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Special Issue "Selected Papers from the 1st International e-Conference on Energies — Whither Energy Conversion? Present Trends, Current Problems and Realistic Future Solutions"

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A special issue of Energies (ISSN 1996-1073).

Deadline for manuscript submissions: closed (31 July 2014)

Special Issue Editor

Guest Editor
Prof. Dr. Enrico Sciubba (Website)

Room 32, Department of Mechanical and Aerospace Engineering, University of Roma Sapienza, Via Eudossiana 18, 00184 Roma, Italy
Phone: +39-06-44585244
Fax: +39 06 44 58 52 49
Interests: turbomachinery CFD; turbulence modelling; exergy analysis of complex systems; engineering applications of AI to thermal conversion processes

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Published Papers (14 papers)

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Research

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Open AccessArticle Hydrogen Storage in Boron Nitride and Carbon Nanomaterials
Energies 2015, 8(1), 319-337; doi:10.3390/en8010319
Received: 10 September 2014 / Accepted: 26 December 2014 / Published: 31 December 2014
Cited by 3 | PDF Full-text (4965 KB) | HTML Full-text | XML Full-text
Abstract
Boron nitride (BN) nanomaterials were synthesized from LaB6 and Pd/boron powder, and the hydrogen storage was investigated by differential thermogravimetric analysis, which showed possibility of hydrogen storage of 1–3 wt%. The hydrogen gas storage in BN and carbon (C) clusters was [...] Read more.
Boron nitride (BN) nanomaterials were synthesized from LaB6 and Pd/boron powder, and the hydrogen storage was investigated by differential thermogravimetric analysis, which showed possibility of hydrogen storage of 1–3 wt%. The hydrogen gas storage in BN and carbon (C) clusters was also investigated by molecular orbital calculations, which indicated possible hydrogen storage of 6.5 and 4.9 wt%, respectively. Chemisorption calculation was also carried out for B24N24 cluster with changing endohedral elements in BN cluster to compare the bonding energy at nitrogen and boron, which showed that Li is a suitable element for hydrogenation to the BN cluster. The BN cluster materials would store H2 molecule easier than carbon fullerene materials, and its stability for high temperature would be good. Molecular dynamics calculations showed that a H2 molecule remains stable in a C60 cage at 298 K and 0.1 MPa, and that pressures over 5 MPa are needed to store H2 molecules in the C60 cage. Full article
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Open AccessArticle Assessment of Energy, Environmental and Economic Performance of a Solar Desiccant Cooling System with Different Collector Types
Energies 2014, 7(10), 6741-6764; doi:10.3390/en7106741
Received: 31 July 2014 / Revised: 1 October 2014 / Accepted: 15 October 2014 / Published: 20 October 2014
Cited by 5 | PDF Full-text (810 KB) | HTML Full-text | XML Full-text
Abstract
Desiccant-based air handling units can achieve reductions in greenhouse gas emissions and energy savings with respect to conventional air conditioning systems. Benefits are maximized when they interact with renewable energy technologies, such as solar collectors. In this work, experimental tests and data [...] Read more.
Desiccant-based air handling units can achieve reductions in greenhouse gas emissions and energy savings with respect to conventional air conditioning systems. Benefits are maximized when they interact with renewable energy technologies, such as solar collectors. In this work, experimental tests and data derived from scientific and technical literature are used to implement a model of a solar desiccant cooling system, considering three different collector technologies (air, flat-plate and evacuated collectors). Simulations were then performed to compare the energy, environmental and economic performance of the system with those of a desiccant-based unit where regeneration thermal energy is supplied by a natural gas boiler, and with those of a conventional air-handling unit. The only solution that allows achieving the economic feasibility of the solar desiccant cooling unit consists of 16 m2 of evacuated solar collectors. This is able to obtain, with respect to the reference system, a reduction of primary energy consumption and of the equivalent CO2 emissions of 50.2% and 49.8%, respectively, but with a payback time of 20 years. Full article
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Open AccessArticle Experimental Investigation and Discussion on the Mechanical Endurance Limit of Nafion Membrane Used in Proton Exchange Membrane Fuel Cell
Energies 2014, 7(10), 6401-6411; doi:10.3390/en7106401
Received: 28 July 2014 / Revised: 16 September 2014 / Accepted: 28 September 2014 / Published: 9 October 2014
Cited by 2 | PDF Full-text (928 KB) | HTML Full-text | XML Full-text
Abstract
As a solution of high efficiency and clean energy, fuel cell technologies, especially proton exchange membrane fuel cell (PEMFC), have caught extensive attention. However, after decades of development, the performances of PEMFCs are far from achieving the target from the Department of [...] Read more.
As a solution of high efficiency and clean energy, fuel cell technologies, especially proton exchange membrane fuel cell (PEMFC), have caught extensive attention. However, after decades of development, the performances of PEMFCs are far from achieving the target from the Department of Energy (DOE). Thus, further understanding of the degradation mechanism is needed to overcome this obstacle. Due to the importance of proton exchange membrane in a PEMFC, the degradation of the membrane, such as hygrothermal aging effect on its properties, are particularly necessary. In this work, a thick membrane (Nafion N117), which is always used as an ionic polymer for the PEMFCs, has been analyzed. Experimental investigation is performed for understanding the mechanical endurance of the bare membranes under different loading conditions. Tensile tests are conducted to compare the mechanical property evolution of two kinds of bare-membrane specimens including the dog-bone and the deeply double edge notched (DDEN) types. Both dog-bone and DDEN specimens were subjected to a series of degradation tests with different cycling times and wide humidity ranges. The tensile tests are repeated for both kinds of specimens to assess the strain-stress relations. Furthermore, Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and Scanning electron microscope (SEM) observation and water absorption measurement were conducted to speculate the cause of this variation. The initial cracks along with the increasing of bound water content were speculated as the primary cause. Full article
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Open AccessArticle A Critical Interpretation and Quantitative Extension of the Sama-Szargut Second Law Rules in an Extended Exergy Perspective
Energies 2014, 7(8), 5357-5373; doi:10.3390/en7085357
Received: 20 May 2014 / Revised: 21 July 2014 / Accepted: 30 July 2014 / Published: 18 August 2014
PDF Full-text (459 KB) | HTML Full-text | XML Full-text
Abstract
Twenty-five years ago, Gaggioli, Sama and Qian published a series of 10 “second law guidelines” for design and process engineers, nicknamed at that time “the Gaggioli-Sama rules”. These guidelines, some of them previously published by Sama between 1980 and 1983, are a [...] Read more.
Twenty-five years ago, Gaggioli, Sama and Qian published a series of 10 “second law guidelines” for design and process engineers, nicknamed at that time “the Gaggioli-Sama rules”. These guidelines, some of them previously published by Sama between 1980 and 1983, are a compilation of “second law errors” to avoid in the design of energy conversion systems. The list was rearranged several times, until a revised version containing 21 rules was published by Sama and Szargut in 1995. Ever since, these guidelines came to be known as “the Sama-Szargut rules”. The rules are a series of well-formulated and insightful suggestions that reflect a thermodynamicist’s idea that the “best design” is the one that minimizes the overall irreversibility in a process or plant, under the prescribed technological constraints. Characteristically, the concept of “optimal system” is completely absent, the emphasis being on the extensive inclusion of second law reasoning into design decisions. A critical analysis of the rules would suggest that all of them be routinely implemented both in new designs and most important in retrofit projects. A survey of some of the current most common energy conversion installations shows that, quite on the contrary, most of the rules are disregarded in practical applications. This paper argues that the reason for this incongruency is the neglection in the engineering design decision of the real cost of installation, operation and decommissioning of a plant, and proposes a rephrasing of the rules in an extended exergy perspective: if the production cost, including the externalities, is measured in units of equivalent primary exergy, the Sama-Szargut rules can be directly interpreted in this sense, and abidance by the rules results in the reduction of the resource cost for any given objective. Full article
Open AccessArticle Circuit Simulation for Solar Power Maximum Power Point Tracking with Different Buck-Boost Converter Topologies
Energies 2014, 7(8), 5027-5046; doi:10.3390/en7085027
Received: 20 June 2014 / Revised: 17 July 2014 / Accepted: 29 July 2014 / Published: 5 August 2014
Cited by 3 | PDF Full-text (882 KB) | HTML Full-text | XML Full-text
Abstract
The power converter is one of the essential elements for effective use of renewable power sources. This paper focuses on the development of a circuit simulation model for maximum power point tracking (MPPT) evaluation of solar power that involves using different buck-boost [...] Read more.
The power converter is one of the essential elements for effective use of renewable power sources. This paper focuses on the development of a circuit simulation model for maximum power point tracking (MPPT) evaluation of solar power that involves using different buck-boost power converter topologies; including SEPIC, Zeta, and four-switch type buck-boost DC/DC converters. The circuit simulation model mainly includes three subsystems: a PV model; a buck-boost converter-based MPPT system; and a fuzzy logic MPPT controller. Dynamic analyses of the current-fed buck-boost converter systems are conducted and results are presented in the paper. The maximum power point tracking function is achieved through appropriate control of the power switches of the power converter. A fuzzy logic controller is developed to perform the MPPT function for obtaining maximum power from the PV panel. The MATLAB-based Simulink piecewise linear electric circuit simulation tool is used to verify the complete circuit simulation model. Full article
Open AccessArticle Current State of Technology of Fuel Cell Power Systems for Autonomous Underwater Vehicles
Energies 2014, 7(7), 4676-4693; doi:10.3390/en7074676
Received: 3 June 2014 / Revised: 8 July 2014 / Accepted: 16 July 2014 / Published: 22 July 2014
Cited by 9 | PDF Full-text (210 KB) | HTML Full-text | XML Full-text
Abstract
Autonomous Underwater Vehicles (AUVs) are vehicles that are primarily used to accomplish oceanographic research data collection and auxiliary offshore tasks. At the present time, they are usually powered by lithium-ion secondary batteries, which have insufficient specific energies. In order for this technology [...] Read more.
Autonomous Underwater Vehicles (AUVs) are vehicles that are primarily used to accomplish oceanographic research data collection and auxiliary offshore tasks. At the present time, they are usually powered by lithium-ion secondary batteries, which have insufficient specific energies. In order for this technology to achieve a mature state, increased endurance is required. Fuel cell power systems have been identified as an effective means to achieve this endurance but no implementation in a commercial device has yet been realized. This paper summarizes the current state of development of the technology in this field of research. First, the most adequate type of fuel cell for this application is discussed. The prototypes and design concepts of AUVs powered by fuel cells which have been developed in the last few years are described. Possible commercial and experimental fuel cell stack options are analyzed, examining solutions adopted in the analogous aerial vehicle applications, as well as the underwater ones, to see if integration in an AUV is feasible. Current solutions in oxygen and hydrogen storage systems are overviewed and energy density is objectively compared between battery power systems and fuel cell power systems for AUVs. A couple of system configuration solutions are described including the necessary lithium-ion battery hybrid system. Finally, some closing remarks on the future of this technology are given. Full article
Open AccessArticle Explicit Expressions for Solar Panel Equivalent Circuit Parameters Based on Analytical Formulation and the Lambert W-Function
Energies 2014, 7(7), 4098-4115; doi:10.3390/en7074098
Received: 21 April 2014 / Revised: 18 June 2014 / Accepted: 23 June 2014 / Published: 26 June 2014
Cited by 10 | PDF Full-text (1123 KB) | HTML Full-text | XML Full-text
Abstract
Due to the high dependence of photovoltaic energy efficiency on environmental conditions (temperature, irradiation...), it is quite important to perform some analysis focusing on the characteristics of photovoltaic devices in order to optimize energy production, even for small-scale users. The use of [...] Read more.
Due to the high dependence of photovoltaic energy efficiency on environmental conditions (temperature, irradiation...), it is quite important to perform some analysis focusing on the characteristics of photovoltaic devices in order to optimize energy production, even for small-scale users. The use of equivalent circuits is the preferred option to analyze solar cells/panels performance. However, the aforementioned small-scale users rarely have the equipment or expertise to perform large testing/calculation campaigns, the only information available for them being the manufacturer datasheet. The solution to this problem is the development of new and simple methods to define equivalent circuits able to reproduce the behavior of the panel for any working condition, from a very small amount of information. In the present work a direct and completely explicit method to extract solar cell parameters from the manufacturer datasheet is presented and tested. This method is based on analytical formulation which includes the use of the Lambert W-function to turn the series resistor equation explicit. The presented method is used to analyze commercial solar panel performance (i.e., the current-voltage–I-V–curve) at different levels of irradiation and temperature. The analysis performed is based only on the information included in the manufacturer’s datasheet. Full article
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Open AccessArticle Production of a Biofuel that Keeps the Glycerol as a Monoglyceride by Using Supported KF as Heterogeneous Catalyst
Energies 2014, 7(6), 3764-3780; doi:10.3390/en7063764
Received: 22 April 2014 / Revised: 4 June 2014 / Accepted: 10 June 2014 / Published: 18 June 2014
PDF Full-text (670 KB) | HTML Full-text | XML Full-text
Abstract
This study describes the results obtained in the synthesis of a biofuel that avoids the production of glycerol by applying supported KF as alkaline heterogeneous catalyst, to generate two moles of fatty acid methyl esters and one mole of monoglyceride from one [...] Read more.
This study describes the results obtained in the synthesis of a biofuel that avoids the production of glycerol by applying supported KF as alkaline heterogeneous catalyst, to generate two moles of fatty acid methyl esters and one mole of monoglyceride from one mol of triglyceride. In this respect, the selective transesterification process of sunflower oil with methanol was carried out with KF (10 wt%) supported on three different solids, Al2O3, ZnO and MgO. The standard experimental conditions employed in the heterogeneous selective methanolysis reaction were: 12 mL of sunflower oil, 2.7 mL of methanol, 0.8 g of catalyst, at 65 °C temperature and one hour of reaction time. In all cases 100% conversion was obtained, with high selectivity values, greater than 90%, and quite suitable viscosity values, 4.5–8.5 cSt. In this way, the best catalytic behavior in the first use was obtained by using Al2O3 as support. However, although in the five consecutive reuses all catalysts exhibited a continuous decrease in their catalytic activities; the lower one was for KF catalyst using MgO as support. In summary, these three KF supported catalysts are very suitable to obtain a new biofuel, similar to conventional biodiesel, applicable to diesel engines. Full article
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Open AccessArticle Solar Air Collectors for Space Heating and Ventilation Applications—Performance and Case Studies under Romanian Climatic Conditions
Energies 2014, 7(6), 3781-3792; doi:10.3390/en7063781
Received: 9 April 2014 / Revised: 12 June 2014 / Accepted: 13 June 2014 / Published: 18 June 2014
Cited by 1 | PDF Full-text (567 KB) | HTML Full-text | XML Full-text | Correction
Abstract
Solar air collectors have various applications: on the one hand, they can be used for air heating in cold seasons; on the other hand they can be used in summer to evacuate the warm and polluted air from residential, offices, industrial, and [...] Read more.
Solar air collectors have various applications: on the one hand, they can be used for air heating in cold seasons; on the other hand they can be used in summer to evacuate the warm and polluted air from residential, offices, industrial, and commercial buildings. The paper presents experimental results of a solar collector air, under the climatic conditions of the Southeastern Europe. The relationships between the direct solar irradiation, the resulting heat flow, the air velocity at the outlet, the air flow rate, the nominal regime of the collector and the efficiency of conversion of solar energy into thermal energy are all highlighted. Thus, it was shown that after a maximum 50 min, solar air collectors, with baffles and double air passage can reach over 50% efficiency for solar irradiation of 900–1000 W/m2. The article also presents a mathematical model and the results of a computational program that allows sizing solar collectors for the transfer of air, with the purpose of improving the natural ventilation of buildings. The article is completed with case studies, sizing the area to be covered with solar collectors, to ensure ventilation of a house with two floors or for an office building. In addition, the ACH (air change per hour) coefficient was calculated and compared. Full article
Open AccessArticle Performance of a 250 kW Organic Rankine Cycle System for Off-Design Heat Source Conditions
Energies 2014, 7(6), 3684-3694; doi:10.3390/en7063684
Received: 11 April 2014 / Revised: 29 May 2014 / Accepted: 9 June 2014 / Published: 13 June 2014
Cited by 5 | PDF Full-text (426 KB) | HTML Full-text | XML Full-text
Abstract
An organic Rankine cycle system comprised of a preheater, evaporator, condenser, turbine, generator, and pump was used to study its off-design performance and the operational control strategy. R245fa was used as the working fluid. Under the design conditions, the net power output [...] Read more.
An organic Rankine cycle system comprised of a preheater, evaporator, condenser, turbine, generator, and pump was used to study its off-design performance and the operational control strategy. R245fa was used as the working fluid. Under the design conditions, the net power output is 243 kW and the system thermal efficiency is 9.5%. For an off-design heat source flow rate (mW), the operating pressure was controlled to meet the condition that the R245fa reached the liquid and vapor saturation states at the outlet of the preheater and the evaporator, respectively. The analytical results demonstrated that the operating pressure increased with increasing mW; a higher mW yielded better heat transfer performance of the preheater and required a smaller evaporator heat capacity, and the net power output and system thermal efficiency increased with increasing mW. For the range of mW studied here, the net power output increased by 64.0% while the total heat transfer rate increased by only 9.2%. In summary, off-design operation of the system was examined for a heat source flow rate which varied by –39.0% to +78.0% from the designed rate, resulting in –29.2% to +16.0% and –25.3% to +12.6% variations in the net power output and system thermal efficiency, respectively. Full article
Open AccessArticle A Biofuel Similar to Biodiesel Obtained by Using a Lipase from Rhizopus oryzae, Optimized by Response Surface Methodology
Energies 2014, 7(5), 3383-3399; doi:10.3390/en7053383
Received: 12 April 2014 / Revised: 12 May 2014 / Accepted: 19 May 2014 / Published: 22 May 2014
Cited by 4 | PDF Full-text (719 KB) | HTML Full-text | XML Full-text
Abstract
A new biodiesel-like biofuel is obtained by the enzymatic ethanolysis reaction of sunflower oil with ethanol, in free solvent media, by using BIOLIPASE-R, a multipurpose alimentary additive from Biocon®-Spain that is a low cost lipase from a strain of Rhizopus [...] Read more.
A new biodiesel-like biofuel is obtained by the enzymatic ethanolysis reaction of sunflower oil with ethanol, in free solvent media, by using BIOLIPASE-R, a multipurpose alimentary additive from Biocon®-Spain that is a low cost lipase from a strain of Rhizopus oryzae. This biofuel is composed by two parts of fatty acid ethyl esters (FAEE) and one of monoglyceride (MG), which in this form integrates glycerol, through the application of the 1,3-selective lipases. Thus, this process minimizes waste generation and maximizes the efficiency of the process because no residual glycerol is produced. Response surface methodology (RSM) is employed to evaluate the main reaction parameters (reaction temperature, oil/ethanol ratio and pH) on the sunflower oil conversion. Water content and amount of lipase were also previously investigated. Regarding the results, we found that it operates optimally with a water content of the reaction medium of 0.15%, 0.05%–0.1% lipase by weight relative to the weight of oil used, 20 °C, volume ratio (mL/mL) oil/ethanol 12/3.5 and pH 12 (by addition of 50 µL of 10 N NaOH solution). These results have proven a very good efficiency of the biocatalyst in the studied selective process. Full article
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Review

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Open AccessReview State of the Art and Future Trends in Grid Codes Applicable to Isolated Electrical Systems
Energies 2014, 7(12), 7936-7954; doi:10.3390/en7127936
Received: 11 August 2014 / Revised: 15 November 2014 / Accepted: 24 November 2014 / Published: 27 November 2014
Cited by 1 | PDF Full-text (1530 KB) | HTML Full-text | XML Full-text
Abstract
Isolated electrical systems lack electrical interconnection to other networks and are usually placed in geographically isolated areas—mainly islands or locations in developing countries. Until recently, only diesel generators were able to assure a safe and reliable supply in exchange for very high [...] Read more.
Isolated electrical systems lack electrical interconnection to other networks and are usually placed in geographically isolated areas—mainly islands or locations in developing countries. Until recently, only diesel generators were able to assure a safe and reliable supply in exchange for very high costs for fuel transportation and system operation. Transmission system operators (TSOs) are increasingly seeking to replace traditional energy models based on large groups of conventional generation units with mixed solutions where diesel groups are held as backup generation and important advantages are provided by renewable energy sources. The grid codes determine the technical requirements to be fulfilled by the generators connected in any electrical network, but regulations applied to isolated grids are more demanding. In technical literature it is rather easy to find and compare grid codes for interconnected electrical systems. However, the existing literature is incomplete and sparse regarding isolated grids. This paper aims to review the current state of isolated systems and grid codes applicable to them, specifying points of comparison and defining the guidelines to be followed by the upcoming regulations. Full article
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Open AccessReview Evaluation of the Wave Energy Conversion Efficiency in Various Coastal Environments
Energies 2014, 7(6), 4002-4018; doi:10.3390/en7064002
Received: 8 April 2014 / Revised: 11 June 2014 / Accepted: 16 June 2014 / Published: 24 June 2014
Cited by 8 | PDF Full-text (1543 KB) | HTML Full-text | XML Full-text
Abstract
The main objective of the present work was to assess and compare the wave power resources in various offshore and nearshore areas. From this perspective, three different groups of coastal environments were considered: the western Iberian nearshore, islands and an enclosed environment [...] Read more.
The main objective of the present work was to assess and compare the wave power resources in various offshore and nearshore areas. From this perspective, three different groups of coastal environments were considered: the western Iberian nearshore, islands and an enclosed environment with sea waves, respectively. Some of the most representative existent wave converters were evaluated in the analysis and a second objective was to compare their performances at the considered locations, and in this way to determine which is better suited for potential commercial exploitation. In order to estimate the electric power production expected in a certain location, the bivariate distributions of the occurrences corresponding to the sea states, defined by the significant wave height and the energy period, were constructed in each coastal area. The wave data were provided by hindcast studies performed with numerical wave models or based on measurements. The transformation efficiency of the wave energy into electricity is evaluated via the load factor and also through the capture width, defined as the ratio between the electric power estimated to be produced by each specific wave energy converters (WEC) and the expected wave power corresponding to the location considered. Finally, by evaluating these two different indicators, comparisons of the performances of three WEC types (Aqua Buoy, Pelamis and Wave Dragon) in the three different groups of coastal environments considered have been also carried out. The work provides valuable information related to the effectiveness of various technologies for the wave energy extraction that would operate in different coastal environments. Full article
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Other

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Open AccessCorrection Correction: Budea, S. Solar Air Collectors for Space Heating and Ventilation Applications—Performance and Case Studies under Romanian Climatic Conditions. Energies 2014, 7, 3781–3792
Energies 2014, 7(9), 6223; doi:10.3390/en7096223
Received: 27 August 2014 / Accepted: 15 September 2014 / Published: 25 September 2014
PDF Full-text (194 KB) | HTML Full-text | XML Full-text
Abstract An inadvertent error was found in the pdf version of paper [1]. On page 3785, Equation (2) was not displayed well and should be read...[...] Full article

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