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J. Sol. Energy Eng. 2018;141(3):031001-031001-10. doi:10.1115/1.4040841.

A solar-assisted liquid desiccant demonstration plant was built and experimentally evaluated. Humidity of the air, density of the desiccant, and all relevant mass flow rates and temperatures were measured at each inlet and outlet position. Adiabatic dehumidification experiments were performed in different seasons of the year under various ambient air conditions. The moisture removal rate m˙v, the mass balance factor κm, and the absorber effectiveness, εabs, were evaluated. An aqueous solution of LiCl was used as liquid desiccant with an initial mass fraction of about 0.4 kgLiCl/kgsol. The mass flow rate of the air through the absorber was about 1100 kg/h. The experimental results showed a reduction in the air humidity ratio in the range of 1.3–4.3 g/kg accompanied with an increase in the air temperature in the range of 3–8.5 K, depending on the inlet and operating conditions. For the air to desiccant mass flow ratio of 82, a mass fraction spread of 5.7% points in the desiccant and a volumetric energy storage capacity of 430 MJ/m3 were achieved. By operating the desiccant pump in an intermittent mode, a mass fraction spread of about 13% points in the desiccant and an energy storage capacity of about 900 MJ/m3 were reached. In addition, the experimental results were compared with results from a numerical model. The numerical model overestimates the heat and mass transfer because it assumes ideal surface wetting and uniform distribution of the circulated fluids.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;141(3):031002-031002-9. doi:10.1115/1.4041401.

The main objective of this study is to identify how climate variability and urbanization influence human comfort levels in tropical-coastal urban environments. San Juan Metro Metropolitan Area (SJMA) of the island of Puerto Rico was chosen as a reference point. A new human discomfort index (HDI) based on environmental enthalpy is defined to determine the energy required to maintain indoor human comfort levels. Regression analysis shows that both temperature and HDI are good indictors to predict total electrical energy consumption. Results showed that over the past 35 years, the average environmental enthalpy have increased, resulting in the increase of average HDI with clear bias due to urbanization. Local scale weather station data show a decreasing rate of maximum cooling per capita at –11.41 kW h/years and increasing of minimum cooling per capita of 10.64 kW h/years; however, for the whole Caribbean region, an increasing trend is observed for both minimum and maximum cooling per capita. To estimate human comfort levels under extreme heat wave events conditions, an event of 2014 was identified. The analysis is complemented by simulations from the weather forecasting system (WRF) at a resolution of 1 km, forced by data from the National Center for Environmental Prediction at 250 km spatial resolution. WRF model results were evaluated against observations showing good agreement for both temperature and relative humidity (RH) and improvements. It also shows that energy per capita in urban areas during a heat wave event can increase to 21% as compared to normal day.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;141(3):031003-031003-12. doi:10.1115/1.4041402.

A solar radiation model is applied to a low temperature water-in-glass evacuated tubes solar collector to predict its performance via computational fluid dynamics (CFD) numerical simulations. This approach allows obtaining the transmitted, reflected, and absorbed solar radiation flux and the solar heat flux on the surface of the evacuated tubes according to the geographical location, the date, and the hour of a day. Different environmental and operational conditions were used to obtain the outlet temperature of the solar collector; these results were validated against four experimental tests based on an Official Mexican Standard resulting in relative errors between 0.8% and 2.6%. Once the model is validated, two cases for the solar collector were studied: (i) different mass flow rates under a constant solar radiation and (ii) different solar radiation (due to the hour of the day) under a constant mass flow rate to predict its performance and efficiency. For the first case, it was found that the outlet temperature decreases as the mass flow rate increases reaching a steady value for a mass flow rate of 0.1 kg/s (6 l/min), while for the second case, the results showed a corresponding outlet temperature behavior to the solar radiation intensity reaching to a maximum temperature of 36.5 °C at 14:00 h. The CFD numerical study using a solar radiation model is more realistic than the previous reported works leading to overcome a gap in the knowledge of the low temperature evacuated tube solar collectors.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;141(3):031004-031004-16. doi:10.1115/1.4041403.

The effects of collector roughness shape on the performance of solar chimney power plant were investigated in this study. The roughness shapes of triangular, curved, and square grooves were chosen and were compared to smooth case. The performance parameters of solar radiation, updraft velocity, temperature distribution, static pressure, power, and Nusselt number were varied. The effects of number, position, height, and width of the grooves on the performance were investigated. The results of this investigation show that the updraft velocity with the triangular groove increases by 1.5 times compared to the smooth case at solar radiation of 1000 W/m2. At solar radiation of 1000 W/m2, the power increases by 169%, 96%, and 19% for triangular, curved, and square grooves, respectively, compared to the smooth case. Moreover, the Nusselt number values with triangular groove and curved groove enhance by 42% and 26%, respectively, compared to the smooth case. The power increases by 1.98% for three grooves instead of using one groove at higher solar radiation. Increasing the groove height by 1.7 times, the power increases by 1.03 times at higher solar radiation. The power enhancement shows less sensitivity to the change of groove width at higher solar radiation.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;141(3):031005-031005-13. doi:10.1115/1.4041404.

An accurate assessment of the amount solar radiation incident at specific locations is highly complex due to the dependence of available solar radiation on many meteorological and topographic parameters. Reunion Island, a small tropical French territory, intends to deploy solar energy technologies rapidly. In this context, the variability and intermittency of solar irradiance in different regions of the island is of immediate interest if the generated energy will be integrated in the existing energy network. This paper identifies different features of spatial and temporal variability of daily global horizontal irradiance (GHI) observed on Reunion Island. For this purpose, trends in the mean daily as well as seasonal variability of GHI were investigated. Furthermore, the intermittency and multifractal behaviors of the spatial daily GHI change were examined. Analyzing this daily variability is crucial to day-ahead forecasting of solar resource for better managing solar integration in the power grid, particularly in small island states with isolated power systems. Results revealed that the difference in cumulative GHI for two successive days ranges between −10 and 10 kW/m2/day while the highest and lowest variability of daily change occurs during summer and winter, respectively. The decorrelation distance, which gives a measure of the distance over which the variability at distinct geographic locations become independent of one another at a given timescale, was also calculated. It was found that the average decorrelation distance for day-to-day GHI change is about 22 km, a smaller value than that calculated by the previous studies using much sparser radiometric networks. The Hurst exponent, fractal co-dimension, and Lévy parameter, which describe solar radiation intermittency, were also evaluated for Reunion Island.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;141(3):031006-031006-8. doi:10.1115/1.4041546.

Particle-based concentrating solar power (CSP) plants have been proposed to increase operating temperature for integration with higher efficiency power cycles using supercritical carbon dioxide (sCO2). The majority of research to date has focused on the development of high-efficiency and high-temperature particle solar thermal receivers. However, system realization will require the design of a particle/sCO2 heat exchanger as well for delivering thermal energy to the power-cycle working fluid. Recent work has identified moving packed-bed heat exchangers as low-cost alternatives to fluidized-bed heat exchangers, which require additional pumps to fluidize the particles and recuperators to capture the lost heat. However, the reduced heat transfer between the particles and the walls of moving packed-bed heat exchangers, compared to fluidized beds, causes concern with adequately sizing components to meet the thermal duty. Models of moving packed-bed heat exchangers are not currently capable of exploring the design trade-offs in particle size, operating temperature, and residence time. The present work provides a predictive numerical model based on literature correlations capable of designing moving packed-bed heat exchangers as well as investigating the effects of particle size, operating temperature, and particle velocity (residence time). Furthermore, the development of a reliable design tool for moving packed-bed heat exchangers must be validated by predicting experimental results in the operating regime of interest. An experimental system is designed to provide the data necessary for model validation and/or to identify where deficiencies or new constitutive relations are needed.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;141(3):031007-031007-10. doi:10.1115/1.4041547.

The current challenge of human society is to meet the large demand of freshwater, which is depleting at a faster rate due to a rapid rise in human population and fast urbanization. Solar still is the economical way to obtain fresh water since it solely requires the energy from the sun alone for its operation, which is abundantly and freely available in nature. The major constraint in conventional solar still (CSS) is to maintain a large surface area of water with a minimum water depth. The best solution for the above constraint is to prefer inclined solar still (ISS) in which the surface area of water is large with a minimum water depth. In order to improvise the performance and efficiency of ISS, numerous works have been incorporated by increasing the free surface area of water. The distillate yield collected from the passive ISS was found as 1000–8100 mL/m2 whereas active ISS produced the distillate yield of 1045–9000 mL/day. In this review, an attempt is made to analyze the present status of different designs in ISS to motivate further research in ISS technology for meeting the demand of fresh water.

Topics: Solar stills , Water
Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;141(3):031008-031008-10. doi:10.1115/1.4041838.

In major region of the world, ample amount of fresh water is required for the drinking purpose as well as for the agricultural and industrial growth. Hence, it is necessary to investigate the alternate clean water extraction technologies to get the potable water from the saline water available at local area or inside the earth. One of the methods used to get the fresh water from the brackish water is solar distillation and the means used is called as a solar still. In the present work, single slope double basin solar still performance has been investigated with and without using Al2O3 nanoparticles at the location 20.61°N, 72.91°E. For the experimentation, two identical single slope double basin solar stills were fabricated with the same basin area. The yield of solar still, one without nanoparticles and the other with Al2O3 nanoparticles, has been measured for various weight concentrations of Al2O3 nanoparticles such as 0.01%, 0.05%, 0.10%, and 0.20%. The results show that the use of nanoparticles in solar still increases the distilled output by 17.6%, 12.3%, 7.2%, and 2.6% for weight concentrations of 0.01%, 0.05%, 0.10%, and 0.20%, respectively, in comparison to the solar still without nanoparticles.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;141(3):031009-031009-9. doi:10.1115/1.4041737.

This paper presents three-dimensional numerical simulation results of the effect of surface tension on two-phase flow over unglazed collector covered with a wire screen. The homogenous model is used to simulate the flow with and without the effect of porous material of wire screen and surface tension. The Eulerian-Eulerian multiphase flow approach was used in this study. The phases are completely stratified, the interphase is well defined (free surface flow), and interphase transfer rate is very large. The liquid–solid interface, gas–liquid interface, and the volume fraction for both phases were considered as boundaries for this model. The results show that the use of porous material of wire screen will reduce the velocity of water flow and help the water flow to distribute evenly over unglazed plate collector. The possibility of forming any hot spot region on the surface was reduced. The water velocity with the effect of surface tension was found higher than the one without this effect, due to the extra momentum source added by surface tension in longitudinal direction. The use of porous material of wires assures an evenly distribution flow velocity over the inclined plate, therefore helps a net enhancement of heat transfer mechanism for unglazed solar water collector application.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;141(3):031010-031010-12. doi:10.1115/1.4041846.

Concentrated solar power (CSP) is a mature and efficient technology to cater the large-scale demand of hot water. Conventional reflectors/mirrors in CSP share 50% of total system cost. High installation as well as O&M cost is the major concern in reflector-based CSP. Apart from the above, manufacturing defects and adverse service environment cause premature degradation of reflectors and substantial drop in efficiency and service life. Performance analysis of an innovative optically concentrated solar water heater (OCSWH) using plurality of Fresnel lenses of poly methyl methacrylate (PMMA) is presented in the work. Size and yield of any solar water heater (SWH) are mainly dependent on its aperture area, output temperature, and mass flow rate, which are termed herein as critical parameters. Series of experimentations is carried out by varying critical design and operating parameters viz. aperture area, outlet temperature, and rate of mass flow, and similar experimentation is also carried out on commercially available flat plate SWH to compare its performance. Loss of heat from riser and header pipes is restricted by application of effective insulation. Substantial improvement in collector efficiency, increase in rate of mass flow, and rise in discharge temperature with reference to flat plate collector are noted. Economics is also studied covering life cycle cost (LCC), life cycle saving (LCS), and energy payback period.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2018;141(3):031011-031011-15. doi:10.1115/1.4041847.

This study presents an experimental nanoparticle synthesis and the numerical analysis of a parabolic trough collector (PTC) operating with olive leaf synthesized TiO2/water nanofluid. The PTC is modeled after the LS-2 collector for various operating conditions. An analysis of the heat transfer and entropy generation in the PTC is carried out based on the first and second laws of thermodynamics for various parameters of nanoparticle volumetric concentration (0 ≤ φ ≤ 8%), mass flow rate (0.1 ≤ m˙ ≤ 1.1 kg/s), and inlet temperatures (350–450 K) under turbulent flow regime. The effect of these parameters is evaluated on the Nusselt number, thermal losses, heat convection coefficient, outlet temperature, pressure drop, entropy generation rate, and Bejan number. The results show that the values of the Nusselt number decrease with higher concentrations of the nanoparticles. Also, the addition of nanoparticles increases the heat convection coefficient of the nanofluid compared to water. The thermal efficiency of the system is improved with the use of the new nanofluid by 0.27% at flow rates of 0.1 kg/s. The entropy generation study shows that increasing the concentration of nanoparticles considerably decreases the rate of entropy generation in the system. It is also observed that increasing the volumetric concentration of nanoparticles at low mass flow rates has minimal effect on the rate of entropy generation. Finally, a correlation that provides a value of mass flow rate that minimizes the entropy generation rate is also presented for each values of inlet temperature and nanoparticle volumetric concentration.

Commentary by Dr. Valentin Fuster

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