This report presents an assessment of the efficiency and power density limitations of thermophotovoltaic (TPV) energy conversion systems for both ideal (radiative-limited) and practical (defect-limited) systems. Thermodynamics is integrated into the unique process physics of TPV conversion, and used to define the intrinsic tradeoff between power density and efficiency. The results of the analysis reveal that the selection of diode bandgap sets a limit on achievable efficiency well below the traditional Carnot level. In addition it is shown that filter performance dominates diode performance in any practical TPV system and determines the optimum bandgap for a given radiator temperature. It is demonstrated that for a given radiator temperature, lower bandgap diodes enable both higher efficiency and power density when spectral control limitations are included. The goal of this work is to provide a better understanding of the basic system limitations that will enable successfiil long-term development of TPV energy conversion technology.

A Monte Carlo code has been developed to simulate individual electron interactions. The code has been instrumental in determining the range of validity for the widely used condensed history method. This task was accomplished by isolating and testing the condensed history assumptions. The results show that the condensed history method fails for low energy electron traiisport due to inaccuracies in energy loss and spatial positioning.

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an integrated test which incorporates four fundamental parameters of TPV direct energy conversion. These are: (1) the TPV devices, (2) spectral control, (3) a radiatiodphoton source, and (4) closed cavity geometry affects. The overall efficiency of the TPV device is controlled by the TPV cell performance, the spectral control characteristics, the radiator temperature and the geometric arrangement. Controlled efficiency measurements and predictions provide valuable feedback on all four. This paper describes and compares two computer codes developed to model 16,l cm2 TPV cells (in a 4x4 configuration) in a cavity geometry. The first code, subdivides the infrared spectrum into several bands and then numerically integrates over the spectrum to provide absorbed heat flux and cell performance predictions (assuming infinite parallel plates). The second, utiliies a Monte Carlo Ray-Tracing code that tracks photons, from birth at the radiation source, until they either escape or are absorbed. Absorption depends upon energy dependent reflection probabilities assigned to every geometrical surface within the cavity The model also has the capability of tallying above and below bandgap absorptions (as a function of location) and can support various radiator temperature profiles. The arrays are fabricated using 0.55 eV InGaAs cells with Si/SiO interference filters for spectral control and at steady state conditions, array efficiency was calculated as the ratio of the load matched power to its absorbed heat flux. Preliiary experimental results are also compared with predictions.

NOTICERecently, the photon Monte Carlo code, RACER-X, was modified to include wavelength dependent absorption coefficients and indices of refraction. This work was done in an effort to increase the code's capabilities to be more applicable to a wider range of problems.These new features make RACER-X useful for analyzing devices like Monolithic Interconnected Modules (MIMs) which have etched surface features and incorporates a back surface reflector (BSR) for spectral controL A series of calculations were perfonned on various MIM structures to determine the impact that surEace features and component reflectivities have on spectraI utilization. The traditionaI concern of "cavity photonics" is replaced with ''intracell photonics" in the MIM design. Like the cavity photonic problem previously discussed [ll, spall changes in optical properties and/or geometry can lead to large changes in spectral utilization. The calculations show that seemingly innocuous surface features (e.g., trenches and grid lines) can significantly reduce the spectral utilization due to the non-normal incident photon flux. Photons that enter the device through a trench edge are rehted onto a trajectory where they will not escape. ?hi leads to a reduction in the number of reflected below bandgap photons that r e m to the mdiator and reduce the specaal utilization. In addition, trenches expose a lateral conduction layer in this particular series of calculations which increase the absorption of above bandgap photons in inactive material.

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A thermophotovoltaic (TPV) efficiency measurement, withii a closed cavity, is an integrated test which incorporates four fundamental parameters of TPV direct energy conversion. These are: (1) the TFV devices, (2) spectral control, (3) a radiation/ photon source, and (4) closed cavity geometry effects. The overall efficiency of the TPV device is controlled by the TPV cell performance, the spectral control characteristics, the radiator temperature and the geometric arrangement. Controlled efficiency measurements and predictions provide valuable feedback on all four. This paper describes and compares two computer codes developed to model 16, 1 cm2 TPV cells (in a 4x4 configuration) in a cavity geometry. The first code, subdivides the infrared spectrum into several bands and then numerically integrates over the spectrum to provide absorbed heat flux and cell electrical output performance predictions (assuming infinite parallel plates). The second code, utilizes a Monte Carlo Photon Transport code that tracks photons, from birth at the radiation source, until they either escape or are absorbed. Absorption depends upon energy dependent reflection probabilities assigned to every geometrical surface within the cavity. The model also has the capability of tallying above and below bandgap absorptions (as a function of location) and can support various radiator temperature profiles. The arrays were fabricated using 0.55 eV InGaAs cells with Si/SiO interference filters for spectral control and at steady state conditions, array efficiency was calculated as the ratio of the load matched power to its absorbed heat flux. Preliminary experimental results are also compared with predictions. an integrated test which incorporates four fundamental parameters of TPV direct energy conversion. These are: (1) the TPV devices, (2) specUal control, (3) a radiatioxdphoton source, and (4) closed cavity geometry effects. The overall efficiency of the TPV device is controlled by the TPV cell performance, the spectral control characteristics, the radiator temperature and the geometric arrangement. Controlled efficiency measurements and predictions provide valuable feedback on all four. This paper describes and compares two computer codes developed to model 16,l an2 TPV cells (in a 4x4 configuration) in a cavity geometry. The first code, subdivides the infrared spectrum into several bands and then numerically integrates over the spectrum to provide absorbed beat flux and cell electrical output performance predictions (assuming infinite parallel plates). The second code, utilizes a Monte Carlo Photon Transport code that tracks photons, from birth at the radiation source, until they either escape or are absorbed. Absorption depends upon energy dependent reflection probabilities assigned to every geometrical surface within the cavity. The model also has the capability of tallying above and below bandgap absorptions (as a function of location) and can support various radiator temperature profiles. The arrays were fabricated using 0.55 eV ...