|Navid Aslfattahi||Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, 50603, Kuala Lumpur|
|Saidur Rahman||-Research Center for Nano-Materials and Energy Technology (RCNMET), School of Science and Technology, Sunway University, Bandar Sunway, Petaling Jaya, 47500, Selangor Darul Ehsan, Malaysia -Departmen|
|Mohd Faizul Mohd Sabri||Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, 50603, Kuala Lumpur|
|A. Arifutzzaman||Research Center for Nano-Materials and Energy Technology (RCNMET), School of Science and Technology, Sunway University, Bandar Sunway, Petaling Jaya, 47500, Selangor Darul Ehsan, Malaysia|
In this study, nanocomposites containing a pre-defined mass ratio of solar salt (NaNO3-KNO3: 60-40 wt.%) dispersed with magnesium oxide (MgO) nanoparticles with nominal sizes of 100 nm were prepared in solid and liquid states. The proposed amounts of sodium nitrate and potassium nitrate were added to certain amounts of ultrapure deionized (DI) water comprising a 5 wt.% concentration of MgO nanoparticles. Afterward, the prepared mixture was placed in a dry oven to mix in a liquid state to obtain well-dispersed nanocomposites. Scanning electronic microscopy (SEM) was conducted to evaluate the uniformity of synthesized, molten salt–based magnesium oxide–nanoparticles, revealing a uniform dispersion. Enthalpy and melting point measurements were performed using differential scanning calorimetry. The experimental results of solar salt–based MgO indicated decreases in melting point and enthalpy by 7% and 12.4%, respectively. The reduction of enthalpy indicated that, with the addition of magnesium oxide to solar salt, the final nanocomposite tends to have more exothermic reactions and enhanced thermal conductivity performance at the melting point. Lower melting points constitute one of the major concerns regarding molten salt–based nanofluids. MgO nanoparticles with a concentration of 5 wt.% have a melting point decreased by 7%. Mass loss and thermal stability measurements were conducted using thermogravimetric analysis (TGA). The experimentally acquired results revealed an increment of decomposition temperature from 734.29°C to 750.73°C, demonstrating the enhancement of thermal stability at high temperatures.
Enthalpy; MgO; Solar salt; Thermal stability
Renewable energy sources are promising replacements for petroleum resources. Demand for green and renewable energy has significantly attracted the interest of scientists, many of whom have studied solar energy as a clean source of energy (Thirugnanasambandam et al., 2010). The productivity of solar thermal systems depends on the efficient conversion of thermal energy from the sun. Light-to-heat conversion at high temperatures (above 300°C) is more desirable to access a broad range of operation temperatures (Kusrini & Kartohardjono, 2019). In solar thermal systems, storage and transportation of thermal energy occurs using heat transfer fluids (HTFs) and thermal energy storage (TES) materials (Hawachi et al., 2014). Inclusion of nanoparticles in HTFs in volume concentrations (typically lower than 10%) generates nanofluids (NFs). The anomalous increments of NFs in thermal conductivity and thermal storage are well-known. Conventional NFs, such as water-based ones, were initially introduced by Choi and Eastman (1995). Eastman et al. (2004) later obtained a 60% enhancement of NF thermal conductivity over water by the dispersion of CuO nanoparticles at a 5% volume concentration. Carbon-based nanostructures and/or ceramic and metallic nanoparticles (NPs) have been utilized as additives to water-based fluid (Huang et al., 2010). Ethylene glycol, engine oil, and organic oil have all been employed as base fluids for NFs (Kalogirou, 2004). These traditional NFs, however, are not applicable to several industrial processes that require the application of fluids that operate at temperatures higher than those suitable for the aforementioned fluids. Lower thermal stability up to 400°C is another drawback of conventional NFs. These limitations have encouraged scientists to study new classes of NFs, a tendency that might lead to enhancing the efficiency of systems and be suitable in terms of cost effectiveness. Molten salts can operate at high temperatures, thus improving procedural efficiency, as they possess efficient thermal properties to operate as TES materials in concentrated solar power (CSP) systems. The low costs of molten salts and their higher thermal stabilities (up to 600°C) constitute a prominent advantage in these types of applications.
On the other hand, the industrial implementation of molten salts is affected by their low thermal conductivity properties (Mahian et al., 2013). The optimization of these thermophysical properties is the key to applying these salts in TES systems and new HTFs in CSP facilities. With the aim of fulfilling this need, Shin and Banerjee developed a new kind of NF eight years ago at the University of Texas at Arlington (Shin & Banerjee, 2011a) by utilizing a mixture of binary inorganic salt (Li2CO3-K2CO3: 62-38 by mol) as the base fluid and SiO2 as the additive. The specific heat capacity (Cp) of the developed mixture revealed more than 100% enhancement with 1% volume concentration of NPs. Shin and Banerjee suggested that applying their NF would reduce costs by 50% with a combination of higher operating temperatures (higher thermodynamic efficiency) and the diminution of materials.
Phase change materials (PCMs) are preferable to TES in terms of large enthalpy changes (Putra et al., 2016). These crucial changes occur during freezing and melting (phase changes). Inorganic eutectic alkali metal PCMs (molten salts) have enormous capabilities as TES (Shukla et al., 2009). Hence, molten salts have been a subject of intense research for scientists around the world. Extensive amounts of researches have focused on organic PCMs, such as paraffin waxes (Huang et al., 2009), but few inorganic PCMs have been investigated for operation in high-temperature applications. Nitrate-based molten salts have received more attention due to their availability in a wide range of temperatures (Lachheb et al., 2016). Solar salt is a eutectic, nitrate-based, alkali molten salt with combination of 60 wt.% sodium nitrate and 40 wt.% potassium nitrate. This type of molten salt has a relatively high melting point and high energy storage density (Vignarooban et al., 2015). According to the literature, solar salt has efficient high thermal stability (Wang et al., 2015). Solar salt has also been utilized as a heat storage medium (Myers et al., 2016). One of its major drawbacks, however, is its relatively low thermal conductivity, which affects its thermal storage performance (Gimenez-Gavarrell & Fereres, 2017).
The present study investigates the effects of magnesium oxide dopant nanoparticles on the thermophysical properties of nitrate-based molten salt. A conventional solar salt (NaNO3-KNO3: 60-40 wt.%) is utilized as a base fluid, and the suspended nanoparticles are MgO (5 wt.%). Well-dispersed molten salt–based NF was synthesized in a two-phase preparation method. The first step was physically mixing and the second step conducted in a melting state using an oven at high temperature. The melting point and enthalpy of alkali metal molten salt with and without nanoparticles were measured using differential scanning calorimetry (DSC). The measured melting point of the synthesized molten salt was compared with the literature to verify its accuracy. The nanostructures of the synthesized molten salt with and without NPs were observed using scanning electronic microscopy (SEM) images. Adding magnesium oxide nanoparticles at 5 wt.% has increased exothermic reactions are the melting point. The melting point also fell by 7%, which prevented the solidification of the solar salts on the walls of heat exchanger surface. This reduction in melting point is one of the main drawbacks of solar salts.
In conclusion, the eutectic alkali metal solar salt dotted with a 5% concentration by volume of MgO nanoparticles and the pure eutectic solar salt were synthesized using a two-phase method. The alkali metal molten salts with and without nanoparticles were mixed physically followed by mixing at melting state in an oven at high temperature. The resultant samples were well-dispersed. Enthalpy and melting point measurements were performed using DSC. The enthalpy of the MSBNFs decreased by 12.4%, which demonstrates a more exothermic reaction at the melting point. The experimentally achieved data indicated a melting point decrement for MSBNFs by 7% in comparison to pure eutectic solar salt. SEM and EDX indicated chain-like structures in the resultant NF, and elemental analysis using EDX showed good dispersion of magnesium oxide nanoparticles. Thermal stability measurements expressed an enhancement of thermal stability in solar salt induced with a 5% concentration by volume of MgO nanoparticles. The experimentally acquired results revealed the increment of decomposition temperature from 734.29°C to 750.73°C.
R. Saidur would like to acknowledge the financial support provided by the Sunway University through the project no. STR-RCTR-RCNMET-001-2019.?
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