Research Papers

J. Sol. Energy Eng. 2018;140(3):031001-031001-9. doi:10.1115/1.4039093.

This work deals with wind turbine wakes in complex terrain. The test case is a cluster of four 2.3 MW wind turbines, sited in a very complex terrain. Their performances are studied through supervisory control and data acquisition (SCADA) data, suggesting a relevant role of the terrain in distorting the wake of the upstream turbines. The experimental evidences stimulate a deeper comprehension through numerical modeling: computational fluid dynamics (CFD) simulations are run, using the Reynolds-averaged Navier–Stokes (RANS) formulation. A novel way of elaborating the output of the simulations is proposed, providing metrics for quantifying the three-dimensional (3D) evolution of the wake. The main outcome of the numerical analysis is that the terrain distorts the wind flow so that the wake profile is severely asymmetric with respect to the lateral displacement. Further, the role of orography singularities is highlighted in dividing the wake front, thus inducing faster wake recovery with respect to flat terrain. This interpretation is confirmed by SCADA data analysis.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;140(3):031002-031002-8. doi:10.1115/1.4039018.

The evaluation of the performance and characteristics of a solar flat-plate collector (FPC) are reported for domestic and industrial requirements in the existing literature. A computer code was developed using matlab to model and evaluate the energetic and exergetic performance of a nanofluid-based FPC for steady-state and laminar conditions. The analysis was performed using practical geometry data, especially the absorber emittance, for a standard collector. Linear pressure losses in manifolds were taken into account, and a more accurate exergy factor corresponding to a correct value of 5770 K for the sun temperature was employed. The results demonstrate that copper–water nanofluid has the potential to augment the internal convection heat transfer coefficient by 76.5%, and to enhance the energetic efficiency of the collector from 70.3% to 72.1% at 4% volume concentration, when compared to the values with water. Additionally, it was revealed that copper nanofluid is capable of increasing the collector fluid's outlet temperature and decreasing the absorber plate's mean temperature by 3 K. The addition of nanoparticles to the water demonstrated a reduction in the total entropy generation by the solar FPC. Furthermore, increasing the nanoparticle size reflected a reduction in the overall performance of the solar collector.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;140(3):031003-031003-7. doi:10.1115/1.4039095.

Solar tracking is a major alternative to increase the electric output of a photovoltaic (PV) module, and therefore, improves the global energy collected by PV systems. Nonetheless, solar-tracking PV systems require more resources and energy than static systems. Additionally, the presence of cloudiness and shadows from near buildings may reduce the profitability of these systems. Therefore, their feasibility must be assessed in order to justify their application. In equatorial latitudes, the sun's movement through the sky is in the zenith East–West axis. It may be advantageous, since the best tilt in such latitudes is the horizontal. In these terms, the main objective of this research is to numerically assess the performance of a PV array with solar tracking and under typical operation conditions in equatorial latitudes. For this, the assessment of the solar resource in Quito was analyzed in first place. Then, the comparison between three solar arrays was studied to evaluate the feasibility of solar tracking (two-axes tracking, horizontal one-axis tracking, and horizontal fixed). Additionally, the impact of cloudiness and shadows in the system was analyzed. The results showed that the horizontal one-axis tracking is the most beneficial option for equatorial latitudes as the two-axes tracking system only surpasses the gains of the one-axis tracking marginally. Furthermore, the use of a strategy to place the PV modules horizontally in cloudy conditions seems to be marginally advantageous. Finally, the shadows created from neighboring buildings in the East and West of the system may reduce considerably the solar irradiation on the PV-array (not the ones in the north and south).

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;140(3):031004-031004-11. doi:10.1115/1.4039272.

Sun-tracking system (STS) is a key factor for solar photovoltaic (PV) future and new answers for the solar market. It will expand large-scale PV projects (PV farms) worldwide, and it is possible to collect more energy from the sun. PV farms consist of thousands of STS that are subject to dynamic loads (wind, snow, etc.), vibrations, and gravitational loads. This paper presents the structural dynamic analysis of a 24 m2 bi-axial STS (azimuth-elevation) at different elevation angles based on its modal parameters (natural frequencies, modal shapes, and modal damping ratios) and dynamic performance indices (modal participation factors (MPF), forcing frequencies, and mechanical quality factors) by means of the finite element analysis (FEA). The simulation results show that the structural dynamic design of the STS meets the desired structural requirements and agrees well with structural dynamic standards (EN 1991-1-4 and ASHRAE). These results can be used for further analysis on optimal design and vibration safety verification for the bi-axial STS (PV applications).

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;140(3):031005-031005-13. doi:10.1115/1.4039274.

This paper focuses on the thermal and energetic behavior of a building located in the Brazilian Amazon Region, a region climatically characterized by elevated temperatures and high humidity levels, where achieving adequate thermal comfort demands a high-energy consumption due to the use of air-conditioning systems. Therefore, different energy conservation measures (ECMs) need to be evaluated to reduce the thermal load for cooling. The use of a thermal insulation material on the west wall and on the roof, and a photovoltaic (PV) system integrated as an architectural element and adapted to the roof of the building are considered. The building is simulated with the software energyplus, with its thermal behavior and energy consumption analyzed for an entire year and for a chosen design day, defined with data measured by a weather station installed close to the building. According to the evaluations carried out, it is determined that the ECMs have a direct and major influence on the reduction of the thermal load for cooling, on the reduction of the effects caused by radiation with the shading on the eaves, and the reduction of the transmittance on the surfaces that were modified in the study. In terms of energy consumption and economic feasibility, the ECMs reach an annual energy saving percentage of 74% for the building chosen as the case study, and the solutions adopted provide the return of the financial investment, proving suitable for energy saving and economically viable for regions with similar climatic characteristics.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;140(3):031006-031006-10. doi:10.1115/1.4039276.

Inverse design of thickness sensitive spectrally selective pigmented coatings that are used in absorbers of solar thermal collectors is considered. The objective is to maximize collection efficiency by achieving high absorptance at solar wavelengths and low emittance at the infrared (IR) wavelengths to minimize heat loss. Radiative properties of these coatings depend on coating thickness, pigment size, concentration, and the optical properties of binder and pigment materials, and a unified radiative transfer model of the pigmented coatings is developed in order to understand the effect of these parameters on the properties. The unified model (UM) relies on Lorenz–Mie theory (LMT) for independent scattering regime in conjunction with extended Hartel theory (EHT) to incorporate the multiple scattering effects, T-matrix method (TMM) for dependent scattering, and effective medium theory (EMT) for very small particles. A simplified version of the UM (SUM) ignoring dependent scattering is also developed for improving computational efficiency. Through the solution of the radiative transfer equation by the four flux method (FFM), spectral properties are predicted. The developed model is used in conjunction with inverse design for estimating design variables yielding the desired spectral emittance of the ideal coating. The nonlinear inverse design problem is solved by optimization by using simulated annealing (SA) method that is capable of finding global minimum regardless of initial guess.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;140(3):031007-031007-13. doi:10.1115/1.4039377.

Computational fluid dynamics (CFD) simulations of wind turbine wakes are strongly influenced by the choice of the turbulence model used to close the Reynolds-averaged Navier-Stokes (RANS) equations. A wrong choice can lead to incorrect predictions of the velocity field characterizing the wind turbine wake and, consequently, to an incorrect power estimation for wind turbines operating downstream. This study aims to investigate the influence of different turbulence models, namely the kε,kω,SSTkω, and Reynolds stress models (RSM), on the results of CFD wind turbine simulations. Their influence was evaluated by comparing the CFD results with the publicly available experimental measurements of the velocity field and turbulence quantities from the Sexbierum and Nibe wind farms. Consistent turbulence model constants were proposed for atmospheric boundary layer (ABL) and wake flows according to previous literature and appropriate experimental observations, and modifications of the derived turbulence model constants were also investigated in order to improve agreement with experimental data. The results showed that the simulations using the k–ε and k–ω turbulence models consistently overestimated the velocity and turbulence quantities in the wind turbine wakes, whereas the simulations using the shear-stress transport (SST) k–ω and RSMs could accurately match the experimental data. Results also showed that the predictions from the k–ε and k–ω turbulence models could be improved by using the modified set of turbulence coefficients.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;140(3):031008-031008-5. doi:10.1115/1.4039330.

Solar desalination is an important way to obtain fresh water. Nowadays, the all-glass evacuated tube with only one open end (blind pipe) is widely used in hot-water system, and the glass-metal evacuated tube is commonly used in solar trough concentrating system in medium temperature area. But both of them have drawbacks for seawater desalination. The former cannot quickly heat seawater to 100 °C, and the latter is expensive and fails to make full use of its high temperature advantage. A new design of a medium-temperature full-glass evacuated tube whose inner tube is wound by a spiral structure is proposed in to achieve a more cost-effective solar trough concentrating system for solar desalination. The sealing stress between inner and outer tube is analyzed and the thermal performance is tested. The results showed that the evacuated tube could withstand temperature of 250 °C, quickly heat seawater, and has good corrosion resistance and can keep a long-term vacuum. Such a tube enables seawater to flow directly without oil heat-exchange system, so it is a more promising solution for seawater desalination.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;140(3):031009-031009-13. doi:10.1115/1.4039273.

Solar dryer with thermal energy storage device is an essential topic for food drying applications in industries. In this work, a two-dimensional (2D) numerical model is developed for the application of solar drying of agricultural products in an indirect type solar dryer. The phase-change material (PCM) used in this work is paraffin wax. The study has been performed on a single set of concentric tube which consists of a finned inner copper tube for air flow and an outer plastic tube for PCM material. The practical domain is modeled using ANSYS, and computer simulations were performed using ANSYS fluent 2015. The air velocity and temperature chosen for this study are based on the observation of indirect type solar dryer experimental setup. From this numerical analysis, the temperature distribution, melting, and solidification fraction of PCM are estimated at different air flow velocities, time, and inlet temperature of air. It is concluded that the drying operation can be performed up to 10.00 p.m. as the PCM transfers heat to inlet air up to 10.00 p.m. and before it got charged up to 3.00 p.m. because of solar radiation. The maximum outlet temperature is 341.62 K (68.62 °C) which is suitable for food drying applications. Higher air flow velocity enhances quick melting of PCM during charging time and quick cooling during recharging of inlet air; therefore, higher air flow velocity is not preferred for food drying during cooling of PCM.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;140(3):031010-031010-17. doi:10.1115/1.4039255.

Maintaining receiver’s thermal stresses and corrosion below the material limits are issues that need careful attention in solar thermal towers. Both depend on heliostats’ aiming points over the central receiver and available direct solar radiation at any instant. Since this technology relies on an unavoidable time-changing resource, aiming points need to be properly manipulated to avoid excessive hot spots. This paper proposes a new aiming point strategy based on a multivariable model predictive control (MPC) approach. It shows an alternative approach by introducing an agent-based group behavior over heliostats’ subsets, which makes possible either concentrating or dispersing solar radiation as required by the MPC algorithm. Simulated results indicate that it is feasible to develop a closed-loop control procedure that distributes solar irradiance over the central receiver according to the predefined heat flux limits. The performance of the proposed approach is also compared with the results found in the available literature that uses a different methodology.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;140(3):031011-031011-15. doi:10.1115/1.4039275.

Earth air heat exchanger (EAHE) systems are inefficient to provide thermal comfort in winter season for semi-arid regions. The performance of such systems could be improved by coupling them with other renewable energy sources. One of the renewable energy technology is rooftop photovoltaic/thermal (PV/T) air collectors which could utilize the incident solar insolation to obtain both electricity as well as useful heat. In the current paper, the thermal performance of an EAHE coupled with a PV/T system has been numerically investigated for climatic conditions of Pilani, Ajmer (India), and Las Vegas (U.S.). For the comparative analysis, a thermodynamic model has been developed and compared with experimental data available in the literature which seems to be in good comparison with the results. Further, a parametric analysis has been carried out for assessing the effect of different operating parameters. Results showed that for the winter season, the maximum cell temperature without any cooling goes up to 54.3 °C, 54.5 °C, and 44.4 °C for Pilani, Ajmer, and Las Vegas, respectively, while with cooling it drops to 43.4 °C, 44.2 °C, and 35.6 °C, respectively, for 0.053 kg/s flow rate. The heating capacity of the EAHE was observed to be improved with PV/T air collector by 23.47 Wh–298.74 Wh, 71.18 Wh–315.93 Wh, and 41.43 Wh–270.75 Wh for the Pilani, Ajmer, and Las Vegas, respectively.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Sol. Energy Eng. 2018;140(3):034501-034501-5. doi:10.1115/1.4039332.

This paper presents an experimental study of solar hydrogen production via electrolysis of water (conversion of solar energy into chemical energy), and thus, a direct link between the electrolyser (Hoffman voltammeter) and two photovoltaic panels should be present. Sodium hydroxide at a low concentration is used in the experiment. The experimental temperature is the temperature of the ambient environment. The experiment was conducted over 45 days spread over most of the months of the year. This represents the first experiment for the Ouargla region with such a long duration. We focused on measuring four important parameters: solar irradiation, the voltage produced by the two photovoltaic panels, the current used in the electrolysis and the flow of hydrogen; these parameters were evaluated throughout the day from 8 am until sunset. This study shows the changes in solar irradiation, voltage, and current, as well as hydrogen flow during the course of a day, month, and the year. It also demonstrates the extent of the negative effect of seasonal temperature on the efficiency of photovoltaic cells and by contrast, the extent of the positive effect of seasonal temperature on the hydrogen production process.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;140(3):034502-034502-7. doi:10.1115/1.4039329.

The set of optical models that is implemented in ray-tracing software determines the accuracy of its output. A sensitivity analysis was carried out using a powerful in-house program, which provides a large number of surface reflectance and scattering models and in addition, can also run spectral simulations. A linear Fresnel collector was selected as a test case together with the most accurate data that can be found in the literature for the optical properties of its components. The test results indicate that simulations based on constant values, such as mostly provided by the manufacturer, are generally inaccurate and a spectral simulation is not essential for thermal applications.

Commentary by Dr. Valentin Fuster

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