Magnetically-accelerated photo-thermal conversion and energy storage based on bionic porous nanoparticles

Recently, the technology of mixing phase change materials with high thermal conductivity fillers was developed, which has allowed thermal energy storage to be implemented in a wide range of industrial technologies and processes. In the present study, a hierarchical bionic porous nanocomposite was prepared, which efficiently merged the nanomaterial characteristics of magnetism and high thermal conductivity in order to form a magnetically-accelerated solar-thermal energy storage method. The morphology and thermo-physical properties of materials were analysed. The experimental outcomes of phase change heat transfer demonstrated that the maximum storage efficiency increases by 102.7% when the hierarchical bionic porous structure is used, and a further 27.1% improvement can be achieved with the magnetic field. At the same time, the heat transfer process of energy storage in hierarchical porous composites under external physical fields is explained by simulation. Therefore, this magnetically-accelerated method demonstrated the superior solarthermal energy storage characteristics within a hierarchical bionic porous structure which is particularly beneficial for the utilisation of solar direct absorption collectors and energy storage technology.


Introduction
The world's energy consumption has increased, due to the drastic development of human society which has caused global warming and environmental pollution [1,2]. As an excellent renewable energy source which can substitute fossil fuels, solar energy has successfully gained attention due to its environmental friendliness and lower cost [3][4][5][6]. Solar-thermal conversion depends on direct absorption solar collectors and has become a good method for harnessing solar energy [7][8][9][10]. In addition, it is the main cause of spreading industrial applications, such as thermal energy storage and electricity and steam generation [11]. Solar-thermal energy storage is considered to be a key feature in sustainable solar-thermal conversion requests [12]. Through improvement in heat storage capacity and solar absorption ability, this is important for widely applying solar-thermal energy storage [13].
Therefore, thermal energy storage structures and materials are two important aspects which are often explored by researchers.
When it comes to energy storage structures, pore-based shape-stabilised composite is more obtainable and profitable for exploring other excellent properties such as thermal conduction and flame retardancy comparing with a single organic or inorganic material, eutectics and mixtures and encapsulated phase change materials (PCMs) [14,15]. Currently, the hierarchical porous material is considered as a developing type of porous material which possesses a variety of levels of structure and porosity [16]. This helps in presenting the unique scale benefits from micro pores to a macro level [17].
Due to their integrated hierarchical porosity with a variety of length scales, they are quite suitable for mass loading and diffusion, electron and ion transport, and light-harvesting, because they are used for converting solar and chemical energy on a wider level [18,19]. The hierarchical porous architecture causes degradation to the structure of energy shortage and fast heat transfer incurred by volume expansion during the charge cycle process [20]. This process is also important for light-harvesting via multiple light reflections and scattering [21]. However, the machines of effect of hierarchically porous materials on their functions are not only simple micro-scale structures, but they also include structural parameters at different scales and interactions with each other [22]. The theory of flow and heat transfer in hierarchically porous materials has not been reported systematically, and there are still great challenges which must be faced in the development of hierarchically porous materials for thermal energy storage [23].
Furthermore, thermal energy storage is achieved due to a change in internal energy material, such as chemical energy, latent heat, and sensible heat [24,25]. From the existing thermal energy shortage methods, latent heat storage is considered as a reliable and efficient method due to the solid-liquid phase material change [26][27][28][29], mostly due to the fact that it has a high density of thermal storage in a small volume change as well as a small temperature region during thermal energy storage [30,31].
Phase change materials have the ability to absorb latent heat at the time of transition into liquid from a solid which is appropriate for storing solar-thermal energy [32,33]. In liquid PCMs, when nanoparticles are added, this results in nanofluid PCMs which possess high storage capacity and heat transfer, compared to one-component PCMs, such as paraffin wax and hydrated salts [34][35][36]. For example, a research experiment was conducted in order to determine the effects of nickel nanoparticle mass concentration on the performance of phase change in PCMs by Oya et al. [37]. The percolation clusters caused a high upsurge in determined heat conduction. Nourani et al. [38] created an original paraffin/Al2O3 mixture which exhibited enhanced thermal conductivity and reliability. However, very little research has been conducted on solar-thermal PCM systems on the basis of nanofluid PCMs, allowing separate expansion of the conversion of solar-thermal efficiency after applying the external physical fields, such as magnetic, sound, electric field [39][40][41], let alone studying the heat transfer process of solar-thermal storage and energy conversion in hierarchically porous structures under external physical fields.
In this study, hierarchically bionic porous phase change materials were prepared by imitating natural systems combining superior thermal conductivity and phase change characters. The prepared hierarchically bionic porous phase change materials were considered, then the thermo-physical properties and optical properties were discovered. Therefore, a magnetically-accelerated solar-thermal energy storage method was suggested. Solid-liquid phase transition research with hierarchically bionic porous phase change materials was conducted, comparing to pure paraffin wax. In addition, the efficiency and storage capacity of various heat transfer performances were determined for the evaluation of the influence of the hierarchically porous structure and magnetism on phase change performances. Finally, the heat transfer process of photo-thermal energy conversion and storage in hierarchically porous materials under an external physical field was verified by simulation.

Material preparation
To create an ultrapure water system in the lab, deionized (DI) water purification was performed, along with other experiments (Arium-mini plus, Sartorius, Germany). Firstly, the iron (III) chloride hexahydrate was thoroughly mixed for 10 minutes in the DI. Next, sodium citrate was added and mixed in the suspension and urea by ultrasonic agitation. Later, the suspension and polyacrylamide were mixed together completely for 15 min. A100 mL flask set at 160°C immersed in a water bath for 8h was used to transfer the final solution. Subsequently, ethanol was used to wash the black precipitates magnetic Fe3O4 composites through magnetic attraction prior to oven drying at 50 °C for 12 h. The MF nanoparticles were gradually mixed into the 40 mmol titanium (IV) tetrafluoride solution which was then stirred for 10 minutes. After this, the solution was moved into and placed in a Teflon-sealed autoclave and maintained at a temperature of 180 °C for 48 h. Finally, the poriferous magnetic TiO2 (MT) was stirred vigorously in liquid paraffin wax (PW) within 5 wt.% to prepare the poriferous paraffin@magnetic TiO2 (PMT). The MF was stirred vigorously in liquid paraffin wax within 5 wt.% to prepare the poriferous paraffin@magnetic Fe3O4 (PMF). All above drugs were purchased from Aladdin Reagent (Shanghai, China) within the analytical reagent grade and were used as received.

Characterisation
To further confirm the particles, the MF and MT nanomaterials were characterised by the scanning of microscopy regarding electrons (SEM, Zeiss Supra 55, Germany) along with transmission electron microscopy (TEM, 2010-JEM, Japan). An X-ray diffraction (XRD) pattern was gained using an X-ray diffractometer (AXS-Bruker GmbH, Germany, D8-Advance). Brunauer-Emmett-Teller (BET) was used in order to gain analysis of the particle size and surface area of the pores (Quantachrome Autosorb-1C-VP, US). Magnetism of nanoparticles was achieved by using a vibrating sample magnetometer (SQUID VSM-MPMS, Design Quantum, USA). The PCM's thermo-physical properties, thermal conductivities, and particular heat volume and optical properties were determined using an ultraviolet-visible-near-infrared spectrophotometer (5000-CARY, Technology of Agilent, USA), a laser thermal diffusivity instrument (LFA 457, Netzsch, Germany), and a differential scanning calorimetry system (F1-204, Netzsch, Germany).

Experimental setup
The solar-thermal energy storage method is inspired by imitating natural systems in the ocean.
Diatoms can track light sources through the holes in pores in order to capture solar energy by reflecting less sunlight. The silk network structure of diatoms not only prevents incident light from escaping and enhances their absorption, but it also enhances their adsorption and storage capacities within flexible and stable structures (Figure 2a). Based on this, bionic phase change composites within a hierarchical porous structure were prepared and a magnetically-accelerated method was developed for energy storage and solar-thermal conversion. The experimental setup is shown in Figure 2b: the light source used in the experiment was a sunlight simulator (CEL HXF300, Ceaulight, China) with a constant solar intensity (1000 W/m 2 ), a magnetism generator (ELE-P80, Elecall, China) whose intensities were measured by a magnetometer (M943, Honor Top of Magnetic Technology, China); Test chamber (an acrylic beaker), thermocouples, a data collector, and a computer form a data collection system. The chamber had a 3.0 cm height and 4.0 cm diameter. In addition, temperature readings were obtained from the five thermocouples inserted in the middle of the sample to record temperature changes and noted by a data collection unit (CA34972, Technology of Agilent).   Figure 4a shows the specific heat capacity in the phase change regions for different phase change composites with the temperature of 10~110°C, which was used to calculate the energy storage capacity.
The value of the specific heat of solid (2.56 J/(ºC·g)) is less than that of liquid (2.97 J/(ºC·g)) of the PW, which are similar to the standard specific heat of paraffin wax (2.60 J/(ºC·g) and 2.89 J/(ºC·g)).
It can be seen that the specific heat of the phase change composites had no obvious change during the melting and freezing process when adding nanoparticles. This indicates that magnetic nanoparticles within small mass concentration in phase change materials have a weak influence on heat storage capacity. Figure 4b displays the thermal conductivities of the PW, PMF and PMT over a temperature range of 30~90°C. The outcome showed a vibrant change in thermal conductivity when the nanoparticles were inserted into both liquid and solid forms of paraffin wax. PMT had a higher thermal conductivity, compared to PMF. PW had greater transmission in the instance of the liquid state in the noticeable range of light (Figure 4c), but the transmission of PMF and PMT was close to zero, due to the robust photo-absorption capacity of the nanostructure. It also can be seen that PMT has superior optical absorption performance. The transmission of different weight percentages of PMT were further characterized (Figure 4d). It can be seen that the transmittance of PMT was decreased with the increase of mass concentration at first, and it has no change when further increasing the mass concentration of nanoparticles, which is in order to determine the best mass concentration (5 wt. %) to conduct the solar-thermal conversion and phase change experiments [2,5].

Solar-thermal conversion and phase change characteristics
In order to further examine the solar-thermal conversion and heat transfer performance of phase change composites, both were determined based on the temperature variation and measured particular heat of the PCMs [2,21]: where Qe is energy conversion capacity (storage or discharge) at time t, S is the direct solar area, qsolar is the solar radiation power, m is the mass, cp is the specific heat capacity, T is the temperature of the PCMs, and ηs and ηr are the heat storage/discharge efficiency. Optimal heat transfer and solar absorption are two requirements for achieving high energy conversion efficiency during the solarthermal conversion process. Therefore, based on the magnetism and absorbance of the bionic hierarchical porous materials, the effect of magnetic nanoparticles on phase change characteristics was investigated, and the mechanism of magnetically enhanced solar-thermal conversion was researched.

Influence of the magnetic nanoparticles on phase change characteristics
The phase change composites melt robustly via straight solar radiation and the localized heating area shifts downwards by heat conduction. The experiments of phase change characteristics under solar illumination with PMF and PMT composites were conducted and compared with PW. Figure 5a shows the change in temperature at the time of changing processes. Compared to PW and PMF, the temperature of PMT increases rapidly during the charging process and has a higher steady temperature

Enhanced magnetically solar-thermal conversion process
Under simulated solar irradiation, when a solid PMT converts the liquid phase and also following a reduction in viscosity, the particles within magnetism become firmly combined to the paraffin. At  Figure 6c, the magnetically controlled heat shift method reflected the improved storage capability, approximately 20.3% greater than that without magnetic field. The thermal-solar storage efficiencies exhibited rapid charging rate improvement in the initial stage with magnetic field strength and the highest storage efficiency was approximately 27.1% greater than that without magnetic field, as shown in Figure 6d. In order to further explain the mechanism of magnetically enhanced solar-thermal conversion, a two-dimensional mathematical model was created using commercial software assuming Newtonian, laminar and incompressible flow. The size and boundary conditions used for the simulation were consistent with the experiment. Material properties were applied to simulation in the form of piecewise functions of experimental values. From the simulation results and using the finite element method (Figure 6e), the PMT's changes in temperature exhibited similar features to the research outcomes. By applying the magnetic field, the PMT temperature increased rapidly. The velocity distribution and magnetic field of PMT liquid showed that the circulation was initiated by using volumetric magnetism power. In addition, the normal volumetric magnetic force formed at the phase interface also increased the melting of paraffin.  Figure 6f with error bar. It can be seen that the numerical average temperature of the PMT was similar to the outcome of the experiment, and the errors are 1.7% without magnetic field and 1.2% with magnetic field.

Conclusion
In the current study, phase change materials with hierarchical bionic porous were formed with the help of a two-step method with paraffin wax and porous magnetic nanoparticles. The morphology,