Transient natural convection induced by the absorption of concentrated solar radiation in high temperature molten salts

Solar-thermal energy systems that involve the deposition of radiation in absorbing high temperature molten salts to harness the entire solar spectrum and achieve high efficiencies and low Levelised Cost Of Energy (LCOE) are of considerable interest for power generation. From a design stand point, to achieve a competitive solar power generation devices, it is imperative to have an accurate knowledge of the inherent physical processes of such a fluid system. Thus under high temperature conditions, detailed understanding of the heat transfer and fluid flow characteristics in an irradiated fluid is considered. The work investigates the spectral dependent heat transfer and fluid dynamics in a thermal storage concept which uniquely combines a volumetric receiver and a single tank thermal store. The Thermal Energy Storage (TES) is protypical of a small scale concept concentrated solar plant. Advances in computing power, has seen Computational Fluid Dynamics(CFD) consolidated as a powerful tool employed by researchers and engineers to simulate real world behaviour and complex phenomena to a certain degree of accuracy with low effort in time, personnel and resources. This thesis is focused on the development of a realistic numerical model capable of predicting the local volumetric absorption of solar radiation in a fluid layer which provides an improved understanding of the hydrodynamic and thermal conditions in an enclosed fluid layer. Computational Fluid Dynamics is used to simulate the transient heat transfer and fluid flow determined by a combined influence of volumetric absorption and natural convection in a high temperature fluid filled enclosure. The enclosure is studied for the specific case in which a high temperature salt is first heated by direct volumetric absorption of the incident solar radiation and secondly by natural convection from a absorber plate located at the bottom of the enclosure whose sole purpose is to absorb all non-absorbed radiation reaching the lower surface. The current models considers the depth dependence absorption of solar radiation based on (i) a solar weighted absorption coe cient (assumed constant over all wavelengths) and (ii) spectral absorption coe cient characterised by wavelength band based on a standard solar spectrum reference. A commercially available CFD Package based on Finite Element Method (FEM), COMSOL Multiphysics is used to discretise and solve the Navier Stokes and energy equation under transients heating conditions for a non Boussinesq condition by accounting for the temperature variable properties of molten salts. A time-dependent and Backward Differentiation Formula (BDF) solver using implicit time-stepping methods is combined with refined mesh to solve the non-linear PDE. Validity of the numerical tool has been conducted, by comparing results from published results found in literature with corresponding numerical results. The mesh element optimum sizes and time steps used conform to those obtained in validation models. Simulations have been conducted for a daily charging period of three hours as used in conjunction with a solar system. The effects of bottom absorber plate, flux Rayleigh number, aspect ratio, variable Air Mass and inclination angle have also been investigated. Numerical results are presented in terms of surface plots, temperature contours, and velocity contours and streamlines which show the thermal field distribution and flow structures, for volumetric absorption of thermal radiation coupled with natural convection. Performance criteria are based on quantification of the level of thermal stratification using the MIX number, the dimensionless exergy and capture efficiency. Three dimensionality effects were studied by considering three dimensional simulation for the same problem. The results show that the present method and models are capable of capturing the main features of the flow and the overall performance of these turbulence models in terms of predicting time-averaged quantities. Results obtained indicate a nonlinear temperature profile consisting of two distinct layers: a surface layer and a bottom layer. The numerical results reveal natural convection in the cavity follows an initial stage, a transitional stage and a quasi-steady stage. Results indicate that volumetric absorption of solar radiation, when coupled to natural convection has a direct influence on the thermal field through the disparities in absorption and emission phenomena. The isotherms and streamlines show that the natural convective heat transfer and flow are quite different from those obtained in differentially heat enclosures. Thus the heat transfer mechanism destroys a symmetry of the system that relates clockwise and counter clockwise flows. Temperature and flow field are found to be greatly influenced by the aspect ratio (H/D) of the store and the flux Rayleigh number. It is found that the predicted heat transfer from the lower surface in the cavity is increased when the simulation is extended from two to three dimensional. Results obtained indicated that increasing the aspect ratio, Air Mass and inclination angle all result in increasing levels of thermal stratification. Natural convection from the lower absorber surface is found to increase with increasing flux Rayleigh number

Heat transfer in a molten salt filled enclosure absorbing concentrated solar radiation

Numerical simulations of the natural convection driven by the direct absorption of concentrated solar radiation by a high temperature molten salt filled enclosures for height to diameter ratios (H/D) of 0.5, 1 and 2 and Rayleigh numbers 107–1011 is presented. The domain of interest consists of a fluid cavity bounded by rigid adiabatic vertical walls, a heat-conducting bottom wall of finite thickness and an open adiabatic top surface, directly irradiated by a non- uniform concentrated solar flux. The salt volume is first heated non-uniformly by direct absorption of solar radiation and subsequently from the lower absorber plate which is heated by the absorption of the radiation transmitted through the salt. A Finite Element Method is used to solve the time dependent two dimensional Navier Stokes equations that includes a depth dependent volumetric heat source and temperature dependent thermophysical of molten salts. Numerical results presented in terms of isotherms and streamlines show a nonlinear temperature profile consisting of distinct layers where thermocapilarity and buoyancy effects are evident. Fluid flow development in the lower layer is found to vary significantly with time and exhibits an initial stage, transitional stage and quasi-steady stages. The magnitude of the natural convection and the duration of each stage is found to decrease as the aspect ratio increases from 0.5 to 2. Calculation of the average heat transfer reveals that the Nusselt Rayleigh number relationship is not uniformly linear and the average heat transfer over the lower boundary surface increased with increasing Ra

Natural convection induced by the absorption of solar radiation: A review

Natural convection primarily driven by the absorption of thermal energy known as penetrative or thermo-convection is a topic that generates attention due to its importance in various physical systems. A very common example of where this process can be found in geophysical systems such as lakes, where radiation induced natural convective transport have been seen to influence water temperature, biological activity and water quality. The present paper reviews previous analytical, experimental and numerical studies reported in literature concerning natural convection driven by absorption of thermal radiation. Many of the reviewed studies were motivated by the interest of investigators to understand the physical processes in volumetric absorption thermal radiation in a fluid layer process and its associated energy transport. In this class of problems, temperature fields are generally described as non-linear and the associated fluid flow is considered rather complex owing to coupling between the direct absorption of radiation and fluid flow. Parametric investigations for the effect of various parameters of interest such as the Rayleigh numbers, Prandtl numbers, spectral nature of incident radiative flux, optical depth, fluid absorptivity, aspect ratio, albedo and boundary emissivity on natural convection have been investigated. The overall aim of the current review is to present a comprehensive review of the previous and recent approaches applied in the investigations of radiation induced natural convection in reservoirs. The paper also aims to contribute to improving the understanding of the physical processes, heat transfer and fluid dynamics associated with the thermal energy deposition into a fluid layer. The paper is also highlight the potential application of this concept to help keep solar energy capture costs to a minimum and inform efficient designs of energy systems based on the concept of direct absorption of thermal energy inside a fluid layer

Transport of nanoparticles in porous media and associated environmental impact: a review.

The release of nanoparticles into the environment occurs at different stages during their life cycle, with significant harmful effects on the human (e.g., lung inflammation and heart problems) and the ecosystem (e.g., soil and groundwater contamination). While colloids (particles >1 micrometre) behaviour in porous media is influenced by filtration, nanoparticles (<100 nanometres) behaviour is driven by Brownian motion and quantum effects. Recognising these disparities is essential for applications like groundwater remediation and drug delivery, enabling precise strategies based on the differing transport dynamics of colloids and nanoparticles. The extent of the impact of nanoparticle release on the environment is strongly influenced by their type, size, concentration, and interaction with porous media. The main factor preventing the use of nanoparticles for environmental remediation and other related processes is the toxicity arising from their uncontrolled distribution beyond the application points. Finding a suitable dosing strategy for applying nanoparticles in porous media, necessary for the correct placement and deposition in target zones, is one of the significant challenges researchers and engineers face in advancing the use of nanoparticles for subsurface application. Thus, further studies are necessary to create a model-based strategy to prevent nanoparticle dispersion in a porous media. In general, this review explores the transport of nanoparticles in porous media concerning its application for environmental remediation. The aim of this study is captured under the following: a) Identifying the properties of nanoparticles and porous media to develop an innovative remediation approach to reclaim contaminated aquifers effectively. b) Identify critical parameters for modelling an effective strategy for nanoparticle-controlled deposition in porous media. This would require a general understanding of the onset and mapping of the different nanoparticle depositional mechanisms in porous media. c) Identify existing or closely related studies using model-based strategies for controlling particulate transport and dispersion in porous media, focusing on their shortcomings

Generation, characteristics and energy potential of solid municipal waste in Nigeria

The generation, characteristics and energy potential of municipal solid waste for power generation in Nigeria is presented in this paper. Nigeria generates 0.44-0.66 kg/capita/day of MSW with a waste density of 200-400 kg/m3 leading to large volumes of poorly managed waste. The direct burning of these wastes as a waste management option in the open air at elevated temperatures liberates heat energy, inert gases and ash which can be conveniently used for power generation and other applications. The net energy yield depends upon the density and composition of the waste; relative percentage of moisture and inert materials, size and shape of the constituents and design of the combustion system. MSW samples used in this study were obtained randomly from different dump sites in selected state capitals, at least one from each of the six geopolitical zones in Nigeria based on the spot sampling method of Corbit. An average calorific value of 17.23 MJ/kg with variable high water content of 20-49% was determined for MSW using a bomb calorimeter and on the basis of an incineration plant of capacity 1500 ton of MSW/day, 700kW/day of power can be generated

Start-up dynamics of vertical axis wind turbines: a review.

Vertical Axis Wind Turbines (VAWTs) are becoming increasingly popular for wind power extraction due to their simpler design, lower manufacturing and maintenance costs. The omni-directionality of these power generating machines make them more suitable for operation both under lower wind speeds and highly transient winds. The key wind power extraction component of VAWTs is their rotor, which is conventionally either Drag-based or Lift-based, with the latter being more widely studied in the published literature due to its higher power coefficient compared to its counterpart. The lift-based rotor comprises of aerodynamically profiled blades, while the drag-based rotor comprises of thin cup-shaped blades. The start-up of both these types of VAWT rotors has been an area of active research in the last decade. These studies have been conducted using numerical and experimental methods focusing on key parameters related to the start-up dynamics of VAWTs. Many of these research studies complement each other's findings, however, there are also a number of aspects where there are disagreements and/or significant knowledge gaps that need to be highlighted in order to accelerate scientific efforts strategically. In the present study, we aim to address these challenges through a thorough and critical review of the published literature on VAWTs' start-up dynamics, leading towards the identification of key knowledge gaps