• Energy Research

Abstract 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.

Abstract This paper presents an Integrated Lumped Parameter Model-Computational Fluid-Dynamics approach for off-design ejector performance evaluation. The purpose of this approach is to evaluate the entrainment ratio, for a fixed geometry, in both on-design and off-design operating conditions. The proposed model is based on a Lumped Parameter Model (LPM) with variable ejector component efficiencies provided by CFD simulations. The CFD results are used for developing maps for ejector component efficiencies in a broad range of operating conditions. The ejector component efficiency maps couple the CFD and the LPM techniques for building an Integrated LPM-CFD approach. The proposed approach is demonstrated for a convergent nozzle ejector and the paper is structured in four parts. At first, the CFD approach is validated by global and local data and seven Reynolds Averaged Navier Stokes (RANS) turbulence models are compared: the k–ω SST showed good performance and was selected for the rest of the analysis. At second, a Lumped Parameter Model (LPM) for subsonic ejector is developed and the ejector component efficiencies have been defined. At third, the CFD approach is used to investigate the flow field, to analyze its influence on ejector component efficiencies and to propose efficiency correlations and maps linking ejector component efficiencies and local flow quantities. In the last part, the efficiency maps are embedded into the lumped parameter model, thus creating the Integrated LPM-CFD (ILPM-CFD) model. The ILPM-CFD model is validated over experimental and CFD data and is compared with LPM Constant Efficiency models showing better performance and a wider range of applicability. The average deviation between the ILPM-CFD results and the experimental and CFD data is less than 0.75%, whereas constant efficiency models showed a deviation up to 30%. The ILPM-CFD model is able to vary the value of the ejector component efficiencies depending on the operating condition, being able to evaluate both the on-design and off-design performance of the ejector.

Abstract 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.

A precise understanding of the relationships between the household characteristics and the residential energy consumption is needed to support the implementation of effective top-bottom energy strategies and to improve the prediction of forecasting models. This paper contributes to the present-day discussion and analyses the building factors, socio-demographic variables and appliances contributing to high-energy expenditures (viz., electrical energy expenditure, thermal energy expenditure and total energy expenditure) in the Italian households. The proposed study builds on an earlier work proposed by the authors, which identified the determinants of the household energy expenditures, based on a nationally representative survey (the “household Budget Survey: microdata for research purposes - 2015″ performed by the Italian National Institute of Statistics). In particular, the present paper completes and extends the previous research by applying the odds-ratio analysis to the previously identified determinants, in order to identify the factors that led to high electricity consumption (viz., electrical energy expenditure, thermal energy expenditure and total energy expenditure). In conclusion, this paper aims to providing a more precise understanding of the factors that certainly affect the energy expenditure.

Abstract 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”.

Abstract Despite the many advantages, ejector refrigeration systems have not been able to penetrate the market because of two prevailing reasons: low coefficient of performance and relevant influence of ejector operation on the performance of the whole system. Indeed, the performance of ejector refrigeration systems depends on the local flow phenomena occurring within the ejector. Thus, improving the performance of ejector refrigeration systems relies on the understanding of the fluid dynamic phenomena at the “component-scale” and on integrating such information at the “system-scale”. This paper contributes to the present discussion regarding the multi-scale modeling of ejector-based systems by proposing an integrated Computational Fluid Dynamic (CFD) - Lumped Parameter Model (LPM) ejector refrigeration system. In particular, ejector performances have been obtained by a validated CFD approach, whereas a LPM approach has modeled the refrigeration cycle. The refrigeration system’s performances, for different boundary conditions, have been evaluated, and the effects of the “local-scale” on the “system-scale” have been commented.

Abstract The large-scale deployment of solar-assisted systems in the residential sector relies on innovative thermal energy storage units. This paper contributes to the present-day discussion by proposing a pilot-scale phase change material storage, whose size has been selected to be coupled with solar-assisted heat pumps. The storage unit consists of a fin-and-tube heat exchanger placed within a tank: water is circulated on one side of the heat exchanger and, on the other side, commercial paraffin RT26 is employed. The storage system is operated considering two heat exchanger configurations (viz., parallel and series configurations) and implementing a broad set of boundary conditions, to test the storage unit under relevant operating conditions. To this end, a novel test rig with electrical resistances (to provide the heating load) and a heat pump (to provide the cooling load) has been designed and build. The results were commented in terms of global and local performances; it was found that the proposed storage, compared with a volume-equivalent water storage, is able to store 65% higher thermal energy. In addition, the dataset obtained in this research is attractive to validate numerical codes of phase change material storage units.

Abstract 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.