Research Papers

J. Sol. Energy Eng. 2018;140(5):051001-051001-10. doi:10.1115/1.4039891.

A greenhouse dryer under forced convection mode is designed and fabricated with the integration of solar collector and variable speed exhaust fan. The developed system is used for bitter gourd flakes drying under three different air mass flow rates (0.0275, 0.0551, and 0826 kg/s). Moisture content of bitter gourd flakes was decreased effectively from 96.8% to 12.2% in 17 h with optimum air mass flow rate 0.0551 kg/s, whereas open sun drying has taken 26 h to reach 20.7% moisture content. The average greenhouse dryer efficiency was found to be 19.7% at 0.0551 kg/s air mass flow rate. Shrinkage (in terms of percentage) of dried bitter gourd flakes was found to be higher as 74% at 0.0275 kg/s air mass flow rate because of higher greenhouse room air temperature. Hardness of dried bitter gourd flakes was found to be highest as 365 g at 0.0275 kg/s air mass flow rate due to less air exchange rate and high inside room temperature. On the basis of statistical analysis, Prakash and Kumar model and Logarithmic model were selected as best drying models for greenhouse and open sun drying, respectively. The dehydration of higher moisture content crops inside developed greenhouse dryer was found to be more consistent. The designed greenhouse system is recommended for small farmers.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;140(5):051006-051006-4. doi:10.1115/1.4039893.

The basis of a novel method for passive solar water heating homologous to the traditional thermosyphon but driven by salinity gradient induced by changes of salinity gradient induced by evaporation at the collector is outlined. Its purpose, likewise than a thermosyphon, is to simplify the transfer of liquid while avoiding the cost and complexity of a conventional pump. However, in this concept, the fluid motion is not obtained from the tendency of a less dense fluid to rise above a denser fluid (natural convection) but rather by taking advantage of the energy released during the spontaneous mixing of the low-concentration (evaporated fraction) solution and the high-concentration (no-evaporated fraction) solution, which have been previously separated into two streams in the evaporator module. Finally, the possibility of driving the thermal osmosis by the strong thermal dependence of the solubility featured by many solutions rather than evaporation is envisaged. One important point in favor of the proposed thermosyphon driven by thermo-osmosis is that makes possible downward heat and mass transfer, i.e., heat and mass transport from the top roofs (where solar collectors are generally placed) to the bottom (inside the homes), and then the use of expensive and voluminous tanks so characteristic of current thermosyphons driven by natural convection is no longer needed.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;140(5):051007-051007-10. doi:10.1115/1.4040064.

A solar heating compound parabolic collector (CPC) using air and palm oil as heat carrier fluid is proposed and analyzed within this study via heat transfer and ray tracing simulations. The system is a linear focusing solar system intended to be used for applications across a broad range of industrial sectors for generating medium temperature heat up to 250 °C. The Monte Carlo ray tracing method was used to predict the optical performances of the receiver. We have developed a simplified thermal model to investigate and analyze the thermal performances of the receiver under different conditions. It has been demonstrated that the investigated receiver satisfactorily matches the heat demand by producing low and medium temperature heat with an annual system efficiency of 45%.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;140(5):051008-051008-8. doi:10.1115/1.4039988.

Solar-to-thermal energy conversion technologies are an important and increasingly promising segment of our renewable energy technology future. Today, concentrated solar power (CSP) plants provide a method to efficiently store and distribute solar energy. Current industrial solar-to-thermal energy technologies employ selective solar absorber coatings to collect solar radiation, which suffer from low solar-to-thermal efficiencies at high temperatures due to increased thermal emission from selective absorbers. Solar absorbing nanofluids (a heat transfer fluid (HTF) seeded with nanoparticles), which can be volumetrically heated, are one method to improve solar-to-thermal energy conversion at high temperatures. To date, radiative analyses of nanofluids via the radiative transfer equation (RTE) have been conducted for low temperature applications and for flow conditions and geometries that are not representative of the technologies used in the field. In this work, we present the first comprehensive analysis of nanofluids for CSP plants in a parabolic trough configuration. This geometry was chosen because parabolic troughs are the most prevalent CSP technologies. We demonstrate that the solar-to-thermal energy conversion efficiency can be optimized by tuning the nanoparticle volume fraction, the temperature of the nanofluid, and the incident solar concentration. Moreover, we demonstrate that direct solar absorption receivers have a unique advantage over current surface-based solar coatings at large tube diameters. This is because of a nanofluid's tunability, which allows for high solar-to-thermal efficiencies across all tube diameters enabling small pressure drops to pump the HTF at large tube diameters.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;140(5):051009-051009-11. doi:10.1115/1.4040076.

The parabolic trough collector (PTC) is one of the most widely deployed concentrating solar power technology in the world. This study aims at improving the operational efficiency of the commercially available LS-2 solar collector by increasing the convective heat transfer coefficient inside the receiver tube. The two main factors affecting this parameter are the properties of the working fluid and the inner geometry of the receiver tube. An investigation was carried out on six different working fluids: pressurized water, supercritical CO2, Therminol VP-1, and the addition of CuO, Fe3O4, and Al2O3 nanoparticles to Therminol VP-1. Furthermore, the influence of a converging-diverging tube with sine geometry is investigated because this geometry increases the heat transfer surface and enhances turbulent flow within the receiver. The results showed that of all the fluids investigated, the Al2O3/Oil nanofluid provides the best improvement of 0.22% to thermal efficiency, while the modified geometry accounted for a 1.13% increase in efficiency. Other parameters investigated include the exergy efficiency, heat transfer coefficient, outlet temperatures, and pressure drop. The analysis and modeling of a parabolic trough receiver are implemented in engineering equation solver (EES).

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;140(5):051010-051010-9. doi:10.1115/1.4040104.

A modeling framework to analyze a wind turbine blade subjected to an out-of-plane transformation is presented. The framework combines aerodynamic and mechanical models to support an automated design process. The former combines the National Renewable Energy Lab (NREL) aerodyn software with a genetic algorithm solver. It defines the theoretical twist angle distribution (TAD) as a function of wind speed. The procedure is repeated for a series of points that form a discrete range of wind speeds. This step establishes the full range of blade transformations. The associated theoretical TAD geometry is subsequently passed to the mechanical model. It creates the TAD geometry in the context of a novel wind turbine blade concept. The blade sections are assumed to be made by additive manufacturing, which enables tunable stiffness. An optimization problem minimizes the difference between the practical and theoretical TAD over the full range of transformations. It does so by selecting the actuator locations and the torsional stiffness ratios of consecutive segments. In the final step, the blade free shape (undeformed position) is found. The model and design support out-of-plane twisting, which can increase energy production and mitigate fatigue loads. The proposed framework is demonstrated through a case study based on energy production. It employs data acquired from the NREL Unsteady Aerodynamics Experiment. A set of blade transformations required to improve the efficiency of a fixed-speed system is examined. The results show up to 3.7% and 2.9% increases in the efficiency at cut-in and rated speeds, respectively.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;140(5):051011-051011-9. doi:10.1115/1.4040065.

The eutectic mixture of MgCl2–KCl molten salt is a high temperature heat transfer and thermal storage fluid able to be used at temperatures up to 800 °C in concentrating solar thermal power systems. The molten salt thermophysical properties are reported including vapor pressure, heat capacity, density, viscosity, thermal conductivity, and the corrosion behavior of nickel-based alloys in the molten salt corrosion at high temperatures. Correlations of the measured properties as functions of molten salt temperatures are presented for industrial applications. The test results of tensile strength of two nickel-based alloys exposed in the molten salt at a temperature of 800 °C from 1-week length to 16-week length are reported. It was found that the corrosion and strength loss is rather low when the salt is first processed to remove water and oxygen.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Sol. Energy Eng. 2018;140(5):054501-054501-6. doi:10.1115/1.4040197.

Radiative properties of transparent insulations made of a layer of parallel, small-diameter, thin-walled, visible light transparent pipes placed perpendicularly to the surface of a flat solar absorber are investigated theoretically. A formula for the radiation heat losses through the insulation is derived based on two main assumptions: the system is in steady-state and the fourth power of the temperature along each pipe is linear. Arguments in favor of the assumptions are given. The formula, combined with standard formulas for the conductive heat flux, enables prediction that a 10 cm thick transparent insulation under insolation of 1000 W/m2, at ambient temperature 20 °C, could theoretically raise the absorber temperature to 429 °C and produce 410 W mechanical power under the ideal Carnot cycle. In order to reach that high energy conversion efficiency, the insulation pipes should have diameter less than 0.5 mm and walls about 5 μm thick, which may be technologically challenging.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;140(5):054502-054502-8. doi:10.1115/1.4040196.

The 2012 European energy efficiency directive supported the development of cogeneration combined heat and power (CHP) and district heating and cooling (DHC) networks, stressing the benefits of a more efficient energy supply, the exploitation of recovered heat, and renewable resources, in terms of fuel consumption and avoided costs/emissions. Policy decisions play a crucial role: technical and environmental feasibility of CHP is clear and well demonstrated, whereas economic issues (fuel prices, incentives, etc.) may influence its actual application. In this framework, the introduction of low-carbon technologies and the exploitation of renewable energies are profitable interventions to be applied on existing plants. This work focuses on a small CHP plant, installed in the 90 s and located within a research facility in Italy, designed to supply electricity and heat/cool through a district network. On the basis of monitored consumption of electricity, heating, and cooling, energy fluxes have been analyzed and an assessment was performed to get a management profile enhancing both operational and economic parameters. The integration of renewable energies, i.e., solar-powered systems for supporting the existing devices, has been evaluated, thus resulting in a hybrid trigeneration plant. Results demonstrate how the useful synergy between CHP and DHC can not only be profitable from the economic point of view, but it can also create conditions to considerably boost the integral deployment of primary energy sources, improving fuel diversity and then facing the challenge of climate change toward sustainable energy networks in the future.

Commentary by Dr. Valentin Fuster

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