A numerical comparison among different water-based hybrid nanofluids on their influences on natural convection heat transfer in a triangular solar collector for different tilt angles

Natural convection inside a triangular solar collector is investigated numerically for different nanofluids and hybrid nanofluids in this study. The individual effects of Al₂O₃–water, carbon nanotubes (CNT)–water, and Cu–water nanofluids are observed for different solid volume fractions of nanoparticles (0%–10%). Three types of hybrid nanofluids are prepared using different ratios of Al₂O₃, CNT, and Cu nanoparticles in water. A comparison is made varying the Rayleigh numbers within laminar range (10³–10⁶) for different tilt angles (0°, 30°, 60°, and 90°) of the solar collector. The inclined surface of the triangular solar collector is isothermally cold and the bottom wall (absorber plate) is isothermally hot, whereas the vertical wall with respect to the absorber plate is considered adiabatic. Average Nusselt numbers along the hot wall for different parameters are observed. Streamlines and isotherm contours are also plotted for different cases. Dimensionless governing Navier–Stokes and thermal energy conservation equations are solved by Galerkin weighted residual finite element method. Better convective heat transfer is found for higher Rayleigh number, solid volume fraction, and tilt angle. In the case of hybrid nanofluid, increasing the percentage of the nanoparticle that gives better heat transfer performance individually results in enhancing natural convection heat transfer inside the enclosure.     

Synthesis and thermal conductivity of Cu2O nanofluids

We apply the chemical solution method to synthesize Cu₂O nanofluids: suspensions of cuprous-oxide (Cu₂O) nanoparticles in water, and experimentally study the effect of reactant molar concentration and nanofluid temperature on the thermal conductivity. Substantial conductivity enhancement up to 24% is achievable with the synthesized nanofluids. The nanoparticle shape is variable by adjusting some synthesis parameters. The thermal conductivity shows both sensitivity and nonlinearity to the reactant molar concentration and the nanofluid temperature.     

Experimental and theoretical study of a single-basin solar sill in Jordan

This paper presents experimental results of a single-basin solar still using various absorbing materials. The still has equal angle double-sloped covers with an effective basin area of 3 m². The experiments were conducted in Amman, Jordan during the months of April and May. The materials used to enhance the absorptivity of water for solar radiation include dissolved salts, violet dye, and charcoal. The salts were potassium permanganate (KMnO₄) and potassium dichromate (K₂Cr₂O₇). Significant increase in still efficiency and productivity was obtained. For example, the addition of potassium permanganate resulted in 26% improvement in efficiency. Also, a comparison between theoretical and measured water productivity is presented. It was found that there is a good agreement between theory and experiment.     

Experimental investigation on the effect of MgO and TiO2 nanoparticles in stepped solar still

This work aims at augmenting the amount of potable water using MgO and TiO₂ in stepped solar still. Experiments were carried out for the climatic conditions of Chennai, India, with two different concentrations of nanofluids inside a stepped basin under three different cases. Results show that there is an improvement in yield of fresh water from stepped solar still by 33.18% and 41.05% using 0.1% and 0.2% volume concentration of TiO₂ nanofluid, respectively, whereas the freshwater yield from stepped still with MgO nanofluids improved by 51.7% and 61.89%. Furthermore, the economic analysis revealed that the cost of potable water from the modified solar still reduced from 0.029 to 0.016 $/kg. Similarly, the useful annual energy, yearly cost per kilogram, and annual cost per kilowatt hour are significantly profitable with the use of MgO nanofluid in the stepped basin and found as 512.46 kWh, 0.025 $/kg, and 0.026 $/kWh, respectively. It is also found that the cost of potable water from the modified still significantly increases as the amount of fresh water produced is decreased with increased fabrication cost of the solar still.     

Chemical bath deposition synthesis of TiO2/Cu2O core/shell nanowire arrays with enhanced photoelectrochemical water splitting for H2 evolution and photostability

In this study, we have developed a facile chemical bath deposition (CBD) method to grow p-type Cu₂O nanoparticles on n-type TiO₂ nanowire arrays (TiO₂ NWAs) to fabricate TiO₂/Cu₂O core/shell heterojunction nanowire arrays (TiO₂/Cu₂O core/shell NWAs). When used as photoelectrode, the fabricated TiO₂/Cu₂O core/shell NWAs show improved photoelectrochemical (PEC) water splitting activity to pure TiO₂ NWAs. The effects of the CBD cycle times on the PEC activities have been studied. The TiO₂/Cu₂O core/shell heterojunction nanowire array photoelectrode prepared by cycling 5 times in the CBD process achieves the highest photocurrent of 2.5 mA cm^?2, which is 2.5 times higher than that of pure TiO₂ NWAs. In addition, the H₂ generation rate of this photoelectrode reaches to 32 μmol h^?1 cm^?2, 1.7 times higher than that of pure TiO₂ NWAs. Furthermore, the TiO₂/Cu₂O core/shell heterojunction nanowire array photoelectrode shows excellent photostability and achieves a stable photocurrent of over 2.3 mA cm^?2 during long light illumination time of 5 h. The enhanced photocatalytic activity of TiO₂/Cu₂O core/shell heterojunction nanowire array photoelectrode is attributed to the synergistic actions of TiO₂ and Cu₂O for improving visible light harvesting, and efficient transfer and separation of photogenerated electrons and holes.     

Efficiency improvement of a solar direct volumetric receiver utilizing aqueous suspensions of CuO

In this paper, an experimental study was performed to investigate the photothermal conversion properties of CuO‐H₂O nanofluid‐based volumetric receiver mainly considering the effects of nanoparticle (NP) concentration, irradiation time, and receiver depth. First, stable aqueous suspensions of CuO with NPs having average diameter close to 10 nm were produced by the precursor transformation method. The spectral transmittances of CuO‐H₂O nanofluids decrease with increasing the NP concentration (0.01‐0.25 wt%) at wavelengths of 200 to 1350 nm. The photothermal conversion performance of CuO‐H₂O nanofluids is sensitive to the receiver depth, irradiation time, and NP concentration. The higher NP concentration causes stronger optical absorption in the upper part and reduces the temperature at the bottom accordingly. The temperature difference between CuO‐H₂O nanofluid and distilled water increased initially and then decreased with the increase of penetration depth, and there existed an optimal depth of 1 cm with respect to the best photothermal conversion performance. The receiver efficiency decreased with increasing the light irradiation time, and an efficiency improvement up to 30.4% was achieved for the 0.25 wt% nanofluid at the optimal depth of 1 cm as compared with water. This work shows that volumetric receivers provide a potential alternative for solar thermal energy utilization versus surface‐based absorber especially under concentrated solar radiation.     

Quasi-solid-state dye-sensitized solar cells employing nanocrystalline TiO2 films made at low temperature

Quasi-solid-state dye-sensitized solar cells with enhanced performance were made by using nanocrystalline TiO₂ films without any template deposited on plastic or glass substrates at low temperature. A simple and benign procedure was developed to synthesize the low-temperature TiO₂ nanostructured films. According to this method, a small quantity of titanium isopropoxide (TTIP) was added in an ethanolic dispersion of TiO₂ powder consisting of nanoparticles at room temperature, which after alkoxide's hydrolysis helps to the connection between TiO₂ particles and to the formation of mechanically stable thick films on plastic or glass substrates. Pure TiO₂ films without any organic residuals consisting of nanoparticles were formed with surface area of 56 m²/g and pore volume of 0.383 cm³/g similar to that obtained for Degussa-P25 powder. The structural properties of the films were characterized by microscopy techniques, X-ray diffractometry, and porosimetry. Overall solar to electric energy conversion efficiencies of 5.3% and 3.2% (under 1sun) were achieved for quasi-solid-state dye-sensitized solar cells employing such TiO₂ films on F:SnO₂ glass and ITO plastic substrates, respectively. Thus, the quasi-solid-state device based on low-temperature TiO₂ attains a conversion efficiency which is very close to that obtained for cells consisting of TiO₂ nanoparticles sintered at high temperature.     

Experimental analysis of SiO2-Distilled water nanofluids in a Polymer Electrolyte Membrane fuel cell parallel channel cooling plate

An experimental report on the thermal performance of Silicone Dioxide (SiO₂) nanofluid coolants based on a PEM fuel cell cooling system is presented. The aim of this study is to evaluate the feasibility of applying these nanofluids coolants as an alternative to conventional distilled water through detailed analysis of thermofluids behaviour in a simulated cooling plate environment. SiO₂ nanoparticles were dispersed in distilled water at 0.1%, 0.3% and 0.5% volume concentrations and tested in a parallel channel cooling plate system. A constant heat load was supplied to simulate a fuel cell stack thermal condition. At inlet flow conditions from 750 to 900 Reynolds number, the SiO₂ nanofluids reduced the average plate temperatures by 15%–20% compared to conventional water coolant. The nanofluids also increased the cooling effectiveness by a similar margin, as well as improving the bulk heat transfer coefficient to a range between 2700 and 4400 W m⁻². ^oC⁻¹. However, the required pumping power was also increased due to the added viscous effect. Through the Advantage Ratio (AR) analysis, it was concluded that the enhancement in heat transfer mechanics was more significant than the penalties in fluid flow dynamics. Thus, the SiO₂ nanofluids and the cooling plate design are possible options for advanced PEM fuel cell thermal management practice in future stack designs.     

Heat Transfer by Nanofluid with Different Nanoparticles in a Solar Collector

The buoyancy flow and heat transfer characteristics inside a solar collector having the flat‐plate cover and sinusoidal corrugated absorber are analyzed numerically. The water‐based nanofluid with alumina and copper nanoparticles is used as the working fluid inside the solar collector. The governing partial differential equations with proper boundary conditions are solved by the finite element method using Galerkin's weighted residual scheme. The behavior of both nanoparticles related to performance such as temperature and velocity distributions, radiative and convective heat transfers, mean temperature, and velocity of the nanofluid is investigated systematically. This performance includes the solid volume fraction, namely ?₁ and ?₂, with respect to Al ₂ O ₃ and Cu nanoparticles. The results show that the better performance of heat transfer inside the collector is found by using the highest ?₂ than ?₁. The result of this study expresses a good agreement with the theoretical result available in the literature. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res, 43(1): 61–79, 2014; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21061     

Experimental evaluation of a single-basin solar still using different absorbing materials

Single-basin solar stills can be used for water desalination. Probably, they are considered the best solution for water production in remote, arid to semi-arid, small communities, where fresh water is unavailable. However, the amount of distilled water produced per unit area is somewhat low which makes the single-basin solar still unacceptable in some instances. The purpose of this paper is to study the effect of using different absorbing materials in a solar still, and thus enhance the productivity of water. Experimental results show that the productivity of distilled water was enhanced for some materials. For example, using an absorbing black rubber mat increased the daily water productivity by 38%. Using black ink increased it by 45%. Black dye was the best absorbing material used in terms of water productivity. It resulted in an enhancement of about 60%. The still used in the study was a single-basin solar still with double slopes and an effective insolation area of 3 m².     

Mathematical modeling and the thermal performance of floating cum tilted-wick type solar still

In this paper, mathematical modeling for the thermal performance of floating cum tilted-wick type solar still has been presented. Explicit expressions for the temperatures of various components of the proposed system and its efficiency have been developed. The effect of mass flow rate due to capillary action of jute wick has been investigated for evaporative, convective, and radiative heat transfer from the evaporating surface to the condensing surface.It has been found that the mass flow rate of 2.5 x 10^?3 kg/s is optimum for effective distillation. Also, the effect of absorptivity of the wick surfaces (floating and tilted-wick) on the productivity of the solar still has been studied and found that α_w1 (absorptivity of the tilted-wick surface) and α_wf(absorptivity of the floating-wick surface) of 0.85 have given higher productivity.For enunciation of the analytical results, numerical calculations have been made using meteorological parameters for a typical winter and summer day in Coimbatore.The theoretical results are in good agreement with the experimental results.     

Factors influencing solar energy collector efficiency

The effects of glazing, solar flux, emissivity and absorptivity of the absorber surface on collector performance have been predicted for different plate temperatures (60°C and 100°C). At low solar flux levels (200–600 W m^?2) double- or triple-glazed collectors are superior to single-glazed collectors. For collectors with selective absorber coatings, the optimal number of glazing panes is two. Higher values of absorptivity and lower values of emissivity are more effective for single-glazed than for double-glazed collectors.     

Experimental investigation on the thermal efficiency and performance characteristics of a flat plate solar collector using SiO2/EG–water nanofluids

Application of nanofluids in thermal energy devices such as solar collectors is developing day by day. This paper reports the results of experiments on a flat plate solar collector where the working fluid is SiO₂/ethylene glycol (EG)–water nanofluid with volume fractions up to 1%. The thermal efficiency and performance characteristics of solar collector are obtained for mass flow rates between 0.018 and 0.045 kg/s. The curve characteristics of solar collector indicate that the effects of particle loading on the thermal efficiency enhancement are more pronounced at higher values of heat loss parameter. The results of this work elucidate the potential of SiO₂ nanoparticles to improve the efficiency of solar collectors despite its low thermal conductivity compared to other usual nanoparticles.     

Estimation of Nusselt number and effectiveness of double‐pipe heat exchanger with Al2O3–, CuO–, TiO2–, and ZnO–water based nanofluids

This paper deals with experimental studies carried out to analyze heat transfer characteristics of Al₂O₃–, CuO–, TiO₂–, and ZnO–water based nanofluids in a double‐pipe, counter flow heat exchanger for different volume concentrations (0.025%, 0.05%, 0.075%, and 0.1%) of the nanofluids. The fabricated double‐pipe heat exchanger is made up of two different materials, viz., copper as the inner tube and unplasticized polyvinyl chloride as the outer tube. The density, viscosity, and thermal conductivity were calculated, and were used to estimate dimensionless numbers, such as Reynolds number, Prandtl number, and Nusselt number, and also to estimate heat exchanger effectiveness. High‐energy ball milling technique was used to prepare nanoparticles and were characterized using X‐ray diffraction, scanning electron microscopy, and transmission electron microscopy. Polyvinyl alcohol (3%) was used as a surfactant for making the nanofluids stable. It was observed from the experiment that with the increase in the volume concentration, thermal conductivity, viscosity, and friction factor increase, whereas the Reynolds number decreases. The experimentally observed data for Nusselt number were formulated into a correlation that matches the data for all these nanofluids within an error of 11.4%. It was found that the highest effectiveness was obtained while using TiO₂–water nanofluids than other nanofluids.     

Soft‐template synthesis of high surface area mesoporous titanium dioxide for dye‐sensitized solar cells

In the present work, 10 to 14 nm titania nanoparticles with high‐packing density are synthesized by the soft‐template method using a range of cationic surfactants including cetyl trimethylammonium bromide (CTAB), Sodium dodecyl sulfate (SDS), and dodecyl trimethylammonium bromide (DTAB). The synthesized nanoparticles are used as a photoanode material in dye solar cells. Density functional theory (DFT) simulations reproduce our experimental results of charge transfer and strong interaction between the TiO₂ and N719. N719‐TiO₂ complex establishes strong electrostatic bonding through H of the dye with the O of TiO₂ surface. Solar cell efficiency of 6.08% with 12.63 mA/cm², 793 mV, and 48.5% for short circuit current density, open circuit voltage, and fill factor, respectively, are obtained under 1 sun illumination for the dye‐sensitized solar cell (DSSC) using a film of mesoporous TiO₂ synthesized from the SDS surfactant. On the other hand, the 21 nm commercial TiO₂ powder (P25) device results in 4.60% efficiency under similar conditions. Electrochemical impedance spectroscopic studies show that the SDS device has lesser charge transport resistance than the other devices because of its higher surface area, packing density, and dye loading capacity. Our results show that employing high packing density‐based TiO₂ nanoparticles represents a commercially viable approach for highly beneficial photoanode development for future DSSC applications.     

Theoretical and Experimental Studies of the Ultra-Thin-Channel Solar Water Collector

Ultra-thin-channel solar water collector efficiency (UCSWC) was investigated theoretically and experimentally. An ultra-thin-channel solar water collector was constructed using several flat plates with an ultra-thin fluid channel formed using an adjustable flexible silicon frame inserted between the absorber plate and bottom plate. The advantages of the ultra-thin-channel solar water collector are low absorber plate temperature and low total water mass flow rate, resulting in considerable collector efficiency improvement with high outlet fluid temperature and low pump power requirement. A simple and general modeling method was developed to predict the collector efficiencies and mean temperatures of the glass cover, absorber plate and fluid. Good agreement was achieved between the calculated and experimental values. The superior collector efficiencies of the UCSWC are obtained as 82.2% and 75.5% for the inlet temperatures 30°C and 70°C, respectively, operating at a total fluid mass flow 8.3 × 10^?3 kg/s and solar radiation incident of 1100 W/m².     

Theory and experimental investigation of a weir-type inclined solar still

A weir-type solar still is proposed to recover rejected water from the water purifying systems for solar hydrogen production. This consists of an inclined absorber plate formed to make weirs, as well as a top basin and a bottom basin. Water is flowed from the top basin over the weirs to the bottom collection basin. A small pump is used to return the unevaporated water to the top tank. Hourly distillate productivity of the still with double- and single-pane glass covers was measured and the latter showed higher production rates. The average distillate productivities for double- and single-pane glass covers are approximately 2.2 and 5.5 l/m²/day in the months of August and September in Las Vegas, respectively. Mathematical models that can predict the hourly distillate productivity are developed. These compared well with the experimental results. Productivity of the weir-type still with a single-pane glass was also compared with conventional basin types tested at the same location. The productivity of the weir-type still is approximately 20% higher. The quality of distillate from the still is analyzed to verify the ability of the still to meet the standards required by the electrolyzers.     

Research on Thermophysical Properties of Nanoliquids Based on SiO<Subscript>2</Subscript> Nanoparticles for Use as a Heat-Transfer Medium in Solar-Thermal Converters

This paper presents an analysis of the modern state of studies of the thermophysical properties of nanofluids and the heat-transfer mechanism in them. The results of experimental studies of obtaining and determining the dynamic viscosity of the nanofluids (SiO₂ + water) with various concentrations of nanoparticles are given. Nanofluids are obtained using a two-stage method in an ultrasonic field with a frequency of 20 kHz. It is shown that, in the SiO₂ + water system, nanoparticles with sizes of 7, 12, and 16 nm are most stable. Various SiO₂ concentrations in the volume range 0.5–5% were tested, and their thermophysical properties were studied for the purpose of using them as a heat-transfer medium in flat-plate solar collectors.     

Surface modification of aligned TiO2 nanotubes by Cu2O nanoparticles and their enhanced photo electrochemical properties and hydrogen generation application

In this work, we report the synthesis of cuprous oxide (Cu₂O) nanoparticles modified vertically oriented aligned titanium dioxide (TiO₂) nanotube arrays through wet chemical treatment of TiO₂ nanotubes and their multi-functional application as enhanced photo electrochemical and hydrogen generation. The synthesized samples were characterized by X-ray diffraction, SEM, TEM, and UV–Vis spectroscopy. The structural characterization revealed that the admixed Cu₂O nanoparticles on the TiO₂ surface did not alter its crystalline structure of vertically oriented aligned TiO₂ nanotube. The photocatalytic performance and hydrogen generation of as synthesized Cu₂O nanoparticles modified aligned TiO₂ nanotube was found to highly depend on the Cu₂O content. The optical characterizations reveal that the presence of Cu₂O nanoparticles extends its absorption into the visible region which improves the photocurrent density in comparison to pristine aligned TiO₂ nanotubes electrodes due to enhanced photoactivity and better charge separation. The optimum photocurrent density and hydrogen generation rate has been found to be 3.4 mA cm^?2 and 127.5 μmole cm^?2 h^?1 in 1 M Na₂SO₄ electrolyte solution under 1.5 AM solar irradiance of white light with illumination intensity of 100 mW cm^?2.     

Flow and heat transfer characteristics on a moving plate in a nanofluid

This paper considers the extended Blasius and Sakiadis problems in nanofluids, by considering a uniform free stream parallel to a fixed or moving flat plate, which has more practical significance. It is assumed that the plate moves in the same or opposite direction to the free stream. The resulting system of nonlinear ordinary differential equations is solved numerically for three types of nanoparticles, namely copper (Cu), alumina (Al₂O₃), and titania (TiO₂) in the water based fluid with Prandtl number Pr = 6.2. The effect of the solid volume fraction parameter φ of the nanofluids on the heat transfer characteristics is investigated. The results indicate that dual solutions exist when the plate and the free stream move in the opposite directions.