PCM-Based for Heat Storage in Solar-Thermal Converter

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PCM-Based for Heat Storage in Solar-Thermal Converter NADHRAH Md Yatim

^1,2,a*

, SITI RAHMAH Md Nizar

^3,b

, MOHD AZMAN Hashim@Ismail

^4,5,c

and SYAHIDA Suhaimi

^6,d

1,3,4,6Applied Physics Programme, Faculty of Science and Technology, Universiti Sains Islam Malaysia, 71800 Nilai, Negeri Sembilan, Malaysia

2,4Frontier Materials Research Group (FMRG), Faculty of Science and Technology, Universiti Sains Islam Malaysia, 71800 Nilai, Negeri Sembilan, Malaysia

a[email protected], ^b[email protected], ^c[email protected],

d[email protected]

Keywords: PCM, solar-thermal converter, paraffin, beeswax, TEG.

Abstract. Solar thermal energy is one of the promising renewable and sustainable energy that have gain research interest. However, the nature of intermittent solar irradiation limits the usage of this energy. Phase change material (PCM) are substance that has the property of absorbing and releasing thermal energy through phase transformation. Combination of graphene foam/PCM composite will be able to absorb heat from solar thermal energy and sustain energy release to thermoelectric generator (TEG) for electrical conversion. Two different PCM material were tested which are petroleum-based paraffin wax and bio-based PCM beeswax. Thermal properties of both materials were measured using DSC and heat absorption were tested under real solar irradiation. This solar- thermal converter showed that graphene/paraffin/beeswax composite is more effective than the paraffin wax or beeswax alone. The recorded results also showed that combination of these petroleum based and bio-based PCM with added graphene foam could retain longer heat than graphene/paraffin wax and individual PCM. The longer heat can be stored in solar-thermal converter device may sustain electricity generation even with absence of solar energy.

Introduction

Solar energy has gained worldwide attention since solar energy is abundant and clean source of energy. It can be harnessed and converted into various kinds of energy including electricity, fuels, and thermal energy through photovoltaic, photochemical, and photothermal processes respectively.

One of the current research interests is the use of heat energy from solar (solar-thermal energy) in generating electricity through thermoelectric generator (TEG) device due to its good high energy efficiency [1]. Efficiency of TEG related to the ZT value, which is temperature dependent. Although TEG performance could be enhance through segmentation [13], fabrication [14], doping and material itself, in solar thermal case maintaining the temperature to TEG is important due to the nature of solar energy in producing intermittent solar irradiation. This limits the usage of this energy and makes them incapable in storing much energy for the continuous needs [2]. Due to this reason, further investigations in harvesting solar-thermal energy hybrid with PCM as thermal storage has been investigate as support for an alternative power source.

Phase change material (PCM) composite has been studied as high performance solar-thermal storage. Graphene has been an ideal support of PCM as it has high photothermal efficiency and great in light absorption [3,4]. Graphene foam that have high macro porosity and low density allow for high weight fraction of the PCM to be incorporated, which enhances heat storage capacity of the composite. To get the optimum efficiency in converting solar heat to electricity, the graphene foam is used as filler in PCM. This composite will boost the capacity of heat storage thus increase energy that can be converted by TEG to generate electricity [5].

Recently, many researchers work on this field as the world starts to concern the benefits of solar thermal conversion combined with source of thermal storage, such as PCM. Akhiani et al. [5] reported the graphene nanoplalets helps to improve the thermal conductivity of palmitic acid 10 times greater

Solid State Phenomena Submitted: 2019-06-04

ISSN: 1662-9779, Vol. 307, pp 297-303 Accepted: 2019-09-17

doi:10.4028/www.scientific.net/SSP.307.297 Online: 2020-07-17

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific.net. (#547228104, Universiti Sains Islam Malaysia-25/11/20,10:35:01)

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than palmitic acid alone by constructed the palmitic acid/graphene nanoplets for the uses of thermal energy storage applications. In the same year also, Mehrali et al. [12] had done the research using the same material, palmitic acid but it gives a little bit difference as palmitic acid used graphene oxide.

In the term of building applications, Amin et al. [4] investigated the thermal properties of graphene/beeswax composite as the reducer of energy consumptions. According to this research, the addition of graphene nanoplatelets improved the thermal conductivity by 0.12% and heat capacity of the composite boosted by 12% than the using of beeswax alone. This eventually proved the facts that the additional of graphene in composite does help in achieving better findings and improvements.

In the other perspectives, besides graphene, graphene oxide also has marked their ability of the usage in the PCM. Yadav et al. [11] in 2016 plotted the result of their research that it is GO/PCM composites the faster and more uniform heating compared to pristine PCM in the exposing the composite to the simulation of light. Recently, in the same year also, Zhang et al. [10] in 2016 reported that the graphene/PCM composite was suitable to act as solar thermal collector. This is due to the capability of graphene/PCM composite in absorbing light and combination of solar-thermal conversion and stored thermal energy capability allow the graphene-PCM composite to act as heat source for solar thermoelectric generator for continuous power generation.

PCM Composite Preparation

PCM materials that used in this study are beeswax and paraffin wax that was categorize as organic source which can provide higher energy storage density and wide melting temperature range [6].

Generally, the system acts as solar thermal converter device, which effectively collects and converting the light energy into thermal energy and the stored thermal energy is released for maintainable consumption. These thermal energies will be stored and keep by PCM materials to gives better performance and can be utilized in generating continuous electricity even the light source is absent.

There are various researches on how to optimize PCM in harvesting solar-thermal energy [7]. As the nature of intermittent solar irradiation and irregularity in assessing light, the percentage of electricity generated from this usually low and limited. Hence, to bridge the gap and optimize solar- thermal energy, this study investigate feasibility of two different source of PCM composite in retain heat for longer usage. The power output and duration of power produced after absence of source is main concerned of this work.

Synthesis and Characterization

The samples were tested in this study are graphene, paraffin wax, beeswax, paraffin wax/beeswax composite and graphene/paraffin wax/beeswax composite. The graphene foam was prepared by an in-situ chemical reduction-induced self-assembly method reported previously [8]. 20.0mL of the graphene oxide (GO) suspension obtained by a modified Hummer’s method with a concentration of 2.5 mg mL^-1 was prepared, and then 0.5g of OA and 1.0g of NaI were added to the GO suspension to obtain graphene. The mixture was sonicated for 10 min and then transferred to an oil bath at 90^oC for 12 hours without stirring. The resulted black cylinder of graphene hydrogel was washed with ethanol and distilled water to discard remaining impurities and then the wet hydrogels were freeze-dried for 2 days to obtain graphene.

The paraffin/beeswax composite was prepared using melt-mixing method to increase the better dispersity among the waxes. The heating for both of waxes was using method of double-boiler to prevent the burning of waxes if direct heating. Both of 0.75 g of paraffin wax and 0.25 g of beeswax were melted separately with 70℃ and 110℃ respectively. The melted waxes were mixed with temperature of 90℃ before that composite is cooled down to room temperature. Preparation of graphene/paraffin wax/beeswax composite was same with the preparation composite of paraffin wax/beeswax except the last part of that step was replaced with the addition of graphene before cooled down to room temperature. The samples were also tested in Differential Scanning Calorimeter (DSC) to get their thermal properties. The specific heat capacity can be obtained from the DSC measurement by:

298 Solid State Science and Technology XXX

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∆𝐶𝐶𝑝𝑝= �^{𝜕𝜕𝜕𝜕}_{𝜕𝜕𝜕𝜕}� (1) where ∆Cp is the heat capacity, ∂H is the enthalpy and ∂T is the temperature difference of end set and onset that obtained from the DSC curve.

The hybrid of solar thermal device was tested by integrate the TEG each of these PCM composites that were used as solar thermal collector. These composites were deposit on the hot end of a TEG (4.0 cm × 4.0 cm) and were exposed to the sunlight while the cold side of TEG was attached to heat sink. The temperature and output voltage were measured when the device was expose to the sun and when sunlight was absent. The magnifying glass was used in this experiment to concentrate the sunlight pass through the composites. The performance of all samples in collecting the heat of sunlight were recorded when source was removed.

Results and Discussions

The results obtained from DSC tabulated in Fig. 1 shows that the pattern of the graph for all three samples without graphene which are paraffin wax, beeswax and the paraffin wax/beeswax composite are quite similar. The specific heat capacity for all samples can be calculated using Eq. 1 and shown in Table 1. Beeswax has melting point of 59.11⁰C but yet produce high enthalpy of 177.28 Jg^(-1).

On the other hand, the graph pattern shown in the paraffin wax and the composite of waxes are similar thus their melting point and enthalpy were not difference much in values. The melting point and enthalpy for the paraffin wax and paraffin/beeswax composite are 60.99 ⁰C, 60.01⁰C and 127.40 Jg^(- 1), 127.66 Jg^(-1) respectively. The composite behaves more toward paraffin characteristics because the composite was mixed using 75% ratio of paraffin wax and 25% of beeswax.

It is shown that the peak of the graphene and the composite of graphene/PCM were not significantly seen. Because graphene base element is carbon, it is expected that graphene has high melting point thus it does not appear within the temperature range of DSC test (<100 ^oC). It is desirable for the solar thermal system can withstand the temperature without change its physical structure. The addition of graphene in the waxes can gives great impact in assist PCM as heat storage.

However, the ability of graphene in assistance of heat absorption can be assess when the source is absent.

Figure 1: Change in heat flow rate with temperature for all samples taken by DSC

60.993 59.1181

60.0092 -2

-1.5 -1 -0.5 0 0.5 1

-20 0 20 40 60 80 100 120

Heat flow (W/g)

Temperature ( ⁰C) Paraffin wax Beeswax

Paraffin/Beeswax composite Graphene

Graphene/Paraffin/Beeswax composite

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Table 1: Heat capacity calculated for all samples

Samples Heat capacity (J/gK)

Paraffin wax 20.100

Beeswax 9.670

Paraffin/Beeswax composite 13.00

Graphene 0.012

Graphene/Paraffin/Beeswax composite 0.044

During the exposing to the sunlight within one hour, it is observed that graphene shows a quick rose in temperature compared to the other two waxes which is paraffin wax and beeswax. This is because graphene has high ability in absorbing light as the structure of graphene itself which have large pores that enhanced light capturing into it [10]. The synthesis of graphene using modified Hummer’s method made the structure in 3D, since modified Hummer’s method was using the oxidation-reduction reaction. Previous research [9] stated that the waxes have the natural properties of high ability in storing energy, but not in the absorbing light. That’s why these properties may give effect when they were exposed to the sunlight. As the same reason also, the temperature of the graphene drops significantly compared to both of waxes, as graphene has high thermal conductivity thus not good in storing heat after sunlight has been removed.

Based on the data tabulated, there were differences in temperature observed between composite without graphene and the composite with the presence of graphene (Fig. 2). There were steadily rise in temperature when paraffin wax/beeswax composite is exposed to the sunlight, but the graphene/paraffin wax/beeswax composite shows the increase of temperature of quite significantly rose. This strongly suggest that graphene in the composite help to collect more energy from the sunlight.

Paraffin wax/beeswax/graphene composite has shown the great improvement in absorbing and storing heat that got from direct sunlight compare to the mixture of paraffin and beeswax composite and their pristine material. Comparison data observed in this research with previous researcher using graphene/paraffin wax [10] using simulation solar light shows similar temperature increased during exposed to sunlight. However, paraffin wax/beeswax/graphene composite in this study could retain more heat after source is removed.

To study the effect of varying irradiation during sunlight measurement, normalization was made as compared with previous study is shown in Fig. 3. the highest data calculated for temperature over the insolation of sunlight were at the 10 minutes of the samples being exposed to the Sun. The temperature that obtain from the samples are influenced by the insolation of sunlight of certain time along the period of the Sun exposure. Based on the previous research [10] that conducted the similar experiment, there were the gradual increase of temperature per insolation due to that research used the constant intensity of light which is 1500Wm^-2 during the experiment, and hence there were no problem in having such irregular peak at certain peak as achieved in this experiment. The smaller values tabulated in graphene/paraffin composite compared to pristine graphene shows that the composite that they formed do not depend on the insolation much as in composite tend to store more the absorb light.

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Figure 2: Temperature change with time taken for all samples

Figure 3: Temperature per insolation against time

Performance on the solar thermal converter hybrid system between graphene/PCM composite and thermoelectric module (TEG) was measured and analyzed based on the output voltage that generated during the presence and absence of sunlight as shown in Fig. 4(a). As a control, the thermoelectric module in the absence of the composite generated maximum voltages of 47 mV and the output voltages quickly dropped to 0 mV within 60 seconds once it was moved from sunlight. For the device using composite of paraffin wax/beeswax, it is shown that the highest voltage generated was at 60 minutes device is being exposing to the sunlight with the value of 58 mV, while for the device using composite of graphene/paraffin/beeswax, the highest voltage generated was at 60 minutes device is being exposing to the sunlight with the value of 83 mV. Solar thermal device using hybrid graphene/paraffin/beeswax with TEM show the higher increment compared to the other composite without graphene. Device using graphene/paraffin/beeswax composite give better reading of increment temperature as the sun was exposed to it. Thus, it suggests that the use of mixture waxes as the PCM composites gives better reading in output values as they have larger ability to absorb and store the heat from the Sun, hence generate more output voltage.

20 30 40 50 60

0 20 40 60 80 100 120

Temperature (ºC)

Time (min)

Beeswax Paraffin wax Graphene

Paraffin Wax + Beeswax

Paraffin Wax + Beeswax + Graphene

Presence of Sunlight Absence of Sunlight

0.01 0.02 0.03 0.04 0.05 0.06

0 10 20 30 40 50 60 70

Temp (⁰C)/ insolation (Wm-2)

Time (min) Graphene

Paraffin+Beeswax

Graphene+Paraffin+Beeswax graphene [ref] (Zhang et al, 2016) graphene/paraffin [ref] (Zhang et al, 2016)

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Most importantly, a unique feature of the composite integrated thermoelectric module led in their prolonged thermal response, which was a result of the heat storage by the composite and which enabled the continuation of the voltage output even when the light was off. As shown in Fig. 4(b), thermoelectric module without composite at the top of its surface give the value of voltage drop drastically decrease to 0 mV within 60 seconds once it was moved from sunlight. Solar thermal device with the presence of both composite, paraffin wax/beeswax and graphene/paraffin wax/beeswax composite recorded the voltage drop to 0 mV with approximately 180 seconds and 220 seconds respectively. The device that have composite of graphene/paraffin wax /beeswax on the top on it taking longer time compared to the other device that having composite without graphene. The electricity is still generated even though light source has been removed for a certain time.

Voltage that had been tabulated during one-hour period exposing to the direct sunlight demonstrated on how much the composites can absorb light and converted to heat and at the same time store that heat for the further utilization. The elongation in voltage drop when there no heat source demonstrated on how much respective samples store heat enabled the continuation of the voltage even the absence of sunlight.

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Figure 4:Voltage output against time for solar thermal device during (a) presence of light, and (b) absence of light

Summary

The implementation of solar thermal conversion device using hybrid graphene/phase change material composite and thermoelectric generator (TEG) are successfully studied using different type of PCM which are paraffin wax and bees wax. PCM composite consist of graphene/paraffin/beeswax shows the highest increment temperature in absorbing and storing heat from direct sunlight compared to the composite of graphene/paraffin wax from previous study and PCM alone. The voltage recorded when exposing to the sun for the graphene/paraffin/beeswax composite were 83mV and 58mV for the composites of waxes in the same light condition give advantages in addition of graphene in the composite. The time taken for voltage drop to 0mV when there is no light expose were 220s and 180s for paraffin/beeswax composite and graphene/paraffin/beeswax composite respectively. Hence, it can be concluded that the PCM composite consist of graphene with two different types of PCM materials have increase the overall device performance due to increase in high absorption of heat and longer time to retain the heat in the absence of solar light.

0 20 40 60 80 100

0 15 30 45 60 75

Voltage (mV)

Time (min) TEM with paraffin/beeswax/graphene

TEM with paraffin/beeswax TEM without composite

0 20 40 60 80 100

3600 3660 3720 3780 3840

Voltage (mV)

Time (s) TEM without composite

TEM with paraffin/beeswax TEM with paraffin/beeswax/graphene

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References

[1] Information on https://arpa-e.energy.gov/?q=slick-sheet-project/solar-thermoelectric-generator.

Advanced Research Projects Agency. 2012. Solar Thermoelectric Generator.

[2] W.H. Chen, P.H. Wu, X.D. Wang, Y.L. Lin, Power output and efficiency of a thermoelectric generator under temperature control, Energy Conversion and Management 127 (2016) 404-415.

[3] G.Q. Qi, C.L. Liang, R.Y. Bao, Z.Y. Liu, W. Yang, B.H. Xie, Polyethylene glycol based shape- stabilized phase change material for thermal energy storage with ultra-low content of graphene oxide, Solar Energy Material & Solar Cells 123 (2014) 171-177.

[4] M. Amin, N. Putra, E.A. Kosasih, E. Prawiro, R.A. Luanto, T.M.I. Mahlia, Thermal properties of beeswax/graphene phase change material as energy storage for building applications, Applied Thermal Engineering 112 (2016) 273-280.

[5] A.R. Akhiani, M. Mehrali, S.T. Latibari, M. Mehrali, T.M. Indra Mahlia, E. Sadeghinezhad, H.S.

Cornelis Metselaar, One-step preparation of form-stable phase change through self-assembly of fatty acid and graphene, J. Phys. Chem. C. 119(40) (2015) 22787-22796.

[6] Sekar, S; Putra, N; Amin, M; and Afriyanti, The utilization of paraffin and beeswax as heat energy storage in infant incubator, Journal of Engineering and Applied Sciences 11(2) (2016) 800-804.

[7] J.H. Ming, X.D. Wang, W.H. Chen, Performance investigation and design optimization of a thermoelectric generator applied in automobile exhaust waste heat recovery, Energy Conversion and Management 120 (2016) 71-80.

[8] L. Zhang, G. Chen, M.N. Hedhili, H. Zhang, P. Wang, Three-dimensional assemblies of graphene prepared by a novel chemical reduction-induced self-assembly method, Nanoscale 4 (2012) 7038- 7045.

[9] T. Kousksou, P. Bruel, A. Jamil, T. El Rhafiki, Y. Zerouli, Energy Storage: Applications and Challenges, Solar Energy Materials and Solar Cells 120 (2013) 59-80.

[10] L. Zhang, R. Li, B. Tang, P. Wang, Solar-thermal conversion and thermal energy storage of graphene foam-based composite, Nanoscale 8 (2016) 14600-14607.

[11] Yadav, A., Barman, B., Kumar, V., Kardam, A.S., Narayanan, S., Verma, A., Madhwal, D., Shukla, P., and Jain V.K, Solar thermal charging properties of graphene oxide embedded myristic acid composites phase change material. AIP Publishing (2016).

[12] Mehrali, M., Latibari, S. R., Mehrali, M., Mahlia, T.M.I., Metselaar, H.S., Naghavi, M.S., and Akhiani, A.R, Preparation and characterization of palmitic acid/graphene nanoplatelets composite with remarkable thermal conductivity as a novel shape-stabilized phase change material, Applied Thermal Engineering (2013) 663-640.

[13] N.Z.I. Mohd Sallehin, N. Md. Yatim, Influence of difference length segmented Bi2Te2.95Se0.05/SnSe0.95I0.05 on thermoelectric’s seebeck coefficient, Advanced Science Letters 23 (5) (2017) 4496-4499.

[14] N.Z.I. Mohd Sallehin, N. Md. Yatim, and S. Suhaimi, A review on fabrication methods for segmented thermoelectric structure, AIP Conference Proceedings 1972, 030003 (2018)