A novel volumetric absorber integrated with low-cost D-Mannitol and acetylene-black nanoparticles for solar-thermal-electricity generation

Solar Energy Materials & Solar Cells 207 (2020) 110366

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A novel volumetric absorber integrated with low-cost D-Mannitol and acetylene-black nanoparticles for solar-thermal-electricity generation Xusheng Zhang a, b, 1, Zheng Du a, b, 1, Yudong Zhu a, c, 1, Chuan Li b, Xianfeng Hu a, Tingbin Yang d, Bin-Bin Yu a, e, Rui Gu d, Yulong Ding b, Zhubing He a, * a

Department of Materials Science and Engineering, Shenzhen Key Laboratory of Full Spectral Solar Electricity Generation (FSSEG), Southern University of Science and Technology, No. 1088, Xueyuan Rd, Shenzhen, 518055, Guangdong, China School of Chemical Engineering, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK c Department of Electronic and Computer Engineering, The Hong Kong University of Science and Technology Clear Water Bay, Kowloon, Hong Kong SAR, China d Southern University of Science and Technology Core Research Facilities, No. 1088, Xueyuan Rd, Shenzhen, 518055, Guangdong, China e Academy for Advanced Interdisciplinary Studies, Southern University of Science and Technology (SUSTech), Shenzhen 518055, China b

A R T I C L E I N F O

A B S T R A C T

Keywords: Volumetric absorber Solar-thermal-electricity generation Phase change materials D-mannitol Acetylene black nanoparticles

Low-medium temperature phase change materials (PCMs) act as a predominant role in one-sun solar thermal energy conversion and storage, attracting increasing attention in recent years. However, highly thermal conductive PCMs relying on the doping of a high dose of thermally conductive particles would limit the light absorption just at its top surface and decrease its bulk thermal storage density. In this work, we designed a volumetric solar absorber integrated with a D-Mannitol (DM) based PCM loaded with a low concentration (0.05 wt%) of acetylene black nanoparticles other than a high concentration of over 5 wt%. Such a method maintains a high phase change enthalpy of DM (295.2 kJ/kg) while achieving an enhanced heat transfer and hence a uniform temperature distribution. Compared to conventional surface absorbers with PCM, the volumetric absorber significantly reduces the temperature difference inside the bulk materials, from 103.1 � C of the surface absorber to 62.7 � C of the volumetric absorber with a thickness of 10 mm. Their photo to thermal charge and discharge performance was investigated for the first time in their based solar thermoelectric generator (STEG) system, which show that the volumetric absorber could reach 198.2 � C on bottom of the PCM and the corresponding open circuit voltage is 0.65 V. Such performance data are significantly better than the corresponding results from the surface absorber of 135.1 � C and 0.45 V, respectively, demonstrating successfully the significant performance enhancement by our volumetric absorber. This paper definitely deepens our understanding of volumetric ab­ sorbers and their based device design and applications in solar-thermal-electricity systems.

1. Introduction As one of the most important renewable and clean energy, solar energy would dominate a considerable share of the future electricity market. However, due to the native demerit of solar light dependent electricity output, photovoltaics can’t supply the market the stable electricity output and herein is scare of energy dispatching ability. As another important part of solar energy conversion, solar thermal can store solar energy as heat and own strong dispatching ability meeting the time dependent demand for electricity. As the key component in the solar thermal electricity generation (STEG), thermal energy storage

(TES) plays a significant role in that system and has attracted tremen­ dous attention in recent years. Phase change materials (PCMs) act as the key material in TES systems, along with another critical component, solar light absorber, owing to its considerable energy storage density, scalable capacity and low cost [1,2]. With different melting temperatures, PCM based TES can be used for STEG systems operated at different temperature ranges. The mediumtemperature STEG systems have attracted increasing attention owing to their significantly lower costs in a one-sun working environment than that of concentrated solar systems [3,4]. For a targeted temperature range of 150–200 � C, D-Mannitol (DM), a non-toxic and relatively

* Corresponding author. E-mail address: [email protected] (Z. He). 1 Equal contributions to this work. https://doi.org/10.1016/j.solmat.2019.110366 Received 6 April 2019; Received in revised form 1 October 2019; Accepted 15 December 2019 Available online 26 December 2019 0927-0248/© 2019 Elsevier B.V. All rights reserved.

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low-cost sugar alcohol, is regarded as one of the most promising PCMs, which has a melting temperature of approximately 165 � C–167 � C [5–8]. More importantly, DM has both a high melting enthalpy of 328.8 kJ/kg [9] and a high density of 1520 kg/m3 [10]. However, it suffers from an inherently low thermal conductivity, which can result in slow heat diffusion and uneven melting. Doping with high thermal conductivity nanoparticles has been extensively adopted in PCMs, which can defi­ nitely improve the thermal conductivity of DM composite [11,12]. On the other hand, compared with PCM, the solar light absorber in the conventional STEG system often has such complex surface structure as metasurface, complicated fabrication process and herein high cost [4, 13,14]. The separate solar light absorber also suffers from limited inci­ dent flux density because of limited superficial surface area, and high heat loss due to considerable re-emission [15]. Simultaneously, the heat transfer between the superficial solar light absorber and the underlying PCM bulk definitely leads to heat loss and so called degraded thermal conductivity at the interface [16,17]. Regarding the above concerns, the conventional structure in TES systems combining the surface solar light absorber and the underlying PCM storage component needs destructive re-design by advantage of PCM only because PCM can not only absorb solar light but also store and transfer light after some composition en­ gineering [18–20]. Beyond any question, the current SA mode is not valid for PCM. In fact, volumetric absorption (VA) directly by PCM can take that responsibility of both absorption and storage. As predicted, VA may offer a 5–10% augment in photo-to-thermal conversion efficiency [21,22], owing to the remarkable reduction in the surface temperature of the absorbers [18,23]. That can decrease in return both radiative and convective heat losses in practical operations [24]. Recently, Fe3O4@Graphene was successfully applied to enable the VA of paraffin wax [18], which demonstrates a typical example for the VA of PCMs in TES systems. Hence, introducing high thermal conductivity dopants in the DM like matrix may also enhance volumetric light absorption along with improved heat transfer in PCM bulk, as well as a reduction in the system cost [25–28]. Over the past few years, many carbon-based nanomaterials have been employed to improve the direct light absorption of PCMs along with their thermal conductivities; examples include carbon nanotube [29–31], graphite and graphene foams [32,33], organic dyes [34], inorganic oxides [25,35,36], and metal nanomaterials [37], have also applied to realize the desired functions of PCMs. However, a minimal amount of work has been done on this topic, particularly in terms of achieving an effective VA of PCMs at low costs over the medium-temperature range. To address the concerns discussed above, we developed a DM based PCM with acetylene black nanoparticles as dopants to realize a volu­ metric solar absorber in a facile and low-cost way. Such key thermal properties as melting points, melting enthalpies, subcooling tempera­ ture, thermal conductivity were characterized in-detail. More impor­ tantly, light absorption along with the corresponding thermal collection effect of those doped DM series were carefully investigated. The DM composites with suitable doping concentration exhibit obvious VA fea­ tures and own significant advantages in the photo to thermal conversion performance over the SA samples. Thereafter, STEG systems based on those composited DMs were designed, built-up and served the valuable performance data of them in both VA and SA mode.

from ALANOD GmbH & Co. KG in Germany, which was cut into an appropriate size for use in the work. 2.2. Preparation of acetylene black nanoparticles @DM composite In a typical experiment, the acetylene black powder was thoroughly mixed with the DM powder to give mixtures with acetylene black con­ centrations of 0.005 wt%, 0.05 wt%, 0.5 wt% and 5 wt%, which are noted as AcM-1, AcM-2, AcM-3, and AcM-4 respectively. The mixtures were then exposed to the Ozone for 2 h in a UV/Ozone Cleaner (NOVASCAN model PSD, United States) to increase the bonding force. The mixtures were balled milled with a planetary ball mill filled with a mixture of Al2O3 balls with three different diameters of 10 mm, 8 mm and 5 mm with a weight ratio of 1:1:2 (Changsha MITR instrument, YXQM-2L, China). To prevent agglomeration during ball milling, 500 μL ethanol (AR 98%) and 10 g treated composite are loaded in a milling jar. The milling speed was fixed at 250 rpm. It takes 3 h to produce the desired acetylene black nanoparticles @DM composite powder. The powders were recrystallized by heating to the melting temperature and cooling to room temperature before use in the experiments. 2.3. Characterizations A Scanning Electron Microscope (SEM) analysis was done on the samples with a field-emission scanning electron microscopy (ZEISS MERLIN, Germany) operated at 3 kV accelerating voltage. The crystal­ lographic phases of the samples were determined with an XRD (Bruker D8 Advance Diffractometer, United States), equipped with the Cu-Kα radiation source and operated at 40 kV. The thermal characterization of the samples was done with a DSC (Mettler Toledo, United States). The DSC was operated at a heating/ cooling rate of 10 � C/min under N2 atmosphere. The thermal stability was studied using thermogravimetric analysis (TGA) function of the DSC, operated between the room temperature to 800 � C at a heating rate of 10 � C/min in an N2 atmosphere. Thermal conductivities of the sam­ ples were determined with a Hot Disk Thermal Constant Analyser (TPS 2500, Sweden) at the room temperature. Both the absorbance of liquid and solid samples were measured from 300 to 2500 nm in a UV-VIS-NIR spectrometer (PerkinElmer Lambda 950, Unites States) which has a heating element installed on the wall of the sample holder. 2.4. Solar thermal experiments To study solar light to thermal conversion, the samples were loaded in quartz cuvettes with 10 beam path length, and then put onto an expandable polystyrene (EPS) holder, as shown in Fig. S1a. The samples were tested under a solar simulator with an AM1.5 filter (Abet Tech­ nology Sun 3000, United States). The heating up process of composite samples was investigated by a thermal detector (FLUKE 480 Pro, United States) to record the temperature distribution. To further investigate the photo to thermal conversion difference between surface absorption (SA) and VA modes, solar to thermal ex­ periments were performed. In a typical experiment, a quartz cuvette filled with pure AcM-2 was taken as a VA sample; another quartz cuvette, also filled with pure AcM-2 but covered at the top surface by a piece of commercial absorber was taken as a SA sample. Both samples were directly placed under a solar simulator for a duration of 400s. During the experiment, solar irradiation was focused and fixed at approximately 915 mW/cm2, and the optical power calibration process is shown in Fig. S1b and Table S1.

2. Method and materials 2.1. Chemicals and reagents D-mannitol (CAS number: 69-65-8, AR 98%) and ethanol (CAS number: 69-65-8, AR 98%) were purchased from Aladdin Reagent (Shanghai) Co., Ltd. acetylene black powder (diameter: 30–50 nm) was purchased from Yingxin Laboratory. All the raw materials were used as received without further purification. A commercial solar absorber eta plus® film coated on a 1.0 mm thick aluminium film was purchased

2.5. Fabrication and characterizations of STEG device An aluminium container without cover was designed to load the sample and placed at the top of the device. A Bi–Te based commercial 2

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thermoelectric module (Thermonamic TEHP1-1264-0.8, China) was integrated into the STEG system as an electricity generator. A finned cooler was attached at bottom to cool down the cold side of the ther­ moelectric module and maintained the temperature at 25 � C. Aluminium fixtures were designed to assemble the device, and each part was divided by a 4 mm thick copper spreader. To decrease the thermal contact resistant, thermal grease (PC Cooler A1, ~7.5 W/mK) was applied to each surface. At last, the surrounding of the device was insulated by glass fiber and EPS, to minimize the heat loss to the environment. The as-fabricated device was placed under the solar simulator, and a Fresnel lens was used to concentrate the light. An optical power meter (CEAULIGHT CEL-NP2000, China) was used to calibrate the focused light power density, which is approximately 1550 w/cm2 and the optical power calibration process was shown in Fig. S1b and Table S2. The thermocouples were embeded in the centre of the copper plates on the top of the thermoelectric module to measure temperature. The output of the thermoelectric module was connected to a digital source multimeter (Keithley 2420, United States) to measure the open-circuit voltage.

listed in Fig. 2a for reference. All the XRD patterns are identical to that of the pure DM [40], and no impurity peak turns up, which confirms that this doping does not change the chemical structure of DM itself and hence keeps its merits for the thermal storage over the medium tem­ perature range. The phase change and crystallization behaviour of PCMs are of great importance to temperature control. The melting–freezing DSC curves of the pure and doped DM are shown in Fig. 2b, which can determine the melting temperature and enthalpy, crystallization temperature, and enthalpy of the PCM used in this work. For the pure DM (dark yellow line), the melting onset (Tmo) and peak (Tmp) temperature are observed at 165.0 � C and 167.4 � C, which is identical to the value reported by A. Mojiri [7], The melting enthalpy (ΔHm) is calculated to be 295.2 kJ/kg, which is comparable to the values reported previously [7,10,41]. Compared with the pure DM, Tmo and Tmp are slightly shifted after introducing of acetylene black particles. The Tmo for AcM samples are observed at 165.3 � C, 165.4 � C, 166.2 � C and 166.5 � C for AcM-1, AcM-2, AcM-3 and AcM-4, respectively, while the Tmp are 168.0 � C, 167.7 � C, 168.8 � C and 169.2 � C for AcM-1, AcM-2, AcM-3, and AcM-4, respectively. The melting enthalpies of AcM series are 293.8 kJ/kg, 287.8 kJ/kg, 277.8 kJ/kg and 270.3 kJ/kg from AcM-1 to AcM-4, which declines with increasing acetylene black particle concentration due to acetylene black particles do not undergo a phase change under this temperature range [26,42]. Fig. 2b also shows the crystallization DSC curves for the pure DM and AcM series. The pure DM shows an onset crystallization temperature (Tco) of 109.4 � C, which is also consistent with previous reports [7]. Such a huge subcooling is undesirable [43,44], which implies solidifi­ cation at a temperature much lower than its melting point and hence leading to a downgrade of thermal energy produced [5,7,45]. The crystallization enthalpy (ΔHc) can be calculated as 213.0 kJ/kg, which is in line with the recent reports [42]. When the subcooled DM is in a metastable liquid state, activation energy must be provided to start nucleation. After nuclei grow to a critical size, a new crystal phase will form spontaneously. Followed by the crystallization, the stored heat will release to the system [46]. This subcooling should be prevented to reduce the temperature decrease during discharge and ensure the heat to be released at the desired temperature [47,48]. Fortunately, the onset crystallization temperatures are found at 119.6 � C, 118.7 � C, 123.0 � C, and 122.4 � C, respectively, The degree of subcooling temperature (ΔTsub) is calculated from the difference between melting onset temperature and crystallization onset temperature. The pure DM exhibits a maximum degree of subcooling

3. Results and discussion SEM images shown in Fig. 1a–e illustrate the surface morphologies of the pure and doped DM with different concentrations of acetylene black nanoparticles from 0.005% to 5% in an exponential increment. In contrast to the pure DM (Fig. 1a), acetylene black nanoparticles are visible in the bulk of doped PCMs after ball milling. As expected, the particles loading increases with an increase in the doping concentration from AcM1 to AcM 4. No visible aggregation of acetylene black nano­ particles is found in AcM-1 and AcM-2 samples possibly due to sufficient spacing between adjacent nanoparticles in the two samples. However, a significant reduction in the spacing between neighboring particles is observed with increasing acetylene black nanoparticle contents, espe­ cially AcM-4, where a large number of particles have agglomerated. Also, cross-sectional SEM images, as shown in Fig. S2, demonstrate a similar dense decoration of the acetylene black nanoparticles for the AcM samples. Fig. 2f shows the photographs of the re-melted DM and AcM samples. The colour turns darker and darker with an augmentation in the concentration of the acetylene black nanoparticles while cloudwhite colour is seen for the pure DM sample. As shown in Fig. 2a, the XRD pattern shows the DM crystal is β-phase [38], with a needle-like shape [39], as shown in Figs. S2a–b. The typical diffraction peaks of pure DM are observed nearly at 14.641� (130), 18.979� (021) and 23.518� (140). The pattern of acetylene black is also

Fig. 1. (a) – (e) The cross-sectional SEM images of pure DM, AcM-1, AcM-2, AcM-3, and AcM-4. (f) Photographs of DM, AcM-1, AcM-2, AcM-3, and AcM-4. 3

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Fig. 2. XRD patterns (a), DSC curves (b), TGA curves (c) and Thermal conductivity (d) of pure DM, acetylene black (AB) and acetylene black doped AcM se­ ries samples.

temperature of about 55.6 � C, whereas the subcooling temperature de­ creases successively to 45.7 � C, 46.7 � C, 43.2 � C and 44.1 � C for AcM samples, because of the involved acetylene black nanoparticles play an important role as an impurity nucleating agent and offer a large surface area and roughness for heat transfer [49]. The crystallization enthalpies for AcM samples are found to be 211.8 kJ/kg, 205.2 kJ/kg, 186.6 kJ/kg and 179.3 kJ/kg, respectively. This reduction in enthalpy is due to the reduced portion of the PCM, which is confirmed by TGA analyses, and the narrow temperature distribution during crystallization can be attributed to better heterogeneous nucleation [28,50]. Table 1 sum­ marizes the results of the DSC analyses. Overall, the phase change characteristics of the AcM samples are similar to those of the pure DM, as no chemical reactions between the pure DM and the acetylene black nanoparticles occurring in the whole process. The thermal stability of the pure DM and AcM composite PCMs are tested by the TGA, and the results are shown in Fig. 2c. It can be seen that the pure DM starts to decompose at ~252 � C, and the final weight loss percentage is nearly 100% at 386 � C, in consistence with the liter­ ature [7,42]. The overall trend of mass loss for different weight per­ centages of Acetylene black nanoparticles is consistent with that pure DM. The AcM samples start to degrade at 243 � C, 209 � C, 234 � C, and 228 � C, respectively. Except for AcM-4, other AcM samples decompose nearly 100% after 378 � C, 354 � C, and 353 � C, respectively. While the AcM-4 reaches a weight percentage of nearly 5% after 360 � C, which is the remaining acetylene black nanoparticles. Thermal conductivity is a critical parameter of PCMs that determines

how fast the charge and discharge processes are. The introduction of a good thermal conductive nanostructure into the pure PCM would usu­ ally enhance the thermal conductivity of the composites [51], but the concentration of nanostructure must be limited to ensure a desired en­ ergy storage density [52,53]. In this study, the use of acetylene black nanoparticles improves the heat transfer performance of the composites, as shown in Fig. 2d. The pure DM exhibits an inherently low thermal conductivity of 0.68 W m 1K 1, which is consistent with the literature [26]. Thermal conductivity is improved with increasing content of the acetylene black nanoparticles, which are 0.77, 0.79, 0.82, and 0.85 W/m 1K 1, respectively. With the 5.0 wt% acetylene black nano­ particles in DM, the improvement is 26.4% higher than the pure DM. The enhanced thermal conductivity is hypothetically caused by two reasons. Firstly, the introduced acetylene black nanoparticles with instinct high thermal conductivity will create paths of lower thermal resistance in the matrix, which would contribute a significant effect on effective thermal conductivity [54]. Secondly, thermal conductivity is strongly depended on the heat flow along a particular direction on the molecular alignment parameter in that direction [55]. The disorder large size acetylene black nanoparticles have more contact with the surrounding crystallization, which leads to a higher alignment parameter, consequently a lower interfacial thermal resistance. The optical absorption of DM based samples over the entire solar spectrum is characterized. Fig. 3a shows the absorption of the composite PCMs in the solid-state over the wavelength of 300 nm–2500 nm. The dark yellow line shows the typical absorption spectrum of pure DM. The integrated absorption over the whole wavelength is only 41.86%. With a small amount of dosing (0.005%wt) of acetylene black, the absorption is enhanced remarkably to over 85% in visible light, which is contributed by the acetylene black nanoparticles. As we know, acetylene black nanoparticles own excellent light absorption and act as good solar light absorber already owing to their intrinsic absorption coefficient and larger surface-to-volume ratio as well as the low density [56,57]. That’s much more effective in solar light absorption than DM. Here, the nanoparticles absorb solar light, convert it into heat and then transfer the heat to the surrounding DM, although DM itself also has some

Table 1 Phase change thermal properties of pure DM and AcM samples. Samples

Tmo(� C)

Tmp(� C)

ΔHm (kJ/ kg)

Tso (� C)

ΔHs(kJ/ kg)

ΔTsub (� C)

Pure DM AcM - 1 AcM - 2 AcM - 3 AcM - 4

165.0 165.3 165.4 166.2 166.5

167.4 168.0 167.7 168.8 169.2

295.2 293.8 287.8 277.8 270.3

109.4 119.6 118.7 123.0 122.4

213.0 211.8 205.2 186.6 179.3

55.6 45.7 46.7 43.2 44.1

4

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Fig. 3. Solid-state (a) and molten-state (b) absorption spectra over the wavelength from 300 nm to 2500 nm of DM, AcM-1, AcM-2, AcM-3 and AcM-4 PCM composites.

absorption. As a result, the DM doped with the little amount of acetylene black nanoparticles boosts the light absorption sharply in contrast to the bare DM. Moreover, good dispersion of the nanoparticles in the DM with small weight concentrations could also benefit the highly efficient VA. Therefore, the absorption is enhanced with increasing doping concen­ tration, which is 90.43%, 92.76%, 94.31%, and 96.13% for AcM-1 to AcM-4, respectively. This effect is in accordance with some reports nanofluids mixed with carbon nanomaterials [58,59], where the ab­ sorption increases sharply as the increase of carbon nanotubes concen­ tration. More than the measurements of them at solid state, the optical absorption at the molten state of DM is also investigated to discover the absorption data under the real work status (Fig. 3b). The DM’s trans­ mittance over the visible light range is enlarged to nearly 90% compared with ca. 70% in the solid state, which may be attributed to the looser molecule alignment of the liquid DM. Comparing with the solid-state situation, lots of changes are observed to the molten state absorption of AcM series. For AcM-1, the absorption over the wavelength from 300 nm to 1300 nm drops sharply to below 80% at its lowest with the in­ tegrated absorption ratio of 85.42%, which suggests that the absorption depends also on chemical interactions between DM molecules and acetylene black nanoparticles besides the distribution of acetylene black nanoparticles in the liquid DM. When the dose is over 0.5 wt%, the absorption in molten state approaches 100%, which could be ascribed to the aggregation of acetylene black nanoparticles at the surface of the molten DM with that level of dose. That also announces that a high concentration of dopants in the molten state of the matrix may result in SA mode, without taking advantage of VA. Conversely, the VA needs lower doping concentration in the DM matrix to avoid that aggregation

at the surface of molten DM liquid in service condition. Among all the samples, the molten-state absorption of AcM-2 is over 95% at the wavelength of 300–1300 nm with the integrated absorption ratio reaching 97.02%, which is desired in practice to achieve a good VA. These results of the doped PCMs absorption working under inservice conditions, in both solid and molten states, are reported here for the first time to our best knowledge. To further clarify the VA capacity, solar thermal tests were per­ formed with an infrared camera to observe the evolution of temperature distribution as a function of charging time (see the experimental section for details). Fig. 4a shows the results for the AcM series along with the penetration depth obtained at four time points in the duration of light soaking. As the duration increases from 0 s to 400 s, the temperature gradient of each kind of AcM is enlarged sequentially (Fig. S3). The top surface temperature of the samples becomes higher when the dose is high as AcM-3 and AcM-4, which also indicates they abide by SA mode. Fig. 4b presents the temperature distribution along with the penetration depth. The temperature gradient of AcM-1 and AcM-2 is obviously smaller than that of AcM-3 and AcM-4. Specifically, from AcM-1 to AcM4, the temperature at the top surface of samples are reached 166 � C, 173 � C, 191 � C, and 208 � C, respectively. However, the top-bottom tem­ perature differences of each sample show opposite, which are 59 � C, 62 � C, 87 � C and 109 � C. As tested in above, the thermal conductivity of the samples is increased with the increase of the nanoparticles loaded, and then the top-bottom temperature difference of the samples should be smaller owing to the better thermal conductivity. So, opposite to that deduction, the experiment result reveals that the top-bottom tempera­ ture difference of each sample in solar light irradiation should be not

Fig. 4. a) The temperature-position-time curves of pure DM and AcM samples under the sunlight (915 mW/cm2). b) The infrared images of AcM samples under the sunlight of 915 mW/cm2 at 400s. 5

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dominated by thermal conductivity only. Radiative heat transfer is responsible to take another important role in heat transfer in our system besides thermal conductivity of acetylene black nanoparticles and DM, which contributes to so called VA. When the dose is low (AcM-1 and AcM-2), the VA mode dominates the heat transfer in the composite system, where the radiative heat transfer takes the main responsibility of heat transfer inside because the penetration depth of solar light can cover 10 mm in the sample’s thickness. The effect of conductive heat transfer is limited by the poor thermal conductivity of DM itself. So, the top-bottom temperature difference of the sample is small, and the sur­ face temperature keeps low, which can suppress the volatilization of DM molecules due to the overhigh temperature at the surface. When the dose is high (AcM-3 and AcM-4), the SA mode dominates the heat transfer inside because of the aggregation of the acetylene black nano­ particles especially at the surface, which leads to overhigh temperature at the surface, for example, 191 � C (AcM-3) and 208 � C (AcM-4). Heat could be delivered from the top to the bottom by only conduction, which is herein less efficient. Although the thermal conductivity of DM matrix is enhanced by the doped the acetylene black nanoparticles from 0.77 (AcM-1) to 0.85 W/m 1K 1 (AcM-4), the conductive heat transfer by the heavy doped DM accounts for large top-bottom temperature differences, such as 87 � C for AcM-3 and 109 � C for AcM-4. The severe thermal resistance occupies a large share of absorbed energy from solar and re­ duces the conversion efficiency of the underlying STEG. The temperature distribution graph of the four kinds of composite PCMs is clearly shown in the inset of Fig. 4b. AcM-2 shows the deepest depth of light penetration while AcM-4 shows the shortest depth. The result may be ascribed to the over-high concentration of acetylene black nanoparticles embedded in DM blocking the light penetration, which could be observed in SEM images Fig. 1 and Fig. S2. This result also declares that the dose as high as 5% may result in superficial other than volumetric optical charging in such composite PCMs as reported in the literature [60–62]. In addition, AcM-2 has a higher mean temperature than AcM-1, probably due to the more appropriate amount of acetylene black nanoparticles as absorption cores in AcM-1 [19].

To further investigate the difference between SA and VA, the opti­ mum AcM-2 was compared in the heat transfer test with the pure DM integrated with a commercial solar selectively surface absorber. Fig. 5a–b shows the temperature distribution graph depending on the penetration depth and light soaking duration. The thermal diffusivity and average temperature of the VA sample are significantly higher than the SA sample, which indicates that VA has a better ability of photothermal conversion and collect thermal energy more effectively, which is in accordance with the previous reports [18]. Fig. 5c presents the temperature distribution along with the pene­ tration depth for two samples. One can see the temperature in the SA sample reaches the peak near the top surface and falls steeply along with the depth. The top surface and the bottom temperature of the SA absorber are 195.9 � C and 92.8 � C, respectively, while there are 173.4 � C and 110.7 � C for the VA absorber. The top-bottom temperature differ­ ence in significantly reduces to 62.7 � C for VA from 103.1 � C for SA. Meanwhile, the peak temperature at the surface of the SA sample could reach 195.9 � C, which is 22.6 � C higher than in the VA sample, while the temperature at the bottom of the SA sample is lower 17.9 � C than the VA sample. The top-bottom temperature is smaller, then more uniform temperature distribution, and hence the thermal storage capacity must be higher for the same medium. Moreover, over high top-surface tem­ perature would lead to evaporate the PCM material much easier and hence limit their applications in higher temperature service environ­ ments. This result definitely confirms the advantages of VA. As shown in Fig. 5d, the SA mode is currently a typical way to collect solar light. In this model, a selective solar absorber film is always used to collect and convert the solar energy into thermal energy, and the heat is then transferred to PCMs only by thermal conduction and convection [63]. Unfortunately, the slow charging rate due to the low thermal conductivity of PCMs will lead to a large top-bottom temperature dif­ ference and a higher heat loss at the surface [63]. In contrast, employing VA is an alternative way to avoid the above problems. Because VA al­ lows good transmittance of light to a considerable thickness, it could allow solar irradiation to be absorbed across the depth. Therefore, heat

Fig. 5. a) and b) The infrared images of SA and VA samples under the sunlight of 915 mW/cm2 at 100s, 200s, 300s, and 400s. c) Temperature-position curves of SA and VA samples under the sunlight (915 mW/cm2) after illuminating 400s. d) Two kinds of models of the solar absorbers for photo-thermal conversion: SA and VA. 6

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would not be accumulated at the surface layer, convection and re-emission [15]. Meanwhile, the radiative heat transfer occurred in VA can also enhance the heat transfer throughout the volume. To test the performance of fabricated volumetric absorber, a STEG system was built-up and photo-to-thermal charge-discharge perfor­ mance was investigated in this work. As far as we know, the device performance based on sugar alcohol-based low and medium tempera­ ture PCMs was hardly investigated although their thermal storage properties have been studied extensively. In our cell structure, the composite DM was attached on a commercial thermoelectric generator (TEG) along with finned cooler at the rear side. Fig. 6a–b shows the schematic and practical picture of the fabricated STEG system. This system also consists of solar simulator, voltage monitor, temperature data acquisition（DAQ） and data recording computer. The solar light was irradiated from the solar simulator and absorbed by our composite DM series in our cells. The heat converted from solar light was collected and stored in the composite DMs. Then, TEG converted the heat flow from the bottom of composite DM layer into electricity based on the Seebeck effect. The cell was kept adiabatic by surrounding of thermally insulation foam so that the absorbed heat could only flow downward to the TEG. As shown in Fig. 6c, the time dependent temperature profile could be divided into four stages: sensible charge, latent charge, latent discharge and sensible discharge. At the sensible charge stage (0–50s ca.), AcM1-3 show comparably fast temperature rise in contrast to that of DM Plus and

AcM-4. DM Plus is the absorber integrated with commercial SA (eta plus®) and pure DM. This contrast distinguishes clearly the difference of the photo to thermal conversion rate between VA and SA [64]. In the second charging stage, the temperature in the PCM samples tends to be saturated gradually and keep a steady-state, which is corresponding to their phase change temperature given by DSC data (Fig. 2b). In the stage, thermal energy is stored as PCM’s latent heat. At the latent charge step, the charging rate of all kinds of cells becomes slow, and the monitored temperature arrives at a peak of around 300s. AcM-2 turns to be the champion from the AcM series. For the two cells in SA mode, AcM-4 and DM Plus, their charging behaviour follow behind an obvious gap of the VA samples (Fig. 6c), which can be ascribed to delay or even miss of the kink points in charging from sensible to latent. If compared with each other, the charging rate of AcM-4 is obviously superior to that of DM Plus owing to AcM-4 still has some effect of VA inside. After the solar simulator is switched off, the monitored temperatures decrease sharply in the latent discharge stage, which indicates that the phase change enthalpy turns to be released by means of crystallization. It must be mentioned that the exothermic temperature range is not in a strict line with the DSC data. The cooling down kink point is around 150 � C, which indicates the occurrence of solidification at a temperature of ca. 150 � C. This phenomenon means that AcM samples have a smaller subcooling degree (<20 � C) in this STEG system, which is also reported in some literature [7]. In contrast, for AcM-4 and DM Plus in SA mode, the kink point is ambiguous or even missed, which also announces they are scarce

Fig. 6. a) The schematic and b) the corresponding practical diagram of the home made STEG system. The charge-discharge temperature curves (c) and open circuit voltage curves (d) of AcM series and DM Plus based cells and derived STEG systems. DM Plus is the absorber integrated with commercial SA (eta plus®) and pure DM. In both (c) and (d), they follow four stages named as sensible charge (light red in the background), latent charge (dark red), latent discharge (light blue) and sensible discharge (dark blue). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 7

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and even missing of solidification course in this stage and directly enter the sensible discharging stage. In the final stage (Sensible Discharge), all the samples are cooled down fast without any plateau, which confirms this stage is dominated by the release of sensible heat. Basing on our STEG system, the open-circuit voltage (Voc) of the thermoelectric module output was measured, which is shown in Fig. 6d. The voltage charging also follows the same trace with the temperature as shown in Fig. 6d. Among all the samples, AcM-2 wins over the other AcM series as well as the typical SA of DM Plus, which becomes much more outstanding in the charging of Voc of TEGs (Fig. 6d) in contrast to the temperature trace. AcM-2 can reach the highest Voc of around 0.65 V, while AcM-4 has the lowest Voc of about 0.5 V. Compared with AcM series, the Voc of DM Plus turns to be the lowest at around 0.4 V and can’t afford a steady output in neither charging nor discharging processes. The final system performance contrast between VA and SA mode further convince us clearly the advantage of VA over SA not only in conversion efficiency but also in stability from solar light to electricity.

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4. Conclusions A low cost and mid-temperature phase change material was suc­ cessfully obtained by facile doping acetylene black nanoparticles into DM, which results in VA rather than conventional SA. The SEM images confirm the acetylene black nanoparticles well dispersed in the DM bulk. The DSC results show the composites have high thermal enthalpy during the phase change, which could get up to 293.8 kJ/kg at 168 � C. Acet­ ylene black nanoparticles could facilitate the nucleation of DM that in­ creases the solidification temperature and improves the thermal management performance. The absorption and transmission spectra of both the pure and doped DM series were for the first time characterized in both solid and molten states, simulating their real service environments. The absorption over the whole solar spectra surges from only 41.86% for the pure DM to over 90% for the doped DM with only 0.05 wt% of commercial acetylene black nanoparticles. Strikingly, depending on those composite PCMs, VA was compared directly with conventional SA. The top surface and bot­ tom temperature of the SA absorber are 195.9 � C and 92.8 � C, respec­ tively, while the two values for the VA absorber are 173.4 � C and 110.7 � C. The lower top-bottom temperature difference reveals the radiative heat transfer dominates VA and has a fast thermal charging rate and high photo-thermal conversion efficiency, compared with SA by means of conductive heat transfer. Basing on their based STEG system, the photo to thermal charge and discharge performance of sugar alcohol like PCMs was for the first time investigated in this work, although their thermal charge and discharge properties have been studied extensively. Both the hot-end temperature and the open-circuit voltage of their based STEG system show ambigu­ ously the advantage of VA over SA. Our work offers an extremely promising strategy for low to medium temperature solar thermal systems under one sun, by advantage of VA mode relying on facile and low cost acetylene black nanoparticles doped DM. The doped DM acts directly as the solar light absorber in VA mode along with the function of thermal storage simultaneously. That benefits to a large extent our in-depth understanding of volumetric absorbers and their future applications in STEG systems. The encouraging results would definitely attract extensive attention of the researchers in related fields. Acknowledgements The authors acknowledge the assistance of SUSTech Core Research Facilities for characterizations in this work. This work is supported by the Shenzhen Key Laboratory Project (No. ZDSYS201602261933302) and the Natural Science Foundation of Shenzhen Innovation Committee (Nos. JCYJ20150529152146471).

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