Solar, wind and logistic substitution in global energy supply to 2050 – Barriers and implications

The sustained rapid growth and learning rates displayed by solar PV and wind electricity generation capacity over recent decades appear to be unprecedented. With these technologies now available at costs competitive with - or below - those of fossil fuel incumbents in many parts of the world, high rates of growth appear likely to continue. In this paper we use ‘top-down’ extrapolation of global trends and simple and transparent models to attempt to falsify the proposition that PV and wind have the potential to achieve dominance in global primary energy supply by 2050. We project future deployment of PV and wind using a logistic substitution model, and examine a series of potentially fundamental constraints that could inhibit continued growth. Adopting conservative assumptions, we find no insuperable constraints across physical and raw materials requirements, manufacturing capacity, energy balance (EROEI), system integration and macro-economic conditions, to this outcome. We also demonstrate synergy with direct air carbon capture and storage (DACCS) that would allow the achievement of global net-zero CO2 emissions by mid-century. Achieving such an outcome would require large scale reconfiguration of the architecture of global and regional energy systems, particularly from 2040 onwards. Low cost primary electricity is likely to be a significant factor in driving such a reorganisation. But given the speed and depth of the transition, hurdles will remain that will require foresight and strategic, coordinated action to overcome

Diffusion quantum Monte Carlo and GW study of the electronic properties of monolayer and bulk hexagonal boron nitride

© 2020 American Physical Society. We report diffusion quantum Monte Carlo (DMC) and many-body GW calculations of the electronic band gaps of monolayer and bulk hexagonal boron nitride (hBN). We find the monolayer band gap to be indirect. GW predicts much smaller quasiparticle gaps at both the single-shot G0W0 and the partially self-consistent GW0 levels. In contrast, solving the Bethe-Salpeter equation on top of the GW0 calculation yields an exciton binding energy for the direct exciton at the K point in close agreement with the DMC value. Vibrational renormalization of the electronic band gap is found to be significant in both the monolayer and the bulk. Taking vibrational effects into account, DMC overestimates the band gap of bulk hBN, while GW theory underestimates it

Quantum Monte Carlo study of the phase diagram of solid molecular hydrogen at extreme pressures.

Establishing the phase diagram of hydrogen is a major challenge for experimental and theoretical physics. Experiment alone cannot establish the atomic structure of solid hydrogen at high pressure, because hydrogen scatters X-rays only weakly. Instead, our understanding of the atomic structure is largely based on density functional theory (DFT). By comparing Raman spectra for low-energy structures found in DFT searches with experimental spectra, candidate atomic structures have been identified for each experimentally observed phase. Unfortunately, DFT predicts a metallic structure to be energetically favoured at a broad range of pressures up to 400 GPa, where it is known experimentally that hydrogen is non-metallic. Here we show that more advanced theoretical methods (diffusion quantum Monte Carlo calculations) find the metallic structure to be uncompetitive, and predict a phase diagram in reasonable agreement with experiment. This greatly strengthens the claim that the candidate atomic structures accurately model the experimentally observed phases.We thank Dominik Jochym for help with the implementation of the BLYP density functional. Financial support was provided by the Engineering and Physical Sciences Research Council (EPSRC), U.K. This research used resources of the Oak Ridge Leadership Computing Facility at the Oak Ridge National Laboratory, which is supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC05-00OR22725. Additional calculations were performed on the Cambridge High Performance Computing Service facility Darwin and the N8 high-performance computing facility provided and funded by the N8 consortium and EPSRC (Grant No. EP/K000225/1). We thank Dominik Jochym for help with the mplementation of the BLYP density functional. Financial support was provided by the Engineering and Physical Sciences Research Council (EPSRC), U.K. This research used resources of the Oak Ridge Leadership Computing Facility at the Oak Ridge National Laboratory, which is supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC05-00OR22725. Additional calculations were performed on the Cambridge High Performance Computing Service facility Darwin and the N8 high-performance computing facility provided and funded by the N8 consortium and EPSRC (Grant No. EP/K000225/1).This is the final version of the article. It first appeared from Nature Publishing Group via http://dx.doi.org/10.1038/ncomms879

Diffusion quantum Monte Carlo study of excitonic complexes in two-dimensional transition-metal dichalcogenides

Excitonic effects play a particularly important role in the optoelectronic behavior of two-dimensional semiconductors. To facilitate the interpretation of experimental photoabsorption and photoluminescence spectra we provide (i) statistically exact diffusion quantum Monte Carlo binding-energy data for a Mott-Wannier model of (donor/acceptor-bound) excitons, trions, and biexcitons in two-dimensional semiconductors in which charges interact via the Keldysh potential, (ii) contact pair-distribution functions to allow a perturbative description of contact interactions between charge carriers, and (iii) an analysis and classification of the different types of bright trions and biexcitons that can be seen in single-layer molybdenum and tungsten dichalcogenides. We investigate the stability of biexcitons in which two charge carriers are indistinguishable, finding that they are only bound when the indistinguishable particles are several times heavier than the distinguishable ones. Donor/acceptor-bound biexcitons have similar binding energies to the experimentally measured biexciton binding energies. We predict the relative positions of all stable free and bound excitonic complexes of distinguishable charge carriers in the photoluminescence spectra of WSe2 and MoSe2

Developing a coding scheme for analysing classroom dialogue across d educational contexts.

The research reported sought to develop a framework for systematically analysing classroom dialogue for application across a range of educational settings. The paper outlines the development and refinement of a coding scheme that attempts to represent and operationalise commonalities amongst some key theorists in the field concerning productive forms of educational dialogue. The team has tested it using video recordings from classroom settings in the UK and Mexico, across age phases, subject areas, and different interactional contexts including whole class, group and paired work. Our Scheme for Educational Dialogue Analysis (SEDA) is situated within a sociocultural paradigm, and draws on Hymes' Ethnography of Communication to highlight the importance of context. We examined how such a tool could be used in practice. We found that concentrating on the ‘communicative act’ to explore dialogue between participants was an appropriate level of granularity, while clustering the 33 resulting codes according to function of the acts helped to highlight dialogic sequences within lessons. We report on the application of the scheme in two different learning contexts and reflect on its fitness for purpose, including perceived limitations. Development of specialised sub-schemes and a version for teachers is underway.This collaborative work was carried out for a project entitled “A Tool for Analysing Dialogic Interactions in Classrooms” (http://tinyurl.com/BAdialogue) funded through the British Academy International Partnership and Mobility Scheme (ref. RG66509), between January 2013 - December 2015. We are most grateful to colleagues on the project teams who made significant contributions and helpful input during development and testing of the scheme and preparation of the manuscript, including Farah Ahmed, Riikka Hofmann, Christine Howe, Ruth Kershner, Fiona Jackson, Karen Littleton, Neil Mercer, Paul Warwick (UK team); Mariana Alarcón, Nube Estrada, Erika Gil, Kissy Guzmán, Flora Hernández, José Hernández, Haydeé Pedraza, Ana Luisa Rubio, Brenda Itzel Sánchez, Ana Laura Trigo, Maricela Velez (Mexico team). We also thank all of the teachers (especially Lloyd and Tania) and students who participated in our previous research from which examples were taken. We appreciate the support of the Economic and Social Research Council, sponsor of most of the UK team’s work in this area over the years

Analytic two-loop form factors in N=4 SYM

The original publication is available at www.springerlink.co