Susan Murray

Greater than 20% radiant heat conversion efficiency of a thermophotovoltaic radiator/module system using reflective spectral control

An InGaAs monolithic interconnected module (MIM) using reflective spectral control has been fabricated and measured in a thermophotovoltaic radiator/module system (radiator, optical cavity, and thermophotovoltaic module). Results showed that at a radiator and module temperature of 1039/spl deg/C and 25/spl deg/C, respectively, 23.6% thermophotovoltaic radiator/module system radiant heat conversion efficiency and 0.79W/cm/sup 2/ maximum thermophotovoltaic radiator/module system power density were obtained. The use of reflective spectral control increased the spectral efficiency and thus the thermophotovoltaic radiator/module system radiant heat conversion efficiency by /spl sim/16% (relative). However, the amount of useful radiation reaching the MIM decreased by /spl sim/7% (relative) compared to using transmissive spectral control. Also, the thermophotovoltaic system radiant heat conversion efficiency and maximum power density using either transmissive or reflective spectral control decreased as the MIM temperature increased. The MIM using reflective spectral control was found to be more sensitive to changes in the MIM temperature than the MIM using transmissive spectral control.

Thermophotovoltaic Converter Performance for Radioisotope Power Systems

The development of lightweight, efficient power for emerging NASA missions and recent advances in thermophotovoltaic (TPV) conversion technology have renewed interest in combining radioisotope heat sources with photovoltaic energy conversion for Radioisotope Power Systems (RPS) for spacecraft. TPV power conversion uses advanced materials able to utilize a broader, spectrally tuned range of wavelengths for more efficient power conversion than Si solar cells. Spectral control, through choices of selective radiant emitters, TPV modules, and filters, is key to high‐efficiency operation. This paper describes performance tests of an array of TPV cells with boundary conditions prototypical of an RPS. TPV performance tests were conducted at prototypical array size (≅100 cm2), emitter temperature (1350 K), and heat rejection temperature (300 K). Test hardware included InGaAs TPV cells at 0.60 eV band‐gap, with tandem plasma/interference filters for spectral control. At the target emitter temperature of 1350 K, a c...

MOCVD growth of lattice-matched and mismatched InGaAs materials for thermophotovoltaic energy conversion

The details of MOCVD growth of lattice-matched (0.74 eV) and lattice-mismatched (0.55 eV and 0.6 eV) InGaAs-based thermophotovoltaic (TPV) devices on InP substrates are discussed. The optimization of growth conditions, structural parameters and run-to-run consistency have played a key role in the development of high quality TPV devices, particularly in the development of lattice-mismatched materials.

Advanced Thermophotovoltaic Devices for Space Nuclear Power Systems

Advanced thermophotovoltaic (TPV) modules capable of producing > 0.3 W/cm2 at an efficiency > 22% while operating at a converter radiator and module temperature of 1228 K and 325 K, respectively, have been made. These advanced TPV modules are projected to produce > 0.9 W/cm2 at an efficiency > 24% while operating at a converter radiator and module temperature of 1373 K and 325 K, respectively. Radioisotope and nuclear (fission) powered space systems utilizing these advanced TPV modules have been evaluated. For a 100 We radioisotope TPV system, systems utilizing as low as 2 general purpose heat source (GPHS) units are feasible, where the specific power for the 2 and 3 GPHS unit systems operating in a 200 K environment is as large as ∼ 16 We/kg and ∼ 14 We/kg, respectively. For a 100 kWe nuclear powered (as was entertained for the thermoelectric SP‐100 program) TPV system, the minimum system radiator area and mass is ∼ 640 m2 and ∼ 1150 kg, respectively, for a converter radiator, system radiator and environ...

Thermophotovoltaic Converter Design for Radioisotope Power Systems

The development of lightweight, efficient power for emerging NASA missions and recent advances in thermophotovoltaic (TPV) conversion technology have renewed interest in combining radioisotope heat sources with photovoltaic energy conversion. Thermophotovoltaic power conversion uses advanced materials able to utilize a broader, spectrally tuned range of wavelengths for more efficient power conversion than solar cells. Spectral control, including selective emitters, TPV module, and filters, are key to high‐efficiency operation. This paper outlines the mechanical, thermal, and optical designs for the converter, including the heat source, the selective emitter, filters, photovoltaic (PV) cells, and optical cavity components. Focus is on the emitter type and the band‐gap of InGaAs PV cells in developing the design. Any component and converter data available at the time of publication will also be presented.

Multi-wafer growth and processing of 0.6-eV InGaAs monolithic interconnected modules

Recent progress in the optical and electrical performance of monolithic interconnected modules (MIMs) has produced an interest in manufacturing large quantities of cells for evaluation. Information resulting from this evaluation is necessary to produce and optimize a TPV system, where a large number of devices with a nominal performance must be available for insertion into series/parallel electrical networks. In this work over 130 wafers comprising three different device designs were grown, with representative wafers from each design processed in a pilot-line manufacturing environment. This paper describes the material growth, device design and processing, and electrical performance of these cells.

Development of Thermophotovoltaic Devices Optimized for High Temperature Operation

The recent interest in thermophotovoltaics (TPV) for space power applications has necessitated consideration of new device designs that are optimized for operation at elevated temperatures in order to be compatible with practical space‐platform power systems. Photovoltaic device performance degrades as operating temperature is increased due to increasing dark current density (Jo). A cooling system is therefore required to maintain a low cell temperature with respect to the emitter (∼1300 K). However, the cold‐side cooling capacity is directly related to the temperature, size and mass of a passive radiator. Therefore, since a high power to weight ratio is a critical factor for spacecraft power system design, the ability to realize devices capable of improved power density at elevated temperatures is critical to minimize the mass of space TPV systems. A simple model has been developed to estimate the effect of temperature on device electrical properties. Design parameters under investigation are the modific...

Effect of Metal Coverage on the Performance of 0.6‐eV InGaAs Monolithic Interconnected Modules

With the device performance of 0.6eV InGaAs monolithic interconnected modules (MIMs) reaching open circuit voltages of 400 mV/junction and achieving excellent quantum efficiency, the next step to improve performance focuses on controlling the parasitic optical absorption in these MIMs. With an integrated spectral control approach, the design of grid finger and interconnect metallization affects both the output power and the optical absorption of the MIM. The effect of metal coverage on the optical and electrical performance of MIMs processed in a multi‐wafer environment is presented.

Flexible solar cells in milliseconds: Pulse Thermal Processing of CdTe devices

Materials for a CdTe solar cell (ITO/CdS/CdTe/Cu/Pt) were sputtered at room temperature onto kapton, then transformed from resistive layers into a working solar cell by Pulse Thermal Processing (PTP), a novel radiant heat treatment developed at Oak Ridge National Laboratory (ORNL). Unlike conventional device fabrication approaches, the solar cell was a complete device, front-to-back contact, prior to heat treatment. In this proof-of-concept approach, the I-V curves for the as-deposited sputtered materials demonstrate little measurable photovoltaic (PV) activity, but achieved a Voc of 634 mV after PTP. Based on process simulations, its estimated that the material/device transformation occurred in under 30 ms, while maintaining the kapton substate at temperatures below 250 °C.