Numerical simulations of atomic-scale disordering processes at impact between two rough crystalline surfaces

Numerical calculations have been used to throw light on the mechanical deformation and the atomic mixing processes taking place when two different metallic systems collide at low temperature. To this end, two semicrystals terminating with a free surface were pushed each against the other at a given relative velocity. Surfaces of different roughness were considered under different impact conditions. Simple mechanical loads on plane surfaces did not induce any significant mixing of atomic species at the interface, observed instead in collisions involving either rough surfaces or plane surfaces undergoing a relative sliding. In the case of rough surfaces, the local contact between the semicrystals is initially sustained by surface asperities. The atoms there located experience thus sudden mechanical loads and an unusual localization of kinetic energy, which enhance their mobility and favor the mixing process. A diffuse interfacial region with a disordered structure correspondingly appears. Its structural features were not significantly modified by the thermal relaxation processes occurring after the compressive load removal

Water Absorption Properties of Cement Pastes: Experimental and Modelling Inspections

An intermingled fractal units’ model is shown in order to simulate pore microstructures as pore fraction and pore size distribution. This model is aimed at predicting capillary water absorption coefficient and sorptivity values in cement pastes. The results obtained are in good agreement with the experimental ones. For validating this model, a comparison with other procedures has been shown. It is possible to establish that the newly proposed method matches better with the experimental results. That is probably due to the fact that pore size distribution has been considered as a whole. Moreover, even though the proposed model is based on fractal base units, it is able to simulate and predict different properties as well as nonfractal porous microstructure

Comparison of Medium-size Concentrating Solar Power Plants based on Parabolic Trough and Linear Fresnel Collectors

Abstract This paper compares the performance of medium-size Concentrating Solar Power (CSP) plants based on an Organic Rankine Cycle (ORC) power generation unit integrated with parabolic trough and linear Fresnel collectors. The CSP plants studied herein use thermal oil as heat transfer fluid and as storage medium in a two-tank direct thermal storage system. The performance of the CSP plants were evaluated on the basis of a 1 MW ORC unit with a conversion efficiency of about 24% and by considering different values of solar multiple and thermal storage capacity. The comparative performance analysis of the two CSP solutions was carried out with reference to the direct solar energy availability of Cagliari, Italy (1720 kWh/m2y) on a yearly basis by means of specifically developed simulation models. The results of the performance assessment demonstrate that CSP plants based on linear Fresnel collectors lead to higher values of electrical energy production per unit area of occupied land. The highest specific energy production of CSP plants based on linear Fresnel collectors is about 55-60 kWh/y per m2 of occupied land and it is achieved with solar multiples in the 1.74-2.5 range and storage capacities in the range of 4-12 hours. The highest specific production of the solutions based on parabolic trough collectors is about 45-50 kWh/y per m2 of occupied land and is achieved with lower solar multiples (around 1.5-2.3). Owing to their better optical efficiency, the use of parabolic troughs gives better values of energy production per unit area of solar collector (about 180-190 kWh/m2 vs. 130-140 kWh/m2)

Use of weather forecast for increasing the self-consumption rate of home solar systems: An Italian case study

With the aim of increasing the self-consumption rate of grid-connected Photovoltaic (PV) home systems, two main options can be implemented: the inclusion of an energy storage system, in particular a battery bank, and the adoption of a Demand Side Management (DSM) strategy. However, both the reshaping of the load consumption curve with the displacement of deferrable loads and the optimal management of the battery bank require estimation of the daily PV generation profile. The assessment of the on-site energy production can be carried out based on weather forecast data. However, the latter are characterized by uncertainty, which may affect the achievable self-consumption rate. This work investigates the influence of weather forecast errors on the performance of home PV systems equipped with a battery bank and characterized by a certain share of deferrable loads. Two different weather forecast services are considered, referring to the annual meteorological conditions occurring in Rome, and energy consumption data for 150 different households are analysed. The self-consumption rate is maximized by solving a suitable optimization problem, while different combinations of relative battery capacity, PV-to-load ratio and share of deferrable loads are considered. Two different approachesâ\u80\u94deterministic and stochasticâ\u80\u94are adopted and compared with an ideal approach where the PV generation profile is perfectly forecasted. The results show that the adoption of the deterministic approach leads to a reduction in the achievable self-consumption rate in the range of 0.5â\u80\u934.5% compared to the ideal approach. The adoption of a stochastic approach further reduces the deviations from the ideal case, especially in the case of consumption profiles with a high share of deferrable loads. Finally, a preliminary economic analysis proves that the use of a battery bank is not yet a cost-effective solution and a price reduction of the current battery prices is therefore required

Modeling and simulation of an isolated hybrid micro-grid with hydrogen production and storage

Abstract This work relates the study of system performance in operational conditions for an isolated micro-grid powered by a photovoltaic system and a wind turbine. The electricity produced and not used by the user will be accumulated in two different storage systems: a battery bank and a hydrogen storage system composed of two PEM electrolyzers, four pressurized tanks and a PEM fuel cell. One of the main problems to be solved in the development of isolated micro-grids is the management of the various devices and energy flows to optimize their functioning, in particular in relation to the load profile and power produced by renewable energy systems depending on weather conditions. For this reason, through the development and implementation of a specific simulation program, three different energy management systems were studied to evaluate the best strategy for effectively satisfying user requirements and optimizing overall system efficiency

Integration of pumped thermal energy storage systems based on Brayton cycle with CSP plants

In this paper, the integration of Brayton cycle PTES systems with Concentrating solar power (CSP) plants is proposed and investigated. Specific mathematical models were developed to simulate the PTES and CSP sections as well as to calculate the thermal profiles of the different TES storage tanks during the charging and discharging phases. As case study, an integrated PTES-CSP system using argon as working fluid and characterized by a nominal power of 5 MW and a nominal storage capacity of 4 equivalent hours of operation is considered. The influence of the main design parameters on two performance indexes, namely, the charge-to-discharge efficiencies of the sole PTES section and the integrated PTES-CSP plant, have been investigated. The results demonstrate that the use of high values of pressure ratio is beneficial for the charge-to-discharge efficiency of the integrated plant, even if too high operating pressures could be detrimental for the design of the solar receiver and the high temperature storage tank. The low temperature TES is a critical component due to its cryogenic operating conditions, but an increase in the minimum temperature should be achieved by increasing the inlet temperature of the LP compressor. A sensitivity analysis on the compressor and turbine efficiencies, maximum and minimum temperatures, circuit pressure drop and working fluid has been carried out. Finally, a feasible design of the PTES-CSP system with a PTES roundtrip efficiency of nearly 52% and a charge-to-discharge efficiency of the integrated PTES-CSP plant of about 36% was proposed

Microstructural defects in nanocrystalline iron probed by x-ray-absorption spectroscopy

An EXAFS (extended x-ray-absorption fine structure) study of nanocrystalline iron prepared by high-energy ball milling is presented. Accurate data analysis has been performed using a recently developed ab initio multiple-scattering method (GNXAS). The crystalline iron EXAFS spectrum, taken as reference, has been analyzed taking proper account of the important multiple-scattering signal. Results of the data analysis are shown to be in good agreement with known structural values. The dramatic decrease of the first-neighbor coordination number found in nanocrystalline iron, as a function of the milling time, is shown to be related to the presence of a large defect density. The corresponding decrease of the coordination numbers of the second and further neighbors, as well as the significant decrease of the multiplicities of the three-atom configurations, confirms this interpretation. A simple model which takes into account the presence of atomic defects is shown to lead to marked reduction of coordination numbers and three-atom multiplicities in agreement with the experimental data. Selected models without lattice defects are shown to be unable to give a quantitative explanation of the intensity reduction of the EXAFS signal found in milled nanocrystalline iron. An in situ annealing of the sample (up to about 1100 °C) milled for the longer time (32 h) is shown to reduce considerably the density of the defects, as expected, although no complete recovering of the crystalline order is found. The transition from α iron to γ iron has been directly observed for such nanocrystalline material

Epidemiology of acute lymphoblastic leukaemia in Sardinia, Italy:Age, sex, and environmental correlates

Using a database of 1974–2003 incident cases of haematological malignancies, we explored the time trend, geographic spread and socio-economic and environmental correlates of ALL incidence in Sardinia, Italy, by sex and age. The age- and sex-standardized (World population) ALL incidence rate was 2.0 per 100,000 (95% CI 1.8 – 2.1) and showed variable trend patterns by sex and age. In the total population, ALL incidence showed an annual per cent change of −1.4% (95% CI -0.59 – -3.34) over the study period, with a knot separating a downward slope in 1974–1996 from an increase in 1996–2003. ALL incidence replicated such pattern in women but not men, whose incidence did not substantially vary over the study period (APC = −2.57%, 95% CI -5.45 – 0.26). Among women, the spatial analysis suggested a clustering of ALL in the southwestern part of the region, whilst only a commune had a high posterior probability of a high ALL incidence among men. Three unrelated communes showed a high posterior probability of ALL at age ≤ 24; only the most populated urban centre showed excess cases at age ≥ 25 years. There was no correlation between the geographic spread of ALL at ages ≤ 24 and ≥ 25 years (p = 0.082). Urban residence was a risk factor for the younger age group. Residences near industrial settlements and in the most populated urban centre were risk factors for subjects aged ≥ 25 years. Our findings suggest age-related differences in ALL aetiology