Research Papers

J. Sol. Energy Eng. 2016;138(6):061001-061001-8. doi:10.1115/1.4034333.

Since the air density reduces as altitude increases, operation of small wind turbines (SWTs), which usually have no pitch adjustment, remains challenging at high altitudes due largely to the reduction of starting aerodynamic torque. By reducing the moment of inertia through the use of hollow blades, this study aims to speed up the starting while maintaining the structural integrity of the blades and high output power. A horizontal axis turbine with hollow blades was designed for two sites in Iran with altitude of 500 m and 3000 m. The design variables are the distributions of the chord, twist, and shell thickness and the improvement of output power and starting are the design goals. Blade-element momentum (BEM) theory was employed to calculate these goals and beam theory was used for the structural analysis to investigate whether the hollow timber blades could withstand the aerodynamic and centrifugal forces. A combination of the goals formed the objective function and a genetic algorithm (GA) was used to find a blade whose output power at a predetermined tip speed ratio (TSR) and the starting performance were high while the stress limit was met. The results show that hollow blades have starting times shorter than solid ones by approximately 70%. However, in the presence of generator resistive torque, the algorithm could not find a blade for an altitude of 3000 m. To solve that problem, the tip speed ratio was added to other design variables and another optimization was done which led to the optimal blades for both altitudes.

Topics: Blades , Optimization , Torque
Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;138(6):061002-061002-9. doi:10.1115/1.4034350.

This paper extends the prescribed-wake vortex lattice method (VLM) to perform aerodynamic analysis of dual-rotor wind turbines (DRWTs). A DRWT turbine consists of a large, primary rotor placed co-axially behind a smaller, secondary rotor. The additional vortex system introduced by the secondary rotor of a DRWT is modeled while taking into account the singularities that can occur when the trailing vortices from the secondary (upstream) rotor interact with the bound vortices of the main (downstream) rotor. Pseudo-steady assumption is invoked, and averaging over multiple relative rotor positions is performed to account for the primary and secondary rotors operating at different rotational velocities. The VLM solver is first validated against experiments and blade element momentum theory results for a conventional, single-rotor turbine. The solver is then verified for two DRWT designs against results from two computational fluid dynamics (CFD) methods: (1) Reynolds-averaged Navier–Stokes CFD with an actuator disk representation of the turbine rotors and (2) large-eddy simulations with an actuator line model. Radial distributions of sectional torque force and angle of attack show reasonable agreement between the three methods. Results of parametric sweeps performed using VLM agree qualitatively with the Reynolds-averaged Navier–Stokes (RANS) CFD results demonstrating that the proposed VLM can be used to guide preliminary design of DRWTs.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;138(6):061003-061003-7. doi:10.1115/1.4034444.

The performance of parabolic trough (PT) receiver tubes (RTs) has a direct impact on concentrated solar power (CSP) plant production. As a result, one major need of operation and maintenance (O&M) in operating plants is to monitor the state of the receiver tube as a key element in the solar field. In order to fulfill this necessity, Abengoa Solar has developed the first existing portable device for measuring the transmittance and reflectance of parabolic trough receiver tubes directly in the field. This paper offers a description of the technical features of the instrument and reviews the issues related to its usability as a workable portable device in operating solar fields. To evaluate its performance, laboratory studies have been carried out using two patterns to determine the accuracy and standard deviation of the measurements, obtaining excellent results. This information is complemented with data collected by O&M using this instrument in solar power plants. Studies have been carried out to determine the effect of both rainfall and artificial cleaning on the increase of transmittance. These values are then compared to those obtained from hand-cleaning, and show important differences. The results are discussed in this paper.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;138(6):061004-061004-8. doi:10.1115/1.4034334.

A major advantage of concentrating solar power (CSP) plants is their ability to store thermal energy at a cost far lower than that of current battery technologies. A recent techno-economic study found that packed rock bed thermal storage systems can be constructed with capital costs of less than 10 United States dollar (USD)/kWht, significantly cheaper than the two-tank molten salt thermal storage currently used in CSP plants (about 22–30 USD/kWht). However, little work has been published on determining optimum rock bed design parameters in the context of a CSP plant. The parametric study in this paper is intended to provide an overview of the bed flow lengths, particle sizes, mass fluxes, and Biot numbers which are expected to minimize the levelized cost of electricity (LCOE) for a central receiver CSP plant with a nominal storage capacity of 12 h. The findings show that rock diameters of 20–25 mm will usually give LCOE values at or very close to the minimum LCOE for the combined rock bed and CSP plant. Biot numbers between 0.1 and 0.2 are shown to have little influence on the position of the optimum (with respect to particle diameter) for all practical purposes. Optimum bed lengths are dependent on the Biot number and range between 3 and 10 m for a particle diameter of 20 mm.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;138(6):061005-061005-8. doi:10.1115/1.4034549.

Solar radiation consists of direct beam, sky diffuse, and reflected radiations from the ground and adjacent surfaces. The amount of diffuse radiation falling on solar collector depends on the view factor of the collector to sky. The reflected radiation striking the collector's surface depends on the reflectivity of the surface, as well as on view factors and the amount of solar radiation reaching the reflecting surfaces. The amount of reflected radiation coming from the ground can be of an appreciable amount, and can be amplified using special reflector surfaces. This study develops general analytical expressions for the sky's view factors as well as factors related to the ground and those between collectors for the deployment of collectors in multiple rows, in three types of solar fields: flat, inclined, and steplike solar fields. All parameters presented in these expressions are measurable (edge-to-edge dimension). The effects of the design parameters such as the tilt of the angle of the collector, the distance between the collectors, the height of the collector, the position of the collector above the ground (as in the case of step-like field), and the inclination of the land of the field (as in the case of an inclined field) on the view factors are numerically demonstrated. The current study also specifies new terms such as the sunny zone and the shadow zone; these zones control the amount of solar radiation reflected onto the collector. As a result, the ground-view factor that depends on the altitude of the solar angle is considered to be a dynamic parameter. The results obtained may be used to estimate the solar radiation incident on all types of solar fields, with the possibility of increasing the incident radiation on a collector by using planar reflectors.

Topics: Solar energy
Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;138(6):061006-061006-8. doi:10.1115/1.4034518.

Thermal storage plays a major role in a wide variety of industrial, commercial, and residential applications when there is a mismatch between the offer and the claim of energy. In this paper, we study numerically the contribution of phase change materials (PCMs) for solar thermal energy storage (TES) in buildings. The studied configuration is a plane solar collector incorporating a PCM layer and coupled to a concrete slab (a roof of a building). The study is conducted for Casablanca (Morocco) meteorological conditions. Several simulations were performed to optimize the melting temperature and the PCM layer thickness. The results show that PCM imposes, on the roof, a temperature close to its melting temperature. The choice of a melting temperature Tmelt = 22 °C (the local indoor temperature Tc is fixed as Tc = 22 °C) limits the losses through the concrete slab, considerably. This last seems to be, nearly, adiabatic, in this case. Also, the energy released by PCM solidification, overnight, increases the outlet temperature of the coolant fluid to 35 °C and the useful flux to 80 W/m2, increasing the efficiency of the solar collector by night. The PCM functioned both as an energy storage material for the stabilization of the coolant fluid temperature and as an insulating material for the building.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;138(6):061007-061007-9. doi:10.1115/1.4034516.

Concentrated solar power (CSP) plants have the potential to reduce the consumption of nonrenewable resources and greenhouse gas emissions in electricity production. In CSP systems, a field of heliostats focuses solar radiation on a central receiver, and energy is then transferred to a thermal power plant at high temperature. However, maximum receiver surface fluxes are low (30–100 W cm−2) with high thermal losses, which has contributed to the limited market penetration of CSP systems. Recently, small (∼4 cm2), laminated micro pin-fin devices have shown potential to achieve concentrated surface fluxes over 100 W cm−2 using supercritical CO2 as the working fluid. The present study explores the feasibility of using these microscale unit cells as building blocks for a megawatt-scale (250 MW thermal) open solar receiver through a numbering-up approach, where multiple microscale unit cell devices are connected in parallel. A multiscale model of the full-scale central receiver is developed. The model consists of interconnected unit cell and module level (i.e., multiple unit cells in parallel) submodels which predict local performance of the central receiver. Each full-scale receiver consists of 3000 micro pin-fin unit cells divided into 250 modules. The performance of three different full-scale receivers is simulated under representative operating conditions. The results show that the microscale unit cells have the potential to be numbered up to megawatt applications while providing high heat flux and thermal efficiency. At the design incident flux and surface emissivity, a global receiver efficiency of approximately 90% when heating sCO2 from 550 °C to 650 °C at an average incident flux of 110 W cm−2 can be achieved.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;138(6):061008-061008-8. doi:10.1115/1.4034517.

In this paper, we propose a numerical study of a tubular solar collector with a U-tube. A three-dimensional numerical model is developed. It was first used in order to study the efficiency of the solar collector and to evaluate the validity of the developed computational fluid dynamics (CFD) model by comparison with experimental results from the literature. For the numerical simulations, the turbulence and the radiation were, respectively, modeled using the standard k–ε model and the discrete ordinates (DO) model. This numerical model was then used to carry out a parametrical study and to discuss the effect of selected operating parameters such as the fluid mass flow rate, the absorber selectivity, and the material properties. Numerical results show that with the increase of the working fluid flow rate from 0.001 kg/s to 0.003 kg/s, the efficiency of the solar collector is improved (from 30% to 35%). Numerical results also show that the filled-type evacuated tube with graphite presents a best result in comparison with those found using the copper fin tube (η increases from 54% to 64%). Finally, we noted that the use of a high selective absorber surface adds to better performance in comparison with the black absorber tube. This is mainly due to the radiation losses reduction.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;138(6):061009-061009-8. doi:10.1115/1.4034743.

Experiments were conducted in summer using two identical photovoltaic (PV) panels at two heights using three roofing types: white, black, and green (vegetated). For experiments at an 18 cm height, the mean power output of the PV-green roof system was 1.2% and 0.8% higher than the PV-black and PV-white roofs, respectively. At a 24 cm height, the benefit of the green roof was slightly diminished with power output for the PV above a green roof being 1.0% and 0.7% higher than the black and white roof experiments, respectively. These results were consistent with measured variations in mean panel surface temperatures; the green roof systems were generally cooler by 1.5–3 °C. A unique aspect of this research is the investigation into the effect of vegetation on the convective cooling of the PV panels. Panel heat transfer coefficients for the PV-green roof were 10–20% higher than for the white and black roof configurations, suggesting a mixing benefit associated with the roughness of the plant canopy. While the best PV performance was obtained by locating PV above a green roof, the relative benefits diminish with distance between the PV and the roof.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;138(6):061010-061010-10. doi:10.1115/1.4034806.

The application of thermo-active foundation (TAF) systems to heat and cool residential buildings is evaluated in this paper. First, a transient three-dimensional finite difference numerical model is developed for the analysis of thermo-active foundations. The numerical model predictions are then validated against experimental data obtained from laboratory testing. Using the validated numerical model, G-functions for TAFs are generated and integrated into whole-building simulation analysis program, energyplus. A comparative analysis is carried out to evaluate TAF systems compared to conventional ground-source heat pumps (GSHPs) to provide heating and cooling for multifamily residential buildings. In particular, the analysis compares the cost-effectiveness of TAFs and GSHPs to meet heating and cooling needs for a prototypical multifamily building in three U.S. climates. Due to lower initial costs associated to the reduced excavation costs, it is found that TAFs offer a more cost-effectiveness than GSHP systems to heat and cool multifamily residential buildings.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;138(6):061011-061011-10. doi:10.1115/1.4034807.

An urbanized version of MM5 (uMM5) was used at a 500 m horizontal grid-resolution to study effects on morning urban mixing depths and near roof-top stability from use of extensive green roofs in Mexico City, which is characterized by large Bowen ratios and high building storages. The model uses urban-morphology data, while building hydrothermal uMM5 input parameters were obtained from measurements over green and nearby conventional roofs. Evaluation of uMM5 predicted values against rooftop and planetary boundary layer (PBL) observations from extensive field measurement campaigns showed that the model performed reasonably well. Additional simulations were carried assuming that the roofs in entire urban neighborhoods were greened. Predicted mixing depths from these simulations, along with observed air pollution concentrations, were then used in a simple box model to evaluate potential green roof impacts on concentration. Results showed that green roofs produced an early morning (7–10 LST) cooling of up to 1.2 °C at rooftop levels, which reduced mixing depths during that period. Effects were greater on a day with weak synoptic forcing that on one 48 h later with strong synoptic forcing. The mixing-depth decreases produced increased box-model pollutant concentrations. While the green roofs did not elevate the observed concentrations of CO, SO2, and NO2 above World Health Organization (WHO) health standards, they did increase PM10, values (which were already above its standard) by as much as 8% from 7 to 9 LST, when local populations are normally exposed to peak concentrations. This study has applications in the analyses of building energy efficiency.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;138(6):061012-061012-10. doi:10.1115/1.4034907.

Traditional learning process in solar radiation modeling usually requires historical data to perform regularization using training and cross-validation approaches. However, in applications where no historical data are available, regularization cannot be performed using traditional techniques. This paper presents a hierarchical Bayesian framework with the extended Kalman filter (Bayesian-EKF) to perform regularization in sequential learning of the artificial neural network (ANN) for solar radiation modeling. A highly stochastic time series for daily solar radiation, the global horizontal irradiance (GHI), is modeled based on different meteorological variables including temperature (T), relative humidity (RH), wind speed (WS), and sunshine duration (SSD). A comparison is made with well-known methods including the ANN-based nonlinear autoregressive with exogenous inputs neural network (NARX-NN) and Wiener filter-based multivariate linear regression (MLR). The method is validated on test data using coefficient of determination (R2) and root mean squared error (RMSE). The proposed technique effectively estimates the noise components in the data and achieves superior performance as compared to the traditional learning processes of NARX-NN and MLR. Moreover, it is more robust to statistical outliers in the data and does not require prior history for training and cross-validation. In the presence of the outliers, the performance of the NARX-NN degrades from R2 = 94.73% to R2 = 85.85% but there is virtually no difference in the case of Bayesian-EKF. Over and above, MLR performs better than NARX-NN but worse than Bayesian-EKF.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;138(6):061013-061013-7. doi:10.1115/1.4034906.

Flange height is between the geometric features that contribute efficiently to improve the diffuser aerodynamic performances. Results obtained from wind tunnel experiments, particle image velocimetry (PIV) measurements, and numerical simulations reveal that at the diffuser inlet section, the wind velocity increases as the flange height increases. Nevertheless, there is an optimal ratio (flange height/inlet section diameter, Hopt/Da ≈ 0.15) beyond it, the flange height effect on the velocity increase diminishes. This behavior can be explained by both the positions of the two contra-rotating vortices generated downstream of the diffuser and the pressure coefficient at their centers. Indeed, it was found that, as the flange height increases, the two vortices move away from each other in the flow direction and since the flange height exceeds (Hopt/Da), they became too distant from each other and from the flange. While the pressure coefficients at the vortices' centers increase with (H/Da), attain a maximum when (Hopt/Da) is reached, and then decrease. This suggests that the wind velocity increase depends on the pressure coefficient at the vortices' centers. Therefore, it depends on the vortices' locations which are in turn controlled by the flange height. In practice, this means that the diffuser could be more efficient if equipped with a control system able to hold the vortices too near from the flange.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;138(6):061014-061014-10. doi:10.1115/1.4034958.

Comparative performance assessment and model validation of the linear Fresnel concentrator (LFC) and the conical solar reflector (CSR) systems were performed under identical operating and climatic conditions. This paper analyzes the amount of heat loss by convective heat transfer (natural or forced) from the receiver to ambient air with and without a glass-reinforced plastic sheet enclosure around the collector assembly. The matlab ordinary differential equation (ode) solvers were used for simulation of the transient states. Mathematical models were generated from energy balance equations of the glass cover, absorber pipe, heat transfer fluid, and storage tank for each system. Thermal and optical analyses of the LFC (with and without an enclosure) and CSR systems were carried out by using the measured and calculated results. Satisfactory agreement was found between the experimental data and predicted results. The given models are suitable to simulate the dynamic energy flow across the different components of the LFC and CSR systems.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2016;138(6):061015-061015-10. doi:10.1115/1.4034926.

The environment of a regularly occupied space can be extensively improved by maximum utilization of natural light/daylight, which is available in abundance. In Indian climate, availability of sufficient day light in both direct and diffused form of radiation can lead to reduction in dependency on artificial lighting thus, decreasing energy demand for artificial lighting system. In this study, an institutional building in New Delhi, India is analyzed for its daylighting characteristics. The academic block of a building comprising all categories of regularly spaces is modeled and simulated using Integrated environmental solutions - virtual environment (IES VE). The objective is to analyze the extent of penetration of natural light into these spaces of the building for reducing energy requirement for artificial lighting by studying a room, which performs the worst as per present case parameters. The conclusion puts forth the optimal solutions for utilizing maximum day light in a work space, complying with standards set forth by building construction council by utilizing the principles for increasing luminous flux level through visual light transmittance, window-to-wall ratio, and controlled usage of artificial lighting. Considering all these factors in the analysis, energy savings and carbon mitigation due to these savings in regularly occupied spaces are finally evaluated.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Sol. Energy Eng. 2016;138(6):064501-064501-6. doi:10.1115/1.4034316.

Ground reflected radiation is one component of the global radiation on photovoltaic collectors in a solar field. This component depends on the view factor of the collector to ground, hence depends on the relative position of the collectors to each other. General analytical expressions and numerical values for the view factor to the ground were developed between flat-plate collectors positioned in a general configuration. Based on the general expression, the view factors to ground for particular collector configurations were derived. For deployment of photovoltaic collectors in multiple rows with common inclination angles, the view factor to ground is rather small, and hence, the reflected radiation from the ground on the collectors may be neglected compared to the direct beam and the diffuse components. However, in some cases the reflected radiation from the ground may constitute an appreciable amount as in snowy area. Bifacial photovoltaic (PV) panels can absorb solar radiation by both the front and the rear sides and are usually deployed vertically. In this case the reflected radiation from the ground on the panels may be appreciable depending on the ground albedo. The mathematical expressions of the different view factors may be used by the solar field designer to estimate the amount of reflected radiation from the ground reaching the collectors for different configurations of solar PV plants.

Commentary by Dr. Valentin Fuster

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