Study of the possibilities of thermal performance enhancement of flat plate solar water collectors by using of nanofluids as heat transfer fluid

The current work represents the simulation results of the thermal performances of flat-plate solar collector with heat transfer fluid–nanofluid (SiO₂ + water with 5% concentration) which is obtained experimentally [1]. The dependence of the outlet temperature and gained useful energy of heat transfer fluid (nanofluid) on the flow rates (10, 15 and 20 L/h) at different ranges of incident solar radiation (500–1000 W/m²) was obtained.     

Investigation of Heat Transfer Enhancement in a Forward-Facing Contracting Channel Using FMWCNT Nanofluids

The turbulent forced convection heat transfer of water/functionalized multi-walled carbon nanotube (FMWCNT) nanofluids over a forward-facing step was studied in this work. Turbulence was modeled using the shear stress transport K-ω model. Simulations were performed for Reynolds numbers ranging from 10,000 to 40,000, heat fluxes from 1,000 to 10,000 W/m², and nanoparticle volume fractions of 0.00% to 0.25%. The two-dimensional governing equations were discretized with the finite volume method. The effects of nanoparticle concentration, shear force, heat flux, contraction, and turbulence on the hydraulics and thermal behavior of nanofluid flow were studied. The model predictions were found to be in good agreement with previous experimental and numerical studies. The results indicate that the Reynolds number and FMWCNT volume fraction considerably affect the heat transfer coefficient; a rise in local heat transfer coefficient was noted when both Reynolds number and FMWCNT volume fraction were increased for all cases. Moreover, the contraction of the channel passage leads to the formation of two recirculation regions with augmented local heat transfer coefficient value.     

Feasibility of air-cooled solar air-conditioning in hot arid climate regions

In air-cooled lithium bromide absorption chillers, the working conditions in the absorber and condenser are shifted to higher temperatures and salt concentrations, thereby increasing the risk of crystallisation in the solution heat exchanger. In this paper, we present the results of numerical investigations performed in order to find out the appropriate operating conditions to avoid salt crystallisation by ensuring an acceptable coefficient of performance. It is shown that, to this purpose, the lowest and highest temperatures in the machine must be modified (evaporation at 11 °C, generator at 90 °C). Based on these results, we evaluated the potential of an integrated solar air-conditioning system for a middle-class house (150 m²) under the climatic conditions of Tunis city.

The chiller generator is supplied with heat via an external pressurised water loop from an insulated tank storing solar heat from an evacuated tube collector field and maintained at a maximum temperature of 110 °C with make-up energy from fossil fuel combustion. It is shown that the collector surface is unlikely to exceed 32 m².     

Thermal analysis of insulated north-wall greenhouse with solar collector under passive mode

An insulated north wall greenhouse dryer has been fabricated and tested for no-load condition under passive mode. Testing has been conducted in two different cases. Case-I is considered for solar collector kept inside the dryer and Case-II is dryer without solar collector. Convective heat transfer coefficient and various heat transfer dimensionless numbers with have been calculated for thermal analysis. The maximum convective heat transfer coefficient is found 52.18?W/m²°C at 14?h during the first day for Case-I. The difference of the highest convective heat transfer coefficient of both cases was 8.34?W/m²°C. Net heat gain inside room curves are uniform and smooth for Case-I, which shows the steady heat generation process due to presence of solar collector inside the dryer. Above results depicts the effectiveness of solar collector and insulated north wall. The selection of suitable crop for drying can be done by analysing article’s result.     

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.     

Pool boiling heat transfer of water and nanofluid outside the surface with higher roughness and different wettability

In order to investigate the effect of surface wettability on the pool boiling heat transfer, nucleate pool boiling experiments were conducted with deionized water and silica based nanofluid. A higher surface roughness value in the range of 3.9 ~ 6.0μm was tested. The contact angle was from 4.7° to 153°, and heat flux was from 30kW/m² to 300kW/m². Experimental results showed that hydrophilicity diminish the boiling heat transfer of silica nanofluid on the surfaces with higher roughness. As the increment of nanofluid mass concentration from 0.025% to 0.1%, a further reduction of heat transfer coefficient was observed. For the super hydrophobic surface with higher roughness (contact angle 153.0°), boiling heat transfer was enhanced at heat flux less than 93 kW/m², and then the heat transfer degraded at higher heat flux.     

Heat Transfer in a Loop Heat Pipe using Diamond-H2O Nanofluid

The main aim of this study is to enhance the thermal performance of loop heat pipe (LHP) charged with nanofluid as the working fluid. Thus, experiments are conducted to investigate heat transfer characteristics of using diamond-H₂O nanofluid with nanoparticle mass concentration ranged from 0% to 3% in a LHP as a working medium for heat input range from 20 W to 60 W. The three-dimensional model, laminar flow and heat transfer governing equations are solved using the finite volume method. The simulations are carried out with three-dimensional model based on the characterization of the working fluid inside the LHP to give an insight into the heat transfer and fluid flow mechanism. The LHP performance is evaluated in terms of temperature distributions and total thermal resistance of LHP. It is inferred that the temperatures obtained at all points in evaporator side of LHP charged with diamond-H₂O nanofluid are lower and reach their steady state faster than LHP charged with pure water. At the constant heat input, test results showed the average decrease of 5.7%?10.8% at nanoparticle mass concentrations ranging from 0.5% to 3% in R_th of LHP as compared with pure water (0%).     

The effects of operation parameter on the performance of a solar-powered adsorption chiller

A solar-powered adsorption chiller with heat and mass recovery cycle was designed and constructed. It consists of a solar water heating unit, a silica gel-water adsorption chiller, a cooling tower and a fan coil unit. The adsorption chiller includes two identical adsorption units and a second stage evaporator with methanol working fluid. The effects of operation parameter on system performance were tested successfully. Test results indicated that the COP (coefficient of performance) and cooling power of the solar-powered adsorption chiller could be improved greatly by optimizing the key operation parameters, such as solar hot water temperature, heating/cooling time, mass recovery time, and chilled water temperature. Under the climatic conditions of daily solar radiation being about 16–21 MJ/m², this solar-powered adsorption chiller can produce a cooling capacity about 66–90 W per m² collector area, its daily solar cooling COP is about 0.1–0.13.     

Performance investigation on a thermal energy storage integrated solar collector system using nanofluid

The effect of Fe nanofluid on the performance enhancement on solar water heater integrated with thermal energy storage system is investigated experimentally. A 0.5% wt fraction of Fe nanoparticle was synthesized with the mixture of water/propylene‐glycol base fluid. The experimental implementation utilized 40‐nm‐size Fe nanoparticle, 15 ° collector tilt angle, and 1.5 kg/min mass flow rate heat‐transfer fluid circulation. The system efficiency reached 59.5% and 50.5% for with and without nanofluid. The water tank temperature was increased by 13 °C during night mode. The average water tank temperature at night mode was 47.5 °C, while the average ambient temperature was 26 °C. The Fe nanofluid improved the system working duration during night mode by an average of 5 h. The techno‐economic analysis results showed a yearly estimated cost savings of 28.5% using the Fe nanofluids as heat transfer fluid. The embodied energy emission rate, collector size, and weight can be reduced by 9.5% using nanofluids. Copyright © 2016 John Wiley & Sons, Ltd.     

A Feasibility Study About Using SiO2 Nanofluid Screen Mesh Wick Heat Pipe for Cooling of High-Power LEDs

Effective and timely heat removal from high-power light-emitting diodes (LEDs) is crucial to their performance and lifetime. The strategy of using a screen mesh wick heat pipe with SiO₂ nanofluid as the working fluid for LED heat dissipation is comprehensively evaluated. An experimental system is set up to study the heat transfer performance of the heat pipe. The obtained experimental results give optimal conditions/parameters for the heat pipe: 60% charging ratio, 30° incline angle, and 1wt% concentration of the nanofluid. Compared with a heat pipe using the secondary distilled water as the working fluid, the thermal resistance of the heat pipe using the SiO₂ nanofluid as the working fluid is generally reduced by around 35–40% for the investigated heat load range of 1–60 W. Based on an equivalent heat conductivity of the SiO₂ nanofluid heat pipe derived from the experimental results, an Icepak modeling effort for the cooling system of a 60-W LED lamp is then expended. The numerical results show that the temperature of the LED lamp remains low and quite uniform across the LED chip region, indicating the technical feasibility of using this class of heat pipes for cooling of high-power LEDs.     

Effects of working fluids on the performance of a bi-directional thermodiode for solar energy utilization in buildings

A series of experiments were conducted to investigate the effects of different working fluids on the behavior and performance of a bi-directional thermodiode. The thermodiode was made up of two rectangular loops mounted between a collector plate and a radiator plate. The loops were filled with a working fluid for effective heat transfer when the thermodiode was forward biased. Five different working fluids were tested with thermal conductivity values ranging from 0.1 to 0.607 W/m-K, thermal expansion coefficient values ranging from 2.54 × 10⁻⁴ to 1.43 × 10⁻³ 1/K, and kinematic viscosity values ranging from 6.5 × 10⁻⁷ to 1 × 10⁻⁴ m²/s. The thermodiode was heated by a radiant heater consisting of 88 halogen lamps that generated a heat flux of about 10³ W/m² on the collector surface. Experimental results indicated that the onset time for natural convection to be induced throughout the diode system did not differ considerably when different working fluids were used. On the other hand the required fluid temperature differences in the loops for the onset of throughflow were quite different and depended strongly upon the viscosity and other properties such as thermal expansion coefficient and specific heat of the working fluid. Of the five fluids tested, water and low-viscosity silicon oil had the highest heat transfer rate. An analytical model was developed to predict and analyze the steady operation of the diode system when different working fluids are used.     

Multiwalled Carbon Nanotube Nanofluid for Thermal Management of High Heat Generating Computer Processor

Minichannel heat sink geometries with varying fin spacing were tested with de‐ionized water and MWCNT (1 wt %) nanofluid to evaluate their performance with flow components of a liquid cooling kit. Four heat sinks with fin spacing of 0.2 mm, 0.5 mm, 1.0 mm, and 1.5 mm were used in this investigation. Heat sink base temperature was analogous to processor operating temperature which was the prime parameter of interest in this investigation. The base temperature decreased by reducing the fin spacing and using multiwalled carbon nanotube (MWCNT) nanofluid. The lowest value of heat sink base temperature recorded was 49.7 ^°C at a heater power of 255 W by using a heat sink of 0.2 mm fin spacing and MWCNT nanofluid as a coolant. Moreover, as a result of reduced fin spacing and using MWCNT nanofluid as a coolant the value of overall heat transfer coefficient increased from 1200 W/m²K to 1498 W/m²K, translating to about a 15% increase. The value of thermal resistance also dropped by reducing the fin spacing and using MWCNT nanofluid. The most important aspect of the study is that the heat sinks and MWCNT nanofluid proved to be compatible with the pump and radiator of the commercial CPU liquid cooling kit. The pump was capable to handle the pressure drop which resulted by reducing the heat sink fin spacing and by using MWCNT nanofluid. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res, 43(7): 653–666, 2014; Published online 11 November 2013 in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21107     

Cooling cogeneration cycle

The existed combined power and cooling cycle operates with ammonia–water mixture as working fluid having low cooling due to the vapor at the inlet of evaporator. It also demands high ammonia concentration at turbine inlet to get cooling and suitable only at low sink temperature (10–12°C). A new cooling cogeneration cycle has been proposed and solved to generate more cooling with adequate power generation from single source of heat with two options in working fluids i.e. ammonia–water mixture and LiBr–water mixture. The results show that an increase in cycle maximum temperature is only supporting the power but not the cooling. A suitable range for separator temperature has been developed and optimized to maximize the total output. From this study, the resulted specific power, specific cooling, cycle power efficiency, cycle coefficient of performance (COP) and cycle energy utilization factor (EUF), plant EUF, and specific area of solar collector are 0.008 kW/m², 0.11 kW/m², 2%, 0.28, 0.3, 0.13 and 8 m²/kW for ammonia–water cycle and 0.04 kW/m², 0.3 kW/m², 9.5%, 0.7, 0.8, 0.37 and 3 m²/kW for LiBr–water mixture plant respectively.     

Numerical and Optical Analysis of Solar Power Level Adaptable Solar Reactor

Solar energy is an abundant renewable energy resource that can be used to provide high process heat necessary to run thermochemical processes for production of various solar fuels and commodities. In a solar reactor, sunlight is concentrated into a receiver through a small opening called the aperture. However, obtaining and maintaining semiconstant high temperatures inside a solar reactor is a challenge. This is because the incident solar radiation can fluctuate depending on the position of the sun and the weather conditions. For fixed aperture size reactors, changes in incident solar flux directly affect the temperature inside the reactor. This paper presents a novel solar reactor with variable aperture mechanism that is designed and manufactured at our lab. Radiation heat transfer analysis of this reactor concept is studied via Monte Carlo (MC) ray tracing. MC ray tracing module is coupled to a steady-state one-dimensional energy equation solver. Energy equation is solved for the wall and gas, accounting for the absorption, emission, and convection. Incoming direct flux values for a typical day are obtained from National Renewable Energy Lab database. Results show that for a perfectly insulated reactor, the average temperature of the working fluid may be kept appreciably constant throughout the day if aperture diameter is varied between 3 cm and 1.5 cm for incoming fluxes starting with 400 W/m² at 05:12 a.m., reaching peak value of 981 W/m² at noon, and eventually receiving 400 W/m² at 6:58 p.m., which can make the solar reactor run about 13 hr continuously at 1500 K semiconstant temperature.     

An improved dynamic test method for solar collectors

A comprehensive improvement of the mathematical model for the so called transfer function method is presented in this study. This improved transfer function method can estimate the traditional solar collector parameters such as zero loss coefficient and heat loss coefficient. Two new collector parameters t and m_fC_f are obtained. t is a time scale parameter which can indicate the heat transfer ability of the solar collector. m_fC_f can be used to calculate the fluid volume content in the solar collector or to validate the regression process by comparing it to the physical fluid volume content if known. Experiments were carried out under dynamic test conditions and then test data were processed using multi-linear regression method to get collector parameters with statistic analysis. A comparison of the collector parameters obtained from the improved transfer function (ITF) method and the quasi-dynamic test (QDT) method is carried out. The results show that the improved transfer function method can accurately obtain reasonable collector parameters. The influence of different averaging time intervals is investigated. Based on the investigation it is recommended to use on line calculation if applicable for the second-order differential term with 6–9 min as the best averaging time interval. The measured and predicted collector power output of the solar collector are compared during a test of 13 days continuously both for the ITF method and the QDT method. The maximum and averaging error is 53.87 W/m² and 5.22 W/m² respectively of the ITF method while 64.13 W/m² and 6.22 W/m² of the QDT method. Scatter and relative error distribution of the measured power output versus the predicted power output is also plotted for the two methods. No matter in either error analysis or scatter distribution, the ITF method is more accurate than the QDT method in predicting the power output of a solar collector.In conclusion, all the results show that the improved transfer function method can accurately and robustly estimate solar collector parameters and predict solar collector thermal performance under dynamic test conditions.     

Thermal Performance Enhancement of L-Shaped Microgrooved Heat Pipe Containing Water-Based Al2O3 Nanofluids

An experimental study was performed to investigate the thermal performance of an L-shaped grooved heat pipe with cylindrical cross section, which contained 0.5 wt% water-based Al₂O₃ nanofluid as the working fluid. The transient performance of the heat pipe and the effect of cooling water temperature on the heat transfer characteristics of the heat pipe were investigated. The outer diameter and the length of the heat pipe were 6 mm and 220 mm, respectively. Experimental results revealed that the temperature of the cooling water has a significant effect on the thermal resistance of the heat pipe containing nanofluids as its working fluid. By increasing the cooling water temperature from 5°C to 27.5°C, the thermal resistance decreases by approximately 40%. At the same charge volume, test results indicated an average reduction of 30% in thermal resistance of heat pipes with nanofluid as compared with heat pipe containing pure water. For transient conditions, unsteady state time for nanofluids was reduced by approximately 28%, when compared with water as the working fluid.     

Experimental performance investigation of modified cavity receiver with fuzzy focal solar dish concentrator

In this paper, thermal performance analysis of 20 m² prototype fuzzy focal solar dish collector is presented. The focal image characteristics of the solar dish are determined to propose the suitable design of absorber/receiver. First, theoretical thermal performance analysis of the fuzzy focal solar parabolic dish concentrator with modified cavity receiver is carried out for different operating conditions. Based on the theoretical performance analysis, the total heat loss (conduction, convection and radiation heat losses) from the modified cavity receiver is estimated. It is observed that the maximum theoretical efficiencies of solar dish collector are found to be as 79.2% for no wind conditions and 78.2% and 77.8% for side-on and head-on winds speed of 5 m/s respectively. Latter, real time analysis of parabolic dish collector with modified cavity receiver is carried out in terms of stagnation test, time constant test and daily performance test. From stagnation test, the overall heat loss coefficient is found to be 356 W/m² K. The time constant test is carried out to determine the influence of sudden change in solar radiation at steady state conditions. The daily performance tests are conducted for different flow rates. It is found that the efficiency of the collector increases with the increase of volume flow rates. The average thermal efficiencies of the parabolic dish collector for the volume flow rate of 100 L/h and 250 L/h are found to be 69% and 74% for the average beam radiation (I_bn) of 532 W/m² and 641 W/m² respectively.     

Effect of light scattering on the performance of a direct absorption solar collector

Recently, a solar thermal collector often employs nanoparticle suspension to absorb the solar radiation directly by a working fluid as well as to enhance its thermal performance. The collector efficiency of a direct absorption solar collector (DASC) is very sensitive to optical properties of the working fluid, such as absorption and scattering coefficients. Most of the existing studies have neglected particle scattering by assuming that the size of nanoparticle suspension is much smaller than the wavelength of solar radiation (i.e., Rayleigh scattering is applicable). If the nanoparticle suspension is made of metal, however, the scattering cross-section of metallic nanoparticles could be comparable to their absorption cross-section depending on the particle size, especially when the localized surface plasmon (LSP) is excited. Therefore, for the DASC utilizing a plasmonic nanofluid supporting the LSP, light scattering from metallic particle suspension must be taken into account in the thermal analysis. The present study investigates the scattering effect on the thermal performance of the DASC employing plasmonic nanofluid as a working fluid. In the analysis, the Monte Carlo method is employed to numerically solve the radiative transfer equation considering the volume scattering inside the nanofluid. It is found that the light scattering can improve the collector performance if the scattering coefficient of nanofluid is carefully engineered depending on its value of the absorption coefficient.     

An experimental investigation on the effect of Al2O3-H2O nanofluid on the efficiency of flat-plate solar collectors

In the present work the effect of Al₂O₃-water nanofluid, as working fluid, on the efficiency of a flat-plate solar collector was investigated experimentally. The weight fraction of nanoparticles was 0.2% and 0.4% and the particles dimension was 15 nm. Experiments were performed with and without Triton X-100 as surfactant. The mass flow rate of nanofluid varied from 1 to 3 Lit/min. The ASHRAE standard was used to calculate the efficiency. The results show that, in comparison with water as absorption medium using the nanofluids as working fluid increase the efficiency. For 0.2 wt% the increased efficiency was 28.3%. From the results it can be concluded that the surfactant causes an enhancement in heat transfer.     

Evaluation of the effect of nanofluid-based absorbers on direct solar collector

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