Published at : 25 Nov 2019
Volume : IJtech
Vol 10, No 6 (2019)
DOI : https://doi.org/10.14716/ijtech.v10i6.3568
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
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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