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The increasing integration of wind energy in power systems can be responsible for the occurrence of over-generation, especially during the off-peak periods. This paper presents a dedicated methodology to identify and quantify the occurrence of this over-generation and to evaluate some of the solutions that can be adopted to mitigate this problem. The methodology is applied to the Portuguese power system, in which the wind energy is expected to represent more than 25% of the installed capacity in a near future. The results show that the pumped-hydro units will not provide enough energy storage capacity and, therefore, wind curtailments are expected to occur in the Portuguese system. Additional energy storage devices can be implemented to offset the wind energy curtailments. However, the investment analysis performed show that they are not economically viable, due to the present high capital costs involved.

Abstract In this study, a numerical analysis of the performance of two Oscillating Water Column (OWC) wave energy converters, at different front and back wall slopes, was carried out. The first device had vertical front and back walls and the second one had its wall slopes of 40° in relation to the horizontal plane. The FLUENT® numerical model, which is based on the Reynolds-Averaged Navier-Stokes (RANS) equations, was used. The Volume of Fluid (VoF) method was employed to take into account free surface flows. The case study comprised a 10 m deep flume with an onshore OWC at its end, equipped with a Wells turbine. A 2D hydrodynamic mathematical model was employed and a 3D air pressure effect inside the OWC air-chamber was considered. Analyses of the hydrodynamics behavior, fluid-structure interaction outside (run-up/down, reflected waves) and inside the chamber (free surface elevation, sloshing) and energy distribution (pneumatic extracted energy, reflected wave energy and energy losses) were conducted. Results showed that the device with inclined walls had the best efficiency in comparison with the other one. However, the latter showed lower variation in efficiency in the wave period range than the former.

Abstract The need to generate thermal and electrical energy, global warming caused by increased emissions of greenhouse gases, rising fossil fuel prices and demand for energy independence, have created a new industry focused on energy production through the use of renewable sources. Among the different options, biomass is the third most important source for obtaining electricity, and is the main source for the production of thermal energy. However, problems related to the low density of the different types of biomass, and the difficulty of transportation and storage, have led to the need to find solid fuels with higher density and greater hardness, known as pellets and briquettes. This paper seeks to develop an analysis of the current situation of the production of pellets, mainly with mixed biomass types, and the possible uses they have, with the main emphasis on the review of different combustion processes.

Abstract Molten salts are a very relevant member of industrial fluids for high temperature applications, such as catalytic medium for coal gasification, molten salt oxidation of wastes, heat transfer fluids or latent, sensible heat storage and solar (CSP) or nuclear power station operations. Available data on thermophysical properties, applications and a discussion of the state of the art for molten alkali carbonates and its mixtures like pure Li2CO3, Na2CO3 and K2CO3, mixtures of Li2CO3-Na2CO3, Li2CO3-K2CO3 (binary eutectics) and Li2CO3-Na2CO3-K2CO3 (ternary eutectic) and nanofluids based in these carbonate melts are presented. These melts are especially suitable for application at higher temperature regimes, like those involving high temperature energy storage, coolants or molten salts oxidations of wastes and therefore the accurate knowledge of their most important thermophysical properties is essential for efficient energy transfer and storage, because of their impact on energy efficiency, namely in energy savings and decrease of carbon footprint. From the analysis performed it can be concluded that the scatter of data found for molten alkali carbonates, added to present and future applications, still justifies further studies on these systems, to support their application as alternative engineering fluids. Additionally, some comments on how to improve present situation of methods and measurements are made, especially in the area of thermal conductivity.

Abstract The world has been experiencing a significant increase in daily average temperatures per decade and climate change scenarios are projecting high probability of more frequent heat waves. In vulnerable regions, like Southern Europe, where most of the residential buildings still rely on natural ventilation for cooling, impact on thermal comfort can be significant in terms of health, well-being and also energy consumption. The question is particularly important for the existing building stock, which was not designed considering the projected future climate conditions and is prone to be subjected to interventions with the purpose of improving thermal performance. The study presents a vulnerability framework and methodology for the assessment of thermal comfort in existing dwellings in the context of climate change. Results relating to a 1960s typical building case study in Lisbon, Portugal, suggest that specific dwelling characteristics, such as orientation, and occupancy profiles are relevant when assessing vulnerability, suggesting significant differences, of up to 91% in discomfort hours on an annual basis. Furthermore, increased insulation seems to be effective in decreasing discomfort, as the best results (48% in discomfort hours decrease) stem from a context of external insulation for a heatwave situation. The methodology can be useful for assessing vulnerability in existing dwellings and its specific conditions. It can also contribute to understanding the effect of energy retrofitting measures in future climate conditions, assisting energy efficiency policies and decision-making regarding retrofit interventions.

In the steel industry, the iron making system deals with large quantities of materials and energy and so it can play a critical role in reducing emissions and production costs. More specifically, excess by-product gases should be used for electricity generation; otherwise, they lead to pollution. A life cycle analysis is performed to compare the environmental impact of an iron making system with a combined cycle power plant (CCPP), to a system producing the same amount of electricity in a coal power plant. The results for a Chinese steel plant show a 33% reduction in the energy conservation and emission reduction potential for the CCPP system, which is thus more environmentally friendly. A mathematical programming formulation is then proposed for optimal scheduling. It incorporates key technological constraints and is sensitive to hourly changing electricity prices. The outcome is a 19% increase in revenue from electricity sales compared to a schedule that does not dynamically adjust to the price profile. The results also show that emissions from by-product gases can be avoided completely. The paper ends with a sensitivity analysis to evaluate the impact of changes in product demand, gas storage and CCPP capacity, and emission cost.

Abstract The study of sustainable energy systems is an interdisciplinary endeavour which entails the analysis of a large amount of diverse data and complex interactions that are better understood if developed from first principles. This paper reviews the approaches to this analysis and presents as a general case study, a fossil free imaginary island whose electricity, heat and mobility demand are fulfilled with sustainable and renewable energies only. The detailed hourly balance between supply and demand highlights the importance of energy storage, which is achieved by reversible hydropower and storage in electric vehicles.

With the concept of climate services rapidly climbing research and research-funding agendas worldwide, the time is ripe for a debate about the objectives, scope and content of such services.