Donald L. Chubb

Rare Earth Doped High Temperature Ceramic Selective Emitters

As a result of their electron structure, rare earth ions in crystals at high temperature emit radiation in several narrow bands rather than in a continuous blackbody manner. This study develops a spectral emittance model for films of rare earth containing materials. Although there are several possible rare earth doped high temperature materials, this study was confined to rare earth aluminum garnets. Good agreement between experimental and theoretical spectral emittances was found for erbium, thulium and erbium-holmium aluminum garnets. Spectral emittances of these films are sensitive to temperature differences across the film. Emitter efficiency is also a sensitive function of temperature. For thulium aluminum garnet the efficiency is 0.38 at 1700 K but only 0.19 at 1262 K.

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...

Rare earth optical temperature sensor

A rare earth optical temperature sensor is disclosed for measuring high temperatures. Optical temperature sensors exist that channel emissions from a sensor to a detector using a light pipe. The invention uses a rare earth emitter to transform the sensed thermal energy into a narrow band width optical signal that travels to a detector using a light pipe. An optical bandpass filter at the detector removes any noise signal outside of the band width of the signal from the emitter.

Thermophotovoltaics for Space Power Applications

Thermophotovoltaic (TPV) energy conversion has long been considered a potential replacement for thermoelectrics in radioisotope powered deep space power systems. In this application, TPV offers significant potential improvements in both efficiency and mass specific power (W/kg), performance which is considered mission enabling for a variety of mission concepts. TPV systems powered by concentrated solar energy have also been proposed for inner planetary solar system missions. This concept takes advantage of TPV’s ability to store energy for shadow periods in the form of heat energy rather than as electrical energy (batteries), as is commonly done for photovoltaic power systems. The simplicity and large number of power cycles offered by the thermal energy storage offers potential system benefits compared to a photovoltaic / battery system. Recent efforts in the development of radioisotope TPV (RTPV) at Creare have resulted in the demonstration of converter efficiencies in excess of 19%. Several independent ...

Rare earth doped yttrium aluminum garnet (YAG) selective emitters

Abstract As a result of their electron structure, rare earth ions in crystals at high temperature emit radiation in several narrow bands rather than in a continuous blackbody manner. This study presents a spectral emittance model for films and cylinders of rare earth doped yttrium aluminum garnets. Good agreement between experimental and theoretical film spectral emittances was found for erbium and holmium aluminum garnets. Spectral emittances of films are sensitive to temperature differences across the film. For operating conditions of interest, the film emitter experiences a linear temperature variation whereas the cylinder emitter has a more advantageous uniform temperature. Emitter efficiency is also a sensitive function of temperature. For the holminum aluminum garnet film the efficiency is 0.35 at 1446 K, but only 0.27 at 1270 K.

Dual layer selective emitter

A selective emitter consisting of two layers separated by a vacuum is analyzed. The bottom layer consists of a selective emitting material such as a rare earth containing crystal on a metal substrate. The top layer, which blocks long wavelength radiation, is a window such as sapphire with a deposited metal film. As a result of reduced long wavelength emission, the theoretical analysis shows that the emitter efficiency can be increased by nearly a factor of 2.

Semiconductor Silicon as a Selective Emitter

Silicon operating in a vacuum is a good candidate thermal emitter since it has a high melting point (1680 K). The semiconductor bandgap, which can provide selective emission, adds to the potential for high operating temperature and, therefore, high radiated power. We present the detailed emitter theory, along with both theoretical and experimental results for spectral emittance of thin (∼1 μm) silicon films on sapphire substrates with a platinum backing. These results show the importance of temperature and film thickness in determining the selective spectral emittance and, with the proper material parameters, can be readily extended to other materials and systems.

Population inversion calculations using near‐resonant charge exchange as a pumping mechanism

Theoretical results are presented for population inversions in a Ca–Xe system. Preliminary results show that it is possible to obtain inversions greater than 1014 m−3 with inversion times up to 8 × 10−7 sec. A possible charge‐exchange laser system using an MPD arc plasma accelerator is also described.

Light Pipe Thermophotovoltaics (LTPV)

In a conventional thermophotovoltaic (TPV) energy converter the radiation from the emitter to the photovoltaic (PV) array is transmitted in a vacuum or air where the index of refraction, n = 1. The intensity of the radiation is proportional to n2. Therefore, the incident intensity on the PV array could be greatly increase if the medium between the emitter and the PV array had n > 1. This light pipe TPV (LTPV) concept was introduced by The Quantum Group at the Third National Renewable Energy Laboratory (NREL) TPV Conference in 1997.This paper presents a theoretical analysis of the LTPV concept. The solution of the one‐dimensional energy equation that includes both thermal conduction and radiation yields the temperature distribution through the light pipe. Applying the analysis to a zinc selenide (ZnSe) light pipe yielded the following result. For an emitter temperature of 1000K the convertible radiation(photon energy >PV bandgap energy) that reaches the photovoltaic(PV) cell is 1 W/cm2. At the same emitter...

Theoretical Comparison of Erbium, Holmium and Thulium Aluminum Garnet Selective Emitters

Spectral control through the use of selective emitters is an important means of improving the efficiency of thermophotovoltaic (TPV) systems. The availability of PV cells having a wide range of bandgap energies, along with the development of selective emitters having a range of emission band energies, gives the developer of TPV power systems a wide range of options. The rare earth aluminum garnet selective emitters developed in our laboratory offer a number of advantages when used in such systems [1–3]. In this paper, we present results of a detailed computational study of the effects of a number of design and operating parameters, including emitter film thickness, scattering within the film, and the temperature gradient across the film thickness, on the performance of three rare‐earth aluminum garnet selective emitters. Specifically, we present emitter efficiency, net emittance, wavelength‐dependent emittance, total output power, and the useful output power, for Er3Al5O12, Ho3Al5O12, and Tm3Al5O12 emitte...