stage of energy-conversion to its corresponding experiment-specific thermodynamical limit, this overview analyses several decades experimental worth of TPV literature.The highest stated efficiencies are close to 40% [6,34,35] of their thermodynamic limit.To produce electricity, TPV cells must be integrated with heat sources like motors or furnaces.The integration of TPV cells with these sources is being studied using a variety of techniques, including as fiber optic coupling, selective emitters, and metamaterials.Overall, the level of TPV cell technology is encouraging, and research is still being done to increase effectiveness and performance.However, there are still many difficult issues to be resolved, such as the creation of affordable materials and integration with useful thermal sources [1,27,36].

Temperature and TPV Cell Efficiency
Temperatures within the emitter as well as the cell have an important effect on TPV energy conversion performance.The adverse impacts of dark current on cell effectiveness appear.Irrespective of the type of emitter, substance, or cell design, the TPV efficiency decreases as temperatures increase because dark current is inversely related to temperature [31,34,36].The suggested dot junction's device, that is comprised up of a great number of extremely small, evenly distributed dots across the device's front, is one of the most effective devices.It makes use of the idea that the output voltage a cell is inversely proportional to the size of the p-n junction [7].This figure indicates the basic parts of a Thermophotovoltaic system.An emitter, a heat source and a photovoltaic converter are the three principal components of a thermophotovoltaic device.The component that emits transmits thermal energy throughout a gap to the photovoltaic cell or grid of photovoltaic cells upon absorbing thermal energy via the heat source.The thermal radiation is then transformed into electrical energy by the photovoltaic cell, which can then be supplied to a load or conditioning circuitry.Reflectors deposited on the bottom of the photovoltaic cell and optical filters between the emitter and the photovoltaic cell are additional typical component [15,18,32].To reduce conduction and convective heat transfer, the optical cavity between the emitter and photovoltaic cell is often kept under pressure.The efficiency of each major component, specifically, where heat refers to the combined efficiency of the solar-to-heat or chemical-to-heat conversion and the thermal efficiency of the heat delivered to the Thermoelectric conversion (the emitter, in the case of the thermophotovoltaic converter), spectral refers to how well the emitted spectrum can match the photovoltaic cell (discussed later), and cavity is the fraction of the thermal expansion coefficient (TEC) surface area [1].

Applications of TPV
In comparison to solar or photovoltaic cell, thermophotovoltaic systems exhibit significant advantages as a source of energy with a continuous working arrangement of blackbody temperatures 500-2000 K. Due to their widespread integration in research field and industry, the TPV technologies are dominating.The structural and functional improvement of parameters like metal contact, thickness, doping concentration, material selective radiator, surface field layer and linked modules contributed to the rise in cell efficiency [14,5,6].The TPV system has attracted a lot attention because of its potential impact as a clean energy generation method that is also effective, useful and favourable to the environment.Depending on whether the thermal energy source is a chemical reaction involving in nuclear fusion, TPV applications can be divided.These thermal sources are classified into four categories solar heat, nuclear heat, waste heat and heat energy via combustion [1,4,15].A thermophotovoltaic system is suitable for a variety of useful purposes, such as solar thermophotovoltaic systems, hybrid electric cars, waste heat recovery.commercial and residential power supplies, and nuclear generators for space applications.Hence, as a result, the primary addition of this study is the thorough analysis of TPV cells, which offers a great blueprint on how to improve upon the present accomplishment and their potential for implementation in energy and power system technology in the future.A few key research topics are mentioned, and the analysis provided some foremost and focused suggestion for the advancement of TPV cells [2,4,5].High-efficiency TPV cells will be very much essential in the future for space application technology.For the long-term running of a small spaceship, solar and nuclear reactors are both suitable power sources.A recent analysis of the main competing advances in the area of space power production is reviewed.The TPV system provides an efficiency up to 40% and offers the extra benefits of being movable, physically static, and obtaining energy directly from the radiant heat source in space power production.Additionally, the TPV device produces a high-power energy density relative to other systems per unit area, making it appropriate for middle electrical systems.By combining nuclear fission sources with radioisotope TPV producers, the power output can be raised even more [5,8,9].The usual heat limit for nuclear reactors starts at 10 kW to MWs.Future planetary colony projects are presently considering using these engines to generate electricity up to MWs.The Institute for Soldier Nanotechnologies was the site of the most recent RTPV research, where modelling and measurement data on RTPV prototype systems were successfully published.Spectral management of photonic crystals is used in terrestrial uses.Additionally, it was stated that an RTPV device built on an InGaAsSb cell had an efficiency approximately 8.25% and an output power of about 40 W. A InGaAs TPV cell of 0.6eV is excellent for space applications, with less than 1% degradation in cell efficiency over time owing to system damage [7,8].

TPV Energy in Space
Thermophotovoltaic (TPV) Technology Development, the final SP-100 contract, included a task study that included Lockheed Martin Space Power Systems' initial investigation into TPV power conversion in nuclear reactors.Based on cell and radiator performance, this study evaluated the radioisotope TPV conversion using a range of specific energies.By showcasing enhanced thermoelectric and Brayton conversion at 20 to, that they advanced the research [34].The projects requiring MWe scale electricity, in-space nuclear fission power sources are being considered.Thermophotovoltaic (TPV) cells may be a mass-efficient choice for turning the heat energy of a nuclear reactor into electrical energy, according to recent research on these cells.TPV cells were created for the first time in the early 1960s, and their uses in space were being discussed as early as 1970 [36].More lately, radioisotope thermoelectric generator (RTG) technology has been viewed as the obvious option for producing a lower specific power than radioisotope TPV (RTPV) [4,34].In 2003, NASA awarded contracts for RTPV research [34].Tests and modelling revealed that, even when radiator mass is taken into account, an RTPV system (at 20% efficiency) would have a higher specific power (16 W/kg) than rival radioisotope power systems (4 W/kg) [4].Since those experiments were done, TPV rates have increased.With a 2400 K emitter and a 300 K cell, new advancements have raised TPV efficacy to as high as 40%.The power conversion assembly (PCA), which includes the TPV energy conversion device in the middle which serves as a little heat exchanger for the heated coolant from the reactor and the frigid coolant from the condenser [4].The two primary uses of TPV inverters in orbit are for RPS and solar thermal systems.Stirling engines and thermoelectric converters are the main rivals for the replacement of more seasoned and inefficient thermoelectric converters in RPS uses.A novel solar thermal power generator called a TPV has been suggested.It could incorporate extremely energetically dense heat energy storage and be used in hostile environments like the Van Allen circles.Thermal energy storage is extremely useful for applications that require a constant supply of heat and electricity, such as solar thermal propulsion systems.[1,4,24].W with Si3N4 thin film 1700K GaSb 6.2% [1] Mo/HfO2Nanostructure 1640K GaSb 5.1% [1] Si/SiO2 stack 1272K InGaSb 6.8% [1]

Recent TPV Concept for Space Power applications
The term "advanced TPV concepts" refers to a variety of strategies for increasing the amount of useful energy transferred from the emitter to the TPV cell, either by altering the emitter itself or the medium between the emitter and cells (LTPV or NF-TPV [39]).Recent advancements have raised TPV efficiency to as high as 40%, with a2400K emitter and a 300k cell, experts believe that a feasible route forward (higher substrate reflectivity and air bridge strategy) will lead to efficiencies as high as 56% at 2500 K emitter temperatures [5].Modelling predicts that if the cell temperature were raised to 600 K to handle a high temperature radiator, the 40% effective TPV would drop to 27% efficiency.High emitter temps, which are most readily attained when using a nuclear reactor, were a crucial component for the increased effectiveness [34].The specific power (W/kg) of the spaceship is intended to be increased by future ideas for space application of TPV conversion [4,37,35].Two components are contributed by the figure of merit (FOM): bulk and force.High efficiency TPV energy conversion is necessary to decrease the quantity of rejected heat, the amount of the radiator heat sink, which makes up the bulk of the TPV generator mass, and so forth.On the contrary, high-power density (W/cm2) is essential to decreasing the heat source area required to generate the necessary minimum output power [4,34].TPV Increase Very High RTPV [24] Multiband High High STPV [24] The power density and various figure of merit (FOMS) of TPV converters are inversely proportional in some in some instances.This is so that there is no energy waste in a TPV setup where some of the radiant heat may be returned to the heat source.Theoretically, increasing TPV conversion efficiency while reducing output electrical power density involves preventing the majority of thermal energy and enabling only a very small spectral range to reach the TPV cell.In a perfect world, novel TPV ideas would concurrently enhance both FOMs.The bulk of them, though, are mainly directed at one of them.Therefore, itis crucial to identify the variables that are crucial for boosting power density or efficiency in order to choose the best TPV idea [4,31,31] 10.Summary and Conclusion A summary of the major technologies that compete with TPV in the area of space power generation have been outlined in brief.Small-medium devices with output powers varying from a few tens to hundreds of We are best suited for TPV technology.Comparable technologies in the ongoing power range, TPV guarantees a greatest specific power.Generators powered by radioisotopes and the sun are the main applications for TPV.A 20% efficacy has already been shown by radioisotope TPV systembased cells working at 1000°C, and 6-8 W/kg of specific powers have been predicted.Presently, the emphasis is on proving long-term dependability, including radiation harm to the semiconductor in the TPV cell.The primary benefit of solar TPV is its capacity to integrate latent heat with a high temperature (> 1400 °C) having the ability to store energy at extremely high concentrations of up to 4 MJ/kg.For solar thermal propulsion uses that call for extremely high impulse and velocity increases, this is particularly crucial.Due to the significant difficulties experienced during its technological growth, the TPV system continues to have a wide market penetration.It is anticipated that TPV technology will stabilize and advance scientifically over the upcoming ten years, enabling it to produce output with a high-power density for future energy.Further advancements in cell efficiency are anticipated over the following few years, with estimates of reaching 40% when compared to conventional power systems, By the utilization of the multi-junction cells to transform lower bandgap photon radiations.Both nearsun mission and deep-space missions can profit from TPV's appealing efficiency advantages.The discrepancy between the TPV spectrum and the cell bandgap is one of the main problems that reduces the effectiveness of the TPV devices.The most of the blackbody spectral radiation must be absorbed by the TPV cell for optimum cell performance, so selecting the correct low bandgap material depending on the temperature of the heat source is very important.Additionally, effective sub-bandgap photon recycling and reflection inside the radiator will greatly raise the conversion efficacy of the TPV device.Reviewing the efficacy of TPV cells revealed that they work at high radiation energies, and that creating TPV cells with the least amount of optical and electrical loss would greatly enhance their output performance.It would be beneficial to perform research and analysis on designs that are best for the near field TPV in order to minimize the electrical and optical losses inside the specific semiconductor layer.

Figure 2 .
Figure 2. Schematic diagram of TPV system Reproduce with permission from ref [1].

Figure 3 .
Figure 3.Primary control of thermophotovoltaic conversion [adapted with permission from ref 17].

Table 1 .
Important features of selected STPV systems

Table 2 .
Important features of selected STPV systems