Thermionic-enhanced near-field thermophotovoltaics

Solid-state heat-to-electrical power converters are thermodynamic engines that use fundamental particles, such as electrons or photons, as working fluids. Virtually all commercially available devices are thermoelectric generators, in which electrons flow through a solid driven by a temperature difference. Thermophotovoltaics and thermionics are highly efficient alternatives relying on the direct emission of photons and electrons. However, the low energy flux carried by the emitted particles significantly limits their generated electrical power density potential. Creating nanoscale vacuum gaps between the emitter and the receiver in thermionic and thermophotovoltaic devices enables a significant enhancement of the electron and photon energy fluxes, respectively, which in turn results in an increase of the generated electrical power density. Here we propose a thermionic-enhanced near-field thermophotovoltaic device that exploits the simultaneous emission of photons and electrons through nanoscale vacuum gaps. We present the theoretical analysis of a device in which photons and electrons travel from a hot LaB6-coated tungsten emitter to a closely spaced BaF2-coated InGaAs photovoltaic cell. Photon tunnelling and space charge removal across the nanoscale vacuum gap produce a drastic increase in flux of electrons and photons, and subsequently, of the generated electrical power density. We show that conversion efficiencies and electrical power densities of 30% and 70W/cm2 are achievable at 2000K for a practicable gap distance of 100nm, and thus greatly enhance the performances of stand-alone near-field thermophotovoltaic devices (10% and 10W/cm2). A key practical advantage of this nanoscale energy conversion device is the use of grid-less cell designs, eliminating the issue of series resistance and shadowing losses, which are unavoidable in conventional near-field thermophotovoltaic devices.Comment: Nano Energy (2019

External luminescence and photon recycling in near-field thermophotovoltaics

The importance of considering near-field effects on photon recycling and spontaneous emission in a thermophotovoltaic device is investigated. Fluctuational electrodynamics is used to calculate external luminescence from a photovoltaic cell as a function of emitter type, vacuum gap thickness between emitter and cell, and cell thickness. The observed changes in external luminescence suggest strong modifications of photon recycling caused by the presence of the emitter. Photon recycling for propagating modes is affected by reflection at the vacuum-emitter interface and is substantially decreased by the leakage towards the emitter through tunneling of frustrated modes. In addition, spontaneous emission by the cell can be strongly enhanced by the presence of an emitter supporting surface polariton modes. It follows that using a radiative recombination model with a spatially uniform radiative lifetime, even corrected by a photon recycling factor, is inappropriate. Applying the principles of detailed balance, and accounting for non-radiative recombination mechanisms, the impact of external luminescence enhancement in the near field on thermophotovoltaic performance is investigated. It is shown that unlike isolated cells, the external luminescence efficiency is not solely dependent on cell quality, but significantly increases as the vacuum gap thickness decreases below 400 nm for the case of an intrinsic silicon emitter. In turn, the open-circuit voltage and power density benefit from this enhanced external luminescence toward the emitter. This benefit is larger as cell quality, characterized by the contribution of non-radiative recombination, decreases.Comment: 44 pages, 8 figures, 1 table, 4 supplemental figure

Drag-free and attitude control for the GOCE satellite

The paper concerns Drag-Free and Attitude Control of the European satellite Gravity field and steady-state Ocean Circulation Explorer (GOCE) during the science phase. Design has followed Embedded Model Control, where a spacecraft/environment discrete-time model becomes the realtime control core and is interfaced to actuators and sensors via tuneable feedback laws. Drag-free control implies cancelling non-gravitational forces and all torques, leaving the satellite to free fall subject only to gravity. In addition, for reasons of science, the spacecraft must be carefully aligned to the local orbital frame, retrieved from range and rate of a Global Positioning System receiver. Accurate drag-free and attitude control requires proportional and low-noise thrusting, which in turn raises the problem of propellant saving. Six-axis drag-free control is driven by accurate acceleration measurements provided by the mission payload. Their angular components must be combined with the star-tracker attitude so as to compensate accelerometer drift. Simulated results are presented and discusse

All-propulsion design of the drag-free and attitude control of the European satellite GOCE

This paper concerns the drag-free and attitude control (DFAC) of the European Gravity field and steady-state Ocean Circulation Explorer satellite (GOCE), during the science phase. GOCE aims to determine the Earth's gravity field with high accuracy and spatial resolution, through complementary space techniques such as gravity gradiometry and precise orbit determination. Both techniques rely on accurate attitude and drag-free control, especially in the gradiometer measurement bandwidth (5-100mHz), where non-gravitational forces must be counteracted down to micronewton, and spacecraft attitude must track the local orbital reference frame with micro-radian accuracy. DFAC aims to enable the gravity gradiometer to operate so as to determine the Earth's gravity field especially in the so-called measurement bandwidth (5-100mHz), making use of ion and micro-thruster actuators. The DFAC unit has been designed entirely on a simplified discrete-time model (Embedded Model) derived from the fine dynamics of the spacecraft and its environment; the relevant control algorithms are implemented and tuned around the Embedded Model, which is the core of the control unit. The DFAC has been tested against uncertainties in spacecraft and environment and its code has been the preliminary model for final code development. The DFAC assumes an all-propulsion command authority, partly abandoned by the actual GOCE control system because of electric micro-propulsion not being fully developed. Since all-propulsion authority is expected to be imperative for future scientific and observation missions, design and simulated results are believed to be of interest to the space communit

Photovoltaic/thermal systems based on concentrating and non-concentrating technologies: Working fluids at low, medium and high temperatures

The present article provides an overview about photovoltaic/thermal systems categorised by the temperature of the working fluid: Low-temperature (lower than 60º C), medium-temperature (between 60 and 90º C) and hightemperature (higher than 90º C). Concerning photovoltaic/thermal-air systems for low-temperature use, the majority of studies involve building-integrated non-concentrating systems with phase change materials and working-fluid temperatures at around 30-55º C. Concerning low-temperature photovoltaic/thermal-water systems, a large number of studies are about non-concentrating configurations appropriate for building-integrated and, in general, domestic applications with working fluids at approximately 50–60º C. Regarding nonconcentrating photovoltaic/thermal systems for medium-temperature use, a large number of references are appropriate for industrial and domestic applications (working fluids: air; water) with around 60-70º C workingfluid temperatures. The literature review about medium-temperature concentrating photovoltaic/thermal systems shows that the majority of investigations concern photovoltaic/thermal-water systems with concentration ratios up to 190X and working fluids at approximately 62-90º C, appropriate for domestic and waterdesalination applications. As for high-temperature concentrating photovoltaic/thermal systems, most of them have concentration ratios up to 1000X, involve parabolic concentrators and use water (as the working fluid) at around 100-250º C. Moreover, in the field of high-temperature photovoltaic/thermal systems, most of the configurations are appropriate for building and industrial applications, and consist of triple-junction or siliconbased photovoltaic/thermal cells. In light of the issues mentioned above, a critical discussion and key challenges (in terms of materials, efficiencies, technologies, etc.) are presented.The authors would like to thank ’’Ministerio de Economía y Competitividad’’ and “Ministerio de Ciencia e Innovación” of Spain for the funding (grant references ENE2016-81040-R and PID2019-111536RBI00). D. Chemisana thanks ’’Institució Catalana de Recerca i Estudis Avançats (ICREA)’’ for the ICREA Acadèmia award. Chr. Lamnatou is Lecturer of the Serra Húnter programme. Figures 1–6: reproduced with permission

Solar Cells Operating under Thermal Stress

Operating a solar cell under thermal stress at temperatures >100°C and up to 500°C seems counterintuitive because conversion efficiency drops dramatically. Even so, there are cases in which solar cells are in high-illumination high-temperature conditions, for near-the-sun space missions and in various terrestrial hybrid systems involving solar-to-thermal energy conversion. This review analyzes the progress of solar cells tested in the laboratory under thermal stress. The fundamental physics governing the thermal sensitivity of solar cells and the main criteria determining the ability of semiconductor materials to survive high temperatures are recalled. Materials and architectures of a selection of the solar cells tested so far are examined. Deviation from the Shockley-Queisser limit at each temperature is used for a fair assessment of the performances. Our analysis reveals the strengths and weaknesses of the existing technologies and the gaps to be filled to develop new classes of solar cells capable of withstanding high temperatures.This work was developed in the frame of the French program Investments for the Future managed by the National Agency for Research under contract ANR-10-LABX-22-01-SOLSTICE. C.L. is lecturer of the Serra Húnter programme. D.C. thanks the Institució Catalana de Recerca i Estudis Avançats (ICREA) for the ICREA Acadèmia, and the Ministerio de Ciencia e Innovación (project PID2019-111536RB-I00)

Energy and Luminous Performance Investigation of an OPV/ETFE Glazing Element for Building Integration

The combination of architectural membranes such as ethylene tetrafluoroethylene (ETFE) foils and organic photovoltaic (OPV) cells offers a wide range of possibilities for building integration applications. This is due to their flexibility, free-shape, variable color and semitransparency, light weight, cost-effectivity, and low environmental impact. In addition, electrical generation is provided. Four configurations of ETFE foils designed to be integrated onto a south façade glazing element were studied for two representative European locations with different climatic conditions: Barcelona and Paris. These configurations comprise a reference one based on a double ETFE foil with a 10 mm air gap in between, and the other three incorporate on the inner ETFE foil either OPV cells covering 50% or 100% of its surface or a shading pattern printed on it covering 50% of its surface. Results show that, in terms of energy, the configuration with higher OPV coverage area is the one achieving the lowest net energy consumption in both locations. However, when looking at the illumination comfort this option results in insufficient illumination levels. Therefore, a tradeoff strategy balancing energy performance and illumination comfort conditions is necessary. Based on that, the best solution found for both cities is the configuration integrating OPV cells covering 50% of the glazing area and for a window to wall ratio of 0.45.This research was supported by the “Generalitat de Catalunya” (grants 2018FI_B1_00136, 2017 SGR 1276 and ICREA Academia) and “Ministerio de Economía y Competitividad” of Spain (grant reference ENE2016-81040-R)

Local density of electromagnetic states within a nanometric gap formed between two thin films supporting surface phonon polaritons

We present a detailed physical analysis of the near-field thermal radiation spectrum emitted by a silicon carbide (SiC) film when another nonemitting SiC layer is brought in close proximity. This is accomplished via the calculation of the local density of electromagnetic states (LDOS) within the gap formed between the two thin films. An analytical expression for the LDOS is derived, showing explicitly that (i) surface phonon polariton (SPhP) coupling between the layers leads to four resonant modes, and (ii) near-field thermal radiation emission is enhanced due to the presence of the nonemitting film. We study the impact of the interfilm separation gap, the distance where the fields are calculated, and the thickness of the nonemitting layer on the spectral distribution of the LDOS. Results show that for an interfilm gap of 10 nm, the near-field spectrum emitted around the SPhP resonance can increase more than an order of magnitude as compared to a single emitting thin layer. Interfilm SPhP coupling also induces a loss of spectral coherence of resonance, mostly affecting the low frequency modes. The effect of the nonemitting film can be observed on LDOS profiles when the distance where the fields are calculated is close to the interfilm gap. As the LDOS is calculated closer to the emitter, the near-field spectrum is dominated by SPhPs with small penetration depths that do not couple with the modes associated with the nonemitting film, such that thermal emission is similar to what is observed for a single emitting layer. Spectral distribution of LDOS is also significantly modified by varying the thickness of the nonemitting film relative to the thickness of the emitting layer, due to an increasing mismatch between the cross-coupled SPhP modes. The results presented here show clearly that the resonant modes of thermal emission by a polar crystal can be enhanced and tuned, between the transverse and longitudinal optical phonon frequencies, by simply varying the structure of the system. This analysis provides the physical grounds to tune near-field thermal radiation emission via multilayered structures, which can find application in nanoscale-gap thermophotovoltaic power generation.publisher versio