Storing hydrogen in solid state is gaining attention enabling for overcoming simultaneously two specific limits of conventional hydrogen storage technologies: storage capacity and safety. Currently, hydrogen is stored as compressed gas (usually, at either 350 or at 700 bar), reaching densities ranging from 24.5 to 41.4 kg/m(3), but requiring a significant compression work and determining several safety issues; otherwise, it can be stored in liquid state under cryogenic conditions (70.8 kg/m(3) at 1 bar and 20 K), but requiring a great amount of energy for lique-faction (about 12.5 kWh/kg) and costs to keep it liquid. In solid state context, physisorption (also said adsorption) mechanism has been recognized as an attractive technique for its capability of storing hydrogen in a porous structure, with weaker bonds, differently from chemisorption mechanism where hydrogen molecules chemically bonds to the absorbing material forming, for instance, metal hydrides. During adsorption, heat is released and thermal management can become an issue when the tank volume is increased. This work aims to numerically investigate the effect of increasing the tank volume on the solid-state hydrogen storage capacity. Moreover, the effect of the tank aspect-ratio is considered. The investigation has been carried out through Computational Fluid Dynamic analyses. The modified Dubinin-Astakhov isotherm model has been taken into account to describe the isotherm sorption during charge, dormancy, and discharge processes. Initially, the model has been validated on a 2.5 dm(3) tank, against experimental results available in the literature; then, the geometry has been scaled by a factor of 10, and several aspect ratio considered. The numerical study was conducted considering three different aspect ratios (i.e., 5, 8.5, and 15), whilst keeping the internal volume constant. In this work, it is shown that larger aspect ratios can lead to improved adsorbed mass and decreased maximum temperatures in the system.

Pumped-Storage Hydropower Plants (PSHPs) are a well-established technology for storing large amounts of energy and enhancing the power systems flexibility. In the framework of Micro and Mini Hydropower, Pumps as Turbines (PaTs) could represent a valid cost-effective solution also for PSHPs. Nonetheless, the lack of a control device could represent a problem, mainly with non-fixed operating conditions. Additionally, PaTs show generally a steep efficiency decrease at part load. Thus, since conventional PaTs do not present adjustable geometries, a preliminary sizing of an adjustable Inlet Guide Vane (IGV), referring to turbine mode, is proposed in this work. The aim of this work is to propose a methodology suitable to modify a baseline PaT introducing an adjustable IGV to perform a high efficient control in terms of flow rate and pressure drop. The machine selected for the proposed study is a centrifugal pump with a specific speed, n(q), equal to 16. The adjustable IGV are inserted between the volute cutwater and the impeller by reducing impeller diameter. To obtain an initial solution for the stationary blades row profile, a review of Francis turbines guide vanes design methods is conducted. Finally, a numerical study on the influence of guide vanes number is performed.

Abstract Hydrogen storage in solid-state is a promising alternative to conventional technologies to overcome some of their limitations, particularly in terms of the amount of hydrogen stored per unit volume. For instance, liquid hydrogen (stored at 1 bar and 20 K) is characterized by a 70.8 kg/m3 volumetric density, but a great amount of energy is required for liquefaction (about 12.5 kWh/kg); whereas, hydrogen stored as compressed gas (350 - 700 bar) at ambient temperature reaches a density ranging from 24.5 to 41.4 kg/m3, however the compression work is significant, and the high storage pressure determines safety issues. Regarding solid-state hydrogen storage, chemisorption is a solution, where hydrogen is stored at moderate pressure and temperature, reaching a volumetric density of 80-110 kg/m3, but between hydrogen and metal hydrides chemical bonds born, which require a significant amount of energy to be break them; moreover, the time required for absorption and desorption is very low. In the context of solid-state storage, carbon materials can be an interesting solution due to their low cost, and their ability to store hydrogen reversibly within a porous structure through physisorption mechanisms, under supercritical conditions. This study numerically investigates the effect of the variation of the tank capacity on the charging, dormancy, and discharging processes. Two similar axi-symmetric geometries are considered by doubling the tank volume from V 1 = 2.5 L to V 2 = 5 L. The physisorption mechanism has been modeled according to the modified Dubinin-Astakhov isotherm model. Initially, the CFD model has been validated on V 1 tank against experimental results available in the literature. Then, simulations have been carried out on V 2 tank to estimate the effect of a storage capacity increase. The simulations on the V 2 tank have been set keeping the same boundary conditions in terms of velocity, density, and temperature at the inlet by doubling the overall amount of hydrogen stored with respect to V 1 tank. The results show corresponding trends in temperature and pressure profiles with only slight differences in their behaviors; in particular, in the final stage of the charging process.

Industrial energy demand in the EU is driven by the thermal energy share, which accounts for the most part of the total energy demand. A large part, about one-third, is at medium temperatures, between 100 and 200 degrees C, and it is still produced by fossil fuels, mainly natural gas. This energy demand could be met by solar thermal energy by using Concentrated Solar Thermal (CST) technologies, suitably combined with thermal energy storage systems. This paper presents a study of CST plant composed by Parabolic Trough Collectors (PTC) combined with Latent Heat Thermal Energy Storage (LHTES) system in shell-and-tubes configuration for the supply of heat for industrial processes. Firstly, the criteria adopted for the sizing and the quasi-static simulation of the system are described. Significant attention is given to evaluate the variation of the Solar Fraction (SF) with the typical parameters of thermal storage, such as the maximum capacity and exchange surface, given a specific location and the Solar Multiple (SM). The methodology has been applied to a dairy processing facility within the agri-food sector with a thermal load of 500 kWth. Then, an economic analysis has been carried out by evaluating the impact of the component's costs, such as the linear parabolic collectors, thermal storage materials and landscape preparation. Finally, the Levelized Cost of Heat (LCOH) has been computed for different thermal storage sizes in order to find the best combination that minimizes it. As a result, despite the overall increase of the SF with the thermal storage capacity, the proposed work shows how the minimum value of the LCOH (7.24 c(sic)/kWh) corresponds to low storage capacity (1 MWh).

Abstract Prediction and control of thermoacoustic instabilities is a major challenge in the development of modern power generation gas turbines and aeroengines. Such instabilities arise from the coupling between flame dynamics and combustor acoustic modes, resulting in severe oscillations that can lead to premature aging of combustor components and structural damage. In many combustors, passive dampers are implemented to increase the acoustic energy dissipation of the system and prevent the onset of these harmful flame-acoustic interactions. In the present study, passive damping systems based on a cavity-backed perforated plate are experimentally analyzed, with a focus on studying the impact of bias flow on the reflection coefficient over a wide range of frequencies. Tests are carried out on two cavity-backed perforated plates characterized by the same porosity but a different number of holes 25 and 49, namely P25 and P49, respectively. It is observed that, for a given plate geometry, a higher bias flow leads to an increase in the plates absorption capacity over a wider range of frequency. This is more pronounced in the P 49 plate configuration. For both tested configurations, comparing the experimental results with Scarpato model proposed in the literature [1], a good match it is observed only for low values of bias flow. The model instead is not able to correctly capture the behavior of the damping systems when higher dissipation is reached.

The exploitation of hydrogen-based technologies for on-board power generation is having a significant impact on the shipping industry due to their potential to support the decarbonization. This paper proposes the modeling and optimization of a fuel-cell (FC) hybrid power system for the case study ferry in order to achieve the best performance on stack degradation over the vessel experimental missions cycle. The FC system degradation has a significant impact on hydrogen consumption, energy efficiency and machinery operational cost. The research target is to build a prescriptive digital twin to provide an holistic design of the fuel cell system. The simulations will consider power distribution between components proposing an energy management strategy during the preliminary design phase. By sizing the components to meet the energy demand of a real mission, this work aims to propose a feasible powertrain solution for the ferry under investigation. The results of this study could be valuable for the further development of fuel cell technology for the maritime industry and provide an effective tool in the sizing of the power generation system on board full electric propulsion vessels.

Hydrogen can play a key role in the gradual transition towards a full decarbonization of the combustion sector, e.g., in power generation. Despite the advantages related to the use of this carbon-free fuel, there are still several challenging technical issues that must be addressed such as the thermoacoustic instability triggered by hydrogen. Given that burners are usually designed to work with methane or other fossil fuels, it is important to investigate their thermoacoustic behavior when fueled by hydrogen. In this framework, the present work aims to propose a methodology which combines Computational Fluid Dynamics CFD (3D Reynolds-Averaged Navier-Stokes (RANS)) and Finite Element Method (FEM) approaches in order to investigate the fluid dynamic and the thermoacoustic behavior introduced by hydrogen in a burner (a lab-scale bluff body stabilized burner) designed to work with methane. The case of CH4-air mixture was used for the validation against experimental results and benchmark CFD data available in the literature. Numerical results obtained from CFD simulations, namely thermofluidodynamic properties and flame characteristics (i.e., time delay and heat release rate) are used to evaluate the effects of the fuel change on the Flame Response Function to the acoustic perturbation by means of a FEM approach. As results, in the H2-air mixture case, the time delay decreases and heat release rate increases with respect to the CH4-air mixture. A study on the Rayleigh index was carried out in order to analyze the influence of H2-air mixture on thermoacoustic instability of the burner. Finally, an analysis of both frequency and growth rate (GR) on the first four modes was carried out by comparing the two mixtures. In the H2-air case the modes are prone to become more unstable with respect to the same modes of the case fueled by CH4-air, due to the change in flame topology and variation of the heat release rate and time delay fields.

Nowadays, mobility represents a key sector to achieve the goal of carbon neutrality. Indeed, the development of hybrid powertrains is contributing to a reduction in the environmental impact of vehicles. One of the most promising energy-saving solutions is regenerative braking, which enables deceleration while recovering energy, otherwise wasted. Even though much scientific community effort has been addressed to the optimization of this technology in the automotive field, the increase of energy storage systems efficiencies enables the overcoming of the constraints related to the reuse of electric energy in railway vehicles. This solution could be extremely useful for those railway vehicles which operate on non-electrified lines, where traction is usually provided by diesel engines. For this reason, the present work focuses on how regenerative braking technology could be exploited in diesel-powered rail applications. In further detail, a diagnostic train working on real railway lines has been considered as a case study. Given the real duty-cycle of the vehicle, a simulation model has been developed with the aim of evaluating the amount of energy recovered during braking phases and, consequently, the fuel saving and the avoided CO2 emissions. As a result, the analysis shows an improved energy efficiency of propulsion system. Compared with a pure diesel operation, it leads to fuel savings of 20%, a reduction of CO2 emissions of 22.3 kg with 23.25 kWh stored in the battery at the end of the route.

Marine energy still plays a marginal role in the current global energy scenario, despite the incessant effort by research for more than thirty years in the exploitation of the so-called blue energy. Among the wide range of marine technologies, wave energy harvesting can play a significant role in view of its potential and Oscillating Water Column (OWC) systems, coupled with Wells turbines, can be considered among the most mature wave energy technology. Due to the oscillating nature of the flow rate in this kind of applications, Wells turbines are affected by dynamic stall, which has significant effects in terms of performance, fatigue, noise and structural integrity of the turbine.Actually, during dynamic stall, the Wells turbine experiences evident high frequency torque fluctuations which overlay on the typical hysteresis loop, mainly during flow deceleration. The amplitudes of these fluctuations are damped as the flow rate decreases toward reattachment. Often these fluctuations are not evident because hysteresis loops are usually provided with phase-averaged data, which can significantly smoothen or even conceal them. Indeed, it is difficult to find in the literature high frequency torque measurements able to show these fluctuations. With the aim to better investigate how the stall triggers this phenomenon, a monoplane Wells turbine has been manufactured in 3D printing and tested in the open wind tunnel of the Polytechnic University of Bari, Italy. The interest of the experimental campaign has been mainly focused on the effects of main parameters of the oscillating inlet flow rate (mean flow rate, amplitude and period of the oscillations, modifying the controlling parameters of the inverter driving the squirrel cage blower) on the performance of the machine. The machine has been firstly investigated under steady state inlet flow conditions, then under dynamic stall conditions. As a result, unsteady torque fluctuations occur during the flow deceleration till the flow reattachment. After the stall, the investigated Wells turbine experiences a drastic reduction of the torque coefficient of about 90%. Moreover, the torque coefficient shows a number of peaks during deceleration phases ranging from 2 to 4. Specifically, the case with the maximum period of the flow rate under investigation (i.e., T = 20 s) shows a greater number of peaks (4) than those related to the other cases (3). Moreover, it has been found that this unsteady behavior is due neither to the mass flow rate crossing the turbine, nor to the stagnation pressure drop, nor to the rotational speed control, which is correctly performed keeping the rotational speed within 1% of the target value. Hence, detecting these oscillations can be relevant in the turbine design phase to enhance the structural strength of the turbine.

Water Distribution Networks (WDNs) represent a noteworthy field for possible implementation of Small Hydropower (SHP), by replacing Pressure Reduction Valves (PRV) with turbomachines, in particular Pump as Turbines (PaTs), to control and regulate the pressure, while harvesting energy otherwise wasted. Different models were developed to predict the performance and select the positioning of the PaTs for the maximum energy recovery but most of them neglect practical aspect such as: power grid limitations and optimal harvesting strategy. In this framework, we intend to propose a new method to select a PaT, defining its optimal working point, by introducing an energy exploitation coefficient. The proposed methodology is based on the experimental results of a real PaT tested in the high capacity hydraulic laboratory at Polytechnic University of Bari. Firstly, the selected commercial centrifugal pump was tested in both pump and turbine modes. Then, three different approaches, for the Best Efficiency Point (BEP) selection, are described and compared in terms of energy exploitation and capacity factor for a WDN. The first consists of selecting the BEP at the average flow rate, the second one considers the probability distribution of the flow rate and the corresponding available hydraulic energy, whereas the latter is based on the highest energy harvesting. By applying energy production, economic and environmental analyses, the new proposed methodology, based on the third approach, shows a remarkable advantage in terms of exploited energy. Indeed a remarkable 60% energy recovery is achieved with 334 ton CO2/year avoided. Furthermore, the impact of the electrical motor on the maximum power generation (cut-off) is considered. Eventually, useful insights for the future PaT selection and installation are discussed.

The upcoming regulations to achieve zero-emission passenger transport present challenges for designing new ferry powertrains. The proposed work investigates the feasibility of using a Proton Exchange Membrane Fuel Cell (PEMFC) power system to power a long-haul ferry. The paper describes the zero-order cell model as well as the method for estimating cell degradation. The stack modeling, heat balance equations, and auxiliary modeling are also presented. The proposed model enables the simulation of the fuel cell under different operating conditions and includes the use of air or oxygen as an oxidizer. A thermal management strategy for the overall PEMFC system is also proposed. The model was calibrated on the characteristic curves of the PEMFC Ballard FCvelocity™ HD6 (150 kW) and validated by reproducing experimental results. Then, a real load profile of a ferry, as well as the proposed powertrain is considered as case study. The presented results are related to a single daily mission and its deterioration throughout the set mission cycle is finally presented.

The purpose of this work is to build a model of diesel engine useful for studying new hybrid solutions for power generation. This kind of study is of significant interest particularly for the marine sector, due to recent limitations on fuel consumption and constraints imposed on the minimum powertrain efficiency. In this paper a variable-speed diesel generator model is presented. The model simulates both the dynamic behavior of the engine and the fuel consumption during the operating cycle. This model was developed using experimental results of bench tests conducted on the VL1716C2-MLL engine by Isotta Fraschini Motori (IFM) SpA. Diesel engine dynamics is based on the formulation found in the technical literature on mean value models and then adjusted through experimental data. Special attention was paid to developing the procedure to find the minimum specific fuel consumption for a given load. Initially, the presented resutls concern the correct response of the model under different load conditions. Then, the improvement in specific fuel consumption (obtained by adjusting the engine speed according to the load), in comparison with fixed speed operation, is shown.

Hydrogen is gaining momentum in the current global energy transition framework. In fact a great and widespread enthusiasm is growing up towards it, as indicated by the current worldwide economic and political strategies, which endorse the carbon neutrality by 2030 and a fast transition to clean energy. Green hydrogen has the potential to create a virtuous cycle for the future renewables-based electricity grids, as it can provide the much-needed flexibility to power systems, acting as a buffer to non-dispatchable renewable generation. Indeed, the excess energy, provided by conventional and renewable power plants, can be stored as hydrogen and then employed to produce electricity (fuel cells or power systems), heat (combustion) or both (co-generation), abating drastically the greenhouse gas production. In this scenario, it is important to understand what benefits could derive from the use of hydrogen. For this reason, the present work not only aims at reviewing the recent updates on hydrogen economy (in terms of the main advantages and drawbacks) but also focuses on determining the impact that this hydrogen may have in various sectors (transport, industry and power generation). Different assessments have been carried out showing how hydrogen can effectively contribute to the carbon neutrality goal. This work points out that hydrogen can be really sustainable if produced via electrolysis powered by renewable energies. Furthermore, for the mobility, the use of fuel cells currently turns out to be less efficient than the adoption of Li-ion batteries, but at the same time far less polluting (CO2,eq) and labor intensive. Finally, a near-term solution to contrast the power generation carbon footprint, namely the blending of fossil fuels with hydrogen, has been investigated. Thus, a real Combined Cycle Gas Turbine power plant has been selected as a case study, in order to assess the impact of the hydrogen employment in terms of power output and emissions with respect to the current status of the plant fueled with 100% natural gas. As a result, using a mixture with 70% CH4 and 30% H2 a remarkable reduction of CO2 can be achieved (0.28 MtCO2/year).

Given the current practice to perform lean-premixed combustion to decrease NOx emissions, thermoacoustic instabilities have become one of the major drawbacks in gas turbine combustors. The necessity to control and limit such a deleterious phenomenon is mandatory to avoid structural damage of the burner. It has been demonstrated that perforated liners, if conveniently designed, can be very effective in reducing acoustic oscillations inside gas turbine combustors. Studying perforated plates traversed by bias flow can give a useful insight on sound absorption properties of liners, rather than investigate complex geometries. The present paper aims to carry out a numerically cost-effective, but reliable, CFD analysis to predict the acoustic impedance of perforated plates traversed by bias flow, and to grasp the details of the sound dissipation process. 2D axisymmetric simulations have been carried out and the governing equations solved by using the commercial code ANSYS Fluent®. Hypotheses, boundaries and operating conditions are described, focusing on the role of the Non-Reflecting-Boundary-Condition (NRBC) and the Transparent-Flow-Forcing condition (TFF) in treating acoustic waves. Numerical results are compared both with linear analytical models and experimental data from a case study, by proving a fast and reliable prediction of the acoustic response. Furthermore, effects of increasing bias flow temperature on the sound absorption property have been investigated, showing an increase in acoustic power losses as temperature rises. The proposed CFD model (2D-axisymmetric) proved to be a valid and versatile tool in evaluating the acoustic response of perforated plates under different operating conditions.

The effects of climate change and global warming are arising a new awareness on the impact of our daily life. Power generation for transportation and mobility as well as in industry is the main responsible for the greenhouse gas emissions. Indeed, currently, 80% of the energy is still produced by combustion of fossil fuels; thus, great efforts need to be spent to make combustion greener and safer than in the past. For this reason, a review of the most recent gas turbines combustion strategy with a focus on fuels, combustion techniques, and burners is presented here. A new generation of fuels for gas turbines are currently under investigation by the academic community, with a specific concern about production and storage. Among them, biofuels represent a trustworthy and valuable solution in the next decades during the transition to zero carbon fuels (e.g., hydrogen and ammonia). Promising combustion techniques explored in the past, and then abandoned due to their technological complexity, are now receiving renewed attention (e.g., MILD, PVC), thanks to their effectiveness in improving the efficiency and reducing emissions of standard gas turbine cycles. Finally, many advances are illustrated in terms of new burners, developed for both aviation and power generation. This overview points out promising solutions for the next generation combustion and opens the way to a fast transition toward zero emissions power generation.

This paper expands on the results of the technical and economic feasibility analysis of substituting existing pressure reduction valves (PRVs) with pumps used as turbines (PaTs) in two real Italian water distribution networks (WDN), chosen as case studies, aiming at effective energy recovery. Water demand variability makes complex the selection of the right pump to be used as a turbine in a WDN maximizing its annual electric energy yield. Hence, this study describes an effective approach that permits us to identify the most suitable pumps, starting from the definition of the best efficiency points at which they should operate in reverse mode.

Even if researchers are working on the exploitation of marine energy for more than thirty years, blue energy is not yet a consolidated reality and still contributes marginally to the world energy mix. For this reason, in the last years, the effort of the scientific community has been significantly intensified to further improve the know-how on marine energy harvesting. The goal is to allow ocean energy to effectively contribute to a more sustainable energy production in the next future. In the wide range of technologies for wave energy harvesting, Oscillating Water Column (OWC) devices are counted among of the most mature ones. Due to the oscillating nature of the generated air flow, which continuously inverts its direction, OWC devices need to be coupled with self-rectifying turbines, such as Wells, impulse, or biradial turbines. Wells turbines can reach high efficiencies, but their performance can show a hysteretic behaviour due to dynamic stall phenomena, especially in presence of high amplitude flow rate oscillations. Moreover, under dynamic stall conditions, during the flow deceleration, the shaft torque evidence the presence of gradually damped fluctuations, which delay the flow reattachment, superposed to the hysteresys loop. In order to better characterize this phenomenon, a new experimental campaign was performed in the open wind tunnel of the Polytechnic University of Bari on a 3D-printed Wells turbine model. The interest is mainly focused on the dependency of these torque fluctuations on the amplitude and frequency of the oscillating flow.

In the open wind tunnel of the Polytechnic University of Bari a new 3D-printed prototype of a Wells turbine is investigated under steady-state and pulsating flow conditions. The flow rate is modified by changing sinusoidally the frequency of the control drive, hence the rotational speed of the suction fan. The Wells turbine is a scaled prototype designed to operate in a 1:10 scaled model of a REWEC3 breakwater for ocean application. The Wells turbine characteristics are evaluated in terms of torque coefficient and pressure drop coefficient vs. flow coefficient. A delayed onset of stall can be observed, with a clockwise hysteretic loop, when the turbine experiences large sinusoidal variation of the flow coefficient at high mass flow rates. The variation of the turbine performance under dynamic flow conditions is crucial for a correct design of the Wells turbine.

In recent years, pumps operated as turbines (PaTs) have been gaining the interest of industry and academia. For instance, PaTs can be effectively used in micro hydropower plants (MHP) and water distribution systems (WDS). Therefore, further efforts are necessary to investigate their fluid dynamic behavior. Compared to conventional turbines, a lower number of blades is employed in PaTs, lowering their capability to correctly guide the flow, hence reducing the Euler’s work; thus, the slip phenomenon cannot be neglected at the outlet section of the runner. In the first part of the paper, the slip phenomenon is numerically investigated on a simplified geometry, evidencing the dependency of the lack in guiding the flow on the number of blades. Then, a commercial double suction centrifugal pump, characterized by the same specific speed, is considered, evaluating the dependency of the slip on the flow rate. In the last part, a slip factor correlation is introduced based on those CFD simulations. It is shown how the inclusion of this parameter in a 1-D performance prediction model allows us to reduce the performance prediction errors with respect to experiments on a pump with a similar specific speed by 5.5% at design point, compared to no slip model, and by 8% at part-loads, rather than using Busemann and Stodola formulas.

The slip phenomenon consists in the deviation of the fluid flow (relative ve locity vector) with respect to the blade congruent angles and is mainly due to the finite number of blades. For this reason, slip becomes significant in pumps operating as turbines (PaTs) being characterized by a lower number of blades compared to conventional turbines highlighting a shortcoming of these devices in energy recovery applications. liven though this topic has been widely investigated in the past for centrifugal pumps, it has been often neglected for hydraulic turbine applications. As described in the literature, a counter rotating vortex (known as eddy vortex) develops inside each vane of a rotating machine and its effect can be superimposed to the main flow characteristics. Moreover, the relative vorticity magnitude, which is twice the angular velocity under the hypothesis of inviscid, incompressible and irrotational flow, is constant regardless of the number of vanes. In this work, 3D inviscid steady flow numerical simulations of a purely radial impeller with zero-thickness blades, being designed according to a logarithmic spiral law, have been carried out with the purpose of bringing out the relationship between the flow deviation and the number of blades, neglecting viscous effects which could hinder the inertial ones. The results show a local deviation of the streamline downstream of the trailing edge when the flow is not confined by the blades. The effect of the flow deviation has been also evaluated by calculating the hydraulic performance of the runners. Four different runners have been investigated (with 28, 14, 7 and 3 blades) at their design point and rotating at two different angular velocities. This allowed to correlate the deviation with the inertial effect and to propose a least-squares fitting curve. As shown in previous works, the inclusion of the slip correction factor in 1-D PaT performance prediction models enhances their accuracy.