1,720,980 research outputs found
Techno-economic analysis of closed OTEC cycles for power generation
No full textThis study aims at offering a techno-economic evaluation of closed OTEC cycles for on-shore installations. A flexible Matlab®suite has been developed to identify plant design parameters (temperature difference of cold and warm seawater, pinch-point temperature difference of evaporator and condenser etc.) that guarantee the maximum value of γ (ratio between electricity output and heat exchangers area). The optimization model is able to handle different working fluids through the addition of specific correlations that consider fluid influence on heat transfer coefficients and turbine performance. Each plant component is technically analyzed and, in particular, plate heat exchangers were considered for evaporator and condenser and sized accurately with Aspen EDR®, while expander was analyzed with the in-house code Axtur. For warm seawater temperature of 28 °C and cold seawater temperature of 4 °C (8500 kg/s taken from 1000 m depth), ammonia cycle is the best solution characterized by efficiency equal to 2.2% and net power output equal to 2.35 MWe. The obtained LCOE (269 €/MWhe) confirms how OTEC technology is not ready to compete in energy market. Nevertheless, remote zones (i.e. small islands archipelagos), which are often characterized by high electricity price, represent interesting scenarios where OTEC technology could be a promising alternative to conventional power production technologies
Feasibility of solar-driven trilateral-like organic Rankine cycle with radial-inflow turboexpander
No full textLow-temperature solar collectors coupled with thermal energy storage can enable stable and carbon-free energy production. This work proposes a fully integrated organic Rankine cycle (ORC) with solar field and thermocline direct energy storage. The organic fluid remains liquid inside the solar field and the thermal energy storage, leading to a trilateral-like thermodynamic cycle. As opposed to other trilateral (flash) cycles, the proposed system distinguishes itself by including a turboexpander to deal with two-phase expansion, leading to higher conversion efficiency. In particular, with the same turbine efficiency, the proposed cycle outperforms alternative integrated ORC-solar field configurations by 1.5–3.8 percentage points in thermodynamic cycle efficiency for maximum temperatures between 400–600K. The equivalent electric energy density also increases by 30% to 60%. The problem of the two-phase turbine is tackled by relying on a recently proposed radial-inflow turbine concept. The centripetal stator leverages the retrograde shape of the saturation curve to achieve a complete liquid-to-vapor expansion. As a result, the rotor can handle dry organic vapors without experiencing mechanical damage or additional losses from two-phase interactions. Preliminary turbine designs, obtained through optimization of a validated meanline method, consistently yield isentropic total-to-static efficiencies exceeding 85%, confirming the potential of the proposed system
Off-design performance of closed OTEC cycles for power generation
Full text linkThe present study illustrates the development of a detailed model to estimate the part-load performance of an ammonia closed OTEC system for on-shore installations. A previously published Matlab® suite is extended by accounting for off-design conditions in terms of variable seawater temperature and mass flow on the cycle performance. The off-design behavior of each component is thoroughly discussed, with particular attention devoted to the single-stage axial-flow turbine, whose performance maps are obtained by means of three-dimensional CFD simulations. Assuming a representative plant sized for warm seawater temperature of 28 °C and cold seawater temperature of 4 °C (8500 kg/s taken from 1000 m depth), the model predicts an annual electricity yield of 15.963 GWhe and LCOE of 316 €/MWhe when including seawater measured data of a simile-Hawaiian site. Moreover, a sensitivity analysis is assessed in order to identify the best design parameters (i.e. warm seawater temperature and cold seawater mass flow rate) that minimize the LCOE for the given location. The new design guarantees a reduction of approximately 11% of the LCOE (284 €/MWhe). The simulation capabilities of the developed model prove it as valuable tool to estimate the OTEC competitiveness in different scenarios
Solar hydrogen production: Techno-economic analysis of a parabolic dish-supported high-temperature electrolysis system
Full text linkThe roadmap for the hydrogen uptake passes through the development of near-zero emission and/or renewable technologies for hydrogen production. This is the rationale for the investigation of renewable power-to-fuel, namely the coupling between high temperature electrolysis and concentrating solar power. The proposed plant is conceived to supply hydrogen for a small refuelling station. It is based on solid oxide electrolyser cell technology, which performs water electrochemical reduction, in order to produce a target of 150 kg d−1 of hydrogen. The plant is integrated with a parabolic dish solar field designed to provide both electricity and thermal energy, necessary for the electrolysis reaction to take place. Specifically, a modular multi-dish configuration is selected, in which electric power is produced by 30 kWel solarized micro-gas turbines placed in the dishes’ focus. In addition to considering a pure renewable power input, a hybridization with natural gas is considered to face the variability of solar resource. Once a yearly H2 yield is estimated, a preliminary economic analysis is carried out and the levelised cost of hydrogen is subsequently obtained. It is found that the system can be operated at a nominal solar-to-hydrogen efficiency above 30%, with a solid oxide electrolysis cell efficiency around 80%. In hybrid conditions, 10 parabolic dishes (9 generating electricity through the micro-gas turbines, 1 supplying heat to the solid oxide electrolysis cell) are needed to produce the target 150 kg d−1 of hydrogen. In conclusion, the competitiveness of the plant is evaluated in comparison with other solar fuels technologies
Small scale solar tower coupled with micro gas turbine
No full textThis paper studies a small-scale CSP system composed of a solar tower and a recuperative air micro gas turbine (i.e. net power in the 100–200 kWe range). A code is developed to determine the optical performance of the heliostat field coupled with a secondary concentrator, while another code computes the thermal engine performance. The 832 m2 heliostat field layout is taken from a real plant, while the secondary optics is studied to maximize the optical-thermal efficiency. The selected secondary concentrator (CPC), with an aperture diameter of 0.5 m and an acceptance angle of 35° tilted of 52.5°, guarantees an overall optical efficiency of 77.9% in design conditions (Spring equinox, solar noon) and of 66.9% on yearly basis. For every Effective DNI (EDNI) and ambient temperature the turbine operation is optimized allowing to achieve a yearly solar-to-electricity efficiency of 16.3%. Summing up the cost of each component, an overall plant cost of about 2300 €/kW (peak) and a LCOE of 175 €/MWh are obtained. A sensitivity analysis on design EDNI, impacting on turbine size, is performed showing that its reduction from 700 W/m2 to 550 W/m2 allows reducing the LCOE down to 158 €/MWh, a value competitive with large-scale solar towers. The possibility of hybridization of plant (i.e. improving the gas turbine power output in selected hours, by means of biomethane or natural gas combustion) was considered to further reduce the LCOE
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
A NUMERICAL MODEL FOR OFF-DESIGN PERFORMANCE CALCULATION OF PARABOLIC TROUGH BASED SOLAR POWER PLANTS
No full textThe paper deals with the development and testing of an innovative code for the performance prediction of solar trough based CSP plants in off-design conditions. The code is developed in MS Visual Basic 6 0 with Excel as user interface The proposed code originates from a previously presented algorithm for on-design sizing and cost estimation of the solar field lay-out, as well as of the main components of the plant, including connecting piping and the steam cycle Off-design calculation starts from data obtained through the on-design algorithm and considers steady-state situations. Both models are implemented in the same software, named PATTO (PArabolic Trough Thermodynamic Optimization), which is very flexible the optical-thermal model of collectors can simulate different kinds of parabolic trough systems in commerce, including a combination of various mirrors, receivers and supports The code is also flexible in terms of working fluid, temperature and pressure range, and can also simulate duce( steam generation plants (DSG) Regarding the power block, a conventional steam cycle with super-heater, eventually a re-heater section, and up to seven regenerative bleedings is adopted. The off-design model calculates thermal performance of collectors taking Into account proper correlations for convective heat transfer coefficients, considering also boiling regime in DSG configurations. Solar plant heat and mass balances and performances at off-design conditions are estimated by accounting for the constraints imposed by the available heat transfer areas in heat exchangers and condenser, as well as the characteristic curve of the steam turbine The numerical model can be used for a single calculation in a specific off-design condition, as well as for a whole year estimation of energy balances with an hourly resolutionThe model is tested towards real applications and reference values found in literature, in particular, focusing on SECS VI plant in the USA and SAM (R) code Annual energy balances with ambient condition taken from TMY3 database are obtained, showing good accuracy of predicted performances. The code potentiality in the design process reveals twofold: it can be used for plant optimization in feasibility studies, moreover it is useful to find the best control strategy of a plant, especially the mass flow of heat transfer fluid in each operating conditio
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