1,721,047 research outputs found
Modelling approaches of micro and small-scale organic Rankine cycle systems: A critical review
No full textOrganic Rankine cycle (ORC) systems are a technology capable of producing electricity and heat from a wide range of energy sources and are particularly well-suited for medium and low-temperature sources. However, an almost infinite number of technical solutions (cycle configurations, working fluids, components, etc.) can be adopted making the full experimental characterisation of ORC operations for each application unfeasible. To overcome the limitations of extensive experimental investigations, numerical tools are often adopted, thereby supporting the design and operation of these plants. Therefore, over the last two decades, many researchers have put their efforts into developing models to elucidate the design and off-design performances of ORC systems. In this paper, the different modelling approaches for the analysis of ORC systems are discussed and a conclusive review is performed concerning the micro and small-scale ORCs. In total, more than 150 works are reviewed with many of them related to models of volumetric machines and assumption-based system modelling. Semi-empirical models of expanders show good capabilities and accuracy (with errors below 5%) while spatial resolution methods for heat exchangers are used to better capture the dynamics of the system. However, only a limited number of papers (10) deal with assumption-free models of the systems to predict their performance considering the actual boundary conditions. In summary, the present review paper provides a clear overview of the advantages and disadvantages of each modelling approach at both component and system levels to provide insights for interested readers in the advanced simulation of micro and small-scale ORC systems
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
Assumption-free modeling of a micro-scale organic Rankine cycle system based on a mass-sensitive method
No full textOrganic Rankine cycle (ORC) systems are one of the most suitable technologies to produce electricity from low-temperature sources. In this paper, the main components of a non-regenerative, micro-scale ORC unit are modeled using the experimental results. These components are then used as functions in the system-level solver developed in MATLAB(C) to predict the performances of the system at off-design conditions. The proposed system solver is based on a novel approach, in which no assumptions are made about the system's state, and only the components' specifications and the real system boundaries that an operator encounters are adopted as inputs. To this end, the conservation of mass is considered in addition to the conservation of energy in the modeling of the system.Using the assumption-free model, the performances of the ORC system are mapped in the range of the experimental data considering the pump and the expander speeds as variables. The results show that the optimum system net electric performance is achieved at the pump and the expander speeds of 400 rpm and 900 rpm approximately. However, the pump is prone to the risk of cavitation due to low subcooling at the condenser outlet at this condition. Moreover, zero superheating is calculated at the expander suction that is not recommended for its operation.Hence, the developed assumption-free, object-oriented, mass-sensitive model has led to the full understanding of the system limitations and losses in the case of waste heat recovery applications. The proposed approach could be extended also to other ORC systems thus mapping their performances at off-design conditions without making artificial assumptions
Assumption-free modeling of a micro-scale organic Rankine cycle system based on a mass-sensitive method
No full textOrganic Rankine cycle (ORC) systems are one of the most suitable technologies to produce electricity from low-temperature sources. In this paper, the main components of a non-regenerative, micro-scale ORC unit are modeled using the experimental results. These components are then used as functions in the system-level solver developed in MATLAB© to predict the performances of the system at off-design conditions. The proposed system solver is based on a novel approach, in which no assumptions are made about the system's state, and only the components’ specifications and the real system boundaries that an operator encounters are adopted as inputs. To this end, the conservation of mass is considered in addition to the conservation of energy in the modeling of the system. Using the assumption-free model, the performances of the ORC system are mapped in the range of the experimental data considering the pump and the expander speeds as variables. The results show that the optimum system net electric performance is achieved at the pump and the expander speeds of 400 rpm and 900 rpm approximately. However, the pump is prone to the risk of cavitation due to low subcooling at the condenser outlet at this condition. Moreover, zero superheating is calculated at the expander suction that is not recommended for its operation. Hence, the developed assumption-free, object-oriented, mass-sensitive model has led to the full understanding of the system limitations and losses in the case of waste heat recovery applications. The proposed approach could be extended also to other ORC systems thus mapping their performances at off-design conditions without making artificial assumptions
Integrating biomass gasification with a steam-injected micro gas turbine and an Organic Rankine Cycle unit for combined heat and power production
No full textProper integration of different energy systems is one of the most effective strategies to achieve higher conversion efficiencies and to reduce emissions in power systems. Hence, in this study a biomass-fuelled Steam Injected micro Gas Turbine (SImGT) coupled to a bottom Organic Rankine Cycle (ORC) unit is investigated to better exploit the primary energy use while lowering the environmental impact. In the proposed integrated system configuration, a Heat Recovery Steam Generator (HRSG) produces steam for both the gasification process and the SImGT. To conduct a realistic simulation, several gas conditioning units are included in the configuration of the integrated system to purify the produced syngas to the allowable limit of mGTs. The impacts of both the mass flow rate of the injected steam and the S/B ratio on the performance of the integrated system have been assessed. Results of the analysis have shown that the produced electrical power increases as the mass flow rate of the injected steam increases, while the net electrical efficiency of the integrated system is penalized marginally. At its maximum net electricity production, the integrated system has an output of 127.6 kWel and 78.7 kWth with 23.6% electrical efficiency when the mass flow rate of the injected steam to the combustion chamber is 25 g/s, which corresponds to the maximum of the investigated range in this study
Investigating the hybridisation of micro-scale concentrated solar trigeneration systems and wind turbines for residential applications using a dynamic model
No full textSolar and wind energy sources are envisioned to play a key role in the transition towards fully renewable energy systems but both suffer from their intermittent nature, which can be relieved by hybridisation. Hence, the benefits of the complementarity of these sources are investigated hereunder by considering a small-scale hybrid trigenerative system consisting of a Linear Fresnel Reflector solar field with a 440 m2 collector area, a 20 kWe/100 kWt organic Rankine cycle unit combined with a latent heat thermal energy storage system, an absorption chiller, and a 20 kWe wind turbine for the provision of electricity, heating and cooling to 10 residential apartments. To this end, a dynamic model and suitable control logic are developed to assess and compare the performances of the proposed hybrid system concerning the solar plant and wind turbine separately.
The results show that hybridisation can extend the annual thermal coverage by renewables by 3.7–6.1% for the investigated locations by increasing the operation of the organic Rankine cycle unit, especially in winter. In particular, the surplus electrical energy from the wind turbine is used to satisfy 6% and 4% more than the separate configuration of the annual thermal and electrical demands for Ancona. As a result of hybridisation, the annual thermal energy consumption of the boiler reduces by 8.3%, 9.9%, and 37.6% for Perugia, Ancona, and Messina respectively, which shows the impact of the profile of the energy sources. At the same time, hybridisation improves grid stability by reducing the excess electricity production 18.2%, 19.2%, and 26.3% for Perugia, Ancona, and Messina respectively.
A sensitivity analysis for Ancona has revealed that the size of the water thermal energy storage tank does not have a significant impact on the electrical and thermal coverage of the hybrid system, while electrical and thermal coverages improve 11.1% and 6.7% respectively by doubling the wind turbine capacity along with a 14.8% drop in the thermal energy consumption by the boiler. In brief, the analysis has shown the complementarity of these energy sources and the benefits of hybridisation for trigenerative systems for residential buildings
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
- …
