• Energy Research

Carbon nanoparticles are very useful in solar thermal applications, since they absorb much of the solar spectrum, are cheap and have excellent optical properties. Carbon nanoparticlesthermal oil-based nanofluid was prepared using two-step method with diphenyl sulfone as surfactant to achieve that nanoparticles remain suspended even at high temperatures. The size particle distribution was studied using two Dynamic Light Scattering systems at room and high temperature and also evaluated before and after exposing the nanofluid to a thermal treatment so that conditions closer to those in real applications were replicated. Moreover, the morphological changes due to the thermal treatment were observed with Transmission Electron Microscopy. Finally, the optical properties as the ballistic transmittance, absorption coefficient and scattering albedo of the base fluid as well as of the nanofluid were measured using a spectrophotometer with and without integrating sphere. The results of this study contribute to the knowledge about these solar nanofluids that are promising alternatives to the conventional solar collectors.

The stability and agglomeration state of nanofluids are key parameters for their use in different applications. Silica nanofluids were prepared by dispersing the nanoparticles in distilled water using an ultrasonic probe, which has proved to be the most effective system and gives the best results when compared with previous works. Results were obtained concerning the influence of the solid content, pH and salt concentration on the zeta potential, electrical double layer, viscosity, elastic and viscous moduli, particle size and light backscattering. Measurement of all these properties provides information about the colloidal state of nanofluids. The most important variable is the solid content. Despite the agglomeration due to high concentration, nanofluids with low viscosity and behaving like liquid were prepared at 20% of mass load thanks to the good dispersion achieved with the ultrasonic treatment. The pH of the medium can be used to control the stability, since the nanofluids are more stable under basic conditions far from the isoelectric point (IEP) and settle at pH = 2. Therefore, stable nanofluids for at least 48 h, with high solid content, can be prepared at high pH value (pH > 7) due to the electrostatic repulsion between particles.

Nanoencapsulated phase change materials (nePCMs) – which are composed of a core with a phase change material and of a shell that envelopes the core – are currently under research for heat storage applications. Mechanically, one problem encountered in the synthesis of nePCMs is the failure of the shell due to thermal stresses during heating/cooling cycles. Thus, a compromise between shell and core volumes must be found to guarantee both mechanical reliability and heat storage capacity. At present, this compromise is commonly achieved by trial and error experiments or by using simple analytical solutions. On this ground, the current work presents a thermodynamically consistent and three-dimensional finite element (FE) formulation considering both solid and liquid phases to study thermal stresses in nePCMs. Despite the fact that there are several phase change FE formulations in the literature, the main novelty of the present work is its monolithic coupling – no staggered approaches are required – between thermal and mechanical fields. Then, the FE formulation is implemented in a computational code and it is validated against one-dimensional analytical solutions. Finally, the FE model is used to perform a thermal stress analysis for different nePCM geometries and materials to predict their mechanical failure by using Rankine’s criterion.

Abstract Synthetic thermal oils are used as heat transfer fluids (HTFs) in different applications, due to their higher working temperature. In this way, one of the applications of interest is the use of thermal oils in Concentrated Solar Power (CSP) plants with Parabolic Trough technology. Nowadays, the HTF known commercially as Therminol VP1 (Solutia Inc.) is being used in CSP plants. This fluid is composed of a eutectic mixture of diphenyl (C 12 H 10 ) and diphenyl oxide (C 12 H 10 O), and it is used as an HTF with a maximum working temperature of 400 °C. However, one of the drawbacks of Therminol VP1 is its low thermal conductivity. In recent years it has been demonstrated that the addition of nanoparticles can improve the thermal properties of HTFs, and they are then called nanofluids. The key factor of nanofluids is their high stability over time. However, at high temperatures it is necessary to add chemically compatible surfactants that do not degrade and endow the nanofluids with stability through steric repulsion even under high temperature conditions. In this work, a carbon black/Therminol VP1 nanofluid was synthesized and stabilized using diphenyl sulfone as a stabilizer. Stability tests after thermal cycling at 400 °C showed the higher performance of this additive compared to others commonly used in the literature. Thermal conductivity, heat capacity, and viscosity of the nanofluids at 3 vol% and 5 vol% were characterized from 50 °C to 350 °C. Finally, the Mouromsteff number was calculated to determine the heat transfer enhancement provided by the use of nanofluids.

Renewable energy has become of great interest over the past years in order to mitigate Global Warming. One of the actions gaining attention is the enhancement of the thermal energy storage capacity of Concentrated Solar Power plants. The addition of nanoencapsulated phase change materials (core-shell nanoparticles) to the already used materials has been proposed for that purpose, due to the possibility of increasing thermal storage through the contribution of both core latent heat and sensible heat. In this work, Atomic Layer Deposition has been used to synthesise SiO2 and Al2O3 nanoscale coatings on tin nanoparticles. The multi-encapsulated phase change materials have been characterised in terms of chemical composition, crystalline structure, particle size, thermal stability and thermal storage capacity. Sn@Al2O3 nanoparticles present the best thermal behaviour as they show the lowest reduction in the phase change enthalpy over 100 cycles due to the oxidation barrier of the coating. Moreover, the specific heat of both nanoparticles and solar salt-based nanofluids is increased, making the nanoencapsulated phase change material suitable for thermal energy storage applications.

Solar energy has become an important renewable energy source for reducing the use of fossil fuels and to mitigate global warming, for which solar collectors constitute a technology that is to be promoted. The use of nanofluids can increase the efficiency of solar into thermal energy conversion in solar collectors. Experimental values for the specific heat, thermal conductivity and viscosity of alumina/water nanofluids are needed to evaluate the influence of the solid content (from 0.25 to 5 v%) and the flow rate on the Reynolds, Nusselt and the heat transfer coefficient. In the laminar flow regime, thermal conductivity enhancement over specific heat decrement is key parameter, and a 2.34% increase in the heat transfer coefficient is theoretically obtained for 1 v% alumina nanofluid. To corroborate the results, experimental tests were run in a flat plate solar collector. A reduction in efficiency from 47% to 41.5% and a decrease in the heat removal factor were obtained using the nanofluid due to the formation of a nanoparticle deposition layer adding an addition thermal resistance to heat transfer. Nanofluids are recommended only if the nanoparticle concentration is high enough to enhance thermal conductivity, but no so high so as to avoid wall deposition.

This research was partially funded by Ministerio de Economia y Competitividad (MINECO) of Spain through the project ENE201677694R. Josep FornerEscrig thanks Ministerio de Economia, Industria y Competitividad of Spain and Fondo Social Europeo for a predoctoral fellowship through Grant Ref. BES-2017-080217 (FPI program) . This work has been developed by participants of the COST Action CA15119 Overcoming Barriers to Nanofluids Market Uptake (NANOUPTAKE) . Nanoencapsulated phase change materials (nePCMs) are one of the technologies currently under research for energy storage purposes. These nePCMs are composed of a phase change core surrounded by a shell which confines the core material when this one is in liquid phase. One of the problems experimentally encountered when applying thermal cycles to the nePCMs is that their shell fails mechanically and the thermal stresses arising may be one of the causes of this failure. In order to evaluate the impact of the uncertainties of material and geometrical parameters available for nePCMs, the present work presents a probabilistic numerical tool, which combines Monte Carlo techniques and a finite element thermomechanical model with phase change, to study two key magnitudes of nePCMs for energy storage applications of tin and aluminium nePCMs: the maximum Rankine's equivalent stress and the energy density capability. Then, both uncertainty and sensitivity analyses are performed to determine the physical parameters that have the most significant influence on the maximum Rankine's stress, which are found to be the melting temperature and the thermal expansion of the core. Finally, both a deterministic and a probabilistic failure criterion are considered to analyse its influence on the number of predicted failures, specially when dispersion on tensile strength measurements exists as well. Only 1.87% of tin nePCMs are expected to fail mechanically while aluminium ones are not likely to resist. Ministerio de Economia y Competitividad (MINECO) of Spain ENE201677694R Ministerio de Economia, Industria y Competitividad of Spain European Social Fund (ESF) European Commission BES-2017-080217 European Cooperation in Science and Technology (COST) CA15119