Small thermophotovoltaic prototype systems

In an earlier paper we reported on a small grid-connected thermophotovoltaic (TPV) system consisting of an ytterbia mantle emitter and silicon solar cells with 16% efficiency (under solar irradiance in standard test conditions, STCs). The emitter was heated up using a butane burner with a rated thermal power of 1.35 kW (referring to the lower heating value). This system produced an electrical output of 15 W, which corresponds to a thermal to electric (direct current) conversion efficiency of 1.1%. In the interim, further progress has been made, and significantly higher efficiencies have been achieved. The most important development steps are: (1) the infrared radiation-absorbing water filter between emitter and silicon cells (to protect the cells against overheating and against contact with flue gasses) has been replaced by a suitable glass tube. By doing this, it has been possible to prevent losses of convertible radiation in water. (2) Cell cooling has been significantly improved, in order to reduce cell temperature, and therefore increase conversion efficiency. (3) The shape of the emitter has been changed from spherical to a quasi-cylindrical geometry, in order to obtain a more homogeneous irradiation of the cells. (4) The metallic burner tube, on which the ytterbia emitter was fixed in the initial prototypes, has been replaced by a heat-resistant metallic rod, carrying ceramic discs as emitter holders. This has prevented the oxidation and clogging of the perforated burner tube. (5) Larger reflectors have been used to reduce losses in useful infrared radiation. (6) Smaller cells have been used, to reduce electrical series resistance losses. Applying all these improvements to the basic 1.35 kW prototype, we attained a system efficiency of 1.5%. By using preheated air for combustion (at approximately 370 °C), 1.8% was achieved. In a subsequent step, a photocell generator was constructed, consisting of high-efficiency silicon cells (21% STC efficiency). In this generator, the spaces between the cells were minimized, in order to achieve as high an active cell area as possible, while simultaneously reducing radiation losses. This new system has produced an electrical output of 48 W, corresponding to a system efficiency of 2.4%. This is the highest-ever-reported value in a silicon-cell-based TPV system using ytterbia mantle emitters. An efficiency of 2.8% was achieved by using preheated air (at approximately 500 °C). An electronic control unit (fabricated of components with low power consumption, and including a battery store) was developed, in order to make the TPV system self-powered. This unit controls the magnetic gas supply valve between gas supply cylinder and burner as well as the high-voltage ignition electrodes. Both the control unit’s own power consumption and the battery-charging power are supplied directly by the TPV generator. A small commercial inverter is used to transfer excess power to the 230 V grid.     

Pyrolysis-field ionization mass spectrometry of modified bagasse fractions

The fractions obtained from bagasse after activation with 17.5% NaOH, alternative alkylation with quarternary ammonium groups and extraction with 80% ethanol, water and 5% NaOH, were analyzed using pyrolysis-field ionization mass spectrometry (Py-FIMS). The proposed degradation products from lignin were mostly symmetric fragments containing two aromatic rings while the compounds formed from polysaccharides were the result of dehydration reactions. From the temperature-resolved Py-FIMS it could be seen that ethanol-lignin starts to degrade at temperatures lower than 180°C before the degradation of the hemicellulose fraction. The lignin markers are eliminated from hemicellulose fractions before the polysaccharide gasification starts. The quarternary ammonium groups are cleaved from the fractions at temperatures lower than 200°C.     

Zehn weitere Briefe zwischen K.E. v. Baer und L.F. und R. Froriep aus den Jahren 1825–1851

Following the publication of 7 letters from Baer to the Frorieps by H.E. Müller-Dietz inNTM N.S. 1 (1993), 2 further letters from Baer and 8 from the Frorieps are published. A curious technical problem in the presentation of the journalNotizen published by Froriep in his Landes-industriecomptoir in Weimar is discussed at length. Froriep recommends his son Robert, to whom Baer sends several enquiries and commissions, which Robert deals with carefully (1831 in Jena, 1849 in Paris). Robert explains in great detail his intention to leave Jena for Berlin in 1831. After his father's death in 1847 he returns to take charge of the Landesindustriecomptoir and begins to cooperate with the newly founded Russian Geographic Society as publisher on behalf of the Society. But about 1850 the cooperation is discontinued by the Russians, and Froriep, who is in economic difficulties, asks Baer (who lives and works in St. Petersburg) urgently (but in vain) to help him with the Society.     

Decision support in early development phases—A case study from machine engineering

In the case study presented in this paper we consider early development phases of a mechanical product. We want to evaluate different concepts and decide which one(s) to pursue. A problem in early phases is that usually no test runs are available. In our case study, based on a standard, there are ways to compute the lifetime distributions of the components of the different concepts. Some parameters needed for these computations are not known precisely. Unfortunately, the lifetime distributions of the components are highly sensitive to these parameters. Our approach is to equip these parameters with distributions. These distributions would be called prior distributions in Bayesian terminology, but no update is possible since no test runs are available. Our approach implies that the distribution of the system lifetime for each concept is random, i.e. we get random elements in the space of lifetime distributions. Using Monte-Carlo simulations, we demonstrate several ways to compare the random lifetime distributions of the concepts. Some of these comparisons use stochastic orderings. We also introduce a new stochastic ordering which is particularly suitable for reliability purposes. Our case study, consisting of three scenarios, allows us to demonstrate some conclusions that can be reached.     

Context relevance assessment and exploitation in mobile recommender systems

In order to generate relevant recommendations, a context-aware recommender system (CARS) not only makes use of user preferences, but also exploits information about the specific contextual situation in which the recommended item will be consumed. For instance, when recommending a holiday destination, a CARS could take into account whether the trip will happen in summer or winter. It is unclear, however, which contextual factors are important and to which degree they influence user ratings. A large amount of data and complex context-aware predictive models must be exploited to understand these relationships. In this paper, we take a new approach for assessing and modeling the relationship between contextual factors and item ratings. Rather than using the traditional approach to data collection, where recommendations are rated with respect to real situations as participants go about their lives as normal, we simulate contextual situations to more easily capture data regarding how the context influences user ratings. To this end, we have designed a methodology whereby users are asked to judge whether a contextual factor (e.g., season) influences the rating given a certain contextual condition (e.g., season is summer). Based on the analyses of these data, we built a context-aware mobile recommender system that utilizes the contextual factors shown to be important. In a subsequent user evaluation, this system was preferred to a similar variant that did not exploit contextual information.     

Formation of compounds by high-flux nitrogen ion implantation in titanium

Polycrystalline titanium was implanted with nitrogen ions at energies from 30 to 60 keV and with doses from 1 × 10¹⁶ to 1.5 × 10¹⁸ N⁺-ions cm^–2 at room temperature. The implanted titanium layers were investigated by high-voltage electron microscopy and transmission high-energy electron diffraction. The formation of titanium nitride, titanium carbonitride and titanium carbide phases were considered in relation to their dependence on nitrogen ion implantation dose. In the dose range from 1 × 10¹⁶ up to 1.5 × 10¹⁸ N⁺-ions cm^–2 the cubic phase -TiN _x was formed. In the dose range between 1 to 2.5 × 10¹⁷ and 1.5 × 10¹⁸N⁺-ions cm^–2 the tetragonal phase -Ti₂N was found in addition to the -TiN _x phase. The lattice structure of these titanium nitride phases is a function of the nitrogen ion implantation dose. Additionally, the presence of titanium carbonitrido TiC _y O _x and titanium carbide TiC _y phases can be proved. The analysed titanium nitride, titanium carbUnitride and titanium carbide phases are represented in a state diagram as a function of implantation dose.     

The effect of reaction conditions on montmorillonite-catalysed peptide formation

Oligomerization of glycine (gly) and diglycine (gly₂) on montmorillonite was performed as cyclic, drying-wetting process at temperatures below 100°C, under varying reaction conditions. The influence of substrate/clay ratio, temperature and pH was found to be different for amino acid (AA) dimerization, cyclic anhydride (CA) formation and peptide chain elongation. High temperatures and neutral pH favour CA formation over diglycine production. An AA/catalyst ratio of 0.2 mmol/g leads to optimal yields. This supports the assumption that amino acid dimerization and CA formation take place at the edges of clay particles. Peptide chain elongation, starting from gly₂, produces higher yields at higher temperatures and neutral pH.