1,720,999 research outputs found
Solar/biomass hybrid cycles with thermal storage and bottoming ORC: System integration and economic analysis
No full textThis paper focuses on the thermodynamic modelling and thermo-economic assessment of a novel arrangement of a combined cycle composed of an externally fired gas turbine (EFGT) and a bottoming organic Rankine cycle (ORC). The main novelty is that the heat of the exhaust gas exiting from the gas turbine is recovered in a thermal energy storage from which heat is extracted to feed a bottoming ORC. The thermal storage can receive heat also from parabolic-trough concentrators (PTCs) with molten salts as heat-transfer fluid (HTF). The presence of the thermal storage between topping and bottoming cycle facilitates a flexible operation of the system, and in particular allows to compensate solar energy input fluctuations, increase capacity factor, increase the dispatchability of the renewable energy generated and potentially operate in load following mode. A thermal energy storage (TES) with two molten salt tanks (one cold and one hot) is chosen since it is able to operate in the temperature range useful to recover heat from the exhaust gas of the EFGT and supply heat to the ORC. The heat of the gas turbine exhaust gas that cannot be recovered in the TES can be delivered to thermal users for cogeneration. The selected bottoming ORC is a superheated recuperative cycle suitable to recover heat in the temperature range of the TES with good cycle efficiency. On the basis of the results of the thermodynamic simulations, upfront and operational costs assessments and subsidized energy framework (feed-in tariffs for renewable electricity), the global energy conversion efficiency and investment profitability are estimated. © 2017 The Author(s)
Effect of nanoparticles on heat capacity of nanofluids based on molten salts as PCM for thermal energy storage
No full textIn this study, different nanofluids with phase change behavior were developed by mixing a molten salt base fluid (selected as phase change material) with nanoparticles using the directsynthesis method. The thermal properties of the nanofluids obtained were investigated. These nanofluids can be used in concentrating solar plants with a reduction of storage material if an improvement in the specific heat is achieved. The base salt mixture was a NaNO3-KNO3 (60:40 ratio) binary salt. The nanoparticles used were silica (SiO2), alumina (Al2O3), titania (TiO2), and a mix of silica-alumina (SiO2-Al2O3). Three weight fractions were evaluated: 0.5, 1.0, and 1.5 wt.%. Each nanofluid was prepared in water solution, sonicated, and evaporated. Measurements on thermophysical properties were performed by differential scanning calorimetry analysis and the dispersion of the nanoparticles was analyzed by scanning electron microscopy (SEM). The results obtained show that the addition of 1.0 wt.% of nanoparticles to the base salt increases the specific heat of 15% to 57% in the solid phase and of 1% to 22% in the liquid phase. In particular, this research shows that the addition of silicaalumina nanoparticles has a significant potential for enhancing the thermal storage characteristics of the NaNO3-KNO3 binary salt. These results deviated from the predictions of the theoretical model used. SEM suggests a greater interaction between these nanoparticles and the salt. © 2013 Chieruzzi et al
CFD analysis of melting process in a shell-and-tube latent heat storage for concentrated solar power plants
No full textA latent heat storage system for concentrated solar plants (CSP) is numerically examined by means of CFD simulations. This study aims at identifying the convective flows produced within the melted phase by temperature gradients and gravity. Simulations were carried out on experimental devices for applications to high temperature concentrated solar power plants. A shell-and-tube geometry composed by a vertical cylindrical tank, filled by a Phase Change Material (PCM) and an inner steel tube, in which the heat transfer fluid (HTF) flows, from the top to the bottom, is considered. The conjugate heat transfer process is examined by solving the unsteady Navier-Stokes equations for HTF and PCM and conduction for the tube. In order to take into account the buoyancy effects in the PCM tank the Boussinesq approximation is adopted. The results show that the enhanced heat flux, due to natural convective flow, reduce of about 30% the time needed to charge the heat storage. A detailed description of the convective motion in the melted phase and the heat flux distribution between the HTF and PCM are reported. The effect of the mushy zone constant is also investigated. © 2015 Elsevier Ltd
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No full textREVISA Interpretation Benchmark based on a Rupther #15 Creep Test at 1100ÉC Performed on a Notched Tube
No full textThe object of this document is to show the results obtained performing a 3rd benchmark calculation for the Rupther test in creep conditions on a notched tube. This exercise is part of task 3 organised in the framework of the REVISA (Reactor Vessel Integrity in Severe Accidents) project (EU FI4S-CT96-0024). The objective of this project is to compare predictive tools for the behaviour of the PWR pressure vessel in case of severe accident such as core melting. Rupther is a simple analytical experiment in which a notched tube is subjected to internal pressure and thermal loading. The thermal loading is an axial gradient and the upper temperature is very high, 1100...C in the present case. The internal pressure is maintained at 0.8 MPa. This benchmark follows calculation exercises performed on tubes without notches. In the present report, the following arguments are provided: a summary of the FEM analysis of the Rupther Test#15 experiment, an evaluation of the rupture time on the basis of different creep damage criteria, a sensitivity analysis based on different methods for the identification of the material parameters and different creep laws
Numerical simulation of a complete charging-discharging phase of a shell and tube thermal energy storage with phase change material
No full textNumerical simulations of a shell and tube energy storage device based on a phase change material (PCM) in vertical position are performed. The heat transfer fluid (HTF) is a diathermic oil and the PCM, made by molten salts, is confined within a closed shell surrounding the tube where the HTF flows. The energy loss through the external wall is included. The test has been carried out within the experimental activity performed by ENEA. A complete cycle is considered: the initial stabilization, the charging phase and the discharging phase. Details of flow behavior within the molten PCM are described highlighting its influence on the device performance
3rd REVISA Benchmark Based on Rupther #15 Creep Test at 1100°C Performed on a Notched Tube. New Calculation
No full textObject of the present document is to show the results gotten in the remaking of the 3rd REVISA benchmark made necessary from the changed thermal conditions of the test-tube. In fact, in consequence of an error of measure of the temperature, the thermal field, which the test-tube was subjected, had been amiss valued. A new benchmark has been carried out in the same conditions of the precedent using a correct thermal field however. The gotten results are, despite everything, very similar to those of the preceding benchmark. The consequence of this is that the considerations effected in precedence remain however valid
Heat capacity of nanofluids for solar energy storage produced by dispersing oxide nanoparticles in nitrate salt mixture directly at high temperature
No full textMolten salts as phase change materials (PCMs) can be used as thermal storage media in concentrated solar power (CSP) plants. The addition of nanoparticles into a base fluid (producing the so called nanofluid) can enhance its thermal properties. The most common technique involves the use of water. We present a new procedure based on high temperature mixing. In particular, different nanofluids were developed by mixing NaNO3-KNO3 (60–40 wt%) solar salt with 1.0 wt% of SiO2, Al2O3 and a mix of SiO2/Al2O3 nanoparticles at 300 °C using a twin screw micro-compounder. The effect of different screw speeds (100 and 200 rpm) and mixing times (15 and 30 min) were studied. The results showed that the nanoparticles induce an increase of the heat of fusion of 1.5–7.4% while the onset temperatures decrease for all the nanofluids independently from the processing conditions (up to 9.7 °C). Moreover, an increase in the specific heat (Cp) is recorded mainly for the nanofluid with SiO2/Al2O3 with a maximum of 52.1% in solid phase and 18.6% in liquid phase after 30 min of mixing at 200 rpm. The same nanofluid showed the highest stored heat. Particle aggregation into clusters in solid state was detected by scanning electron microscopy (SEM) but smaller aggregates resulted for higher mixing times and screw speed related to the highest Cp. Moreover, smaller grains in the nanofluids were detected with respect to the base salt morphology. Thus, the nanofluid produced with SiO2/Al2O3 nanoparticles at 200 rpm for 30 min gives the best overall performances. This work showed that nanofluids with enhanced thermal properties can be obtained with an innovative mixing process directly at high temperature, eliminating the water evaporation step. © 2017 Elsevier B.V
A New Phase Change Material Based on Potassium Nitrate with Silica and Alumina Nanoparticles for Thermal Energy Storage
No full textIn this study different nanofluids with phase change behavior were developed by mixing a molten salt base fluid (KNO3 selected as phase change material) with nanoparticles using the direct synthesis method. The thermal properties of the nanofluids obtained were investigated. Following the improvement in the specific heat achieved, these nanofluids can be used in concentrating solar plants with a reduction of storage material. The nanoparticles used (1.0 wt.%) were silica (SiO2), alumina (Al2O3), and a mix of silica-alumina (SiO2-Al2O3) with an average diameter of 7, 13, and 2–200 nm respectively. Each nanofluid was prepared in water solution, sonicated, and evaporated. Measurements of the thermophysical properties were performed by DSC analysis, and the dispersion of the nanoparticles was analyzed by SEM microscopy. The results obtained show that the addition of 1.0 wt.% of nanoparticles to the base salt increases the specific heat of about 5–10 % in solid phase and of 6 % in liquid phase. In particular, this research shows that the addition of silica nanoparticles has significant potential for enhancing the thermal storage characteristics of KNO3. The phase-change temperature of potassium nitrate was lowered up to 3 °C, and the latent heat was increased to 12 % with the addition of silica nanoparticles. These results deviated from the predictions of theoretical simple mixing model used. The stored heat as a function of temperature was evaluated for the base salt, and the nanofluids and the maximum values obtained were 229, 234, 242, and 266 J/g respectively. The maximum total gain (16 %) due to the introduction of the nanoparticles (calculated as the ratio between the total stored heat of the nanofluids and the base salt in the range of temperatures 260–390 °C) was also recorded with the introduction of silica. SEM and EDX analysis showed the presence of aggregates in all nanofluids: with silica nanoparticles they were homogenously present while with alumina and silica-alumina also zones with pure salt could be detected. © 2015, Chieruzzi et al
Thermo-economic Assessment of an Externally Fired Hybrid CSP/biomass Gas Turbine and Organic Rankine Combined Cycle
No full textThis paper focuses on the thermo-economic analysis of a hybrid solar-biomass CHP combined cycle composed by a 1.3-MW externally fired gas-turbine (EFGT) and a bottoming organic Rankine cycle (ORC) plant. The primary thermal energy input is provided by a hybrid concentrating solar power (CSP) collector-array coupled to a biomass boiler. The CSP collector-array is based on parabolic-trough concentrators (PTCs) with molten salts as the heat transfer fluid (HTF) upstream of a fluidized-bed furnace for direct biomass combustion. Thermal-energy storage (TES) with two molten-salt tanks (one cold and one hot) is considered, as a means to reducing the variations in the plant's operating conditions and increasing the plant's capacity factor. On the basis of the results of the thermodynamic simulations, upfront and operational costs assessments, and considering an Italian energy policy scenario, the global energy conversion efficiency and investment profitability are estimated for 2 different sizes of CSP arrays and biomass furnaces. The results indicate the low economic profitability of CSP in comparison to only biomass CHP, because of the high investment costs, which are not compensated by higher electricity sales revenues. © 2017 The Authors
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