Original article
Enhancing the efficiency of a symmetric flat-plate solar collector via the use of rutile TiO2-water nanofluids

https://doi.org/10.1016/j.seta.2020.100783Get rights and content

Abstract

Flat-plate solar collectors are one of the cleanest and most efficient heating systems available. This experimental study explores the effect of replacing water with surfactant-free rutile TiO2–water nanofluids as the working fluid in a symmetric flat-plate solar collector. The efficiency of the collector was investigated in Aghajari, a city in the south of Iran. Analysis was performed according to the ASHRAE standard, taking into account the flow rate of the heat transfer fluid (HTF), the solar irradiance, and the temperature difference between the inlet and outlet. Results show that the use of TiO2–water nanofluids can improve thermal efficiency relative to water. As the solar irradiance or the HTF flow rate increases, the effect of nanoparticle addition on the collector efficiency gains becomes more pronounced. The maximum efficiency of the collector, when filled with a 1 wt% TiO2-water nanofluid, is found to be approximately 78%; this represents maximum and average efficiency gains of 9.80% and 6.64%, respectively, relative to the water baseline. In addition, the maximum efficiency gains are 17.41%, 27.09%, and 33.54% for 1 wt%, 3 wt%, and 5 wt% nanoparticle concentration, respectively.

Introduction

Solar energy is one of the cleanest energy resources available. Photovoltaic (PV) systems convert sunlight into electricity directly, whereas solar thermal systems concentrate the heat from solar radiation through collectors, after which the heat is transferred to a heat transfer fluid (HTF) to produce either hot water or electricity. Such collectors are considered an integral part of solar thermal systems. One of the most common types of solar collectors is the flat-plate solar collector, which operates at relatively low temperatures and therefore delivers sub-optimal efficiency [1], [2]. To increase the efficiency of such collectors, various methods have been proposed, with the use of HTFs with improved heat transfer properties being a preferred approach. For example, nanofluids, which are suspensions of nanoparticles (whose sizes are under 100 nm), can be added to HTFs to enhance its thermal properties and, hence, the efficiency of the overall collector.

For that reason, the use of nanofluids in solar collectors has attracted growing interest in recent years [3], [4]. Otanicar et al. [5] experimentally compared the use of different nanofluids as HTFs in direct absorption solar collectors. They reported that, in comparison with pure water, when nanofluids were used as HTFs, the thermal efficiency of the solar collector increased by up to 5%. Yousefi et al. [6] conducted a large empirical study of the use of nanofluids, such as Al2O3-water with a weight fraction of 2–4%, in a flat-plate solar collector. They noted that, for a nanofluid HTF with weight fraction of 2%, the efficiency gains can be up to 28.3% in comparison with pure water. Yousefi et al. [7] reported that the thermal efficiency of a flat-plate solar collector increased by up to 28.3% when a Al2O3 nanofluid with volume fraction of 0.2% was used to replace pure water. After exploring the effect of pH of nanofluid-based HTFs, they found that by optimizing the pH value, the efficiency of flat-plate solar collectors can be maximized [7]. Hey et al. [8] experimentally studied the effect of the size and concentration of copper nanoparticles in Cu-water HTFs. They reported that adding Cu nanoparticles to pure water increased the collector efficiency by up to 23%. Jabari Moghadam et al. [9] experimentally investigated the effect of CuO nanoparticle addition (into water) on the efficiency of a flat-plate solar collector. They found that when a CuO-water nanofluid with volume fraction of 0.4% and flow rate of 1 kg/min was used as the HTF, the collector efficiency rose by 21.8%. They also reported that nanoparticle addition often increased the collector efficiency and recommended an optimum value of the HTF flow rate for each condition.

Lu et al. [10] investigated the thermal performance of an open thermosyphon in high-temperature evacuated tubular solar collectors. Experiments were conducted with both pure water and water-CuO nanofluids as the working HTFs. This study highlighted the role of nanofluids in improving the performance of solar collectors, particularly by increasing the evaporating heat transfer coefficients by 30%. Noghrehabadi et al. [11], [12] investigated the use of SiO2-water nanofluids in a novel 3-D conical solar collector and a square flat-plate collector. They showed that increasing the HTF flow rate could directly affect the overall performance gain. Sint et al. [13] theoretically estimated the efficiency of a CuO-water nanofluid-based flat-plate solar collector. They showed that using CuO-water nanofluids can enhance the collector performance by up to 5%. Ozsoy and Corumlu [14] experimentally examined the effect of silver-water nanofluid utilization on the thermal performance of a thermosyphon heat-pipe evacuated tube solar collector. They observed that the THP charged with a silver-water nanofluid maintains its desirable heat transfer characteristic in the THP experiments. In addition, it was observed that adding nanoparticles to a baseline HTF can increase both natural and mixed convection as well as conductive heat transfer [15]. Sundar et al. [16] experimentally investigated the effect of aluminum oxide–water nanofluid utilization as the working HTFs. The volume fractions used were 0.1% and 0.3%. As the HTF flow rate and the nanoparticle volume fraction increase, the thermal efficiency was also found to increase, producing a maximum efficiency gain of 28.7%. In experiments, Sardarabadi et al. [17] reported that when a SiO2-water nanofluid was used as the HTF (instead of water), the thermal efficiency of a photovoltaic thermal unit rises by up to 12.8%. Jouybari et al. [18] examined the effect of silica-nanoparticle addition on the performance of a flat-plate solar collector containing porous channels. They concluded that for a volume fraction of 0.6%, the thermal efficiency was about 81%.

Mirzaei [19] experimentally investigated the thermal characteristics of a flat-plate solar collector filled with a CuO nanofluid. It was concluded that the collector efficiency can increase by up to 15.2%, 17.1% and 55.1% for flow rates of 1, 2, and 4 L/min, respectively. Accordingly, it was reported that in comparison with pure water, the use of Cu-water and Al2O3-water HTFs increases the thermal efficiency of a flat-plate solar collector by more than 20%, at the optimum HTF flow rate [20], [21]. Elsheikh et al. [22] conducted an advanced review of the application of nanofluids in solar energy systems. They discussed the application of different nanofluids in solar collectors, solar photovoltaic systems, solar water desalinations, and other solar devices. Their study showed that the use of nanofluids as HTFs can increase the thermal efficiency at almost all operating conditions. Gupta et al. [23] investigated the effects of a low-temperature Al2O3-water nanofluid on the absorption parameters of a solar collector. They reported that using the nanofluid as the HTF boosts the optical properties as well as the collector efficiency. Kilic et al. [24] conducted an experimental study on the effect of TiO2 nanoparticle addition on the thermal efficiency of a flat-plate solar collector. The experiment was carried out for 0.2 wt% nanofluid at flow rate of 0.033 kg/s. The results showed that replacing pure water with a nanofluid causes the thermal efficiency to increase by up to 34.45%. Said et al. [25] found that as the TiO2 nanoparticle volume fraction increases from 0.1% to 0.3%, the thermal efficiency of a flat-plate solar collector increases by up to 56%. In addition, they found that changes to the HTF flow rate influences the thermal efficiency of the collector significantly. Tiwari et al. [26] experimentally showed that the use of a Al2O3 nanofluid as the HTF enhances the thermal efficiency of a flat-plate solar collector by up to 30%. To achieve this maximum efficiency gain, the volume fraction of Al2O3 was 0.15%. Gangadevi et al. [27] showed that in a flat-plate solar collector, when a Al2O3 nanofluid was used as the HTF rather than water, the efficiency gain was approximately 30%. They also reported that as the volume fraction of Al2O3 nanoparticles increases, the thermal efficiency of the collector improves.

In this experimental study, the effect of TiO2-water nanofluids on the thermal performance and efficiency of a symmetric square flat-plate solar collector was investigated. Water and rutile nanofluids with varying nanoparticle concentrations were considered. In comparison with previous studies on the experimental investigation of the efficiency gain of flat-plate solar collectors arising from TiO2 nanoparticle addition, the main novelty of this work is that experiments were conducted at different operating conditions, including a wide range of HTF flow rates and nanoparticle concentrations. In the present work, eight different HTF flow rates (ranging from 0.005833 to 0.046667 L/s) were used. Another novel aspect is our use of rutile nanofluids. Furthermore, the dimensions of the solar collector, the length and diameter of the absorber tube, the inclination of the plate, and the height of the collector are all unique aspects of the present study.

Section snippets

Methodology and materials

In this section, the geometrical properties of the flat-plate solar collector are introduced. Moreover, the geographical details of the location where the experiments were conducted are specified. Then, the experimental setup is presented. In addition, the procedure used to prepare the rutile nanofluid mixtures is discussed.

Experimental procedure

To quantify the collector efficiency, the ASHRAE Standard 93–86 [28] was used. This is one of the most established standards available for analysis of the performance of stationary solar collectors. Accordingly, it has been used in many previous studies of solar collectors [7], [8], [10], [30]. The efficiency of a collector is given by the ratio of the useful energy produced to the incoming energy to the collector. The incoming energy in a solar water heater or a solar flat-plate collector

Results and discussion

After calibrating the devices and setting up the laboratory equipment, experiments were conducted in mild and sunny weather. These tests were repeated over several days, resulting in the collection of multiple data sets. The data were collected every 15 min, from 08:00 to 16:00, in steady state or quasi-steady state. The data included the efficiency and performance of the collector, as well as the thermal and temperature performance under different conditions.

The experiments were carried out on

Conclusions

This experimental study was conducted to explore the effect of replacing pure water with surfactant-free rutile TiO2-water nanofluids as the working fluid in a symmetric square flat-plate solar collector. Focus was placed on comparing these different HTFs in terms of the collector efficiency and the temperature difference between the inlet and outlet. The experimental measurements were performed according to ASHRAE Standard 93–86 at different environmental conditions. The following conclusions

CRediT authorship contribution statement

Mojtaba Moravej: Conceptualization, Methodology, Writing - original draft. Mehdi Vahabzadeh Bozorg: Writing - original draft, Validation. Yu Guan: Data curation. Larry K.B. Li: Data curation, Writing - review & editing. Mohammad Hossein Doranehgard: Data curation. Kun Hong: Investigation. Qingang Xiong: Supervision, Writing - review & editing.

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.

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