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Optimization of silver/water-based porous wavy direct absorption solar collector
Abstract
Direct Absorption Solar Collectors (DASCs) are a widely utilized technology in residential applications. However, having known the limitation in DASC size, the efficiency must be enhanced by applying effective modifications and optimizing design parameters. In this study, the performance of a wavy bottom-shaped collector filled with an aluminum porous medium was investigated and the most influential characteristic parameters are specified. Then a design for DASC using Polyvinylpyrrolidone-coated silver nanofluid is proposed and characteristic parameters are optimized based on the full factorial design of the experiment methodology. The model consists of four primary factors, including nanofluid volume concentration (C=0.025%,0.05%,0.1%), porosity (ε=0.8,0.88,0.95), bottom wave amplitude (A=2.5,5,7.5mm), and bottom wavenumber (λ=15,30,60m−1). The results indicated that lowering porosity and increasing nanofluid concentrations improves collector efficiency, whereas rising the wave amplitude and wavenumber causes a higher pressure drop. Additionally, by employing the full factorial design, the main and interaction effects of factors on the efficiency and pressure drop of DASC as the response variables are evaluated. Thus, an optimum value is observed for wave amplitude to reach maximum efficiency and minimize pressure drop. By integrating a porous medium and a wavy bottom with nanofluid, the efficiency of DASC is enhanced from 52 to 93.7%, paving the way for their use in residential applications.
... Owing to the suboptimal outlet temperature of DASC, the wavy direct absorption solar collector is selected for this study. Bozorgi et al. [49] conducted a numerical examination, optimizing the geometry and porosity of the Wavy Direct Absorption Solar Collector (WDASC) with porous media. In the current study, the recommended collector without porous media is adapted to provide for the system's thermal energy requirements. ...
... (1), known as the instantaneous efficiency curve [50]. For this study, simulation outcomes from the author's previous research, specifically Bozorgi et al. [49], are utilized to establish the instantaneous efficiency curve for the WDASC. The coefficients derived from the instantaneous efficiency enabled the simulation of the collector within the TRNSYS software environment. ...
Addressing indoor thermal comfort in buildings located in hot and humid climates is a persistent challenge, demanding innovative cooling solutions that are both efficient and environmentally responsible. This paper introduces a new integration of a Phase Change Material (PCM)-based solar desiccant cooling system with an adsorption chiller, a setup designed to enhance indoor thermal comfort conditions while utilizing renewable energy sources. This research simultaneously leverages the heating and cooling capacities of the adsorption chiller, leading to an efficient system performance. The TRNSYS software is used to evaluate the system's behavior in a typical house with moderate cooling demand and characterized by high relative humidity. Two types of solar collectors, the evacuated tube and Wavy Direct Absorption Solar Collectors (WDASC), are evaluated to supply the required thermal energy, providing a comprehensive assessment of the system’s efficiency under different configurations. Additionally, the study conducts a comprehensive Life Cycle Assessment (LCA) to quantify the Global Warming Potential (GWP) of the systems. The results confirm the proposed system's capability to maintain indoor thermal comfort within acceptable Predicted Mean Vote (PMV) and Predicted Percentage of Dissatisfied (PPD) ranges. The evacuated tube collector showed marginally better performance (average PMV: 0.018, average PPD: 6.7 %) compared to the WDASC (average PMV: 0.075, average PPD: 7.3 %). Additionally, the evacuated tube collector demonstrates superior energy (75.68 %) and exergy (10.82 %) efficiencies compared to the WDASC. The system utilizing the evacuated tube collector surpasses the WDASC configuration, achieving a higher Coefficient of Performance (COP) of 0.47 compared to 0.41, and emitting lower GWP values of 0.0194 kg-CO2 per kWh of cooling capacity compared to 0.0201 kg-CO2. An analysis of Embodied Energy reveals that the adsorption chiller is the most significant contributor to lifecycle energy consumption, with a quicker Energy Payback Time (EPBT) for the evacuated tube system (2.75 years) compared to the WDASC system (3.94 years). This research thus contributes a highly efficient, sustainable, and environmentally friendly solution to the challenge of cooling in buildings, setting a new standard for future green building practices.
... The impact of internal design on solar collectors has also been extensively studied. For example, Bozorgi et al. [26] examined a wavyshaped collector, affecting the temperature and velocity profiles within the system. Additionally, we reference the work by Chen et al. [27], where attention was given to nanofluids situated in an annular space between two circular glasses, resulting in a thinner nanofluid layer compared to a standard setup. ...
This review article focuses on the impact of working fluid characteristics, geometrical parameters and the operating coefficients in thermal efficiencies of direct absorption solar collectors (DASCs). Regarding working fluid parameters, the review emphasized the importance of type of base fluid, nanoparticle properties, such as material, size, concentration and shape. Geometrical parameters, including the collector area, aspect ratio, tube diameter and collector panel design, were identified as crucial design considerations for maximizing DASC efficiency. Additionally, the review highlighted the significance of various operating parameters. Solar irradiance was identified as a critical factor, emphasizing the importance of optimizing the collector's orientation and tilt angle to maximize solar exposure. The fluid flow rate within DASCs was also recognized as a key parameter influencing heat transfer and overall performance. Increasing the thermal flow measurement of nanomaterials has been predicted to improve the heat transfer, ultimately leading to improved thermal efficiency. By carefully considering and optimizing these parameters, it is possible to maximize the efficiency and effectiveness of DASCs, thereby advancing the utilization of solar energy for sustainable and renewable power generation.
Nowadays, there is wide acceptance among core energy experts and the research community that solar collectors have a critical role to play in the renewable energy sector. With the high-energy conversion rate associated with this solar energy harvesting technology, there is an urgent need to review various ways through which the heat transfer process can be improved. Researchers have mainly focused their attention on working fluid enhancement, thereby neglecting other techniques that can equally contribute to the heat transfer enhancement process in solar collectors. This has made studies in other directions obsolete. The primary objective of the present study is to provide an up-to-date overview of the latest advancements and novel approaches in enhancing the heat transfer process in solar collectors. Both conventional and innovative techniques were investigated, exploring their effectiveness in enhancing thermal performance and overall system efficiency. A comprehensive literature review of recent studies on heat transfer enhancement techniques in solar collectors was undertaken. Key challenges, knowledge gaps, and limitations were identified, and potential future research options were recommended. This holistic review will serve as a guide for researchers and key energy players, providing insight into the avalanche of research options available in the field of solar thermal collectors.
As a sustainable and widely available source of clean energy, solar energy has great potential for solving the aforementioned issues. Nanofluids (NFs)‐based direct absorption solar collectors can convert solar energy into thermal energy, which has been widely applied in energy‐thermal conversion and wastewater purification. However, the low photothermal modulation efficiency of NFs limits the industrialization of solar collectors. Herein, the Fe 3 O 4 @Au composite structure is constructed using coprecipitation method. The effect of different parameters, such as Au loading, concentration of NFs, external magnetic field, and flow rate on the optical and photothermal properties of the Fe 3 O 4 @Au NFs and Fe 3 O 4 NFs are systematically studied. The results reveal that the solar energy‐weighted absorption fraction of Fe 3 O 4 @Au reaches 97.54% with a low concentration of 0.0667 vol%. Besides, the heat distribution can be controlled using an external magnetic field owing to the magnetic properties of Fe 3 O 4 in water evaporation devices. The evaporation rate of Fe 3 O 4 evaporator and Fe 3 O 4 @Au evaporator is 1.8 and 2.3 kg m ⁻² h ⁻¹ , respectively, confirming their excellent and stable solar energy harvesting ability. The prepared Fe 3 O 4 @Au NFs are a promising approach in efficient photothermal conversion and thermal management in solar energy harvesting.
In this study, attention is focused to examine experimentally how utilization of porous foams, as well as nanofluids, may improve thermal performance of a residential–type direct absorption solar collector (DASC). The experiments are undertaken for SiC ceramic as the porous foam and SiC–water suspension as the nanofluid. Three levels are chosen for the reduced temperature difference, the porous foam, and the volumetric flowrate while four levels are selected for the nanoparticle concentration. To determine the relative importance of each of the involved parameters on thermal efficiency of the DASC, an ANOVA is undertaken. The best thermal performance is achieved in a case having both the metal foam and the nanoparticles. The maximum rises in thermal efficiency due to the insertion of the porous foam and the inclusion of the nanoparticles are recorded to be 14% and 37%, respectively. However, it is found that the volumetric flowrate is the dominant parameter in this system in a way that the inclusion of the porous foam or the nanoparticles deteriorates its role. Comparison of the results of the two porous foams demonstrates that the foam having higher porosity and pore density leads to higher thermal efficiency.
With an increasing global population, energy demand is rising as well. The limitation of fossil fuels, climate change, and environmental concerns due to the usage of these fuels, have led to considerable attention to renewable energies in the last few decades. Solar energy and biomass are two of the best available sources of renewable energy in most parts of the world. However, each of them suffers from some drawbacks. Solar radiation is only available for a limited time during a day, and its availability can be intermittent or reduced depending on the weather and season. On the other hand, extracting energy from biomass demands a huge feedstock, which may not be readily available in all locations and seasons. Transporting biomass would not only increase costs but would also increase carbon monoxide emissions from fossil fuel consumption by transport systems. A combination of both sources is a promising method to lower greenhouse gas emissions and reliable energy investment. The hybridization of these energy resources and applications in a power plant has attracted the attention of researchers in recent years. This study presents an in-depth review of the latest advances in integrating solar and biomass energy in power plants and summarizes and discusses the past effort and the current status of hybrid power plants, their performance, different feasible combinations, and challenges. This review can be a useful reference to investigate the performance of a hybrid solar-biomass power plant in terms of energy, environmental, economical aspects, and conduct readers to future work and novel ideas in this field. A thorough review of the literature reveals that despite that several studies have been conducted in this field, there is currently only one operational hybrid solar-biomass power plant; Termosolar Borges, with a capacity of 22.5 MWe, which is located in Les Borges, Spain. However, some small-scale prototypes are constructed and aimed to extend to a large scale. In terms of possible hybridization scenarios and performance, among solar energy technology, concentrated solar power is a more suitable and proven technology than PV for the hybridization with biomass fuels. Among technologies applied to utilize power from biomass fuels, combustion has proven to be a more economically viable input heat for typical power plants than gasification. In contrast, gasification offered a better option for a larger scale, as the overall cost decreased significantly with scale.
The performance of nanofluid-based solar collector depends on different factors like nanoparticle material, base fluid, mass fraction of nanoparticles, height of the collector, length of the collector, incident solar flux, mass flow rate of the nanofluid in the collector. In order to quantify the effect of nanoparticle material, optical properties of different nanofluids (different nanoparticles dispersed in DI water) were measured using spectrophotometer. After studying the optical signatures of the nanofluids, solar-weighted absorptivity of the nanofluids was calculated to evaluate the efficient nanoparticle material for harnessing solar irradiation. It has been found that amorphous carbon nanoparticles-based fluid has highest absorptivity at low mass fractions and is suitable for harnessing solar energy. Further, the performance of nanofluid-based solar collector has been investigated numerically using amorphous carbon nanoparticles. For the purposes of this analysis, a two-dimensional steady-state heat transfer model has been developed for a collector in which the nanofluid flows horizontally and is heated with normally incident solar irradiation. During the analysis carried out in this study, five factors were chosen (height of the collector, nanofluid mass flow rate, solar irradiation incident flux, nanoparticle mass fraction, and length of the collector). The influence of variation in these five factors on the overall performance of the solar collector (i.e., outlet temperature) was analyzed using the Taguchi method. In order to carry to this analysis, the values of these five factors were varied at three levels. Based on these variations, the L18 standard orthogonal array was prepared, and the matrix containing the 18 sets of combinations was organized. The results showed that two factors (length of the collector, and the incident solar flux) exhibited a strong increasing trend, while one factor (nanoparticle mass fraction) exhibited an optimizing trend on the output (outlet collector temperature). Further, from the calculations it has been observed that 60 mg L⁻¹ is an optimum mass fraction at which maximum collector efficiency can be achieved for the length of the collector (= 1 m), height of the collector (= 10 mm), incident flux (= 1000 W m⁻²), mass flow rate (= 0.010 kg s⁻¹).
In the present work, we report the thermophysical properties of ethylene glycol and water mixture based silver nanofluids. The thermo physical properties such as thermal conductivity, viscosity, density and specific heat are measured using KD2 Pro thermal properties analyser, capillary viscometer, electronic weighing balance and differential scanning calorimeter respectively. The thermal conductivity increases with the increase in particle concentration and temperature. The maximum enhancement of thermal conductivity observed is approximately ~12% for 0.15 vol% at 50°C. The higher thermal conductivity of the particle, Brownian motion and clustering of the particles could be the possible reason for the improvement in thermal conductivity of the nanofluid which is consistent with the published literature. The viscosity and density increases with increase in particle concentration and decreases with increase in temperature. The specific heat decreases with increase in particle concentration and increases with increase in temperature.
The efficiency of conventional tube- in- plate type solar collectors is limited due to higher heat losses for surface based solar energy absorption and indirect transfer of heat from hot absorber surface to working fluid having poor heat transfer properties flowing through tubes. In this paper, a prototype direct absorption solar collector having gross area 1.4 m2 working on volumetric absorption principle is developed to investigate the effect of using Al2O3-H2O nanofluid as heat transfer fluid at different flow rates. Experimentation was carried using distilled water and 0.005% volume fractions of 20 nm size Al2O3 nanoparticles at three flow rates of 1.5, 2 and 2.5 lpm. ASHRAE standard 93-86 was followed for calculation of instantaneous efficiency of solar collector. Use of nanofluid improves the optical and thermo physical properties that result into an increase in the efficiency of the collector in all cases of using nanofluids in place of water. Collector efficiency enhancement of 8.1% and 4.2% has been observed for 1.5 and 2 lpm flow rate of nanofluid respectively. Optimum flow rate of 2.5 and 2 lpm towards maximum collector efficiency have also been observed for water and nanofluid respectively.
Solar thermal collectors are applicable in the water heating or space conditioning systems. Due to the low efficiency of the conventional collectors, some suggestions have been presented for improvement in the collector efficiency. Adding nanoparticles to the working fluid in direct absorption solar collector, which has been recently proposed, leads to improvement in the working fluid thermal and optical properties such as thermal conductivity and absorption coefficient. This results certainly in collector efficiency enhancement. In this paper, the radiative transfer and energy equations are numerically solved. Due to laminar and fully developed flow in the collector, the velocity profile is assumed to be parabolic. As can be observed from the results, outlet temperature of collector is lower than that obtained using uniform velocity profile. Furthermore, a suspension of carbon nanohorns in the water is used as the working fluid in the model and its effect on the collector efficiency is investigated. It was found that the presence of carbon nanohorns increases the collector efficiency by about 17% compared to a conventional flat-plate collector. In comparison with the mixture of water and aluminium nanoparticles, a quite similar efficiency is obtained using very lower concentration of carbon nanohorns in the water.
Due to its renewable and nonpolluting nature, solar energy is often used in applications such as electricity generation, thermal heating, and chemical processing. The most cost-effective solar heaters are of the "flat-plate" type, but these suffer from relatively low efficiency and outlet temperatures. The present study theoretically investigates the feasibility of using a nonconcentrating direct absorption solar collector (DAC) and compares its performance with that of a typical flat-plate collector. Here a nanofluid-a mixture of water and aluminum nanoparticles-is used as the absorbing medium. A two-dimensional heat transfer analysis was developed in which direct sunlight was incident on a thin flowing film of nanofluid. The effects of absorption and scattering within the nanofluid were accounted for. In order to evaluate the temperature profile and intensity distribution within the nanofluid, the energy balance equation and heat transport equation were solved numerically. It was observed that the presence of nanoparticles increases the absorption of incident radiation by more than nine times over that of pure water. According to the results obtained from this study, under similar operating conditions, the efficiency of a DAC using nanofluid as the working fluid is found to be up to 10% higher (on an absolute basis) than that of a flat-plate collector. Generally a DAC using nanofluids as the working fluid performs better than a flat-plate collector, however, much better designed flat-plate collectors might be able to match or outperform a nanofluids based DAC under certain conditions.
In this paper, we present a comprehensive analytical and experimental investigation for the determination of the effective thermal conductivity (ke), permeability (K) and inertial coefficient (f) of high porosity metal foams. In the first part of the study, we provide an analysis for estimating the effective thermal conductivity (ke). Commercially available metal foams form a complex array of interconnected fibers with an irregular lump of metal at the intersection of two fibers. In our theoretical model, we represent this structure by a model consisting of a two-dimensional array of hexagonal cells where the fibers form the sides of the hexagons. The lump is taken into account by considering a circular blob of metal at the intersection. The analysis shows that ke depends strongly on the porosity and the ratio of the cross-sections of the fiber and the intersection. However, it has no systematic dependence on pore density. Experimental data with aluminum and reticulated vitreous carbon (RVC) foams, using air and water as fluid media are used to validate the analytical predictions.The second part of our paper involves the determination of the permeability (K) and inertial coefficient (f) of these high porosity metal foams. Fluid flow experiments were conducted on a number of metal foam samples covering a wide range of porosities and pore densities in our in-house wind tunnel. The results show that K increases with pore diameter and porosity of the medium. The inertial coefficient, f, on the other hand, depends only on porosity. An analytical model is proposed to predict f based on the theory of flow over bluff bodies, and is found to be in excellent agreement with the experimental data. A modified permeability model is also presented in terms of the porosity, pore diameter and tortuosity of our metal foam samples, and is shown to be in reasonable agreement with measured data.
Nanofluid-based direct absorption of solar heat results in thermal efficiencies superior to conventional solar thermal technology. In addition, convection of nanofluid can be sustained pump-free in the collector. In this article, we study an aqueous magnetic nanofluid capable to establish the photothermal convection in a lab-scale direct absorption solar collector equipped with a solenoid. The nanofluid consisted of 60-nm Fe2O3 particles dispersed in distilled water at concentration in the range 0.5% wt.-2.0% wt. An empirical model of the photothermal convection was developed based on the experiments. The model accounted for magnetic and thermophoretic forces acting within the nanofluid. The nanofluid with up to 2.0% wt. iron oxide nanoparticles obtained the velocity of ∼5 mm/s under the magnetic field of up to 28 mT. This resulted in the maximum thermal efficiency of the collector equal to 65%.
Nanofluids show great interest in the direct absorption solar collector (DASC) due to their excellent optical, thermal, and flow properties. However, the stability of nanofluid in the terms of the long period and the working temperature has limited its commercial application in DASCs. In this work, ultra-stable carbon quantum dot (CQD) nanofluids were prepared by microwave heating polyethylene glycol 200 (PEG-200), which shows excellent stability over a long period (such as 30 days) and repeatable performance at the working temperature. The solar thermal conversion efficiency η increases with extending the microwave duration t, and η can reach 81% at 1 sun and t = 24 min, which is triple the base fluid PEG-200 (η = 27%). In addition, the solar intensity and working temperature have little effect on η due to the ultra-stability of CQD nanofluids. Finally, η can be further enhanced by changing the base fluid or mixing with the plasmonic nanofluids. These results show that the CQD nanofluids can be a potential working fluid used in solar thermal applications.
It has been reported that plasma nanofluids are beneficial to improve the performance of a direct absorption solar collector (DASC). For this purpose, the solar absorption of gold sphere nanoparticles (NPs) in the near-infrared spectral region and gold nanorods in the visible spectral region are needed to be focused. Based on this issue, we propose a core-shell composite nanorod and investigate its optical properties to illustrate its potential for improving the solar absorption efficiency of the DASC. The results show that the core-shell composite nanorod improves the absorption efficiency both in the near-infrared and visible spectral region. Further study reveals that the absorption mechanism of composite nanorods can be attributed to localized surface plasmon resonance (LSPR) and propagating surface plasmon resonance (PSPR). In addition, in view of the discussion about the geometrical size of composite nanorods and the calculation of composite nanorod suspension, the solar absorption efficiency can be as high as 95.3%. Notably, the imperfect shell during the experimental synthesis will greatly reduce the absorption capacity of the composite nanorods. The results show that the core-shell composite nanorod has great potential in the DASC applications.
Nanofluid offers remarkable thermal and optical properties favourable for direct solar absorption. The nanofluids prepared by the conventional two-step synthesis method have low colloidal stability, while that synthesized through the one-step method is costly. Hence the nanofluid synthesized using an economical one-step method has great significance. In the present study, a highly stable C-dot/water nanofluid was synthesized using an economical one-pot synthesis method. The optical characterisation, corrosion analysis and cost estimation of the nanofluid were conducted. The influence of C-dot/water nanofluid on the performance of direct absorption solar collector was analyzed. The direct absorption parabolic solar collector employed with C-dot/water nanofluid yielded a maximum thermal efficiency of 73.41% at Reynolds number of 2952, while that for water was 15.79%. Thermodynamic analysis of the system and cost estimation of the nanofluid was performed to establish its commercial suitability in various solar thermal devices. The maximum exergy destruction was found to be 924.3 W and was more or less constant at all flow rates. The main highlight of the new C-dot/water nanofluid is its significantly high colloidal stability and was found to be stable for more than six months. The corrosion rate of the new C-dot/water nanofluid was obtained as 0.094 mm/year, while that for the base fluid was 0.372 mm/year. With superior optical performance, corrosion resistance, and low production cost, the C-dot nanofluid has the potential to be a prospective working fluid in various direct absorption solar thermal systems.
Direct absorption solar collectors (DASC) with nanofluid represent a new direction in solar thermal technology that is simpler yet more efficient than conventional equipment. In this work, we report details of performance for a custom tubular DASC with a carbon-based nanofluid. The collector was tested experimentally following a standard procedure and using a multiphase CFD-model of the device. The experiments were carried out in a range of flow rates 2... 10 l/min, nanoparticle concentrations 0.0015... 0.082%wt., temperature differences (up to 29.3 degrees), and radiant heat fluxes. We found that, at a particle concentration of 0.01%, the collector demonstrated the average thermal efficiency of 80%. For the comparable temperature differences, the efficiency of DASC was 5.8... 37.9% higher than a collector with similar geometry but a surface absorption of light energy.
The CFD-model, validated against our experiments, depicts flow patterns in the DASC focusing on nanoparticles’ deposition. Less than 5% of particles deposit under local flow restrictions at flows above 6 l/min. The deposition patterns from the CFD-model correlate to the experimental observations.
In this work, adapted from the concept of cavity, a new model of receiver tube presented and used in one direct-absorption solar collector (DASC) equipped with two linear parabolic reflectors (LPR). Using both the insulated wall and the cavity-like receiver tube minimize the energy loss in this new-presented receiver tube. Its other advantage is the increase of sun-rays path length created by the sequential reflections of rays in the receiver tube which leads to the attenuation of nanoparticle agglomeration. Therefore, the effect of mentioned two advantages of receiver tube as well as the effect of nanoparticles is analyzed separately. The new presented receiver tube with the absorbent walls is utilized in surface-base solar collector (SBSC) to survey the effect of using the nanoparticle. Also, to investigate the effect of using new-presented absorber tube in SBSC, the obtained results are compared with the conventional type of SBSC. For these purposes, after the sun-rays tracing conducted by a mathematical method, the numerical simulation is carried out by COMSOL software to investigate some effective parameters. Accordingly, using nanoparticle in the new-presented DASC enhances the efficiency by 5% than SBSC. Also, using the new presented cavity-like receiver tube in SBSC augments the efficiency up to 2% rather than conventional LS-2 collector. Finally, as the prominent finding of the current work, the highly-efficient DASC (i.e., 81%) can be achieved using the lower percentage of nanoparticle (i.e., α=0.07).
Direct absorption solar collector (DASC) is regarded as one of the most promising next-generation solar energy collection technology. Most researches focus on the photothermal performance of working fluids. While the optical boundary condition, which is another important factor influencing the efficiency of DASC, receives little attention. In this paper, the ethylene glycol based TiN nanofluids are used as the research objective. The temperature-dependent optical properties of nanofluids are experimentally investigated in detail, and when the temperature increases from 0 °C to 60 °C, the optical absorption performance of fluids could enhance ~50%, which means that the heated fluids has stronger absorption capability. To improve the photothermal conversion efficiency of collector system, two types of irradiation directions have been studied for the collector, and different heat transfer mode of each type has been experimentally analyzed. The experimental results show that the added nanoparticles can significantly enhance the photothermal conversion efficiency of solar collectors. When the concentration of TiN is 0.003 wt.%, the photothermal conversion efficiency of bottom irradiation mode achieves ~45%, much higher than that of side irradiation. However, the side irradiation collector can save ~40% of time to reach steady-state compared with the bottom irradiation collector. Moreover, two kinds of collectors have a uniform temperature field (~10 °C difference between different depth) over 1.0 cm irradiation depth. Consequently, the prospects for possible applications of ethylene glycol based TiN nanofluids in high-efficiency DASC are presented.
In this study, an artificial neural network (ANN) adaptive neuro-fuzzy inference system (ANFIS) model was developed to predict the performance of a low-temperature direct absorption solar collector (DASC) enhanced with nanofluids. The accuracy and the effect of different types of clustering methods such as Subtractive Clustering and Fuzzy c-means (FCM) clustering investigated. Various inlet temperatures, weight percentage of graphene nanoplatelets nanofluids based deionized water (0.0005, 0.001, 0.005 wt%) and flow rates (0.0075, 0.015, 0.0225 kgs−1) were tested. Five inputs used for the ANFIS model: the inlet temperature, rate of solar radiation, ambient temperature, mass flow rate and weight percentage of nanofluid. The ANN provides a single output: the efficiency of the collector. In total, 1536 data points utilized for modeling, including 1075 for the ANN training process and 461 for testing. Overall, the ANFIS model was very accurate at predicting the collector performance. This was indicated by the root mean square error (RSME), the mean absolute percentage error (MAPE), coefficient of determination (R2), and mean bias error (MBE), which calculated to be 0.008, 0.031, 0.999 and 7e-4 respectively. The ANFIS model agrees well with the experimental results.
Nanofluids are an emerging technology in heat transfer field. Nanofluids can be used in a variety of industrial applications because of improved thermo-physical properties. In this field, the nano sized materials are suspended with the base fluid. This nanoparticles suspended base fluid enhances heat transfer properties such as thermal conductivity, dynamic viscosity and density. Thus, the enhanced properties of base fluid may enhance the efficiency of diffuser heat exchanger. This will lead to the increase in testing time of the engine. In this study a methodology to estimate the thermal properties of nanofluids is discussed.
Variable morphology metal foams, embedded in phase change materials (PCMs), enhance heat transfer performance without sacrificing the volume available for the PCM. The present study is a numerical investigation of the effect of aluminum metal foam, having varying morphology, on the unidirectional melting process of n-octadecane under pulsating heat flux boundary condition. The interdependencies of metal foam morphology, mean porosity, pulse duration, average heat flux, and system size on the progress of melt fraction and isoflux surface temperature are studied. The effect of two graded and one uniform metal foam morphologies, three pulses, two average wall heat fluxes, two system sizes, and five different mean porosities are analyzed. The positive gradient of metal foam porosity (i.e., the increase in the porosity with distance from the isoflux surface) limits the increase in the surface temperature by more than 15°C when compared with the uniform metal foam. As well as, graded metal foam reduces temperature fluctuation under pulse heating load by more than 20%. However, the positive gradient of metal foam has adverse effects on the overall melting time. The effect of metal foam morphology on heat transfer is observed to increase with the average heat flux, system size, and mean porosity of metal foams. In the presence of heating from the top, as discussed in the present study, melting processes are driven by heat conduction due to the limited presence of advection. Therefore, the significance of enhancement in effective thermal conductivity is higher in the presence of isoflux boundary on the top surface.
Solar water heating is one of the most efficient solar technologies in the domestic sector. The most important component of the solar thermal systems is the solar collector, which converts solar radiation to useful thermal energy. There are many types of solar collectors, which are categorized based on the operating temperature (low, medium and high temperatures) or the working fluid (gas or liquid). One of the newest types of solar collectors is direct absorption solar collector in which solar radiation is absorbed through the working fluid, unlike other collectors that use the surface absorber or indirectly absorb the solar radiation. The common working fluid for DASC is the suspension of metal, metal oxide or carbon nanomaterials in solar common fluids (water, EG, PG and Therminol VP-1). In this review paper, the effect of design and operating parameters on the thermal performance of low-temperature direct absorption solar collector is summarized. Using the numerous studies done in this field, the efficiency enhancement of DASC by variation of the collector geometric properties, the flow properties (the flow rate and Reynolds number), the working fluid properties (the base fluid, the nanoparticle material and size, and the nanofluid concentration), and the collector design is identified. This paper also identifies the current challenges facing the direct absorption solar collectors and the future recommendations for developing and commercializing these collectors.
In this study, the performance of the hybrid nanofluid of alumina/fly ash-based nanofluid and silica/fly ash-based nanofluid in the direct absorption solar collector is compared. SiO2, Fe2O3, Al2O3 and CaO are main components of the fly ash. The effect of different proportions of major components in fly ash and flow rate on the thermal and exergy efficiency is studied. Microchannel-based flat plate solar collector is used for the experimentation with a channel height of 800 microns. Experiments are conducted to evaluate the thermal efficiency, pumping power, performance evaluation criteria, entropy generation rate and exergy efficiency of fly ash-based nanofluids in direct absorption solar collector. The experimental results revealed that the thermal efficiency of the alumina/fly ash (80:20)-based nanofluid for direct absorption solar collector is 72.82% while silica/fly ash (80:20) nanofluids showed 59.23% thermal efficiency. Exergy efficiency achieved by the alumina/fly ash (80:20)-based nanofluids is 73%. This is significantly more than the silica/fly ash-based nanofluids. Silica/fly ash (80:20)-based nanofluids achieved an exergy efficiency of 68.09%. The study revealed that an increase in the concentration of alumina in the fly-ash nanofluid will increase the thermophysical property and efficiency of the nanofluid and an increase in the silica concentration will lead to decrease in the thermophysical property and efficiency of the fly ash-based nanofluid.
The current research proposes the idea of using water‐saturated metal oxide foams and water‐based nanofluids as solar absorber in the direct absorption solar collectors (DASCs). Specifically, the novel solar collector design utilizes copper oxide (CuO) porous foam and nanoparticle with high optical properties and is expected to have enhanced thermal performance than the conventional collectors utilizing pure water. The finite volume technique is used to solve the governing equations of flow and heat transfer in the radiative participating media. Also, to establish the reliability and accuracy of numerical solutions, the obtained results are compared with the corresponding numerical and experimental data. The computations are carried out for different nanoparticle volume fractions, foam pore sizes, working fluid mass flow rates, and both porous layer thicknesses and positions (inserted at the lower or upper wall of the collector). It is found that the efficiency of DASC partially/fully filled with metal oxide foam is maximized when the collector is completely filled with it. Compared with the water flow, the numerical results show that the collector efficiency using CuO nanofluid and metal oxide foam is improved by up to 26.8% and 23.8%, respectively. Moreover, considering the second law of thermodynamics, the use of CuO nanofluids in the DASC seems to be more effective than the use of CuO porous foam. The use of CuO porous foam and nanoparticle in a low‐temperature direct absorption solar collector is numerically investigated and to the best of our knowledge, there has been no experimental or numerical study on the application of metal oxide foam in this type of solar collector. Considering both first and second laws analysis, a performance number is defined and evaluated for both porous foam‐filled and nanofluid‐based collectors.
For high-temperatures surfaces, surface absorbers possess a higher evaporation rate while volumetric receivers have a higher solar vapor generation efficiency by localizing high temperatures to the interior of the receiver and thus reduce surface losses. In this work, the solar vapor generation rates and efficiencies of several direct absorption solar collectors were compared by using reduced graphene oxide/graphene oxide and silver nanofluids as evaporators, two of which deserve to be noticed. One is the hybrid nanofluids containing reduced graphene oxide decorated with silver nanoparticles in volumetric solar absorbers and the other one is hybrid nanofluids containing reduced graphene oxide with silver floating on the surface, combining surface absorbers and volumetric ones. The results show that relative efficiency monotonously increases with the light density (e.g. correspondingly changes within 61.5% at 1 sun, 62% at 2 suns, 64% at 3 suns and 69% at 4 suns for 0.45 mg/ml reduced graphene oxide) and nanofluid concentration. The further experiments reveal that nanofluid-assisted vapor generation efficiencies of hybrid nanofluids containing reduced graphene oxide with silver floating on the surface were higher, reaching 91.6% at 3 suns, than the others due to its higher absorbance and plasmonic effect of the nanoparticles and high thermal conductivity of graphene nanosheets. This demonstration of hybrid nanofluids containing reduced graphene oxide with silver floating on the surface holds the promise of significantly expanding the potential applications in desalination, water treatment and power generation.
In this study, energy and exergy efficiency of residential-type direct absorption solar collector using PVP-coated silver nanofluid has been evaluated experimentally. First, stability and thermophysical and optical properties of nanofluid have been considered using the theoretical and experimental methods. Then, outdoor thermal performance of collector is investigated using the experimental setup based on EN12975-2. Measurement of optical properties of nanofluid using the spectrophotometry method show that the extinction coefficient of the base fluid is increased up to 6.24 cm-1 using 1000 ppm PVP-coated silver nanoparticles. Results of energy analysis show that the collector efficiency is increased by increase of flowrate and concentration of nanofluid asymptotically. It is observed that exergy efficiency is firstly increased by nanofluid concentration and then, decreased after reaching the optimum value. The optimum concentration was 500 ppm for all flowrates. The variation of exergy efficiency by reduced temperature difference is similar to volume fraction. The optimum exergy efficiency is obtained at 0.01 m2K/W. The decrease of exergy efficiency by flowrate indicated that exergy losses due to pressure drop have the significant effect on the collector performance.
In this study, energy and entropy analysis of a residential-type direct absorption solar collector using hybrid Fe3O4/SiO2 nanofluid is evaluated experimentally. The hybrid nanofluid samples are prepared in the different volume ratios of Fe3O4/SiO2 (25:75, 50:50 and 75:25) and different volume fractions (500 ppm, 1000 ppm and 2000 ppm). The appropriate nanofluid samples for using as the working fluid of the collector are chosen based on the results of stability and optical properties of nanofluid. Then, outdoor thermal performance of collector is investigated using the experimental setup based on EN12975-2. Measurement of nanofluid optical properties using the spectrophotometry method shows that the extinction coefficient of 2000 ppm hybrid Fe3O4/SiO2 nanofluid is on average 10 cm−1 higher than that of the base fluid. Results of energy analysis display that the collector efficiency is increased by mass flow rate and volume fraction of nanofluid asymptotically. The asymptotic value is about 83% for 2000 ppm hybrid Fe3O4/SiO2 nanofluid. The findings indicate that the variation of exergy efficiency of a direct absorption solar collector with the volume fraction and mass flow rate is similar to energy efficiency. The enhancement of exergy efficiency is 66.4% for mass flow rates of 0.0225 kg s−1 by increasing the volume fraction from 0 to 2000 ppm. It is also observed that dimensionless entropy generation number is decreased by nanofluid volume fraction and by mass flow rate. The lowest entropy generation number is obtained in the mass flow rate of 0.0225 kg s−1 and the volume fraction of 2000 ppm. The variation of Bejan number by volume fraction shows that the contribution of pressure drop in entropy generation is insignificant.
In this paper, the experimental results of the performance test of a direct absorption solar collector using PVP (polyvinylpyrrolidone) coated silver nanofluid as working fluid were obtained at different operating conditions such as different nanofluid volume fractions, different tilt angles, and different rate of flows. The dimensional analysis technique was utilized to develop an empirical correlation for collector efficiency. Also an empirical correlation is introduced for calculating Nusselt number in terms of Reynolds and Prandtl numbers to specify the amount of heat loss in direct absorption solar collector. Also, a comparison between experimental and correlated results was carried out. The results showed that the bias errors were 2.46% and 5.98% for Nusselt number and collector efficiency, respectively. Consequently, Average Standard Deviation (ASD) values reached 7.31% and 18.04% for both, respectively. The correlations presented can be used in calculating Nusselt number and collector efficiency for performance analysis of direct absorption solar collectors. It helps to enhance design calculations of this new type of solar collectors.
In the current study, thermal efficiency of a direct absorption solar collector, using single-walled carbon nanohorn (SWCNH)-water nanofluid as the working fluid, is numerically investigated. An in-house, parallel, multi-relaxation time, lattice Boltzmann method (MRT-LBM) code is developed, and the effects of nanofluid concentration, presence of aluminum absorber sheet and its position on design parameters of the collector are discussed. Results are discussed for two cases: with and without the absorber plate, and the effects of nanoparticles concentration (φφ), nanofluid’s mass flow rate (View the MathML sourceṁ), collector height (H), absorber plat position (Y) and radiation heat flux (q″q″) on efficiency and maximum temperature of the collector are studied. The importance of these parameters have been proven and it was shown that, in some cases, optimal working conditions exist. It has been demonstrated that low concentrations of nanoparticles can improve the collector efficiency, while its larger amounts can lead to negative performance. The absorber plate highly improved the collector performance especially when the working fluid is the pure water. Simultaneous use of nanofluid with the absorber plate is not recommended except for low mass flow rates. Also, the bottom of the collector is the best position for the absorber plate in order to gain the maximum efficiency.
In this paper, the best wavy profile for the bottom plate of a wavy nanofluid-based direct absorber solar collector (WDASC) is obtained by numerical and statistical methods. For this aim, thermal performance of a WDASC is evaluated by finite element method (FEM) using FlexPDE commercial package software and Al2O3-water nanofluid is considered as the working fluid in the collector. It is considered that bottom wavy plate remain in a constant temperature while the top flat plate is in constant heat flux due to solar radiation and the sides wall are insulated. To find the best wavy profile for the bottom plate, 9 different cases proposed by CCD are designed and by response surface method (RSM) analysis the best profile is introduced. Results show that the best case involves minimum possible wave amplitude (Am) in average wave numbers (λ) for the wavy wall.
In this paper, thermal performance of a wavy nanofluid-based direct absorption solar collector (DASC) is evaluated using finite element method (FEM) by FlexPDE commercial package software. After presenting the governing equations and solving them by FEM, TiO2, Al2O3 and CuO nanoparticles are considered to make water-based nanofluid in the collector. Due to three different wall types in the problem (bottom wavy wall, flat top wall and vertical side walls) and three different boundary condition types (insulated, constant heat flux and constant temperature), the problem is solved for the more appropriate conditions. For all tested nanofluids, the effect of nanoparticles volume fraction is also investigated on the local/average Nusselt numbers. Results show that wavy collectors have larger maximum Nusselt number compared to flat plate collectors. Furthermore, the most Nusselt numbers are reported for TiO2 nanoparticles in the little volume fractions.
A one-dimensional transient heat transfer analysis was carried out to analyze the effects of the Nanoparticle (NP) volume fraction, collector height, irradiation time, solar flux, and NP material on the collector efficiency. The numerical results were compared with the experimental results obtained by silver nanofluids to validate the model, and good agreement was obtained. The numerical results show that the collector efficiency increases as the collector height and NP volume fraction increase and then reaches a maximum value. An optimum collector height (∼10 mm) and particle concentration (∼0.03%) achieving a collector efficiency of 90% of the maximum efficiency can be obtained under the conditions used in the simulation. However, the collector efficiency decreases as the irradiation time increases owing to the increased heat loss. A high solar flux is desirable to maintain a high efficiency over a wide temperature range, which is beneficial for subsequent energy utilization. The modeling results also show silver and gold nanofluids obtain higher photothermal conversion efficiencies than the titanium dioxide nanofluid because their absorption spectra are similar to the solar radiation spectrum.
The melting process of a nano-phase change material (or nano-PCM) in a square enclosure filled with a porous medium was investigated numerically and analytically. The dimensionless continuity, Darcy–Brinkman momentum, and energy equations were solved using the finite element method. One vertical wall of the square enclosure was heated at a constant temperature (Th), while all the other walls were insulated. The numerical results were adopted for a wide range of Rayleigh number (106⩽Ra⩽5×107), Darcy number (10-8⩽Da⩽100), and the volume fraction of nanoparticles (ϕ = 0%, 10%, and 20%). The results were expressed in terms of isothermal lines, streamlines, and Nusselt number. In addition, a scale analysis of the governing equations was performed to verify the numerical results. A validation was performed between the present study and previously reported results in the literature; a good agreement was achieved. The numerical results indicated that the melting process is improved by increasing Ra, ϕ, and Da. The scale analysis successfully predicted the behavior of the meting process of nano-PCM embedded in the porous medium.
Application of nanofluids as a potential working fluid in solar-to-thermal energy conversion systems has shown remarkable improvements in solar power systems. Herein, a combined numerical and experimental study has been conducted on a nanofluid direct absorption collector utilizing three types of nanoparticles (i.e., graphite, magnetite, and silver) dispersed in deionized water as the absorbing medium. To increase the dispersion stability, surface modification was performed on the nanoparticles prior to the preparation of nanofluids via the two-step method and the optical characteristics of nanofluids were experimentally evaluated prior to their use in the DASC. A two-dimensional computational fluid dynamics simulation model was developed to solve the radiative transfer in particulate media and heat transfer equations. Considering the absorption and scattering within the nanofluid medium, the nanofluid temperature distribution within the collector was evaluated. Simultaneously, experiments were performed on a direct absorption collector to validate the numerical model and to investigate the effect of solar flux intensity, nanoparticle concentration and flow rate on the collector thermal performance. According to the results, nanofluids promoted the thermal and exergy efficiencies by 33–57% and 13–20%, respectively than the base fluid.
In this numerical work, natural convection of CuO–water nanofluid and pure water in a cavity submitted to different heating modes on its vertical walls, is analyzed using the Lattice Boltzmann Method (LBM). The effective thermal conductivity and viscosity of nanofluid are calculated by KKL (Koo–Kleinstreuer–Li) correlation. The influence of pertinent parameters such as Rayleigh number (Ra = 103–106), Hartmann number (Ha = 0–80), heat generation or absorption coefficient (q = − 10, − 5, 0, 5, 10) and nanoparticle volume concentration (ϕ = 0–0.04) on the flow and heat transfer characteristics has been examined. In general, by considering the role of Brownian motion, the enhancement in heat transfer is observed at any Hartman and Rayleigh numbers. In addition, the heat generation or absorption influences the heat transfer in the cavity at Ra = 103 more than other Rayleigh numbers as the least effect is observed at Ra = 106.
Today, with growth of population and increasing dependence of industry and technology on fossil energy, all communities and countries will face the challenge of energy in the future. Among all types of renewable energies, solar energy is a proper alternative to fossil fuels. For it is available in most parts of the world. In this study, the experimental investigation of volumetric solar collector’s performance which is applied in domestic hot water has been shown for the first time, by making a laboratory sample of a volumetric collector, using innovative application of graphene nanoplatelets/deionized water with weight fractions of 0.0005, 0.001 and 0.005. The experiments were carried based on EN 12975-2 at different inlet temperatures and three mass flow rate of 0.0075, 0.015 and 0.225 kg/s. The results showed that the collector efficiency increases by the nanofluid weight fraction increase. According to the results, the maximum collector efficiency is obtained at 0.015 kg flow rate for both the base fluid and nanofluids. The zero-loss efficiencies using nanofluids with weight fractions of 0.0005, 0.001 and 0.005 were 83.5%, 89.7% and 93.2%, respectively; whereas the zero-loss efficiency was 70% using the base fluid.
The enhancement of performance by increasing the thermal efficiency of a direct absorption solar collector based on an alumina–water nanofluid is the prime target of the present research. The base panel of the collector channel is subject to either a non adiabatic or an isothermal wall condition both of which introduce two new physical parameters. Analytical solutions for the temperature field are worked out in both cases for a two dimensional steady-state model recently outlined in the literature. The desired increase in the temperature of the heat transferring nanofluid is achieved either by slightly rising the heat transfer coefficient of the bottom panel coating or by prescribing a bottom surface temperature. As a consequence of the increase in the final outlet mean temperature, the solar collector thermal efficiency is found to be enhanced via increasing the new physical parameters as compared to the traditional adiabatic wall case. For instance, 85.63% thermal efficiency of solar collector is achievable for non adiabatic bottom panel by adding suspended aluminum nanoparticles into the pure water. Even better than this, considering isothermal base panels, 100% efficiency is attained more rapidly with lesser base temperatures in the presence of higher nanoparticle volume fractions.
Solar water heating systems are the most economical and large scale application of solar energy in residential buildings. In order to enhance the efficiency of these systems, Direct Absorption Solar Collector (DASC) which used nanofluids with appropriate optical and heat transfer properties as absorbing medium, has been recently proposed. In this study, a prototype of this new type of collector was built with applicability for domestic solar water heater. Different volume fractions of copper oxide nanoparticles in water and ethylene glycol mixture (70%:30% in volume) as the base fluid were prepared and their thermo-physical and optical properties were presented. The procedure of EN 12975-2 standard was used for testing the thermal performance of the collector. The tests were performed in different flowrates from 54 to 90 l/h (0.015–0.025 kg/s) and two different internal surfaces (black and reflective) of bottom wall. The efficiency of the collector with black internal surface is about 11.4% more than that of with reflective internal surface using the base fluid at 90 l/h flowrate. The collector efficiency is increased by increasing nanofluid volume fraction and flowrates. The nanofluids improved the collector efficiency by 9–17% than the base fluid. Based on the results, the performance of this new kind of collector as the main part of solar water heater appears promising, leading to considerably higher efficiencies.
A nanofluid filled direct absorption solar collector (DASC) system in which incident sunlight is absorbed directly by a working fluid, provides a promising alternative to conventional solar collectors. Most of the previous numerical and experimental studies evaluated the effect of various nanofluids on the thermal performance of a pre-designed collector, and did not consider the effect of varying collector dimensions on its overall performance. In this study, a numerical model of nanofluid flow and temperature distribution in a DASC is proposed by solving the radiative transfer equations of particulate media and combining conduction and convection heat transfer equations. Response surface methodology (RSM) was then applied to understand the effect of varying dimensions on thermal efficiency and entropy generation of the DASC collector design. Based on the produced response surfaces, multi response optimization was performed to find the collector optimized geometry within the studied range of dimensions.
The conventional tube-in-plate type flat plate solar collectors have low efficiency and higher heat losses due to surface based solar energy absorption and indirect transfer of heat from hot absorber surface to working fluid flowing through tubes. A full scale direct absorption solar collector having gross area of 1.4 m2 and working on volumetric absorption principle was developed to perform experimental study using thin film of Al2O3-H2O nanofluid. Use of nanofluid as working fluid improves the optical and thermo physical properties that result into an increase in the efficiency of the collector. Experimentation was carried using four different volume fractions of 20 nm Al2O3 nanoparticles, 0.001%, 0.005%, 0.01% and 0.05%. ASHRAE standard 93-86 was followed for calculation of instantaneous efficiency of solar collector. Improvement in efficiency of solar collector has been recorded in all four cases of using nanofluids in place of water. Instantaneous efficiency enhancement of 22.1%, 39.6%, 24.6% and 18.75% has been observed for 0.001%, 0.005%, 0.01% and 0.05% volume fraction respectively. The experimental results also indicated that collector efficiency peaked at certain volume fraction, and decreased for lower and higher values of volume fraction.
Applications of nanofluids in direct absorptions of solar radiation in volumetric solar collector have been investigated in this research. Radiative transport equations along with mass, momentum and energy equations are solved together to simulate operating characteristics of direct absorption solar collector. Different diameter and volume fractions of graphite nanoparticles are investigated and for each case, efficiency of solar receivers in absorption of solar energy, impacts on the harvested solar energy, irradiation spectrum distribution and irradiation energy level versus the depth of the flow have been studied. Moreover, for a proposed low-temperature solar collector, increase in outlet temperature, convective losses, and costs are evaluated. Results of this study show that by using graphite nanofluids having volume fraction around 0.000025%, it will be possible to absorb more than 50% of incident irradiation energy by just about 0.0045 $/L increase in cost, while pure water solar collector can only absorb around 27% of incident irradiation energy.
Sustainable energy generation is one of the most important challenges facing society today. Solar energy is one of the best sources of renewable energy with minimal environmental impact which offers a solution. The present work investigates the heat transfer performance and entropy generation of forced convection through a direct absorption solar collector. The working fluid is Cu–water nanofluid. The simulations focus specifically on the effect of solid volume fraction of nanoparticle and Reynolds number on the mean Nusselt number, mean entropy generation, Bejan number and collector efficiency. Also Isotherms and heatfunction are presented for various solid volume fraction and inertia force. The governing partial differential equations are solved using penalty finite element method with Galerkins weighted residual technique. The results show that both the mean Nusselt number and entropy generation increase as the volume fraction of Cu nanoparticles and Reynolds number increase. The results presented in this study provide a useful source of reference for enhancing the forced convection heat transfer performance while simultaneously reducing the entropy generation.
In the past decade nanotechnology has developed in many directions. Nanofluid is a mixture of nanosized particles dispersed in fluids. Nanofluids are new generation heat transfer fluids used in heat exchangers for energy conservation. Viscosity is an important property particularly concerning fluids flowing in a tube in heat exchangers. In this regard, an attempt has been made to review the available empirical and theoretical correlations for the estimation of viscosity of nanofluids. The review also extended to preparation of nanofluids, nanoparticle volume concentration, nanofluid temperature, particle size and type of base fluid on viscosity of nanofluids. The available experimental results clearly indicate that with the dispersion of nanoparticles in the base fluid viscosity increases and it further increases with the increase in particle volume concentration. Viscosity of nanofluid decreases with increase of temperature.
Solar energy is one of the best sources of renewable energy with minimal environmental impact. Direct absorption solar collectors have been proposed for a variety of applications such as water heating; however the efficiency of these collectors is limited by the absorption properties of the working fluid, which is very poor for typical fluids used in solar collectors. It has been shown that mixing nanoparticles in a liquid (nanofluid) has a dramatic effect on the liquid thermophysical properties such as thermal conductivity. Nanoparticles also offer the potential of improving the radiative properties of liquids, leading to an increase in the efficiency of direct absorption solar collectors. Here we report on the experimental results on solar collectors based on nanofluids made from a variety of nanoparticles (carbon nanotubes, graphite, and silver). We demonstrate efficiency improvements of up to 5% in solar thermal collectors by utilizing nanofluids as the absorption mechanism. In addition the experimental data were compared with a numerical model of a solar collector with direct absorption nanofluids. The experimental and numerical results demonstrate an initial rapid increase in efficiency with volume fraction, followed by a leveling off in efficiency as volume fraction continues to increase. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3429737]
In this letter, we report an experimental correlation [Eqs. ( 1a , 1b ) or ( 1c )] for the thermal conductivity of Al2O3 nanofluids as a function of nanoparticle size (ranging from 11 nm to 150 nm nominal diameters) over a wide range of temperature (from 21 to 71 °C). Following the previously proposed conjecture from the theoretical point-of-view (Jang and Choi, 2004), it is experimentally validated that the Brownian motion of nanoparticles constitutes a key mechanism of the thermal conductivity enhancement with increasing temperature and decreasing nanoparticle sizes.
In the present paper, we have investigated experimentally the influence of both the temperature and the particle size on the dynamic viscosities of two particular water-based nanofluids, namely water–Al2O3 and water–CuO mixtures. The measurement of nanofluid dynamic viscosities was accomplished using a ‘piston-type’ calibrated viscometer based on the Couette flow inside a cylindrical measurement chamber. Data were collected for temperatures ranging from ambient to 75 °C, for water–Al2O3 mixtures with two different particle diameters, 36 nm and 47 nm, as well as for water–CuO nanofluid with 29 nm particle size. The results show that for particle volume fractions lower than 4%, viscosities corresponding to 36 nm and 47 nm particle-size alumina–water nanofluids are approximately identical. For higher particle fractions, viscosities of 47 nm particle-size are clearly higher than those of 36 nm size. Viscosities corresponding to water-oxide copper are the highest among the nanofluids tested. The temperature effect has been investigated thoroughly. A more complete viscosity data base is presented for the three nanofluids considered, with several experimental correlations proposed for low particle volume fractions. It has been found that the application of Einstein’s formula and those derived from the linear fluid theory seems not to be appropriate for nanofluids. The hysteresis phenomenon on viscosity measurement, which is believed to be the first observed for nanofluids, has raised serious concerns regarding the use of nanofluids for heat transfer enhancement purposes.