Optimal synthesis of heat-and-power systems: the operating line method

The physical and mathematical models on which the operating line method (OLM) (H. A. Irazoqui, Chem. Engng Sci. 41, 1243–1255 (1986); P. A. Aguirre, E. O. Pavani and H. A. Irazoqui, Chem. Engng Sci. 44, 803–816 (1989)) for the optimal synthesis of heat-and-power systems is built, are discussed in depth. These models include the heat exchange ‘modes’ allowed and the general features of the type of solution sought in order to reach an optimal scheme for the total energy systems in chemical plants. A thorough development of the mathematical technique used to tackle the optimization problem is also made. This development comprises the derivation of the necessary and sufficient conditions for optimality.     

Understanding and characterizing nestedness in mutualistic bipartite networks

In this work we present a dynamical model that successfully describes the organization of mutualistic ecological systems. The main characteristic of these systems is the nested structure of the bipartite adjacency matrix describing their interactions. We introduce a nestedness coefficient, as an alternative to the Atmar and Patterson temperature, commonly used to measure the nestedness degree of the network. This coefficient has the advantage of being based on the robustness of the ecological system and it is not only describing the ordering of the bipartite matrix but it is also able to tell the difference, if any, between the degree of organization of each guild.     

Microcutting culture and morpho-physiological changes during acclimation in two Lycium chilense cytotypes.

Lycium chilense, a deciduous perennial shrub, is one of the endangered native species of Patagonia due to sheep overgrazing. Chances of recolonization by seeds are scarce due to the limited density of propagules in the soil and very specific requirements for germination. The objective was to develop an in vitro propagation protocol that would help to perform reestablishment of this species in degraded areas of the Patagonian steppe. Seeds came from two provenances with different somatic number due to differences in ploidy level. Defoliated microcuttings were planted in test tubes with different growing media and taken to a growth chamber. Rooting percentage did not differ between origins, but higher values were encountered for medium without hormones. Subcultures increased significantly rooting percentage and reduced time to rooting. The leaves from micropropagated plants were thinner, did not exhibit hairs, and had poorly developed palisade parenchyma and less epicuticular waxes. In vitro leaves had lower stomatal density and their stomata were less functional when compared to acclimated leaves. A repopulation program of Lycium chilense based on microcutting culture, specially using subcultures, is feasible.     

When intermittent power production serves transient loads

Renewable power generation exhibits notorious intermittence. The power load varies daily and also seasonally. The topic of renewable generation, storage and grid interfacing is complex in that it brings into one setting many diverse interests and technologies. Our long-term goal is to help define ways to profitably increase renewable generation. In this paper, we focus on normal day for a grid operator, PJM, (Pennsylvania, New Jersey and Maryland). The variability of wind and (and assumed) solar outputs require a certain capability for load following or storage. Using dynamic modelling, we estimate the variability of the wind output and we simulate a projected solar penetration of 3% of new capacity. To save for eventual use every unit of energy thus generated, a storage system must have the capability to levelize the supply of renewable power. The capacity requirements for storage and generation of such a system are mapped out in 1 min intervals, and are used to define the capacities and ramp rates for a hypothetical pumped storage plant. Knowledge of weather patterns may be helpful to plan dispatch and storage of renewable energy. The results of a brief excursion into the difficult topic of weather patterns are recorded here too.     

Preparation and characterization of polyvinylidene fluoride nanofibrous membranes by forcespinning™

Nanofibers of polyvinylidene fluoride (PVDF) were prepared using solutions of PVDF with acetone and dimethylacetamide. The solutions were prepared at different concentrations (18, 21.5, and 25% wt) of PVDF. The nanofiber membranes were produced by using the Forcespinning? method. Parameters such as the orifice size and spinneret angular velocity were varied. The produced membranes were characterized using scanning electron microscopy, differential scanning calorimetry, and X‐ray diffraction to determine the effect of varying parameters in the Forcespinning? process on fiber diameter, bead formation, thermal stability, polymorphism, and morphology of the fibers and overall structure of the membrane. It was observed that polymer concentration played a key role in fiber and bead formation; at higher concentrations (such as 25 wt%), the fiber diameter increased but the bead formation decreased. The prepared composite membranes have potential applications on separators for Li‐ion batteries, ultra filtration membranes, and proton conductivity membranes for fuel cells applications. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers     

Biodegradability of polyethylene–starch blends prepared by extrusion and molded by injection: Evaluated by response surface methodology

Low density polyethylene (LDPE)–starch blends were prepared by extrusion. Starch content ranged between 0 and 50% under different conditions of temperature and extrusion speed. Each blend was injected into a commercial mold of a 250 mL cup. Cups were cut into 5 cm × 5 cm coupons. Biodegradability was assessed placing the coupons in the middle of a 50 cm pile of compost. Samples were recovered, washed, dried, and weighed after 25, 50, 75, 100, and 125 days under compost. Weight loss was determined and structural modifications were evaluated by SEM. Mechanical properties as tensile strength, elongation to break, and yield point were assessed before and after compost treatment. Experimental design and characterization were performed using a central composite design (CCD) and results were modeled with surface response methodology. SEM analysis revealed fractures and pores as a consequence of microorganism degradation. Pure LDPE samples remained unchanged. Mechanical, physical, and thermal properties of LDPE–starch blends are slightly different from that of pure LDPE. Environmental exposure, measured by accelerated intemperism, does not modify blend properties; consequently, they are suitable for the same industrial applications of LDPE.     

Antioxidant activity and genotoxic effect on HeLa cells of phenolic compounds from infusions of Quercus resinosa leaves

Water infusions of mature and fresh Quercus resinosa leaves were evaluated for antioxidant activity and genotoxic effects on HeLa cells. Native Mexicans used to drink Q. resinosa leaves tea as a refreshing beverage. The air dried leaves were pulverised and boiled in water, then their phenolic content and condensed tannins were determined. The chromatographic profile of 15 phenolic components in Quercus leaves infusions was also determined by HPLC. In vitro analysis of antioxidant capacity of leaves infusion extracts were performed by the DPPH method and the deoxyribose assay. The genotoxicity of Q. resinosa leaves extracts was evaluated on HeLa cells as well as its underlying mechanism by the single-cell electrophoresis assay (comet assay). Results show that fresh leaves infusions increase the oxidative process and other damage to DNA in transformed human cells. Fresh leaves from Q. resinosa may serve as a potential source of phenolics with anticancer activity.     

High resolution FIB-TEM and FIB-SEM characterization of electrode/electrolyte interfaces in solid oxide fuel cells materials

A Focused Ion Beam (FIB)/lift-out technique was used to prepare site-specific thin samples of the cathode/electrolyte interface of Intermediate Temperature Solid Oxide Fuel Cells (IT-SOFC) materials. The cathode under study was a nanostructured perovskite of composition La_0.4Sr_0.6Co_0.8Fe_0.2O_3-δ (LSCF) deposited by spin coating on a Ce_0.9Gd_0.1O_2-δ (CGO) supporting substrate. We compared the results for a 15 μm and a 5 μm thickness cathode layers, before and after a thermal treatment of 1000 h at 500 °C, with the aim of simulating operation conditions. Both, Transmission (TEM) and Scanning (SEM) Electron Microscopy, coupled with Energy Dispersive Spectroscopy (EDS) systems, were used to characterize the composition and nanostructure at both sides of the cathode/electrolyte interface. To our knowledge, this is the first time that a semi-coherent interface between LSCF and CGO was observed by Electron Diffraction as well as by High Resolution TEM in many points at the interfacial boundary. A large difference in total contact area was observed between the thickest and the thinnest cathode layers, despite they present the same composition and nano sized structure. The real contact area in the 5 μm cathode sample is around 50% less than in the 15 μm sample due to the presence of pores at the interface. This observation may partially explain the difference in resistivity observed for these two half cells assemblies. On the other side, no differences were found comparing composition and nanostructure at the interface before and after the thermal treatment. Thus, this study becomes fundamental to understand the role played by the interface for improving the performance of IT-SOFC under long time operation conditions: a necessary premise for its real application.