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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

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

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