• Energy Research

Abstract The paper proposes a method to evaluate the potential for electric power production at any site of possible geothermal interest. Accounting for geological data of the reservoirs, the method allows the computation of the available electrical power of the investigated site. Electrical energy production from geothermal sources is realized through different techniques, such as single flash and double flash, dry steam, and binary ORC plants. The technique chosen to be the most productive is determined by analyzing a specific range of geofluid properties, mainly temperature and pressure. Moreover, each plant typology has a global efficiency that may be correlated to geofluid enthalpy by empiric relations available in literature. The proposed evaluation method brings together all these correlations, yielding the power availability from a geosource, once its temperature and pressure are known. The method takes as input the geofluid available flow rate, its pressure, temperature and non-condensable gas content. It defines the best plant option from these parameters, calculates its global efficiency and finally returns the actual available power. For sites of geothermic interest, such as the volcanic island of Ischia in Southern Italy, the results of the application of this new method clearly highlight the most suitable zones for power plants installations.

The use of pure vegetable oils in Micro Gas Turbine can damage the injection system or the combustion chamber causing undesired vibrations. An ideal solution would consist in having an available tool able to forecast and/or follow in real time the vibrational state of the combustion device. The present paper describes tests performed on a low emission Micro Gas Turbine for power generation, fueled with different liquid fuels, including commercial diesel oil and its blends with pure rapeseed oil. A particular attention was paid both on the emissions and on the measurements of the micro vibrational distributions and their correlation under the different fueling conditions using a new signal processing based on a nonlinear method and chaos analysis. We observed that the overall behavior of the MGT fueled with the blends was good, and the emission concentrations of CO, NOx and Total Particle Matter were comparable to the pure diesel oil ones. Moreover, the chaos analysis and the proposed methodology came out as a possible tool for the real-time characterization of the combustion process of the MGT and to individuation of the fuel supplied.

Ethanol can be used as an alternative fuel for spark-ignition (SI) engines to increase the octane number and oxygen content of ethanol/gasoline blends, thereby reducing dependence on fossil fuels and the exhaust emissions of incomplete combustion products. Although it is widely agreed that ethanol can reduce CO and HC exhaust emissions, the literature on ethanol and NOX emissions is far from conclusive; hence there is a need for an in-depth, updated review of ethanol/gasoline blends in SI engines and the relative production of NOX emissions. In light of that, the present work aims to provide a comprehensive literature review on the current state of ethanol combustion in SI engines to shed definitive light on the potential changes in NOX emissions under various operating conditions. The first part of this paper discusses the feasibility of ethanol as an alternative transportation fuel, including world production and ethanol production processes. The physicochemical properties of ethanol and gasoline are then compared to analyze their effects on combustion efficiency and exhaust emissions. Then, the pathways of NOX formation inside the cylinder of SI engines are discussed in depth. Finally, we review and critically discuss the effects of ethanol concentration in blends and different engine parameters on NOX formation.

This paper explains a numerical optimization of a novel screw expander-based solar thermal electricity plant to evaluate the energetic benefits in specific case studies. In the proposed solar electricity generation system, which is based on the steam Rankine cycle, water is used as working fluid and storage, parabolic trough collectors as a thermal source and screw expander as power machine. Such solar system offers major advantages over conventional power plants adopting steam turbines: low operating pressures, good exploitation of low temperature heat sources, acceptable efficiency in energy conversion with steam-liquid mixtures and reduced size. Since screw expanders can operate at off-design working conditions in several situations when installed in direct steam generation solar plants, the chief purpose of the present study is to develop a thermodynamic model to analyse the energy performance of the planned solar power system when off-design operating conditions befall. To assess maximum efficiency of the whole power plant at part-load operating conditions, numerical optimization is then performed in a specific range of fluctuating evaporation temperatures under fixed condensation pressures. Keywords: Steam screw expander, Solar thermal power efficiency, Direct steam generation, Part-load behavior, Polytropic expansion phase

Abstract Combined heat and power (CHP) generation is a fundamental practice to reduce primary energy consumptions and mitigate the related greenhouse gas emissions. Cogeneration plants are even more important in the Hospital sector, as they give the opportunity to operate regardless of the electric power grid, so ensuring energy availability also during grid faults while providing significant cost savings to the public healthcare system. The integration of a battery energy storage with the CHP system could further increase the expected advantages by enabling more flexible operating strategies, further energy reliability and lower operating costs. Therefore, this research paper addresses the development of a specific methodology for the energetic and economic assessment of a grid connected CHP plant assisted by a battery energy storage. More specifically, an evaluation algorithm has been developed and then coupled to a genetic evolutionary algorithm. Then, a vector optimization problem has been solved to find the optimal configurations of a modular cogeneration plant (i.e. size and number of CHP gas engines; battery size) which maximize the primary energy saving while minimizing the payback period. Results based on electricity and heat demand of a hospital facility demonstrate how the battery energy storage system allows the shifting of the Pareto frontier towards better economic results if compared with Pareto solutions found when the optimization problem does not consider a battery storage supporting the CHP plant. The proposed methodology provides an effective and flexible procedure for the optimal configuration and detailed analysis of CHP plants integrated by an electrochemical based energy storage system. In fact, the effects of a high number of interacting energetic and economic parameters are considered together with the technical constraints required to extend battery lifetime in real applications.

Screw expanders are volumetric machines particularly suitable in energy conversion with steam–liquid mixtures and for the exploitation of low temperature heat sources. This study explored the main criteria to evaluate the thermodynamic advantages and exergetic assessment of an innovative solar electricity generation system: Screw expanders are utilized as power machines and parabolic trough collectors as a thermal source. Such a direct steam generation solar system, which is based on the Rankine cycle, offers benefits in comparison with usual power plants with dynamic expanders: The best exploitation of low temperature heat sources, satisfactory thermal efficiency with steam–liquid mixtures, lower evaporation pressures, and reduced size. Under real working conditions, screw expanders can work at part-load operating conditions; thus, the chief purpose of the present paper was to analyze the exergetic advantages of the planned solar power system when solar radiation and off-design working conditions fluctuate. Initially, the polytropic expansion phase with a specific numerical model is described to evaluate the energy losses that affect the thermodynamic performance of screw expanders when installed in the planned renewable energy power plant. Subsequently, to explore the exergy harnessing in the exhausted steam at off-design working conditions and then to appraise the maximum exergetic efficiency of the proposed screw expander-based solar thermal electricity plant, numerical optimization was performed in a broad range of evaporation and condensation temperatures.

AbstractThis work proposes a hybrid system based on gas turbine or steam/gas plant integrated with a solar field either to provide a part of the heat addition or to replace the combustor in the case that the only solar heat input is enough to ensure that the working fluid reaches the turbine inlet temperature. The system can operate in a pure combustion mode when solar irradiance is weak during some hours in the day or seasons.A thermodynamic model for a hybrid solar gas-turbine power plant is presented. The system consists of a micro gas turbine (100 - 110kW) supported by a solar field to integrate heat for the regeneration and combustion through heat exchangers. In this way, a hybrid thermal energy supply is considered, combining the use of biogas with thermal power from a solar field. The methodology is based on a combined thermodynamic - CFD based approach. The overall plant performance is estimated by the Thermoflex software, a modular program that allows assembling a model with several components. In a second phase, the authors have carried out a CFD analysis of the reacting flow through the micro GT combustor when supplied with a typical biogas from anaerobic digestion in replacement of the conventional natural gas, in order to check the combustion effectiveness and the environmental impact of the gas turbine at several load levels and ambient conditions. The CFD modeling is performed by using the Fluent flow solver.