Elsevier

Energy Conversion and Management

Volume 126, 15 October 2016, Pages 1-11
Energy Conversion and Management

Analysis of utilizing Graphene nanoplatelets to enhance thermal performance of flat plate solar collectors

https://doi.org/10.1016/j.enconman.2016.07.061Get rights and content

Highlights

  • The effect of Graphene on the efficiency of flat plate solar collectors has been evaluated.

  • Colloidal stability of nanofluid in different levels of acidity is tested to get the highest performance.

  • Influence of Graphene on the enhancement of thermal conductivity of water has been assessed.

  • Maximum kinematic viscosity in the range of operating temperatures has been obtained.

  • Results of theoretical studies uphold the efficiency increase obtained by experimental tests.

Abstract

It is the aim of the present paper to investigate the effect of Graphene nanofluid on thermal performance of flat plate solar collectors. Therefore, the influence of Graphene Nanoplatelets on thermal efficiency of a closed-cycle solar system consisted of a flat plate solar collector has been investigated and its hot water production capability has also been tested. To start, the structure of Graphene has been evaluated by Field Emission Scanning Electron Microscopy imaging and UV–vis spectroscopy. Afterward, by utilizing deionized water as the base fluid, Graphene/Water nanofluid has been prepared by a two-step method with different mass fractions (0.01 and 0.02 wt%). To prevent from sedimentation, the colloidal stability of the nanofluid has been tested in different pHs and its best level has been obtained. Moreover, the thermal conductivity, kinematic viscosity and the influence of Graphene/Water nanofluid on the efficiency of the flat plate solar collector have been studied thoroughly. The results indicate that dispersing Graphene in the base fluid can increase thermal efficiency of the solar collector up to 18.87%. As an obvious result, the outlet temperature and absorbed energy of water heater in various states (deionized water, 0.01 and 0.02 wt% nanofluid) has been measured and the temperature of water in the solar heater reached up to 71 °C. Eventually, the theoretical thermal efficiency of the solar collector has been calculated and compared with the experimental one, and the results have been closed to each other.

Introduction

Limitation of using fossil fuel resources and the pernicious effects of their wrong usage by human beings for our nature made researchers in scientific and industrial communities concentrate on renewable energies such as wind, solar, and geothermal like the research done by Chiari and Zecca [1]. Also Pfenninger and Keirstead [2] studied the above items in order to find a suitable scenario for the power systems of Britain. Due to the cleanness and especially its availability, solar energy has been known as a suitable case among other sources and many studies have been done in this area such as the research done by Aydin et al. [3]. With regard to its accessible property in every place of our world, solar energy is used in various applications, for instance, in production of hot water, electricity, air conditioning and so on.

Enough knowledge about solar radiation can be helpful to make systems for heating water in industry or for household consumptions and valuable case studies have been done in the recent years like Halawa et al. [4]. Therefore, heating water by solar water heaters can be considered as the most economical and easiest approach. It is necessary to mention that collectors are the most important part of each solar system which their main role is absorbing solar radiation, converting it to the heat and then transferring the converted energy to the working fluids which flow in pipes or channels. Detailed information about collectors were mentioned by Visa et al. [5]. One of the problems of such systems is how to boost their thermal performance. Stanciu and Stanciu [6] determined the optimized angle for installing collectors but their results indicate that better solutions are needed. Li et al. [7] studied nanofluids which are suspensions prepared by dispersing nanoparticles, rods or tubes in the base fluids. Also the high stability of nanofluids has been investigated by Farbod et al. [8]. Furthermore, Zhu et al. [9] studied the effects of nanoparticles on thermal conductivities of Iron(II,III) oxide [Fe3O4] nanofluids. Moreover, Soleimani et al. [10] analyzed heat transfer capability of nanofluids in semi-annulus enclosure. In addition, Sheikholeslami and Ganji [11] investigated heat transfer of nanofluid flow between parallel plates. Moreover, the effect of electric field on the behavior of nanofluid was studied by Sheikholeslami et al. [12]. Finally, the influence of magnetic field on the heat transfer of nanofluid has been investigated by Sheikholeslami et al. [13].

Improvement of heat transfer capabilities of solar systems seems to be vital since, up to now, human beings have not been successful to broadly utilize such systems economically. To overcome this shortage, many researchers take for example Sarsam et al. [14] investigated many solutions such as applying nanofluids. And nowadays by developing nanotechnology this manner can be applicable for most of the researchers. They studied techniques of increasing outlet temperature of a small collector and compared it by a real size one. They concluded that nanofluid is the only way that results more thermal efficiency and Copper(II) oxide nanoparticles [CuO] have the highest performance in comparison with Silicon dioxide [SiO2], Titanium dioxide [TiO2] and Aluminium oxide [Al2O3]. The influence of Al2O3, Zinc oxide [ZnO] and Magnesium oxide [MgO] on the efficiency of cylindrical solar collectors has been showed by Li et al. [15] that ZnO/H2O by ϕ = 0.2% is the best choice among others. Liu et al. [16] studied the thermal performance of a small thermosiphon system experimentally by dispersing Carbon nanotubes [CNT] in water and they obtained its optimized mass fraction. The effect of Al2O3 with water as a base fluid on flat plate solar collectors have been analyzed by Yousefi et al. [17]. Also, Yousefi et al. [18] studied the influence of Multi walled carbon nanotubes [MWCNT] with water as a base fluid on flat plate solar collectors. In the first case when particle diameter (dp) was 15 nm with 0.2 wt% and 0.4 wt%, collector efficiency for 0.2 wt% was higher than 0.4 wt%. In the second experiment which was done for MWCN, 0.4 wt% nanofluid without any surfactants had the highest thermal performance in comparison to other states. Moghadam et al. [19] tested the effect of CuO/H2O nanofluids on flat plate solar collector with ϕ = 0.4% and dp = 40 nm by different ranges of mass flows. They concluded that by ṁ = 0.016 kg/s, the system performance can be raised up to 21.8%. Among all of the experimental researches, the effect of Al2O3/H2O nanofluid with ϕ = 5% on glazed collectors by sinusoidal absorbers was numerically investigated by Nasrin et al. [20] and they could increase convectional heat transfer up to 19% for nanofluid and 12% for the base fluid. Moreover, Tiwari et al. studied the effect of Al2O3/H2O nanofluid with ϕ = 2% and dp = 0.5 nm on flat plate solar collectors theoretically [21] and their results showed that the system performance can be increased up to 31.64% in comparison with water as the base fluid. Experimental scrutiny which was carried out by various researchers such as Said et al. on CNTs [22] indicates that carbon nanostructures and especially grapheme can be considered as suitable choices in order to be utilized in nanofluids. Subsequently, Zhu et al. investigated thermal conductivity of graphite nanoparticles [23]. Last but not least, heat transfer performance of Graphene nanofluids for fully developed turbulent flow has been investigated by Yarmand et al. [24].

For their higher thermal conductivity compared with other nanoparticles and also because their density is lower than metals or metal oxides like the afore-mentioned done experiments, carbon nanostructures are our main priority. All in all, the main goal of this study is interpreted as creating a high quality solar water heating system with acceptable performance for household consumption that can be used even in adverse climate conditions. Therefore, attempts have been made to enhance thermal performance of a flat plate solar collector and a solar water heater by using Graphene Nanoplatelets [Gnp] which can be considered as a suitable choice among other nanomaterials based on the achieved results explained in the subsequent parts. And finally in order to check the obtained experimental data, theoretical thermal efficiency have been calculated and compared to experimental results.

Section snippets

Preparation of Gnp/H2O nanofluid

The two-step method is known as one of the standard and ordinary techniques for preparing nanofluids like the research done by Haddad et al. [25]. After making nanomaterials by every existed method such as exfoliation, and Chemical vapor deposition (CVD), they should be dispersed in the base fluid. There are some ways for dispersion such as ultrasound method selected for production of Gnp/H2O nanofluids. As a prototype, 10−2 g of Graphene nanoplatelets has been added and dispersed into 5 × 10−5 m3

Experimental setup

In this step, four components of the considered solar system consisted of a flat plate solar collector, a water heater, an inlet temperature regulator system and a thermometer have been manipulated and also two other instruments are introduced on the basis of Fig. 7 as follows:

After preparation of flat plate solar collector, a temperature regulator system is required in order to prepare different inlet temperatures of the working fluid into the collector and also a water heater is needed to

Result and discussion

It is necessary to mention that the period of collecting experimental solar data lasted only two months with the exception of weekends and the process of designing and manufacturing the introduced solar system was done during six months. Moreover, the required tests of nanomaterial and nanofluids were carried out for over four months. In order to investigate the experimental and even theoretical performance of solar systems, having enough knowledge about variation of environmental factors such

Conclusion

In the present paper, the effect of Graphene nanoplatelets on the performance of flat plate solar collectors has been investigated experimentally and theoretically. The ultrasound method has been chosen to disperse Gnps in deionized water and the results indicate that Gnps can increase thermal efficiency of solar collectors. The experiments show that the max UV–vis absorbance of the solution relates to the dispersion of Gnp in the base fluid. Also, the colloidal stability test in terms of

References (39)

Cited by (0)

View full text