Elsevier

Solar Energy

Volume 204, 1 July 2020, Pages 32-47
Solar Energy

Optical, stability and energy performance of water-based MXene nanofluids in hybrid PV/thermal solar systems

https://doi.org/10.1016/j.solener.2020.04.063Get rights and content

Highlights

  • Formulated new water/MXene nanofluid for optical filtration.

  • Comparison of CTAB and SDBS surfactants effect on the optical properties.

  • Improved performance of PV/T system by using the water/MXene nanofluid.

  • Optimized thermal efficiency in hybrid PV/T system is obtained.

Abstract

Solar thermal collectors have been recognized as promising devices for solar energy harvesting. The absorbing properties of the working fluid are crucial because they can significantly influence the efficiency of the solar thermal collectors. The performance of photovoltaic-thermal (PV/T) systems can be optimized by applying nanofluids as working fluids. MXene is a newly developed 2-D nanomaterial that has proven excellent potential in electrical applications with a lack of research in the thermal and optical applications. The present work extensively studied the optical potential of the water/MXene nanofluids with respect to the variation of MXene concentrations (0.0005–0.05 wt%) and types of surfactant (CTAB or SDBS) used in a hybrid PV/T system. The relationship between the investigated parameters was evaluated through an experimental correlation. The evaluation of the nanofluids in term of the transmittance was conducted through the Rayleigh method. The MXene concentrations and the types of the surfactant play predominant role in the transmittance, absorbance and dispersion stability of the water/MXene nanofluids. The corresponding effects due to these factors become the most noticeable in the wavelengths of 300–1350 nm. Low concentration of the MXene and shorter path lengths lead to higher transmittance. The application of the low concentration of water/MXene nanofluids as the optical filtration in a hybrid PV/T system yields a higher performance compared to a conventional PV/T system. Therefore, this research work provides novelty value in understanding the impacts of using water/MXene nanofluid in the hybrid PV/T solar collectors to harness additional energy.

Introduction

World energy demand has enhanced drastically over the last few decades due to the enormous growth of the globe economy and it is expected to increase by 60% by the year 2030 (Kuang et al., 2016). The scale of the emission of CO2 reported for 2018 (33.1G tons) reveals the enormity of the impact and consumption of fossil fuels on environmental issues, including globe climate change, air pollution, ozone layer depletion, and acidic rain (Kuang et al., 2016). Considering that the energy from the hourly solar flux incident on the Earth’s surface is greater than the world energy demand of one year (Xu, 1996), the expectation of the society at large-scale is that the renewable and sustainable energy generation technologies may both fulfill the global energy demand and overcome the detrimental impact on the environment. Approximately 3,400,000 EJ of the solar radiation is estimated to reach the earth in one year, which is 7500 times the world energy demand of one year (450 EJ) (Justo et al., 2013).

Due to the eco-friendly nature and safety aspects of solar energy as well as the fact that it is an abundant source of energy without a limitation, almost 70% of world energy demand is expected to be supplied by solar energy technologies by 2100 (Quoilin et al., 2013). Solar thermal and photovoltaic (PV) technologies are the two main approaches used for solar energy harvesting (Gorji and Ranjbar, 2017). Solar PV technologies are capable of direct conversion of the sunlight to electricity, while solar thermal systems use the harvested solar energy to heat water, air or other fluids. According to the global installed capacity of the solar energy (about 70%), solar thermal systems are more popular compared to PV technologies. The thermal collector, which handles the photo-thermal conversion of solar spectrum, is the critical component of a solar thermal system (Foley et al., 2015). Solar thermal collectors can be categorized into three classes based on the operating temperature: (i) collectors with low-temperature operation such as flat-plate and evacuated-tube collectors, (ii) collectors with medium-temperature operation such as parabolic troughs, and (iii) collectors with high-temperature operation, comprising of power towers and dish-concentrators (Phelan et al., 2013). The above classification is based on the temperature of the working fluid into the thermal collectors. For instance, the flat-plate type thermal collectors can achieve fluid temperatures up to ~100 °C. The main challenge of producing an effective thermal collector is selecting a suitable absorber and a working fluid for efficient conversion of the incident solar radiation into thermal energy (Minardi and Chuang, 1975). Using the appropriate working fluid can potentially optimize the efficiency of the thermal collectors.

Nanofluids have provided outstanding improvements in terms of optical and thermo-physical properties over traditional HTFs. Mu et al. (2010) have demonstrated experimentally that a noticeable amount of the visible light is transmitted through the water/SiO2 nanofluid, and the TiO2 and ZrC nanofluids absorb most of the solar spectrum (ZrC has the highest absorbance). Sani et al. (2010) have reported that 100% of solar energy was absorbed by water/single-walled carbon nanotubes at a loading of 0.05 g/L and a penetration path of 1 cm. Mercatelli et al. (2011) evaluated the extinction coefficient of the water/single-walled carbon nanotubes at a fixed wavelength of 632.8 nm, and found that it varies linearly with the concentration of single-walled carbon nanotubes. Taylor et al. (2011) conducted a comprehensive research on the amount of solar radiation absorbed by different types of water-based nanofluids containing graphite, copper, silver, aluminum, and gold nanoparticles. They have reported absorbance values higher than 95% for all the studied nanofluids at a 10-cm collector depth. The extinction coefficient is ass measure of the light absorbing strength of a substance at a particular wavelength. Taylor et al. (2011) have theoretically investigated the improvement of the extinction coefficient. They compared the measured extinction coefficient of the studied nanofluids with the values obtained using Maxwell-Garnett and Rayleigh scattering approximation models. They concluded that the Maxwell-Garnett model gives better prediction of the results at longer wavelengths when compared with short wavelengths (visible range). Said et al. (2015) evaluated the impacts of concentration and/or size of TiO2 nanoparticles on the extinction coefficient through Rayleigh approach. They have reported that the smaller particle size (less than 20 nm) has negligible impact on the optical properties of the nanofluids and the relationship between the volume fraction and extinction coefficient is linear. Tyagi et al. (2009) reported that the solar radiation absorbance of water/aluminum nanofluid is nine times that of pure water. According to them, as the absorbance and transport of solar energy can be targeted simultaneously in a volumetric absorption collector, nanofluids can enhance the energy efficiency and lessen the energy loss. Jing et al. (2015) studied the optical and thermal properties of silica/water nanofluid at different nanoparticles sizes. The authors reported that for a nanoparticles size of 5 nm and a volume fraction of 2%, the transmittance of the nanofluid was up to 97%, which is nearly the transmittance of a pure water. However, the thermal conductivity was higher by 20% for the water/silica nanofluid. Using CFD simulation to apply their results on a PV/T system, they have found that water/silica at size of 5 nm and volume fraction of 2% is the best for a PV/T system.

Lee et al. (2012) adopted a Mont Carlo algorithm combined with the Mie scattering theory to assess the optical properties of the nanofluids. Taylor et al. (2012) designed optical filters using nanofluids with suspended core/shell nanoparticles for PV/T systems. They achieved higher efficiency of solar energy harvesting compared to that of conventional optical filters. Recently introduced two-dimensional materials (called MXene family), which are comprised of early transition metal carbides/nitrides, represent supreme thermo-physical properties (Naguib et al., 2011). Chemical etching is conducted on the materials so-called Mn+1AXn phases to selectively remove the A layers, where A is mostly adopted from group IIIA/IVA of the periodic table, M represents a transition metal, X indicates C or N and n is denoted as 1, 2, or 3. The resulted MXene and its composites exhibit a promising feature of electromagnetic radiation absorption capability, which is contributed by their high electromagnetic interface (EMI) shielding effect in nature, as reported by Shahzad et al. (2016). Their finding has driven the great efforts given by the other scientists in studying the relationship between the MXene and certain electromagnetic waves, such as the sunlight. Excellent radiation absorption and the subsequent heat generation done by these materials could possibly render them to become the supreme material for light-to-heat (photo-thermal) conversion.

Several attempts have been made to assess the optical performance of the nanofluids (Mercatelli et al., 2011, Taylor et al., 2011, Said et al., 2015, Tyagi et al., 2009, Jing et al., 2015). The unique applicability of the nanofluids in the hybrid PV/T solar collectors is mainly due to their ability of transmitting solar radiation, demanding the comprehensive research that is related to the optical characterization on these materials. The transmittance of nanofluids is very important optical parameter to determine their utility for optical filtration. The principle of using filtration device in the PV panels is to selectively transmit the solar radiation in its beneficial range. Consequently, the efficiency of a hybrid PV/T system strongly depends on the transmittance of optical filter. Meanwhile, the dispersibility of the nanofluids are also closely related to their light transmittance capability. The developed nanofluids in current study, represent promising dispersibility performance based on the zeta potential measurement values. The preparation of a good dispersion of the water-based MXene nanofluids is the most challenging part. Surfactants could be used to achieve homogeneous water/MXene nanofluids. Hence, in this study, two different surfactants, sodium dodecyl benzene sulphonate (SDBS) and cetyltrimethylammonium bromide (CTAB) were used separately to prepare water/MXene nanofluids for the application in a hybrid PV/T system. The prepared nanofluids work as the optical filters of solar radiation. Zeta potential measurement, Fourier transform infrared (FTIR) spectroscopy, UV–Vis spectroscopy, and morphology determination were conducted to determine the suitability of the as prepared nanofluids for applications in hybrid PV/T systems. After the characterization of the water/MXene nanofluids they were incorporated into a hybrid PV/T system.

The novelty of the current study is the development of promising water/MXene nanofluids using two different types of surfactants as a new optical filter with high stability for enhancement of optical performance in a hybrid PV/T system. For the analytic evaluation of the developed nanofluids, a modified Rayleigh method was used in solar thermal applications. A new correlation between the viscosity and stability of the as-prepared water/MXene nanofluids was developed. The transmittance spectra of the water/MXene nanofluids proved that the type of surfactants can influence the transmittance spectra effectively. The water/MXene nanofluid prepared using the SDBS surfactant displays higher spectral transmittances when compared to that prepared using the CTAB surfactant. For both surfactants, an inverse relationship was observed between the MXene nanoflakes concentration and the transmittance spectra of the prepared water/MXene samples. The influence of the nanoflakes concentration on the transmittance spectra was significant. The effect of the surfactant and the nanoparticles-concentration was most observed in the range of 300 – 1350 nm, which corresponds to the Ultraviolet (UV), the Visual (Vis) and the Near Infrared (NIR) ranges. Also, the as prepared water/MXene nanofluids have a good extinction coefficient of ~1.5 Lg−1 cm−1, revealing sufficient light absorption capability even at a low concentration of MXene in the nanofluids. The experimentally acquired results indicate that absorbance is directly proportional to the concentration of MXene nanoflakes. To the best of authors’ knowledge, there are a very limited number of experimental studies undertaken to investigate the band gap of MXenes. In this study, the band gap energy of the nanofluids with the highest concentration of 0.05 wt% MXenes dispersed using CTAB or SDBS are determined using the Tauc method employing the Kubelka-Munk function. For the water/MXene nanofluid with a low concentration of 0.0005 wt%, the efficiency of the hybrid PV/T system is about 20%. Thus, the findings of the present study reveal that optical filtration using the water/MXene nanofluid at a low concentration in a hybrid PV/T system can provide a superior performance compared to the standalone PV system.

Section snippets

Methodology

The water/MXene nanofluid as optical filtration for hybrid PV/T systems was evaluated experimentally. The impact of the concentration of the nanoparticles and the type of surfactant on the optical properties and stability of the nanofluid were investigated. After preparation of the water/MXene nanofluids, the characterization and the optical properties were determined as a function of the nanoparticles concentration. Finally, the electrical and thermal performance of a hybrid PV/T system using

Zeta potential

Immediately, after the preparation of the water/MXene nanofluids, the dispersion quality of the MXene nanoparticles in water was assessed by the determination of the average zeta potential for each sample. An average absolute value of the zeta potential of over 30 indicates the presence of a good dispersion and a highly stable nanofluid. The higher the average absolute value of zeta potential, the higher the stability of the dispersion of the nanofluid. Table 3 demonstrates the measured average

Conclusions

The effects of the concentration of MXene (Ti3C2) nanoflakes and the surfactants of CTAB and SDBS on the optical properties of the water/MXene nanofluids are evaluated. The dispersion stability of the water/MXene nanofluids is determined through visual inspection and by monitoring the transmittance change with time. The stability assessment conclusions are summarized as:

  • Observation from visual inspection showed high degradation of the nanofluids at all concentrations after five days.

  • Monitoring

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.

Acknowledgements

R. Saidur would like to acknowledge the financial support provided by Sunway University, Malaysia, through the project No. STR-RCTR-RCNMET-001-2019. In addition, the authors appreciate the financial support provided by King Fahd University of Petroleum & Minerals (KFUPM), Saudi Arabia, through the project no. DSR-IN161059.

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