177 research outputs found
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
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 âGeneralitat de Catalunyaâ (grants 2018FI_B1_00136, 2017 SGR 1276 and ICREA Academia) and âMinisterio de EconomĂa y Competitividadâ of Spain (grant reference ENE2016-81040-R)
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