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Optical absorption measurements of oxide nanoparticles for application as nanofluid in direct absorption solar power systems – Part I: Water-based nanofluids behavior
... Both annealed and nonannealed CuO x NP powders were then separately re-dispersed in ethylene glycol (EG) at different volume fractions and subjected to sonication to achieve homogenous dispersions. In an extensive study by Milanese et al. [73], the researchers investigated a broad array of metal oxide water-based NFs, encompassing commercially available Al 2 O 3 , CuO, and TiO 2 , as well as CeO 2 , ZnO, and Fe 2 O 3 , prepared via the hydrolytic synthesis method. For instance, the CeO 2 NPs were synthesized by dissolving 2.6 mmol Ce(NO 3 ) 3 ·6H 2 O in methanol, followed by the addition of acetylacetone. ...
... Many reports use absorbance to characterize NFs; however, this parameter does not allow for a direct comparison with other NFs and cannot be used directly in simulations [58,78]. The transmittance of the water-based metal oxide NFs of Al2O3, CuO, TiO2, ZnO, CeO2, and Fe2O3 prepared by Milanese et al. [73] at 0.05-1 vol% were measured over 200-1300 nm using a 3 mm path-length quartz cuvette. As depicted in Figure 7a, at 0.05 vol%, the six materials exhibited three transmittance patterns where CuO and Fe2O3 transmitted light only in the near-IR region at wavelengths above 800 nm and 600 nm, respectively, while TiO2, ZnO, and CeO2 transmitted more in the Vis and near-IR region at wavelengths above ~380 nm; the last material, Al2O3, had the lowest performance with most of the light transmitted. ...
... The extinction Many reports use absorbance to characterize NFs; however, this parameter does not allow for a direct comparison with other NFs and cannot be used directly in simulations [58,78]. The transmittance of the water-based metal oxide NFs of Al 2 O 3 , CuO, TiO 2 , ZnO, CeO 2 , and Fe 2 O 3 prepared by Milanese et al. [73] at 0.05-1 vol% were measured over 200-1300 nm using a 3 mm path-length quartz cuvette. As depicted in Figure 7a, at 0.05 vol%, the six materials exhibited three transmittance patterns where CuO and Fe 2 O 3 transmitted light only in the near-IR region at wavelengths above 800 nm and 600 nm, respectively, while TiO 2 , ZnO, and CeO 2 transmitted more in the Vis and near-IR region at wavelengths above~380 nm; the last material, Al 2 O 3 , had the lowest performance with most of the light transmitted. ...
Owing to their superior optical and thermal properties over conventional fluids, nanofluids represent an innovative approach for use as working fluids in direct-absorption solar collectors for efficient solar-to-thermal energy conversion. The application of nanofluids in direct-absorption solar collectors demands high-performance solar thermal nanofluids that exhibit exceptional physical and chemical stability over long periods and under a variety of operating, fluid dynamics, and temperature conditions. In this review, we discuss recent developments in the field of nanofluids utilized in direct-absorption solar collectors in terms of their preparation techniques, optical behaviours, solar thermal energy conversion performance, as well as their physical and thermal stability, along with the experimental setups and calculation approaches used. We also highlight the challenges associated with the practical implementation of nanofluid-based direct-absorption solar collectors and offer suggestions and an outlook for the future.
... Unlike conventional heat transfer fluids, nanofluids are better absorbers of radiation over the ultra-violet and visible light region of the solar radiation spectrum. This feature enables them as a suitable alternative for solar energy applications; they improved solar radiation absorption in the direct absorption solar collectors (DASC) system [16][17][18]. Nanoparticles are mainly composed of metals, non-metals, metal oxide, and other compounds [19][20][21][22]. ...
... Nanofluids are utilized for heat transfer purposes in solar thermal systems [27][28][29] and have been prominently applied in solar collectors because of its ability to enhance the solar collector's optical properties. Tubular solar collectors have employed Al 2 O 3 /water, ZnO/water, and MgO/water as nanofluids [17], evacuated tube solar collectors use SWCNT/water [30] and MWCNT/water [31], DASC uses Sand/PG-water [32] and ZnO/PG-water nanofluid [33], flat plate solar collector uses Cu/water [34], and Cu-synthesized/EG [35], and CuO/water nanofluid [7], and many more. The specific heat capacity of nanofluids is a measure of the capacity of the nanofluid to retain heat. ...
To characterize the performance of nanofluids for heat transfer applications in solar
systems, an accurate estimation of their specific heat capacity (SHC) is of paramount
importance. To this end, having such properties of nanofluids via computational
approaches has gained attention as an effective method to eliminate the timeconsuming
process of experimental investigations. This study focuses on modeling the
SHC of different carbon-based and metal oxide-based nanoparticles dispersed in
various base fluids. Herein, we propose a novel data-driven dynamic model based on
the Gaussian process regression (GPR) technique in comparison with the random
forest (RF) approach and generalized regression neural network (GRNN) to predict the
SHC of nanofluids. The developed models employ the solid volume fraction ( ),
temperature (T), mean diameter of nanoparticle (Dp), and SHC of base fluid ( ) as the
input parameters. The data has been collected from 10 reliable references. The results
showed that the GPR model (R=0.99974, RMSE=0.01506 J/K.g) shows superior
performance than the results of the RF (R=0.99761, RMSE=0.04598 J/K.g) and GRNN
(R=0.99563, RMSE=0.06085 J/K.g). The results proved that the developed model
would accurately estimate the SHC of the studied nanofluids. In addition, the sensitivity
analysis of the dependence of input variables on the SHC of nanofluids revealed that
the mean diameter of nanoparticles and the SHC of base fluid are the major critical
factors in the determination of SHC of nanofluids.
... Higher surfactant influence and nanofluid absorptance were observed at higher nanomaterial sizes. Yaseen et al. [66] reported a degradation in the refractive index and transmittance of cerium oxide (CeO 2 )-based nanofluid after 7 days from preparation. Higher absorption was also reported at higher nanomaterial sizes. ...
How to make the best use of the unique properties of nanomaterials, in different fluidic applications, has gained a lot of attention in the last few decades. Dispersing the nanomaterials in a conventional fluid can provide a wide range of alternative thermal, rheological, and optical properties for the new nano-enhanced fluids (nanofluids), depending on different affecting parameters. Different metal, metal oxide, and carbon-based nanomaterials were used for this purpose. This chapter provides a step forward in understanding the optical behavior of metal-oxide-based nanofluids and how metal oxides play a major role in controlling the optical response of several nanofluids. This flexible controlled optical behavior has met great interest to increase the absorption of solar energy from one side and work as a spectral-selective filter for solar radiation from another side. This chapter, in different sections, deals with a number of topics in this regard starting from the techniques used for the synthesis of nanofluids, passing by the parameters influencing the optical behavior of nanofluids, the analytical evaluation of optical properties, the optical behavior of metal-oxide-based nanofluids, and ending by their applications in different solar-harvesting-related systems.
... For readers' interest similar research on nanofluids was reported by researchers mentioned in Refs. [16][17][18][19][20][21][22][23][24][25][26]. ...
... Since most widely-available DASC working fluids (water, ethylene glycol, and oil) have relatively weak absorption, nanoparticles (NPs) with a broad absorption band in solar emission spectrum [7][8][9][10][11] are typically added to the solar collector base fluid. Such mixtures, termed nanofluids, can be formed by exploring NPs of different types of materials, including dielectrics [12], semiconductors [13,14], pure metals [15,16], doped metals [17][18][19][20], metal oxides [21][22][23], metal nitrides [24], in order to improve thermophysical properties of base fluids. In particular, Chen et al. showed that solar thermal conversion efficiency can be increased by up to 95% by adding carbon multi-walled nanotubes (0.02 wt%) into a base fluid [25], while the addition of graphene oxide/Ag NPs to water leads to the increase of DASC efficiency by 91.6% [26]. ...
Titanium nitride (TiN) nanoparticles (NPs) look very promising for solar energy harvesting owing to a strong plasmonic absorption with the maximum in the near-infrared range. However, the synthesis of TiN nanofluids is very challenging as one has to combine the plasmonic feature and long-term colloidal stability to withstand harsh conditions of direct absorption solar collectors (DASC). Here, we explore solutions of bare (ligand free) TiN NPs synthesized by pulsed laser ablation in acetone as the nanofluid. We show that such NPs are low size-dispersed (mean size 25 nm) and exhibit a broad absorption peak around 700 nm, while their negative charge ensures a prolonged electrostatic stabilization of solutions. Solar weighted absorption coefficient of such TiN nanofluids reaches 95.7% at very low volume fractions (1.0 × 10 − 5), while nanofluid temperature can be increased up to 29 • C under 1.25-sun illumination. Our data evidence that the thermal efficiency of a DASC using TiN nanofluid is 80% higher compared to Au-based counterparts. The recorded high photothermal efficiency and excellent colloidal stability of TiN nanofluids promises a major advancement of DASC technology, while laser-ablative synthesis can offer easy scalability and relative cost-efficiency required for the implementation of systems for solar energy harvesting.
... Hu et al. [26] analyzed the effect of NP size and concentration on boiling performance of SiO 2 -ethylene glycol and water mixture nanofluid. Milanese et al. [27] measured the optical absorption of Fe 2 O 3 -water nanofluid in direct absorption solar power systems. Pourfattah et al. [28] numerically investigated the effect of attack angle of ribs and Al 2 O 3 NPs on the heat transfer enhancement of turbulent flow in a tube. ...
Three-dimensional nickel-water nanofluid flow and heat transfer in a prismatic glass louver are analyzed under various solar irradiation conditions and different nanoparticle sizes. Our innovative louver serves for dual-purpose: solar energy harvest and illumination. To meet natural daylighting requirement, the nanofluid is extremely dilute. The solar radiation and illumination were analyzed by the Monte Carlo method and the acquired solar energy absorption distribution is input as a source in the conjugate heat convection and conduction modeling executed by COMSOL. Temperature-dependent thermophysical properties and various nanofluid models are considered. The temperature rise and distribution under various combinations of solar air-mass models, nanoparticle sizes, and flow conditions are scrutinized. It is found that the nanofluid properties are not appreciably affected by the different nanofluid models because the nanofluid considered is dilute. The harvested solar thermal power and its efficiency are strong functions of the nanoparticle diameter and the flow Reynolds number.
... Nonlinear optics (Stepanov, 2016), localized surface plasmon resonance (Linic et al., 2015), heat generation (Govorov & Richardson, 2007), thermal stability (Zhao et al., 2010), chemo-electronic , electrical conductivity (Lin et al., 2015), high mechanical stiffness , and magneto-elastic properties are some reported properties of metal nanoparticles. Metal oxides also have unique properties, such as high conductivity in semiconductors (Franke et al., 2006), photocatalytic activity (Katwal et al., 2015), superparamagnetism (Ling et al., 2015), minimal thermal contact resistance (Sarafraz & Hormozi, 2016), and enhanced optical absorptivity (Milanese et al., 2016). These exclusive properties are valuable in biomedical and pharmaceutical applications. ...
The exclusive properties of metal and metal oxide nanoparticles can offer significant benefits in biomedical and pharmaceutical applications. Metal nanoparticles such as gold, silver, and copper can be developed as particulate nanomedicines to treat various diseases. Also, metal oxide nanoparticles, including iron oxide, copper oxide, and zinc oxide are also used as nanomedicines to treat a wide range of diseases. These nanoparticles can be used as photocatalytic antimicrobial agents, magnetic resonance contrast imaging agents, for cell labeling, and as controlled drug delivery systems. This chapter discusses the properties and synthesis approaches of metal nanoparticles as well as their characteristics for diabetes treatment. In addition, the efficacy of chemically synthesized and phytosynthesized metal nanoparticles as nanomedicines, and their mechanism for diabetes treatment are also discussed.
... The absorption properties of CuO nanofluids were measured at 200-2500 nm by using the spectral transmission method and it was found that the nanofluids enhanced absorption coefficient as compared to that of base fluids. Milanese et al. [122] prepared various water-based metal oxide nanofluids including Al 2 O 3 , CuO, TiO 2 , ZnO, CeO 2, and Fe 2 O 3 with different volume fractions, and the optical properties of these six kinds of metal oxide nanofluids were measured at indoor air temperature conditions. The experimental results showed that the optical absorption coefficients of the various water-based metal oxide nanofluids were significantly different, and the optical absorption properties of the CuO and Fe 2 O 3 nanofluids were better than those of the other considered situations. ...
Building envelopes consist of transparent and non-transparent components, and pose distinct opportunities and challenges in energy conservation. Glazing envelopes, which are transparent, are mainly responsible for the lighting and ventilation of buildings, but the acoustic performance may be weakened. Since glazing envelopes suffer from the defects of high solar transmittance, poor thermal insulation, and low thermal inertia, their energy-saving technologies are significantly different from those of non-transparent envelopes. Different energy-saving technologies have been studied to improve the optical and thermal performance of glazing envelopes. However, there is a lack of a review study involving the phase transition process and the improvement of photo-thermal transmission in glazing envelopes containing phase change materials. The present work provides a comprehensive overview of research advances in optical transmittance, thermal resistance, and thermal inertia along with photo-thermal transmittance in glazing envelopes, with a special focus on the integration of phase change materials. The study reveals that measurement and numerical models are inadequate to study photo-thermal transmission. Besides, it is identified that there is a research gap in the acoustic performance of glazing systems incorporating phase change materials, and there is a lack of database on the optical properties of phase change materials containing nanoparticle.
... As a widely used renewable cleaning energy, solar energy is inexhaustible which magnitudes 1.2× 10 5 TW [1]. It is imperative to study the utilization and conversion of solar energy, so that there are numerous researches on photothermal conversion systems which depend on the superior photothermal properties of working fluid [2][3][4][5][6]. Among the plenty kinds of working fluid (water, glycols, oils, phase change materials (PCMs), etc.), paraffin, as a typical PCM, has attracted researchers attention for its excellent energy storage performances [7]. ...
By the widely application in solar energy conversion systems for phase change materials (PCM), the photothermal properties of PCM are important for the efficiency of systems. As a typical PCM, the applications of paraffin in solar utilization are limited by it poor photothermal properties. In order to improve the photothermal performance of pure PCM, Zn–TiN binary nanoparticles are chosen for the enhancement in the present work. Furthermore, the photothermal properties of Zn–TiN binary nanoparticles enhanced phase change material (NePCM) with various volume fraction are evaluated by thermal properties, capacity of solar energy absorbed, and photothermal conversion performance. In addition, the stability of NePCM effected by dispersants and the temperature variation regularity of samples effected by solar radiation intensity and temperature monitoring locations are also studied. As the results: CTAB is a suitable dispersant for the NePCM. Under the same concentration (0.001 vol%), Zn–TiN binary nanoparticles can improve the solar absorption capacity of pure PCM significantly which enhanced almost 8 times. However, due to the aggregation of nanoparticles, the efficiency of photothermal conversion decreases with the increase of volume fraction, therefore, 0.001 vol% Zn–TiN binary NePCM gives the maximum efficiency of 22%, that is twice than that of the pure PCM. Photothermal conversion performance improved with the increase of intensity of solar radiation. Under the intensity of 600 W/m², the peak temperature of 0.01 vol% Zn–TiN binary NePCM is 62.57 °C which appears in the center of the sample.
... Besides, with the recent drastic improvement in the light absorption capability of the heat medium due to the development of NF technology, the use of a VASC has been actively suggested. Recently, NFs like metallic [19e21], metallic oxide [19,20,22], carbon series [21,23], and others [24] with low concentration can be used to improve the photothermal conversion performance. The improvement of the solar absorption using NF technique is based on the inherent high solar absorption, absorption, scattering, and surface plasmon resonance. ...
In this study, the characteristics of volumetric absorption for solar harvesting using a Fe3O4@polyacrylic acid (PAA) nanofluid (NF) are investigated experimentally. The concentration of the Fe3O4@PAA NF was varied from 0 to 0.2wt%, and its mass flow rate was set to 0.0025 and 0.005 kg/s. As a result, the average efficiency of the solar collector at the Fe3O4@PAA NF of 0.05wt% was the highest at the mass flow rates of 0.0025 kg/s and 0.005 kg/s and the improvement ratio of average efficiency was 1.15 and 1.19, respectively, compared to water. The collector performance of the solar thermal harvesting improved owing to the improvement in the solar absorption and heat transfer, as well as the uniform temperature at the receiver tube as the concentration of the Fe3O4@PAA NF increased to a concentration of 0.05wt%. However, the collector performance of the solar thermal harvesting decreased for the 0.05wt% Fe3O4@PAA NF because of the increase in heat loss by the non-uniform temperature at the receiver tube and heat transfer. The increase in mass flow rate can reduce the heat loss by the decrease in temperature in the receiver tube; consequently, the efficiency of the solar collector using NFs is improved.
... The magnetic nanofluid-based collector exhibited 25% and 12% higher efficiency than conventional and selective surface absorber systems, respectively. Milanese et al [78] experimentally measured the optical properties of different metal oxide nanofluids (Al 2 O 3 , CuO, TiO 2 , ZnO, CeO 2 and Fe 2 O 3 ) for low-temperature DASCs. The CuO and Fe 2 O 3 nanofluids exhibited the highest extinction efficiency and least transmittance among the other nanofluids. ...
The evolution of nanofluids over the years have opened new research opportunities in the field of renewable energy. Research on the optical properties of nanofluids for application in direct absorption solar collectors (DASCs) is progressing at a burgeoning speed. In a direct absorption solar collector system, nanofluid with high optical absorptivity will convert the incident solar energy to the thermal energy of the fluid. The dispersed nanoparticles in the fluid acts in the process by the phenomenon of absorption and scattering. Studies conducted on the optical property characterization of monocomponent nanofluids have become saturated. Moreover, the photothermal efficiency of the nanofluid could be enhanced by using multicomponent nanofluids. Nanofluid prepared by varying materials, shapes, and size of nanoparticles could tune the absorption spectra of the bulk fluid to improve the photothermal efficiency. A hybrid nanocomposite can similarly enhance the absorptivity due to the synergy of materials present in the nanocomposite particle. In this review, a comprehensive survey on the synthesis and optical characterization of different blended and hybrid nanocomposite nanofluids have been performed. Besides, works on different mono nanofluids that have been studied over the years have been reviewed.
... Nanofluids was only studied as a single heat transfer working fluid in the initial stage [15], so their research mainly focused on thermal conductivity [16], viscosity [17], clustering phenomenon [18], and various thermal behaviors [19,20]. With the in-depth exploration of the optical properties of nanoparticles [21,22], the research of using nanofluids as light absorbers in DASC has been widely carried out [23]. However, the nonuniformity of the nanofluid's temperature during the photothermal process severely limits its optical absorption and heat transfer performance. ...
Direct absorption solar collectors (DASC) are extremely attractive in solar energy utilization. In this paper, starting from these two aspects, graphene-based nanofluids, including single-layer graphene (SLG) and graphene oxide (GO), are prepared to enhance solar absorption and photothermal conversion performance. The influence of nanofluids' concentration and two different irradiation modes: traditional DASC and reverse radiation DASC (RI-DASC) on the photothermal conversion performance of graphene-based nanofluids has been studied. The addition of a small amount of SLG or GO significantly improves the photothermal conversion efficiency of base fluid, and it increases along with the increase of the concentration. Furthermore, the RI-DASC mode has a more uniform thermal field distribution and the higher photothermal conversion efficiency than the DASC mode. In DASC mode, the photothermal conversion efficiency of pure water is 17.00%. By changing the irradiation mode, the introduction of GO and SLG nanofluids (100ppm) increased the photothermal conversion efficiency by about 172% and 189%, reaching 46.26% and 49.13%. According to the photothermal-thermoelectric conversion experiment, the positive correlation between the output power of the TE module and the heating state of the nanofluid makes it possible to adjust the nanofluid in real-time. This work presents a feasible way to enhance solar energy absorption and improve the photothermal conversion efficiency of nanofluids for DASC.
... These materials have a lower density than metals and show better stability within the fluid. 22 Milanese et al 23 have evaluated the optical properties of Al 2 O 3 , CuO, TiO 2 , ZnO, CeO 2 , and Fe 2 O 3 nanofluids using spectrophotometry analysis. They saw CuO and Fe 2 O 3 nanofluids with higher absorption solar properties than the other nanofluids. ...
Today, the application of nanoparticles with controlled morphologies in solar energy conversion and storage systems is highlighted. This study focused on the assessment of Fe2O3 water-based nanofluids in volumetric solar absorbers and compared the performance of three shapes (blade, spherical, and octahedral), and three concentrations of nanoparticles (0.005, 0.01, and 0.02 vol%). First, a set of homogeneous nanofluids were prepared using a probe ultrasonic apparatus and by adding an anionic surfactant. In addition to long-time stability and narrow particles size distribution, obtained nanofluids presented a considerable light absorption capability in the visible region. It was seen that the light absorption spectrum was broadened with increasing the nanofluid concentration. Moreover, for each concentration, the blade nanoparticles improved the extinction coefficient and the solar-weighted absorption fraction relatively higher than the other shapes. For example, at a concentration of 0.02 vol% and penetration distance of 1 cm, the blade-nanofluid absorbed 96.2% of incident solar energy, which was higher than the spherical nanofluid with 93.5% and octahedral nanofluid with 87.2%. The results of thermal tests conducted in a set of bath direct solar collectors showed that the highest solar–thermal conversion efficiency after 1 hour of light exposure was related to 0.02% blade-nanofluid (82%). In these conditions, the growth of bulk temperature was observed 2.31 times higher than pure water.
... Schematic diagram showing the operating principle of a concentrated solar power (CSP) of a parabolic trough collector, adapted from [136].Milanese et al.[137] reported optical absorption measurements of different fractions of metal oxides in water NFs in a Direct Absorber Solar Collector (DASC) and concluded that in order to minimize pumping power consumption and maintenance costs, which are closely related to the viscosity of the NF, the best NP was the TiO since at a very low concentration of 0.05 vol.% was capable to absorb completely solar radiation within 1 centimeter of depth. Also, Milanese et al.[138] studied the use of NFs of several metal oxide NPs in a Direct DASC and report optical absorption measurements that make these NFs very suitable for operations at very high temperature, thus encouraging the usage of gas-based NFs as heat transfer fluid in the aforementioned CSP plants.Some authors studied the application of the NFs in solar collectors. ...
The nanofluids (NFs) are known as a colloidal suspension where nanoparticles (NPs) are dispersed in conventional base fluids (BFs). This suspension has been frequently used to augment the heat transfer (HT) capacity of a fluid by studying their thermophysical properties. This review outlines the most recent advances on the NFs thermal properties and on their applications that are being used in different fields of engineering, ranging from nanomedicine to renewable energies. In the latter, several relevant advances have been achieved in mobility and propulsion with relevant application in defense and military technologies. Hence, specific applications of NFs in electronics cooling, heat exchangers, heat pipes, solar energy, nanomedicine, space technology, among others, have been reviewed and presented. Overall, the research performed in this subject has shown that several parameters can impact the thermal properties of the NFs including the BF, NPs size, concentration, shape and NPs fabrication procedures. In this review, the most relevant parameters are identified and debated, to provide guidance and inspire researchers to develop low cost NFs with high thermal properties. In addition, the models for predicting the specific heat, viscosity, thermal conductivity and density of NFs are listed and evaluated in a comprehensive way. Based on the results that the literature presents, there are still many models to be validated and explored. Finally, the challenges and several unresolved issues related to NFs flows are discussed and possible solutions are suggested to solve them.
... Besides, with the recent drastic improvement in the light absorption capability of the heat medium due to the development of NF technology, the use of a VASC has been actively suggested. Recently, NFs like metallic [19e21], metallic oxide [19,20,22], carbon series [21,23], and others [24] with low concentration can be used to improve the photothermal conversion performance. The improvement of the solar absorption using NF technique is based on the inherent high solar absorption, absorption, scattering, and surface plasmon resonance. ...
In this study, the thermal performance of a surface absorption solar collector (SASC) and a volumetric absorption solar collector (VASC) using water and Fe3O4 nanofluid was experimentally investigated. As a result, the heat removal factor (FR) and overall heat loss coefficient (UL) of the SASC were increased with the concentration and mass flow rate of the Fe3O4 nanofluid. The limit-normalized temperature difference (LNTD) of the SASC was reduced compared with that of water because of increased heat loss due to the improved heat transfer performance of the Fe3O4 nanofluid. In addition, the thermal and exergy efficiencies of the SASC using the Fe3O4 nanofluid were lower than those using water. However, in the case of the VASC, FR increased while UL decreased with the increasing mass flow rate of the Fe3O4 nanofluid. When the mass flow rate and concentration of the Fe3O4 nanofluid increase, its LNTD increased beyond water. The thermal and exergy efficiencies of the VASC using the Fe3O4 nanofluid were higher than those using water. Moreover, the maximum thermal and exergy efficiencies was the maximum when the 0.05 wt% Fe3O4 nanofluid was used in the VASC.
... Furthermore, nanofluids with various types of nanoparticles, such as metal oxides, have been widely studied for use in solar energy systems. As a result, nanofluid is an attractive choice for the working fluid of direct absorption solar collectors efficiently converting sunlight into heat [8,9]. It was found that nanofluids showed enhanced thermophysical properties, such as thermal conductivity, specific heat, viscosity and convective heat transfer, relative to base fluids like oil or water. ...
Recently, renewable energies have attracted the significant attention of scientists. Nanofluids are fluids carrying nano-sized particles dispersed in base fluids. The improved heat transfer by nanofluids has been used in several heat-transfer applications. Nanofluids' stability is very essential to keep their thermophysical properties over a long period of time after their production. Therefore, a global approach including stability and thermophysical properties is necessary to achieve the synthesis of nanofluids with exceptional thermal properties. In this context, the objective of this paper is to summarize current advances in the study of nanofluids, such as manufacturing procedures, the mechanism of stability assessment, stability enhancement procedures, thermophysical properties, and characterization of nanofluids. Also, the factors influencing thermophysical properties were studied. In conclusion, we discuss the application of nanofluids in solar collectors.
... Milanese et al. [122] investigated the optical behavior of various water-based nanofluids applied in solar power systems. In all experiments, transmittance grew, passing from the visible to the infrared region. ...
Providing optimal operating conditions is one of the major challenges for effective heating or cooling systems. Moreover, proper adjustment of the heat transfer fluid is also important from the viewpoint of the correct operation, maintenance, and cost efficiency of these systems. Therefore, in this paper, a detailed review of recent work on the subject of conventional and novel heat transfer fluid applications is presented. Particular attention is paid to the novel nanoparticle-based materials used as heat transfer fluids. In-depth comparison of environmental, technical, and economic characteristics is discussed. Thermophysical properties including thermal conductivity, specific heat, density, viscosity, and Prandtl number are compared. Furthermore, the possible benefits and limitations of various transfer fluids in the fields of application are taken into account.
... Multiple studies have been carried out to examine the properties of solar radiation absorption by nanofluids of different com-positions. These studies aimed to determine the absorption coefficients of visible, IR, and UV radiation in nanofluids, determining the integral indicators of thermal radiation absorption, the dynamics of nanofluid samples heating, and the temperature distribution in them during photothermal heating [14][15][16][17][18][19][20] . In these works, nanofluids demonstrate an advantage over the dispersing media, i.e., the base fluid. ...
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 addition, the augmentation of nanoparticles affects the optical properties of the base fluids which leads to increment of the energy absorption of the sun, up to 9 times. Therefore, utilizing the direct absorption scattering approach is one of the methods to enhance the efficiency of collectors, in which the solar radiations are directly absorbed by the fluid flow in the collector (Sani et al. 2011;Colangelo et al. 2013;Otanicar et al. 2010;Taylor et al. 2011;Karami et al. 2014;Visconti et al. 2016;Otanicar et al. 2009;Bohren and Huffman 1998;Milanese et al. 2016b). Lenert and Wang (2012) optimized the conversion of solar energy into heat in volumetric nanoscale receivers numerically and experimentally. ...
Solar energy is one of the cleanest sources of renewable energy that is easily accessible in vast geographical areas. As the efficiency of the common solar collectors is very low and is limited by the absorption properties of working fluid, enhancing the thermal performance of these collectors is one of the major challenges of developing parabolic solar thermal power plants. In recent decades, researches revealed that utilizing nanofluid as a novel working fluid has a dramatic effect on the thermophysical and optical properties of the fluid. In this study, the flow and temperature fields of water/magnetite and water/aluminum nanofluids are evaluated by solving the steady form of governing equations including conservation laws of mass, volume fraction transport equation, momentum equation, energy equation, and radiation transfer equation. Moreover, the two-phase Buongiorno model is utilized and Brownian motion, thermophoresis effects, and magnetophoresis movement are taken into account in the nanofluid simulation. The numerical results demonstrate that increasing nanofluid volume fraction and flow rate can increase the thermal performance of the collector tube. It is found that the thermal efficiencies reach maximum values of 151.03% and 158.58% for water/aluminum and water/magnetite nanofluids, respectively. Furthermore, increasing the volume fraction from 0 to 0.3% leads to rise of 24.41% and 21.36% in the maximum temperature of the collector. The effects of different parameters such as nanoparticle volume fraction, flow rate, and nanoparticle kind on the collector thermal and electrical efficiencies, thermal distribution, and entropy production have been studied.
... Milanese, et al. [254] experimentally assessed the optical merit of several water-based NFs using six different nanomaterials (CuO, CeO 2 , TiO 2 , Al 2 O 3 , ZnO and Fe 2 O 3 ) to determine their potential use in DASC. The extinction distance, transmittance and/or absorbance measurements were performed for all the NFs using volumetric concentrations of 0.05-1% for each of the NPs. ...
In the last decade, nanofluids have set significant milestones as efficient working fluids in the field of solar energy conversion to meet rising energy demand. Research on thermophysical properties, long-term stability, and rheology is progressing to achieve effective practical deployment of nanofluids in renewable solar photo-thermal energy conversion sectors (i.e., solar collectors). Nonetheless, researchers and engineers are having a difficult time coping with nearly infinite culpable variables influencing the output of various types of nanofluids. This paper aims to provide an up-to-date analysis of the developments and challenges of widely used water-based nanofluids, with a focus on formulation methods, main properties (thermophysical, stability, and rheological), and effective implementation in low temperature solar collector systems. Previous experimental and numerical studies on the subject have been compiled and thoroughly scrutinized, providing crucial phenomena, mechanisms, flaws, and responsible parameters for achieving stable and optimized thermal properties that integrate with heat transfer performance. It has been discovered that optimizing the critical factors leads to superior behavior of the nanofluids, which results in improved thermal efficiency of the solar collectors. Finally, emerging concerns are identified, as are potential recommendations to resolve existing problems in the field for future advancement that would mobilize rapid progress and practical engineering use of water based nanofluids on solar collectors.
... Numerical and experimental studies on nanofluids and their applications, such as solar thermal, photovoltaic, internal combustion engines and oil recovery [18], confirmed an increase in heat transfer performance of about 10% compared to traditional heat transfer fluids, such as water or water-glycol mixtures [19][20][21][22][23][24][25]. Even if a small quantity of nanoparticles is added at a volume ratio of less than 1%, the thermal conductivity increases of about 10% compared to traditional heat transfer fluids, such as water or water-glycol mixtures while the energy efficiency of PVT systems using SiO 2 -water nanofluids at 1 and 3% in volume ratio concentration was increased by 3.6% and 7.9% respectively, compared to pure water. ...
Nanofluids have great potential to improve the heat transfer properties of liquids, as demonstrated by recent studies. This paper presents a novel idea of utilizing nanofluid. It analyzes the performance of a HVAC (Heating Ventilation Air Conditioning) system using a high-performance heat transfer fluid (water-glycol nanofluid with nanoparticles of Al2O3), in the university campus of Lecce, Italy. The work describes the dynamic model of the building and its heating and cooling system, realized through the simulation software TRNSYS 17. The use of heat transfer fluid inseminated by nanoparticles in a real HVAC system is an innovative application that is difficult to find in the scientific literature so far. This work focuses on comparing the efficiency of the system working with a traditional water-glycol mixture with the same system that uses Al2O3-nanofluid. The results obtained by means of the dynamic simulations have confirmed what theoretically assumed, indicating the working conditions of the HVAC system that lead to lower operating costs and higher COP and EER, guaranteeing the optimal conditions of thermo-hygrometric comfort inside the building. Finally, the results showed that the use of a nanofluid based on water-glycol mixture and alumina increases the efficiency about 10% and at the same time reduces the electrical energy consumption of the HVAC system.
... The nanofluids improved collector efficiency by 9-17% than the base fluid. [429] DASC power systems Several water-based nanofluids (Al 2 O 3 , CuO, TiO 2 , ZnO, CeO 2 , and Fe 2 O 3 ) ...
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.
... The direct absorption solar collector (DASC) is a current production of solar collectors into which solar radiation is directly absorbed via a transport medium [13]. In the 1970s, DASC was initially introduced for improving the efficiency of collectors; the system incorporates solar flux in the employed fluid [14][15][16] directly. ...
A numerical attempt of the two-phase (non-homogeneous) nanofluid approach towards the convection heat transfer within a 3D wavy direct absorber solar collector is reported. The solar collector is permeated by a water-Al2O3 nanofluid and contains a wavy glass top surface that is exposed to the ambient atmosphere and a flat steel bottom surface. The left and right surfaces are maintained adiabatic. The governing equations of the Navier–Stokes and energy equations for the nanofluid are transformed into a dimensionless pattern and then solved numerically using the Galerkin weighted residual finite-element technique. Validations with experimental and numerical data are performed to check the validity of the current code. Impacts of various parameters such as the number of oscillations, wave amplitude, Rayleigh number and the nanoparticles volume fraction on the streamlines, isotherms, nanoparticle distribution, and heat transfer are described. It is found that an augmentation of the wave amplitude enhances the thermophoresis and Brownian influences which force the nanoparticles concentration to display a nonuniform trend within the examined region. Furthermore, the heat transfer rate rises midst the growing wave amplitude and number of oscillations. More importantly, such enhancement is observed more significantly with the variation of the wave amplitude.
... The nanofluids were composed of nanoparticles of different metal oxides in the base fluid of water. The absorbance and extinction distance were measured experimentally with quartz cuvette of 3 mm path length [72]. The spectral transmittance of Ag-water nanofluid was measured with UV-670 double beam UV-Vis spectrophotometer at atmospheric condition. ...
Methods for the determination of the optical properties of both conventional and hybrid nanofluids are presented. The optical properties considered included the extinction coefficient, absorptance, transmittance, and reflectivity. Experimental, theoretical, and numerical approaches for the evaluation of the optical properties are reviewed. The effects of nanoparticle concentration, size, and shape on the optical properties are discussed. The influence of the base fluid on the optical properties is examined. The dependence of the optical properties on the wavelength is investigated. Results of various analytical and numerical procedures and their validation with the experimental measurements are presented. It is concluded that the optical behavior of nanofluids can be optimized by simultaneous consideration of proper size, shape, concentration, nanoparticles material, base fluid, weather condition, and the wavelength range. Hybridization is found to be a promising choice for improving the optical properties of nanofluids. Hybrid nanofluids received an increasing attention recently and there is need for further research in the direction of hybrid nanofluids and their optical properties. As water is commonly used as the base fluid extensive literature is found for water base nanofluids. On the other hand, non-water-based nanofluids with enhanced optical properties are still potential future research areas.
... In an investigation of optical absorption by Milanese et. al [26] , IO nanofluids were compared to other metallic NP's such as CeO 2 , ZnO, TiO 2 , Al 2 O 3 and CuO. It was found that the IO nanofluid was the most efficient of the metallic NF's in terms of absorbed incident radiation. ...
Photo-thermal evaporation of nanofluids has potential applications in solar desalination, micro-CHP (combined heat and power) and domestic off-grid disinfection. In this research, we reproduced the process experimentally using 110-nm iron oxide particles dispersed in water. At an initial lab-scale stage, under the artificial radiation of 6.9 suns, we observed that the boiling nanofluid destabilizes to a suspension of 6µm agglomerates of nanoparticles and that up to 30% of the particles escape the system with the steam. At the prototype stage, we boiled the fluid in a solar concentrator producing 35 g/min steam with an efficiency of around 60%, which is sufficient to drive a small turbine. The optimum concentration of the nanoparticles was 3 wt%. To supplement the experiments, we developed a simplified model for engineering calculations of the solar steam generation rate. The model corresponds well to the experiments deviating by only 8%.
... Hu et al. [27] analyzed the effect of NP size and concentration on boiling performance of SiO 2 -ethylene glycol and water mixture nanofluid. Milanese et al. [28] measured the optical absorption of Fe 2 O 3 -water nanofluid in direct absorption solar power systems. Shen et al. [29] investigated the effect of ultrasonic waves on heat transfer in Al 2 O 3 -water nanofluid under natural convection and pool boiling. ...
It is well known that addition of some nanoparticles (NPs) to a base fluid can enhance solar radiation absorption; however, its influence on simultaneous solar energy harvesting and daylighting is seldom studied. Water is nearly transparent to visible (VIS) light but highly absorbing of ultraviolet (UV) and infrared (IR). It is necessary to examine the performance of water-based nanofluid in a glass louver for dual purpose-illumination and energy harvesting. First, we investigated the absorption and scattering efficiencies of NPs commonly used in solar energy research including Ni, SiO 2 , Fe 2 O 3 , Al 2 O 3 , TiO 2 and ZnO; and found that Ni NPs are the most desirable because they have an excellent balance between UV and IR absorption and VIS transmission. Then the spectral coefficients of absorption and scaled isotropic scattering with different NP sizes and concentrations in Ni-water nanofluids were scrutinized. Results show that the higher the NP size or concentration is, the higher are the absorption and scaled isotropic scattering coefficients. A dilute 0.0 0 0 04 vol% Ni-water nanofluid with particle diameter of 80 nm was found to absorb more solar energy and provide required daylighting. Under AM1.5 model, a glass louver filled with such a nanofluid can transmit 46.5% solar VIS for daylighting and harvest 65.7% of the total solar energy, which is a 25.9% increase as compared to pure water.
... graphite, graphene oxide, nanotubes) (Delfani et al., 2016;Gorji and Ranjbar, 2017b;Hordy et al., 2014;Karami et al., 2014;Khosrojerdi et al., 2017;Li et al., 2020;Luo et al., 2014;Otanicar et al., 2010;Shende and Ramaprabhu, 2016), metal oxides (e.g. Al, Ti, Cu, Fe, Zn, Ce and Si oxides) (Gorji and Ranjbar, 2017b;Gupta et al., 2015b;Hatami and Jing, 2017;Karami et al., 2015;Milanese et al., 2016;Turkyilmazoglu, 2016;Xu et al., 2015) and metallic nanoparticles. Nanofluids containing more than one type of nanomaterials, or composite nanomaterials, have been found to be more adaptable, offering potential performance improvements (Akilu et al., 2018;Dhinesh Kumar and Valan Arasu, 2018;Menbari et al., 2017;Ranga Babu et al., 2017;Yu and Xuan, 2018). ...
To utilize nanofluids for direct absorption solar collectors (DASCs), they need to maintain their performance and physicochemical stability with exposure to solar radiation. In the present studies, three water-based nanofluids were characterized under light exposure, including silver nanoprisms (AgNPrs), silica coated silver nanoprisms (SiO2@AgNPr) and silica (SiO2) nanoparticles. Their temperature profiles and stability were monitored using simulated sunlight (SSL) and natural sunlight exposure (NSL), quantified by UV-vis spectroscopy and, in the case of SSL, characterized by transmission electron microscopy (TEM). With SSL both silver nanofluids showed an increase in maximum temperature of approximately 40 – 45°C, with a photo-conversion efficiency of about three times greater than the SiO2 nanofluid and water base-fluid. Stability tests showed the SiO2@AgNPr nanomaterial to be morphologically unstable, with the AgNPrs etching over a period of several hours. The AgNPrs showed a higher tendency to aggregation than SiO2@AgNPr nanofluids when exposed to NSL sunlight over a two-week period. Contrarily, the latter exhibited notable changing in shape, consequently effecting the absorption band position. The results highlight strongly the need for stability trials under realistic conditions for the development of nanofluids for direct solar absorption.
... In recent years, many researches have been made on the excellent heat transfer performance of various nanofluids [4,5]. The research findings showed that the superior thermal physical properties of nanofluids ensure their potential applications in solar systems [6], especially the direct absorption solar collector (DASC) system, which could absorb the solar radiation directly and volumetrically [7][8][9]. As most commonly used HTFs are weak absorbers over the ultra-violet and visible ranges of the solar spectrum, the nanofluids could considerably improve the optical and photothermal properties of these transparent fluids. ...
This part presents a comprehensive analysis of previous and recent research and advances related to the thermo-optical properties of metal oxide nanoparticles. Modern nanotechnologies produce various nanoparticles from different oxides with unique properties. Metal oxide nanoparticles have significant advantages over other nanoparticles due to their special optical properties and high oxide melting and evaporation temperatures, which allows them to be stable in a high-temperature surrounding. The thermo-optical properties of oxide nanoparticles have recently received significant attention due to their various applications in solar energy conversion, laser processing, photocatalytic applications, photothermal therapies. The development of modern high-temperature photonics, photocatalysis, and laser technologies in recent years requires the study and use of the optical parameters of metal oxide nanoparticles. The analysis of thermo-optical properties of oxide Al2O3, TiO2, NiO, ZnO and other conventional oxide nanoparticles and their dependences on temperature has been carried out. These results can be used for the development and application of various optical and laser devices and technologies based on the use of thermo-optical properties of metal oxide nanoparticles, as well as for high-temperature nanophotonics and nanotechnology.
The topic of energy saving is a constant in everyday life, and it is widespread all over the world. Space heating using solar panels is the most used renewable source of energy, but the application of solar energy for cooling the fluids used for refrigeration is growing very fast. Among the techniques used for refrigeration, this work focused on Desiccant Cooling. In particular, with the use of dynamic simulation software, it was possible to study the heat supplied and the energy consumption of a Heating Ventilation Air Conditioning (HVAC) system of a university building and to compare consumption with those of a Desiccant Cooling system applied to the same building. Four different cases were simulated: two related to the HVAC system, one of which operates with water and glycol and the other one with nanofluid, and the other ones to the Desiccant Cooling system with both types of fluids mentioned above. Keeping the same energy demand of the building in all the simulations, it was found that in summer the Desiccant Cooling system had higher performance than the traditional HVAC system and that the use of the nanofluid in both types of conditioning systems further increased the performance of 21%. Simulations were carried out using TRNSYS software.
As a typical renewable energy source, solar energy is important in addressing current energy challenges. The combination of nanofluids and direct absorption solar collectors is an efficient approach to harness solar energy. This study utilized cuttlefish ink, which is a natural biomass material, to prepare deionized water-based cuttlefish ink nanofluids using a two-step method, and the thermal conductivity and optical properties of nanofluids were systematically investigated. The results showed that the addition of cuttlefish ink nanoparticles improves the heat transfer and optical absorption abilities of the base fluid. Furthermore, photothermal conversion experiments on cuttlefish ink nanofluids were performed using a solar simulator and natural sunlight. When the mass fraction was 0.02 %, the cuttlefish ink nanofluid achieved a maximum overall photothermal efficiency of 60.2 % under solar simulator irradiation, which was 52.0 % higher than that of deionized water. Moreover, the maximum temperature rise of cuttlefish ink nanofluids under natural sunlight was 33 °C, corresponding to an increase of 17.4 °C compared to that of deionized water. Cuttlefish ink is low-cost, easily available, and environmentally friendly. This study suggests that cuttlefish ink nanofluids are promising candidates for direct absorption solar collectors systems.
With the development of solar energy utilization and nanotechnology, nanofluid heat collection has attracted significant attention. The solar plasmonic nanofluids with significant local surface plasmon resonance (LSPR) effect have exhibited superior behaviors in thermophysical properties, optical properties, and photothermal conversion efficiency. This paper reviews the research progress of plasmonic nanofluids in solar collectors and PV/T systems and compares them with conventional nanofluids for the first time. The plasmonic nanofluids exhibit excellent light absorption capacity and controllable light intensity and resonance frequency. Due to the existence of the LSPR effect, plasmonic nanofluids show significant absorption peak, which can meet the requirements of broadband solar energy absorption and have better usage value than conventional nanofluids in solar heat collection. Many factors affect the photothermal conversion efficiency of plasmonic nanofluids, such as particle size, concentration and flow rate. Using hybrid nanofluids can significantly broaden the extinction spectrum and further enhance light absorption. To clarify the specific mechanism of plasmonic nanofluids, improve their stability and make them more suitable for different types of solar heat collection, more experimental exploration is needed.
This work focuses on a potential decarbonising solution for heating and hot water, namely enhanced volumetric based solar thermal capture with silver nanofluids combined with thermal storage. This was shown to be a feasible solution for UK homes < median demand of 12,000 kW h y<sup>-1</sup>, > average roof area of 15 m<sup>2</sup> , space for a thermal store and utilising an enhanced efficiency collector > 70 % efficiency. Morphologically distinct nanofluids were synthesised with different optical properties, including for the first time experimentally, a novel broadband silver absorber produced by combining three silver nanofluids. The morphology was assessed using transmission electron microscopy (TEM), the potential absorption efficiency estimated using UV-vis and UV-vis-IR spectrophotometry and the performance measured in static testing utilising a solar simulator. To assess the nanofluids suitability for the application, one potential stabilisation strategy, namely SiO<sub>2</sub> coating was evaluated. The stability of the nanofluids, colloidal (storage in the dark at 4 °C ≤ 603 days), elevated temperature (70 °C), under simulated sunlight and natural sunlight was measured. The stability of the novel broadband absorber under flow conditions was also evaluated. The uncoated silver nanofluids showed a measured efficiency in the static solar simulator of ≈ 84 % for the novel silver broadband absorber and ≈ 85 % for the less dilute silver nanoprisms similar to the estimations from UV-vis spectrophotometry (< 10 % difference). In contrast the SiO<sub>2</sub> coated silver nanoprisms had a measured performance of 72 % – 77 % lower than the estimations obtained from the UV-vis spectrophotometry. The stability testing showed the nanofluids to be morphologically unstable and in the case of the silver nanoprisms unstable to aggregation in the longer term natural sunlight tests. Coating with SiO<sub>2</sub> reduced the morphological stability questioning the use of this stabilisation strategy. Although this work has highlighted the importance of application relevant stability testing it is currently not possible to recommend the use of these silver nanofluids for solar and elevated temperature applications.
The impact of utilizing Nanofluids with different metal oxide nanoparticles on heat transfer enhancement and fluid flow for concentration photovoltaic thermal the CPVT collector with Fresnel lens is demonstrated using CFD simulation. With the rising temperature, the electrical efficiency and output power of Multi-Junction Solar Cells drop. Four different types of nanofluids and water are utilized as cooling fluids to disperse heat from Multi-Junction Solar Cells by moving the fluid within the mini-channel beneath the PV Solar Cells. In the current analysis, the Reynolds numbers range from 2000 to 18000. For all forms of cooling fluids, the average Nusselt numbers are augmented by rising Reynolds numbers. The CFD results demonstrated that the Nusselt number of all nanofluid cooling fluids is greater than that of water. When compared to water in a CPVT collector, water-SiO2 nanofluid provides the maximum heat transfer improvement of 12.5%. The silicon dioxide nanoparticles have high thermal conductivity, with the water-SiO2 nanofluid exhibiting the greatest heat transfer increase, followed by water-Al2O3, water-ZnO, water-CuO, and water. The average top surface temperature of the mini-channel for the CPVT collector is reduced by 7% when water-SiO2 nanofluid is used as a cooling fluid and by 3.4% when water is used. The usage of water-SiO2 nanofluid has a significant impact on the heat dissipation with high heat transfer enhancement from Multi-Junction Solar Cells in CPVT collector, leading to an increase in electrical efficiency.
In this study, silicon carbide/indium-tin-oxide (SiC/ITO) hybrid nanofluids were synthesized, and their solar energy absorption and photo-thermal conversion performance were experimentally investigated. Furthermore, to determine the optimal mixing ratio of the SiC and ITO nanofluids, the solar energy absorption ratio based on their optical transmittance was calculated at various mixing ratios. As a result, the optimal SiC/ITO hybrid nanofluid presented lower optical transmittance than the single nanofluids and showed a higher solar energy absorption ratio over a wide optical wavelength range. In addition, it presented a higher saturation temperature point compared to single nanofluids. The SiC/ITO hybrid nanofluids with a broadband light absorption spectrum had higher saturation temperature points and photo-thermal conversion efficiency than the SiC nanofluid at all mixing ratios. At the SiC:ITO mixing ratio of 8:2, the photo-thermal efficiency of the hybrid nanofluid was maximum, reaching 34.1%, which was 38.7% higher than that of the SiC nanofluid. This study can provide effective solar heat harvesting technologies in various applications to solve the critical energy shortage problem.
Nanofluids, due to their superior thermal properties, have immense applications in heat transfer process. In view of this, nanofluids, as working fluids in solar thermal systems, have gained importance. This review emphasizes the properties of nanofluids for solar thermal applications as well as typical nanomaterials and analyses experimental and numerical investigations on solar thermal systems utilizing nanofluids along with typical experimental setups and calculation methods used for determination of the performance of nanofluids in the same. The effect of nanoparticles concentration, flowrate, ambient temperature, solar intensity and inlet temperature on the solar thermal system performance utilizing nanofluids is also discussed. Further, challenges occurring during application of nanofluids in solar thermal systems are specified along with recommendations for scaling up of nanofluid-based solar thermal systems.
The photothermal properties of nanoparticles (NP), i.e. their ability to convert absorbed light into heat, have been studied since the end of the past century mainly on gold NP. In the new millennium these studies developed into a burst of research dedicated to the photothermal ablation of tumors. However, beside this strictly medical theme, researches have flourished also in the connected areas of photothermal antibacterial surface coatings, gels and polymers, of photothermal surfaces for cell stimulation, as well as in purely technological areas that do not involve medical biotechnology. These include the direct conversion of solar light into heat, a more efficient sun generation of steam and the use of inkjet printed patterns of photothermal NP for anticounterfeit printing based on temperature reading, to cite a few. After an analysis on the photothermal effect (PTE) and of its mechanism, this minireview briefly considers the antitumoral therapy theme and dedicates in deep to all the other technological and biomedical applications of the PTE, with a particular attention to the photothermal materials whose NP have joined those based on Au
In this study, the effect of simultaneous use of vortex generator and nanofluid inside the pipe with the aim of increasing the heat transfer rate has been investigated. There are two single-phase and two-phase methods for modeling nanofluid flow. Due to the fact that in the basic fluid operation, a separate phase and nanoparticles are also a separate phase, so the use of two-phase models for modeling the behavior of this nanoscale is more accurate. The effects of Peclet number, Reynolds number, nanoparticlesize and nanofluid mean concentration on the distribution of nanoparticles have been evaluated. The valuesof thermal conductivity and viscosity as the main thermo physical properties of nanofluids changedacross different layers of the liquid due to the heterogeneous distribution of concentration. It wasobserved that an increase in the Peclet number caused heterogeneity in the distribution of the properties.The achieved nanoparticle distribution has been implemented for analysis of nanofluid using two-phasemixture model. It was found that the effect of nanofluid concentration on the Nusselt number was morenoticeable in lower Reynolds numbers due to the insignificant effect of flow momentum on heat transfer.
The paper summarizes measures to enhance solar phase change thermal energy storage (TES), starting from two key points in solar phase change TES research: solar phase change materials (PCM) and solar phase transition devices. Firstly, different types of PCM and their advantages and disadvantages are introduced. To improve thermal performance, the research progress of different dimensional thermal conductivity additives, photothermal composite phase change materials (CPCM) and phase change microcapsules is reviewed. Then, the characteristics of commonly used solar phase transition TES devices and the research status of solar energy device improvement with fins and multi‐stage phase transition TES is summarized for the past few years. It is hoped to provide methods and ideas for the design and research of solar phase change TES systems, and to increase the overall heat transfer properties through the combination of multiple optimization methods. Finally, the future direction and research focus of solar phase change TES technology are put forward.
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Global demands for sustainable energy are rising rapidly due to the ever-increasing energy crisis. Efficient energy management is one of the feasible methods to solve the existing energy crisis, and has attracted extensive attention. Pulsating heat pipe (PHP), as a potential thermal management system, has broad application prospects due to its simple structure, low cost and excellent heat transfer performance. Numerous researchers are not only committed to enhance heat transfer performances of PHPs from the aspects of working fluids, but also to further understand the operation mechanisms. The purpose of this review is to summarize and discuss the research status and development trends of working fluids, operation mechanisms, and applications of PHPs in recent years. This review highlights the latest research progress in the area of nanofluids and the recent advances in applications of nanofluids in PHPs, including the preparation and stability improvement methods of nanofluids, thermophysical and rheological behaviors of nanofluids, mechanisms responsible for thermal conductivity enhancement in nanofluids, and heat transfer mechanisms in PHPs with nanofluids. In addition, operation mechanisms of PHPs are discussed from the aspects of theoretical models of PHPs and flow regimes in PHPs. Moreover, recent advances in applications of PHPs in electronic cooling, solar collectors, waste heat recovery and machining process are presented. Subsequently, the optical properties and photothermal conversion performances of nanofluids are also introduced in details. Finally, the existing challenges and suggested directions on PHPs with nanofluids are discussed.
Solar energy is an appropriate alternative to replace fossil fuel. Making high efficient use of solar energy is a critical issue. As a kind of solar collecting medium, nanofluid shows excellent optical and photothermal conversion properties. In this work, lampblack ink, a type of traditional Chinese ink, was used as heat collecting medium for photothermal conversion. Two-step method was taken to prepared water-based lampblack ink nanofluids at mass fraction of 0.01%, 0.02%, 0.05%, 0.1% and 0.2% respectively. The thermal conductivity of lampblack ink nanofluid was mearsured. The transmission spectrum of lampblack ink nanofluid was tested by uv–vis-nir spectrophotometer. Extinction coefficient and solar weighted absorption fraction were also calculated. Dispersion stability of nanofluid was also considered. Photothermal conversion experiments were taken under solar simulator and natural sunlight. The results showed that lampblack ink nanofluids have better photothermal conversion performance than DI water. With increasing of mass fraction, photothermal conversion performance of lampblack ink nanofluid increased and then decreased. The maximum photothermal conversion efficiency could reach 76.7% under solar simulator, which was 26.4% higher than that of DI water. This work could provide new insights on the application of photothermal such as solar collectors for an effective and low-cost way to address the critical energy shortages challenges.
In this paper, [email protected]2O3 nanoparticles (NPs) were prepared through the atomic layer deposition (ALD) technique with Al2O3 nano-layers serving as anti-corrosion encapsulants. Then, the core-shell structured nano-composites were dispersed into Therminol 66 (TH66) to form oil-based plasmonic nanofluids with mass concentrations varied from 0 to 0.04 wt%. The suspension stability, optical absorption properties and photo-thermal conversion performance of oil-based nanofluids were experimentally tested and evaluated. Besides, the finite difference time domain method was used to simulate the optical absorption of [email protected]2O3 NPs with different core-shell sizes and volume concentrations. Results demonstrated that the optical absorption capability of oil-based plasmonic nanofluids was enhanced with increasing the nanomaterial concentration. At the approximate optimum concentration of 0.04 wt%, the temperature of plasmonic nanofluid could be increased up to 90.5 °C after an irradiation time of 45 min at the solar intensity of 920 W/m². The deposition of Al2O3 layer can enhance the optical absorption by intensifying and broadening the absorbance spectras of Ag NPs accompanied by red-shift due to the localized surface plasmon resonance (LSPR) effect. This study provides a promising option to produce plasmonic [email protected]2O3 nano-composites at large scale for applications in solar thermal energy harvesting.
Background
Today, by the arrival of new sustainable energy technology, the provision of energy for the global population has turned into a significant issue for societies. Meanwhile, photovoltaic/thermal (PV/T) solar collectors, as one of the most advanced types to produce electricity and heat simultaneously, can be applied with nanofluid as the working fluid.
Methods
In this research, PVP coated silver nanofluid was prepared in three volume concentrations being 250, 500 and 1000 ppm by two-step method to determine the stability and thermal conductivity, experimentally. Then, the performance of PV/T solar collector is analyzed by TRNSYS software to study electrical and thermal efficiency and also output electrical and thermal energy in different months, flowrates (25, 50, 75, 100, 150 and 200 l/h) and nanofluid’s concentration.
Results
Based on the results, the optimum flowrate and nanofluid’s concentration are obtained 50 l/h and 1000 ppm PVP coated silver nanofluid. At last, the effect of tilt angle on the output thermal and electrical energy is determined. According to the results, by changing tilt angle in different months, the performance of PV/T solar collector can be ameliorated.
Conclusion
This paper can be heeded as a novel approach to overcome the lack of solar radiation in winters by improving the performance of PV/T solar collectors.
In the present work, a nanofluid based volumetrically absorbing solar receiver having reflecting inner surfaces has been tested under outdoor conditions. Carbon soot nanoparticles from used motor oil dispersed in paraffin oil forms the working fluid. Results show that steady-state thermal efficiency peaks at an optimum nanoparticles volume fraction (ηth = 59 ± 4% at fv = 1%). Furthermore, the as-prepared nanofluid shows excellent stability i.e. it retains its optical characteristics and particle size distribution even after undergoing pumping and thermal cycles and moving in flow loops (circulation through pipes/valves) during on-sun testing. Moreover, the as-prepared nanofluid has negligible impact on the surface and optical properties of solar receiver constituent materials. Overall, the proposed receiver design and the as-prepared nanofluid stability represent significant steps towards realization of practical nanofluid based volumetric absorption solar thermal systems.
This article deals with the photothermal conversion performance of solar glycol-multiwall carbon nanotubes based nanofluids were studied with various weight concentrations and time. A broad investigation was performed with various nanofluid samples with the help of a sun simulator. The measurement was conducted with solar glycol-MWCNTs based nanofluids with 0.1%, 0.2%, and 0.3% volume concentration respectively. The heat loss was reduced by insulation material, whereas the nanofluid samples were heated with the help of a sun simulator. The difference in temperature of 7.7 C was attained for the 0.3 vol.% nanofluid as compared with solar glycol. Compared with solar glycol the temperature of MWCNT-SG based nanofluid at the volume concentration of 0.3% was augmented by about 69.21% with a light irradiation time of 10 min. The experimental results confirm that the SG\MWCNTs based can be used as the right replacement for heat transfer fluids for direct absorption solar collector system applications.
Nanofluids with full-spectrum absorption properties are highly desirable for direct solar thermal energy conversion applications. In this work, Ag and CsWO3 nanofluids, which exhibit absorption both in the visible and near-infrared (NIR) region, are integrated to obtain two-component hybrid nanofluids. The hybrid nanofluids show broad band absorption with a solar weighted absorption fraction of 99.6%, compared to 18% and 54% for the base liquid (ethylene glycol) and CsWO3 nanofluids, respectively. The highest photo-thermal conversion performance for the hybrid nanofluids is obtained with Ag/CsWO3 weight ratio of 3/7. The solar thermal conversion efficiency of the optimum hybrid nanofluids is 67%, 10% and 15% higher than single Ag and CsWO3 nanofluids. The two-component hybrid nanofluid provides an alternative for making the best use of solar energy.
Enhancing solar photovoltaic and thermal conversion performances may help develop more environmentally friendly hybrid photovoltaic/thermal (PV/T) systems that can be used in applications ranging from household to industrial scales. Owing to their enhanced thermal and optical properties, nanofluids have proven to be good candidates for designing PV/T systems with superior performances. As smart nanofluids, magnetic nanofluids (MNFs) can further enhance the performances of PV/T systems under external magnetic fields. This paper reviews recent developments in enhancing the electrical and thermal performances of PV/T systems using magnetic nanofluids. Various parameters affecting the performances are highlighted, and some areas for further investigations are discussed. The reviewed literature shows that PV/T systems with MNFs are promising. However, their performances need further investigation before they can be used in applications.
High-efficiency photo-thermal conversion is an attractive approach for solar thermal energy to alleviate energy crisis. In this work, MWCNT-H2O nanofluids were prepared and its photo-thermal conversion properties were characterized. Moreover, an optical fiber as internal light source was introduced into direct absorption solar collector (DASC), in which the photo-thermal conversion performance of the stored MWCNT-H2O nanofluids was systematically explored. The experimental results showed that the photo-thermal conversion performance of MWCNT-H2O nanofluids was improved by introducing optical fiber as internal light source into the DASC and a maximum photo-thermal conversion efficiency of 65.4% was acquired at the MWCNTs concentration of 0.010 wt.% and the optical fiber location of 15 mm. This study indicated that the MWCNT-H2O nanofluids applied in the DASC with optical fiber was a potential candidate for solar thermal energy.
Different types of ionic liquid-based nanofluids have been studied by the researchers since the last two decades. Most of the studies have focused on the effect of nanoparticles on the enhancement optical properties (absorption, transmittance, scattering, and extinction coefficient) of the corresponding ionic liquid-based nanofluids. However, optical properties of ionic liquid-based fluid have considerable contribution to absorbance in nanofluids. Therefore, it is necessary to study absorption properties of the different ionic liquid-based fluid of corresponding nanofluids to utilize ionic liquid-based nanofluids in solar thermal applications. The effect of the anion and cation radius in the ionic liquid on the enhancement absorption properties of the corresponding nanofluids is investigated. Through the measurement results, we find that as the cation radius or anion radius increases, the transmittances of the corresponding nanofluids decrease sharply, which is beneficial to enhance absorption properties of Fe3O4/ionic liquid nanofluids. At the same time, we also find that the large viscosity increase the stability of ionic liquid nanofluids, which can affect the absorption properties. Understanding the effect of ILs base fluid on enhancement absorption properties of the corresponding nanofluids will pave the road for improving the efficiency for direct absorption solar collectors.
Hybrid nanofluids have an advanced application prospect in the community of heat transfer fluids and particularly for the direct absorption solar collectors (DASCs). Therefore, stable ethylene glycol-based SiC-MWCNTs nanofluids with mass fractions ranging from 0.01% to 1% were prepared in this study. Combined with the unique properties of these two nanomaterials, the studied hybrid nanofluids displayed excellent stability, optical properties and photothermal conversion properties. The purpose of this paper is to simultaneously achieve the enhanced stability and high solar-thermal conversion efficiency of hybrid nanofluids used for DASC applications. The stability of hybrid nanofluids was confirmed. In addition, the hybrid nanofluids displayed an excellent solar irradiation absorption capacity in both visible and near-infrared regions (200–1100 nm). The fact proved that the hybrid nanofluid was effective working fluid in DASCs, where 0.5 wt% SiC-MWCNTs nanofluids could absorb 99.9% solar energy at only 1 cm path length. In addition, the solar-thermal conversion efficiency of hybrid nanofluids increased with the mass concentration. The maximum value of solar-thermal conversion efficiency was found to be 97.3% on 1 wt% SiC-MWCNTs nanofluid at 10 min, which was 48.6% higher than that of pure EG. The application potentials of SiC-MWCNTs hybrid nanofluids in low-temperature DASCs system were presented.
In this paper, the effects of adding nanoparticles (including Ag) to a fluid media for improving free convection heat transfer were analysed. The free convective heat transfer was assumed to be in laminar flow regime, and the corresponding calculations and solutions were all done by the integral method. Water, as a Newtonian fluid, was considered as the base and all relevant thermo physical properties of the nanofluids were considered to be unvarying. The calculations performed and the graphs generated showed that, in general, the addition of nanoparticles to the fluid media resulted in an increment or improvement of its heat transfer coefficient. With increase in the concentration of the nanoparticles, the heat transfer rate of the fluid also increased. The increment in heat transfer is also dependent on the nanoparticles’ thermal conductivity and the viscosity theory which was utilized in the calculations. In this study, four different theories were used to calculate the viscosities of the nanofluids. The effects of viscosity on the nanofluids’ thermal conductivity were apparent from the calculations which were performed for nanoparticle concentrations of 4% or less.
ABSTRAK: Kajian ini menganalisis kesan penambahan nanopartikel Ag ke dalam media bendalir bagi tujuan pembaikkan pemindahan haba perolakan bebas. Perolakan bebas diandaikan berada di zon aliran laminar, di mana penyelesaian dan pengiraan telah dilakukan mengunakan kaedah kamilan. Air yang merupakan cecair Newtonian, dianggap sebagai asas dan sifat terma fizikal nanocecair dianggapkan tidak berubah. Mengikut pengiraan yang dilakukan dan graf yang diplotkan, umumnya penambahan nanopartikel kepada media bendalir menyebabkan peningkatan dan pengembangan pekali pemindahan haba. Kadar pemindahan haba meningkat dengan nanopartikel. Peningkatan pemindahan haba juga bergantung kepada pengalir haba nanopartikel dan teori kelikatan yang digunakan. Di dalam kajian ini, empat jenis teori telah digunakan bagi pengiraan kelikatan nanopartikel. Kesan kelikatan ke atas pengalir haba nanocecair sangat jelas kelihatan. Kesemua pengiraan telah dilakukan ke atas kepekatan kurang dari 4%.
KEYWORDS: Ag nanofluid, heat transfer, natural convection.
Poor prognostication leads to continuation of potentially irrelevant, expensive and harmful medications when time is short, for example, statins. Similarly, necessary medications are over-prescribed in large amounts, when the patient may only have days to live. When a patient dies, all these drugs are destroyed, even those in original sealed packs. The cumulative cost of destroyed medication is huge - at a time when the NHS can ill- afford such wastage.
To assess the extent of irrelevant prescriptions in a cohort of patients admitted to a hospice for terminal care. To analyse the annual cost of irrelevant prescribing and drugs for destruction after a patients death in a UK inpatient 12 bedded hospice.
A retrospective survey of:▸ All drugs brought into the hospice at the time of terminal admission ▸ Analysis of key medication groups, highlighting those irrelevant in a patient with weeks or days to live ▸ Cost analysis of medications deemed irrelevant by two experienced hospice physicians ▸ Cost analysis of drugs destroyed after a patients death. Expected outcomes:▸ Prescribing patterns at the end of life, highlighting extent of futile prescriptions. ▸ Estimated cost to the NHS of destroyed drugs following a year of patient deaths.
Prescribing is an art, and medicines are harmful to patients if not relevant. Lack of continuity in primary care and infrequent review of dying patients may lead to prescribing of inappropriate and costly drugs. This a burden to the patient and to the NHS, with large amounts of medication being destroyed after death.
The study is in progress - when finalised, a total cost of destroyed drugs in one UK hospice will indicate a massive national problem. Suggestions will be made as to how this cost burden could be reduced, underpinned by more appropriate prescribing when time is short.
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.
The optical properties of high quality single crystal epitaxial zinc oxide thin films grown by pulsed laser deposition on c-plane sapphire substrates were studied. It was found that annealing the films in oxygen dramatically improved the optical and electrical properties. The absorption coefficient, band gap, and exciton binding energies were determined by transmission measurements and photoluminescence. In both the annealed and the as-deposited films excitonic absorption features were observed at both room temperature and 77 K. In the annealed films the excitonic absorption peaks were substantially sharper and deep level photoluminescence was suppressed. (C) 1999 American Institute of Physics. [S0021-8979(99)02611-0].
The optical characterization is reported of a new fluid consisting of single-wall carbon nanohorns and ethylene glycol for solar energy applications. Carbon nanohorns play a significant role in enhancing sunlight absorption with respect to the pure base fluid. The obtained results are compared with those obtained for fluids suspending more conventional carbon forms, i.e. carbon-black particles. We found that nanohorn spectral features are far more favorable than those of amorphous carbon for the specific application. This result shows that carbon nanohorn-based nanofluids can be useful for increasing the efficiency and compactness of thermal solar devices, reducing both environmental impact and costs.
The optical properties of human skin, subcutaneous adipose tissue and human mucosa were measured in the wavelength range 400–2000 nm. The measurements were carried out using a commercially available spectrophotometer with an integrating sphere. The inverse adding–doubling method was used to determine the absorption and reduced scattering coefficients from the measurements.
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Suspensions of nanoparticles (i.e., particles with diameters < 100 nm) in liquids, termed nanofluids, show remarkable thermal and optical property changes from the base liquid at low particle loadings. Recent studies also indicate that selected nanofluids may improve the efficiency of direct absorption solar thermal collectors. To determine the effectiveness of nanofluids in solar applications, their ability to convert light energy to thermal energy must be known. That is, their absorption of the solar spectrum must be established. Accordingly, this study compares model predictions to spectroscopic measurements of extinction coefficients over wavelengths that are important for solar energy (0.25 to 2.5 μm). A simple addition of the base fluid and nanoparticle extinction coefficients is applied as an approximation of the effective nanofluid extinction coefficient. Comparisons with measured extinction coefficients reveal that the approximation works well with water-based nanofluids containing graphite nanoparticles but less well with metallic nanoparticles and/or oil-based fluids. For the materials used in this study, over 95% of incoming sunlight can be absorbed (in a nanofluid thickness ≥10 cm) with extremely low nanoparticle volume fractions - less than 1 × 10-5, or 10 parts per million. Thus, nanofluids could be used to absorb sunlight with a negligible amount of viscosity and/or density (read: pumping power) increase.
Nanofluids are found to have good stability and useful optical and thermal properties as direct sunlight absorbers in solar collectors. The inherent hydrophobic nature of carbon nanotubes was overcome using a new dispersion procedure (treating carbon nanotubes with base media) to prepare nanofluids. To the authors' knowledge, this is the first application of aqueous suspension based on alkaline functionalized carbon nanotubes as an absorber fluid in a sunlight harvesting device. Dispersion stability and optical properties of the nanofluid were estimated. Spectral absorbance analysis confirms the relative stability of prepared nanofluids versus sediment time. The extinction coefficient of aqueous suspensions of functionalized carbon nanotubes shows remarkable improvement compared to the base fluid even at low particle loadings. We also demonstrate thermal conductivity improvements of up to 32% by adding only 150 ppm functionalized carbon nanotubes to water as the absorbing medium. Their promising optical and thermal properties, together with the appropriate stability of nanofluids, make them very interesting for increasing the overall efficiency of low-temperature direct absorption solar collectors.
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]
By use of a wedge-shaped cell providing an absorbing layer tapering in thickness from less than one wavelength of visible light at one end to approximately 20µm at the other end, we have measured the Lambert absorption coefficient for water in the spectral region between 4000 and 288 cm-1. After proper initial alignment of the cell windows had been established by the observation of interference fringes in the visible, we measured film thicknesses at various positions along the wedge by interferometric methods, employing convenient wavelengths in the infrared. We present the results of the study in graphical and tabular form.
Wide-band-gap oxides such as SrTiO3 are shown to be critical tests of theories of Schottky barrier heights based on metal-induced gap states and charge neutrality levels. This theory is reviewed and used to calculate the Schottky barrier heights and band offsets for many important high dielectric constant oxides on Pt and Si. Good agreement with experiment is found for barrier heights. The band offsets for electrons on Si are found to be small for many key oxides such as SrTiO3 and Ta2O5 which limit their utility as gate oxides in future silicon field effect transistors. The calculations are extended to screen other proposed oxides such as BaZrO3.ZrO2,HfO2,La2O3,Y2O3,HfSiO4, and ZrSiO4. Predictions are also given for barrier heights of the ferroelectric oxides Pb1-xZrxTiO3 and SrBi2Ta2O9 which are used in nonvolatile memories. © 2000 American Vacuum Society.
The present paper reports the experimental results and the potential performance of the investigation on flat solar thermal collectors using nanofluids as innovative heat transfer fluids for solar energy applica- tions. The straight use of heat-transfer nanofluids in traditional solar flat panel revealed some technical issues, due to the nanoparticles sedimentation. Therefore, sedimentation has been investigated both in standard solar flat panels and modified ones made from transparent tubes. The results of the first tests showed that the main sedimentation parameter is the flow velocity and to better control it a standard flat panel was modified changing the cross-section of the lower and top header of the panel, that have been tapered to keep constant the fluid axial velocity. The modification of the panel shape (patent pend- ing) enabled a negligible particles deposit.
After different nanofluids were tested on the panel prototype, water–Al2O3 was chosen as heat transfer fluid. All tested nanofluids were prepared in batch and their thermal conductivity and convective heat transfer coefficient were measured prior of their use as heat transfer fluid in the solar panel. A thermal conductivity enhancement up to 6.7% at a concentration of 3 vol% was observed, while the convective heat transfer coefficient increased up to 25%.
a b s t r a c t The work reported in this paper shows the experimental results from a study on diathermic oil based nanofluids. Diathermic oil finds application in renewable energy, cogeneration and cooling systems. For example, it is used in solar thermodynamic or biomass plants, where high efficiency, compact vol-umes and high energy fluxes are required. Besides diathermic oil is very important in those applications where high temperatures are reached or where the use of water or vapor is not suitable. Therefore an improvement of diathermic oil thermo-physical properties, by using of nanoparticles, can increase the performance of the systems. In literature there are not many experimental data on diathermic oil based nanofluids because many experimental campaigns are focused on water nanofluids. Samples of nanofluids, with nanoparticles of CuO, Al 2 O 3 , ZnO and Cu, having different shapes and concentrations varying from 0.0% up to 3.0%, have been produced and their thermal conductivity has been measured by means of hot-wire technique, according to the standard ASTM D 2717-95. Measurements were carried out to investigate the effects of volume fraction, particle size of nanoparticles on the thermal conductivity of the nanofluid. The effect of temperature has been also investigated in the range 20–60 °C. A dependence was observed on the mea-sured parameters and the results showed that the heat transfer performance of diathermic oil enhances more than water with the same nanoparticles.
In this paper a new equation for calculating the nanofluid viscosity by considering the Brownian motion of nanoparticles is introduced. The relative velocity between the base fluid and nanoparticles has been taken into account. This equation presents the nanofluid viscosity as a function of the temperature, the mean nanoparticle diameter, the nanoparticle volume fraction, the nanoparticle density and the base fluid physical properties. In developing the model a correction factor is introduced to take into account the simplification that was applied on the boundary condition. It is calculated by using very limited experimental data for nanofluids consisting of 13 nm Al2O3 nanoparticles and water and 28 nm Al2O3 nanoparticles and water. The predicted results are then compared with many other published experimental results for different nanofluids and very good concordance between these results is observed. Compared with the other theoretical models that are available in the literature, the presented model, in general, has a higher accuracy and precision.
The one-dimensional quantum-confinement effect in α-Fe 2 O 3 ultrafine nanorod arrays is studied. The formation of bundles of ultrafine hematite nanorods is understood by considering the crystal structure of ΒFeOOH, which occurs in nature as the mineral akaganeite. Every third layer is only two-thirds occupied, with rows of oxygen atoms missing along the c-axis. The results show that the bandgap criteria for direct photo-oxidation of water by solar irradiation without an applied bias would be satisfied for such purpose built nanomaterials.
Mainly by changing the cerium species to be homogeneously precipitated from cerous ion to colloidal cerianite, a special sol–gel process was arranged to synthesize nanocrystalline cerium oxide. The specimen powder was characterized with XRD, SEM, HRTEM, ICP-OES, and UV–vis spectrophotometry. HRTEM detected monodispersed cerianite crystallites with typical size of 3–5 nm, which is a favorable characteristic for many applications. The direct optical band gap estimated from ultraviolet absorption spectrum of the ceria powder is 3.56 eV, which is blue-shifted evidently with respect to the bulk material (Eg = 3.19 eV) and may indicate quantum-size confinement effect in the crystallites. The results show that forced hydrolysis of urea can be used to destabilize the nanocrystal sol, leading to the formation of loosely bound cerianite aggregate.
The effect of the size, shape, and structure of gold and silver nanoparticles on the dependence of their extinction and integral
scattering spectra on the dielectric environment has been investigated. Calculations were performed using the Mie theory for
spheres and nanoshells and the T-matrix method for chaotically oriented bispheres, spheroids, and s cylinders with hemispherical ends. The sensitivity of plasmon resonances to variations in the refractive index of the environment
in the range 1.3–1.7 for particles of different equivolume size, as well as to variations in the thickness of the metal layer
of nanoshells, was studied. For nanoparticles with an equivolume diameter of 15 nm, the maximal shifts of plasmon resonances
due to variation in the refractive index of the environment are observed for bispheres and the shifts decrease in the series
nanoshells, s cylinders or spheroids, and spheres. For particles 60 nm in diameter, the largest shifts of plasmon resonances occur for
nanoshells and the shifts decrease in the series bispheres, s cylinders or spheroids, and spheres. All other conditions being the same, silver nanoparticles are more sensitive to the
resonance tuning due to a change in the dielectric environment.
Metal oxide (CeO2, SnO2) sols were prepared in such a way as to hinder the inorganic condensation reactions. Acetylacetorte was added to cerium nitrate solutions before hydrolysis with ammonia solution, while solvolysis of anhydrous tin chloride with methanol was used for preparing SnO2 sols. The sols were then injected into dodecylamine at 160 degrees C, followed by heating at the resulting temperature for 1 h. A clear sol was obtained, from which the products were extracted by precipitation with methanol and dried. Analysis by X-ray diffraction and TEM demonstrated the presence of CeO2 and SnO2 nanocrystals, whose size depended on the metal concentration in the starting sol, ranging from 2.6 to 3.5 nm for CeO2 and from 1 to 1.9 nm for SnO2. The nanocrystals were soluble after the synthesis and redispersable in organic solvents, forming stable suspensions. The process appears to be easily generalizable to systems having similar hydrolytic chemistry. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008).
The double-aperture method has been used to determine the nonlinearity correction for a new spectrophotometer having a precision of +/-4 x 10(-5) transmittance units. The random and systematic errors of the method are discussed, and techniques are described that yield the additive nonlinearity correction to the high level of precision required for this spectrophotometer. The correction was found to be independent of source polarization, free from interference errors, but slightly dependent on wavelength.
It is well-known that inorganic nanocrystals are a benchmark model for nanotechnology, given that the tunability of optical properties and the stabilization of specific phases are uniquely possible at the nanoscale. Copper (I) oxide (Cu(2)O) is a metal oxide semiconductor with promising applications in solar energy conversion and catalysis. To understand the Cu/Cu(2)O/CuO system at the nanoscale, we have developed a method for preparing highly uniform monodisperse nanocrystals of Cu(2)O. The procedure also serves to demonstrate our development of a generalized method for the synthesis of transition metal oxide nanocrystals. Cu nanocrystals are initially formed and subsequently oxidized to form highly crystalline Cu(2)O. The volume change during phase transformation can induce crystal twinning. Absorption in the visible region of the spectrum gave evidence for the presence of a thin, epitaxial layer of CuO, which is blue-shifted, and appears to increase in energy as a function of decreasing particle size. XPS confirmed the thin layer of CuO, calculated to have a thickness of approximately 5 A. We note that the copper (I) oxide phase is surprisingly well-stabilized at this length scale.
Standards and Best Practice in Absorption Spectrometry
- C Burgess
- T Frost
C. Burgess, T. Frost, Standards and Best Practice in Absorption Spectrometry,
Blackwell Science, Hoboken, 1999, ISBN 0-632-15313-5.
- M Yin
- C.-K Wu
- Y Lou
- C Burda
- J T Koberstein
- Y Zhu
- S O'brien
M. Yin, C.-K. Wu, Y. Lou, C. Burda, J.T. Koberstein, Y. Zhu, S. O'Brien, Copper
Oxide Nanocrystals, J. Am. Chem. Soc. 127 (2005) 9506-9511.
Optical properties of human skin, subcutaneous and mucous tissues in the wavelength range from 400 to 2000 nm
- A N Bashkatov
- E A Genina
- V I Kochubey
- V V Tuchin
A.N. Bashkatov, E.A. Genina, V.I. Kochubey, V.V. Tuchin, Optical properties of
human skin, subcutaneous and mucous tissues in the wavelength range from
400 to 2000 nm, J. Phys. D.: Appl. Phys. 38 (2005) 2543-2555.