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Research Papers

J. Sol. Energy Eng. 2017;139(6):061001-061001-6. doi:10.1115/1.4037378.

Ion-implantation is an advanced technology to inject dopants for shallow junction formation. Due to the ion-induced sputtering effect at low implant energy where dopants tend to accumulate at the silicon surface, the excess ion doses can be easily removed via a surface chemical wet etching process. By taking advantage of the dose limitation characteristic, we proposed a novel method to form shallow emitters with various dopant densities. Two integration flows have been investigated: (1) wet etch after implantation before junction anneal and (2) wet etch after implantation and junction anneal. The two integration flows observed a difference in the density of doping impurities during the thermal process, which is related to the substrate recombination rates. Selective emitter (SE) structures with the two types of integration flows were characterized. Comparing the blanket emitter and SE structures with two types of etching methods, the device with wet etch before annealing process achieved the best effective carrier lifetime of 53.05 μs, which leads to a higher short circuit current density. Hence, this SE cell demonstrated a better blue response and shows an improvement in the conversion efficiency.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2017;139(6):061002-061002-11. doi:10.1115/1.4037380.

Leading edge erosion is a considerable threat to wind turbine performance and blade maintenance, and it is very imperative to accurately predict the influence of various degrees of erosion on wind turbine performance. In the present study, an attempt to investigate the effects of leading edge erosion on the aerodynamics of wind turbine airfoil is undertaken by using computational fluid dynamics (CFD) method. A new pitting erosion model is proposed and semicircle cavities were used to represent the erosion pits in the simulation. Two-dimensional incompressible Reynolds-averaged Navier–Stokes equation and shear stress transport (SST) k–ω turbulence model are adopted to compute the aerodynamics of a S809 airfoil with leading edge pitting erosions, where the influences of pits depth, densities, distribution area, and locations are considered. The results indicate that pitting erosion has remarkably undesirable influences on the aerodynamic performance of the airfoil, and the critical pits depth, density, and distribution area degrade the airfoil aerodynamic performance mostly were obtained. In addition, the dominant parameters are determined by the correlation coefficient path analysis method, results showed that all parameters have non-negligible effects on the aerodynamics of S809 airfoil, and the Reynolds number is of the most important, followed by pits density, pits depth, and pits distribution area. Meanwhile, the direct and indirect effects of these factors are analyzed, and it is found that the indirect effects are very small and the parameters can be considered to be independent with each other.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2017;139(6):061003-061003-14. doi:10.1115/1.4037742.

Beam-down concentrating solar tower (BCST) is known for its merits in easy installation and maintenance as well as lower convection heat loss of the central receiver (CR) when comparing to a traditional concentrated solar tower system. A point-line-coupling-focus (PLCF) BCST system using linear Fresnel heliostat (LFH) as the first stage concentrator (heliostat) and hyperboloid/ellipsoid reflector as the tower reflector (TR) is proposed and theoretically analyzed and compared in this paper. Theoretical investigation on the ray concentrating mechanism with two commonly used reflector structures, namely, hyperboloid and ellipsoid, is conducted utilizing Monte Carlo ray-tracing (MCRT) method. The objective of this study is to reveal the achievable optical performance of these types of TRs in the PLCF system considering the effect of LFH tracking errors on TR astigmatism as well as the differences of optical efficiency factors and power transmission in a large-scale biomimetic layout. Results indicate that the ellipsoid system is superior in terms of interception efficiency over the hyperboloid system due to smaller astigmatism at the CR aperture, especially at larger facet tracking error. However, the ellipsoid reflector shows significantly lower TR shading efficiency resulting from the larger TR surface area compared to that of the hyperboloid reflector. The total optical efficiency of the hyperboloid system is always better than that of the ellipsoid system, and this efficiency gap decreases as the ratio ε increases. The hyperboloid TR is proved to be more promising and practical for the PLCF system.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2017;139(6):061004-061004-10. doi:10.1115/1.4037745.

Variable aperture mechanisms are being used in many fields including medicine, electronics, fluid mechanics, and optics. The main design characteristics of these aperture concepts are the use of multiple blades regulating aperture area and consequently the incoming medium flow. Manufacturing complexities primarily depend on the concept geometry, material, and the process application requirements. Design of a variable aperture demands meticulous methodology and careful consideration of the application field. This paper provides an in-depth methodology on the design of a novel iris mechanism for temperature control in high temperature solar thermal receivers and solar reactors. Such methodology can be used as a guideline for iris mechanisms implemented in other applications as well as in design of different apparatuses exposed to high temperature. Optical simulations in present study have been performed to demonstrate enhanced performance of the iris mechanism over conventional Venetian blind shutter serving as optical attenuators in concentrating solar power systems. Results showed that optical absorption efficiency is improved by 14% while reradiation loss through the aperture is reduced by 2.3% when the iris mechanism is used. Correlation for adaptive control of aperture area was found through computational surface area measurement. Experimental testing with a 7 kW solar simulator at different power levels demonstrated the performance of the mechanism to maintain stable temperature under variable flux.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2017;139(6):061005-061005-8. doi:10.1115/1.4037381.

An opportunity for increasing the parabolic solar power plant efficiency is substituting the actual subcritical Rankine power cycles with the innovative s-CO2 Brayton cycles. In this paper, three configurations are assessed: the recompression cycle (RC), the partial cooling with recompression cycle (PCRC), and the recompression with main compression intercooling cycle (RCMCI), with one reheating stage. The thermodynamic parameters are optimized with three algorithms: SUBPLEX, UOBYQA, and NEWUOA, and the results validated with thermoflow Software. The parabolic troughs and linear Fresnel solar collectors are studied with different heat transfer fluids (HTFs): Solar Salt, HITEC XL, Therminol-VP1, Syltherm 800, and Therminol 75. The dual-loop solar field (SF), combining thermal oil and molten salt (MS) in the same solar plant, is also analyzed. The plant power output and plant energy efficiency are translated into SF aperture area and cost at design point. From the point of view of the plant efficiency and SF cost, the PTC and LF solar collector with Solar Salt as HTF coupled to a s-CO2 Brayton RCMCI cycle is selected as the optimum design solution and compared with the actual PTC Rankine solar plant performance at design point. The total recuperator conductance (UA) plays an important role in optimizing the plant performance, limited by the minimum heat exchangers (HX) pinch point. The UA increment could compensate the HX pressure drop and the compressor inlet temperature (CIT) increment, both impacting very negatively in the s-CO2 plant performance.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2017;139(6):061006-061006-6. doi:10.1115/1.4037746.

A light-mixing module consisting of a compound parabolic concentrator (CPC) and a light-mixing tube is proposed herein to realize a uniform and efficient solar-lighting system. In this lighting system, the sunlight collected into a fiber and then guided to an indoor destination is the principal light source, while an auxiliary light source including multiple red, green, blue, and white (RGBW) light-emitting diodes (LEDs) is controlled by an auto-compensating module. To mix the principal and the auxiliary sources and to realize the uniform illumination, the light-mixing tube was coated with BaSO4 and optimized as a cylindrical tube. The design of the light-mixing tube is described and discussed in this article. According to the simulated results, the uniformity and the optical efficiency of the designed light-mixing tube are 82.9% and 85.7%, respectively, while from the experimental results, the uniformity of 85.9% and the optical efficiency of 83.3% have been obtained. In terms of the common indoor-lighting standards and the specifications of commercial components used in lighting systems, the proposed light-mixing module has demonstrated the high uniformity and acceptable optical efficiency. Additionally, since the main components of the light-mixing module can be designed as plastic optics, a cost-effective light-mixing module and a profitable lighting system can be realized. Thus, the performance and the price of the proposed light-mixing module fit the demands of the illumination market, while the proposed system shows the potential for indoor solar-lighting applications.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2017;139(6):061007-061007-7. doi:10.1115/1.4037743.

In this paper, an online estimation algorithm of the source term in a first-order hyperbolic partial differential equation (PDE) is proposed. This equation describes heat transport dynamics in concentrated solar collectors where the source term represents the received energy. This energy depends on the solar irradiance intensity and the collector characteristics affected by the environmental changes. Control strategies are usually used to enhance the efficiency of heat production; however, these strategies often depend on the source term which is highly affected by the external working conditions. Hence, efficient source estimation methods are required. The proposed algorithm is based on modulating functions method (MFM) where a moving-horizon strategy is introduced. Numerical results are provided to illustrate the performance of the proposed estimator in open-and closed-loops.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2017;139(6):061008-061008-8. doi:10.1115/1.4037744.

Control algorithms for rotor load mitigation are today generally adopted by industry. Most of them are based on the Coleman transformation, which is easy to implement and bears satisfactory results when the rotor is balanced. A multitude of causes, e.g., blade erosion, dirt, and especially pitch misalignment, may create significant imbalances. This gives birth to undesirable vibrations and reduced control performance in terms of load mitigation. In this paper, an alternative transformation is introduced, able to detect and quantify the rotor load harmonics due to aerodynamic imbalance. Next, a control algorithm, capable of targeting rotor imbalance itself and simultaneously lowering rotor loads, is presented. The effectiveness of the proposed solution is confirmed through simulations in virtual environment.

Topics: Stress , Blades , Rotors , Signals
Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2017;139(6):061009-061009-14. doi:10.1115/1.4037815.

The Net-Zero Energy Residential Test Facility (NZERTF) was designed to be approximately 60% more energy efficient than homes meeting the 2012 International Energy Conservation Code requirements. The thermal envelope minimizes heat loss/gain through the use of advanced framing and enhanced insulation. A continuous air/moisture barrier resulted in an air exchange rate of 0.6 air changes per hour at 50 Pa. The home incorporates a vast array of extensively monitored renewable and energy efficient technologies including an air-to-air heat pump system with a dedicated dehumidification cycle; a ducted heat-recovery ventilation (HRV) system; a whole house dehumidifier; a photovoltaic system; and a solar domestic hot water system. During its first year of operation, the NZERTF produced an energy surplus of 1023 kWh. Based on observations during the first year, changes were made to determine if further improvements in energy performance could be obtained. The changes consisted of installing a thermostat that incorporated control logic to minimize the use of auxiliary heat, using a whole house dehumidifier in lieu of the heat pump's dedicated dehumidification cycle, and reducing the ventilation rate to a value that met but did not exceed code requirements. During the second year of operation, the NZERTF produced an energy surplus of 2241 kWh. This paper describes the facility, compares the performance data for the 2 years, and quantifies the energy impact of the weather conditions and operational changes.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2017;139(6):061010-061010-9. doi:10.1115/1.4037384.

Operation of solar photovoltaic (PV) systems under high temperatures and high humidity represents one of the major challenges to guarantee higher system’s performance and reliability. The PV conversion efficiency degrades considerably at higher temperatures, while dust accumulation on PV module together with atmospheric water vapor condensation may cause a thick layer of mud that is difficult to be removed. Therefore, thermal management in hot climates is crucial for reliable application of PV systems to prevent the efficiency to drop due to temperature rise. This research focuses on the utilization of phase-change materials (PCM) for passive thermal management of solar systems. The main focus is to explore the effect of utilization of PCM-based cooling elements on the thermal behavior of solar PV modules. This paper presents the mathematical modeling and validation of PV modules. Both simulation and experimental data showed that the significant increase in PV peak temperature in summer affects the module’s efficiency, and consequently produced power, by 3% compared to standard testing condition (STC) as an average over the entire day, while it goes up to 8% and 10% during peak noon hours in winter and summer, respectively.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2017;139(6):061011-061011-7. doi:10.1115/1.4037816.

In this work, the indirect solar drier (IDSD) is used for drying Thymus by using four techniques. First, the whole leaves of Thymus are dried using the IDSD without using phase change material (PCM). The obtained results of this experiment show that the moisture content of Thymus leaves decreases slowly in the first three days, which is considered abnormal behavior compared with most studied dried plants in the literature. In order to reduce the drying time, the Thymus leaves are cut before drying without using the PCM. The results indicated that cutting Thymus leaves reduces the drying time by 55.6%. To increase the operating time of the IDSD and control the drying temperature as well, the IDSD is integrated with paraffin wax as a PCM. This reduces the drying time of cut leaves by 50% compared with the system without using the PCM. Moreover, eleven mathematical models are tested in order to select the best one describing the drying behavior of whole and cut leaves in the IDSD without and with using the PCM. As a result of the abnormal behavior of drying Thymus, most of the examined mathematical models failed to describe the drying behavior of Thymus satisfactory. Therefore, a new mathematical model (four-parameter logistic model) is introduced aimed to well describe the drying behavior of Thymus.

Topics: Drying , Solar energy
Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2017;139(6):061012-061012-10. doi:10.1115/1.4037905.

This paper evaluates the effectiveness of phase change materials (PCMs) for the improvement of summer thermal comfort in lightweight buildings. Experiments have been carried out using PCM in the form of DuPont Energain wallboards in combination with a roof. Two factors influencing the effectiveness of PCM (thickness and location of PCM layer) have been investigated. An experimental study was carried out using two identical test cavities submitted to Casablanca weather. Thermal performance such as the roof surface temperatures and heat flux densities, through the envelope, have been studied. The results indicated that, compared with reference room (without PCM), the thermal storage allows solar radiation to be stored and released up to 6–7 h after solar irradiation; this has effects on both the reduction of daily temperature swings (up to 2 °C) and heat flux (more than 88%). It has been proved that the PCM with a thickness of 10.52 mm on the outer face of the roof has good thermal insulation effect and energy efficiency potential.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Sol. Energy Eng. 2017;139(6):064501-064501-4. doi:10.1115/1.4037821.

An improvement in performance of hybrid solar system with use of liquid metal alloy GaInSn as a coolant has been demonstrated in this study. The performance of hybrid system using GaInSn, water, and air as coolants under different operating conditions such as ambient temperature, different inlet velocities of fluid, solar irradiations is analyzed using simulation software ANSYS and the results obtained are validated using experiments. The results obtained from simulations and experiments are used for determining important performance criteria such as electrical efficiency, thermal efficiency, and exergy efficiency. The exergy, thermal, and electrical efficiencies were found to be at least 11%, 12%, and 12% higher for hybrid system using GaInSn as a coolant when compared to hybrid systems using air and water as coolants, respectively.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2017;139(6):064502-064502-3. doi:10.1115/1.4037904.

Reverse osmosis (RO) technique is one of most efficient methods to desalinate the salt water. An accurate and detailed experimental method was established to analyze the performance of the unpotable water RO process, which is the solution-diffusion and mass transfer theory. The RO is going to be operated using an alternative energy source, solar energy. The solar cells are utilized to run the RO unit with a single vertical membrane. DC current is produced by a solar panel which produces 18 maximum volt and 4.25 maximum DC current. The solar panel was utilized to run out the DC booster pump without using an inverter. The production of freshwater is the major studied parameter in this project with other parameters, such as the efficiency of the RO unit and the salt rejection percentage (610 max PPM). The experimental work provides a more detailed understanding of solar RO unit in order to be utilized in the remote areas.

Commentary by Dr. Valentin Fuster

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