Computational study of the thermal conductivity in defective carbon nanostructures

We use non-equilibrium molecular dynamics simulations to study the adverse role of defects including isotopic impurities on the thermal conductivity of carbon nanotubes, graphene and graphene nanoribbons. We find that even in structurally perfect nanotubes and graphene, isotopic impurities reduce thermal conductivity by up to one half by decreasing the phonon mean free path. An even larger thermal conductivity reduction, with the same physical origin, occurs in presence of structural defects including vacancies and edges in narrow graphene nanoribbons. Our calculations reconcile results of former studies, which differed by up to an order of magnitude, by identifying limitations of various computational approaches

Concentrated Photovoltaics

70 σ.Στόχος της παρούσας διπλωματικής είναι η βιβλιογραφική ανασκόπηση της τεχνολογίας των συγκεντρωτικών φωτοβολταϊκών (CPV) και στη συνέχεια η μελέτη του ρόλου που μπορούν να παίξουν στην παραγωγή πράσινης, φθηνής ηλεκτρικής ενέργειας σε μεγάλη κλίμακα. Αρχικά, γίνεται μία σύντομη ιστορική αναδρομή στην ιστορία της φωτοβολταϊκής τεχνολογίας, και στη συνέχεια παρουσιάζεται αναλυτικά η τεχνολογία των συγκεντρωτικών φωτοβολταϊκών δίνοντας έμφαση στις διαφορές τους με τα συμβατικά κύτταρα πυριτίου. Έπειτα, παρουσιάζονται οι πιο σημαντικές κατασκευαστικές διαδικασίες (LPE και MOCVD) καθώς και επίκαιρα ζητήματα που αφορούν τις διαδικασίες αυτές. Ειδικό βάρος δίνεται στις τρέχουσες και μελλοντικές δυνατότητες εμπορικής ανάπτυξης των CPV: Παρουσιάζονται σενάρια μελλοντικής υλοποίησης της τεχνολογίας των CPV, καθώς και το τι έχει γίνει μέχρι σήμερα. Αναδεικνύεται ειδικά ο ρόλος των πολυσυνδετικών ηλιακών κελιών(multi-junction solar cells) σαν καταλύτης στη παραγωγή ηλεκτρικής ενέργειας που μπορεί να ανταγωνιστεί ευθέως τις συμβατικές πηγές (άνθρακας, υγραέριο, πυρηνικά καύσιμα). Στο τέλος, αναφέρονται συνοπτικά εφαρμογές μεγάλης κλίμακας. ΘεόδωροςThis diploma thesis is a bibliographical review about Concentrated Photovoltaics (CPV) and the major role they could play in generating solar power on an industrial-size scale. So far, there hasn't been any greek bibliography concerning concentrated photovoltaics and this thesis aims to close that gap. Initially, a small introduction about photovoltaics in general is made, and consenquently CPV technology is explained in detail. Special emphasis is given on its differences with conventional silicon cells. Afterwards, the most important and promising manufactoring processes and techniques are presented(such as LPE and MOCVD) and current challenges and difficulties concerning those techniques are pointed out. Its current and future economic prospects are laid down, especially in comparison to conventional solar cells. Also, we explain in detail why multijunction solar cell technology is an enabler for low cost power generation using concentrated photovoltaic systems. Finally, several current, large scale applications are presented.Θεόδωρος Μ. Μπέλλο

Atomistic potential for graphene and other sp2^2 carbon systems

We introduce a torsional force field for sp$^2$ carbon to augment an in-plane atomistic potential of a previous work (Kalosakas et al, J. Appl. Phys. {\bf 113}, 134307 (2013)) so that it is applicable to out-of-plane deformations of graphene and related carbon materials. The introduced force field is fit to reproduce DFT calculation data of appropriately chosen structures. The aim is to create a force field that is as simple as possible so it can be efficient for large scale atomistic simulations of various sp$^2$ carbon structures without significant loss of accuracy. We show that the complete proposed potential reproduces characteristic properties of fullerenes and carbon nanotubes. In addition, it reproduces very accurately the out-of-plane ZA and ZO modes of graphene's phonon dispersion as well as all phonons with frequencies up to 1000~cm$^{-1}$.Comment: 9 pages, 6 figure

Life-cycle carbon emissions and energy return on investment for 80% domestic renewable electricity with battery storage in California (U.S.A.)

This paper presents a detailed life-cycle assessment of the greenhouse gas emissions, cumulative demand for total and non-renewable primary energy, and energy return on investment (EROI) for the domestic electricity grid mix in the U.S. state of California, using hourly historical data for 2018, and future projections of increased solar photovoltaic (PV) installed capacity with lithium-ion battery energy storage, so as to achieve 80% net renewable electricity generation in 2030, while ensuring the hourly matching of the supply and demand profiles at all times. Specifically—in line with California’s plans that aim to increase the renewable energy share into the electric grid—in this study, PV installed capacity is assumed to reach 43.7 GW in 2030, resulting of 52% of the 2030 domestic electricity generation. In the modelled 2030 scenario, single-cycle gas turbines and nuclear plants are completely phased out, while combined-cycle gas turbine output is reduced by 30% compared to 2018. Results indicate that 25% of renewable electricity ends up being routed into storage, while 2.8% is curtailed. Results also show that such energy transition strategy would be effective at curbing California’s domestic electricity grid mix carbon emissions by 50%, and reducing demand for non-renewable primary energy by 66%, while also achieving a 10% increase in overall EROI (in terms of electricity output per unit of investment)

REGULATIONS ON PHOTOVOLTAIC MODULE DISPOSAL AND RECYCLING.

Environmental regulations can have a significant impact on product use, disposal, and recycling. This report summarizes the basic aspects of current federal, state and international regulations which apply to end-of-life photovoltaic (PV) modules and PV manufacturing scrap destined for disposal or recycling. It also discusses proposed regulations for electronics that may set the ground of what is to be expected in this area in the near future. In the US, several states have started programs to support the recycling of electronic equipment, and materials destined for recycling often are excepted from solid waste regulations during the collection, transfer, storage and processing stages. California regulations are described separately because they are different from those of most other states. International agreements on the movement of waste between different countries may pose barriers to cross-border shipments. Currently waste moves freely among country members of the Organization of Economic Cooperation and Development (OECD), and between the US and the four countries with which the US has bilateral agreements. However, it is expected, that the US will adopt the rules of the Basel Convention (an agreement which currently applies to 128 countries but not the US) and that the Convection's waste classification system will influence the current OECD waste-handling system. Some countries adopting the Basel Convention consider end-of-life electronics to be hazardous waste, whereas the OECD countries consider them to be non-hazardous. Also, waste management regulations potentially affecting electronics in Germany and Japan are mentioned in this report