Mejora de las propiedades térmicas de sal solar mediante adición de nanopartículas

Treball Fi de Màster Universitari en Eficiència Energètica i Sostenibilitat. Curs 2013/2014El principal objetivo de este proyecto final de máster es estudiar la mejora de las propiedades térmicas de sales empleadas para el almacenamiento de energía en centrales termosolares. Dicha mejora se obtiene mediante la adición de partículas de tamaño nanométrico (nanopartículas)

Increment of specific heat capacity of solar salt with SiO2 nanoparticles

Thermal energy storage (TES) is extremely important in concentrated solar power (CSP) plants since it represents the main difference and advantage of CSP plants with respect to other renewable energy sources such as wind, photovoltaic, etc. CSP represents a low-carbon emission renewable source of energy, and TES allows CSP plants to have energy availability and dispatchability using available industrial technologies. Molten salts are used in CSP plants as a TES material because of their high operational temperature and stability of up to 500°C. Their main drawbacks are their relative poor thermal properties and energy storage density. A simple cost-effective way to improve thermal properties of fluids is to dope them with nanoparticles, thus obtaining the so-called salt-based nanofluids. In this work, solar salt used in CSP plants (60% NaNO3 + 40% KNO3) was doped with silica nanoparticles at different solid mass concentrations (from 0.5% to 2%). Specific heat was measured by means of differential scanning calorimetry (DSC). A maximum increase of 25.03% was found at an optimal concentration of 1 wt.% of nanoparticles. The size distribution of nanoparticle clusters present in the salt at each concentration was evaluated by means of scanning electron microscopy (SEM) and image processing, as well as by means of dynamic light scattering (DLS). The cluster size and the specific surface available depended on the solid content, and a relationship between the specific heat increment and the available particle surface area was obtained. It was proved that the mechanism involved in the specific heat increment is based on a surface phenomenon. Stability of samples was tested for several thermal cycles and thermogravimetric analysis at high temperature was carried out, the samples being stable.Universitat Jaume I (project P1-1B2013-43

Mechanical cleaning of food soil from a solid surface: A tribological perspective

In this work, a tribological approach was used to distinguish the synergistic effects of mechanical removal and chemical removal (i.e. dissolution) of a layer of representative food soil from a solid surface, using a tribometer, Mini Traction Machine (MTM). Gravimetric and wear measurements of the soil were used to calculate the cleaning rates of burnt tomato puree on a stainless-steel disc, and the corresponding frictional characteristics offers insight of the mechanical removal. The cleaning due to soil dissolution (chemical removal) was quantified by UV–Vis measurements. The overall cleaning rates of food soil featured a linear reduction in mass over time, with a scaled removal rate k = 0.0046 s−1 (5 N applied force and 100 mm s−1 relative velocity), for most cases studied. It was observed that the cleaning rate can be improved with an increasing mechanical load or speed (50% from 1 to 2.5 N and 13% from 50 to 100 mm s−1), but is independent of the initial mass. UV–Vis measurements show that by increasing the load or speed the removal of chunks of burnt tomato puree was enhanced more than removal attributed to dissolution. Similar values of cleaning rates for most experimental parameters were extracted from both the gravimetric and wear measurements. Adhesion and cohesion measurements of the burnt tomato puree were conducted with a micromanipulator. It was found that adhesion forces are higher than cohesion for short soaking times, but for longer times the adhesion forces became weaker and with the additional shear rate in the MTM cleaning experiment, adhesion failure was observed in many cases by the end of the experiment. Indentation measurements showed the change in mechanical properties of the food foulant with a few minutes of soaking in water

Modelling the dissolution of structured particles for enhanced wash performance

Dissolution of solid particulates is a critical step in a wide range of processes across different industrial sectors including food, pharmaceutical and the domestic sector. Typical particulate product forms are flakes, pellets, tablets and granules. The latter is one of the most common forms of solid detergents made by the popular spray drying process. The dissolution kinetics of particles have been of interest to researchers for the last centuries. Particle dissolution consists of a complex sequence of phenomena, including wetting, sinking, disintegration and finally dissolution. The internal structure of spray dried particles plays a key role in the dissolution process. However, there is still a need for a thorough understanding of the influence that the internal structure of the particles has on the dissolution behaviour of each of the particle components. Particularly, for complex formulations of detergent powders. This forms the main motivation of this Ph.D. study. Powder detergents are usually formulated with a wide range of components with different functions in the end-use application. In light of this, simplified blown powders produced with three main components present a good model of porous powders in order to investigate the effect of structure on the dissolution kinetics. Different particle structures were obtained by varying the fraction of the components and key production parameters (such as primary particle size of components and air injection) during the spray drying process. These model particles were used in this study, which consists of three main areas. Firstly, the characterization of the bulk powders has been done by combining computational image analysis with traditional characterization methods. This combination has proven to be an excellent approach to evaluate both qualitatively and quantitatively the internal structure of the particulate materials. The variations of intra-particle porosity were detecting by both methods, however, the pore size distributions showed a different range of pore sizes in the mercury porosimetry and x-ray tomography. The latter was combined with a novel approach for the evaluation of the spatial distribution of the pores within the particles. This allowed for the identification of the key parameters for the production of particles with thick shells, micronised sodium sulphate and high binder content. Secondly, the dissolution process of structured particles consists of a complex sequence of physical and chemical mechanisms, which can be affected by various factors. As a result, controlled dissolution analyses were performed with UV-Vis spectroscopy and electrical conductivity measurements. This combination of techniques allowed for the individual evaluation of the dissolution kinetics of the particle constituents (sodium sulphate and binder). From the use of these two techniques, the influence of particle structure was readily observed in that the dissolution profiles of the binder and the salt did not coincide depending on the structure. Lastly, a one-dimensional model has been developed in order to simulate the dissolution profiles observed from the dissolution kinetics experiments with the addition of structural parameters (such as shell thickness, in the case of hollow spherical particles and a radial distribution of components) evaluated from the image characterization of the structured particles. This model accounts for the central cavity that the hollow spherical particles contain as well as the mass transfer resistance arising from the presence of one of the binder components (sodium silicate). With these main threads of study the structural parameters were correlated with the dissolution behaviour of the powders

Influence of microwave assisted freezing parameters on ice crystal growth

Final quality of frozen foods is determined by the shape and size of the formed ice crystals, as they might cause irreversible damage to the cellular structure –crystals larger than cells will break them appart, impairing quality. Recent studies using microwave assisted freezing (MAF) – a novel alternative freezing technology – have shown that microwave radiation can influence the ice crystal formation leading to crystal size reduction. Two concepts have been proposed regarding the mechanism of action such as ”NITOM” concept (Nucleation Induced by Temperature Oscillation caused by MWs) and the ”NIMIW” (Nucleation Induced by constant or pulsed MIcroWaves power). The present study aimed to enlighten the influence of different microwave assisted freezing parameters on the ice crystal growth. A Response Surface Method (RSM) has been used to evaluate the effect of three process parameters (i.e. MW pulse width, power and cooling rate) on final crystal size, correlating processing conditions to final crystal sizes and setting the basis for further analysis