Comprehensive Experimental Study on the Thermophysical Characteristics of DI Water Based Co 0.5 Zn 0.5 Fe 2 O 4 Nanoﬂuid for Solar Thermal Harvesting

: The thermophysical properties of water-based Co 0.5 Zn 0.5 Fe 2 O 4 magnetic nanoﬂuid were investigated experimentally. Consequently, the viscosities of 0.25 wt% and 1 wt% Co 0.5 Zn 0.5 Fe 2 O 4 nanoﬂuid were 1.03 mPa · s and 1.13 mPa · s, each greater than that of the 20 ◦ C base ﬂuid (water), which were increased by 7.3% and 17.7%, respectively. The Co 0.5 Zn 0.5 Fe 2 O 4 nanoﬂuid thermal conductivity enhanced from 0.605 and 0.618 to 0.654 and 0.693 W / m · ◦ C at concentrations of 0.25 wt% and 1 wt%, respectively, when the temperature increased from 20 to 50 ◦ C. The maximum thermal conductivity of the Co 0.5 Zn 0.5 Fe 2 O 4 nanoﬂuid was 0.693 W / m · ◦ C at a concentration of 1 wt% and a temperature of 50 ◦ C. Furthermore, following a solar exposure of 120 min, the photothermal energy conversion e ﬃ ciency of 0.25 wt%, 0.5 wt%, 0.75 wt%, and 1 wt% Co 0.5 Zn 0.5 Fe 2 O 4 nanoﬂuids increased by 4.8%, 5.6%, 7.1%, and 4.1%, respectively, more than that of water. ﬂuid) at di ﬀ erent weight concentrations (0.25 wt%, 0.5 wt%, 0.75 wt%, and 1 wt%). TEM and XRD experiments were conducted to examine the nanostructure of the Co 0.5 Zn 0.5 Fe 2 O 4 nanoparticles.


Introduction
Thermal energy has taken an exceptional role in human transformation of the environment since the dawn of time. Due to rapid technological development, annual average electrical energy per capita consumption increased drastically to as much as 3132.5 kWh in 2014, creating a global energy crisis [1]. There has been an unprecedented burning of conventional fossil fuels to meet this enormous energy demand in developed countries [1]. However, to meet this demand in an environmentally sustainable manner, the world must surmount multiple challenges, e.g., discovering affordable and sustainable alternative green energy sources. Among various energy sources, solar energy shows the most potential as a solution to this pressing problem. However, while the technology for obtaining heat from solar energy has been in use for a long time, the scope of its application and systems has been very limited because of low energy conversion efficiency and low energy density.
In many thermal energy systems, heat transfer efficiency is the most important factor in increasing system efficiency. Methods to enhance the heat transfer are divided into two classifications: active and passive enhancement methods [2]. In active heat transfer enhancement, the system requires additional external energy input, e.g., through mechanical stirring, vibration, circulation, and external magnetic or external fields. However, these approaches are usually expensive and are inapplicable under certain operating conditions. In passive heat transfer enhancement, heat transfer improvement is accomplished by altering properties of the working fluids, changing surface area available for heat Table 1. Summary of magnetic nanofluid preparation.

Reference
Particles Diameter (nm) Base Fluid Concentration [27] Fe 3 O 4 6.7 Kerosene 0-7.8 vol% [28] α-Fe 2 O 3 5 Glycerol 0.155-0.75 vol% [29] γ- Li et al. [40] reported a remarkable dependence of Fe 3 O 4 magnetic nanofluid properties on the magnetic field strength and its direction. They result posits that a direction of the magnetic field aligned with the temperature distribution of nanofluid has more potential for improving thermal conductivity than when it is perpendicular. The energy transfer efficiency enhanced because nanoparticles in the nanofluid tended to arrange a chainlike formation along the magnetic field direction that aligned with temperature distribution in a nanofluid. Natami et al. [41] investigated a magnetic effect on convective heat transfer of Fe 3 O 4 nanofluid in a horizontal pipe. They concluded the heat transfer significantly increases in the laminar regime with a high concentration of Fe 3 O 4 nanofluid. Furthermore, in some cases, heat transfer decreases while Hartmann's number increases. They assumed that this was because the accompanying decrease in Brownian motion was caused by the strong magnetic field. An investigation by Sun et al. [3] examined the magnetic effect on convective heat transfer of Fe 3 O 4 nanofluid in a pipe. Heat transfer via 0.9 vol% Fe 3 O 4 nanofluid enhanced convective heat transfer by 9.16% at Re = 1080 compared with the base fluid. They reported the convective heat transfer enhancement from the Fe 3 O 4 nanofluid was more notable in a gradient magnetic field than a uniform magnetic field. Based on their results, the flow frictional resistance of Fe 3 O 4 -water in a thermal system is less significant than the improvement to heat transfer. In a study on surface cooling based thermomagnetic convection, Zablotsky et al. [42] found that the cooling intensity of a ferrofluid improved significantly around the maximal magnetic field intensity with more intense heat transfer.
The finite supply of conventional fuels combined with the issue of climate change has placed humanity on the cusp of restricting the use of fossil fuels. Thus, research on developing systems powered by renewable energy and on improving the system performance of such systems is crucial. Particularly critical studies are on improving heat transfer and heat absorption efficiency of systems that produce heat energy via renewable energy. Against this background, there has been a radical increase in the study of magnetic nanofluids in solar energy harvesting systems within the research community. Furthermore, this flourishing attraction within this field has led to numerous studies and the creation of research literature on magnetic nanofluids such as Fe 3 O 4 , Fe 2 O 4 , and ND-Co 3 O 4 . However, while the thermal and magnetic properties of various magnetic nanofluids have been extensively investigated, there are still uninvestigated magnetic nanofluids (e.g., Co 0.5 Zn 0.5 Fe 2 O 4 nanofluid) that may have promising properties. The key objective of this work is to experimentally investigate and analyze the thermal properties of diluted water (DI-water) based Co 0.5 Zn 0.5 Fe 2 O 4 magnetic nanofluid. To thoroughly characterize the Co 0.5 Zn 0.5 Fe 2 O 4 particles, XRD, transmission electron microscopy (TEM) imaging, and VSM tests were conducted. Through this study, we aim to increase the understanding of Co 0.5 Zn 0.5 Fe 2 O 4 magnetic nanofluid, which is a new applicable nanofluid, and to investigate its applicability in various systems.

Co 0.5 Zn 0.5 Fe 2 O 4 Nanofluid
Nanofluids are complex colloids that disperse functionalized nanomaterials in broadly used working agents such as refrigerant, thermal oil, antifreeze, and water. In a nanofluid, solid materials within the nanofluid tend to settle to the bottom of the liquid due to the force of gravity. This precipitation of nanomaterials can cause a direct unfavorable influence on the operation of a thermal system such as clogging the pump and polluting the heat transfer surface. Thus, the manufacture of nanofluids with superior dispersal dynamics is a major concern in nanofluid application.
The procedure for manufacturing Co 0.5 Zn 0.5 Fe 2 O 4 nanofluid includes synthesis of the Co 0.5 Zn 0.5 Fe 2 O 4 nanoparticle and homogenous nanoparticle distribution in the base fluid, which constitutes the two-step manufacturing method extensively used at both the research and industrial level. Co 0.5 Zn 0.5 Fe 2 O 4 nanoparticles are prepared via chemical coprecipitation using cobalt chloride (CoCl 2 ), zinc sulfate (ZnSO 4 ), and iron (III) chloride (FeCl 3 ). DI water and oleic acid are used A second mixture was prepared using 56 mL of NaOH (4 mol/L) and 18.75 mL of DI water. The second mixture was added dropwise to the first mixture with magnetic mixing at 80 • C. The precipitated Co 0.5 Zn 0.5 Fe 2 O 4 nanoparticles were heated at 80 • C. After heating for 1 h, mixture was maintained at room temperature until its temperature reached 25 • C. After chemical reaction and cooling, the Co 0.5 Zn 0.5 Fe 2 O 4 nanoparticle precipitation was separated from the reactants using a magnetic bar and washed in DI water at least three times. Subsequently, pure Co 0.5 Zn 0.5 Fe 2 O 4 nanoparticles were coated with the oleic acid surfactant at a pH maintained within 10-11 using NaOH. To disperse the Co 0.5 Zn 0.5 Fe 2 O 4 nanoparticles in a DI water base fluid, the nanoparticles and base fluid were dispersed by using ultrasonication at 20 kHz for 2 h. The nanofluid was prepared at concentrations of 0 wt%, 0.25 wt%, 0.5 wt%, 0.75 wt%, and 1 wt%. Figure 1 presents a picture of Co 0.5 Zn 0.5 Fe 2 O 4 nanofluid of varying weight concentrations (0 wt %, 0.25 wt%, 0.5 wt%, 0.75 wt%, and 1 wt%). As illustrated in Figure 1, with an observation test, it can be seen that Co 0.5 Zn 0.5 Fe 2 O 4 nanofluid is relatively dark brown compared to the base fluid. The nanofluid samples exhibit good dispersion stability, as sedimentation and separation have not been observed in the suspensions after multiple experiments at various temperatures over the course of two months. For the chemical coprecipitation method, 8.10 gr of FeCl3•6H2O, 1.78 gr of CoCl2•6H2O, and 2.15 gr of ZnSO4•7H2O were dissolved in 225 mL of DI water and heated until the temperature reached 80 °C. A second mixture was prepared using 56 mL of NaOH (4 mol/L) and 18.75 mL of DI water. The second mixture was added dropwise to the first mixture with magnetic mixing at 80 °C. The precipitated Co0.5Zn0.5Fe2O4 nanoparticles were heated at 80 °C. After heating for 1 h, mixture was maintained at room temperature until its temperature reached 25 °C. After chemical reaction and cooling, the Co0.5Zn0.5Fe2O4 nanoparticle precipitation was separated from the reactants using a magnetic bar and washed in DI water at least three times. Subsequently, pure Co0.5Zn0.5Fe2O4 nanoparticles were coated with the oleic acid surfactant at a pH maintained within 10-11 using NaOH. To disperse the Co0.5Zn0.5Fe2O4 nanoparticles in a DI water base fluid, the nanoparticles and base fluid were dispersed by using ultrasonication at 20 kHz for 2 h. The nanofluid was prepared at concentrations of 0 wt %, 0.25 wt %, 0.5 wt %, 0.75 wt %, and 1 wt %. Figure 1 presents a picture of Co0.5Zn0.5Fe2O4 nanofluid of varying weight concentrations (0 wt %, 0.25 wt %, 0.5 wt %, 0.75 wt %, and 1 wt %). As illustrated in Figure 1, with an observation test, it can be seen that Co0.5Zn0.5Fe2O4 nanofluid is relatively dark brown compared to the base fluid. The nanofluid samples exhibit good dispersion stability, as sedimentation and separation have not been observed in the suspensions after multiple experiments at various temperatures over the course of two months. To conduct the experiment satisfactorily, preliminary test was carried out to decide the optimal concentration of the Co0.5Zn0.5Fe2O4 nanofluid. The Co0.5Zn0.5Fe2O4 nanofluid with a lower concentration than 0.1 wt % exhibited insignificant thermal conductivity and photothermal energy conversion performance, therefore it cannot be acceptable. Moreover, Co0.5Zn0.5Fe2O4 nanofluid with a higher concentration caused serious problems over its merits such as high viscosity and low dispersion stability. For example, too high a concentration of nanofluid is too sensitive to the high temperature, thus it can cause pump and pipe clogging in the thermal system.
The crystal structure of Co0.5Zn0.5Fe2O4 nanoparticles was tested using an X-ray diffractometer on the 2θ scale using CuKα radiation (λ = 0.15443 A, 16 mA, and 40 kV) at room temperature, with the beam angle ranging from 20 to 100° and a step time of 1 s. The reflection of the diffraction pattern were classified and investigated by an X'Pert 3 MRD XL diffractometer (Malvern Panalytical, Malvern, United Kingdom). In order to assess the size of Co0.5Zn0.5Fe2O4 nanoparticles, Scherrer's Equation (2) [43] was used. kλ d= βcosθ (2) where d is the diameter of the nanoparticle (nm), k = 0.9 is the Scherrer constant, λ is the wavelength of the X-ray source (0.15406 nm), β is the full width of the powder reflection, and θ is the peak position for reflection. To conduct the experiment satisfactorily, preliminary test was carried out to decide the optimal concentration of the Co 0.5 Zn 0.5 Fe 2 O 4 nanofluid. The Co 0.5 Zn 0.5 Fe 2 O 4 nanofluid with a lower concentration than 0.1 wt% exhibited insignificant thermal conductivity and photothermal energy conversion performance, therefore it cannot be acceptable. Moreover, Co 0.5 Zn 0.5 Fe 2 O 4 nanofluid with a higher concentration caused serious problems over its merits such as high viscosity and low dispersion stability. For example, too high a concentration of nanofluid is too sensitive to the high temperature, thus it can cause pump and pipe clogging in the thermal system.
The crystal structure of Co 0.5 Zn 0.5 Fe 2 O 4 nanoparticles was tested using an X-ray diffractometer on the 2θ scale using CuKα radiation (λ = 0.15443 A, 16 mA, and 40 kV) at room temperature, with the beam angle ranging from 20 to 100 • and a step time of 1 s. The reflection of the diffraction pattern were classified and investigated by an X'Pert 3 MRD XL diffractometer (Malvern Panalytical, Malvern, UK). In order to assess the size of Co 0.5 Zn 0.5 Fe 2 O 4 nanoparticles, Scherrer's Equation (2) [43] was used.
Energies 2020, 13, 6218 where d is the diameter of the nanoparticle (nm), k = 0.9 is the Scherrer constant, λ is the wavelength of the X-ray source (0.15406 nm), β is the full width of the powder reflection, and θ is the peak position for reflection. The size and morphology of the synthesized Co 0.5 Zn 0.5 Fe 2 O 4 nanoparticles were examined via high-resolution TEM image tests. The TEM tests were conducted using a JEM-2100F (JEOL LTD, Akishima, Japan) analytical electron microscope. Using the TEM results, the distribution and diameter of the Co 0.5 Zn 0.5 Fe 2 O 4 nanoparticles in the base fluid (DI water) were analyzed in ImageJ software 1.8.0.
An important point of focus with magnetic nanoparticles is magnetization property that is nanoparticles direction and it can control with a direction of magnetic field. The magnetic properties are categorized based on their reaction in a magnetic field. Magnetization of Co 0.5 Zn 0.5 Fe 2 O 4 nanoparticles tested using an 8600 VSM Series (Lake Shore Cryotonics, Westerville, OH, USA) vibrating sample magnetometer. The applied magnetic field ranged from −10,000 to 10,000 Gs at room temperature (25 • C).
A concern for further development and application of nanofluids is the poor nanofluid dispersion stability, which can aggressively impact the rheological and thermophysical properties of working fluid. A Zeta potential analyzer (ELSZ-2000, Otsuka Electronics Co., Ltd., Osaka, Japan), which works on the dynamic light scattering (photon correlation technique) principle, was utilized to examine dispersion stability of the Co 0.5 Zn 0.5 Fe 2 O 4 nanofluid. To obtain precise data on stability, the samples were tested at least three times at room temperature (25 • C). In addition to the zeta potential test, an observation test was conducted on the samples to inspect the dispersion stability of Co 0.5 Zn 0.5 Fe 2 O 4 nanofluid.

Experimental Setup
An AvaSpec-ULS2048 spectrometer and an AvaLight light source (Avantes, Appledoorn, The Netherlands) were utilized to examine the optical characteristics of the Co 0.5 Zn 0.5 Fe 2 O 4 nanofluid. Tests were conducted in the UV-Vis spectral wavelength range 300-1300 nm at room temperature (25 • C). To verify the reliability of the results obtained, each sample was tested in a 10 mm wide quartz cuvette at least three times.
Of the nanofluid characteristics, viscosity is a particularly critical property, which indicates fluid's resistance to flow and directly affects pressure drop and pump power consumption in a heat transfer process. The viscosity of Co 0.5 Zn 0.5 Fe 2 O 4 nanofluid was measured using an SV-10 viscometer (A&D, Limited, Tokyo, Japan), which uses the tuning fork measurement method. The operating temperature of the test samples was varied from 20 to 50 • C to examine the temperature effect on viscosity of Co 0.5 Zn 0.5 Fe 2 O 4 nanofluid.
KD2-Pro (Decagon Devices, Inc., Pullman, WA, USA) analyzer was utilized to measure the thermal conductivity of the Co 0.5 Zn 0.5 Fe 2 O 4 nanofluid. The KD2-Pro thermal properties analyzer experiments were performed by applying the transient hot-wire technique. As presented in Figure 2, a metal probe was immersed in the test samples, which had full contact with the surrounding test sample. Furthermore, a thermal constant bath was connected to a double jacket beaker to stabilize the inside temperature. The measured data was verified by testing samples at least four times at 20 min intervals. To examine temperature effect on the thermal conductivity of Co 0.5 Zn 0.5 Fe 2 O 4 nanofluid, the temperature of nanofluid was varied from 20 to 50 • C. Figure 3 presents picture and schematics of the PTEC (photothermal energy conversion) experiment setup. The capacity of the sample beaker was 40 mL, and it was placed directly beneath a halogen lamp functioning as a solar simulator. The incident solar irradiance and ambient temperatures were fixed at 650 W/m 2 and 20 • C, respectively. Thermal insulation with 0.025-0.04 W/m· • C was applied to prevent heat loss. To absorb solar energy in the nanofluid, the top of the sample container was made with glass. The temperature gradient of the nanofluid in the sample beaker was examined by using three K-type thermocouples, which was placed at the top, middle, and bottom of the sample container. PTEC efficiency is a measure of how much solar energy is converted to the heat in the sample fluid. The PTEC efficiency can be expressed as Equation (3) [44] Energies 2020, 13, 6218 where m nf and c nf are the nanofluid mass and specific heat, ∆T is nanofluid average temperature increase (∆T = ∆T top + ∆T middle + ∆T bottom ), I is solar intensity, A is area exposed to the solar energy source, and ∆t is the exposure time.
Energies 2020, 13, x FOR PEER REVIEW 6 of 18  The capacity of the sample beaker was 40 mL, and it was placed directly beneath a halogen lamp functioning as a solar simulator. The incident solar irradiance and ambient temperatures were fixed at 650 W/m 2 and 20 °C, respectively. Thermal insulation with 0.025-0.04 W/m•°C was applied to prevent heat loss. To absorb solar energy in the nanofluid, the top of the sample container was made with glass. The temperature gradient of the nanofluid in the sample beaker was examined by using three K-type thermocouples, which was placed at the top, middle, and bottom of the sample container. PTEC efficiency is a measure of how much solar energy is converted to the heat in the sample fluid. The PTEC efficiency can be expressed as Equation (3) [44] nf nf where mnf and cnf are the nanofluid mass and specific heat, ∆ is nanofluid average temperature increase (∆ = ∆ top + ∆ middle + ∆ bottom), I is solar intensity, A is area exposed to the solar energy source, and ∆t is the exposure time.   The capacity of the sample beaker was 40 mL, and it was placed directly beneath a halogen lamp functioning as a solar simulator. The incident solar irradiance and ambient temperatures were fixed at 650 W/m 2 and 20 °C, respectively. Thermal insulation with 0.025-0.04 W/m•°C was applied to prevent heat loss. To absorb solar energy in the nanofluid, the top of the sample container was made with glass. The temperature gradient of the nanofluid in the sample beaker was examined by using three K-type thermocouples, which was placed at the top, middle, and bottom of the sample container. PTEC efficiency is a measure of how much solar energy is converted to the heat in the sample fluid. The PTEC efficiency can be expressed as Equation (3) where mnf and cnf are the nanofluid mass and specific heat, ∆ is nanofluid average temperature increase (∆ = ∆ top + ∆ middle + ∆ bottom), I is solar intensity, A is area exposed to the solar energy source, and ∆t is the exposure time.  The PTEC efficiency of Co 0.5 Zn 0.5 Fe 2 O 4 nanoparticles can also be defined using the specific absorption rate (SAR): the solar energy per unit mass absorbed by the nanoparticles, which can be calculated using Equation (4) [45] where m p and m bf are nanoparticles and base fluid mass sample, respectively; c bf is the base fluid specific heat; ∆T bf and ∆T nf are temperature elevation of the base fluid and nanofluid, respectively, at a given time ∆t.
Energies 2020, 13, 6218 7 of 17 Table 2 presents the specification of measurement sensors used in this study. Thermal conductivity meter with an error of ±5% was used to measure the thermal conductivity and viscometer with an error of 1% was used to measure the viscosity of nanofluid. Besides, the irradiance meter with an error of ±5% was used to measure the irradiance of light. Moreover, the calculated error of measurements was too little, so that it was insignificant to present in this study.

Results and Discussion
A DI water based Co 0.5 Zn 0.5 Fe 2 O 4 nanofluid was prepared via chemical coprecipitation in a two-step manufacturing technique. The following section describes the Co 0.5 Zn 0.5 Fe 2 O 4 nanoparticles characterization. Furthermore, magnetic, morphological, and thermophysical properties of the Co 0.5 Zn 0.5 Fe 2 O 4 nanofluid were experimentally investigated. The PTEC characteristics of the Co 0.5 Zn 0.5 Fe 2 O 4 nanofluid are discussed and compared to those of DI water.

Characterization of Co 0.5 Zn 0.5 Fe 2 O 4 Nanofluid
The synthesized Co 0.5 Zn 0.5 Fe 2 O 4 nanoparticles were examined via XRD, which is typically applied to examine the crystal structure of nanoparticles to identify crystalline forms in material. To perform the test, an X-ray was beamed at the particle and the scattering intensity in an outward direction was measured. Figure 4 presents an outline of the formation of Co 0.5 Zn 0.5 Fe 2 O 4 nanoparticles in this study. All the peak reflections in the XRD pattern were classified and refinement of the lattice parameter is denoted by (111), (200), (211), (300), (310), (311), and (332). The strongest reflection is observed from the (311) plane, indicating cubic spinel structure characteristics [46,47]. To calculate the average and maximum particle diameter of the Co 0.5 Zn 0.5 Fe 2 O 4 nanoparticles, Scherrer's equation [43] was applied, and the maximum and average diameters of the Co 0.5 Zn 0.5 Fe 2 O 4 nanoparticles were 18.07 nm and 14.73 nm, respectively.       The ferrofluid's performance is directly affected by its magnetic characteristics. The VSM spectrum of the Co 0.5 Zn 0.5 Fe 2 O 4 nanoparticles is presented in Figure 6, with the VSM curve indicating that the One of the primary concerns in nanofluid preparation is the prevention of nanoparticle agglomeration and settlement in the nanofluid. The nanofluid dispersion stability is a fundamental property that significantly influences the thermal and rheological characteristics of sample nanofluid.  One of the primary concerns in nanofluid preparation is the prevention of nanoparticle agglomeration and settlement in the nanofluid. The nanofluid dispersion stability is a fundamental property that significantly influences the thermal and rheological characteristics of sample nanofluid. The absolute value of the zeta potential measurement is extensively used in various methods for examining nanofluid dispersion stability. Generally, an absolute zeta potential of >30 mV is considered a reasonable absolute zeta potential value for obtaining a well-dispersed nanofluid. Figure 7 presents the Co 0.5 Zn 0.5 Fe 2 O 4 nanofluid zeta potential analysis. The average absolute zeta potential of the Co 0.5 Zn 0.5 Fe 2 O 4 nanofluid was −54.03 mV, indicating high dispersion stability and that the Co 0.5 Zn 0.5 Fe 2 O 4 nanoparticles were well and uniformly dispersed in the DI water. Intensity (a.u.)

Zeta potential (mV)
1st measurement 2nd measurement 3rd measurement Figure 7. Zeta potential analysis of the Co0.5Zn0.5Fe2O4 nanofluid. Figure 8 illustrates the optical absorption behavior of the Co0.5Zn0.5Fe2O4 nanofluid in the UV-Vis wavelength range of 300-1300 nm, and shows that all samples of the Co0.5Zn0.5Fe2O4 nanofluid exhibit superior optical absorption in comparison with the base fluid. Furthermore, optical absorption spectra of the Co0.5Zn0.5Fe2O4 nanofluid elevated with increasing nanofluid concentration. In all Co0.5Zn0.5Fe2O4 nanofluid samples, the optical absorption decreased around a wavelength of 700-1200 nm, however the optical absorption of DI water increased in this range. It was observed that adding Co0.5Zn0.5Fe2O4 nanoparticles to DI water changes the optical absorption characteristics of the DI water, improving absorption ability in the working fluid.   Table 3 presents the relation between dispersion stability of the nanofluid and zeta potential value [48]. From Table 3, the Co 0.5 Zn 0.5 Fe 2 O 4 nanofluid can be considered as having great dispersion stability based on its absolute zeta potential.  exhibit superior optical absorption in comparison with the base fluid. Furthermore, optical absorption spectra of the Co0.5Zn0.5Fe2O4 nanofluid elevated with increasing nanofluid concentration. In all Co0.5Zn0.5Fe2O4 nanofluid samples, the optical absorption decreased around a wavelength of 700-1200 nm, however the optical absorption of DI water increased in this range. It was observed that adding Co0.5Zn0.5Fe2O4 nanoparticles to DI water changes the optical absorption characteristics of the DI water, improving absorption ability in the working fluid.

Viscosity and Thermal Conductivity of the Co0.5Zn0.5Fe2O4 Nanofluid
The viscosity of working fluid is one of the most important and fundamental properties, which can directly influence the thermal efficiency of system, operating cost, and applications area. Figure   Figure 8. Optical absorption of the Co 0.5 Zn 0.5 Fe 2 O 4 nanofluid at different concentrations.

Viscosity and Thermal Conductivity of the Co 0.5 Zn 0.5 Fe 2 O 4 Nanofluid
The viscosity of working fluid is one of the most important and fundamental properties, which can directly influence the thermal efficiency of system, operating cost, and applications area. Figure 9 presents the viscosity of the Co 0.5 Zn 0.5 Fe 2 O 4 nanofluid according to the nanofluid concentration at different temperatures. Moreover, the viscosity of the Co 0.5 Zn 0.5 Fe 2 O 4 nanofluid increased as the nanofluid concentration increased, in contrast, the viscosity decreased with increasing its temperature.
Energies 2020, 13, x FOR PEER REVIEW 11 of 18 9 presents the viscosity of the Co0.5Zn0.5Fe2O4 nanofluid according to the nanofluid concentration at different temperatures. Moreover, the viscosity of the Co0.5Zn0.5Fe2O4 nanofluid increased as the nanofluid concentration increased, in contrast, the viscosity decreased with increasing its temperature. For instance, the viscosities of 0.25 wt % and 1 wt % Co0.5Zn0.5Fe2O4 nanofluid were 1.03 mPa•s and 1.13 mPa•s, respectively, which increased by 7.3% and 17.7%, respectively, in comparison with that of DI water at 20 °C. The measured viscosity of the water was 0.96 mPa•s and 0.59 mPa•s at 20 °C and 50 °C. It can be assumed that adding Co0.5Zn0.5Fe2O4 nanoparticles to the DI water increases viscosity of the working fluid. Moreover, viscosities of 0.25 wt % and 1 wt % Co0.5Zn0.5Fe2O4 nanofluid decreased by 68.8% and 73.8% when the temperature decreased from 20 to 50 °C, which were 0.61 and 0.65 mPa•s, respectively. This could be explained by the enhanced collision between the nanoparticles in the Co0.5Zn0.5Fe2O4 nanofluid [49]. The maximum viscosity of Co0.5Zn0.5Fe2O4 nanofluid was 1.13 mPa•s at 1 wt % and 20 °C, respectively.
To investigate thermal conductivity and its enhancement of the Co0.5Zn0.5Fe2O4 nanofluid, the conductivity of samples were measured according to temperature and concentration. The decreased by 68.8% and 73.8% when the temperature decreased from 20 to 50 • C, which were 0.61 and 0.65 mPa·s, respectively. This could be explained by the enhanced collision between the nanoparticles in the Co 0.5 Zn 0.5 Fe 2 O 4 nanofluid [49]. The maximum viscosity of Co 0.5 Zn 0.5 Fe 2 O 4 nanofluid was 1.13 mPa·s at 1 wt% and 20 • C, respectively.
To investigate thermal conductivity and its enhancement of the Co 0.5 Zn 0.5 Fe 2 O 4 nanofluid, the conductivity of samples were measured according to temperature and concentration. The enhancement from thermal conductivity at the same temperature can by expressed by Equation (5).
where k bf and k nf are the thermal conductivity of the base fluid and Co 0.5 Zn 0.5 Fe 2 O 4 nanofluid, respectively. The effects of temperature and concentration on the variation of thermal conductivity in the Co 0.5 Zn 0.5 Fe 2 O 4 nanofluid are presented in Figure 10. It has observed that the thermal conductivity improved when the concentration and operating time increased. The thermal conductivity of  Figure 11 illustrates the enhancement of thermal conductivity in the Co0.5Zn0.5Fe2O4 nanofluid with various nanofluid concentrations at 20-50 °C. The enhancement of thermal conductivity in the Co0.5Zn0.5Fe2O4 nanofluids were 3.1%, 4.9%, 7.5%, and 9.2% at concentrations of 0.25 wt %, 0.5 wt %, 0.75 wt %, and 1 wt %, respectively, at 50 °C. Brownian motion is the random collision between nanoparticles intensified under high concentration and temperature, hence improving the activation of nanofluid and heat transfer of the Co0.5Zn0.5Fe2O4 nanofluid.   Figure 11 illustrates the enhancement of thermal conductivity in the Co 0.5 Zn 0.5 Fe 2 O 4 nanofluid with various nanofluid concentrations at 20-50 • C. The enhancement of thermal conductivity in the Co 0.5 Zn 0.5 Fe 2 O 4 nanofluids were 3.1%, 4.9%, 7.5%, and 9.2% at concentrations of 0.25 wt%, 0.5 wt %, 0.75 wt%, and 1 wt%, respectively, at 50 • C. Brownian motion is the random collision between nanoparticles intensified under high concentration and temperature, hence improving the activation of nanofluid and heat transfer of the Co 0.5 Zn 0.5 Fe 2 O 4 nanofluid. Figure 11 illustrates the enhancement of thermal conductivity in the Co0.5Zn0.5Fe2O4 nanofluid with various nanofluid concentrations at 20-50 °C. The enhancement of thermal conductivity in the Co0.5Zn0.5Fe2O4 nanofluids were 3.1%, 4.9%, 7.5%, and 9.2% at concentrations of 0.25 wt %, 0.5 wt %, 0.75 wt %, and 1 wt %, respectively, at 50 °C. Brownian motion is the random collision between nanoparticles intensified under high concentration and temperature, hence improving the activation of nanofluid and heat transfer of the Co0.5Zn0.5Fe2O4 nanofluid.  Figure 12 presents the variation of temperature in the DI water and the Co0.5Zn0.5Fe2O4 nanofluid according to the exposure time for various concentrations. To examine the PTEC efficiency of the Co0.5Zn0.5Fe2O4 nanofluid, the incident solar irradiance and ambient temperature were maintained at The maximum temperature increase for the Co 0.5 Zn 0.5 Fe 2 O 4 nanofluid was 7.7% more than that for the DI water, and it was obtained at 0.75 wt%. This enhanced PTEC performance is primarily caused by the superior thermal conductivity and optical absorption of the Co 0.5 Zn 0.5 Fe 2 O 4 nanofluid. Furthermore, temperature of the Co 0.5 Zn 0.5 Fe 2 O 4 nanofluid elevated as the concentration increased, except for 1 wt% Co 0.5 Zn 0.5 Fe 2 O 4 nanofluid. It can be inferred that this is primarily because the highly concentrated high-temperature nanoparticles in the 1 wt% Co 0.5 Zn 0.5 Fe 2 O 4 nanofluid agglomerated easily. Based on the PTEC test results for the Co 0.5 Zn 0.5 Fe 2 O 4 nanofluid in this study, 0.75 wt% was the optimal concentration for PTEC applications.

PTEC Characterization of the Co0.5Zn0.5Fe2O4 Nanofluid
For a thorough investigation of the PTEC performance, three K-type temperature sensors were placed in the sample container. By measuring the temperature variation of the three thermocouples, which were installed in the vertical direction, the PTEC performance and temperature gradient of the bulk fluid under sunlight can be investigated. Figure 13 presents the temperature gradient of the DI water and Co 0.5 Zn 0.5 Fe 2 O 4 nanofluid with different concentrations. The temperature at the top of the bulk fluid was much higher than that at the middle and bottom due to the nanofluid solar weighted absorption fraction [22]. In this study, the temperature gaps between the top and bottom portions were 14.5 • C, 17.6 • C, 18.2 • C, 18.8 • C, and 18.6 • C, respectively, for DI water and 0.25 wt%, 0.5 wt%, 0.75 wt%, and 1 wt% Co 0.5 Zn 0.5 Fe 2 O 4 nanofluid. In addition, the temperature gap between the top and bottom position rose slightly with the concentration increase. The highest temperature gap was 18.8 • C, which was observed in the 0.75 wt% Co 0.5 Zn 0.5 Fe 2 O 4 nanofluid. The nanofluid concentration with the highest temperature increased the performance of PTEC. absorption fraction [22]. In this study, the temperature gaps between the top and bottom portions were 14.5 °C, 17.6 °C, 18.2 °C, 18.8 °C, and 18.6 °C, respectively, for DI water and 0.25 wt %, 0.5 wt %, 0.75 wt %, and 1 wt % Co0.5Zn0.5Fe2O4 nanofluid. In addition, the temperature gap between the top and bottom position rose slightly with the concentration increase. The highest temperature gap was 18.8 °C, which was observed in the 0.75 wt % Co0.5Zn0.5Fe2O4 nanofluid. The nanofluid concentration with the highest temperature increased the performance of PTEC.   Figure 14 presents the PTEC efficiency of the DI water and the Co0.5Zn0.5Fe2O4 nanofluid at different concentrations. The PTEC efficiency of the Co0.5Zn0.5Fe2O4 nanofluid grows with an increase of concentration. After 120 min, the lowest PTEC efficiency was 47.9%, which was observed in the DI water. At the time, the PTEC efficiency of the Co0.5Zn0.5Fe2O4 nanofluid was 52.7%, 53.5%, 55%, and 52.2% at concentrations of 0.25 wt %, 0.5 wt %, 0.75 wt %, and 1 wt %, respectively. The increase in PTEC efficiency of the Co0.5Zn0.5Fe2O4 nanofluid was 4.8%, 5.6%, 7.1%, and 4.1% at concentrations of 0.25 wt %, 0.5 wt %, 0.75 wt %, and 1 wt %, respectively, in comparison with DI water. The highest PTEC efficiency of Co0.5Zn0.5Fe2O4 nanofluid was 55%, and was observed at a concentration of 0.75 wt %. As shown in Figure 12, the temperature in the Co0.5Zn0.5Fe2O4 nanofluid increased as the concentration increased, except for concentration of 1 wt % Co0.5Zn0.5Fe2O4; and the similar trend was observed in the variation in the PTEC efficiency.  Figure 14 presents the PTEC efficiency of the DI water and the Co 0.5 Zn 0.5 Fe 2 O 4 nanofluid at different concentrations. The PTEC efficiency of the Co 0.5 Zn 0.5 Fe 2 O 4 nanofluid grows with an increase of concentration. After 120 min, the lowest PTEC efficiency was 47.9%, which was observed in the DI water. At the time, the PTEC efficiency of the Co 0.5 Zn 0.5 Fe 2 O 4 nanofluid was 52.7%, 53.5%, 55%, and 52.2% at concentrations of 0.25 wt%, 0.5 wt%, 0.75 wt%, and 1 wt%, respectively. The increase in PTEC efficiency of the Co 0.5 Zn 0.5 Fe 2 O 4 nanofluid was 4.8%, 5.6%, 7.1%, and 4.1% at concentrations of 0.25 wt%, 0.5 wt%, 0.75 wt%, and 1 wt%, respectively, in comparison with DI water. The highest PTEC efficiency of Co 0.5 Zn 0.5 Fe 2 O 4 nanofluid was 55%, and was observed at a concentration of 0.75 wt%. As shown in Figure 12, the temperature in the Co 0.5 Zn 0.5 Fe 2 O 4 nanofluid increased as the concentration increased, except for concentration of 1 wt% Co 0.5 Zn 0.5 Fe 2 O 4 ; and the similar trend was observed in the variation in the PTEC efficiency.
PTEC efficiency of the Co0.5Zn0.5Fe2O4 nanofluid was 4.8%, 5.6%, 7.1%, and 4.1% at concentrations of 0.25 wt %, 0.5 wt %, 0.75 wt %, and 1 wt %, respectively, in comparison with DI water. The highest PTEC efficiency of Co0.5Zn0.5Fe2O4 nanofluid was 55%, and was observed at a concentration of 0.75 wt %. As shown in Figure 12, the temperature in the Co0.5Zn0.5Fe2O4 nanofluid increased as the concentration increased, except for concentration of 1 wt % Co0.5Zn0.5Fe2O4; and the similar trend was observed in the variation in the PTEC efficiency.   Energies 2020, 13, x FOR PEER REVIEW 15 of 18 Figure 15 presents the variation in the SAR according to exposure time at different concentrations of the Co0.5Zn0.5Fe2O4 nanofluid. The maximum SAR was achieved during the initial stage at 20 min, and the SAR decreased with exposure time for all concentrations. Furthermore, the SAR decreased rapidly with an increase in Co0.5Zn0.5Fe2O4 nanofluid concentration, which is similar to the previous studies on the SAR of different nanofluids [44,45]. The highest SAR for the Co0.5Zn0.5Fe2O4 nanofluid was 1.34 kW/g at a concentration of 0.25 wt % and an exposure time of 20 min. The SAR for the 0.25 wt % and 1 wt % Co0.5Zn0.5Fe2O4 nanofluid decreased from 1.34 and 0.44 to 0.61 and 0.13 kW/g, respectively, under an incident solar irradiance of 650 W/m 2 .

Conclusions
In this study, Co0.5Zn0.5Fe2O4 nanoparticles were manufactured by using the chemical coprecipitation technique. The Co0.5Zn0.5Fe2O4 nanoparticles were suspended in DI water (the base fluid) at different weight concentrations (0.25 wt %, 0.5 wt %, 0.75 wt %, and 1 wt %). TEM and XRD experiments were conducted to examine the nanostructure of the Co0.5Zn0.5Fe2O4 nanoparticles. Furthermore, the magnetic characteristics were examined using a VSM test at 25 °C. The thermal characteristics of the Co0.5Zn0.5Fe2O4 nanofluid were investigated, including absolute zeta potential, optical absorption, viscosity, thermal conductivity, and PTEC. The following results were obtained: • From the XRD results, the average and maximum diameter of the Co0.5Zn0.5Fe2O4 particles were 14.73 nm and 18.7 nm, respectively. The saturation magnetization of the Co0.5Zn0.5Fe2O4

Conclusions
In this study, Co 0.5 Zn 0.5 Fe 2 O 4 nanoparticles were manufactured by using the chemical coprecipitation technique. The Co 0.5 Zn 0.5 Fe 2 O 4 nanoparticles were suspended in DI water (the base fluid) at different weight concentrations (0.25 wt%, 0.5 wt%, 0.75 wt%, and 1 wt%). TEM and XRD experiments were conducted to examine the nanostructure of the Co 0.5 Zn 0.5 Fe 2 O 4 nanoparticles. Furthermore, the magnetic characteristics were examined using a VSM test at 25 • C. The thermal characteristics of the Co 0.5 Zn 0.5 Fe 2 O 4 nanofluid were investigated, including absolute zeta potential, optical absorption, viscosity, thermal conductivity, and PTEC. The following results were obtained: In the case of a magnetic nanofluid, it is important to develop a new alternative method to synthesize nanoparticles over previous complicated techniques that require expensive equipment. Moreover, it is necessary to study to replace the expensive and complex manufacturing nanoparticles and a method to secure high dispersion stability of the magnetic nanofluid in thermal systems. Presumably, the downside of the Co 0.5 Zn 0.5 Fe 2 O 4 nanofluid is a high viscosity, which can increase the pressure drop and pump power consumption in a heat transfer system. The magnetic field effect on thermal characteristics and PTEC performance of Co 0.5 Zn 0.5 Fe 2 O 4 nanofluid will be the focus of a forthcoming study.