0

IN THIS ISSUE

# Newest Issue

### Research Papers

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

Sorry! You do not have access to this content. For assistance or to subscribe, please contact us:

• TELEPHONE: 1-800-843-2763 (Toll-free in the USA)
• EMAIL: asmedigitalcollection@asme.org
Sign In