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Research Papers

J. Sol. Energy Eng. 2018;141(1):011001-011001-8. doi:10.1115/1.4039098.

A design of a solar tracker with a new tracking method based on computer vision techniques is presented in this paper. The proposed method extracts the sun position (orientation θ, elevation φ) in real time from hemispherical sky images using a processing techniques and then drives a pair of motors to move solar panels (or heliostats) toward the sun. To ensure a wide field of view, a camera equipped with a fisheye lens is used to acquire whole sky images. The advantages of such a system are the high sensitivity to brightness compared to traditional photosensors-based trackers. Thus, the system becomes more efficient and able to determine the sun position even during cloudy days. It also operates independently of time and position which makes it reliable in case of mobile solar stations, contrary to systems based on astronomical equations.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;141(1):011002-011002-12. doi:10.1115/1.4040840.

Solar harvesting designs aim to optimize energy output per unit area. When it comes to choosing between rooftop technologies for generating heat and/or electricity from the sun, though, the literature has favored qualitative arguments over quantitative comparisons. In this paper, an agnostic perspective will be used to evaluate several solar collector designs—thermal, photovoltaic (PV), and hybrid (PV/T) systems—which can result in medium temperature heat for industry rooftops. Using annual trnsys simulations in several characteristic global locations, it was found that a maximum solar contribution (for all selected locations) of 79.1% can be achieved for a sterilization process with a solar thermal (ST) system as compared to 40.6% for a PV system. A 43.2%solar contribution can be obtained with a thermally coupled PV/T, while an uncoupled PV/T beam splitting collector can achieve 84.2%. Lastly, PV and ST were compared in a side-by-side configuration, indicating that this scenario is also feasible since it provides a solar contribution of 75.2%. It was found that the location's direct normal incident (DNI) and global horizontal irradiation (GHI) are the dominant factors in determining the best technology for industrial heating applications. Overall, this paper is significant in that it introduces a comparative simulation strategy to analyze a wide variety of solar technologies for global industrial heat applications.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;141(1):011003-011003-8. doi:10.1115/1.4040842.

The accuracy of radiometric temperature measurement in radiatively heated environments is severely limited by the combined effects of intense reflected radiation and unknown, dynamically changing emissivity, which induces two correlated and variable error terms. While the recently demonstrated double modulation pyrometry (DMP) eliminates the contribution of reflected radiation, it still suffers from the shortcomings of single-waveband pyrometry: it requires knowledge of the emissivity to retrieve the true temperature from the thermal signal. Here, we demonstrate an improvement of DMP incorporating the in situ measurement of reflectance. The method is implemented at Paul Scherrer Institute (PSI) in its 50 kW high-flux solar simulator and used to measure the temperature of ceramic foams (SiSiC, ZrO2, and Al2O3) during fast heat-up. The enhancement allows DMP to determine the true temperature despite a dynamically changing emissivity and to identify well-documented signature changes in ZrO2 and Al2O3. The method also allows us to study the two dominant error sources by separately tracking the evolution of two error components during heat-up. Furthermore, we obtain measurements from a solar receiver, where the cavity reflection error limits measurement accuracy. DMP can be used as an accurate radiometric thermometer in the adverse conditions of concentrated radiation, and as a diagnostic tool to characterize materials with dynamic optical properties. Its simple design and ability to correct for both errors makes it a useful tool not only in solar simulators but also in concentrated solar facilities.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;141(1):011004-011004-9. doi:10.1115/1.4040839.

A solar heating system in greenhouse driven by Fresnel lens concentrator is built in this study. This system uses a soil thermal storage for greenhouse to supply heat in the absence of sunlight, ensuring the safety of the growth of crops. The structure and working principle of the device are introduced in this paper. The underground soil temperature was tested, compared with the indoor and outdoor temperature. The experimental testing result is given. A research shows that when the heating pipe buried 1.65 m underground, the time of heat transfer to the ground is about 5 days. The overall temperature rise of the soil is about 4 °C. In the condition of the coldest weather without additional energy supplement, the greenhouse's temperature is guaranteed above 8 °C, which can ensure the minimum temperature requirements of crop growth. According to the structural parameters of the existing system, the simulation of underground soil heat transfer and heat storage performance was carried out. Then, the temperature curves of different buried depths of the tube are given. The soil temperature steady time in different pipe-buried depths of heat storage temperature is theoretically calculated. It is proved that, to achieve the seasonal thermal storage in this system, the buried depth of the pipe should be over 2.5 m.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;141(1):011005-011005-10. doi:10.1115/1.4041097.

Advanced and model-based control techniques have become prevalent in modern wind turbine controls in the past decade. These methods are more attractive compared to the commonly used proportional-integral-derivative (PID) controller, as the turbine structural flexibility is increased with multiple and coupled modes. The disturbance accommodating control (DAC) is an effective turbine control approach for the above-rated wind speed region. DAC augments the turbine state-space model with a predefined disturbance waveform model, based on which the controller reduces the impact of wind disturbances on the system output (e.g., rotor speed). However, DAC cannot completely reject the wind disturbance in certain situations, and this results in steady-state regulation errors in the turbine rotor speed and electric power. In this paper, we propose a novel wind turbine pitch control using optimal control theory. The obtained feedback and feedforward control terms function to stabilize the turbine system and reject wind disturbances, respectively, derived systematically based on the Hamilton–Jacobi–Bellman (HJB) equation. Simulation results show that the proposed method achieves desired rotor speed regulation with significantly reduced steady-state errors under turbulent winds, which is simulated on the model of the three-bladed controls advanced research turbine (CART3) using the FAST code.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;141(1):011006-011006-13. doi:10.1115/1.4041154.

This study presents an experimental investigation on the effects of winglets on the near wake flow around the tip region and on the tip vortex characteristics downstream of a 0.94 m diameter three-bladed horizontal axis wind turbine (HAWT) rotor. Phase-locked 2D particle image velocimetry (PIV) measurements are performed with and without winglets covering 120 deg of azimuthal progression of the rotor. The impact of using winglets on the flow field near the wake boundary as well as on the tip vortex characteristics such as the vortex convection, vortex core size, and core expansion as well as the resultant induced drag on the rotor are investigated. Results show that winglets initially generate an asymmetric co-rotating vortex pair, which eventually merge together after about ten tip chords downstream to create a single but nonuniform vortex structure. Mutual induction of the initial double vortex structure causes a faster downstream convection and a radially outward motion of tip vortices compared to the baseline case. The wake boundary is shifted radially outward, velocity gradients are diffused, and vorticity and turbulent kinetic energy levels are significantly reduced across the wake boundary. The tip vortex core sizes are three times as big compared to those of the baseline case, and within the vortex core, vorticity and turbulent kinetic energy levels are reduced more than 50%. Results show consistency with various vortex core and expansion models albeit with adjusted model coefficients for the winglet case. The estimated induced drag reduction is about 15% when winglets are implemented.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;141(1):011007-011007-12. doi:10.1115/1.4041098.

The ribbed three-dimensional solar air heater (SAH) model is numerically investigated to estimate flow and heat transfer through it. The numerical analysis is based on finite volume approach, and the set of flow governing equations has been solved to determine the heat transfer and flow field through the SAH. For detailed analysis, rib chamfer height ratio (e′/e) and rib aspect ratio (e/w), two innovative parameters, have been created and considered along with the commonly used roughness parameter, i.e., relative roughness height, e/D. The parameters e′/e, e/w, and e/D are varied from 0.0 to 1, 0.1 to 1.5, and 0.18 to 0.043, respectively, but the value of P/e is kept constant for the entire investigation at 12. A good match is seen in Nusselt number (Nu) and friction factor (f) by comparing the predicted results with the experimental ones. With the variation of roughness parameters, distinguishable change in Nu and f is obtained. The highest value of thermohydraulic performance parameter (TPP) observed is 2.08 for P/e, e′/e, e/w, and e/D values of 12, 0.75, 1.5, and 0.043, respectively, at Re of 17,100. The developed generalized equation for Nu and f has shown acceptable percentage deviation under the studied range of parameters.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;141(1):011008-011008-10. doi:10.1115/1.4041099.

As per the estimates of the world health organization (WHO) by 2025, about half of the world's population shall inhabit water-stressed areas. Water purification through usage of solar energy is a clean and lucrative option to ensure access to clean and safe drinking water. In most of the solar energy-driven desalination systems, evaporation of water is one of the key processes. In this direction, we propose that addition of nanoparticles into the water (owing to their enhanced thermo-physical properties and optical tunability) could significantly enhance the evaporation rate and thus the pure water yield. In the present work, we have developed a detailed theoretically model to predict (and quantify) the evaporation rates when water/nanoparticles dispersion directly interact with solar irradiance. In order to clearly gauge the effects of adding nanoparticles, two systems have been studied (i.e., the one with and the other without nanoparticles dispersed in water) under similar operating conditions. Theoretical calculations show that addition of even trace amounts of nanoparticles (volume fraction = 0.0001) into water can significantly enhance (57–58% higher than the pure water case) the evaporation rates and the pure water yield. Furthermore, a detailed parametric study involving host of parameters influencing the evaporation rate reveals that nanoparticle volume fraction, ambient temperature, and solar irradiance are the most impacting parameters. Finally, the results of the developed theoretical model have been compared with the experimental results in the literature, the two have been found to be in good agreement except at some nanoparticle volume fractions.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;141(1):011009-011009-7. doi:10.1115/1.4041100.

This paper evaluates the on-sun performance of a 1 MW falling particle receiver. Two particle receiver designs were investigated: obstructed flow particle receiver versus free-falling particle receiver. The intent of the tests was to investigate the impact of particle mass flow rate, irradiance, and particle temperature on the particle temperature rise and thermal efficiency of the receiver for each design. Results indicate that the obstructed flow design increased the residence time of the particles in the concentrated flux, thereby increasing the particle temperature and thermal efficiency for a given mass flow rate. The obstructions, a staggered array of chevron-shaped mesh structures, also provided more stability to the falling particles, which were prone to instabilities caused by convective currents in the free-fall design. Challenges encountered during the tests included nonuniform mass flow rates, wind impacts, and oxidation/deterioration of the mesh structures. Alternative materials, designs, and methods are presented to overcome these challenges.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;141(1):011010-011010-12. doi:10.1115/1.4041101.

The optical performance of solar central tower (CT) systems on hillsides of mountain areas is investigated based on the biomimetic spiral heliostat field distribution algorithm. The optical efficiencies and the field characteristics of different hillside solar field configurations are examined. The effect of various geometric parameters such as hillside tilt angle and the location of the receiver on the optical efficiency of the field are investigated and documented. The study is based on generating a 25 MWth power plant at the location of Sierra Sun Tower in California, USA, using Planta Solar 10 (PS10) heliostats' parameters. This study is performed numerically using a specially developed code using matlab software. The biomimetic spiral distribution pattern and the particle swarm optimization (PSO) method were used to obtain optimum solar fields. The spiral distribution shape factors were optimized for pursuing maximum annual weighted field efficiency. It is found that the annual optical weighted field efficiency for hillside solar fields is always lower than that for a flat field for same receiver height. On the other hand, the field land area for small hillside-slopes is smaller than that of a flat field area. It is found that there is an optimum field tilt angle where the land area is minimum. The minimum field area for the system studied in this paper was associated with (15 deg) field tilt angle. Furthermore, it was found that as the tower height increases the annual optical field weighted efficiency increases until it reaches a peak value. It was also found that, the closer the tower to the beginning of the heliostat field, the higher the field efficiency with less number of heliostats and less land area.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;141(1):011011-011011-14. doi:10.1115/1.4041102.

The stacking axis locations for twist and taper distributions along the span of a wind turbine blade are optimized to maximize the rotor torque and/or to minimize the thrust. A neural networks (NN)-based model is trained for the torque and thrust values calculated using a computational fluid dynamics (CFD) solver. Once the model is obtained, constrained and unconstrained optimization is conducted. The constraints are the torque or the thrust values of the baseline turbine blade. The baseline blade is selected as the wind turbine blade used in the National Renewable Energy Laboratory (NREL) Phase VI rotor model. The Reynolds averaged Navier–Stokes (RANS) computations are done using the FINE/turbo flow solver developed by NUMECA International. The k-epsilon turbulence model is used to calculate the eddy viscosity. It is observed that achieving the same torque value as the baseline value is possible with about 5% less thrust. Similarly, the torque is increased by about 4.5% while maintaining the baseline thrust value.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;141(1):011012-011012-8. doi:10.1115/1.4041103.

The objectives of sustainable building design are to provide the comfort to the occupants and to eliminate negative environmental impacts of its operations. In this regard, windows play a crucial role in saving energy used for electrical lights and enhancing the indoor visual environment. Excessive sunlight penetration through the windows could increase the heat gains and create the uncomfortable visual environment. Hence, external shading devices, such as solar screens, control the sunlight penetration and minimize its negative effects. The objectives of this research are to provide new insight into the impact of installing the solar screen on the indoor visual environment and heat gain through the window. Experimental measurements are conducted in extreme weather month and window direction, in June and for West facing façade window. Three design patterns of the solar screen were considered with perforation ratios of 12.5%, 25%, and 35%. Without a solar screen, the results show that there is a significant illuminance level variation in the indoor space, between 200 and 2250 Lux. However, if a solar screen with 12.5% perforation ratio is installed, the illuminance level in entire indoor space becomes uniform, it is maintained at 400 Lux during the daytime, and thereby visual comfort is attained. Additionally, the heat gain through the window is decreased by 52.8%, and the window is prevented from heating up during the daytime.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;141(1):011013-011013-7. doi:10.1115/1.4041157.

A previously developed model of a concentrating solar power plant has been modified to accommodate doping the heat transfer fluid (HTF) with nanoparticles. The model with its unalloyed HTF has been validated with actual operating data beforehand. The thermo-physical properties of the HTF were modified to account for the nanoparticle doping. The nanoparticle content in the HTF was then varied to evaluate its influence on solar power generation. The model was run to simulate plant operation on four different days representing the four different seasons. As the nanoparticle concentration was increased, heat losses were slightly reduced, transient warm up heat was increased, transient cool down heat was reduced, and the overall impact on power generation was trivial. Doping HTFs with nanoparticles does not seem promising for solar thermal power generation from a performance perspective. Moreover, doping HTFs with nanoparticles involves many other operational challenges such as sedimentation and abrasion.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;141(1):011014-011014-5. doi:10.1115/1.4041158.

An innovative solar sterilizer design to be fixed on the focus of concentrated solar parabolic dish collector (CSPDC) was presented. The specifications of CSPDC were 4.8 m diameter and 1.8 m focal point according to the manufacturer. With this size, the sterilization can be achieved at a flowrate of 2.2 l/min enough to satisfy the daily potable water requirement for a community of 50 people. The design was tested from the sample collected form Ain Arzat water spring in Dhofar region, Oman. The polluted water samples were tested chemically and biologically before and after using the solar sterilizer. It was found that all water samples are polluted biologically and cannot be directly consumed. By solar sterilization, the samples chemically become closer to the Omani Standards of the first quality level, thus making the design of the solar sterilizer a safe and cost-effective tool for water sterilization.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;141(1):011015-011015-6. doi:10.1115/1.4041260.

This paper presents the simulation and modeling of the concentrated solar power (CSP) plant for multipurpose applications at Borg El Arab in Egypt. The plant produces 1 MWe and 250 m3 of distilled water using steam turbine and electric generator. The purpose of using different applications is to improve the overall efficiency and the coefficient of performance of the plant. The trnsys simulation platform was used for simulating the thermal performance of the solar power and desalination plant covering the parabolic trough concentrator (PTC), storage tank with an integrated steam generator, a backup unit, steam turbine, electric generator, and two effects desalination unit. The temperature and energy profiles of the plant were investigated for the PTC, steam generator and the electric generator. The results prove that the simulation could be used to support the operation of the CSP plant and for improving the performance of the cogeneration plant at Borg El Arab.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;141(1):011016-011016-14. doi:10.1115/1.4041104.

A building energy simulation study is carried out to analyze the performance of a triple-hybrid single-effect vapor absorption cooling system (VACS) operated by solar, natural gas, and auxiliary electricity-based cogeneration. A high capacity small office building subjected to different climatic conditions is considered. The system is designed to continuously maintain a specified building comfort level throughout the year under diverse environmental conditions. Simulations are done at different generator temperatures to investigate the performance in terms of total annual electric energy consumption, heating energy, and the coefficient of performance (COP). The performance of the present VACS is compared with the conventional compression-based system, which demonstrates the electric energy and cost saving potentials of the proposed VACS. Simulation outcomes are well-validated against benchmark data from national renewable energy laboratory and energy conservation building code. Interestingly, it is found that beyond a certain collector area, surplus energy savings can be acquired with the present triple-hybrid VACS as compared to the compression-based cooling. Results also show that COP of the simulated system is in line with experimental values available in the literature. Finally, recommendations are given to operate the complete system on solar and biomass resources, which provide encouraging opportunity for agriculture-based countries.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;141(1):011017-011017-8. doi:10.1115/1.4041105.

Distributed generation (DG) technology has been growing rapidly in industries as this technology can increase the overall efficiency to the power systems. Improper placement and sizing can lead to power losses and interrupt the voltage profile of distribution systems. Studies have been done to solve the DG placement and sizing problem considering several factors, and one of the common factor is minimizing the power losses. However, it is not adequate by only considering the power losses, whereas, the costs of the generation, investment, maintenance, and losses of the distribution system must be taken in consideration. In this research, DG chosen to study is photovoltaic (PV) type which is monocrystalline and thin-film. Costs of operation planning with respect to the power losses is considered which include the costs of investment, maintenance, power loss, and generation that are determined for optimal placement and sizing of DG. The proposed method improved gravitational search algorithm (IGSA) is used in the matlab environment to find the optimal placement and sizing of DG and is tested with the IEEE 34-bus system. The performance of IGSA is then compared with gravitational search algorithm (GSA) and particle swarm optimization (PSO) to find out which algorithm gives the best fitness value and convergence rate. The purpose of this research is to identify the operation planning cost based on the optimization results and improves the optimal placement and sizing of DG in future, to provide maximum economical, technical, environmental benefits, and increase the overall efficiency to the power system.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Sol. Energy Eng. 2018;141(1):014501-014501-5. doi:10.1115/1.4041156.

The financial sustainability and the profitability of wind farms strongly depend on the efficiency of the conversion of wind kinetic energy. This motivates further research about the improvement of wind turbine power curve. If the site is characterized by a considerable occurrence of very high wind speeds, it can become particularly profitable to update the power curve management. This is commonly done by raising the cut-out velocity and the high wind speed cut-in regulating the hysteresis logic. Doing this, on one side, the wind turbine possibly undergoes strong vibration and loads. On the other side, the energy improvement is almost certain and the point is quantifying precisely its magnitude. In this work, the test case of an onshore wind farm in Italy is studied, featuring 17 2.3 MW wind turbines. Through the analysis of supervisory control and data acquisition (SCADA) data, the energy improvement from the extension of the power curve in the high wind speed region is simulated and measured. This could be useful for wind farm owners evaluating the realistic profitability of the installation of the power curve upgrade on their wind turbines. Furthermore, the present work is useful for the analysis of wind turbine behavior under extremely stressing load conditions.

Commentary by Dr. Valentin Fuster

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