A solar collector is a device which helps to harnesses solar radiation into useful form of energy. Among various solar collectors, the parabolic trough collector (PTC) is considered to be the best ...option for medium temperature (150–400°C) heat requirements. The popularity of solar parabolic trough technology has generated interest in higher efficiency energy recovery potential. The absorber tube, also called the receiver tube or heat collection element, is one of the main functional units of a solar PTC in addition to other elements like parabolic mirrors, metal support structure and tracking unit assembly. Parabolic mirrors reflect to a focal point and concentrate falling sun rays onto an absorber tube, which is made up of a long metallic structure covered by an evacuated glass envelope to reduce convective heat transfer losses. Many techniques were attempted to enhance the heat transfer potential in the receiver tube portion of a solar PTC which includes techniques such as half insulation receiver, cavity receivers, vacuum outer shell, inclusion of inserts, baffles, artificially roughened sinks, selective coatings etc. In some of the research works, nanoparticles were also used to enhance heat transfer properties of the heat transfer fluid. In this paper, detailed review of the experimental and numerical works carried out on heat transfer enhancement techniques which focus on minimization of heat loss, use of turbulators, addition of nanofluid and selective coatings in the receiver tube of a solar PTC are presented. Further the major reasons for heat loss in the receiver tube and comparative study of various heat transfer enhancement techniques are summarised.
Water scarcity has resulted in a pressing need to develop an inexpensive and efficient desalination technology for fresh water production, especially in developing countries. Apart from other ...effective desalination technologies, humidification-dehumidification desalination process is predominantly adopted in the past decade to meet water deficiency due to its varied advantages in system construction, maintenance and operation. In this paper, a comprehensive review of the various advancements carried out experimentally for improving the performance of the solar humidification-dehumidification desalination system has been presented. The major factors contributing for enhanced system performance such as mass flow rate of heat transfer fluid, forced convection mechanism, multi-pass technique, recirculation mechanism, use of nano heat transfer fluid and phase change material are discussed. Further, the effect of solar intensity, ambient temperature, wind velocity, stage-wise heating and cooling on freshwater productivity is also reported. It was found that easy construction and maintenance of flat plate collector and higher performance index of evacuated tube collector make them as the predominant technologies adopted for heating the water and air heat transfer fluid's in solar humidification-dehumidification desalination system respectively. Further, it is identified that there exists a large potential for improving the fresh water yield through hybrid systems which involve use of geothermal or biomass energy, integration of solar stills with humidification-dehumidification desalination system, coupling of solar still, Fresnel lens solar collector and parabolic trough collector respectively with the flat plate collector and evacuated tube collector systems. The scope for further improvement in individual system and process parameters have been discussed.
•Specified solar technologies adopted for HDH desalination are studied.•Effect of water and air preheating are examined.•Significant system factors for performance improvement are identified.•Scope for system design improvement has been provided.
Fresh leaves used for culinary and medicinal purposes are highly perishable by nature. Quite often, post-harvest losses occur and there is noticeable deterioration in quality due to moisture enhanced ...enzymatic and microbial activity, climate changes, improper handling, delayed transportation, improper storage and delay in sales. To preserve leaves for a longer duration and to ensure their easy availability for off-seasonal use without considerable deterioration in nutrient levels, an appropriate drying method is essential for the removal of moisture to a safe activity value. Low moisture content of products made from dried leaves helps improve their shelf life, reduce shipping weight and minimize the transportation cost. A controlled heat treatment process is employed for the removal of the required water content from the leaves. An optimized drying process is necessary not only for the preservation of leaves to achieve concentrated nutrients, but also to minimize the energy consumption to make it eco-friendly. The optimized process of drying leaves is to ensure desired final moisture content retaining the original high level of nutrients as that of fresh leaves. The selection parameters for the drying technique of individual leaves is based on local climatic conditions, drying air temperature, relative humidity of air, drying time, size, shape and age of leaves, etc. The present review work explores the influencing parameters on water loss in leaves, drying kinetics, various available drying methods, range of operating conditions, and the effect of different drying methods on nutritional properties. This research paper highlights the best fit thin-layer models employed for drying of different leaves. The major challenges faced by the drying industry such as energy conservation while drying, emission reduction and hot spots for possible future research are also reviewed in this paper.
•Natural drying methods have lesser control over rate of drying and the quality of dried product. Mechanical dryers and active dryers have higher rate of drying which can also be controlled to meet quality requirements/standards.•In case of leaf drying, as the major drying time involves the removal of internal moisture, falling rate is the predominant drying mechanism involved.•The energy required for drying increases with humidity in the air. In natural drying, dry climate results in faster drying and less energy for drying process. In humid locations, increase in temperature of air is required to decrease the relative humidity.•Higher inlet dry bulb temperature, lesser relative humidity and optimum air flow rate results in higher drying rate. Operating conditions such as moderate air temperature (40-60 °C), lower relative humidity, and higher air velocity play vital role in ensuring an efficient drying process. At these optimum conditions, minimum loss of nutrients, desirable final moisture content, and maximum possible exergetic efficiency are observed.•Various mathematical models have been proposed by various researchers for drying of leaves. The Page, and Midilli and Kucuk models give the best fit in describing the drying behavior of many leaves.
The continuous technological development in the field of solar thermal application is a pressing need to harvest and utilize the solar energy in an efficient manner. Development of a flat plate ...cooking unit (FPCU) intended for the indirect mode of solar cooking has been attempted. In this paper, the performance of the FPCU integrated with the latent heat thermal energy storage system is studied experimentally in order to replace the conventional mode of biomass or LPG based cooking. The heat transfer fluid (HTF), Therminol 55 and the phase change material (PCM), D Mannitol is used in the experimental set-up. The food medium energy utilization, instantaneous heat transfer rate and detailed energy balance for the cooking unit integrated with storage tank is reported. In addition, computational fluid dynamic (CFD) analysis using ANSYS Fluent software is also performed on the newly developed FPCU to analyze the cooking period heat transfer behavior. The results of CFD analysis is validated with experimental results and the results show that the average heat transfer coefficient during the cooking process is found to be approximately 100 W/m2K.
•TES system assisted solar flat plate cooking unit performance is carried out.•Energy balance analysis and various heat losses incurred are studied in detail.•Results of cooking unit CFD analysis are validated with the experimental results.
The enhancement in heat transfer characteristics of a copper sintered wick heat pipe with surfactant free CuO nanoparticles dispersed in DI water is experimentally studied. The effect of heat input, ...tilt angle and weight fractions of nanoparticles on the heat pipe thermal resistance, heat transfer coefficient in evaporator and condenser sections, thermal conductivity and thermal efficiency are investigated. The experimental results are evaluated for the vapor temperatures directly measured at the center core of heat pipe. Interestingly, a temperature difference of 5.1°C is observed between the heat pipe surface and the vapor core in the evaporator section. Results showed a reduction in the thermal resistance of 66.1% and enhancement in the heat transfer coefficient and thermal conductivity of 29.4% and 63.5% is respectively, observed for 1.0wt.% of CuO/DI water nanofluid at 45° tilt angle compared with heat pipe kept at horizontal position. Similarly, the thermal efficiency is also improved by 24.9% for the same tilt angle and weight fraction of CuO nanofluid. The use of nanoparticles and tilt angle enhances the operating range and thermal performance of heat pipe when compared with that of the heat pipe with DI water.
The effect of varying wick structures viz. mesh, sintered and composite wick (sintered-mesh) on the thermal enhancement of cylindrical heat pipes is experimentally investigated. In addition, the ...investigation focused on the effect of inclination angle and heat input of heat pipe. Surfactant free CuO nano-fluid with a mass concentration of 1.0% is used as a working fluid. The energy and exergy analysis of heat pipe was also conducted at various conditions. To analyze the distinctive performance of composite heat pipe, a heat pipe is filled with DI water and the obtained results are compared with nanofluid results. The maximum heat transfer capability of composite heat pipe is improved by 35.71% and 18.75% compared with mesh and sintered wicks. The composite heat pipe with CuO nanofluid as working fluid instead of DI water improves the heat transport capacity by 11.76%. Surface temperature of heat pipe significantly reduces by varying the wick structure viz. mesh, sintered and composite wick. The composite heat pipe with 1.0 mass% of CuO nanofluid obtained 3.7 °C reduction in surface temperature at evaporator section compared with DI water. Thermal resistance of heat pipe is gradually reduces with increasing inclination angle. The maximum reduction is observed for composite wick, sintered and mesh wick heat pipes are 47.50, 43.70 and 24.39% respectively at 45° inclination angle compared with horizontal axis.
•Powder mixing and sol–gel method is used for nanofuel preparation.•Performance of diesel engine using nanofuel additive is carried out.•Significant decrease in pollutants emission is found.
The ...emission norms on internal combustion engine are becoming more and more stringent with a view to improve the ambient air quality. Emissions of Particulate Matter and Oxides of Nitrogen are the focus of today’s diesel emission control technologies. Nanometal oxide additives are reported to be effective in lowering diesel emissions. The diesel fuel formulation along with engine development can achieve clean combustion of the diesel fuel. The study is based on the effect of nanometal oxide additives like manganese oxide and copper oxide to diesel fuel. Sol–gel method was adopted for preparation for nanometal oxide additives. Metal oxide additive are doped with diesel fuel. Scanning Electron Microscope (SEM) images confirmed the presence of nanoparticle in the metal oxide additive. The changes in diesel fuel properties (viscosity, flash point and fire point) due to introduction of nanometal oxide additive were observed. The diesel fuel with nanometal oxide additive presented a marginal increase in performance. Exhaust emissions for the diesel fuel with nanometal oxide additive showed significant decrease in levels of pollutants emission.
The present research of the article is used to describe the microwave sintering of nitinol composite and its surface topography analysis by atomic microscopy (AFM). Scanning electron microscopy (SEM) ...and energy dispersive spectroscopy (EDS) technique is used to analyze the characterization and alloying composition of the composite. Silicon carbide is the reinforcing particle between nickel and titanium. Wear test is conducted to investigate the worn surface morphology of the sintered specimen. The synthesized composite has exceptional material properties such as ultimate tensile strength of 1020MPa, 16% of elongation to fracture, 7.51 g/cc of density and hardness of 310 HV. Low wear rate of 0.01 mg/m is perceived at minimum load and minimum sintering time. Surface roughness is varied from 22.87 to 54.05nm at different sintering time. Maximum roughness height of 327.6 nm is observed in the surface profile of the composite.
•Performance of solar PV system combined with free cool storage system is studied.•Instantaneous and cumulative energy stored in the storage tank are evaluated.•Cooling potential during the ...operational hours of the solar PV panel is studied.
The increase in operating temperature of the solar photovoltaic (PV) module results in loss of conversion efficiency. Active or passive based cooling methods are commonly used to remove heat and thus the performance of PV module is enhanced. In this paper, cooling of solar PV module is achieved with the help of free cool thermal energy stored in the phase change based storage system in the night and early morning hours. The performance of the solar PV system combined with free cool thermal energy storage system containing encapsulated phase change materials is studied. The free cool energy is stored when the ambient air available during early morning hours is in the range of 20–25°C and allowed to flow over the encapsulated phase change materials (PCMs). The HS29 PCM used as PCM in the storage tank stores the cool energy at a temperature range of 29–30 °C and the same is supplied through the bottom of the PV panel during day time hours to keep the panel at a lower temperature. The instantaneous energy and cumulative energy stored in the storage tank during charging process and the cooling potential during the operational hours of the solar panel with different mass flow rates of air is studied and the results are presented.
The role of method of synthesis on the size of flakes, dispersion stability and thermophysical properties of aqua based reduced graphene oxide (rGO) nanofluids is investigated in this study. It is ...prepared by two different synthesis methods, namely hydrazine reduced graphene oxide (rGO/H) and sodium borohydride reduced graphene oxide (rGO/S). The size of rGO/H and rGO/S flakes is found as ~ 0.1 and ~ 0.2 μm, respectively. A set of 10, 100 and 300 mg L
−1
concentrations of rGO/H and rGO/S nanofluids are prepared using DI water, and the stability of rGO/H nanofluids is much better than rGO/S nanofluids due to the smaller size of flakes and the existence of –OH and –COOH groups. An enhancement in thermal conductivity of about 15.8% is found for 300 mg L
−1
concentrations of rGO/H nanofluid at 75 °C. The viscosity of rGO/H nanofluids shows a negligible difference with DI water when the temperature is above 50 °C, whereas the viscosity increases with the concentrations of rGO/S nanofluids and always exhibits a higher value than base fluid.
