Abstract
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.PACS65.: Thermal properties of condensed matter; 65.20.-w: Thermal properties of liquids; 65.20.Jk: Studies of thermodynamic properties of specific liquids
Highlights
The global warming and terawatt challenge are probably two of the most complex issues that humanity have to face during the twenty-first century [1]
Actual commercial hightemperature thermal energy storage (TES) systems are based on solar salt and its main application is in concentrated solar power (CSP) plants in which solar salt is used as a sensible TES in two tank configuration as well as, in some cases, heat transfer fluid
Salt-based nanofluids with other nanoparticle concentrations present weak specific heat capacity increment, with enhancement values near the accuracy given by the differential scanning calorimetry (DSC) system
Summary
The global warming and terawatt challenge are probably two of the most complex issues that humanity have to face during the twenty-first century [1]. In order to comply with these assessments, improvement of energy efficiency and conservation as well as development of low-carbon energy technologies must play a lead role. The development of cost-effective energy storage technologies is one of the key parameters for improving energy management and providing availability and dispatchability. Thermal energy storage (TES) systems play a key role in energy storage. The main advantages of TES systems based on inorganic salts are their competitive costs, high stability at high temperatures, as well as being environmentally friendly. Their main disadvantages are the low thermal properties and corrosion problems. Actual commercial hightemperature TES systems are based on solar salt (eutectic mixture of NaNO3 and KNO3) and its main application is in concentrated solar power (CSP) plants in which solar salt is used as a sensible TES in two tank configuration as well as, in some cases, heat transfer fluid
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