Experimental investigation of nanoparticles concentration, boiler temperature and flow rate on flow boiling of zinc bromide and acetone solution in a rectangular duct

Despite the increase in heat transfer properties of nano-fluids, they are not currently used in vapour absorption refrigeration systems (VARS), and there is little literature on the flow boiling behaviour of concentrated salt solutions with nano-particle suspension. A potential novel working fluid solution for a vapour absorption refrigeration unit capable of utilising very low grade waste heat is acetone and zinc bromide, and this fluid is investigated here as the salt solution with graphene nanoparticles in suspension in flow boiling similar to that found in VARS. Nanoparticle concentration, boiler temperature, and flow rate are investigated. The Rohsenow constant in the flow boiling correlation for the nanofluid acetone/ZnBr2 with graphene on a stainless steel surface is found to be 0.217. By increasing the particle concentration from 0 to 05 vol%, heat flux and heat transfer coefficient on the heated surface increase from 8638 W/m2 and 106 W/m2 K to 13164 W/m2 and 167 W/m2 K, respectively. The steady pressure of the system increases with increasing loading of the nanoparticles and consequently the saturation temperature increases. This is because of the increased vapour generation as a consequence of improved heat transfer properties. Heat transfer coefficient is linearly proportional to temperature difference between the fluid and wall (e.g. increases from 78 W/m2 K to 145 W/m2 K when the temperature difference increase from 102 K to 135 K) in the range tested and the heat flux correspondingly reflects a quadratic relationship with temperature difference. Increasing nanofluid flow rate reduces both the production of acetone in the condenser and the salt concentration in the strong solution reservoir. Regarding properties of the fluid, the density and the specific heat follow the simple mixture combination rule; the thermal conductivity of the nanofluid increases by 4.5% with increasing the loading the particles to 0.5 vol%, following reasonably well the correlation of Suganthi et al. (2014); the viscosity increases linearly with concentration of nanoparticles (e.g. increases from 3.22 m Pa s to 4.5 m Pa s by increasing the concentration from 0 to 0.5 vol%); the stability of the nano-salt-fluid is affected by the density of the base fluid. The nanofluid showing good stability for 4 h and during the circulation of the fluid in the rig. Over the range of temperatures tested, the salt solution demonstrates characteristics of nucleate boiling behaviour and offers significant improvement over the properties of the base fluid in terms of boiling effectiveness, indicating that it will provide improved operation in a VARS situation

Catalysis and evolution on cycling of nano-structured magnesium multilayer thin films

This paper explores the hydrogen cycling properties of Mg/Cr and Mg/V multilayer thin films and studies the effect of chromium and vanadium transition metal catalysts on the cycling properties of thick magnesium coatings. Two transition-metal catalysed magnesium-based multilayer PVD coatings are compared with a non-catalysed magnesium control sample. The (micro-)structural evolution of the thin film coatings into fine, flakey powders is studied in depth using XRD, SEM and TEM and the hydrogen storage properties of all three materials are assessed using volumetric, gravimetric and calorimetric methods focussing on the effect of the microstructure and composition of the coatings on the hydrogen storage kinetics. It was found that the chromiumcatalysed coating had the most favourable hydrogen storage kinetics with an activation energy for the dehydrogenation reaction of 65.7±2.5 kJ mol-1 and a hydrogen capacity of 6.1±0.3 wt%. The mechanism of the dehydrogenation reaction of the catalysed samples was studied using the CV and JMAK kinetic models and it was found that the catalyst material influenced not only the hydrogen storage kinetics but also the mechanism of the reaction

CFD assessment of the effect of nanoparticles on the heat transfer properties of acetone/ZnBr2 solution

A potential novel working fluid for vapour absorption refrigeration utilising very low grade waste heat, is based on acetone and zinc bromide as the salt solution. A Computational Fluid Dynamics (CFD) model is presented of the fluid with zinc oxide nano-particles in a flat tube flow. A two phase type of model represents the zinc oxide nano-particles as a distinct fluid phase. The cases of laminar and turbulent flow are explored numerically for a wide range of acetone and nanoparticles concentrations. The velocity is varied between 1.5 and 6 ms−1, representing typical heat exchanger conditions. Reynolds number depends significantly on the solution concentration. Heat transfer coefficient increases with Re, by turbulent mixing, and with the concentration of nanoparticles and of acetone by the enhanced thermal diffusivity. The shear wall stress is not affected by changing the concentration of nano-particles. The nano-fluid is demonstrated to work well for heat transfer enhancement over the base fluid; the further issue of suspension of the nano-particles in the solution is explored experimentally. The nano-fluid can be achieved by ultra-sonic excitation, with a settling time in the order of several hours. Subject to the particle suspension time being increased, this fluid combination is a good candidate for the application considered

CFD multiphase modelling of the acetone condensation and evaporation process in a horizontal circular tube

With increasing demands on energy efficiency, the use of low grade waste heat using vapour absorption refrigeration systems (VARS) are receiving renewed interest. One idea is to use the combination of acetone and zinc bromide as the salt solution, which allows use of temperatures in the order of 10s of C above ambient conditions. This work numerically models acetone phase change in the evaporator and condenser in order to indicate how improvements can be made in these components of the system. ANSYS® Fluent finite volume method CFD is used to produce volume of fluid (VOF) and mixture multiphase flow models to investigate the evaporation and the condensation of acetone in a horizontal circular tube. Different velocities and temperatures were taken in each process to explore the effect of these variables in the system. A user defined function (UDF) is used to calculate the volume fraction of the phases. For the evaporation case, the heat transfer coefficient increases with increasing velocity and the temperature difference between the inlet flow and the wall, as expected. The mass transfer rate decreases with increasing the flow rate or decreasing the wall temperature, from 0.045 kg/m 3 .s at 0.01 m/s to 0.016 kg/m 3 .s at 0.06 m/s and it drops from 0.044 to 0.023 kg/m 3 .s by changing the temperature just from 300 to 298 K. This demonstrates a reduction in specific heat transfer to the liquid despite the higher wall heat transfer coefficient. In the condenser, vapour quality decreases along the tube as liquid acetone is created with reduced flow rate. Vapour volume fraction at the outlet section drops from 0.74 to 0.168 by increasing the ingoing velocity from 0.01 to 0.06 m/s. Increasing the rate of condensation will increase the liquid in the evaporator, which increase the evaporation rate then increase the performance of the VARS. This demonstrates the importance of controlling the temperature and the flow rate in the VARS for generate more refrigerants

Optimum community energy storage for renewable energy and demand load management

While the management of PV generation is the prime application of residential batteries, they can deliver additional services in order to help systems to become cost-competitive. They can level-out the demand and potentially reduce the cost and emissions of the energy system by reducing demand peaks. In this study, community energy storage (CES) is optimised to perform both PV energy time-shift and demand load shifting (using retail tariffs with varying prices blocks) simultaneously. The optimisation method obtains the techno-economic benefits of CES systems as a function of the size of the community ranging from a single home to a 100-home community in two different scenarios for the United Kingdom: the year 2020 and a hypothetical zero emissions target. It is demonstrated that the levelised cost and levelised value of CES systems reach intermediate values to those achieved when both applications are performed independently. For the optimal performance of a battery system being charged from both local PV plants and the grid, our results suggest that the battery should be sized suitable to ensure it can fully discharge during the peak period

Numerical study of a multiple-segment metal foam-PCM latent heat storage unit: Effect of porosity, pore density and location of heat source

This study numerically investigates the performance of the melting process for a PCM based heat storage system under the effect of different variables in a vertical container with a copper metal foam. Different cases were studied and compared including the effects of variable porosities and pore densities, non-equilibrium porous medium model, a multiple-segment metal foam case and different heater locations in the system on the liquid fraction and temperature as presented by contour plots and diagrams. The results show high performance for the copper foam-PCM unit compared with on its own PCM, for reducing the melting time by almost 85%. By changing the location of constant temperature heater from the bottom to the side and top surface, the melting time decreases by 70.5% and 4.7%, respectively. By using a multiple-segment porous system, the melting time reduces by 3.5% compared with the case of uniform porosity. Furthermore, the more accurate non-equilibrium numerical model shows a 7.4% difference in the melting time compared with the equilibrium model. This study optimises the design to improve practical application performance and to reduce waste energy

Thermo-physical properties of the nano-binary fluid (acetone–zinc bromide-ZnO) as a low temperature operating fluid for use in an absorption refrigeration machine

© 2019, Springer-Verlag GmbH Germany, part of Springer Nature. The current technical note is an expression for extending aspects of the previous work of Ajib and Karno [1] which related to the thermophysical properties of acetone / ZnBr2. The study covers the thermal conductivity of a solution which appears to be a promising fluid for operating vapour absorption refrigeration (VAR) systems from a low temperature source. It covers also an investigation of acetone / ZnBr2 – ZnO nanofluid including the preparation, stability, structure and properties, a zinc based nanoparticle being chosen in order to reduce chemical interactions. Furthermore, this study illustrates an extension of the log p, T diagram of the acetone zinc bromide up to 1.39 bar. The results show that the thermal conductivity drops with increasing salt concentration. With increasing nanoparticles, the density, viscosity and the thermal conductivity increase, as expected, but the heat capacity drops. Both theoretical and experimentally derived formulae for ZnO nano fluid conductivity from the literature are seen to produce good correspondence to the conductivity measured here, but in the case of the theoretical formula, the influence of particle morphology is seen to be significant. The results indicate that converting the acetone / ZnBr2 to a nanofluid provides a potential improvement of performance of this fluid in the vapour absorption refrigeration system, but that suspension stability is difficult to attain

Modelling a kinetic deviation of the magnesium hydrogenation reaction at conditions close to equilibrium

A model has been derived for the magnesium hydrogenation reaction at conditions close to equilibrium. The reaction mechanism involves an adsorption element, where the model is an extension of the Langmuir adsorption model. The concept of site availability (σs) is introduced, whereby it has the capability to reduce the reaction rate. To improve representation of σs, an adaptable semi-empirical equation has been developed. Supplement to the surface reaction, a rate equation has been derived considering resistance effects. It was found that close to equilibrium, surface resistance dominated the reaction

A system design for distributed energy generation in low temperature district heating (LTDH) networks

Project SCENIC (Smart Controlled Energy Networks Integrated in Communities) involves connecting properties at the University of Nottingham’s Creative Energy Homes test site in a community scale, integrated heat and power network. Controls will be developed to allow for the most effective heat load allocation and power distribution scenarios. Furthermore, the system will develop the prosumer concept, where consumers are both buyers and sellers of energy in both heat and power systems. This paper describes the initial phase of project SCENIC, achieving truly distributed generation within a heat network. The first of its kind, the system has a four pipe network configuration, consisting of a network flow loop to supply heat to homes, and a generation loop to collect energy from residential heating systems and supply it to a centralised thermal store. To achieve the design, IES-VE steady state heat load and dynamic building modelling have been used. A pre-insulated Rehau Rauthermex piping diameter was sized using flow rate calculations. Pipe diameter is reduced in line with distance from the central pump and associated pressure losses. The diameter ranges from 40 to 25mm, with a heat loss as low as 7.0 W/m. In addition, flow rates will fluctuate below a maximum of 1.99 l/s. Danfoss – 7 Series BS flatstations have been selected as the network-building heat interface units (HIU), to satisfy a calculated peak design heating loads of between 36.74 and 44.06 kW. Furthermore, to enable the prosumer concept and associated business models an adapted Danfoss Flatstations – 3 Series BS was selected to interface the distributed heat sources with the network. This paper gives details of the novel system configuration and concept, energy flows, as well as calculation and modelling results for the heat network. A premise is given to maintaining low temperatures in the network to ensure system efficiency in line with the latest research thinking

Substitutional effect of Ti-based AB2 hydrogen storage alloys: A density functional theory study

Stability of AB2 alloy in Laves phases C14 and C15 were studied by first-principle density functional theory simulations. A range of different combinations of B and C elements in the Ti1−xCxB2 alloys were considered. The formation energies of these alloys generally increase with the unit cell volumes of alloys. The volume also affects the stability of the corresponding metal hydride. We find that the formation energies and the hydrogenation enthalpies of AB2 alloys are likely to be determined by at least three factors: electronegativity, atomic radius and covalent radius. The enthalpies of AB2 hydrides increase with increasing compositionally-averaged electronegativity and volume change upon hydrogenation. However, the enthalpies of AB2 hydrides decrease with increasing compositionally-averaged atomic and covalent radii. This study provides useful insights for future exploration of AB2-type alloys for hydrogen storage applications