Heat transfer in a directly irradiated solar receiver/reactor for solid–gas reactions

Particle laden solar receivers can be used at high temperatures for efficient heat transfer and fuel generation via chemical reactions. A theoretical analysis of a directly irradiated, particle laden, solar receiver is presented here and compared with experiments. The radiation characteristics of the particles are approximated using a method, which adapts Mie theory to certain cases where a solar receiver is used with seeded particles of variable sizes and shapes. Based on this model carbon black particles whose effective radius, r_p, is less than 100 nm are inefficient in absorbing solar energy and the most suitable particle sizes is in the same range as the wavelengths of the radiation (100 nm < r_p < 1000 nm). The heat transfer coefficient between the particles and the gas was calculated using a refined limiting sphere model developed for the transition regime between molecular and continuum transfer. Previous models assume that there are no gas molecule collisions in the energy transfer layer and the mean free path of the gas molecules is equal to the thickness of this layer. The present model accounts for molecule collisions in the energy transfer layer and therefore enables the thickness of this layer to be larger than one mean free path length. The model was extended to estimate the Nusselt number for gases with several atoms as well as for monatomic gas. A code to simulate the flow and heat transfer in the receiver was developed, utilizing the models for heat transfer from sunlight to the particles and from the particles to the gas. The receiver simulations show good agreement with the wall temperature distribution measured in experiments, but the gas exit temperature in the model was significantly lower than the measured value. This discrepancy could be due to limitations of the simulation code and the particle heat transfer models. The simulation suggests that changing the Nusselt number and particle radius have a small influence on the receiver wall and gas temperatures. Increasing the particle cloud concentration improves the receiver heat transfer up to a threshold value; further increase of the particles concentration has only a marginal influence on the receiver’s heat transfer. This result from the receiver modeling was in a good agreement with solar experiments.     

Charging performance of automotive batteries—An underestimated factor influencing lifetime and reliable battery operation

Dynamic charge acceptance and charge acceptance under constant voltage charging conditions are for two reasons essential for lead-acid battery operation: energy efficiency in applications with limited charging time (e.g. PV systems or regenerative braking in vehicles) and avoidance of accelerated ageing due to sulphation. Laboratory tests often use charge regimes which are beneficial for the battery life, but which differ significantly from the operating conditions in the field.     

A comparison between 24‐hour decays and Butler–Volmer Corrosion Rates

Twenty‐four‐hr decays are commonly used to assess the performance of cathodic protection on reinforced concrete. This approach has been empirically demonstrated by the performance of systems in the field. More recently an approach has been used to extrapolate corrosion rates from the decays and applied current density. This paper compares the two approaches using laboratory data and field data from a large number of cathodically protected structures to assess the natural variability in potentials and corrosion rates.     

Identification of Lactobacillus curvatus TMW 1.624 dextransucrase and comparative characterization with Lactobacillus reuteri TMW 1.106 and Lactobacillus animalis TMW 1.971 dextransucrases

Recently, it was affirmed that the exopolysaccharides (EPSs) of Lactobacillus curvatus TMW 1.624, Lactobacillus reuteri TMW 1.106 and Lactobacillus animalis TMW 1.971 improve the quality of gluten-free breads and that they can be produced in situ to levels enabling baking applications. In this study we provide insight into the molecular and biochemical background of EPS production of these three strains. EPS formation strongly correlated with growth and took place during the exponential phase. Gtf genes were heterologously expressed, purified and their enzymatic properties as well as the structures of the EPSs formed were compared. Structural comparison of EPS formed by heterologously expressed glucosyltransferases (Gtfs) and of those formed by the wildtype lactobacilli confirmed that the respective genes/enzymes were identified and examined. The glucan formed by L. animalis Gtf was identified as a linear low molecular weight dextran. Optimal enzymatic conditions were pH 4.4 and 45 °C for the L. reuteri Gtf and pH 4.4 and 31 °C for L. curvatus Gtf. The Gtf from L. animalis had an optimal pH of 5.8 and displayed more than 50% of activity over a broad temperature profile (22–59 °C). The three Gtfs were stimulated by various mono- and divalent metal ions, dextran, as well as levan to different extents.