Research Papers

J. Sol. Energy Eng. 2018;140(4):041001-041001-6. doi:10.1115/1.4039331.

Phase change materials (PCMs) used in the building walls constitute an attractive way to reduce the energy consumption and to increase the occupant's thermal comfort. However, there are some challenges to be faced among which the critical one is the PCM layer location allowing the greater heat flux reduction. In this work, the potential of PCM wallboards is evaluated experimentally using a heated reduced scale cavity including walls with or without PCM in a laboratory conditions. The cavity at reduced scale provides the flexibility to test most kinds of wall constructions in real time and allows faster installation and dismantling of the test walls. Three different PCM layer locations inside the walls are examined in terms of heat flux reduction and outside surface temperatures. The results confirm that the PCM layer reduces the peak heat flux compared to a reference wall (wall without PCM). Indeed, the PCM layer hugely affects the peak heat flux when it is placed on the inner face of the walls, near to the heat source. At this location, the peak heat flux reduction, compared to the reference wall, is 32.9%. Furthermore, for numerical validation purpose, the outside overall heat coefficient of the cavity outside walls is determined based on the experimental data.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;140(4):041002-041002-5. doi:10.1115/1.4039427.

Wireless sensor network (WSN) is widely used in a variety of applications including habitat monitoring, military surveillance, environmental monitoring, scientific applications, etc. The major limitation of WSN is that sometimes it is not feasible to replace or recharge the battery once it gets fully exhausted and thus, it limits the lifetime of WSN. One of the possible solutions to overcome this limitation is to incorporate any energy harvesting device, which can use the alternative energy sources to charge the battery. However, the processing temperature and the performance of energy harvesting devices limit their applications. In this paper, low temperature and high performance single-sided silicon heterojunction (SHJ) solar cells are fabricated with 13% efficiency using hot-wire chemical vapor deposition (HWCVD) method. This paper also describes an energy management model that successfully addresses the various issues in the existing energy harvesting models. In order to implement the proposed model, the results show that the high efficiency SHJ solar cells are best suitable candidate as an energy harvesting device that can be incorporated inside the node. The subsequent analysis shows that the consumed power per day by the node can be successfully recovered from the SHJ solar cells, if the sunlight is available only for 25 min in a day with 100 mW/cm2 intensity. This clearly indicates that the node's battery will remain fully charged if the above said condition is satisfied, which seems to be very feasible. Finally, one can conclude that the node functioning will remain active till the battery lifetime i.e., approximately 30 years for Li-ion battery.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;140(4):041003-041003-9. doi:10.1115/1.4039351.

This research is intended to design and manufacture a multilayer solar distiller at a promising cost. The solar distiller manufactured has the same design as simple water distillers, which are based on the principle of evaporation and condensation with a different energy cycle, where the processes of evaporation and condensation are completely isolated. The obtained results showed that the amount of produced water has increased by 60% compared to the traditional solar distillers, where the system is not isolated. No catalysts were used, and the areas of the evaporation and condensation have also been increased leading to the production of distilled water under natural conditions and low cost. A comparison between the theoretical and experimental results is performed. The productivity was as follows: 8.45, 11.04, 12.20, 21.44, 18.69, 16.15, and 14.49 L/day in January, February, March, September, October, November, and December, respectively.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;140(4):041004-041004-18. doi:10.1115/1.4039350.

This paper presents a comprehensive study of the evaluation of the effect of spar cap fiber orientation angle of composite blades with induced bending–torsion coupling (IBTC) on the aero-structural performance wind turbines. Aero-structural performance of wind turbines with IBTC blades is evaluated with the fatigue load mitigation in the whole wind turbine system, tower clearances, peak stresses in the blades, and power generation of wind turbines. For this purpose, a full E-glass/epoxy reference blade has been designed, following the inverse design methodology for a 5-MW wind turbine. An E-glass/epoxy blade with IBTC and novel, hybrid E-glass/carbon/epoxy blades with IBTC have been designed and aeroelastic time-marching multibody simulations of the 5-MW turbine systems, with the reference blade and the blades with IBTC, have been carried out using six different randomly generated turbulent wind profiles. Fatigue-equivalent loads (FELs) in the wind turbine have been determined as an average of the results obtained from the time response of six different simulations. The results reveal that certain hybrid blade designs with IBTC are more effective in fatigue load mitigation than the E-glass–epoxy blade with IBTC, and besides the fiber orientation angle, sectional properties of hybrid blades must be adjusted accordingly using proper number of carbon/epoxy layers in the sections of the blade with IBTC, in order to simultaneously reduce generator power losses and the FEL.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;140(4):041005-041005-7. doi:10.1115/1.4039447.

A hybrid algorithm that combines genetic programming (GP) and genetic algorithms (GAs) that deduce a closed-form correlation of building energy use is presented. Throughout the evolution, the terms, functions, and form of the correlation are evolved via the genetic program. Whenever the fitness of the best correlation stagnates for a specific number of GP generations, the GA optimizes the real-valued coefficients of each correlation in the population. When the GA, in turn, stagnates, correlations with optimized coefficients and powers are passed back to the GP for further search. The hybrid algorithm is applied to the problem of predicting energy use of a U-shape building. More than 800 buildings with various foot-print areas, relative compactness (RC), window-to-wall ratio (WWR), and projection factor (PF) values were simulated using the VisualDOETM energy simulation engine. The algorithm tries to minimize the difference between simulated and predicted values by maximizing the R2 value. The algorithm was able to arrive at a closed-form correlation that combines the four building parameters, accurate to within 4%. The methodology can be easily used to model any type of data behavior in any engineering or nonengineering application.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;140(4):041006-041006-10. doi:10.1115/1.4039550.

Phase change materials (PCMs) are investigated in this study as an option to reduce the surface temperature of the photovoltaic (PV) cell during sunshine hours to enhance the electrical efficiency of the cells. For this purpose, thermal energy balance model of the PV panel is integrated with PCM enthalpy model. The simulated results of the model have been validated with experimental results from the literature. The results indicate that PCM can be effectively used for limiting the temperature rise of the PV cell, thus increasing the efficiency of the PV cell up to 10%. Peak temperature of the PV cell can be reduced from 86 °C to 57 °C during the hottest summer month. It has observed that maximum benefits can be obtained when PCM melting point is selected in such a way that there is 10–12 °C difference between melting point of PCM and average minimum ambient temperature of the hottest summer month. PCM selected in such way will also require minimum mass. In current study, PCMs with melting points of 40 °C and 44 °C provide the best result compared to the PCMs having melting points of 35 °C and 30 °C with average minimum ambient temperature of 28 °C.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;140(4):041007-041007-8. doi:10.1115/1.4039655.

This paper presents a mobile testing rig developed for small wind turbine (SWT) experimental work to orchestrate, cost-effectively, turbine performance characterization in both controlled wind inflow speeds and turbulent ambient flows. It facilitates off-grid testing of up to a 1 kW wind turbine. It is a dual-purpose machine that can be towed behind a vehicle to conduct steady-state tests (track testing) or be parked to collect unsteady field data (field testing), all with the same rotor and instrumentation. Its mechanical design included computational fluid dynamics (CFD) analysis to gauge the potential impact of towing vehicle disturbance on the free stream available to the rotor. To provide a compelling platform for full rotor speed control, a reconfigurable control system coupled to an electric vehicle controller with regenerative braking technology has been modeled and implemented into its electrical design. Uncertainty analysis has also been rigorously conducted to project the error bounds pertaining to both precision and bias components of the testing results. The rig has been tested in a towed scenario and blade element momentum (BEM) simulations have been compared with the actual aggregate performance curves obtained experimentally. Future work involves testing in unsteady winds, for which the rig was ultimately designed in order to better understand unsteady rotor performance and adaptive design.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;140(4):041008-041008-11. doi:10.1115/1.4039631.

The issue of reflector soiling becomes more important as concentrating solar thermal power plants (CSP) are being implemented at sites subject to high dust loads. In an operational power plant, a trade-off between reducing cleaning costs and cleaning related collector availability on the one hand and keeping the solar field cleanliness (ξfield) high to minimize soiling induced losses on the other hand must be found. The common yield analysis software packages system advisor model (SAM) and greenius only allow the input of a constant mean ξfield and constant cleaning costs. This oversimplifies real conditions because soiling is a highly time-dependent parameter and operators might adjust cleaning activities depending on factors such as soiling rate and irradiance. In this study, time-dependent soiling and cleaning data are used for modeling the yield of two parabolic trough plant configurations at two sites in Spain and Morocco. We apply a one-year soiling rate dataset in daily resolution measured with the tracking cleanliness sensor (TraCS). We use this as a basis to model the daily evolution of the cleanliness of each collector of a solar field resulting from the application of various cleaning strategies (CS). The thus obtained daily average ξfield is used to modify the inputs to the yield analysis software greenius. The cleaning costs for each CS are subtracted from the project's financial output parameters to accurately predict the yield of a CSP project over its lifetime. The profits obtained with different CSs are compared in a parameter variation analysis for two sites and the economically best CS is identified. The profit can be increased by more than 2.6% by the application of the best strategy relative to a reference strategy that uses a constant cleaning frequency. The error in profit calculated with constant soiling and cleaning parameters compared to the simulation with variable soiling and cleaning can be as high as 9.4%. With the presented method, temporally variable soiling rates and CS can be fully integrated to CSP yield analysis software, significantly increasing its accuracy. It can be used to determine optimum cleaning parameters.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;140(4):041009-041009-14. doi:10.1115/1.4039426.

The demand for affordable, environment-friendly, and reliable water conditioning systems has led to the introduction of several standalone and/or hybrid alternatives. The technology of desiccant evaporative cooling (DEC) has proven to be dependable and has gained success at places where initially it was deemed unfeasible. Today, a number of related technologies and configurations are available. Among them, solar-assisted desiccant cooling system (SADCS) offers a cheap eco-friendly alternative, especially in hybrid configurations. Most studies have investigated the performance of numerous SADCS configurations in specific climatic conditions; however, at the global- and system-level scale, no such study is available. The current study investigates five different SADCS configurations using equation-based object-oriented modeling and simulation approach in five different climatic conditions. The selected climatic conditions cover a wide range of global weather data including arid/semiarid (Karachi), dry summer tropical (Adelaide), and mesothermal (Sao Paulo, Shanghai) to continental conditions (Vienna). The performance of all selected SADCS configurations (ventilation cycle, recirculation and ventilated-recirculation cycles, dunkle and ventilated-dunkle cycle) is analyzed for specified cooling design day of the selected cities. A uniform system control strategy based on the idea of displacement distribution (ventilation) system is used for each configuration and climatic zone. By monitoring their performances based on the values of cooling capacity (CC) and coefficient of performance (COP), the best SADCS configuration is proposed for each considered climatic condition in the world. The results revealed that the climates of Vienna, Sao Paulo, and Adelaide favor the use of ventilated-dunkle cycle configuration with average COP of 0.36, 0.84, and 0.93, respectively, while ventilation cycle based on DEC configuration suits the climate of Karachi and Shanghai with an average COP of 2.32 and 2.90, respectively.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;140(4):041010-041010-8. doi:10.1115/1.4039605.

This paper mainly focuses on the design of solar concentric parabolic cooker with proper arrangement of phase change material (PCM) heat storage system. The receiver is a hollow concentric cylinder with inner and outer radii being 0.09 m and 0.1 m, respectively. The thickness or the gap between the two layers of the receiver is 0.01 m and is filled with heat transfer oil. The outer layer of the receiver is surrounded by the vertical cylindrical PCM tubes of diameter 0.025 m. The three modes of heat transfer, radiation, convection, and conduction, are explained and analyzed by heat transfer network. The schematic view of the receiver is shown with the help of sketchup software. The performance parameters, heat loss factor, optical efficiency factor, cooking power of the solar cooker, were calculated with and without PCM in the receiver. 7.74 W m−2 and 2.46 W m−2 are the heat loss factors, and 0.098 and 0.22 are the optical efficiency factors of the solar cooker without and with PCM presented in the receiver. The optical efficiency factor of the solar cooker with PCM receiver is two times more than that receiver without PCM. The cooking power of the solar cooker with PCM receiver is 125.3 W which is 65.6 W more than that of the cooking power without PCM receiver. From these results, it can be concluded that the design of PCM solar cooking system can expand the applicability of solar cookers as a compatible cooking solution for cooking applications instead of using fossil fuel based cooking systems.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;140(4):041011-041011-7. doi:10.1115/1.4039657.

In this study, the CO2-based photovoltaic–thermal hybrid system has been investigated with an objective to increase the power generation efficiency in photovoltaic solar panel and to improve the performance of supercritical CO2 solar Rankine cycle system (SRCS). From a previous study, an improvement of 2% of power generation efficiency was confirmed via experimental investigation. In this study, the temperature distribution on the CO2-based photovoltaic–thermal hybrid system has been numerically and experimentally investigated and confirmed with referenced experimental results. Particularly, in this study, the one-dimensional (1D) calculation of CO2 flow in the cooling tube and three-dimensional (3D) calculation of temperature distribution on the surface of the photovoltaic solar panel are conducted. The typical summer and winter weather conditions are used as the calculation references to investigate the effect of temperature distribution of the photovoltaic solar panel. The results show that the trend of temperature distribution from calculation was confirmed with the experimental data both in summer and winter conditions. Furthermore, in summer condition, the CO2 temperature was increased to a maximum of 28 °C.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;140(4):041012-041012-10. doi:10.1115/1.4039748.

Wind turbines are subjected to fatigue loading during their entire lifetime due to the fluctuating excitation from the wind. To predict the fatigue damage, the design standard IEC 61400-1 describes how to parametrize an on-site specific wind climate using the wind speed, turbulence, wind shear, air density, and flow inclination. In this framework, shear is currently modeled by its mean value, accounting for neither its natural variance nor its wind speed dependence. This very simple model may lead to inaccurate fatigue assessment of wind turbine components, whose structural response is nonlinear with shear. Here we show how this is the case for flapwise bending of blades, where the current shear model leads to inaccurate and in worst case nonconservative fatigue assessments. Based on an optimization study, we suggest modeling shear as a wind speed dependent 60% quantile. Using measurements from almost one hundred sites, we document that the suggested model leads to accurate and consistent fatigue assessments of wind turbine blades, without compromising other main components such as the tower and the shaft. The proposed shear model is intended as a replacement to the mean shear, and should be used alongside the current IEC models for the remaining climate parameters. Given the large number of investigated sites, a basis for evaluating the uncertainty related to using a simplified statistical wind climate is provided. This can be used in further research when assessing the structural reliability of wind turbines by a probabilistic or semiprobabilistic approach.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;140(4):041013-041013-11. doi:10.1115/1.4039551.

Few studies have been implemented to evaluate whether the renewable energy generation could fit into industrial locations in Saudi Arabia. We completed this feasibility study to investigate whether using photovoltaic (PV) solar arrays to power industrial cities at Saudi Arabia is economically feasible. The case study is a factory in Zulfi city, Riyadh Region. We used National Renewable Energy Laboratory's modeling tool, system advisor model (SAM) to evaluate the economic benefits of using a 150 kW DC PV system to cover 100% of the factory monthly power demand. Over 25 years, the system is estimated to generate about 6,000,000 kWh of electricity whose net savings are $398,000 (1 US$ is equal to about 3.75 Saudi Riyals) represented by a discounted cash flow. The proposed system will save the factory around $304,000 that would have to be paid in electric bills and will eliminate considerable amount of CO2 emissions. Sensitivity analysis has been conducted to determine the effects of underlying parameters on the economic feasibility of the proposed system. Levelized cost of electricity (LCOE) generated and net present value (NPV) are used as indicators of proposed system feasibility. The results indicate that these projects can be profitable under some certain assumptions and can potentially be generalized for all industrial locations in Saudi Arabia.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Sol. Energy Eng. 2018;140(4):044501-044501-6. doi:10.1115/1.4039347.

An experimental study is conducted on wind turbine wakes and their effects on wind turbine performances and operation. The test case is a wind farm located on a moderately complex terrain, featuring four turbines with 2 MW of rated power each. Two interturbine distances characterize the layout: 4 and 7.5 rotor diameters. Therefore, it is possible to study different levels of wake recovery. The processed data are twofold: time-resolved series, whose frequency is in the order of the hertz, and supervisory control and data acquisition (SCADA) data with 10 min of sampling time. The wake fluctuations are investigated adopting a “slow” point of view (SCADA), on a catalog of wake events spanned over a long period, and a “fast” point of view of selected time-resolved series of wake events. The power ratios between downstream and upstream wind turbines show that the time-resolved data are characterized by a wider range of fluctuations with respect to the SCADA. Moreover, spectral properties are assessed on the basis of time-resolved data. The combination of meandering wind and yaw control is observed to be associated with different spectral properties depending on the level of wake recovery.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;140(4):044502-044502-5. doi:10.1115/1.4039416.

This technical brief presents a study on the effectiveness of the bottom insulation of a salinity gradient solar pond (SGSP) in Melbourne, Australia. Insulation is applied at the bottom of a SGSP in order to minimize the heat loss from the SGSP to the ground underneath. But selection of optimum thickness of the insulation to extract the best thermal performance of an SGSP is a challenge as insulation involves significant investment. Hence, modeling heat loss from SGSP to the ground before and after applying the insulation is thus very essential. In this study, a layer of polystyrene is used as insulation at the bottom of SGSP. The temperature distribution in the SGSP and ground below it, the efficiency of the SGSP and the heat removal from SGSP are estimated for the SGSP without insulation and with insulation of different thicknesses. The results show that the insulation definitely reduces the heat loss from the SGSP to the ground, but to a certain extent. Insulation beyond a certain thickness is proved to be ineffective in increasing the efficiency or reducing the heat loss to ground and thus unable to enhance the thermal performance of the SGSP.

Commentary by Dr. Valentin Fuster

Design Innovation Paper

J. Sol. Energy Eng. 2018;140(4):045001-045001-7. doi:10.1115/1.4039658.

We present the design and characterization of a high flux solar simulator (HFSS) based on metal halide lamps and built from commercially available components. The HFSS that we present was developed to support the evaluation of a solar thermochemical reactor prototype. The HFSS consists of an array of four independent lamp/reflector modules aimed at a common target location. Each module contains one 2500 We lamp and one electroformed ellipsoidal reflector having an interfocal distance of 813 mm. The modules are oriented with an angle relative to the target surface normal vector of 24.5 deg. Design simulations predicted that the peak flux of this HFSS would be 2980 kWth/m2, with a total power delivered to a 6-cm target of 3.3 kWth, for a transfer efficiency of 33.3%. Experimental characterization of the HFSS using optical flux mapping and calorimetry showed that the peak flux at the focal plane reached 2890±170 kWth/m2, while the total power delivered was 3.5±0.21 kWth for a transfer efficiency of 35.3%. The HFSS was built at a material cost of ∼$2700.00/module and a total hardware cost of ∼$11,000.00 for the four-lamp array. A seven-lamp version of this HFSS is predicted to deliver 5.6 kWth to a 6 cm diameter target at a peak flux of 4900 kWth/m2 at a hardware cost of ∼$19,000.00 ($3400.00/kWth delivered, $1100.00/kWe).

Commentary by Dr. Valentin Fuster

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