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The high density excitation dynamics in waveguides of a ladder-type poly(p-phenylene) (LPPP) conjugated polymer are studied. Results show that amplified spontaneous emission causes a rapid depletion of the excited state at excitation densities over 1.6 x 10(super 18)/cu cm in LPPP films deposited on a glass substrate. For LPPP films deposited on an indium-tin-oxide substrate, bimolecular excitation annihilation was the dominant recombination channel for excitation densities over 10(super 18)/cu cm.

The mechanism of gas heat conduction at arbitrary Knudsen numbers and for a wide variety of geometries was investigated using a modified temperature-jump model. The proposed model takes into account the distinction between an inner convex surface and an outer concave surface enclosing the inner surface. It was found that the the modified temperature-jump model is equivalent to the interpolation formula of Sherman which implies the equivalence to solutions of the Boltzmann equation derived through the four momenta approach. The model was extended to the free-molecule environment.

Geometric and thermodynamic arguments are used to derive upper limits on the performance of a solar energy collection system, consisting of an axisymmetric heliostat field, a solar tower, secondary optics and a black receiver. Performance limits on collected power, concentration, and work output are presented. Performance of tower systems with several secondary optics options is compared: tower-top Compound Parabolic Concentrator (CPC), Tailored Edge-Ray Concentrator (TERC) approximated by a cone, and Cassegrainian with ground-level CPC or Compound Elliptic Concentrator (CEC). Optimized ray tracing is used to generate the design parameters of the secondary concentrators that yield the highest optical efficiency. The results show that the tower-top Cone provides the best performance regarding both concentration and efficiency, except for very large fields. The Cassegrainian designs come in second, but become equal and even better than the Cone for large fields. The results for the Cassegrainian are sensitive to the value of the reflectivity, due to the additional reflections incurred. The choice of a CEC is better than a CPC for the terminal concentration in a Cassegrainian system, but the difference is small. The suitability of the different design options for high-temperature solar applications is discussed. The recommendations regarding optical configuration depend on field size, as well as on application-specific constraints.

We model the decomposition reaction of a solid powder into a solid plus gas in a mixed homogeneous open solar reactor and derive criteria for stability. Instability may occur via a feedback mechanism whereby gas produced in the decomposition reaction reduces the inflow, which leads to even higher reaction rate. For a variety of reactions proposed for solar application, we list the limits of stable operation as the maximum fraction of solid educt in the input carrier gas both for complete as well as for partial decomposition.

We consider volumetric thermal absorbers exposed to nonuniform irradiance. The fluid flows perpendicular to the absorber surface. The fluid transport from front to rear surface elements is connected in a parallel configuration. We investigate the optimum adjustment of the local mass flux or, equivalently, the local absorber temperature. Three different strategies are compared: (1) strategy of uniform mass flux, (2) strategy of uniform exit temperature, and (3) a strategy with a locally optimized temperature-flow profile. Significant improvement of strategy (2) compared to (1) and of strategy (3) compared to (2) is observed for the case of a black-body absorber which is isothermal in the direction normal to the absorber surface. We find it to be advantageous to include even regions where the stagnation temperature is below the desired exit temperature.

Thermal surface absorbers convert all incident radiation to heat at a single local temperature. The fluid flows perpendicular to the radiation's propagation direction. In contrast, in volumetric absorbers the fluid flows parallel to the radiation's propagation direction, and the absorber may exhibit temperature stratification along the radiation's direction. This raises the question whether reabsorption of parts of the thermal emission coming from the hotter absorber sections renders the volumetric absorber superior to the surface absorber. For the case of isotropic radiation, we compare the efficiency of the volumetric and the surface absorber with each other and with an isothermal absorber. We find that the nonselective volumetric absorber is less efficient than the nonselective surface absorber for the assumption of perfect heat transfer between absorber and fluid. Thus we conclude that in practical high-flux applications, the superiority of volumetric absorbers stems from the enhanced heat transfer area. If the fluid flows against the direction of radiation propagation the volumetric absorber is slightly more efficient than if it flows with this direction. We also discuss the effect of the two-flux approximation on simulation of isotropic volumetric absorbers.

In stationary heat-loss experiments, the thermal losses by gas conduction of an evacuated flat-plate solar collector (EFPC) were experimentally determined for different values of interior gas pressure. The experiments were carried out with air and argon in the pressure range from [10.sup.-3] to [10.sup.4] Pa. For air, loss reduction sets in at 100 Pa, whereas at 0.1 Pa heat conduction is almost completely suppressed. Using argon as filling gas, gas conduction is reduced by 30 percent (compared to air) at moderate interior pressures of 1000 Pa. With decreasing pressure this reduction is even greater (50 percent reduction at 10 Pa). A theory was developed to calculate thermal losses by gas conduction in an EFPC: Fourier's stationary heat conduction equation was solved numerically (method of finite differences) for the special geometry of the collector. From kinetic gas theory a formula for the pressure dependency of the thermal conductivity was derived covering the entire pressure range. The theory has been validated experimentally for the gases air and argon. Calculations for krypton and xenon show a possible gas conduction loss reduction of 60-70 percent and 75-85 percent (with respect to air, depending on gas pressure), corresponding to a reduction of the overall collector losses of up to 40 percent.

Grazing incidence x-ray diffraction studies of glass substrate-deposited TiN(x)O(y) thin films enable an analysis of the changes in film density rho(mass) with composition and pressure of the reactive gas. The rho(mass) value is unchanged with N2 and O2 partial pressures, while it is enhanced with a reduction in total gas pressure. The optical wavelength of the coatings is affected by porosity, which affects the metallic nature of the TiN and TiO phases.

Parameters are identified by a new method in the grazing incidence X-ray reflectometry, based on a nonlinear least-squares approach with on-line low-pass filtering, which also suits the measured reflectivity curve as a function of the incidence angle. The filter is applied in the angular frequency domain. The global solution is determined by the automatic restarting. The data having large scattering vectors is assessed using the roughness model's generalization. An accuracy of 0.2 nanometers is determined with the thickness.

The performance of series-connected multigap photovoltaic cells under various meteorological conditions was evaluated. Cells for a fixed design point were compared to cells that were optimally adjusted to an actual operating condition. The degree of mismatch between the design and operation conditions were then calculated numerically. Results show that the sensitivity of multigap cells to meteorological conditions is almost the same as that of one-gap cells.

Discusses a model of a solar thermophotovoltaic (STPV) device both for improving the efficiency of one-band-gap photovoltaic cells by matching the photon energy to the band gap and for concentrating diffuse radiation. Definition of thermophotovoltaic; Investigation of a general SPTV system with ideal components; Characteristics of a one-band-gap photovoltaic cell.

We propose a method for the global optimization of stochastic functions. During the course of the optimization, a probability distribution is built up for the location and the value of the global optimum. The concept of information entropy is used to make the optimization as efficient as possible. The entropy measures the information content of a probability distribution, and thus gives a criterion for decisions: From several possibilities we choose the one which yields the most information concerning location and value of the global maximum sought.

For parabolic trough solar collectors with tubular absorbers, we design new tailored secondary concentrators. The design is applicable for any rim angle of a parabolic reflector. With the secondary, the concentration can be increased by a factor of more than 2 with a compact secondary reflector consisting of a single piece, even for the important case of a rim angle of 90 deg. The parabolic reflector can be used without changes; the reduced absorber is still tubular but smaller than the original absorber and slightly displaced toward the primary.