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Open Access Publications from the University of California
Cover page of New approach to waste-heat energy harvesting: pyroelectric energy conversion

New approach to waste-heat energy harvesting: pyroelectric energy conversion

(2019)

© 2019, The Author(s). Harvesting waste heat for useful purposes is an essential component of improving the efficiency of primary energy utilization. Today, approaches such as pyroelectric energy conversion are receiving renewed interest for their ability to turn wasted energy back into useful energy. From this perspective, the need for these approaches, the basic mechanisms and processes underlying their operation, and the material and device requirements behind pyroelectric energy conversion are reviewed, and the potential for advances in this area is also discussed.

Cover page of Paradise of novelty-or loss of human capital? Exploring new fields and inventive output

Paradise of novelty-or loss of human capital? Exploring new fields and inventive output

(2019)

© 2018 The Author(s). Does a person become more or less creative when exploring a new field? Exploring new fields exposes a person to new knowledge that might increase the novelty of inventive output; at the same time, exploration means a lack of prior expertise and a learning challenge that might harm the value of that output. Using new combinations as a measure of novelty and citations as a measure of value, we demonstrate correlations between exploring new fields and increased novelty-but decreased value-in an inventor-firm fixed effects panel. The negative effect of exploring new fields on value is muted when the novice collaborates with experts or uses the scientific literature in the new field. We find consistent results using an unintended change in noncompete labor law as an exogenous influence on exploring new fields. The research illustrates two opposite influences of exploration on creative output and suggests how inventors can reduce the downside of entering a new field.

Cover page of Low-copy nuclear sequence data confirm complex patterns of farina evolution in notholaenid ferns (Pteridaceae)

Low-copy nuclear sequence data confirm complex patterns of farina evolution in notholaenid ferns (Pteridaceae)

(2019)

© 2019 Elsevier Inc. Notholaenids are an unusual group of ferns that have adapted to, and diversified within, the deserts of Mexico and the southwestern United States. With approximately 40 species, this group is noted for being desiccation-tolerant and having “farina”—powdery exudates of lipophilic flavonoid aglycones—that occur on both the gametophytic and sporophytic phases of their life cycle. The most recent circumscription of notholaenids based on plastid markers surprisingly suggests that several morphological characters, including the expression of farina, are homoplasious. In a striking case of convergence, Notholaena standleyi appears to be distantly related to core Notholaena, with several taxa not before associated with Notholaena nested between them. Such conflicts can be due to morphological homoplasy resulting from adaptive convergence or, alternatively, the plastid phylogeny itself might be misleading, diverging from the true species tree due to incomplete lineage sorting, hybridization, or other factors. In this study, we present a species phylogeny for notholaenid ferns, using four low-copy nuclear loci and concatenated data from three plastid loci. A total of 61 individuals (49 notholaenids and 12 outgroup taxa) were sampled, including 31 out of 37 recognized notholaenid species. The homeologous/allelic nuclear sequences were retrieved using PacBio sequencing and the PURC bioinformatics pipeline. Each dataset was first analyzed individually using maximum likelihood and Bayesian inference, and the species phylogeny was inferred using *BEAST. Although we observed several incongruences between the nuclear and plastid phylogenies, our principal results are broadly congruent with previous inferences based on plastid data. By mapping the presence of farina and their biochemical constitutions on our consensus phylogenetic tree, we confirmed that the characters are indeed homoplastic and have complex evolutionary histories. Hybridization among recognized species of the notholaenid clade appears to be relatively rare compared to that observed in other well-studied fern genera.

Cover page of Validation of Pronghorn friction-dominated porous media thermal-hydraulics model with the SANA experiments

Validation of Pronghorn friction-dominated porous media thermal-hydraulics model with the SANA experiments

(2019)

© 2019 Elsevier B.V. A significant challenge in the core modeling of pebble bed reactors (PBRs) is the complex fuel-coolant structure. At the expense of approximating local flow and heat transfer effects, porous media models can provide medium-fidelity predictions of complicated thermal-fluid systems with significantly less computational cost than high-fidelity Computational Fluid Dynamics (CFD) models. This paper presents a new porous media code, Pronghorn – a fast-running core simulator intended to accelerate the design and analysis cycle for PBRs and provide boundary conditions for systems-level analysis. This paper describes the physical models in Pronghorn and demonstrates the capability of a friction-dominated model for predicting gas-cooled PBR decay heat removal by presenting simulation results for all 52 of the steady-state axisymmetric German SANA experiments, which include two different fluids and three different types of pebbles. The pebble temperature in all 52 cases is predicted with a mean error (predicted minus experimental) of +22.6 °C with standard deviation of 54.6 °C. To demonstrate Pronghorn's capability for modeling bed-to-plenum heat and mass transfer, one open-plenum SANA experimental case is also simulated. A code-to-code comparison with Flownex and GAMMA shows that Pronghorn is comparable in accuracy to other porous media simulation tools, with the additional advantages of 1) an arbitrary equation of state; 2) 3-D unstructured mesh capabilities; and 3) multiphysics coupling to other Multiphysics Object-Oriented Simulation Environment (MOOSE) applications. Finally, the effect of several porous media closure selections, in particular the porosity, the near-wall treatment for effective solid thermal conductivity, the interphase drag and heat transfer, and the fluid thermal dispersion, on temperature predictions are quantified.

Cover page of An approach for modeling tachyons with gravitation

An approach for modeling tachyons with gravitation

(2019)

This work expands previous efforts, within the classical theories of Special and General Relativity, to include tachyons (faster-than-light particles) along with ordinary (slower-than-light) particles at any energy. The objective here is to construct a Hamiltonian that includes both the particles and the gravitational field that they produce. We do this with a linear approximation for the Einstein field equations; and we also assume a time-independent gravitational metric implied by a static picture of the particles’ motion. The resulting formulas will allow serious modeling to test the idea that cosmic background neutrinos may be tachyons, which can produce the observed gravitational effects now ascribed to some mysterious Dark Matter.

Cover page of Environment or food: Modeling future land use patterns of miscanthus for bioenergy using fine scale data

Environment or food: Modeling future land use patterns of miscanthus for bioenergy using fine scale data

(2019)

© 2019 Elsevier B.V. Using fine-scale panel data and an econometric model, we predict land use change in the Midwestern United States if a new bioenergy crop, Miscanthus × Giganteus (miscanthus), is introduced. To explain farmers' current crop choices, we use a local, limited dependent variable regression based on soil and weather characteristics. To this model, we add miscanthus as a new crop, based on its place dependent BioCro model-predicted yield. We find that the vast majority of land used to grow miscanthus will come from land now used for non-major crops, pasture, woodland, and other uses. This implies that miscanthus can help mitigate climate change by displacing oil usage without causing food conflict.

Cover page of A new continuum model of the incoherent interface compared with growth of a spinel rim on an olivine grain

A new continuum model of the incoherent interface compared with growth of a spinel rim on an olivine grain

(2019)

© 2019 Elsevier B.V. In a polymorphic change in which the phases differ only by a reversible difference in specific volume, kinematics requires a unit mass to suffer deviatoric strain in the instant it is transformed. Unlike the Eshelby stress–free strain, this strain is a property of the motion. Its existence must be considered when formulating the constitutive relation for the product of an incoherent transformation. To show this, two models are compared: in both, the (Nabarro) condition of vanishing shear stress is imposed at the incoherent interface; they differ only in the treatment of the deviatoric strain at issue. In the existing model, deviatoric stress within a unit mass of product is determined by total deviatoric strain from its initial state as parent phase. In the new model, lattice reconstruction is assumed to erase all memory within the unit mass of deviatoric strain suffered before, or during, its transformation. The existing model is not consistent with experiments on the olivine spinel–phase change in single crystals. It predicts that when the pressure applied exceeds a critical value, samples should transform completely at almost constant rate; instead, growth is seen to slow, and may even cease. The new model predicts this. Without adjustable constants, fair agreement is obtained with experiments on samples having 75–200 ppmw of water. Because elastic deformation by itself can explain those observations, the very thin rims seen on even drier samples suggest that water may be essential to lattice reconstruction in this phase change.