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The heat transfer and flow characteristics of water-based fly ash nanofluid for concentrations of 0.1–1.5 vol.% that flow through a copper tube at an inlet fluid temperature of 303 K with constant heat flux boundary conditions are estimated numerically. Stability, viscosity, and thermal conductivity of fly ash nanofluid have been determined experimentally and compared with correlations available in the literature. STAR CCM + software was used to solve the governing equations using the finite volume method (FVM). Water and stable fly ash nanofluid were used as working fluids for the Reynolds number range of 4500–16,000. The Nusselt number and friction factor at 1.5% volume concentration are greater than the base fluid by 36% and 9.4%, respectively. The fly ash nanofluid showed a performance index (η ) value of more than 1 at all concentrations undertaken, indicating that the use of fly ash nanofluid in heat transfer applications is beneficial. The highest η value is determined to be 1.5 approximately for a nanofluid concentration of 1.5 vol.%. The fly ash nanofluid exhibits enhanced heat transfer performance contrasted to SiO2 nanofluid. The utilization of fly ash particles for heat transfer enhancement can aid in the reduction of environmental pollution to a certain extent. Novel correlations are suggested for the estimation of fly ash nanofluid Nusselt number and friction factor. [ABSTRACT FROM AUTHOR]

Abstract Parabolic trough collector (PTC) is the most established solar concentrating technology worldwide. The conventional parabolic trough collectors are used in various applications of medium and high-temperature levels. However, there are numerous studies which investigate alternative designs. The reasons for examining different PTC configurations regard the thermal efficiency increase, the reduction of the manufacturing cost and the development of more compact designs. The objective of this review paper is to summarize the existing alternative designs of PTC and to suggest the future trends in this area. Optical and thermal modifications are examined, as well as the use of concentrating thermal photovoltaic collectors. The optical modifications include designs with secondary concentrators, stationary concentrators and strategies for achieving uniform heat flux. The thermal modifications regard the use of nanofluids, turbulators and the use of thermally modified receivers with insulation, double-coating and radiation shields. The concentrating thermal photovoltaics are systems with flat or triangular receivers which can operate in low or in medium temperature levels with the proper alternative designs. It has been found that there are many promising choices for designing PTC with higher thermal performance and lower cost. The conclusions of this work can be used as guidelines for future trends in linear parabolic concentrating technologies. [ABSTRACT FROM AUTHOR]

Highlights • The use of nanofluids increases both the thermal and electrical performance. • The collector can operate up to 200 °C with oil and nanofluid. • At 50, 100 and 150 °C, the thermal efficiency was 45.0%, 46.8% and 39.7% respectively. • At 50, 100 and 150 °C, the electric efficiency was 5.8%, 6.6% and 3.9% respectively. • At 100 °C, the thermal and electric enhancements were 1.66% and 5.17% respectively. Abstract The objective of this paper is the investigation of a thermal photovoltaic concentrating collector operating with and without nanofluids. The use of nanofluid Syltherm 800/copper oxide with 5% volumetric concentration is examined and it is compared with the base fluid. The examined thermal photovoltaic has a parabolic concentrator and an inverted flat insulated receiver. The analysis is conducted with SolidWorks Flow Simulation which is able to perform the thermal, optical and flow analysis. According to the present study, the system is able to operate for inlet fluid temperatures up to 200 °C which is a promising result. Moreover, it is found that the use of nanofluid leads to enhancement both in thermal and electrical performance for all the examined cases (different combinations of flow rates and inlet temperatures). For an inlet temperature at 100 °C and flow rate 540 L/h with nanofluid, the thermal efficiency and the electrical efficiency were found at 46.84% and 6.60% respectively. For this case, the enhancements in the thermal, electrical, total and exergy efficiencies were found at 1.66%, 5.17%, 2.08% and 3.05% respectively. [ABSTRACT FROM AUTHOR]

The use of nanofluids in parabolic trough collectors is one of the most promising techniques for enhancing their performance. The objective of this work is to investigate the use of various nanoparticles (Cu, CuO, Fe2O3, TiO2, Al2O3 and SiO2) dispersed in thermal oil (Syltherm 800). A detailed parametric analysis is performed for flow rates from 50 to 300 L min−1, for inlet temperatures from 300 K to 650 K and for nanoparticle concentrations up to 6%, while the impact of the solar irradiation level on the thermal efficiency enhancement is also investigated. Moreover, a new index for the working fluid evaluation in solar collectors is introduced. The analysis is conducted with a developed thermal model in Engineering Equation Solver. According to the final results, the most efficient nanoparticle is the Cu, with CuO, Fe2O3, TiO2, Al2O3 and SiO2 to follow, respectively. It is found that the higher enhancement is observed for lower flow rates, higher inlet temperatures and higher nanoparticle concentrations, while it is approximately constant for the different solar irradiation levels. For the typical operating conditions with 150 L min−1 flow rate and 600 K inlet temperature, the thermal efficiency enhancement is found 0.31, 0.54 and 0.74% for Cu concentrations 2, 4 and 6%, respectively. [ABSTRACT FROM AUTHOR]

Solar energy exploitation is one of the most important weapons for facing the recent environmental and energy management dangers. Concentrating solar collectors can produce useful heat in medium and high-temperature levels. So, they can be used in a great variety of applications as space-cooling, industrial heat, chemical processes and electricity production. The use of nanofluids is one of the most effective ways for enhancing the performance of the concentrating collectors. In this paper, the use of nanofluids as working fluids in concentrating solar collectors is investigated in detail. The examined collector types are the following: Evacuated tube collectors with booster reflector, concentrating thermal photovoltaics, compound parabolic concentrator, parabolic trough collector, linear Fresnel reflector and solar dishes. Moreover, the use of nanofluid-based solar concentrating collectors in applications (heating, cooling, electricity, and trigeneration) is investigated in this work. The emphasis is given to the determination of the thermal efficiency enhancement of the collector with the use of the nanofluids in every case. The recent trends in nanofluid utilization are given in this work. Furthermore, the challenges and the future work about the nanofluid-based solar systems are discussed in details. [ABSTRACT FROM AUTHOR]

Parabolic trough collector is one of the most usual solar collectors for applications up to 400 °C. The thermal enhancement of this collector concentrates a lot of interest and various techniques are tested in order for the thermal efficiency to be maximized with a reasonable penalty in the pressure drop. The use of nanofluids as working fluids, as well as the use of flow turbulators, mainly inserts and internal fins or tube dimples are the main techniques which are examined. The objective of this work is to give a complete literature review of the existing studies on this domain and to present a numerical comparative analysis between the most usual thermal enhancement techniques. More specifically, the use of oil-based nanofluids with 6% CuO is compared with the use of internal rectangular fins in the absorber, while the combination of these techniques is also examined. The analysis is conducted with a validated CFD model in SolidWorks Flow Simulation for various fluid temperature levels. According to the final results, the use of nanofluids leads to 0.76% thermal efficiency enhancement, the use of internal fins to 1.10% and the combination of these techniques to 1.54%. Moreover, emphasis is given in the pressure drop of the examined cases and in the evaluation criteria which are used in every case. [ABSTRACT FROM AUTHOR]

The use of solar dish thermal collectors is a promising choice for designing sustainable energy systems. The use of nanofluids is a new way for enhancing the thermal performance of solar collectors because of their improved thermal properties. The objective of this study is to investigate the use of mono and hybrid nanofluids in a solar dish collector in order to determine which kind of nanofluids leads to higher performance enhancements. The analysis is conducted with a developed thermal model in Engineering Equation Solver and the collector is studied thermally and exergetically. The examined hybrid nanofluid has as base fluid syltherm 800 with 1% Cu and 1% TiO2. Moreover, the examined mono nanofluids are the syltherm 800 with 2% Cu and syltherm 800 with 2% TiO2. The investigated solar dish collector has a spiral absorber and it is examined for inlet temperatures from 25 °C up to 300 °C with a flow rate of 200 L/h. According to the final results, the use of hybrid nanofluid leads to higher thermal efficiency enhancement compared to the mono nanofluids because of the higher increase in the Nusselt number in the flow. More specifically, the use of the hybrid nanofluids leads to 0.99% mean thermal efficiency enhancement compared to the pure oil case, while the use of Oil/Cu and Oil/TiO2 lead to 0.42% and to 0.56% mean thermal efficiency enhancement, respectively. Moreover, the exergy efficiency is found enhanced with the use of all nanofluids. The mean exergy efficiency enhancement is 1.21% with the hybrid nanofluid, while it is 0.73% with Oil/TiO2 and 0.53% with Oil/Cu. [ABSTRACT FROM AUTHOR]

Solar linear Fresnel reflector (LFR) is a promising concentrating technology, which presents important advantages such as the low investment cost, the reduced wind loads and the relatively low land utilization factor. The objective of this work is to investigate an innovative way for enhancing the thermal performance of LFR, especially at high temperatures. The utilization of nanofluid as heat transfer fluid is the investigated thermal enhancement method and more specifically the use of CuO nanoparticle dispersed on Syltherm 800 (6% volumetric concentration). The examined collector has total net aperture equal to 154 m 2 and concentration ratio of 58.36. The primary reflectors are curved mirrors, the secondary reflectors have compound parabolic shape and the receiver is an evacuated tube. The operation with nanofluid is compared to the operation with pure thermal oil for various inlet temperatures from 350 K up to 650 K and flow rate equal to 200 L/min. According to the final results, the maximum thermal efficiency enhancement with the nanofluid is close to 0.8%, while the pumping work demand is increased up to 50% with the nanofluid. Various criteria like the exergy efficiency, overall efficiency and entropy generation are applied in order to evaluate the nanofluid utilization properly. Finally, the operation with nanofluid is found to be beneficial, especially in high-temperature levels. The analysis is conducted with SolidWorks Flow Simulation with a validated model. [ABSTRACT FROM AUTHOR]

Solar energy utilization is vital in order to achieve the sustainability and reduce the utilization of conventional energy sources. The use of nanofluids in concentrating solar collectors is a promising solution in order to achieve high performance. This paper examines the use of various nanofluids as working fluids in a solar dish collector with smooth and corrugated absorber tube. The use of Al 2 O 3 , Cu, CuO and TiO 2 dispersed on thermal oil and water is presented in this work. The examined concentrations of these nanoparticles are up to 5% in this study. The solar collector is investigated exergetically in order to evaluate together the useful thermal output and pressure losses. The analysis is performed with a developed 1-D thermal model and experimental results are used for the model validation for operation with water. According to the final results, the use of oil-based nanofluids leads to higher exergetic efficiency, while the use of water-based nanofluids leads to higher thermal performance. Moreover, the use of Cu nanoparticle is the most suitable exergetically among the examined cases. Generally, the exergetic efficiency of the examined collector is maximized for Cu-oil based nanofluid with the corrugated absorber and it is about 12.29%. [ABSTRACT FROM AUTHOR]

The objective of this paper is to investigate a solar cooling system with an absorption chiller driven by nanofluid based flat plate collectors. Pure water and water/Cu nanofluid (2% volumetric concentration) are the examined working fluids on the solar field and they are compared for steady-state conditions, as well as for daily operation. The system is optimized for steady-state conditions using a multi-objective procedure with energetic and exergetic criteria. The mean thermal efficiency enhancement by the use of nanofluids it is found close to 2.5%. The optimized system is further examined on a daily basis for operation with pure water and nanofluid. According to the final results, the use of nanofluids in the solar collector is able to enhance the exergetic performance of the system close to 4.0% and to increase the refrigeration production approximately 0.84% on a daily basis. The analysis is conducted with a developed model in Engineering Equation Solver. [ABSTRACT FROM AUTHOR]

The main objective of this work is to investigate the utilization of nanofluids in the solar field in order to achieve higher system performance. An Organic Rankine Cycle (ORC) driven by solar parabolic trough collectors (PTCs) is the examined system. Four different nanoparticles are examined (Al 2 O 3 , CuO, TiO 2 and Cu) in the base fluid (Syltherm 800), as well as the pure thermal oil is examined as working fluid. The examined ORC is a regenerative cycle and four organic fluids are also tested (toluene, MDM, cyclohexane and n-pentane). In every combination between the organic fluid in ORC and working fluid (nanofluid) in the solar field, an optimization procedure is followed. The concentration of every nanoparticle and the pressure ratio (pressure in the turbine inlet to critical pressure) are the optimization parameters. According to the final results, toluene is the best organic fluid and CuO is the most suitable nanoparticle. The combination of these two working fluids leads to 167.05 kW electricity production and to 20.11% system efficiency with concentration 4.16%. The enhancement by the use of nanofluids is found up to 1.75% compared to the respective case with pure thermal oil and this result indicates that the use of them is able to improve the performance of solar driven ORCs. For the other nanoparticles and Toluene in the ORC, Cu, Al 2 O 3 and TiO 2 lead to 166.18 kW, 165.72 kW and 165.60 kW electricity productions respectively with optimum concentrations 3.98%, 2.51% and 2.57% respectively. [ABSTRACT FROM AUTHOR]

The objective of this study is to estimate the yearly thermal and electrical enhancement of a hybrid PV operating with nanofluids. The examined nanofluid is Cu/water and it is compared with water as working fluid for various operating conditions. An integrated solar thermal system with a hybrid PV and a storage tank is examined for twelve typical days in order its yearly performance to be determined. Different storage tank volumes are investigated because of the storage capacity influence on the thermal and the exergetic performance of the system. According to the final results, the storage tank of 150 L is found to be the most suitable solution for the hybrid PV of 2 m 2 collecting area, using exergetic criteria. For this case, the yearly thermal performance enhancement is about 4.35%, while the electrical and exergetic enhancements are 1.49% and 3.19% respectively. Moreover, it is found that the yearly enhancement is higher in the cases with greater storage tanks. The study is performed with a developed thermal model in EES (Engineering Equation Solver) which is validated with experimental literature results. The climate data has been taken from the literature for Athens, Greece. [ABSTRACT FROM AUTHOR]

The objective of this work was to optimize and to evaluate a solar-driven trigeneration system which operates with nanofluid-based parabolic trough collectors. The trigeneration system includes an organic Rankine cycle (ORC) and an absorption heat pump operating with LiBr-H2O which is powered by the rejected heat of the ORC. Toluene, n-octane, Octamethyltrisiloxane (MDM) and cyclohexane are the examined working fluids in the ORC. The use of CuO and Al2O3 nanoparticles in the Syltherm 800 (base fluid) is investigated in the solar field loop. The analysis is performed with Engineering Equation Solver (EES) under steady state conditions in order to give the emphasis in the exergetic optimization of the system. Except for the different working fluid investigation, the system is optimized by examining three basic operating parameters in all the cases. The pressure in the turbine inlet, the temperature in the ORC condenser and the nanofluid concentration are the optimization variables. According to the final results, the combination of toluene in the ORC with the CuO nanofluid is the optimum choice. The global maximum exergetic efficiency is 24.66% with pressure ratio is equal to 0.7605, heat rejection temperature 113.7 °C and CuO concentration 4.35%. [ABSTRACT FROM AUTHOR]

It has been more than two decades since the discovery of 'nanofluids' – mixtures of common liquids and solid nanoparticles with at least one dimension below 100 nm in size. While colloidal suspensions of particles (which include larger particles) have been studied for several decades, the term 'nanofluids' designates fluid systems that have enhanced thermal and optical properties. Although barriers to their commercial adoption remain, the field of nanofluids has continued to grow. Many studies have considered the effects of adding nanoparticles on the thermal efficiency and exergy efficiency of renewable energy systems particularly solar systems, however, few have investigated their potential for emission reductions. Critically, since renewable energy technologies aim to reduce the environmental impact of energy systems, this review focuses on whether nanofluids provide a net environmental benefit. Thus, in addition to providing a comprehensive overview of this body of literature from an environmental perspective, this review also highlights areas for future work that could help ensure that nanofluids have a net positive environmental impact in renewable energy systems going forward. [Display omitted] • The applications of nanofluid in renewable energy systems are discussed in detail. • Nanofluid preparation, characterization, related principles and technologies are presented. • Green synthesis methods of nanofluid preparation, CO 2 emissions, and life cycle assessment are explored. • The key challenges associated with the use of nanofluid in practical applications are listed. • Tackling the environmental impacts of nanofluid would pave way for broader applications. [ABSTRACT FROM AUTHOR]

• A nanofluid-based thermal PV coupled a heat pump is investigated. • This system produces space heating and net electricity. • The system is optimized with a multi-objective procedure. • Water/Cu nanofluid leads to 4.80% energy efficiency enhancement. • The heating production is increased by 5.75% with water/Cu nanofluid. The objective of this paper is to investigate a solar assisted heat pump which is driven by nanofluid-based hybrid PV collectors for space-heating and electricity production purposes. The use of nanofluids, as working fluids in the solar collector, enhances both the thermal and electrical performance in order to have higher useful outputs. Practically, the use of nanofluids enhances the heat transfer rates inside the flow and so higher thermal output are produced in the collector and the photovoltaic cell produces higher electricity because it is cooled efficiently by the nanofluids. Water/Cu and water/Al 2 O 3 are found to be the most efficient nanofluids and they are tested in the total system. The system is optimized for steady-state conditions and the optimum design is examined on a daily basis for the pure water case and the nanofluids cases. The optimization procedure is a multi-objective procedure which is conducted using energy, exergy and financial criteria. According to the final results, the use of water/Cu nanofluid is the best case which presents 4.80% higher energy system efficiency and 0.66% higher exergy system efficiency compared to the pure water case. The use of water/Al 2 O 3 leads to an enhancement of 4.34% and 0.19% for energy and exergy efficiency respectively. For the case with water/Cu, the heating production during the winter is found 7423 kWh th and the net electricity production 1068 kWh el. The analysis is performed with a developed thermal/thermodynamic model in Engineering Equation Solver. [ABSTRACT FROM AUTHOR]

From the last few decades, due to the escalating requirement of heat and electricity, the relative utilization of Photovoltaic/thermal (PV/T) system has enlarged as compared to the photovoltaic or solar thermal system alone due to better performance of combined PV/T systems when compared to the conventional ones. PV/T hybrid systems generate both electrical and thermal energy concurrently, giving it a broad variety of applications. Recently, different nanoparticles mixed with base fluid (Nanofluid) have been used in these hybrid systems resulting in attracting the attention of numerous researchers. Due to its improved thermal conductivity nanofluid can be used as an optical filter and proficient coolant in PV/T systems. In the last years, much research has been focused on this domain because it is crucial for the design of sustainable and environmentally friendly energy systems. The primary focus of this review is to accomplish the briefing of diverse PV/T technologies at the outset together with aspects of their efficiencies, structure, thermal governing expressions and their applications as well as the latest technologies and parameters which profoundly affect the PV/T collector performance. We have also reviewed the various positive and adverse effects on the environment safety level by applying these solar techniques for the energy production as compared to the conventional energy sources techniques. As the solar techniques are tremendously helpful for the environment in several ways but at the same deployment of these resources on the wide-scale may lead to adverse implications for the environment. Moreover, the variations in the performance efficiency of the PV/T system using nanofluids, and finally, the suggestions for future research directions are presented. The outcome of this review paper would give an overview of further enhancements in solar PV/T systems with high practicability for extensive relevance in energy supply at all levels in the vicinity of future. [ABSTRACT FROM AUTHOR]

In the past decade, nanotechnology's rapid developments have created quite a lot of prospects for researchers and engineers to check up on. And nanofluids are important consequences of this progression. Nanofluids are created by suspending nanoparticles with average diameters below 100 nm in conventional heat transfer carriers such as water, oil, ethylene glycol, etc. Nanofluids are considered to offer substantial advantages over usual heat transfer fluids. When dispersed in a uniform way and suspended stably in the base fluids, a minimal amount of nanoparticles can significantly improve the thermal properties of host fluids. Present work attempts to address this challenge considering state-of-the-art advances in understanding, discussing, and mitigating problems about nanofluids' stability. Stable and highly conductive nanofluids are produced by generally, one-step and two-step production methods. Both approaches suffer from problems with the nanoparticles' agglomeration to be an important one. Thus, numerous numerical models and the principal physical phenomena affecting the stability (fundamental physical principles that govern the interparticle interactions, clustering and deposition kinetics, and colloidal stability theories) have been analyzed. Concerning the particles' dynamic motion, the significance of different forces in nanofluid in particulate flows such as drag, lift (Magnus and Saffman), Brownian, thermophoretic, Van der Waals, electrostatic double-layer forces are investigated. Furthermore, an overview of nanofluids' thermophysical properties, physical models, and heat transfer models is​ included in this work. In order to realize the unexpected discoveries and overcome classical models' limitations, several researchers have suggested new physical concepts and mechanisms, and they have created new models to enhance the transport properties. This review study includes numerous aspects of the nanofluids' science by investigating applications, thermal properties and giving critical chronological milestones about the nanofluids' evolution. Also, the present review discusses in detail various modeling and slip mechanisms for the heat transfer of nanofluids. Potential novel 2D materials as nanofluids have also been discussed and reported. A brief overview of the potential applications utilizing nanofluids has been reviewed, and future research gaps have been reported. Furthermore, recommendations were extracted regarding current scientific gaps and future research directions to cover the physical phenomenon, stability, thermophysical properties, overview of some applications, and the limitations hindering these nanofluids' deployment. The review is presumed to be valuable for scholars and researchers working in the area of numerical simulations of nanofluids and experimental aspects and help them understand the fundamental physical phenomena taking place during these numerical simulations and experiments and explore the potential of nanofluids both in academia and industry. [Display omitted] [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 this study, the thermal performance of a solar dish concentrator with different cavity receivers and nanofluids was experimentally studied. The first step in this work is a detailed literature review of the studies about the solar dish cavity receivers. The second step is the experimental investigation of a solar dish concentrator with a cubical cavity receiver using Al 2 O 3 /oil nanofluid and pure thermal oil. The last step of this work is the comparison of the found results with others from the literature in order to make an overall overview of cavity receivers. The finally presented results of the dish concentrator with different cavity receivers (including cubical, cylindrical, and hemispherical cavities) using different nanofluids (including Al 2 O 3 /oil, MWCNT/oil, and SiO 2 /oil nanofluids are reported based on a same experimental setup that was designed and built by authors of this research. The results reveal that the hemispherical and the cubical cavities are the most effective designs, while the cylindrical cavity presents lower performance. Moreover, it was found that the use of nanofluids always leads to thermal performance enhancement. More specifically, it was found that the mean thermal efficiency enhancement with the use of nanofluids is 12.90% with the hemispherical cavity, 5.84% with the cubical cavity and 1.44% with the cylindrical cavity. • A cubical cavity receiver was experimental investigated using Al 2 O 3 /oil nanofluid. • Performance comparison was conducted with pure oil and nanofluid. • Performance comparison of cubical, cylindrical, and hemispherical cavities was presented. • A detailed literature review related to the cavities and nanofluid was conducted. • The highest mean thermal efficiency enhancement was calculated equal to 12.90% using hemispherical cavity and nanofluid. [ABSTRACT FROM AUTHOR]

Advanced heat removal technologies are critical for high-performance automotive engines. The conventional fluids being used today are based on a mixture of distilled water (DW) and ethylene glycol (EG), which widens the operational temperature range but at the same time limits the heat removal. Therefore, the use of nanofluids for improving heat transfer performance has soared over the past few years. The problem is that most of the reports highlight the short-term heat transfer results which may not be true over time. In this paper, a suggested best practice for analyzing the usage of nanofluids in heat transfer applications is presented, specifically for an actual car radiator. This work investigates the use of aluminum oxide (Al 2 O 3) and titanium dioxide (TiO 2) nanoparticles dispersed in DW and EG at 50:50 volumetric proportions. The choice of these oxide-based nanofluids is motivated by their anti-corrosive properties that are usually not analyzed or discussed in most of the articles. Furthermore, the emphasis is given on the presentation of a comprehensive characterization of the nanofluids including thermophysical properties (size, density, viscosity, thermal conductivity, corrosive behavior) and long-term stability (zeta potential) which are essential for an end-user to have. The results showed a maximum enhancement of the thermal performance by 24.21% using Al 2 O 3 at a volume fraction of 0.3%. Friction factor and performance evaluation criterion (PEC) for the radiator experiments are calculated in order to determine the penalty in the pressure drop and to evaluate it properly. Finally, it is found that the values of PEC lie in the range of 1.03–1.31 which indicates significant flow enhancement. • A critical review on stability and application of nanofluids (NFs) in a radiator is presented. • The effect of surfactant concentration on stability is investigated. • Thermophysical properties of Al 2 O 3 –DW/EG and TiO 2 -DW/EG NFs are experimentally investigated. • Effect of NFs with respect to corrosion for the pipping system of the radiator is investigated. [ABSTRACT FROM AUTHOR]

In the current study, the thermal performance of a solar dish concentrator using a cylindrical cavity receiver is experimentally investigated using different working fluids. The examined solar working fluids are two kinds of nanofluids (Al 2 O 3 /thermal oil and SiO 2 /thermal oil) and pure thermal oil. The investigated experimental setup includes the dish concentrator, the cylindrical cavity receiver, heat exchangers and a hydraulic circuit system. The thermal performance of the solar system is compared by application of the examined nanofluids with the pure thermal oil. The results indicated that the average thermal efficiency of the cylindrical cavity receiver using the Al 2 O 3 /thermal oil nanofluid has the highest value compared to application of the SiO 2 /thermal oil nanofluid and pure thermal oil operation in the steady-state period. Moreover, the thermal loss coefficient ( U L ) of the cylindrical cavity receiver using the Al 2 O 3 /thermal oil nanofluid has a lower value than the application of the SiO 2 /thermal oil nanofluid and pure thermal oil. Thus, the application of the Al 2 O 3 /thermal oil nanofluid is recommended due to the higher thermal efficiency compared to the other examined working fluids. Finally, three experimental models are suggested for the cylindrical cavity receiver using the Al 2 O 3 /thermal oil nanofluid, SiO 2 /thermal oil nanofluid, and pure thermal oil. [ABSTRACT FROM AUTHOR]

Abstract The objective of this work is the investigation of two oil-based nanofluids in a hemispherical cavity receiver of a solar dish concentrator. The investigated nanofluids are Alumina/oil and Silica/oil, while the used oil is the Behran thermal oil. The two nanofluids are examined experimentally and they are energetically and exergetically compared in order to determine their performance in the solar system. Thermal performance of the hemispherical cavity receiver was studied in the steady-state condition. The results elucidated that the average cavity thermal performance has improved with the application of Alumina/oil nanofluid compared to the application of Silica/oil nanofluid, and pure oil as the solar heat transfer fluid. It was found the lower heat loss coefficient of the hemispherical cavity receiver could be achieved using the Alumina/oil nanofluid than the Silica/oil nanofluid and pure oil as the solar working fluids. Three models were presented for cavity thermal performance versus the parameter T i n − T a m b I b e a m using investigated nanofluids, and base fluid. Exergy efficiency and overall thermal efficiency of the hemispherical cavity receiver was highest using Alumina/oil nanofluid than two other investigated heat transfer fluids. Finally, it was concluded that hemispherical cavity receiver with Alumina/oil nanofluid is the best selection from the energy and exergy viewpoint. Highlights • It was an experimental study of nanofluids in a solar dish collector. • A hemispherical cavity receiver was used as the dish absorber. • Al 2 O 3 /thermal oil and SiO 2 /thermal oil nanofluids were tested. • Performance of the nanofluids was compared with the pure oil. • Three thermal efficiency models were suggested for the nanofluids and the pure oil. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • It was an experimental study of nanofluid in a solar dish collector. • A hemispherical cavity receiver was used as the dish absorber. • MWCNT/thermal oil nanofluid was tested as the solar working fluid. • Performance of the nanofluid was compared with the pure oil. • Two thermal efficiency models were suggested for the nanofluid and the pure oil. Abstract Nowadays, nanofluids are introduced as an effective technique for enhancing the thermal efficiency in solar collectors. In this work, the MWCNT/oil nanofluid with 0.8% nanoparticle mass fraction is investigated experimentally in a solar dish concentrator coupled to a hemispherical cavity receiver. The main objective of this research is to examine the effect of the nanofluid application for improving the solar thermal performance of the hemispherical cavity receiver. The results revealed that thermal efficiency of the hemispherical cavity receiver has averagely increased equal to 12.93%, using application of the MWCNT/oil nanofluid compared to the application of the basefluid. In the steady state periods, the average thermal efficiency of 76.23% and 67.50% were calculated for the hemispherical cavity receiver using the MWCNT/oil nanofluid and pure oil, respectively. It was conducted that the receiver heats gain and the thermal efficiency of the cavity receiver has a similar trend with the working fluid difference temperature. Two experimental models were suggested based on the overall performance versus T in - T amb I beam for the MWCNT/oil nanofluid and the pure oil. Finally, it is found that the use of the examined nanofluid is able to enhance the thermal performance of the examined collector about 13%. Also, in this study, group method of data handling (GMDH) artificial neural networks is applied in order to model the dependency of thermal efficiency and the cavity heat gain of the Hemispherical Cavity Receiver on the mentioned factors. [ABSTRACT FROM AUTHOR]

The present study is a 4E (Energy, Exergy, Economic, and Environmental) analysis of a nanofluid-based parabolic trough collector (PTC) which is conducted with a developed model in MATLAB. MXene (Ti 3 C 2) nanoparticles were added to the silicon oil at weight concentrations of 0.05, 0.08, and 0.1%. The result was the important enhancement of the thermal conductivity of the nanofluid on the rand of 70%–89% for the highest concentration. According to the results, the PTC's optical efficiency was estimated at 79%, while thermal and exergy efficiencies of 74.71% and 22.44% with 0.10 wt% concentration were recorded on July 17, 2019. It is found that the MXene based silicone oil can reduce the system's cost by 0.021 M$ and enhance the energy gained by 1.51% at 0.10 wt% concentration compared to pure oil. Also, the annual CO 2 mitigation from the energy side varies from 2.25 to 2.30-ton CO 2 /year, while the annual environmental gain ranged between 32.73 and 33.28 $/year and 9.065 to 9.102 $/year for the energy and exergy studies respectively. [Display omitted] • A nanofluid-based parabolic trough collector is studied with MXene. • The collector thermal efficiency enhancement is found up to 1.1%. • For the day July 17, 2019, the exergy efficiency was improved from 0.07% to 15.16%. • For energy analysis, the annual CO 2 mitigation varies from 2.26 to 2.30 ton CO 2. • The environmental gain from the energy analysis varies from 32.73 to 33.28 $/year. [ABSTRACT FROM AUTHOR]

• Latest developments in solar thermal applications using nanofluids are reviewed • Low to medium temperature solar collectors are summarized • Improvements in solar collectors using nanofluids as working fluid is compiled • Application of solar collectors in buildings is reviewed The efficient exploitation of solar irradiation is one of the most encouraging ways of handling numerous environmental concerns. Solar collectors are suitable devices that capture solar irradiation and convert it into thermal energy and electricity. In the last years, the nanofluids used in solar thermal systems have been studied as a useful technique for enhancing the solar collectors' performance and establishing them as viable and highly efficient systems. The present review paper aims to summarize and discuss the most important numerical and experimental studies in nanofluid-based solar systems for application at low and medium temperature levels, while the emphasis on the fundamental physical phenomena that occur. In the first part, numerous numerical models and the principal physical phenomena affecting the heat transfer rate in the nanofluid have been analyzed. More specifically, the importance of different forces in nanofluid flows that exist in particulate flows such as drag, lift (Magnus and Saffman), Brownian, thermophoretic, Van der Waals, electrostatic double-layer forces are considered. Moreover, an overview of the thermophysical properties, physical models, heat transfer models, and evaluation criteria of nanofluids are included in this work. In the second part, which is the main part of this work, a comprehensive review is performed to gather and discuss the new advantages in the nanofluid-based solar collectors that operate at low and medium temperatures. More specifically, the examined solar systems are the flat plate collectors, the evacuated tube collectors, the direct absorption collectors, and the thermal photovoltaic systems, while the investigated applications are space-heating, space-cooling, household hot water production, desalination, industrial activities, and power generation. The aforementioned collectors and applications are the most usual in the real systems, indicating the importance of the present work. Moreover, the emphasis is given in the thermal, exergy, economic, and environmental evaluation of the studied systems, as well as in the discussion of the possible limitations of the use of nanofluids like the lack of long-term stability, the agglomeration of nanoparticles, and the increased pumping work due to the increased pressure drop. Finally, it is found that the nanofluid utilization usually enhances the collector efficiency up to 5%, while higher enhancements can be found in thermal photovoltaics. Moreover, it is concluded that there is a need to emphasize issues such as stability and the use of eco-friendly solar systems. Lastly, the field's future trends are highlighted, and a clear image of the present situation and the next steps in the field are given. [ABSTRACT FROM AUTHOR]