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The flow in volumetric absorbers is investigated using a simple mathematical model. It is found that there are several restrictions and failure mechanisms that are inherent to the volumetric absorber, regardless of the precise structural details, material properties, etc. The heat that the absorber can extract safely is limited by flow-related constraints. Multiple steady solutions exist for certain parameter values: a “fast” solution corresponding to a low exit temperature, a “slow” solution which is unstable, and a “choked” solution for which the absorber is near to stagnation temperature. The existence of multiple solutions may lead to abrupt local “switching” and absorber failure. For a given irradiance applied to the absorber, the existence and the character of the solutions are determined by a single dimensionless parameter, the Blow parameter B. Neglecting the variation of the hydraulic resistivity with temperature may lead to a dangerous overestimate of the receiver’s ability to sustain irradiation. For reasonable efficiencies control of mass flow or outlet temperature of the absorber, rather than pressure control, may be required.

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A general model of a solar thermophotovoltaic device is discussed 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. First we assume ideal components to calculate theoretical maximum efficiency. It corresponds to that of a perfect selective absorber in conjunction with a Carnot-engine ranging from 53% for 1 sun to 85% for the highest possible irradiance of 5×104 suns. The improvement over an ideal one-gap device is roughly a factor of 2. Consideration of available materials shows that any improvement in efficiency can be expected only for high irradiance of 1000 suns. The sensitive parameter is the selectivity of the absorber-emitter. Concentration of diffuse solar radiation is not feasible. Perspectives appear not much better than for existing technologies such as photovoltaics or solar tower applications.

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The emission dynamics in photoexcited planar conjugated polymer waveguides is investigated at high excitation densities. Using femtosecond pump/probe experiments and photoluminescence spectroscopy we investigate the interplay of nonlinear radiative and nonradiative recombination processes. Amplified spontaneous emission (ASE) leads to an ultrafast depletion of the excited state at excitation densities above 1018 cm−3 in an ladder-type poly(p-phenylene) film deposited on a glass substrate. Owing to the different waveguide structure ASE is not observed for the same polymer deposited on an indium–tin–oxide (ITO)-coated substrate. Instead, we observe nonradiative bimolecular annihilation with a coefficient γ=4.2×10−9 cm3 s−1. Our results demonstrate that even in the absence of a resonator collective stimulated emission can be much more efficient than nonradiative recombination. A mandatory prerequisite, however, is a suitable waveguide design. The use of ITO as a hole-injecting contact is problematic due to its high refractive index and its relatively high losses.