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Accepted Manuscripts

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research-article  
Apurv Kumar, Jin-Soo Kim and Wojciech Lipinski
J. Sol. Energy Eng   doi: 10.1115/1.4040290
Radiation absorption is investigated in a particle curtain formed in a solar free-falling particle receiver. An Eulerian-Eulerian granular two-phase model is used to solve the two-dimensional mass and momentum equations by employing computational fluid dynamics (CFD) to find particle distribution in the curtain. The radiative transfer equation is subsequently solved by the Monte-Carlo (MC) ray-tracing technique to obtain the radiation intensity distribution in the particle curtain. The predicted opacity is validated with the experimental results reported in the literature for 280 and 697 µm sintered bauxite particles. The particle curtain is found to absorb the solar radiation most efficiently at flowrates upper-bounded at approx. 20 kg s-1 m-1. In comparison, 280 µm particles have higher average absorptance than 697 µm particles (due to higher radiation extinction characteristics) at similar particle flowrates. However, as the absorption of solar radiation becomes more efficient, non-uniform radiation absorption across the particle curtain and hydrodynamic instability in the receiver are more probable.
TOPICS: Radiation (Physics), Particulate matter, Absorption, Irradiation (Radiation exposure), Solar energy, Computational fluid dynamics, Solar radiation, Ray tracing, Hydrodynamic stability, Momentum, Radiative heat transfer
research-article  
Zhang Guojun, Liu Changyu and Li Dong
J. Sol. Energy Eng   doi: 10.1115/1.4040291
Conjugate laminar natural convection heat transfer and air flow with radiation of tube solar receiver with glass window was numerically investigated. The discrete ordinates method was used to solve the radiative transfer equation. And the three dimensional steady-state continuity, Navier-Stokes and energy equations were solved. The temperature difference based on environment and high temperature surface of receiver is varied from 100K to 1000K. The influence of the surface emissivity, heating temperature, convective coefficient and convective temperature of environment on the heat transfer from the receiver with glass window has also been investigated. The numerical results indicated that the highest temperature of glass window increases and the high temperature area becomes wide, with the temperature of heating wall and surface emissivity increasing. Adopting higher convective coefficient of glass window can reduce the peak magnitude of temperature distribution on glass window of tube receiver up to 45%.
TOPICS: Glass, Radiation (Physics), Natural convection, Numerical analysis, Solar energy, Temperature, Heat transfer, Heating, High temperature, Emissivity, Radiative heat transfer, Air flow, Steady state, Temperature distribution
research-article  
Moucun Yang, Yuezhao Zhu, Wei Fu, Garth Pearce and Robert A. Taylor
J. Sol. Energy Eng   doi: 10.1115/1.4040273
The design and construction of solar concentrators heavily affects their cost, heat utilization and optical efficiency. Current trough concentrators use an equivalent uniform beam with a metal grid sub-structure. In this conventional design, there is surplus stiffness and strength, which unnecessarily increases the overall weight and cost of the structure. This paper describes a variable cross-section structural optimization approach (with the EuroTrough design, including safety factors, taken as an example) to overcome this issue. The main improvement of this design comes from keeping the beams rigid and strong near the two ends (at the torque box structure) while allowing the middle of the structure to be relatively weak. Reducing the cross-sectional area of the central beams not only reduces the amount of material needed for the structure but also reduces the deflection of the reflector. In addition, a new connection structure between two neighboring concentrator elements was designed to reinforce the structure. The simulated results show that the concentrator's structural weight (including the torque box, endplates, and cantilever arms) are reduced by 13.5% (i.e. about 133kg per 12-metre long element). This represents a meaningful capital and installation cost savings while at the same time improving the optical efficiency.
TOPICS: Cross section (Physics), Structural optimization, Solar energy concentrators, Design, Weight (Mass), Torque, Heat, Metals, Safety, Construction, Cantilevers, Deflection, Stiffness
research-article  
Himanshu Sainthiya, Narendra Beniwal and Navneet Garg
J. Sol. Energy Eng   doi: 10.1115/1.4040238
Photovoltaic (PV) cells exhibit long term degradation, when its temperature exceeds a certain limit. On the other hand, decreasing the temperature results in lower PV cell efficiency. The aim of this paper is to demonstrate the improvements in the output power and efficiency of PV modules using a cooling system based on flowing water on the front surface. Front surface cooling method with the help of a water pumping system is one of the most promising methods for cooling the PV cells. With poly-crystalline PV cells, different water flow rates are experimented, and the output power and the efficiency are computed for different weather conditions. These experiments yield that the cell efficiency is improved by approximately 27.3% in winter conditions and 27.6% in summer conditions.
TOPICS: Cooling, Water, Temperature, Flow (Dynamics), Cooling systems
research-article  
Iqra Ayub, Anjum Munir, Abdul Ghafoor, Waseem Amjad and Muhammad Salman Nasir
J. Sol. Energy Eng   doi: 10.1115/1.4040206
Baking is an energy intensive unit operation. The thermal application of solar energy is getting attention in food processes by eliminating the facts of interrupted supply and fluctuated costs of nonrenewable energy sources. This study has been carried out for the design and development of solar bakery unit which comprises of a 10 m2 Scheffler reflector focusing all the beam radiations on a secondary reflector that further concentrate the beam radiations towards the heat receiver of solar bakery unit to heat up the air circulated through baking chamber employing a PV operated fan. CFD based 3D simulation was performed to analyze the design for uniform air and heat distribution in the baking chamber. The system designed configurations gave quite good results for airflow distribution. The receiver temperature reached between 300-400°C while temperature at the inlet of baking chamber was in the range of 200-230oC, sufficient for most of the products to be baked. The maximum available solar power at receiver was calculated to be 3.46 kW having an average efficiency of 63%. Series of experiments were conducted for the baking of cakes and total energy available in baking chamber was about 3.29kW and cake utilized 0.201kW energy to be baked. The average value of energy utilization ratio was found to be 45%. As a base, the study would lead to the development of an appropriate and low cost solar baking units for the maximum retention of quality parameters and energy saving.
TOPICS: Solar energy, Food products, Heat, Temperature, Design, Air flow, Simulation, Computational fluid dynamics, Fossil fuels, Solar power
Technical Brief  
Paolo Sdringola, Stefania Proietti, Davide Astolfi and Francesco Castellani
J. Sol. Energy Eng   doi: 10.1115/1.4040196
The 2012 European Energy Efficiency Directive supported the development of cogeneration (CHP) and district heating and cooling (DHC) networks, stressing the benefits due to a more efficient energy supply, the exploitation of recovered heat, and renewable resources, in terms of fuel consumption and avoided costs/emissions. Policy decisions play a crucial role: technical and environmental feasibility of CHP is clear and well demonstrated, whereas economic issues (fuel prices, incentives, etc.) may influence its actual application. An increase from 10% to a potential 24% in the major economies is expected for the CHP share of electricity by 2030. In this framework, the introduction of low-carbon technologies and the exploitation of renewable energies are profitable interventions to be applied on existing plants. This work focuses on a small CHP plant, installed in the 90s and located within a research facility in Italy, supplying electricity and heat/cool through a district network. On the basis of monitored consumption of electricity, heating and cooling, energy fluxes have been assessed and an optimization process was performed to get a management profile enhancing both operational and economic parameters. The integration of renewable energies, i.e. solar-powered systems for supporting the existing devices, has been evaluated thus resulting in a hybrid tri-generation plant. CHP and DHC are confirmed as useful tools to match heating/cooling and electricity demands, to integrate distributed renewable generation, to enhance fuel diversity, and so to face the challenge of climate change towards sustainable energy networks in the future.
TOPICS: Optimization, Renewable energy, Cogeneration plants, Combined heat and power, Heat, Cooling, Fuels, Flux (Metallurgy), Energy efficiency, Carbon, Heating and cooling, Central heating, Energy sustainability, Solar power, Heating, Fuel consumption, Climate change, Emissions
Technical Brief  
Marek Lewkowicz, Sameh AlSaqoor, Ali Alahmer and Gabriel Borowski
J. Sol. Energy Eng   doi: 10.1115/1.4040197
Selected properties of transparent insulations made of a layer of parallel, small-diameter, thin-walled, visible light transparent pipes placed perpendicularly to the surface of a ?at solar collector were investigated theoretically. A formula for the balance radiation heat losses through the insulation was derived based on the two main assumptions: the system is in steady state and the fourth power of the temperature along each pipe is linear. This formula, combined with standard formulas for the conductive heat ?ux, enables to predict that under typical insolation of 1200 W/m2, a 10 cm thick transparent insulation could lead to the temperature of the absorber reaching 484 °C and could give 530 W mechanical power under the assumption that the heat is transformed into mechanical power according to the ideal Carnot cycle. Moreover, the results showed that higher values of the aspect ratio are needed for the satisfactory energy conversion e?ciency.
TOPICS: Modeling, Optimization, Thermal insulation, Transparency, Insulation, Pipes, Heat, Temperature, Light, Radiation (Physics), Visible spectrum, Energy conversion, Solar collectors, Heat losses, Steady state, Carnot cycle
research-article  
Hamid Khakpour Nejadkhaki and John Hall
J. Sol. Energy Eng   doi: 10.1115/1.4040104
A modeling framework to analyze a wind turbine blade subjected to an out-of-plane transformation is presented. The framework combines aerodynamic and mechanical models to support an automated design process. The former combines the NREL AeroDyn software with a genetic algorithm solver. It defines the theoretical twist angle distribution (TAD) as a function of wind speed. The procedure is repeated for a series of points that form a discrete range of wind speeds. This step establishes the full range of blade transformations. The associated theoretical TAD geometry is subsequently passed to the mechanical model. It creates the TAD geometry in the context of a novel wind turbine blade concept. The blade sections are assumed to be made by additive manufacturing, which enables tunable stiffness. An optimization problem minimizes the difference between the practical and theoretical TAD over the full range of transformations. It does so by selecting the actuator locations and the torsional stiffness ratios of consecutive segments. In the final step, the blade free shape (undeformed position) is found. The model and design support out-of-plane twisting, which can increase energy production and mitigate fatigue loads. The proposed framework is demonstrated through a case study based on energy production. It employs data acquired from the NREL Unsteady Aerodynamics Experiment. A set of blade transformations required to improve the efficiency of a fixed-speed system is examined. The results show up to 3.7% and 2.9% increases in the efficiency at cut-in and rated speeds, respectively.
TOPICS: Design methodology, Modeling, Blades, Wind turbines, Design, Stiffness, Geometry, Energy generation, Wind velocity, Stress, Actuators, Aerodynamics, Fatigue, Optimization, Shapes, Computer software, Genetic algorithms, Additive manufacturing
research-article  
Eric Chekwube Okonkwo, Muhammad Abid and Tahir A.H. Ratlamwala
J. Sol. Energy Eng   doi: 10.1115/1.4040076
The parabolic trough collector (PTC) is one of the most widely deployed concentrating solar power technology in the world. This study aims at improving the operational efficiency of the commercially available LS-2 solar collector by increasing the convective heat transfer coefficient inside the receiver tube. The two main factors affecting this parameter are the properties of the working fluid and the inner geometry of the receiver tube. An investigation was carried out on six different working fluids: pressurized water, supercritical CO2, Therminol VP-1 and the addition of CuO, Fe3O4, and Al2O3 nanoparticles to Therminol VP-1. Furthermore, the influence of a converging-diverging tube with sine geometry is investigated because this geometry increases the heat transfer surface and enhances turbulent flow within the receiver. The results showed that of all the fluids investigated, the Al2O3/Oil nanofluid provides the best improvement of 0.22% to thermal efficiency while the modified geometry accounted for a 1.13% increase in efficiency. Other parameters investigated include the exergy efficiency, heat transfer coefficient, outlet temperatures and pressure drop. The analysis and modeling of a parabolic trough receiver are implemented in engineering equation solver (EES).
TOPICS: Heat transfer, Fluids, Numerical analysis, Geometry, Parabolic troughs, Water, Heat transfer coefficients, Supercritical carbon dioxide, Thermal efficiency, Nanofluids, Pressure drop, Solar collectors, Concentrating solar power, Turbulence, Magnetite, Nanoparticles, Exergy, Convection, Modeling, Temperature
research-article  
Hamid Ajdad, Yousra Filali Baba, Ahmed Al Mers, Ossama Merroun, Abdelfattah Bouatem, Noureddine Boutammachte, Soukaina El Alj and Sara Benyakhlef
J. Sol. Energy Eng   doi: 10.1115/1.4040064
A solar heating compound parabolic collector (CPC) using air and palm oil as heat carrier fluid is proposed and analysed within this study via heat transfer and ray tracing simulations. This collector is placed above a linear Fresnel solar field prototype with an area of approximately 70m2 erected at the platform of the Green Energy Park (32° 14' N 7° 57' O), southern Morocco (Benguerir). The system is a linear focusing solar system intended to be used for applications across a broad range of industrial sectors for generating medium temperature heat up to 250°C. The Monte Carlo ray tracing method was used to predict the optical performances of the receiver. We have developed a simplified thermal model to investigate and analyze the thermal performances of the receiver under different conditions. It has been demonstrated that the investigated receiver satisfactorily matches the heat demand by producing low and medium temperature heat with an annual system efficiency of 45%.
TOPICS: Heat, Temperature, Heat transfer, Fluids, Simulation, Engineering prototypes, Engineering simulation, Solar energy, Ray tracing, Renewable energy, Solar heating, System efficiency
research-article  
Xiankun Xu, Xiaoxin Wang, Peiwen Li, Yuanyuan Li, Qing Hao, Bo Xiao, Hassan Elsentriecy and Dominic Gervasio
J. Sol. Energy Eng   doi: 10.1115/1.4040065
The eutectic mixture of MgCl2-KCl molten salt is a high temperature heat transfer and thermal storage fluid able to be used at temperatures up to 800 oC in concentrating solar thermal power systems. The molten salt thermophysical properties are reported including vapor pressure, heat capacity, density, viscosity, thermal conductivity, and the corrosion behavior of nickel-based alloys in the molten salt corrosion at high temperatures. Correlations of the measured properties as functions of molten salt temperatures are presented for industrial applications. The test results of tensile strength of two nickel-based alloys exposed in the molten salt at a temperature of 800 oC from one-week length to sixteen-week length are reported. It was found that the corrosion and strength loss is rather low when the salt is first processed to remove water and oxygen.
TOPICS: Heat transfer, Fluids, Concentrating solar power, Thermal energy storage, Temperature, Corrosion, Nickel, Alloys, High temperature, Water, Oxygen, Tensile strength, Viscosity, Thermal conductivity, Heat capacity, Solar thermal power, Density, Vapor pressure
research-article  
Justin P Freedman, Hao Wang and Dr. Ravi Prasher
J. Sol. Energy Eng   doi: 10.1115/1.4039988
Solar-to-thermal energy conversion technologies are an increasingly promising segment of our renewable energy technology future. Today, concentrated solar power plants provide a method to efficiently store and distribute solar energy. Current industrial solar-to-thermal energy technologies employ selective solar absorber coatings to collect solar radiation, which suffer from low solar-to-thermal efficiencies at high temperatures due to increased thermal emission from selective absorbers. Solar absorbing nanofluids (a heat transfer fluid seeded with nanoparticles), which can be volumetrically heated, are one method to improve solar-to-thermal energy conversion at high temperatures. To date, radiative analyses of nanofluids via the radiative transfer equation have been conducted for low temperature applications and for flow conditions and geometries that are not representative of the technologies used in the field. In this work, we present the first comprehensive analysis of nanofluids for concentrated solar power plants in a parabolic trough configuration. This geometry was chosen because parabolic troughs are the most prevelant CSP technologies. We demonstrate that the solar-to-thermal energy conversion efficiency can be optimized by tuning the nanoparticle volume fraction, the temperature of the nanofluid, and the incident solar concentration. Moreover, we demonstrate that direct solar absorption receivers have a unique advantage over current surface-based solar coatings at large tube diameters. This is because of a nanofluid’s tunability, which allows for high solar-to-thermal efficiencies across all tube diameters enabling small pressure drops to pump the heat transfer fluid at large tube diameters.
TOPICS: Solar energy, Nanofluids, Parabolic troughs, Energy conversion, Nanoparticles, Concentrating solar power, Heat transfer, Fluids, Coatings, High temperature, Emissions, Absorption, Solar radiation, Radiative heat transfer, Flow (Dynamics), Temperature, Geometry, Pressure drop, Renewable energy, Low temperature, Pumps
research-article  
Francisco J. Arias and August Salvador De Las Heras
J. Sol. Energy Eng   doi: 10.1115/1.4039893
The basis of a novel method for passive solar water heating homologous to the thermosyphon but driven by induced salinity which circulates a fluid without the necessity of a mechanical pump and with inverse natural convection is outlined. The \emph{brinesyphon} operates -as its homologous thermosyphon, by harnessing the tendency of a less dense fluid to rise above a denser fluid to cause fluid motion through a collector as in a thermosyphon, but with two exceptions: buoyancy is controlled by induced salinity gradients an not by thermal gradients, and second, as result, natural convection is in the opposite direction than its homologous thermosiphon, i.e., hot fluid flows down and cold fluid rises up. A brinesyphon may be more suitable for solar domestic water-heating systems than the thermosyphon because the direction of flow allows to transport hot fluid from the roof where solar collector are placed to the bottom ( e.g., inside the house) without any kind of mechanical pumping system.
TOPICS: Pressure, Fluid dynamics, Flow (Dynamics), Buoyancy, Fluids, Hot water heating, Natural convection, Pumps, Solar collectors, Solar energy, Osmosis, Roofs, Temperature gradient, Solar water heating
research-article  
Kalameshwar Patil, Kaushik S.C. and Sat Narayan Garg
J. Sol. Energy Eng   doi: 10.1115/1.4039656
Light pipes are popularly used for transporting outdoor sunlight into deep spaces of the building and hence use of artificial lighting could be substantially reduced. Performance prediction of a light pipe is an essential step before its use in buildings, so that energy saving potential of the light pipe could be quantified. This paper deals with experimental validation of three existing semi empirical models for light pipes, with different aspect ratios, installed on a windowless test room, at IIT Delhi, New Delhi. Two new semi empirical models, based on the existing correlations are developed. The better performing new model is used for the prediction of internal illuminance, energy saving potential and CO2 mitigation potential of light pipe system for the test room. New model found to perform better with mean bias error and root mean squared error of 0.076 and 0.01 respectively. Monthly average energy saving potentials of the light pipe - fluorescent tube light system are found to be 50 % for continuous dimming control, 38% for three step on-off control. Results show that the light pipe-fluorescent tube light system, with different lighting controls, could reduce CO2 emissions to 15-50%.
TOPICS: Composite materials, Energy conservation, Pipes, Climate, Errors, Carbon dioxide, Space, Brightness (Photometry), Sunlight, Emissions, Structures
research-article  
Zakaria El Jaouhari, Youssef Zaz, Salah Moughyt, Omar El Kadmiri and Zakaria El Kadmiri
J. Sol. Energy Eng   doi: 10.1115/1.4039098
A design of solar tracker with a new tracking method based on computer vision techniques is presented in this paper. The proposed method extracts the sun coordinates (orientation ?, elevation f) in real time from hemispherical sky images using an image processing algorithm and then drives a pair of motors to move solar panels (or heliostat) toward the sun. To ensure a wide field of view, a camera equipped with a fisheye lens is used to acquire the whole sky images. The advantages of such a system are the high sensitivity to brightness comparing to the traditional photo sensors based trackers that making the system more efficient and able to determine the sun position even in cloudy days. It also operates independently of time and position which makes it reliable in case of mobile solar station, contrary to systems based on astronomical equations.
TOPICS: Design, Solar energy, Computers, Image processing, Sensors, Lenses (Optics), Motors, Brightness (Photometry), Algorithms

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