Performance assessment of linear Fresnel solar reflector using MWCNTs/DW nanofluids
Introduction
Solar energy is used in several fields, including industrial and household applications. The most popular uses are electricity production, water heating, air conditioning, drying, and desalination. Solar collectors convert sunlight into thermal energy, where these solar devices are designed to collect solar energy transmitted by radiation and transfer it to a heat carrier fluid in the form of heat. This heat energy can then be used in several areas [1,2]. Solar collectors are two types according to the temperature range as follow:
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Low temperature for non-concentrating collectors which are known for their low-temperature applications, where the most famous technology is the flat solar collector (FPCs) [1];
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High temperature concentrating collectors includes linear Fresnel solar reflectors (LFRs) [[3], [4], [5]], parabolic trough solar collectors (PTCs) [[6], [7], [8]], heliostat field collectors (HFCs) [9], and Parabolic dish collectors (PDCs) [10].
Among the high temperature solar collectors, LFRs show a great interest for various applications because of its good compromise in terms of cost and performance. This type of solar concentrators is considered an emerging concentrated solar power (CSP) technology [11,12]. The operation principle of this type of solar collector is based on a plane mirror system [5,13]; the orientation of each of these mirrors revolves around a pivot following the path of the sun to concentrate the direct sunlight towards absorber tubes [14,15]. Compared to their competitive technology, i.e. PTCs, linear Fresnel reflectors have the advantages of simple production and lower construction costs [3,16]. However, the main drawbacks of this technology are the lower optical efficiency and the higher sensitivity to optical and tracking errors [17].
On the other hand, LFR technology presents the advantage to produce thermal energy from medium to high temperature levels. Such technology can be also used in various applications including industrial and building applications. In this context, small scale LFRs can provide the required solar thermal energy for space heating and domestic water production. This was studied by some scientific studies in the literature such as Ghodbane [4,16] and Sultana [18,19]. Based on the work of Ghodbane [4,16], it was observed that the thermal efficiency of the experimental device has exceeded 29%.
In the literature, various research works have been carried out to study and improve the performance of LFRs. Moreover, many researchers have investigated different configurations and designs to enhance the LFR performance. Recently, a comprehensive review study was carried out by Bellos combining the manufacturing side, the optical behavior and the thermal balance of these solar reflectors [3]. Another alternative to improve the thermal performance of LFR is to use nanofluids which can enhance the thermal properties and therefore increase the heat transfer inside the absorber tubes.
With regard to nanofluids, the first works dealing with heat transfer in the presence of nanoparticles practically started in 1995 with the study of Choi and Eastman [20], which later allowed to determine the nanofluids thermophysical properties. Nanofluids have been under extensive research for more than two decades due to the theoretically predicted enhancements and experimentally observed improvements in the thermophysical properties [[21], [22], [23]]. Nanoparticles, due to their increased thermal conductivity compared to the base fluids, results in increased heat exchange [24]. For heat transfer applications, increasing nanoparticle loading results in both advantages and challenging properties of the nanofluids. For forced conventional heat transfer applications, the enhancement in thermal conductivity is an advantage, while the increase in viscosity and higher density of the nanoparticles results in increased pressure drop and required pumping power [25,26]. In addition, aggregation happens due to strong van der Waals interactions between nanoparticles, which is still a technical challenge faced by researchers for preparing a homogenous suspension. The settlement and clogging of micro-channels are resulted due to the agglomeration and sedimentation of nanoparticles, but they also result in reducing the suspension characteristics such as the thermal properties [21,22,27]. In this respect, ensuring the colloidal stability of nanofluids and maintaining stability for longer periods is crucial.
Due to the great importance of nanofluid technologies at the moment, there is a tendency to use nanofluids in many solar applications as demonstrated by the studies carried out by Bellos et al. [28], Loni et al. (2018) [[29], [30], [31]], Said et al. [32] and Sabiha et al. [33]. These studies have shown the use of nanofluids in several fields, in a clean and environment-friendly way.
Industrial grade Multi-walled carbon nanotubes (MWCNTs) are selected for this study. They are a specific form of carbon nanotubes in which several single-walled carbon nanotubes are nested within each other when mixing nanoparticles of MWCNTs in water as the base fluid can improve the thermal conductivity [34]. Multiple studies have addressed the importance of (MWCNTs) in several industrial fields. Qu et al. studied the performance of the photo-thermal conversion and the optical absorption properties of hybrid CuO-MWCNTs/water nanoparticles in order to harvest direct solar thermal energy [35]. In addition, Nasrin et al. performed an experimental and numerical study on the cooling system of PVT using MWCNTs/water nanofluid [36]. Also, Yu et al. executed an experimental study on the rheological properties of MWCNT/water nanofluid with low concentrations [37]. Moreover, Abdallah et al., analyzed the performance for hybrid system PV/T working on low concentration MWCNT/water nanofluid [38].
Few scientific studies have addressed the improvement of the efficiency of the linear solar reflectors using nanofluids. Among these studies, Bellos and Tzivanidis conducted a study that allows the improvement of the thermal efficiency of a LFR reflector based on Syltherm 800/CuO nanofluid with volumetric nanoparticle concentration equal to 6% [39]. With flow rate equal to 200 L min−1 and inlet temperature between 350 K and 650 K, the thermal efficiency of the studied device improved from 0.22% to 0.78%. In addition, Bello et al. have investigated the thermal performance of a linear Fresnel collector where the Syltherm 800/CuO was used as working fluid [40]. With 200 L min−1 flow rate and inlet temperature equal to 600 K, the improvement in thermal efficiency of the studied cases is 0.28, 0.61 and 0.82% for the cases 4% nanofluid in the smooth absorber, the use of internal fins with pure oil and the use of 4% nanofluid in finned absorber, respectively. But these values for improvements remain weak values. These two studies are considered the most important works conducted on the LFR solar collector using nanotechnology in working fluids.
In this study, MWCNTs/DW nanofluids are used to improve the thermal efficiency of LFRs, by suspending industrial-grade MWCNTs nanoparticles in DW. Stability and thermophysical properties of the MWCNTs/DW nanofluids have been experimentally carried out. The experimental data was used for the numerical study. In addition, a numerical model is developed to determine the heat behavior of MWCNTs/DW nanofluid compared with DW. Energy balance equations at copper pipe were analyzed and simplified based on the finite difference method. A comprehensive thermal analysis was performed to examine the various aspects of the solar reflector using nanofluids.
Section snippets
Materials
Industrial grade MWCNTs was procured from Nanostructured & Amorphous Materials Inc. (NanoAmor), USA. These nanoparticles were purchased because of their low cost and non-toxicity, which has never been reported in the literature before as per the author’s knowledge. Table 1 below shows the physical and chemical properties of these nanoparticles 1. DW was prepared in the labs. Arabic Gum (AG) was used as a surfactant acquired from Sigma Aldrich, Germany.
Preparation and characterization of nanofluids
Two-step method was used to prepare
Experimental setup
The experimental setup is a small LFR made at the Institute of Mechanical Engineering, Saad Dahlab University in Blida, Algeria [4,16]. The manufacturing cost of this prototype was estimated at 346.332 EUR in 2016.
As shown in Fig. 1, the manufactured model consists of five basic parts [16]:
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The reflective mirrors: they are eleven flat rectangular reflective mirrors, whose main role is to reverse sunlight coming directly from the sun towards the receiver system;
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The receiver tube: it is a four
Numerical model
In this work, a numerical study is carried out considering all the dimensions and optical characteristics of this experimental setup. It is noted that the numerical model is developed using Matlab, where the energy equations were analyzed and approximated. In addition, the results obtained from this program have been confirmed in previous studies [4,49].
The performance predicting of the studied concentrator at a given location requires consideration of the climatic conditions. Fig. 2
Numerical solution and model validation
In order to investigate and approximation Eqs. (19), (21), the finite difference method with the backward numerical scheme was adopted. The final form of the equations after simplification and analysis is:
Results and discussion
This paper focuses on taking into account the influence of the MWCNTs/DW nanofluids characteristics on the thermal behavior of a small linear Fresnel concentrator, where the mass flow of the working fluid is 0.015 kg s−1. This subject is of great importance in terms of improving the amount of heat transfer between the copper absorber tube and the MWCNTs/DW nanofluids on the one hand, and reducing the thermal loss towards the outside environment on the other hand.
Conclusion
Linear Fresnel reflector is an emerging technology that is characterized by low cost, but its thermal efficiency is still considered low compared to other CSP technologies. One of the alternatives to alleviate this problem is to enhance heat transfer and thermal performance by using nanofluids. In this context, the heat transfer is analyzed in a small linear Fresnel collector to study the thermal performance when using nanofluids. The working fluids used in this study are DW, MWCNTs/DW (0.05%),
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
Dr. Zafar Said would like to thank the University of Sharjah, Projects #18020406118 for its financial support.
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