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Since the early 2000s, organic Rankine cycle (ORC) technology has experienced rapid development and market uptake. More than 4.5 GW of total ORC power plant capacity has been installed since then. Due to its flexibility, suitability for small- to medium-scale installations, its applicability to low- to medium-temperature heat sources, and compact design, ORC technology is already considered as state-of-the-art. However, to further increase the technical and economic potential of this technology, significant research is needed to further improve efficiency and other key performance indicators, as well as to address environmental and safety aspects. Therefore, it is important to research new technologies and to foster innovations for improving performance; but it is of similar importance to resolve operational challenges with applied research that focuses closer to the market, and the technology provider and user needs. This vision article outlines the current state-of-the-art and presents current research trends. Parts of this article summarize the research progress reported at the 6th International Seminar on ORC Power Systems (ORC2021) and accompanies the associated special issue published following this event. The article highlights research trends at the concept level, but also at the component and system levels and, therefore, provides a holistic overview for the interested reader regarding the current challenges and potential future research activities in this area. Beyond the variety of cycle concepts in Section 2.1, the different heat sources and applications in Section 2.2, rotating device and turbomachinery (pumps and expansion devices) options in Section 2.3, current R&D trends for working fluids are presented in Section 2.4. This editorial concludes with a more applied perspective on ORC systems, covering process control and a general perspective on the technology in Section 2.5. With an increasing number of ORC plants operating in the field, their integration within a wider energy system is also of increasing importance, and this is addressed in Section 2.6. All prior elements are levers toward the grand challenges in ORC technology: increasing efficiency, reducing investment costs, as well as increasing sustainability, environmental performance, and system compactness.

This vision article accompanies a special issue of Applied Thermal Engineering dedicated to the 16th Conference on Sustainable Development of Energy, Water and Environment Systems (SDEWES) held in Dubrovnik in 2021, and summarizes a selection of papers presented at the conference. At the focal point are a range of topics related to thermal processes as these arise in energy production, storage, utilization and conservation, covering fundamental research, the development of technical solutions for diverse sustainable engineering applications, technoeconomic analyses, and issues relating to the potential and integration of technologies from higher-level approaches. Thermal processes are the basis of numerous sustainable engineering applications and their understanding and improvement are increasingly required in the context of improved resource use efficiency and reduced environmental impact. Applications of interest include thermal systems used in buildings, thermochemical processes, seawater treatment, thermal storage solutions and renewable energy resource use. Emerging challenges in this space have given an opportunity to scientists, researchers and engineers to actively contribute to the development of relevant technological solutions, which are covered briefly in the present article.

It is known that the performances of multi-phase reactors depend on the operating parameters (the temperature and the pressure of the system), the phase properties, and the design parameters (the aspect ratio (AR), the bubble column diameter, and the gas sparger design). Hence, the precise design and the correct operation of multi-phase reactors depends on the understanding and prediction of the fluid dynamics parameters. This paper contributes to the existing discussion on the effect of operating and design parameter on multi-phase reactors and, in particular, it considers an industrial process (e.g., the LOPROX (low pressure oxidation) case study, which is typical example of two-phase bubble columns). Based on a previously-validated set of correlations, the influence of operating and design parameter on system performances is studied and critically analyzed. First, we studied the effects of the design parameter on the liquid-gas interfacial area, by keeping constant the fluid physical-chemical properties as well as the operating conditions; subsequently, we discussed for a fixed system design, the influence of the liquid phase properties and the operating pressure. In conclusion, this paper is intended to provide guidelines for the design and scale-up of multi-phase reactors.

Rapid population growth and the finite nature of fossil-fuel resources have given rise to an urgent interest in sustainable energy development. Thermal technologies include several devices and systems able to improve energy use, recycle resources and waste, and harness renewable energy sources. In particular, thermal technologies applied to renewable sources have been improving over the years in terms of performance costs have reduced, however, several challenges remain that need to be overcome. This vision article is concerned with the prevailing challenges for thermal technologies applied to renewable energy; specifically, solar drying technologies, focussing on medium and large-capacity solar drying systems, thermal devices for waste heat and gas treatment/recovery, and thermochemical technologies for the valorization of biomass to fuels. Firstly, a brief description of each technology is provided while remarking on their importance in energy transition and resource recovery scenarios. Subsequently, key challenges are identified and promising directions and areas for exploration and future research are suggested. The most common critical challenges for the further development and deployment of these technologies include improved designs and material use to decrease final costs and minimize environmental impact. In particular, managing this process within a circular economy perspective would be necessary for improved sustainability. Incorporating renewable energy sources with thermal technologies to reduce the need for fossil fuels is of major interest globally, and optimizing the combined use of mathematical, computational, and experimental tools is the most promising approach for accelerated understanding and development in this space.

It is known that the transportation sector accounts for a considerable share of the emissions and the primary energy consumption of the countries as a whole, thus determining an increas-ing attention towards the decarbonisation pathways of the transportation sectors. The energy consumption at the country-scale can be interpreted as the integral of the socio-demographic layer and the behavior spectrum. Thus, ad-hoc policy schemes need to rely on multi-scale ap-proaches, describing the household-scale and, subsequently, scaling-up towards the country-scale. In this long-term aim and perspective, the present communication contributes to the ex-isting discussion regarding relationships between the household/socio-demographic character-istics and the transportation patterns. In particular, focusing on the Italian case study, this communication explores the relationships between the household/socio-demographic variables and the carbon dioxide emissions related to the private transportation sector. To this end, this paper build on micro-data obtained by the Italian Institute of statistics and it applies a four-step statistical method to select suitable variables, explore the significant determinants and perform an household segmentation. It is found that the geographic area (in terms of the macro-scale as well as the micro-scale geographic locations) as well as income-related variables are likely to be factors influencing the carbon dioxide emissions related to the private transportation sector.

The transport sector represents approximately 30% of the final energy consumption in the Eu-ropean Union. Cars, trucks and light vehicles are responsible for 70% of the final energy con-sumed in the transport sector (source: Eurostat). The process of decarbonisation of mobility is represented by a combination of technological, behavioural and policy dynamics to be effec-tively fostered and combined in order to maximise their impact. The introduction provides a review of the main mobility and energy integrated planning ap-proaches represented by Sustainable Urban Mobility Plans (SUMPs) and Sustainable Energy and Climate Action Plans (SECAPs), as well as a brief overview of the expected impacts of disruptive innovation and their inclusion in the revised approach towards mobility planning in Europe. This picture provides the framework for the presentation of the papers selected for the 2020 EPEE special issue on sustainable mobility, and their contribution to the progress of economic research to increase the knowledge of innovation trends that strongly influence the evolution of urban mobility networks.

The large-scale deployment of ejector refrigeration systems (i.e., solar-based ejector refrigeration systems), although representing a promising alternative compared with mechanical compressor ones, is hindered due to limitations regarding ejector control modes. Indeed, ejectors are fluid-dynamics controlled devices and, because of their fixed geometry, they operate at their highest efficiency in a narrow range of operating conditions, which is in contrast with the dynamic pressure and temperature levels characterizing real applications. In this context, variable geometry ejectors (VGE) represent a promising solution to increase the flexibility and operation range of this component. The present study aims to extend the present body of knowledge regarding VGE systems, evaluating the impact of a spindle-provided ejector operated with R290 on the performance of the refrigeration system. The analysis has been carried out using an integrated lumped parameter/CFD approach, thus linking the local flow properties and global performances. Different spindle positions have been tested to assess how the different nozzle area ratios affect both the entrainment ratio and the critical pressure. Results showed that increasing primary nozzle area ratio the system can effectively reduce the thermal input, increasing the average COP at the expanse of a lower critical pressure. In conclusion, using a moving spindle control system might ensure an improvement of the ejector performance.

It is generally admitted that the “global-scale” behavior of bubble columns is imposed by the “local-scale” phenomena. For this reason, understanding the fluid dynamics in bubble columns relies on the precise knowledge of the so-called “birth and life” of bubbles. A-priori knowledge of the bubble sizes and shapes is required to characterize the “local-scale”, to understand the “global-scale”, to set-up and validate numerical models, as well as to support scaling-up methods towards the “industrial-scale”. This paper contributes to the present-day discussion by proposing an experimental research devoted to clarify the relationships between the bubble sizes and shapes, the integral flow parameters, and the liquid phase properties. The experimental study, based on a bubble-identification methods, was performed in a “large-scale” bubble column (inner diameter equal to 0.24 m, height equal to 5.3 m) operated in the batch and in the counter-current modes with pure (deionized water) and binary (mixture of ethanol and deionized water) liquid phases. The system was operated in the pseudo-homogeneous flow regime with superficial gas velocities in the range of 0.0037–0.0188 m/s and superficial liquid velocity, in the counter-current mode, equal to −0.066 m/s. In the different experimental runs, bubble size distributions and shapes were obtained at different radial and axial locations. The experimental observations have been presented, compared with literature correlations, used to develop novel correlations (to be applied in practical applications), compared with previously obtained experimental data and interpreted in a multi-scale point of view. The comprehensive dataset obtained within this research may be used to improve the validation of numerical approaches and, in particular, to tackle the unsolved issue of developing break-up and coalescence kernels.

The applications of ejectors are many and encompass the refrigeration, the power generation and the chemical sectors. On one hand, ejector technology needs limited maintenance, has low operational costs and has no restrictions concerning the working fluids; on the other hand, the complex single- and multi-phase fluid dynamics make ejector design and performance prediction a real challenge. This perspective explores the main advancements in ejector technology and proposes a critical discussion with an outlook for the future research; the proposed discussion is grounded on the multi-scale relationship between the “local-scale” phenomena and the “component-scale” performances. After a look at the past, this perspective examines the ongoing research activities and achievements regarding four state-of-the-art research areas, namely refrigeration systems, power conversion plants, chemical process and technology and computational methods. For the different research areas discussed, directions and opportunities for the future research activities are appraised. Finally, this perspective defines a fundamental challenge that needs be addressed in the forthcoming long-term activities: “the multi-scale ejector challenge”.

Fuel poverty is a multi-scale and a multidisciplinary topic, as it relates the demand-side to the supply-side, the “household-scale” to the “country-scale”, and it involves social and economic boundary conditions. This paper focuses on the Italian case and contributes to the present-day discussion by introducing a novel measure of fuel poverty that considers details on household energy expenditure and its impacts on aggregated regional and national levels. The proposed measure of fuel poverty couples the “household-scale” to the “country-scale” and is based on an integrated statistical-lumped parameter approach. This approach compares the theoretical household thermal requirements for heating purposes―obtained by a lumped parameter model―with the real thermal energy expenditure―obtained by a national-representative survey. Subsequently, the proposed measure of fuel poverty is related to the socio-demographic and geographic variables by using by a statistical approach, structured in four techniques: ordinary least squares method, (b) variance inflation factor, (c) least absolute shrinkage and selection operator and (d) classification and regression tree approach. It is found that fuel poverty, in the Italian case, is mainly related to the geographical dimension, rather than being related to the socio-demographic dimension. This results is of practical importance and provide a rational basis for policy makers when planning strategies to tackle the incidence of fuel poverty in Italy.

The coupling between heat pumps and renewable energy sources is a recognized strategy to reduce primary energy consumption at the household level. This paper contributes to the present-day discussion concerning solar-assisted heat pumps for heating/cooling and to produce domestic how water. In particular, this study presents the results of a field study concerning a novel solar-assisted dual-source multifunctional heat pump, installed in a detached house in Milan. The proposed system couples hybrid photovoltaic/thermal (PVT) panels with a multifunctional and reversible heat pump. The heat pump is equipped with “air-source” and “water-source” evaporators, connected in series and operated alternatively, based on the ambient conditions, system parameters and operating modes. In addition, the PVT panels are used, by employing two storage tanks, to produce domestic hot water and to provide a heat source to the “water-source” evaporator. The proposed system has been tested experimentally, showing interesting and promising results: the system has been able to maintain high efficiencies in the different seasons and has been able to use the solar energy to support the production of domestic hot water. It was found that the use of the “water-source” evaporator allowed to significantly increase the performance of the system and to avoid the defrost cycles.

To achieve the decarbonization objectives described in the Paris Agreement, thermal appliances’ electrification is considered an essential factor. Unfortunately, the substitution of thermal appliances with electric alternatives will inevitably increase the load on the national electric system, thus, it is crucial to perform analyses to assess these policies’ impact. This paper, using as a reference the electrification pathways studied by Besagni et al. employing the MOIRAE (bottom-up MOdel to compute the energy consumption of the Italian REsidential sector) model, calculates the energy and power increase in the residential sector in 2030 due to the substitution of different thermal appliances and discusses potential policies to take into account the effect of their implementation on the Italian electric system. Even if the current generation capacity can sustain the electrification process without problems, investments will be needed to limit the possibility of congestions on transmission lines connecting the country’s northern and southern parts.

It is known that the global performances of ejector-based systems (viz., at the “global-scale”) depend on the local flow properties within the ejector (viz., at the “local-scale”). For this reason, reliable computational fluid-dynamics (CFD) approaches, to obtain a precise and an a-priori knowledge of the local flow phenomena, are of fundamental importance to support the deployment of innovative ejector-based systems. This communication contributes to the existing discussion by presenting a numerical study of the turbulent compressible flow in a supersonic ejector. In particular, this communication focuses on a precise knowledge gap: the comparison between 2D and 3D modelling approaches as well as density-based and pressure-based solvers. The different approaches have been compared and validated against literature data consisting in entrainment ratio and wall static pressure measurements. In conclusion, this paper is intended to provide guidelines for researchers dealing with the numerical simulation of ejectors.

Within the broader discussion regarding the decarbonisation of the household sector, ejector refrigeration is attracting a growing attention. This communication contributes to the present day discussion concerning the performances and the perspectives in ejector refrigeration systems. Based on a very large dataset, gathered from the previous literature (encompassing a wide range of system design, operating conditions and refrigerants), this paper proposes a comprehensive comparative analysis. First, the current trends in ejector refrigeration systems, refrigerants and performances are presented. Second, the relationships between ejector performances, refrigerants and boundary conditions (in terms of non-dimensional temperatures, to ensure generality of the proposed analysis) are presented. In conclusion, this paper is intended to provide guidelines for perspective researchers and practitioners interested in selecting suitable ejector-based systems.

What are the required costs to sustain the electrification of the residential sector? What are the achievable primary energy savings? This paper tries to answer these questions, for the Italian residential sector, by providing coupled energetical and economic evaluations of the electrification pathways. To this end, this paper extends MOIRAE, a bottom-up modelling approach previously proposed by the authors. First, the input data have been upgraded by coupling, using ad-hoc statistical methods, different datasets provided by the Italian Institute of Statistics. Second, to estimate households’ time-variation, a socio-demographic model has been developed, validated, and implemented. Third, an economic model of fixed and variable costs for electrical and thermal appliances has been implemented. Subsequently, the modelling approach has been calibrated against detailed consumption data for the different Italian regions and validated against historical data. Finally, MOIRAE has been employed to unveil the electrification pathways with and without household budget constraints, aiming at replacing natural gas, LPG, diesel, and fuel oil energy carriers with electrical energy. For the different scenarios investigated, the changes in primary energy consumptions and the variation of variable and fixed costs have been included to consider both the energetic and the economic point of view.

Although representing a promising alternative to mechanical compressor driven systems, ejector refrigeration has not widely spread due to its limits due to the system operation's rigidity. Ejectors are fluid-dynamics controlled devices and because of their fixed geometry, operates at their best efficiency in a narrow range of operating conditions, which is often in contrast with the dynamic pressure and temperature levels of real refrigeration systems. Variable geometry ejector represents a promising solution to provide the ejector cycle with increased flexibility and an overall performance improvement. The present study aims to extend knowledge on VGE systems, evaluating the impact of a spindle-provided ejector operated with R290 on the refrigeration system's performance. The analysis has been carried out using an integrated model which allowed us to interpret the results by CFD visualization. Eight spindle positions (in the range of 0–7 mm) have been tested for different primary and secondary boundary conditions to assess how the different nozzle area ratio (in the range of 2.25–4.41) affects the entrainment ratio and the critical temperature. Results showed how increasing primary nozzle area ratio moving the spindle towards the mixing chamber can effectively reduce the thermal input, increasing the average COP and reducing the critical discharge temperature. For instance, a + 33% increase of the nozzle area ratio, enhanced the COP by an average + 57.1% but lowered the average critical temperature by −6.7 K. Fluid dynamics significantly affect the entrainment process and pump-effect phenomenon with particular attention to the primary flow expansion. In detail, the entrainment ratio benefitted from over-expanded jets, whilst the compression effect increased for an adapted jet. In the end, the performance curves have been obtained, which correlate the critical temperature conditions to the system performance.

The efficiency of ejector-based systems (the “system-scale”) relies on the behaviour of the ejector (the “component-scale”), related to the flow phenomena within the component itself (the “local-scale”). As a consequence of this multi-scale connection, the precise prediction of the “local-scale” is of fundamental importance to sustain the design of commercially viable ejector-based systems. Although it is widely accepted that computational fluid-dynamics can achieve the prediction of the “local-scale” (CFD) modelling approaches, a broad agreement regarding the performances of numerical methods is not reached: different authors applied different methods, and a complete validation is missing so far. This paper contributes to the current discussion and closes the knowledge gap by assessing the performances of a CFD approach for single-phase supersonic ejectors. To this end, a comprehensive validation has been conducted, encompassing a wide range of ejector designs, boundary conditions and working fluids; besides, a screening of modelling approaches is conducted, encompassing a wide range of mesh criteria, geometrical modelling (2-Dimensional and 3-Dimensional approaches), solvers (density-based and pressure-based) and turbulence models (k-e RNG and k-ω SST). The extensive comparison with experimental data allowed assessing and determining the influence of mesh criteria, geometrical modelling, solvers and turbulence models. In particular, k-ω SST has shown the best agreement with the experimental measurements concerning both global and local flow quantities, with an average entrainment ratio error of 14% and a maximum of 20%, under on-design operating mode. Finally, the simulation outcomes have been further post-processed to derive ejector component efficiencies, to contribute to the present discussion regarding closures in lumped parameter ejector modelling approaches. In conclusion, this paper thoroughly assesses the performance of a CFD model for single-phase ejector simulations and poses precise guidelines to be applied in future research activities and to support the design of ejector-based systems.

The increasing share of renewable energy sources on the supply side, as well as the so-called electrification pathways on the demand side, has led to peculiar challenges for electrical systems: Indeed, the increasing load demand has to be balanced from the supply-side viewpoint. In particular, the residential sector contributes to nearly 26% of the final energy consumption in Europe, suggesting that a further understanding of households’ consumptions and load profiles is needed to support an energy transition. In this context, this paper contributes to the existing discussion by proposing a sociodemographic analysis of Italian households’ load profiles using a smart metering experimental study, while also considering the households in energy poverty conditions. For the sake of generality, results are presented based on a previously proposed household segmentation of the Italian residential sector. The outcomes point out three prominent peaks on load profiles for all the identified clusters, with a notable distinction in intensity. Where children are present, a higher load profile is noted, reaching a maximum value of 600 W of absorbed power between 19:30 and 22:30. Conversely, households in an energy poverty condition show a relatively regular load profile, ranging from a minimum of 110 W of absorbed power in baseload conditions at night time to a maximum of 280 W in the evening hours. The findings in this paper are in agreement with existing research in the field, and accordingly the study proposes a better focus on domestic appliances and sociodemographic parameters.

A precise understanding of the relationships between the household characteristics and the transportation expenditures is of paramount importance to support bottom-up policies, aiming at defining decarbonisation pathways keeping into account the household budget constraints. Despite the considerable amount of research activities carried out during the last decades, an agreement regarding the factors influencing the transportation expenditures is far from being reached. This paper contributes to the present-day discussion, focusing on the Italian case study, by analyzing the relationships between the private, public and total transportation expenditure and the socio-demographic and geographical dimensions. The impact that the household characteristics have on the transportation expenditures have been explored by coupling (a) the ordinary least squares method, to determine the relationship between the variables, (b) the variance inflation factor, to check for multicollinearity issues, (c) the least absolute shrinkage and selection operator, to select variable. Subsequently, a segmentation of the Italian households is proposed, by using a segmentation-tree approach and the outcomes of the previous analysis. It is found that the geographic area (in terms of the macro-scale as well as the micro-scale geographic locations) as well as income-related variables are likely to be factors influencing the transportation expenditures. These observations may serve as bottom-layer for the forthcoming studies regarding decarbonisation of the transportation sector, considering also the household budget constraints.

Since the early 1990s, the pulsating heat pipe (PHP) has emerged as one of the most innovative, effective and potentially more convenient passive two-phase heat transfer systems, thanks to its good performance, versatility, and construction simplicity. On the other hand, the PHP is characterized by complex thermohydraulic behaviour that still presents a true challenge to designers, which has led to significant interest by a growing number of researchers. The technological readiness level (TLR) of this technology is quite broad depending on the application: for instance, the industrial community is starting to consider the PHP as a reliable solution for electronic cooling in ground conditions, while implementations in the cryogenic temperature range and in space environments is also being extensively explored. This vision paper aims at shedding light on the current knowledge and prediction capability of PHP numerical models, on unsolved phenomenological issues, on the current technological challenges and the future perspectives of this fascinating heat transfer device. Specifically, after a general introduction and a brief overview of the current knowledge and the open issues of PHPs, special focus is devoted to the following topics: flat-plate PHP assessments; advancements in PHP modelling and simulation; flow stabilization techniques; non-conventional fluids subdivided into fluid mixtures, self-rewetting fluids, nanofluids; cryogenic applications, space applications, and finally the newest frontiers of flexible PHPs. Each section is accompanied by a brief roadmap providing directions for future research based on key challenges, which are also gathered and summarized in the final outlook section.

Two-phase bubble columns are equipment used to bring one or several gases into contact with a liquid phase. Despite the simple system design, bubble columns are characterized by complex fluid dynamic phenomena at different scales; for this reason, their correct design, operation and scale-up rely on the precise estimation of global and local fluid dynamics properties. In this respect, multi-phase Computational Fluid Dynamics (CFD), in the Eulerian multi-fluid framework, is particularly useful to study the fluid dynamics in multi-phase reactors. Within this approach, the accurate prediction of the fluid dynamics depends on the correct modeling of (a) the momentum exchange between the phases, (b) the effects of the dispersed phase on the turbulence of the continuous phase, and (c) the bubble coalescence and break-up phenomena. Furthermore, the global and the local fluid dynamic properties are related to the prevailing flow regime, i.e., the homogeneous flow regime and the heterogeneous flow regime. This paper mainly focuses on the homogeneous flow regime, which can be classified as “pseudo-homogeneous” or “mono-dispersed homogeneous”, depending on the prevailing bubble size distribution. The numerical modeling of the “pseudo-homogeneous” flow regime has been discussed in our previous papers (i.e., modeling closures and suitable boundary conditions); conversely, this paper contributes to the existing discussion on the modeling closures by investigating the “mono-dispersed homogeneous” flow regime in “small-scale” and “large-scale” bubble columns. To this end, two test cases have been considered: (a) a “small-scale” bubble column (a test case taken from the previous literature); (b) a large-scale bubble column (a test case experimentally studied within this paper by image analysis, optical probe and gas holdup techniques). In particular, this paper studies the effects of the interfacial forces and bubble induced turbulence modeling within the Eulerian two-fluid approach. Three-dimensional transient simulations have been performed and the numerical results were compared with experimental data (both local and global fluid dynamics parameters). The results have been critically analyzed and the reasons for the discrepancies between the numerical results and the experimental data have been identified and may serve as a basis for future studies. Likewise, recommendations on suitable closures as well as guidelines for future studies have been provided. In conclusion, this paper extends the validation of a previously proposed set of closure relations (validated for the “pseudo-homogeneous” flow regime in a “large-scale” annular gap bubble column) to the “mono-dispersed homogeneous” flow regime in “small-scale” and large-scale bubble columns.

It is known that the fluid dynamics and transport phenomena in bubble columns depend mainly on the bubble column design (i.e., the column diameter, aspect ratio, and gas sparger openings) and the liquid phase properties. In this communication, we contribute to present-day discussion through an experimental study concerning the combined effects of the gas sparger design and liquid phase properties on both the gas holdup and the main flow regime transition. The experimental study concerning gas holdup measurements was conducted in a large-diameter and large-scale bubble column (with a height of 5.3 m and inner diameter of 0.24 m) operated in the batch mode. Air was used as the dispersed phase (using gas superficial velocities in the range 0.004–0.20 m/s), and various water–monoethylene glycol (MEG) solutions were employed as binary liquid phases. The water–MEG solutions tested have viscosities between 0.9 mPa·s and 7.97 mPa·s, densities between 997.086 kg/m 3 and 1094.801 kg/m 3 , and surface tension between 0.0715 N/m and 0.0502 N/m. Two gas spargers were tested: (a) a spider sparger (“ coarse gas sparger ”) and (b) a needle sparger (“ fine gas sparger ”). The former produced a poly-dispersed homogeneous flow regime resulting in a concave gas holdup curve, whereas the latter produced a mono-dispersed homogeneous flow regime resulting in an S-shaped gas holdup curve. It was observed that the mono-dispersed bubble size distribution stabilized the homogeneous flow regime. The addition of MEG produced different effects depending on the gas sparger design. The addition of MEG in the “ coarse gas sparger ” configuration produced what is usually referred to as “ dual effect of viscosity ”: depending on the MEG concentration, the homogeneous flow regime was stabilized/destabilized, and thus, the gas holdup increased/decreased. Conversely, the addition of MEG in the “ fine gas sparger ” changed the shape of the gas holdup curve from an S-shape to concave, thus rendering it similar to the ones produced by “ coarse gas sparger ”. We speculate that viscous solutions reduce the influence of the inlet conditions in large-diameter and large-scale bubble columns; this is a matter of future research.

The use of carbon dioxide as refrigerant is attracting a growing attention and is a cutting-edge research topic. In spite of its many advantages, carbon dioxide has a major shortcoming, viz., low critical temperature. Owing to the low critical temperature, carbon dioxide cycles encompass both the sub-critical and the trans-critical operation conditions; the trans-critical operating conditions are characterized by high thermodynamic losses, requiring particular attention in the integrated component/system design criteria. In this perspective, in recent years, ejector technology has been widely recognized as a promising technical solution to support the deployment of carbon dioxide cycles, by reducing throttling losses. Unfortunately, the large variation in system operations as well as the changes in sub-critical and trans-critical operating conditions makes the numerical simulation of carbon dioxide ejector-based system a cutting-edge challenge. This paper contributes to the present day discussion on the validation of lumped parameter models for carbon dioxide ejectors. A model taken from the literature has been tested against literature data and the equilibrium assumptions, underlying the modelling approach have been tested.

Carbon dioxide (CO2, R744) is a natural working fluid with interesting thermophysical properties that have stimulated strong attention by the academic and industrial communities for a broad range of energy applications. The technology readiness level of CO2-based energy systems is very diverse due to the increasing consideration that the fluid has been receiving since the 1990s. Hence, the state of the art in CO2 energy research spans from fundamental thermofluid and chemistry science to commercial system innovations. After a brief compendium on ongoing activities, this paper proposes a roadmap for CO2 energy research with reference to the cooling, heating and power sectors. The key knowledge gaps and the main challenges at system and component levels are critically discussed. Pathways to advance the understanding and the technological maturity of CO2 energy systems are also outlined.

Proton exchange membrane fuel cells (PEMFCs) are considered a promising candidate to replace internal combustion engines. The unconsumed hydrogen, in PEMFC systems, is released at the anode exit and a recirculation can be applied to increase the fuel utilization; to this end, a recirculation system is needed and ejector technology is a promising technique, considering its many advantages. Unfortunately, ejectors are characterized by extremely complex fluid dynamic phenomena and a small deviation from the optimum operating condition might drastically lower the performances of the ejector itself and, consequently, of the whole ejector-based system. For this reason, multi-scale models—taking into account both the “local-scale” and the “component-scale” fluid dynamics phenomena—should be applied to evaluate the performance of ejector-based systems. In this paper, we contribute to the existing discussion concerning multi-scale modeling techniques and we propose an integrated lumped parameter- Computational Fluid Dynamics model to investigate the performance of convergent-nozzle ejectors for the anode recirculation in PEMFC systems. The integrated approach is based on a lumped parameter model (able to estimate the “component-scale” performance) with variable ejector component efficiencies, provided by Computational Fluid Dynamics simulations (able to predict the “local-scale” phenomena). Computational Fluid Dynamics simulations have been used to investigate the ejector “local-scale” fluid dynamic phenomena and to formulate correlations for ejector component efficiencies, thus linking ejector component efficiencies to the “local-scale” phenomena. In the first part of the paper, the integrated lumped parameter-Computational Fluid Dynamics approach has been formulated, validated and compared with different constant efficiency models, showing better performance and a wider range of applicability. In the second part of the paper, the integrated approach has been included in a complete PEMFC system model (considering both electro-chemical and pressure-drop effects). It has been demonstrate that a small deviation from the optimum operating condition of the ejector lower the performances of the whole system. The Integrated lumped parameter model-Computational Fluid Dynamics approach, because of the variable ejector component efficiencies, has been able to correctly consider the off-design performance of an ejector based system; conversely, constant ejector component efficiency models cannot correctly predict the performance of the PEMFC system. In conclusion, the use of variable ejector component efficiency models is needed in order to (a) provide a realistic model of the system and (b) analyze the performance in both for on- design and off-design performance. In addition, the proposed paper also provides a demonstration for the implementation of modeling involving both fluid dynamics and electro-chemical analysis in the context of fuel cells.

Ejector refrigeration has not been able to penetrate the market because of the low coefficient of performance and because of the high influence of ejector performance on the efficiency of the refrigeration system. Improving the performance of ejector refrigeration systems relies on the understanding of the fluid dynamic phenomena, which can be obtained through computational fluid-dynamics approaches. Unfortunately, no agreement has been reached yet on the closures for the turbulence modeling in the Reynolds Averaged Navier-Stokes (RANS) approach. This paper contributes to the existing discussion by presenting a numerical study of the turbulent compressible fluid in a supersonic ejector. Seven RANS turbulence closures have been compared and, in addition, the turbulence models were tested under different near-wall modeling options in order to investigate the wall treatment effect on the numerical results. The numerical results have been validated by literature data consisting in entrainment ratio and wall static pressure measurements, for different ejector geometries and operating conditions. As a result, the k-ω SST model shows better performance in terms of global and local flow phenomena predictions for the different ejector geometries and operating conditions: (a) on the global point of view, the entrainment ratio is well predicted with a maximum relative error equal to about 10%, (b) on the local point of view, the shock wave position, the pressure recovery and the wall static pressure values are well predicted. The results, taking into account previous numerical studies, suggest the use of the k-ω SST model to simulate ejector fluid dynamics.

An understanding of the fluid dynamics and the transport phenomena in bubble columns (in the homogeneous and heterogeneous flow regimes) is of fundamental importance to support the design and scale-up methods. In this respect, multiphase Computational Fluid-Dynamics (CFD) simulations in the Eulerian multi-fluid framework are particularly useful to study the fluid dynamics in large-scale reactors; in particular, this study concerns the modeling of the fluid dynamics in bubble columns within the boundaries of the homogeneous flow regime. Reliable predictions of the homogeneous flow regime with this approach are, however, limited up to now. One important drawback is that usually the needed closure models for the interphase forces, turbulence and coalescence and break-up are selected case-by-case, which hinder improvement of the predictive value. A set of closure relations has been collected at the Helmholtz-Zentrum Dresden-Rossendorf that represents the best available knowledge and may serve as a baseline model for further investigations. In this paper, the validation of this set of closure relations has been extended to the pseudo-homogeneous flow regime—characterized by a wide spectrum of bubble sizes and typically associated with the large sparger openings used in industrial applications—in large-scale bubble columns, thus establishing a first step towards the simulation of industrial-scale reactors. To this end, the benchmark considered is a comprehensive dataset obtained for a large-scale bubble column, which has been built accordingly with the well-known scale up criteria (large-diameter, high aspect ratio and large sparger openings). The numerical approach has been tested in its fixed-poly-dispersed formulation (considering the two- and four-classes approaches to represent the dispersed phase) and considering the coalescence and break-up closures. The results suggest that the correct simulation of the fluid dynamics in the bubble column requires the definition of coalescence and break-up closures. The results have been critically analyzed and the reasons for the discrepancies between the numerical results and the experimental data have been identified and may serve as basis for future studies.

A precise estimation of the bubble size distribution (BSD) is required to understand the fluid dynamics in gas-liquid bubble columns at the &ldquo, bubble scale,&rdquo, evaluate the heat and mass transfer rate, and support scale-up approaches. In this paper, we have formulated a population balance model, and we have validated it against a previously published experimental dataset. The experimental dataset consists of BSDs obtained in the &ldquo, pseudo-homogeneous&rdquo, flow regime, in a large-diameter and large-scale bubble column. The aim of the population balance model is to predict the BSD in the developed region of the bubble column using as input the BSD at the sparger. The proposed approach has been able to estimate the BSD correctly and is a promising approach for future studies and to estimate bubble size in large-scale gas&ndash, liquid bubble columns.

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