Search

Summary: Nanofluid can be used in a CPV/T solar collector to boost electrical and thermal performances as this technology has drawn great attention from researchers over the last decades. In a CPV/T system, the amount of collected heat could be significantly higher than the amount of electrical power. Combining thermoelectric generator (TEG) and nanofluid‐based CPV/T system may result in better electrical performance than CPV/T system alone. In the present work, a nanofluid‐based CPV/T‐TEG hybrid system with a cooling channel was designed and tested, and the obtained performance was compared with conventional cooling methods [ie, natural cooling (CPV/TEG) and water cooling (WCPV/T‐TEG) methods]. At the optimum value of solar concentration, C = 14.6, the electrical performance of the nanofluid‐based concentrated photovoltaic/thermal‐thermoelectric generator (NCPV/T‐TEG) configuration was found to be ~89% higher than the standard PV modules. For the same concentration, the electrical performance of the above configuration was found to be ~13.9% and ~8.4% higher than CPV/TEG and WCPV/T‐TEG configurations, respectively. In addition, the overall thermal energy of the NCPV/T‐TEG was found to be higher by 4.98% compared to WCPV/T‐TEG hybrid system. The NCPV/T‐TEG configuration was found to produce 92.47%, 41.06%, and 8.8% higher daily exergy compared to standard PV cell, CPV/TEG, and WCPV/T‐TEG, respectively. Overall, the proposed design of the NCPV/T‐TEG hybrid system has the potential for further development in high‐concentration solar systems. [ABSTRACT FROM AUTHOR]

Different properties of nanofluids have been studied by the researchers since the last two decades. Most of the studies have focused on the thermal properties of the nanofluids. However, optical properties have considerable contribution to heat absorbance in nanofluids. Therefore, it is necessary to study the different parts of solar spectrum (optical properties) to utilize nanofluids in solar thermal applications. The optical properties (absorption, transmittance, scattering, and extinction coefficient) based on metal, metal oxide, carbon nanotubes, graphite, and graphene have been reviewed thoroughly in variation with particle size and shape, path length, and volume fraction. The present investigational outcomes about the nanofluids showed that optical solar absorption increased accordingly with increasing nanoparticle size and volume concentration. However, there were some conflicting results on the effects of nanoparticle size on absorption, in which the particle size has an insignificant effect on overall optical properties. Moreover, it was observed that path length has some remarkable effects over optical absorption of nanofluids. The experimental results revealed that the transmittance of nanofluids has indirect relation with nanoparticle size, volume fraction, and path length. The scattering of light is directly proportional to the volume concentration and particle size of metallic particles. Overall, results of various elements showed that the presence of large particles and particle agglomerates leads to significant amount of light scattering. As a result, overall extinction coefficient will be increased. Therefore, an optimization of these properties need to be maintained for stable and cost-effective nanofluid. [ABSTRACT FROM AUTHOR]

Application of nanofluid to increase the thermal efficiency of a traditional solar collector is getting tremendous attention among the scientific community. Al 2 O 3 –water nanofluid, as a working fluid and its effect on the energy and exergy efficiencies of a flat plate solar collector was examined experimentally. Volume fraction used for this study was 0.1% and 0.3%, while the size of the nanoparticles was ∼13 nm. Experiments were carried out using a stable nanofluid which was obtained by controlling the pH of the solution over a period of 30 days. The mass flow rates of the nanofluid varied from 0.5 to 1.5 kg/min. Energy and exergy efficiencies of a flat plate solar collector using water and nanofluids as working fluids were matched. The results revealed that nanofluids increased the energy efficiency by 83.5% for 0.3% v/v and 1.5 kg/min, whereas the exergy efficiency was enhanced by up to 20.3% for 0.1% v/v and 1 kg/min. Thermal efficiency of the system was found to be more than 50% compared to the existing system available in the literature. New findings on the stability and exergy analysis of the solar collector system operated with a pH controlled nanofluid are reported. [ABSTRACT FROM AUTHOR]

An experimental study was performed to determine the thermal efficiency of an Evacuated Tube Solar Collector (ETSC) using water based Single Walled Carbon Nanotubes (SWCNTs) nanofluids. Experiments were carried out using SWCNTs nanofluids having volume concentrations of 0.05, 0.1, and 0.2 vol.%. The performance of the collector was compared with SWCNTs nanofluid and water using the flow rates of 0.008, 0.017, and 0.025 kg/s. The experiments were undertaken according to ASHRAE standard 93-2003. The results show that, the collector efficiency improved with SWCNTs nanofluids compared to water as a working fluid. The maximum efficiency found to be 93.43% for 0.2 vol.% SWCNTs nanofluids at a mass flow rate of 0.025 kg/s. The collector efficiency shows greater enhancement with the increasing volume fractions of SWCNT nanoparticles and flow rate. In conclusions, results suggest that SWCNTs nanofluids can be used as the working fluids in an ETSC to absorb heat from solar radiation and to convert solar energy into thermal energy efficiently. [ABSTRACT FROM AUTHOR]

Solar energy is the most available, environmental friendly energy source and renewable to sustain the growing energy demand. Solar energy is captured by solar collectors and an evacuated solar collector is the most efficient and convenient collector among various kinds of solar collectors. In this paper, a comprehensive literature on why evacuated collector is preferable, types of evacuated collectors, their structure, applications and challenges have been reviewed. Latest up to date literature based on journal articles, web materials, reports, conference proceedings and thesis have been compiled and reported. Applications of evacuated solar collectors in water heating, heat engines, air conditioning, swimming pool heating, solar cooker, steam generation and solar drying for residential and industrial sectors have been summarized and presented. Collector efficiency of different types of evacuated collectors and their performance based on different working fluids have been reported as well. Based on the available literature, it has been found that an evacuated tube collector has higher efficiency than the other collector. An evacuated tube collector is also very efficient to be used at higher operating temperature. There are few challenges that have been identified and need to be addressed carefully before installing an evacuated tube solar collector. However, after critically analyzing the available literature, authors have presented some future recommendations to overcome the barriers and for enhanced performance of an evacuated tube solar collector. [ABSTRACT FROM AUTHOR]

To overcome the environmental impact and declining source of fossil fuels, renewable energy sources need to meet the increasing demand of energy. Solar thermal energy is clean and infinite, suitable to be a good replacement for fossil fuel. However, the current solar technology is still expensive and low in efficiency. One of the effective ways of increasing the efficiency of solar collector is to utilize high thermal conductivity fluid known as nanofluid. This research analyzes the impact on the performance, fluid flow, heat transfer, economic, and environment of a flat-plate solar thermal collector by using silicon dioxide nanofluid as absorbing medium. The analysis is based on different volume flow rates and varying nanoparticles volume fractions. The study has indicated that nanofluids containing small amount of nanoparticles have higher heat transfer coefficient and also higher energy and exergy efficiency than base fluids. The measured viscosity of nanofluids is higher than water but it gives negligible effect on pressure drop and pumping power. Using SiO nanofluid in solar collector could also save 280 MJ more embodied energy, offsetting 170 kg less CO emissions and having a faster payback period of 0.12 years compared to conventional water-based solar collectors. [ABSTRACT FROM AUTHOR]

Numerical study of heat transfer phenomena has become a major field of research nowadays. In engineering applications, different boundary conditions arise which have various effects on heat transfer characteristics. For the present work, a triangular-shape cavity has been analyzed for the sine-squared thermal boundary condition which is common in practical cases. The augmentation of heat transfer has been done by introducing a nanofluid inside the cavity. Different solid volume fractions(φ = 0, 0.05, 0.1, 0.2)of water-CuO, water-Al2O3, and water-TiO2nanofluid have been tested for the cavity with a wide range of Rayleigh number(Ra = 105–108)and for dimensionless time(τ = 0.1 to 1). The Galerkin weighted residual finite-element method has been applied for the numerical solution, and numerical accuracy has been checked by code validation. The heat transfer augmentation for different nanofluids has been done in the light of local(NuL)and overall Nusselt number(Nuav), and the results have been presented with streamline, isotherm, and related contours, in graphs and charts. It has been found that variable boundary condition has significant effect on flow and thermal fields and increase of solid volume fraction enhances the heat transfer. [ABSTRACT FROM PUBLISHER]

Recent studies specify that designated nanofluids may increase the proficiency of direct absorption solar thermal collectors. To determine the efficiency of nanofluids in solar applications, their capability to change light energy to thermal energy must be identified (i.e., the absorption spectrum of the solar material). In view of that, this study compares model predictions to spectroscopic measurements of extinction coefficients over wavelengths that are important for solar energy (200– 1100 nm). In the first decade of nanofluid research, most of the focus was on measuring and modeling the fundamental thermophysical properties of nanofluids (i.e., thermal conductivity, density, viscosity, and convection coefficients). Lately, considerable focus is given to the fundamental optical properties of nanofluids. However, the effect of particle size, shape, and volume fraction of nanoparticles as well as alternation of the base fluids, which can significantly affect scattering and absorption, have not been addressed to date in the literature. In this study, the effects of size and concentration of TiO2nanoparticles on the extinction coefficient were analyzed using the Rayleigh approach. The results show that smaller particle size (<20 nm) has a nominal effect on the optical properties of nanofluids. Volume fraction is linearly proportionate to the extinction coefficient. Considering a nanoparticle size of 20 nm, almost 0% transmissivity is obtained for wavelengths ranging from 200 to 300 nm. However, a sudden increase of 71% in transmissivity is noted from 400 nm, gradually increasing to 88% and becoming similar to that of water at 900 nm. Promising results are observed for volume fractions below 0.1%. [ABSTRACT FROM AUTHOR]

A numerical investigation has been carried out in this paper to study the effect of varying Lewis number on the nature of heat and mass transfer in a triangular solar collector at unsteady condition. The triangular solar collector consists of two inclined glass covers and an absorber plate. The situation studied here resembles heat transfer phenomena in solar collectors and hence a thorough study on the effect of different parameters dictating this kind of heat transfer is essential. Bottom corrugated wall is subjected to high temperature and high concentration. Finite element method was employed to solve the unsteady dimensionless governing equations of continuity, momentum, energy and concentration of the problem. Calculations were carried out for Raleigh number ranging from 104 to 106 and for dimensionless time parameter = 0.1, 0.5 and 1. For the conditions mentioned here the effect of varying local and overall Nusselt and Sherwood number at those conditions are discussed. A comprehensive explanation follows each result. [ABSTRACT FROM AUTHOR]

Highlights: [•] Single wall carbon nanotubes (SWCNTs) based nanofluids as an absorbing medium. [•] SWCNTs nanofluid could reduce the entropy generation by 4.34%. [•] SWCNTS can enhance the heat transfer coefficient by 15.33%. [•] SWCNTS should a small penalty in the pumping power by 1.20%. [•] More efficient solar collectors as a building technology to reduce primary energy. [ABSTRACT FROM AUTHOR]

Highlights: [•] A new concept of a cogeneration system is proposed and investigated. [•] The system comprises solar collector, PV, SOFC and heat exchanger. [•] 83.6% Power and heat generation efficiency has been found at fuel cell mode. [•] 85.1% Efficiency of SOSE has been found at H2 production mode. [•] The heat to power ratio of SOFC mode has been found about 0.917. [Copyright &y& Elsevier]

Abstract: The present review is an extensive perspective of the research progress arisen in the performance of solar collector using nanofluids. The increase in the price of fossil fuels and rapid depletion of conventional energy sources are among the major energy concerns. Solar collector, as a kind of green and renewable energy device, can help us stay out of these energy concerns. Low efficiency and high cost of solar collectors compared with the conventional devices persuade scientists and engineers to make effort to increase performance of solar collectors. Nanofluid – the suspension of nanoparticles into a basefluid – has predominant characteristics because of nanoparticles' small size and high surface area. Many researchers evaluated these special properties of nanofluids, using several methods and techniques. Mathematical and numerical methods are practiced and experimental methods come to validate the results. Using nanofluid instead of conventional fluid improves heat transfer as well as optical and thermal properties, efficiency, transmittance and extinction coefficient of solar collector. Based on comprehensive studies, it has been also realized that the thermal properties of nanofluid such as thermal conductivity have significant effect on improving the efficiency of direct solar absorption collectors. On the other hand, using nanofluid is a big challenge in terms of economical aspects. Moreover, there is a lack of study on the effect of nanofluid's optical properties such as transmittance and extinction coefficient on the performance of solar collector. Similarly, effort should be made to perform two-phase analysis of nanofluid and study properties of nanofluid with more than one type of nanoparticle. [Copyright &y& Elsevier]

Highlights: [•] By using nanofluid, smaller and compact solar collector can be produced. [•] The average value of 220MJ embodied energy can be saved. [•] The payback period of using nanofluid solar collector is around 2.4years. [•] Around 170kg less CO2 emissions in average for nanofluid solar collector. [•] Environmental damage cost is lower with the nanofluid based solar collector. [ABSTRACT FROM AUTHOR]

Abstract: As conventional energy sources like fossil fuels are getting rare, cost of energy production has become higher as well as the concern of environmental pollution by burning of fossil fuels among the developed and developing nations. Solar energy is the most vastly available energy and very effective in terms of energy conversion. The most common solar thermal collector used is the black surface as radiant absorber but the thermal energy efficiency is low. In this study, the effect of nanofluid has been analyzed by using as working fluid for direct solar collector. The extinction coefficient of water based aluminum nanofluid has been investigated and evaluated by varying nanoparticle size and volume fraction. The particle size has minimal influence on the optical properties of nanofluid. On the other hand, the extinction coefficient is linearly proportionate to volume fraction. The improvement is promising within 1.0% volume fraction and the nanofluid is almost opaque to light wave. [Copyright &y& Elsevier]

A specific part of the incident sun light on the PV panel that is responsible for the overheating of the cells might be filtered using nanofluids-based optical filter. The goal of this particular analysis is to, numerically, look into the functionality of the hybrid Photovoltaic/thermal (PV/T) solar system accompanied with an optical filtration channel (PV/OF). This analysis is among the first studies to evaluate the impact of many optical filtration managing parameters on the functionality of the hybrid PV/OF system at different atmospheric conditions and solar concentrations using the water/silver nanofluid as a working fluid. The thermal, optical, and electrical models have been developed to assess the common performance of the hybrid system. The effects of the loading and size of the nanoparticles, the height of the optical nanofluid channel and the rate of flow on the common performance of the system at different atmospheric conditions have been examined. The numerical results were validated with experimental data readily available in the literature. Comparatively to the individual PV system, the hybrid system has declared better general performance. This specific enhancement was pronounced at higher atmospheric temperatures and solar concentrations. At an atmospheric temperature of 45 °C and solar concentration of 5, the hybrid system was able to produce both electrical and thermal energies with efficiencies of 8.4% and 66.7% respectively, at the same time, where the standalone PV system was showing an electrical efficiency of 3.7%. Nevertheless, the electrical performance was not better at atmospheric temperatures lower than 34.1 °C in addition to unity solar concentration. At 25 °C, the hybrid system shows electrical and thermal efficiencies of 10.7% and 57.7%, respectively, compared to an electrical efficiency of 11% for the standalone system. The results of the study reveal that the hybrid PV/OF system can be an excellent replacement for the single PV system and its potential becomes more significant at the high atmospheric temperatures and solar concentrations. • The fluid-based optical filtration is a significant solution for the PV overheating problems. • Studying effect of optical nanofluid controlling-parameters at different weather conditions. • The system is highly recommended at high temperatures and solar concentrations. • Up to 100% enhancement in the electrical efficiency compared to the single PV. • More solar obstruction at higher loadings and sizes of nanoparticles and layer-thicknesses. [ABSTRACT FROM AUTHOR]

• Influence of optical nanofluid and nanoPCM on performance of different PV/T systems. • Distinguishing the difference between cooling and filtration effects of optical nanofluid. • Optical filtration enhances the overall performance of PV/T system. • An air gap between the optical filtration and PV enhances thermal performance. • NanoPCM layer resists the heat transfer from PV panel. Analytical and numerical solutions were developed to assess the effects of introducing a nanofluid for optical filtration (OF) above the PV module and a layer of a nano-enhanced phase change material (nanoPCM) below the PV panel, on the overall performance of a photovoltaic-thermal solar collector. The analytical study employed six models (M1–M6) with different arrangements of the cooling channel, optical filtration channel, nanoPCM and the number of optical filtration passes. The optical fluid demonstrated an enhancement in the performance by cooling the PV module by convection beside the optical filtration effect. A numerical study was undertaken to distinguish the effects of the optical fluid in enhancing the performance of the photovoltaic-thermal system. The numerical study employed three extra models (N1–N3), which use cooling and optical filtration only. The optical filtration fluid channel and the PV module are separated by an airgap in N-1, while in N-2 the optical filtration channel is directly above the PV module. Optical filtration is not used in N-3. The effect of the channel height, nanoparticle concentration, and mass flow rate on the performance was evaluated. The analytical results show that the use only of a nanoPCM decreases the overall efficiency (thermal + electrical) by about 6.7% (neglecting the storage effects of the nanoPCM). However, the introduction of optical filtration makes the decrease in efficiency very small (<1%). Moreover, using the optical filtration alone improves the overall efficiency by 6–12%. The thermal performance can be improved if the optical fluid channel and the PV panel are separated by an airgap. However, this may reduce the electrical efficiency. [ABSTRACT FROM AUTHOR]

This study aims to investigate water extraction process from a solar cooling system using a vapor absorption chiller under variable fresh air ratios. The system consists of an evacuated tube solar collector, lithium bromide absorption chiller and a fan coil unit (FCU). A parametric study is carried out to investigate the effects of flow rate of the fluid in the collector, solar insolation, fresh air volume ratio, temperature and humidity on the system performance and rate of water production. The operating conditions for the best performance are identified in this work. The results showed maximum collector efficiency of 0.66 at an optimum flow rate of the collector fluid of 0.3 kg/s at A c = 28 m 2 , T f = 45 °C, I = 800 W/m 2 and R = 50%. For the same conditions, useful energy to the generator was found to be 14.8 kW and water production rate was 8 L/h. Using the climate data of a typical day of August for Dhahran, Saudi Arabia, the findings indicated that the chiller COP and water production rate, respectively, reached maximum (0.73 and 6.6 L/h) at noon when the incident solar flux is peak (935 W/m 2 ) for 45% fresh air volume ratio. [ABSTRACT FROM AUTHOR]

The use of TiO 2 –water nanofluid as a working fluid for enhancing the performance of a flat plate solar collector has been studied. The volume fraction of the nanoparticles was 0.1% and 0.3% respectively, while the mass flow rates of the nanofluid varied from 0.5 to 1.5 kg/min, respectively. Thermo-physical properties and reduced sedimentation for TiO 2 -nanofluid was obtained using PEG 400 dispersant. The results reveal the impact and importance of each of these parameters. Energy efficiency increased by 76.6% for 0.1% volume fraction and 0.5 kg/min flow rate, whereas the highest exergy efficiency achieved was 16.9% for 0.1% volume fraction and 0.5 kg/min flow rate. Results showed that the pressure drop and pumping power of TiO 2 nanofluid was very close to the base fluid for the studied volume fractions. [ABSTRACT FROM AUTHOR]

Abstract: Optical properties of nanofluids are vital for calculating performance of a Direct Absorption Solar Collector (DASC). Characteristics of nanofluids are not constant; they vary with time and growth of nanoparticles. For current investigation, nanofluids were prepared to obtain considerable stability. Stability ratio of our nanofluids was 100 times larger than the threshold limit. Here, we have investigated aggregation process and its effect on optical characteristics of the nanofluids using Transmission Electron Microscopy (TEM) imaging, Dynamic Light Scattering (DLS) approach and UV–visible spectroscopy. Steps of aggregation are broadly described with TEM images. Our results indicate that extinction coefficients of the nanofluids reduce rapidly with time within visible to near IR region. Quasi Crystalline (QC) and Rayleigh Approaches were used to compare the experimental behavior of optical properties of nanofluids. It was found that both of these approaches are weak to predict the optical behavior, especially at UV region and scattering of light is found responsible for high extinction with the experimental results. More experimental effort is still required to get an appropriate explanation. [Copyright &y& Elsevier]

Highlights: [•] CuO nanofluid could reduce the entropy generation in solar collector by 4.34%. [•] CuO nanofluid enhances the heat transfer coefficient by 22.15%. [•] Using CuO nanofluid in solar collector has small penalty in pumping power by 1.58%. [•] Enhancing efficiencies via increasing volume fraction, mass flow rate and density. [Copyright &y& Elsevier]

Abstract: Compared to thermal conductivity and convection studies with nanofluids; the optical and radiative properties of nanofluids have received much less interest. However, very recently, the number of studies on radiative heat transfer in nanofluids has been increasing. This is due to the fact that, in general, a composite nanofluid has different properties than those found in either the base fluid or the particles. At high temperatures, knowledge of the resultant radiative properties becomes increasingly significant. The concept of using direct absorbing nanofluid (suspension formed by mixing nanoparticles and a liquid) recently been shown numerically and experimentally to be an efficient method for harvesting solar thermal energy. Nanofluid is a product of emerging field of nanotechnology, where nanoparticles (1–100nm in size) are mixed with conventional base fluids (water, oils, glycols, etc.). Nanofluids as an innovative class of heat transfer fluids represent a rapidly emerging research field where nano-science and thermal engineering coexist. Nanofluids are considered to be a two-phase system, comprised of a solid and a liquid phase. Compared to the base fluids like water or oil, nanofluids feature enhanced thermo-physical properties such as thermal diffusivity, viscosity, thermal conductivity, convective heat transfer coefficients, and optical properties. They offer unprecedented potential in many applications. Recent development in solar thermal collectors is the use of nanofluids to absorb the light directly. There is much current work going on the use of nanoparticles in several applications. With thousands of papers published every year, a comprehensive literature survey is impossible, and only selected representative publications are cited in this paper, particularly as they concern fundamental scientific insights on the fundamental optical properties of nanofluids. [Copyright &y& Elsevier]

• Different heat transfer fluids used to enhance thermal efficiency and storage capacity for parabolic trough collector are reviewed. • Preparation methods and stability issues with obtaining stable heat transfer fluids are discussed. • Thermophysical properties of liquid, solid and gaseous HTFs suitable for PTC and their stability at high temperature are summarized. • Prospects of parabolic trough collector are discussed. Solar thermal energy conversion is gaining more attention among researchers due to the recent development in nanofluids and molten salt technology. Among various solar collectors, parabolic trough collector has received significant attention from researchers due to their operating temperature range (150-800 °C) feasible for power generation. Parabolic trough collector is currently having a higher number of installations compared to other concentrated solar power technology around the globe. Most of the conventional heat transfer fluid used in PTC have poor heat transfer and light to heat conversion properties. Therefore, it is advantageous to enhance the thermophysical properties of heat transfer fluid to improve the overall efficiency of the system. Well-engineered nano-enhanced heat transfer fluid is advantageous because a very low mass fraction of nanoparticles brings considerable enhancement in thermophysical properties. This paper focuses on the most recent advancement in heat transfer fluids, their preparation and stability issues when doped with nanoparticles. Various heat transfer fluids currently used in parabolic trough collectors and the nano-enhanced heat transfer fluids having the properties better than conventional heat transfer fluids are compared and their preparation methods and properties are discussed. Enhancement of thermophysical properties of molten salts by doping nanoparticles and their enhancement in thermal stability at high temperature, the possibility of using mono and hybrid nanofluid, ionic liquids, gaseous heat transfer fluid and vegetable oil as the heat transfer fluid in parabolic trough collectors are the key highlights of this review. Image, graphical abstract [ABSTRACT FROM AUTHOR]

• 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. 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. [ABSTRACT FROM AUTHOR]

• Experimental hybrid nanofluid based flat plate solar collector was studied. • New hybrid nanofluid increased the outlet temperature compared with water. • A significant enhancement in thermal conductivity of new hybrid nanofluid was observed. • Using new hybrid nanofluid improves the FPSCs efficiency by about 20% higher than DW at medium temperature differences. Covalent Functionalized-Multi wall carbon nanotubes (CF-MWCNTs) and Covalent Functionalized-graphene nanoplatelets (CF-GNPs) with hexagonal boron nitride (h-BN) were suspended in distilled water to prepare the hybrid nanofluids as working fluids inside the Flat Plate Solar Collector (FPSC). Different concentrations of the hybrid nanoparticles were considered and Tween-80 (Tw-80) was used as a surfactant. The stability and thermophysical properties were tested using different measurement tools. The structural and morphological properties were examined using FTIR, XRD, UV–vis spectrometry, HRTEM, FESEM, and EDX. The thermal efficiency of FPSC were tested under different volumetric flow rates (2 L/min, 3 L/min, and 4 L/min), whereas the efficiency of the collector was determined based on ASHRAE standard 93-2010. As a result, the most thermal-efficient solar collector improved up to 85% with hybrid nanofluid as the absorption medium at 4 L/min flow rate. Increment in nanoparticles' concentrations enhanced thermal energy gain and resulted in higher fluid outlet temperature. [ABSTRACT FROM AUTHOR]

Nanofluids have been identified as one of the potential solar energy absorbing media to improve the performance of the photovoltaic-thermal (PV/T) system. Use of a nanofluid above the PV panel through optical filtration may improve the performance of PV/T systems. The aim of this study is to extend the current knowledge of the parameters affecting the optical behavior of water/silver nanofluid through analytical and experimental investigation. The effect of the optical path length and the concentration of the nanoparticles on the transmittance of water/Ag nanofluid were evaluated and an experimental correlation describing the relation was developed. The Rayleigh method was used to analytically evaluate the transmittance of nanofluid. Experiments were conducted at five concentrations of Ag from 0.0005 wt % to 0.05 wt % using a UV–vis spectrophotometer at path lengths of 2 mm, 5 mm, and 10 mm. Experimental results were used to develop a correlation on the transmittance of water/silver nanofluid with the concentrations and optical path length of nanofluid. The transmittance of water/Ag nanofluid depends on the optical path length and concentration of the nanoparticles. The transmittance is higher at shorter path lengths and low concentrations. The effects are most pronounced in the spectral range from 250 nm to 1400 nm. The performance of a PV/T system with optical filtration showed better overall performance than the standalone PV system at low concentration. The outcomes of this study are useful in understanding the effect of using an optical filtration layer in PV/T solar collectors in capturing more energy. • Experimental study to evaluate the spectral transmittance of the water/Ag nanofluid for optical filtration purposes. • Side by side, studying the effect of the nanoparticles concentration and the optical length on the transmittance. • Analytical evaluation for the effect of the nanoparticles size on the overall transmittance of the nanofluid. • Correlation developed to relate between the studied parameters and the transmittance of the nanofluid. • PV/thermal systems with optical filtration shows higher performance than standalone PV systems. [ABSTRACT FROM AUTHOR]

The design of high performance photovoltaic/thermal solar system involves many parameters related to thermal, economic and reliability issues. The current paper provides a comprehensive review for the recent studies that have discussed the design and cooling considerations to build a high performance hybrid Photovoltaic/Thermal (PV/T) system. The review highlights the importance of the nanofluids and nano phase change materials (nanoPCMs) as an alternative for the traditional ways of cooling and storage. In addition, the application of the nanofluids in optical filtration has been covered. The main conclusions that have been reached at the end of the review show that cooling from both sides of the PV, with involvement of the optical filtration using nanofluid; will result in high improvements in the performance of the PV/T system. In addition, change of the channels cross section to honeycomb and induction of nanoPCM for storage and cooling purposes can provide better distribution of the thermal load. [ABSTRACT FROM AUTHOR]

Highlights • Simple-design SWH with PCM for rural areas is studied. • Fluctuating climatic conditions are integrated. • Optimum system variables were selected. • New thermal performance index is presented. • 32% of missed thermal energy to user can be decreased. Abstract This work presents a detailed analysis of an improved Integrated Collector Storage Solar Water Heater (ICSSWH). This type of device is well suited for rural areas of Morocco because of its low cost, simplicity and compact structure. The innovation targeted in this system lies in the integration of a latent storage system by using a layer of phase change materials (PCM) in its lower part. Indeed, this integration is likely to increase the thermal energy delivered to the user during the night. The overall performance of the system depends on external climate data, type of PCM used and its mass, and flow rate of water. N-eicosane is considered as PCM in this application while hourly weather data corresponding to the city ER-RACHIDIA is used for the analysis. A detailed 2-D transient simulation has been established to optimize the system performance by studying the effect of different design variables and operating conditions. A deep analysis was also made to understand the PCM melting and solidification processes for a better exploitation of this storage technique. Optimized results are obtained when a mass flow rate of 0.0015 kg/s is used with a PCM thickness of 0.01 m and a set temperature of 313 K. [ABSTRACT FROM AUTHOR]

Concentrating solar power (CSP) has received significant attention among researchers, power-producing companies and state policymakers for its bulk electricity generation capability, overcoming the intermittency of solar resources. The parabolic trough collector (PTC) and solar power tower (SPT) are the two dominant CSP systems that are either operational or in the construction stage. The USA and Spain are global leaders in CSP electricity generation, whereas developing countries such as China and India are emerging by aggressive investment. Each year, hundreds of articles have been published on CSP. However, there is a need to observe the overall research development of this field which is missing in the current body of literature. To bridge this gap, this study 1) provides a most up-to-date overview of the CSP technologies implemented across the globe, 2) reviews previously published review articles on this issue to highlight major findings and 3) analyzes future research trends in the CSP research. Text mining approach is utilized to analyze and visualize the scientific landscape of the research. Thermal energy storage, solar collector and policy-level analysis are found as core topics of discussion in the previous studies. With a holistic analysis, it is found that direct steam generation (DSG) is a promising innovation which is reviewed in this study. This paper provides a comprehensive outlook on the CSP technologies and its research which offers practical help to the future researchers who start to research on this topic. [ABSTRACT FROM AUTHOR]

The use of solar thermal collectors is one of the promising options for heating and/or cooling due to the green nature (pollution free) and abundance of the solar energy in many parts of the world. Improvement of thermal performance of solar collectors is important for better energy conversion. In this regard, many studies reported in literature tried to modify geometry of the collectors. On the other hand, nanofluids as heat transfer fluids could be a promising option for the efficiency enhancement of solar collectors. In this study, heating performance of an evacuated tube solar collector system initially operated with water is considered for improvement using nanofluid. The collector (about 20 kW heating capacity) is a part of an absorption cooling system. The aim of the present study is to analyze the effect of Single Walled Carbon Nanotube–water nanofluid on the collector performance. Thermal efficiency improvement is assessed based on comparative analyses between collector working with ordinary water and that with nanofluid. The results indicate that up to 56.7% and 66% of efficiencies are observed when the collector is operated with water and 0.2 vol% nanofluid, respectively. Therefore, nanofluids are promising to enhance the efficiencies of solar collectors. [ABSTRACT FROM AUTHOR]

Solar thermal collectors have significant importance due to its wide use in solar thermal technology. Augmentation of heat transfer is a key challenge for solar thermal technology. A quarter circular solar thermal collectors is investigated throughout the paper introducing carbon nanotube (CNT)–water nanofluid in the cavity. Tilt angle of this type of collector plays a vital role and heat transfer can be maximized for a particular tilt angle and solid volume fraction of the nanofluid. Galerkin weighted residual of FEM has been applied for the numerical solution of the problem. Grid independency test and code validation have been assessed for the accuracy of numerical solution. In this paper a wide range of solid volume fraction (δ = 0 to δ = 0.12) and tilt angle (ϕ = 0 to ϕ = 60°) has been investigated for Rayleigh number (Ra = 10 5 –10 8 ) with varying dimensionless times. It has been found that both solid volume fraction and tilt angle play vital roles for the augmentation of heat transfer and a good heat transfer characteristic can be obtained by compromising between these two parameters. The results are shown using streamline, isotherm contour and related graph and chart. [ABSTRACT FROM AUTHOR]

Highlights: [•] Exergy efficiency of parabolic trough system is 10% while energy efficiency is 30%. [•] Overall exergy efficiency can increase up to 30% using PCM with the solar collector. [•] PCM with higher melting point temperature give higher exergy efficiency of the system. [Copyright &y& Elsevier]

Abstract: Nanofluids have been introduced for the enhancement in the heat transfer phenomena in the last few years. In this paper a corrugated bottom triangular solar collector has been studied introducing water based nanofluids inside the enclosure. The corrugated bottom is kept at a constant high temperature whereas the side walls of the triangular enclosure are kept at a low temperature. Three types of nanoparticles are taken into consideration: Cu, Al2O3, and TiO2. The effect of solid volume fraction (ϕ) of the nanoparticle of nanofluid has been studied numerically by Galerkin weighted residual method of finite element for a wide range of Grashof number (Gr) 104–106. Calculations are carried out for ϕ=0, 0.05, 0.08, and 0.1 and dimensionless time, τ=0.1, 0.5, and 1. For the specified conditions streamlines and isotherm contours are obtained and detailed results of the interaction between different parameters are studied using overall Nusselt number. It has been found that both Grashof number and solid volume fraction have significant influence on streamlines and isotherms in the enclosure. It is also found that heat transfer increased by 24.28% from the heated surface as volume fraction ϕ increases from 0% to 10% at Gr=106 and τ=1 for copper water nanofluid. [Copyright &y& Elsevier]

Abstract: Experimental investigations have been carried out for obtaining the thermophysical properties of 60:40 (by mass) ethylene glycol/water mixture and water based alumina nanofluids. The effect of density and viscosity on the pumping power for flat plate solar collector has been investigated as well. Nanofluids of 0.05–0.1%v/v concentrations were prepared and characterized. Water based alumina nanofluids were found more preferable against sedimentation and aggregation than ethylene glycol/water mixture based nanofluids. The measured thermal conductivities of both types of the nanofluids increased almost linearly with concentration and are consistent in their overall trend with previous works done at lower concentrations by different researchers. In contrast to thermal conductivity, viscosity measurements showed that the viscosity of the Al2O3–water nanofluids exponentially decreases with increasing temperature. Furthermore, the measured viscosities of the Al2O3–water nanofluids showed a non-linear relation with concentration even in the low volume concentration except 0.05%v/v at below 40°C. On the other hand, Al2O3–EG/water mixture exhibited Newtonian behavior. Existence of a critical temperature was observed beyond which the particle suspension properties altered drastically, which in turn triggered a hysteresis phenomenon. The hysteresis phenomenon on viscosity measurement, which is believed to be the first observed for EG/water-based nanofluids, has raised serious concerns regarding the use of nanofluids for heat transfer enhancement purposes. Results suggest that nanofluids can be used as a working medium with a negligible effect of enhanced viscosity and/or density. Results also show that the pressure drop and pumping power of the nanofluid flows are very close to that of the base liquid for low volume concentration. [Copyright &y& Elsevier]

Enhancing the efficiency of low-cost solar-powered desalination technologies such as solar stills (SS) is essential to ensuring sustained access to potable water in water-stressed areas. In the present work, the potential enhancement of SS performance through the addition of different concentrations of activated carbon nanoparticles (ACNPs) to the standard black paint used to coat the absorber plate was explored. SSs were fabricated with absorber coated using black paint doped with 0, 5, 10 and 15 wt% ACNPs (denoted as CSS, MSS-5, MSS-10 and MSS-15, respectively) and the effect on water temperature and distillation efficiency determined. ACNPs were synthesized by crushing dead Kigelia Africana leaves and pyrolysis of the leaf powder in a muffle furnace at 500 °C. Absorber surface roughness analysis illustrated increasing roughness with increasing the loading of ACNPs in the nano-paint, leading to significant increase in heat transfer. Significant improvement in average water temperature (12.5%) and absorber (14.5%) was noted in the modified SS coated with 15 wt% ACNP over the CSS. The augmented temperature of this still enhanced the peak hourly and full-day water yield of MSS-15 by ⁓39.3%, compared to the conventional SS. The water quality analysis shows that the distilled water is appropriate for drinking. In conclusion, the synergetic effect of the high thermal conductivity and excellent solar absorption of the ACNP-based solar absorber improves the performance of the SS unit. [Display omitted] • AC nanoparticle was prepared from pyrolysis of dead Kigelia Africana leaves. • AC exhibited highly porous structure and excellent solar absorption behavior. • AC with loading of 5, 10 and 15 wt% were mixed in black paint and coated on absorber. • 15 wt% RGO doped in black-paint coated absorber show maximum water yield. • Water quality analysis showed the distilled water is suitable for drinking. [ABSTRACT FROM AUTHOR]

The objective of the present research work is to investigate a novel high-efficiency nanofluid in a solar dish concentrator by using the numerical model developed. The working fluids examined consisted of soybean oil-based MXene nanofluid of different concentrations (i.e. 0.025, 0.075 and 0.125 wt%) and pure soybean oil. The studied nanofluids yielded excellent thermal properties such as high thermal conductivity and heat capacity, which were two particular factors rendering them excellent candidates for solar thermal applications. The solar dish collector was evaluated for three different cavity receivers including cubical, hemispherical and cylindrical shapes. Then, thermal analysis was performed with a developed numerical model in steady-state conditions, which was validated by using experimental results. Meanwhile, the thermal properties of the oil-based nanofluid were described after the experiments. The analysis was parametric in nature, thereby studying the system performance on a daily basis. According to the analysis, the hemispherical cavity receiver led to maximum thermal efficiency with the nanofluid used. In particular, the daily mean thermal efficiency with nanofluid of 82.66% and the mean equivalent efficiency of 82.46% were achieved, while the mean enhancement was 0.6%. However, the enhancements were higher with the use of other cavities due to the higher thermal losses shown in such cases. Moreover, the equivalent efficiency proved that the increased pumping work due to the use of nanofluid could not overcome the thermal enhancement, thereby improving the overall performance of the solar collector. • Novel soybean oil-based MXene nanofluid is investigated in a solar dish collector. • The hemispherical cavity revealed as the best candidate for all examined working fluids. • The daily mean thermal efficiency with nanofluid was recorded as 82.66%. • Low volumetric flow rates in the developed nanofluid enhance the thermal efficiency. • The daily yield of the examined solar dish collector is 9.07 kWh. [ABSTRACT FROM AUTHOR]

In the last decade, nanofluids have set significant milestones as efficient working fluids in the field of solar energy conversion to meet rising energy demand. Research on thermophysical properties, long-term stability, and rheology is progressing to achieve effective practical deployment of nanofluids in renewable solar photo-thermal energy conversion sectors (i.e., solar collectors). Nonetheless, researchers and engineers are having a difficult time coping with nearly infinite culpable variables influencing the output of various types of nanofluids. This paper aims to provide an up-to-date analysis of the developments and challenges of widely used water-based nanofluids, with a focus on formulation methods, main properties (thermophysical, stability, and rheological), and effective implementation in low temperature solar collector systems. Previous experimental and numerical studies on the subject have been compiled and thoroughly scrutinized, providing crucial phenomena, mechanisms, flaws, and responsible parameters for achieving stable and optimized thermal properties that integrate with heat transfer performance. It has been discovered that optimizing the critical factors leads to superior behavior of the nanofluids, which results in improved thermal efficiency of the solar collectors. Finally, emerging concerns are identified, as are potential recommendations to resolve existing problems in the field for future advancement that would mobilize rapid progress and practical engineering use of water based nanofluids on solar collectors. • Water-based nanofluids are studied comprehensively. • Formulation methods and crucial thermo-physical properties are critically assessed. • Stability of the nanofluids is investigated in detail with associated factors. • Application of the nanofluids on low temperature solar collectors is scrutinized. • Existing challenges are disclosed, and future recommendations are made. [ABSTRACT FROM AUTHOR]

[Display omitted] • Oil-based nanofluids are analyzed comprehensively. • Formulation methods and crucial thermo-physical properties are critically assessed. • Stability of the nanofluids is studied comprehensively with associated factors. • Application of the nanofluids on concentrating solar collectors is reviewed. • Existing challenges are revealed, and future recommendations are drawn. Nanofluids have exhibited noteworthy advancement as efficient working fluids in the last decade towards the field of solar energy conversion field to deal with escalating global energy demand. Research developments on thermo-physical, long-term stability and rheology are moving ahead to achieve practical deployment in renewable solar photo-thermal conversion sectors (i.e., solar collectors). Nevertheless, researchers and engineers are encountering many difficulties dealing with nearly infinite culpable variables impacting performance of several categories of NFs. This work attempts to offer an up-to-date review on advances and challenges of oil-based nanofluids focusing on formulation, key properties (stability, thermal conductivity, c p , and viscosity) and effective implementation in concentrating solar collector devices. Previous experimental and numerical studies on the topics are compiled and acutely scrutinized providing essential phenomena, mechanisms, shortcomings, responsible parameters to obtain stable and optimized thermal properties integrating with heat transfer performance. It is observed that optimization of the critical factors leads to superior behavior of the nanofluids, which in turns generates enhanced thermal performance of the solar collectors. Lastly, existing challenges are reported along with recommendations to address the issues for further developments in the field which would mobilize rapid innovation and practical engineering practice of nanofluids. [ABSTRACT FROM AUTHOR]