Roland A. Lowe

High efficiency indium gallium arsenide photovoltaic devices for thermophotovoltaic power systems

The development of indium gallium arsenide (Eg=0.75 eV) photovoltaic devices for thermophotovoltaic power generation is described. A device designed for broadband response had an air mass zero efficiency of 11.2 % and an internal quantum yield of over 90% in the range of 800 to 1500 nm. Devices designed for narrow‐band response have also been developed. Both structures are based on a n/p junction which also makes them applicable for integration into indium phosphide based, monolithic, tandem solar cells for solar photovoltaic applications.

Rare earth garnet selective emitter

Thin film (Ho)‐yttrium aluminum garnet (YAG) and Er‐YAG emitters with a platinum substrate exhibit high spectral emittance in the emission band (eλ≊0.75, 4I15/2–4I13/2, for Er‐YAG and eλ≊0.65, 5I7–5I8 for Ho‐YAG) at 1500 K. In addition, low out‐of‐band spectral emittance, eλ<0.2, suggest these materials would be excellent candidates for high efficiency selective emitters in thermophotovoltaic (TPV) systems operating at moderate temperatures (1200–1500 K). Spectral emittance measurements of the thin films were made (1.2<λ<3.0 μm) and compared to the theoretical emittances calculated using measured values of the spectral extinction coefficient.

InGaAs PV device development for TPV power systems

Indium gallium arsenide (InGaAs) photovoltaic devices have been fabricated with bandgaps ranging from 0.75 eV to 0.60 eV on Indium Phosphide (InP) substrates. Reported efficiencies have been as high as 11.2 percent (AMO) for the lattice matched 0.75 eV devices. The 0.75 eV cell demonstrated 14.8 percent efficiency under a 1500 K blackbody with a projected efficiency of 29.3 percent. The lattice mismatched devices (0.66 and 0.60 eV) demonstrated measured efficiencies of 8 percent and 6 percent respectively under similar conditions. Low long wavelength response and high dark currents are responsible for the poor performance of the mismatched devices. Temperature coefficients have been measured and are presented for all of the bandgaps tested.

Solar thermophotovoltaic (STPV) system with thermal energy storage

A solar thermophotovoltaic (STPV) system has both terrestrial and space applications because thermal energy storage can be utilized. Excellent properties (heat of fusion=1800 j/gm and melting temperature=1680 K) make silicon the ideal thermal storage material for an STPV system. Using a one dimensional model with tapering of the silicon storage material, it was found that several hours of running time with modest lengths (∼15 cm) of silicon are possible. Calculated steady‐state efficiencies for an STPV system using an Er‐YAG selective emitter and ideal photovoltaic (PV) cell model are in the range of 15%–17%. Increasing the taper of the storage material improves both efficiency and power output.

Emittance theory for thin film selective emitter

Thin films of high temperature garnet materials such as yttrium aluminum garnet (YAG) doped with rare earths are currently being investigated as selective emitters. This paper presents a radiative transfer analysis of the thin film emitter. From this analysis the emitter efficiency and power density are calculated. Results based on measured extinction coefficients for erbium‐YAG and holmium‐YAG are presented. These results indicate that emitter efficiencies of 50% and power densities of several watts/cm2 are attainable at moderate temperatures (<1750 K).

Radiative performance of rare earth garnet thin film selective emitters

In this paper we present the first emitter efficiency results for the thin film 40% Er‐1.5% Ho YAG (Yttrium Aluminum Garnet, Y3Al5O12) and 25% Ho YAG selective emitter at 1500 K with a platinum substrate. Spectral emittance and emissive power measurements were made (1.2<λ<3.2 μm). Emitter efficiency and power density are significantly improved with the addition of multiple rare earth dopants. Predicted efficiency results are presented for an optimized (equal power density in the Er, 4I15/2−4I13/2@ 1.5 μm, and Ho, 5I7−5I8@ 2.0 μm emission bands) Er‐Ho YAG thin film selective emitter.

Comparison of selective emitter and filter thermophotovoltaic systems

At the NASA Lewis Research Center we have developed a systems model for a general thermophotovoltaic (TPV) system. The components included in the model are a solar concentrator, a receiver, a thermal storage module, an emitter, a protective window, a filter, and a photovoltaic (PV) array. The system model requires the wavelength dependence of the optical properties of the components, together with the PV cell spectral response and the cell current‐voltage characteristics. With these inputs, the system efficiency, the emitter or filter efficiencies, the PV cell efficiency, the emitter operating temperature, and the cell output power density are calculated.In this paper we compare the performance of a variety selective emitter and filter TPV systems. The overall system model is based on the solar TPV system being developed jointly by McDonnell‐Douglas and NASA. In the current study, the concentrator, receiver, and storage parameters are fixed; only the characteristics of the emitter/filter and the PV cell a...

Thermophotovoltaic energy converters based on thin film selective emitters and InGaAs photovoltaic cells

This paper presents the results of an investigation to demonstrate thermophotovoltaic energy conversion using InGaAs photovoltaic cells, yttrium‐aluminum‐garnet‐ (YAG‐) based selective emitters, and bandpass/reflector filters, with the heat source operating at 1100 °C. InGaAs cells were grown on InP by organometallic vapor phase epitaxy with bandgaps of 0.60 and 0.75 eV and coupled to Ho‐, Er‐, and Er‐Tm‐doped YAG selective emitters. Infrared reflector and/or shortpass filters were also used to increase the ratio of in‐band to out‐of‐band radiation from the selective emitters. Efficiencies as high as 13.2% were recorded for filtered converters.

Pulsed laser illumination of photovoltaic cells

In future space missions, free electron lasers (FEL) may be used to illuminate photovoltaic receivers to provide remote power. Both the radiofrequency (RF) and induction FEL produce pulsed rather than continuous output. In this work, the authors investigate space power solar cell response to pulsed laser light which simulates the RF FEL format. The results indicate that if the pulse repetition is high, cell efficiencies are only slightly reduced compared to constant illumination at the same wavelength. The frequency response of the cells is weak, with both voltage and current outputs essentially DC in nature. Comparison with previous experiments indicates that the RF FEL pulse format yields more efficient photovoltaic conversion than does an induction FEL format.

The effect of temperature on the radiative performance of Ho-YAG thin film selective emitters

In this paper, the authors present the thermophotovoltaic (TPV) converter emitter efficiency results for a thin film 25% Ho YAG (yttrium aluminum garnet, Y/sub 3/Al/sub 5/O/sub 12/) selective emitter from 1000-1700 K with a platinum substrate. Spectral emittance and emissive power measurements were made (1.2</spl lambda/<3.2 /spl mu/m) and used to calculate the radiative efficiency. The radiative efficiency and power density of rare earth doped selective emitters are strongly dependent on temperature and experimental results indicate an optimum temperature (1650 K for Ho YAG) for TPV applications.