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[en] Highlights: •Organic Flash Regenerative Cycles were introduced (OFRCs). •A technical and economic comparison with traditional Organic Flash Cycle (OFCs) was carried out. •Different organic fluids were analyzed. •Results show that specific costs of OFRCs are lower than OFCs. •OFRCs can be a valid alternative to ORCs for low temperature heat recovery. -- Abstract: Organic Flash Cycles (OFCs) can improve the overall efficiency of waste heat recovery or geothermal systems due to a better match of the hot and cold heat transfer curves. However, the lower mean temperature difference between the heat transfer curves implies larger exchanger areas and therefore higher heat exchanger costs. In order to reduce the exchanger size, a new cycle configuration is introduced in this paper, consisting in a new type of organic flash regenerative cycle (OFRC) for heat source temperatures in the range 80–170 °C. The regeneration allows to recover part of the enthalpy of the liquid phase from the flash evaporator increasing the temperature of the liquid at the exchanger inlet, thus reducing the exchanger size. The thermodynamic performance of OFRCs are practically the same as of the OFC, but the unit cost of the system per kW installed power can be 20% lower. A variety of working fluids was tested and results have shown that long molecular chain alkanes provide the best thermodynamic efficiency, but those fluids have the main drawback of a low vapor density, resulting in very large expansion devices and condensers. R601a is the working fluid featuring the best tradeoff between thermodynamic efficiency and components size in the heat source temperature range between 80 °C and 170 °C. The comparison of the OFRC with conventional ORCs has shown the thermodynamic superiority of the OFRC with every tested fluid. Finally the cost analysis has highlighted that OFRCs specific cost has the same magnitude as ORCs for mini and micro scale plants.
[en] Highlights: • A small scale solar ORC was investigated during a year-long simulation. • The system was operated without a thermal storage. • High flexibility thanks to a sliding-velocity control and volumetric expander. • Influence of ORC and solar field parameters considered. • Strong influence of concentration factor and system inertia. - Abstract: In this paper the dynamic behavior of a small low-concentration solar plant with static Compound Parabolic Collectors (CPC) and an ORC power unit with rotary volumetric expander has been analyzed. The plant has been simulated in transient conditions for a year-long operation and for three different sites respectively located in northern, central and southern Italy, in order to evaluate the influence of the latitude on the production. Hourly discretized data for solar radiation and for ambient temperature have been used. The adoption of a sliding-velocity control strategy, has allowed to operate without any storage system with a solar multiple (S.M.) of 1, reducing the amplitude of the solar field and simplifying the control system. Different collectors tilt angles and concentration factors, as well as thermodynamic parameters of the cycle have been tested, to evaluate the optimal working conditions for each locality. Results highlighted that specific production increased with the concentration ratio, and with the decrease of latitude. The comparison with the steady-state analysis showed that this type of control strategy is suited for those configurations having a smaller number of collectors, since the thermal inertia of the solar field is not recovered at all during the plant shut-down phase.
[en] A method of thermo-economic analysis for the choice of optimal thermodynamic parameters of steam bottoming cycles in combined cycle power plants is presented. By keeping the thermodynamic aspects separated from the economic aspects, this method allows designers to easily perform a sensitivity analysis of the change in the economic parameters
[en] Highlights: • We made a CFD simulation with a validated model. • We analyzed the influence of the geometrical parameters of the collector. • We established a correspondence between the Nusselt number and the characteristic dimensions and parameters of the collector. - Abstract: In this paper a methodology is proposed to estimate thermal heat losses inside compound parabolic collectors (CPC) to be used in designing and validating new collectors' concepts and materials. CFD simulations were carried out on different CPCs, taking into account the effective working conditions and the presence of radiative heat transfer as well as the absence of adiabatic walls. The CFD model was validated considering a previous work reported in literature. The results were employed to develop some correlations by interpolation of numerical data, to express the Nusselt number on the receiver. We used these correlations to calculate heat losses of the receiver and to show the influence of different parameters such as the shape of receiver itself, tilt angle and concentration ratio. The variation of terms of the correlation as a function of characteristic length and concentration was studied. These results might be employed for a preliminary estimation procedure of a CPC collector efficiency and to propose sizing criteria of general validity for this class of devices.
[en] A performance and economic analysis of an existing combined heat and power plant with an internal combustion engine and district heating for the Faculty of Engineering of the University of Perugia is presented. Results of the first 15 months of operation are shown and discussed with reference to daily performance of the combined heat and power plant, which shows how electric efficiency is only slightly affected by ambient temperature. A comparison of evaluation indexes for cogeneration plants is made with particular attention to the energy index (EI) which is used by Italian legislation as an evaluation parameter to decide if a combined heat and power plant can have access to financial benefits.A cost-benefits analysis based on the first year of operation was made to decide eventual changes in heat and power management, in order to reduce pay-back period and increase the internal rate of return of the investment. (Copyright (c) 1998 Elsevier Science B.V., Amsterdam. All rights reserved.)
[en] Highlights: ► Life cycle was assessed for both concentrated solar power and photovoltaic systems. ► The PV plant has a higher environmental impact than the CSP plant. ► The Global Warming Potential is lower for the CSP than for the PV plant. ► The energy payback time is lower for the CSP than for the PV plant. -- Abstract: Solar energy is an important alternative energy source to fossil fuels and theoretically the most available energy source on the earth. Solar energy can be converted into electric energy by using two different processes: by means of thermodynamic cycles and the photovoltaic conversion. Solar thermal technologies, sometimes called thermodynamic solar technologies, operating at medium (about 500 °C) and high temperatures (about 1000 °C), have recently attracted a renewed interest and have become one of the most promising alternatives in the field of solar energy utilization. Photovoltaic conversion is very interesting, although still quite expensive, because of the absence of moving components and the reduced operating and management costs. The main objectives of the present work are: •to carry out comparative technical evaluations on the amount of electricity produced by two hypothetical plants, located on the same site, for which a preliminary design was made: a solar thermal power plant with parabolic trough collectors and a photovoltaic plant with a single-axis tracking system; •to carry out a comparative analysis of the environmental impact derived from the processes of electricity generation during the whole life cycle of the two hypothetical power plants. First a technical comparison between the two plants was made assuming that they have the same nominal electric power and then the same total covered surface. The methodology chosen to evaluate the environmental impact associated with the power plants is the Life Cycle Assessment (LCA). It allows to analyze all the phases of the life cycle of the plants, from the extraction of raw materials until their disposal, following the “from cradle to grave” perspective. The environmental impact of the two power plants was simulated by using the software SimaPro 7.1, elaborated by PRé Consultants and using the Eco-Indicator 99 methodology. Finally, the results of the analysis of the environmental impact are used to calculate the following parameters associated to the power plants: EPBT (Energy Pay-Back Time), CO2 emissions and GWP100 (Global Warming Potential over a 100 year time horizon).
[en] The production of hydrogen combined with carbon capture represents a possible option for reducing CO2 emissions in atmosphere and anthropogenic greenhouse effect. Nowadays the worldwide hydrogen production is based mainly on natural gas reforming, but the attention of the scientific community is focused also on other gas mixtures with significant methane content. In particular mixtures constituted mainly by methane and carbon dioxide are extensively used in energy conversion applications, as they include land-fill gas, digester gas and natural gas. The present paper addresses the development of an innovative system for hydrogen production and CO2 capture starting from these mixtures. The plant is based on steam methane reforming, coupled with the carbonation and calcination reactions for CO2 absorption and desorption, respectively. A thermodynamic approach is proposed to investigate the plant performance in relation to the CH4 content in the feeding gas. The results suggest that, in order to optimize the hydrogen purity and the efficiency, two different methodologies can be adopted involving both the system layout and operating parameters. In particular such methodologies are suitable for a methane content, respectively, higher and lower than 65%
[en] Highlights: • Renewable HPS for the train start-up within feeding durations. • Dynamic modelling of the modern HPS applied to traction systems. • Port-Controlled Hamiltonian (PCH) design for supercapacitors’ charge/discharge operation. • Experimental validation and applicability of HPSs for energy management in eco-tractions. - Abstract: Electrochemical capacitors, called supercapacitors (SCs) or ultracapacitors, are devices conveniently used for embedded electrical energy management owing to their huge capacitance, low internal resistance and flexible control through power electronic conversion. This paper proposes a main power supply of hybrid Wind Generator (WG)–SC within the train station for feeding the traction onboard SC through specified limited feeding transit durations. Onboard SCs provide the train with the requested start–up self–energy. The hybrid WG–SCs system is an environmental–friendly source that enables the independency on national grid and guarantees an efficient bidirectional power transfer for energy management with enhanced dynamic performance. Therefore, the dynamic modelling and the experimental analysis of the modern hybrid WG–SCs used for managing the charge/discharge operation of SCs at Unity Power Factor (UPF) mode are presented. For this purpose, the Port–Controlled Hamiltonian (PCH) methodology is deduced and explicitly presented. Simulation results, via MATLAB™, reveal that the proposed PCH control methodology can be successfully implemented to ensure acceptable system dynamic behavior. Numerical results are validated with experimental measurements to investigate the significance of the PCH approach for the energy management operation in eco-tractions.
[en] In the last years the interest in hydrogen as an energy carrier is significantly increased both for vehicle fuelling and stationary energy production by fuel cells. The benefits of a hydrogen energy policy are the reduction of the greenhouse effect and the centralization of the emission sources. Moreover, an improvement to the environmental benefits can be achieved if hydrogen is produced from renewable sources, as biomass. The present study relates the development of an innovative system for hydrogen production and CO2 capture starting from syngas. The plant is based on the carbonation and calcination reactions for CO2 absorption and desorption, respectively. In the carbonation reactor also steam methane reforming and CO-shift take place to enhance the hydrogen production. By means of a thermodynamic analysis, the system has been optimized in terms of the amount of the hydrogen produced and its purity. Different syngas compositions have been tested. The results confirm the effectiveness of the system proposed, which provides 99% H2 purity and zero CO2 emission in the case of the syngas derived from Battelle Columbus Laboratories gasifier