Wolfgang Spirkl

Inherent limitations of volumetric solar receivers

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.

Optical constants and film density of TiNxOy solar selective absorbers

Thin TiNxOy films were deposited on glass substrates in an activated reactive evaporation process. The influence of the reactive gas composition and pressure during deposition on the film density ρMass was investigated by grazing incidence x‐ray‐diffraction measurements. The density increases from 3.8 to 4.8 g/cm3 when lowering the total gas pressure ptotal from 10−3 h Pa down to 2.5×10−4 h Pa, whereas the ratio of the partial gas pressures of nitrogen and oxygen has no measurable effect on the film density. Data on the refractive index are presented, obtained from reflectance and transmittance measurements in the optical wavelength range (λ=0.3–2.5 μm). Bruggeman effective medium theory was applied to explain the optical behavior n(λ) as a result of the low density of the coatings. The high porosity of the films suppresses the metallic character of the phases TiN and TiO contained in the TiNxOy film. Its dielectriclike behavior in the infrared region is essential for low thermal emittances when deposite...

Nonlinear Emission and Recombination in Conjugated Polymer Waveguides

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 i...

Dynamic solar collector performance testing

Abstract A dynamic solar collector model in conjunction with a dynamic parameter identification and performance prediction method is presented. It promises to make possible solar collector (loop) testing under instationary outdoor meteorological conditions, variable inlet temperature and variable volume flow rate. Measurements for three test sequences were made outdoors under instationary conditions on a 6 m2 flat plate collector array with double glazing. A multi-node model with three parameters (effective area, effective loss coefficient andthermal capacity) is used. Collector parameters are determined for one sequence and the collector performance is then predicted for the remaining two sequences using the same model with the already determined parameters. The error of predicting the temperature difference across the collector, averaged over a sequence, is found to lie at or below 0.2 K. A computer program package providing the parameter identification and performance prediction algorithms as well as the collector model is available.

Performance limitations of rotationally symmetric nonimaging devices

An upper limit on concentration for any optical device has previously been derived from the conservation of etendue. In this contribution we derive more stringent upper limits for the efficiency and the concentration of rotationally symmetric optical devices that are a consequence of skewness conservation. If the desired source and target have different skewness distributions, then losses or dilution or both will limit the performance of the optical system. We calculate the limiting curve of efficiency versus concentration and provide a design example that is virtually at this limit. We conjecture that even rotationally symmetric problems may benefit from asymmetric optical systems.

Optimized reflectors for non-tracking solar collectors with tubular absorbers

Abstract We present an approach to find optimal reflector shapes for non-tracking solar collectors under practical constraints. We focus on cylindrical absorbers and reflectors with translational symmetry. Under idealized circumstances, edge ray reflectors are well known to be optimal. However, it is not clear how optimal reflectors should be shaped in order to obtain maximum utilizable energy for given operating temperatures under practical constraints like reflectivity less than unity, real radiation data, size limits, and gaps between the reflector and the absorber. For a prototype collector with a symmetric edge ray reflector and a tubular absorber, we derive from calorimetric measurements under outdoor conditions the optical efficiency as a function of the incidence angle. Using numerical optimization and raytracing, we compare truncated symmetric edge ray reflectors, truncated asymmetric edge ray reflectors and free forms parametrized by Bezier splines. We find that asymmetric edge ray reflectors are optimal. For reasonable operating conditions, truncated asymmetric edge ray reflectors allow much better land use and easily adapt to a large range of roof tilt angles with marginal changes in collector construction. Except near the equator, they should increase the yearly utilizable energy per absorber tube by several percent as compared to the prototype collector with symmetric reflectors.

Optimized compact secondary reflectors for parabolic troughs with tubular absorbers

Parabolic trough type solar power plants could reach higher efficiencies by using secondary reflectors, which increase the concentration of the solar irradiance onto the absorber tube. Recently, compact secondary concentrators have been proposed which are constructed according to the edge ray principle; they are composed of an involute part and an edge ray reflector with a form close to a straight line. Using these reflector forms as a starting point, we optimize numerically various secondary concentrators and investigate their performance by means of raytracing, taking into account reflectivity losses, shading and the effective solar angular distribution, i.e. the sunshape. We investigate the performance of secondary concentrators with the outer part approximated by a straight line. The entire characteristic curve of collection efficiency versus concentration was investigated, for both a pillbox sunshape and a Gaussian sunshape. It is found that the secondary reflector significantly improves the concentration and is essentially optimal in a wide class of shapes, even at efficiencies close to unity.

General model for testing solar domestic hot water systems

Abstract For modelling solar domestic hot water (SDHW) systems, a plug flow store model is presented. It is used in conjunction with the dynamic system testing algorithm to predict the long term performance of SDHW systems from short term test data. The basic property of a plug flow model is its capability of modelling drawoffs without any mixing inside the store. As an extension, the model developed in this paper covers the range between pure plug flow and full mixing. The degree of draw off mixing is characterized by a continuous parameter instead of a (discrete) number of thermal nodes which actually is defined neither for a plug flow model nor for a real system. Less than ten model parameters characterize a system under test. It was found that the parameters can be identified in short time (two to four weeks outdoors) for a large class of systems, e.g. thermosyphon systems or integrated collector store (ICS) systems, with load side heat exchanger, auxiliary heater, and heat pipe collectors. A summary of experimental results for different systems is given.

Optimized secondary concentrators for a partitioned central receiver system

We present secondary concentrators with non-regular shapes for increasing the concentration of radiation from a given field of heliostats, well suited for partitioning the receiver into several units, arranged side by side. For a general heliostat field with a non-axisymmetric directional distribution of the radiation at the entrance aperture of the secondary concentrator, concentrators with non-regular shape can significantly increase the concentration as compared to their symmetric analogs. Our optimizations indicate best results for concentrators based on rectangular entrance and exit apertures. The concentration may be increased by a factor of 2.3 at an optical efficiency of 90%. If the shape of the exit aperture is required to be close to circular, concentrators based on non-regular hexagonal apertures may reach concentration higher than their symmetric analogs by a factor of 1.3. For the given radiation, concentrators with polygonal apertures perform significantly better than concentrators with smooth elliptic apertures.

In situ characterization of solar flat plate collectors under intermittent operation

The Bayerisches Zentrum fur angewandte Energieforschung Bayern (ZAE) performs an in situ test of eight different solar domestic hot water systems in Oberpfalz, Bayern. We present results of the evaluation of the collector loop from one system. This system has been chosen because due to the relatively low load the variability of the collector loop temperature is expected to be insufficient for testing purposes. The system under test consisted of the collector field and the piping from the cellar to the roof. First, we use a dynamic collector model which ignores the extra pipes and thus implicitly takes into account the pipes by modified parameter values. It is shown that this model can be inaccurate if the input data such as the irradiance and the inlet temperature do not vary sufficiently and independently from each other. Second, we use a model which explicitly takes into account the pipes, using the overall loss coefficient and the total specific heat capacity of the pipes as new parameters. It is shown that it is possible to identify the values of these additional parameters. The overall accuracy improves with the extended model. The models without and with pipes differ mainly in dynamic effects associated with the energy stored in the pipes during stops of the collector loop pump.