Brian S. Good

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.

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).

Temperature‐dependent efficiency calculations for a thin‐film selective emitter

Given the degree of current interest in thin‐film thermophotovoltaic (TPV) devices, a realistic detailed estimate of the efficiency of these devices is of great importance. In this paper we describe computer calculations of the efficiency of a rare‐earth garnet selective emitter, based on the theoretical development of Chubb et al., who describe emitter efficiencies calculated assuming a constant temperature across the emitter thickness. It is known from experiment, however, that in many cases of interest the temperature is not constant. We have therefore extended this earlier work by computing numerically the emitter efficiency when the temperature is not uniform across the emitter thickness. We have restricted our attention to temperature profiles that are linear across the emitter thickness. This assumption is based on earlier simplified iterative computations of the temperature profile using a simplified source term, though these results are suggestive rather than definitive. Computed results for emit...

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.

Effect of temperature gradient on thick film selective emitter emittance

A temperature gradient across a thick (⩾.1 mm) film selective emitter will produce a significant reduction in the spectral emittance from the no temperature gradient case. Thick film selective emitters of rare earth doped host materials such as yttrium-aluminum-garnet (YAG) are examples where temperature gradient effects are important. In this paper a model is developed for the spectral emittance assuming a linear temperature gradient across the film. Results of the model indicate that temperature gradients will result in reductions the order of 20% or more in the spectral emittance.

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...

Effects of geometry on the efficiency of TPV energy conversion

The importance of view factors in determining the efficiency of a thermophotovoltaic (TPV) energy conversion system was pointed out at the first NREL TPV meeting by D. C. White and H. C. Hottel. In this paper we consider the effect of view factors on efficiency for both planar- and cylindrical-geometry TPV systems. One method for reducing the view factor radiation loss is to introduce reflecting surfaces at locations where the radiation loss occurs. We also consider the effect of these additional reflectors on efficiency. To determine the cavity efficiency (efficiency of emitter, window, filter and lateral reflector in combination) a set of nine algebraic equations for the radiation fluxes incident on each of the cavity components must be solved. The solution depends on the wavelength-dependent optical properties of the components, as well as the view factors between components. Once the fluxes are obtained as functions of wavelength, the cavity efficiency can be calculated. Results to be presented will i...

Optimization study of selective emitter thermophotovoltaic systems

At the NASA Lewis Research Center, the authors have developed a systems model for a general thermophotovoltaic (TPV) system. The components included in the model are a solar concentrator, receiver, emitter, window, filter and photovoltaic (PV) array. The system model requires the concentrator and receiver efficiencies, the wavelength dependence of the optical properties of the components, together with the emitter temperature, and the PV cell spectral response and current-voltage characteristic. With these inputs, the system efficiency and power output are calculated. For a selective emitter TPV system it is the emitter spectral emittance that is the major determinant of system performance. In this paper, the authors present results of an optimization study of a selective emitter TPV system. They vary the emitter temperature, the spectral emittances and emission bandwidth of the emitter, the PV cell bandgap energy, the cell back-surface reflectivity, and the long-wavelength emission band limit of the emitter, and discuss the effects that the variation of each of these parameters has on system performance.

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.