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In recent years, liquid air energy storage (LAES) has gained prominence as an alternative to existing large-scale electrical energy storage solutions such as compressed air (CAES) and pumped hydro energy storage (PHES), especially in the context of medium-to-long-term storage. LAES offers a high volumetric energy density, surpassing the geographical constraints that hinder current mature energy storage technologies. The basic principle of LAES involves liquefying and storing air to be utilized later for electricity generation. Although the liquefaction of air has been studied for many years, the concept of using LAES “cryogenics” as an energy storage method was initially proposed in 1977 and has recently gained renewed attention. With the growing need for alternative energy storage methods, researchers have increasingly explored the potential of cryogenic media, leading to the development of the first LAES pilot plant and a growing body of research on LAES systems. However, one notable drawback of LAES is its relatively low round-trip efficiency, estimated to be around 50–60% for large-scale systems. However, due to its thermo-mechanical nature, LAES offers versatility and can be easily integrated with other thermal energy systems or energy sources across a wide range of applications. Most of the existing literature on LAES focuses on thermodynamic and economic analyses, examining various LAES configurations, and there is a clear lack of experimental studies in this field. This paper aims to conduct a comprehensive review of LAES technology, with a focus on the performance enhancement of these systems. Future perspectives indicate that hybrid LAES solutions, incorporating efficient waste energy recovery sections, hold the most promise for enhancing the tech-no-economic performance of standalone LAES systems.

The International Energy Agency states that geothermal energy technologies could meet 15% of the global electricity demand growth, provided cost reductions continue. Organic Rankine Cycle (ORC) systems are expected to play a key role in achieving this ambitious target. Recognized for their effectiveness in converting low-to-moderate temperature heat, ORC systems are already in use in numerous installations. The performance of ORC systems is primarily influenced by operational conditions and the choice of working fluid. A key system design challenge arises from the operational conditions of ORC systems, which are closely tied to the design and sizing of heat exchange components. This study examines the effect of the pinch point temperature difference, and the approach point temperature on the thermodynamic performance of a low-temperature ORC, with cycle efficiency and the total heat transfer area of the evaporator serving as the main performance indicators. The analysis uses a parametric approach to assess ORC performance by varying pinch point and approach point temperatures for a range of suitable working fluids. An optimal design region is identified, where the trade-off between thermal efficiency and heat exchanger size is most advantageous. These results offer valuable theoretical insights for low-temperature ORC design, highlighting the importance of selecting pinch point and approach point temperatures that strike a balance between thermal and economic goals.

Approximately 45% of power generated by conventional power systems is wasted due to power conversion process limitations. Waste heat recovery can be achieved in an Organic Rankine Cycle (ORC) by converting low temperature waste heat into useful energy, at relatively low-pressure operating conditions. The ORC system considered in this study utilises R-1234yf as the working fluid; the work output and thermal efficiency were evaluated for several operational pressures. Plate and shell and tube heat exchangers were analysed for the three sections: preheater, evaporator and superheater for the hot side; and precooler and condenser for the cold side. Each heat exchanger section was sized using the appropriate correlation equations for single-phase and two-phase fluid models. The overall heat exchanger size was evaluated for optimal operational conditions. It was found that the plate heat exchanger out-performed the shell and tube in regard to the overall heat transfer coefficient and area.

In recent years, packed-bed systems for large-scale applications have emerged as a highly promising design for Thermal Energy Storage systems because of their high thermal efficiency and economic feasibility. Large-scale application systems typically include packed-bed thermal energy stores as essential components, enabling effective integration with renewable energy and processed heat. The packed-bed systems investigated in this paper utilise Magnesia as the storage medium and optimised parameters, which have previously been identified through research involving charging and discharging cycles of both the hot and cold storage systems when air is the heat transfer fluid. This includes solid particle diameters of 0.004 m, a material porosity of 0.2, an aspect ratio of 1 for the storage tank, and a mass flow rate of 13.7 kg/m3. This paper aims to present a comparative analysis of the influence of alternative heat transfer gases, namely air, argon, carbon dioxide, helium, hydrogen, and nitrogen, on the performance of Pumped Thermal Energy Storage hot and cold storage systems. The performance of the six gases in the storage system was evaluated using an axisymmetric model simulated with COMSOL Multiphysics 5.6 software, with the total energy stored and the capacity factor serving as key performance indicators. The results revealed that carbon dioxide gas was the most promising heat transfer fluid and that the packed bed could be operated efficiently over 72% and 76% of its range for hot and cold systems, respectively. Hydrogen, nitrogen, and air performed similarly but less adequately than carbon dioxide and had operating ranges of 55% and 75% for hot and cold storage. Helium and argon had the poorest performance, with optimal charging and discharging rates corresponding to 50% and 66%.

Simulations are presented for flow around pairs of circular cylinders at a Reynolds number of 3900. The flow is assumed to be two-dimensional and incompressible in nature and the simulations are performed using a RANS (Reynolds Averaged Navier Stokes) approach with a k-ε model. Simulations are performed for three different configurations of the cylinders: A tandem configuration where the line joining the centre of the cylinders is parallel to the mean flow direction; side-by-side, where the centre line is perpendicular to the mean flow direction; and staggered where the centre line is an angle α to the flow direction. Simulation results are presented for cylinder separations ranging from 1.125 to 4 diameters and for values of α between 10° and 60°. The results are presented and discussed in terms of the lift and drag coefficients, the Strouhal number, the vorticity field and the flow regimes observed. The results and flow regimes are also compared to previous observations at lower Reynolds numbers to investigate the Reynolds number dependence of the phenomena.

Model validation is an essential part of CFD-based projects. Despite being successfully employed for decades, the level and extent of CFD model validation details vary significantly in the published literature, which, in turn, adversely affects the repeatability and usefulness of published models and data. This study explores the various challenges associated with validating CFD models of thermodynamic components, namely, the compressors and their performance evaluation. The methodology involves blade generation through TurboGrid and BladeGen, mesh generation to ensure computational efficiency, and pre-processing with CFX to define boundary conditions and turbulence models, all within ANSYS 2024 R1. Three case studies are discussed, each assessing different compressor configurations and common challenges encountered during the model validation stage. Based on the case studies, a number of recommendations are presented relating to best practices in terms of both the use of published materials to validate new models and the level of detail required for experimental or simulation publication to ensure they can be replicated or used to validate a new model.

The Improved Delayed Detached-Eddy Simulation (IDDES) is a modification of the original Detached-Eddy Simulation (DES) design to incorporate Wall Modeled Large Eddy Simulation (WMLES) capabilities and to extend the class of flows suitable for this methodology. For thin attached boundary layers, typically seen in external aerodynamic flows, the DES branch of the model is active, whereas with thick boundary layers, typically seen in internal flows and also wake flows, the WMLES branch is active, thus providing a numeric method suited to handling most flow cases automatically. The flow over a triangular bluff body is used to validate the suitability of the IDDES model and compare the results with experimental, DDES, and LES data. The IDDES model is found to be relatively accurate when compared with the experimental results, with recirculation length, streamwise velocity, and Reynolds stresses all showing good agreement with the experimental data. However, when compared with the DDES model, there is a ~4% overprediction of the recirculation length using the same mesh and numerical scheme. The code, with its extra complexity, is also ~3% slower to solve. The IDDES model has also been tested against different meshes, and the results show that even for a coarse mesh, there is still good agreement with the experimental data.

Curved surfaces are a feature of many engineering applications, and as such, the accurate prediction of separation and reattachment from a curved surface is of great engineering importance. In this study, improved delayed detached eddy simulation (IDDES) is used, in conjunction with synthetic turbulence injection using the synthetic eddy method (SEM), to investigate the boundary layer separation from a curved backward-facing step for which large eddy simulation (LES) results are available. The commercial code Star CCM+ was used with the k-ω shear stress transport (SST) variation of the IDDES model to assess the accuracy of the code for this class of problem. The IDDES model predicted the separation length within 10.4% of the LES value for the finest mesh and 25.5% for the coarsest mesh, compared to 36.2% for the RANS simulation. Good agreement between the IDDES and LES was also found in terms of the distribution of skin friction, velocity, and Reynolds stress, demonstrating an acceptable level of accuracy, as has the prediction of the separation and reattachment location. The model has, however, found it difficult to capture the pressure coefficient accurately in the region of separation and reattachment. Overall, the IDDES model has performed well against a type of geometry that is typically a challenge to the hybrid RANS-LES method (HRLM).

Commonly, researchers have investigated many factors that impact the performance of air conditioning and refrigeration systems, such as varied cooling configurations, operating conditions and optimization of specific system components. Although there is an abundance of research detailing the importance of working fluid selection, very few studies focus on how the working fluid selection influences the performance of the individual components of the system, such as the compressor. In this paper, the performances of a selection of working fluids are compared through a centrifugal compressor using CFD. The working fluids considered are R1234ze, R1234yf, R152a, R444a, R445a, R290 and R600a and were selected due to suitability as replacements to R134a. Each fluid, including R134a, was compared based on the performance of a centrifugal compressor with fixed inlet conditions across two operational speeds. The results indicate that R1234ze and R1234yf demonstrated the best performance as replacements to R134a, achieving the highest overall pressure ratios. Additionally, R1234ze also displayed similar power required through the compressor to R134a indicating greater suitability as a drop-in replacement. The working fluids R444a and R445a both displayed performance similar to that of R134a across both operational speeds, indicating reasonable suitability as a replacement to R134a. Alternatively, R152a, R290 and R600a displayed reduced performance compared to R134a and subsequently, are not suitable replacements based on the compression system considered in this study. As well as considering the observed differences in the performance from the selected working fluids, the implications of the results for industrial applications are also considered, along with avenues for further work.

Blood is a shear-thinning non-Newtonian fluid in which the viscosity reduces with the shear rate. When simulating arterial flow, it is well established that the non-Newtonian nature is important in the smallest vessels; however, there is no consistent view as to whether it is required in larger arteries, such as the carotid. Here, we investigate the importance of incorporating a non-Newtonian model when applying a plaque deposition model which is based on near-wall local haemodynamic markers: the time-averaged near wall velocity and the ratio of the oscillatory shear index to the wall shear stress. In both cases the plaque deposition was similar between the Newtonian and non-Newtonian simulations, with the observed differences being no more significant than the differences between the selected markers. More significant differences were observed in the haemodynamic properties in the stenosed region, the most significant being that lower levels of near-wall reverse flow were observed for a non-Newtonian fluid.