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Research Papers

J. Sol. Energy Eng. 2016;138(3):031001-031001-10. doi:10.1115/1.4032682.

This paper outlines a novel elevation linear Fresnel reflector (ELFR) and presents and validates theoretical models defining its thermal performance. To validate the models, a series of experiments were carried out for receiver temperatures in the range of 30–100 °C to measure the heat loss coefficient, gain in heat transfer fluid (HTF) temperature, thermal efficiency, and stagnation temperature. The heat loss coefficient was underestimated due to the model exclusion of collector end heat losses. The measured HTF temperature gains were found to have a good correlation to the model predictions—less than a 5% difference. In comparison to model predictions for the thermal efficiency and stagnation temperature, measured values had a difference of −39% to +31% and 22–38%, respectively. The difference between the measured and predicted values was attributed to the low-temperature region for the experiments. It was concluded that the theoretical models are suitable for examining linear Fresnel reflector (LFR) systems and can be adopted by other researchers.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;138(3):031002-031002-13. doi:10.1115/1.4032545.

The objective of this study is to investigate how different volumetric projection techniques used in actuator-line modeling affect the unsteady blade loads and wake turbulence statistics. The two techniques for the body-force projection radius are based on either (i) the grid spacing or (ii) the combination of grid spacing and an equivalent elliptic blade planform. An array of two National Renewable Energy Laboratory 5-MW turbines separated by seven rotor diameters is simulated for 2000 s (about rotor 300 revolutions) within a large-eddy simulation (LES) solver of the neutral and moderately convective atmospheric boundary layer (ABL). The statistics of sectional angle of attack (AOA), blade loads, and turbine power histories are quantified. Moreover, the degree of unsteadiness of sectional blade loads in response to atmospheric and wake turbulence is computed via a reduced frequency based on the rate-of-change in sectional AOA. The goal of this work is to make the wind energy community aware of the uncertainties associated with actuator-line modeling approaches.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;138(3):031003-031003-10. doi:10.1115/1.4032683.

Solar air conditioners (A/Cs) have attracted much attention in research, but their performance and cost have to be optimized in order to become a real alternative to conventional A/C systems. In this study, a hybrid solar A/C is simulated using the transient systems simulation program(trnsys), which is coupled with matlab in order to carry out the optimization study. The trnsys model is experimentally validated prior to the optimization study. Two optimization problems are formulated with the following design variables: solar collector area, solar collector mass flow rate, solar thermal energy storage volume, and solar electrical energy storage size. The genetic algorithm (GA) is selected to solve the single-objective optimization problem and find the global optimum design for the lowest electrical consumption. To optimize the two objective functions simultaneously, energy consumption and total cost (TC), a multi-objective genetic algorithm (MOGA) is used to find the Pareto curve within the design variables' bounds while satisfying the constraints. The overall cost of the optimized solar A/C design is also compared to a standard vapor compression cycle (VCC). The results show that coupling trnsys and matlab expands trnsys optimization capability in solving more complex optimization problems. The results also show that the optimized solar hybrid A/C is not very competitive when the electricity prices are low and no governmental support is provided.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;138(3):031004-031004-12. doi:10.1115/1.4032684.

This study compares the time-varying rotor thrust and shaft power characteristics of a yawed floating offshore wind turbine (FOWT) predicted by three different open-source aerodynamic models. These models involve the blade-element-momentum (BEM) and the general dynamic wake (GDW) methods implemented in the design code fast developed by NREL, and a higher fidelity free-wake vortex model (FWVM) that is capable of modeling the unsteady skewed helical wake development of the yawed rotor. The study is based on the NREL 5 MW baseline rotor installed on the MIT tension-leg platform (TLP) operating with different rotor yaw angles and under regular sea wave conditions. Both the undisturbed wind speed and rotor speed are maintained constant throughout the analysis, though different sea wave heights and periods are considered. Initially, the motions of the FOWT under both axial and yawed rotor conditions are estimated in a time domain using fast. These motions are then prescribed to winds, an open-source FWVM developed by the University of Massachusetts Amherst, to determine the aerodynamic rotor thrust and power as a function of time. Both TLP surge and pitch motions are noted to impact the rotor thrust and power characteristics considerably. The three models have consistently shown that the TLP motion exhibits a negligible impact on the time-averaged rotor shaft thrust and power of the yawed rotor. On the other hand, the cyclic component of rotor thrust and power are found to be significantly influenced by the wave state at all yaw angles. Significant discrepancies between the predictions for this cyclic component from the three models are observed, suggesting the need of further research through experimental validation to ensure more reliable aerodynamics models are developed for floating wind turbine design software packages.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;138(3):031005-031005-11. doi:10.1115/1.4032772.

Solar cooling systems might represent a viable alternative for space cooling in residential buildings because the peak of cooling demand matches the availability of solar radiation. The use of adsorption chillers in this field gives another environmental benefit, since they employ natural refrigerants as water. However, the design of such systems is critical because it relates to several parameters and cannot be easily accomplished using traditional tools. In this work, a dynamic model for the simulation of a small solar cooling system employing adsorption chillers has been evaluated. The model, realized with the commercial software trnsys, has been implemented to quantify the effect of different operational and design parameters on the overall performances of solar cooling systems in three different Italian cities (Milan, Rome, and Messina). Particular focus was put on the comparison of different heat rejection systems, which was found to be a critical aspect in the design of such systems. In addition, an economic analysis has been performed for an optimized system, in order to evaluate the payback time of the systems compared to a traditional air conditioning system and provide indication on the possible outlooks by means of a sensitivity analysis.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;138(3):031006-031006-8. doi:10.1115/1.4032794.

In this paper, four key design parameters with a strong influence on the performance of a high-solidity variable pitch vertical axis wind turbine (VAWT) operating at low tip-speed-ratio (TSR) are addressed. To this aim, a numerical approach, based on a finite-volume discretization of two-dimensional (2D) unsteady Reynolds-averaged Navier–Stokes (URANS) equations, on a multiple sliding mesh, is proposed and validated against experimental data. The self-pitch VAWT design is based on a straight-blade Darrieus wind turbine with blades that are allowed to pitch around a feathering axis, which is also parallel to the axis of rotation. The pitch angle amplitude and periodic variation are dynamically controlled by a four-bar linkage system. We only consider the efficiency at low and intermediate TSR; therefore, the pitch amplitude is chosen to be a sinusoidal function with a considerable amplitude. The results of this parametric analysis will contribute to define the guidelines for building a full-size prototype of a small-scale wind turbine of increased efficiency.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;138(3):031007-031007-9. doi:10.1115/1.4032758.

The design of a solar field is one of the crucial aspects when a solar tower system is realized. In general, shading and blocking effects, which are the main causes of solar power losses, are minimized displacing the heliostats each other quite distant, with typical land coverage less than 20%, and thus, strongly limiting the construction of these plants to low value lands. A new method is proposed here to improve the collected energy for solar tower systems with high land coverage (greater than 30%), based on the chance for each heliostat to rotate about the normal passing through the center of its surface. Then, shading and blocking are minimized by optimization of the relative orientations. To this aim, a small solar field composed of 150 rectangular flat heliostats has been considered, and its performances with and without the proposed optimization have been computed and compared for a wide variety of cases. In particular, a systematic analysis is presented to study the effect of the shape of the heliostats on the solar field performance: in a series of simulations, maintaining constant the area of each heliostat, the ratio between its two sides has been varied in a range between 1 (squared heliostats) and 3 (very stretched heliostats), and optimized and nonoptimized systems have been compared. Also, the total energy collected by the solar field has been calculated for optimized and nonoptimized heliostats' orientations, considering towers of different heights. Finally, the real PS10 solar plant has been considered, demonstrating that also for an optimized, very low coverage plant (about 14%), heliostats rotation can still improve the energy collection efficiency by a non-negligible amount.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;138(3):031008-031008-10. doi:10.1115/1.4032887.

Concentrating photovoltaic (CPV) is a solution that is gaining attention worldwide as a potential global player in the future energy market. Despite the impressive development in terms of CPV cell efficiency recorded in the last few years, a lack of information on the module's manufacturing is still registered among the documents available in literature. This work describes the challenges faced to fabricate a densely packed cell assembly for 500× CPV applications. The reasons behind the choice of components, materials, and processes are highlighted, and all the solutions applied to overtake the problems experienced after the prototype's production are reported. This article explains all the stages required to achieve a successful fabrication, proven by the results of quality tests and experimental investigations conducted on the prototype. The reliability of the components and the interconnectors is successfully assessed through standard mechanical destructive tests, and an indoor characterization is conducted to investigate the electrical performance. The fabricated cell assembly shows a fill factor as high as 84%, which proves the low series resistance and the lack of mismatches. The outputs are compared with those of commercial assemblies. A cost breakdown is reported and commented: a cost of $0.79/Wp has been required to fabricate each of the cell assembly described in this paper. This value has been found to be positively affected by the economy of scale: a larger number of assemblies produced would have reduced it by 17%.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;138(3):031009-031009-14. doi:10.1115/1.4033026.

The field performance of a low-flow internally cooled/heated liquid desiccant air conditioning (LDAC) system is investigated in this paper. The quasi-steady performance (sensible and latent heat transfer rates, coefficient of performance (COP), and uncertainties) of the LDAC system is quantified under different ambient air conditions. A major contribution of this work is a direct comparison of the transient and quasi-steady performance of the LDAC system. This paper is the first to quantify the importance of transients and shows that, for the environmental and operating conditions in this paper, transients can be neglected when estimating the energy consumption of the LDAC system. Another major contribution of this work is the development and verification of a new method that quantifies (with acceptable uncertainties) the quasi-steady performance of a LDAC system from transient field data using average data.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;138(3):031010-031010-8. doi:10.1115/1.4033111.

Site-specific assessment of wind turbine design requires verification that the individual wind turbine components can survive the site-specific wind climate. The wind turbine design standard, IEC 61400-1 (third edition), describes how this should be done using a simplified, equivalent wind climate established from the on-site distribution functions of the horizontal mean wind speeds, the 90% quantile of turbulence along with average values of vertical wind shear and air density and the maximum flow inclination. This paper investigates the accuracy of fatigue loads estimated using this equivalent wind climate required by the current design standard by comparing damage equivalent fatigue loads estimated based on wind climate parameters for each 10 min time-series with fatigue loads estimated based on the equivalent wind climate parameters. Wind measurements from Boulder, CO, in the United States and Høvsøre in Denmark have been used to estimate the natural variation in the wind conditions between 10 min time periods. The structural wind turbine loads have been simulated using the aero-elastic model FAST. The results show that using a 90% quantile for the turbulence leads to an accurate assessment of the blade root flapwise bending moment and a conservative assessment of the tower bottom for-aft bending moment and low speed shaft torque. Currently, IEC 61400-1 (third edition) neglects the variation in wind shear by using the average value. This may lead to a nonconservative assessment of blade root flapwise fatigue loads, which are sensitive to wind shear. The results in this paper indicate that using a 75% quantile for the wind shear at each wind speed bin leads to an appropriate, but conservative, assessment of the fatigue loads. However, care should be taken when using this approach for components where low or negative wind shears can lead to large fatigue loads. This is the case for some drivetrain components where a lower quantile may be required.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;138(3):031011-031011-11. doi:10.1115/1.4033112.

In this paper, passive cooling strategies have been investigated to evaluate their effectiveness in reducing cooling thermal loads and air conditioning energy consumption for residential buildings in Kingdom of Saudi Arabia (KSA). Specifically, three passive cooling techniques have been evaluated including natural ventilation, downdraft evaporative cooling, and earth tube cooling. These passive cooling systems are applied to a prototypical KSA residential villa model with an improved building envelope. The analysis has been carried using detailed simulation tool for several cities representing different climate conditions throughout KSA. The impact of the passive cooling systems is evaluated on both energy consumption and electrical peak demand for residential villas with and without improved building envelope for five cities, representatives of various climate conditions in KSA. It is found that both natural ventilation and evaporative cooling provide a significant reduction in cooling energy use and electrical peak demand for the prototypical villa located in dry KSA climates such as that of Riyadh and Tabuk. Natural ventilation alone has reduced the cooling energy end-use by 22%, while the evaporative cooling system has resulted in 64% savings in cooling energy end-use. Moreover, the natural ventilation is found to have a high potential in all KSA climates, while evaporative cooling can be suitable only in hot and dry climates such as Riyadh and Tabuk. Finally, the analysis showed that natural ventilation provided the lowest electrical peak demand when applied into the improved envelope residential buildings in all five cities in KSA.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Sol. Energy Eng. 2016;138(3):034501-034501-7. doi:10.1115/1.4033110.

In order to maximize the total power generation of a wind farm, several control strategies based on tilt angle, yaw angle, and cone angle were investigated numerically using computational fluid dynamics (CFD) simulation. The full rotor model (FRM) of 5 MW wind turbine was used to simulate the wake in the wind farm. According to the comparison of different cases' power generations and velocity fields, the result indicates that appropriate strategies based on tilt angle and positive yaw angle have effective improvements on the power output of whole wind farm, but changing cone angle and opposite yaw angle result in negative effects.

Commentary by Dr. Valentin Fuster

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