A detailed summary of the most relevant aspects of the thermodynamics of a shallow solar still is presented, including historical features not often found in the literature. Solar distillation has grown from applying empirical knowledge to advanced modeling and simulation. Geometrical, environmental and operational parameters of the solar still to heat transfer phenomena including evaporation and condensation, are taken into account in this overview, giving a comprehensive structure and classification to the study of solar stills from the thermodynamic point of view. The article describes global parameters, such as solar radiation, wind speed and thermal insulation among others and how they have been taken into account in the literature. Also, a distinction between internal and external heat transfer phenomena is proposed for clarification. Exergy balance is included to account for thermodynamic imperfections in the several processes inside the solar still. KeywordsSolar Still, Overview, Thermodynamics, Modeling, Exergy, Heat Transfer J. C. Torchia-Núñez et al. 247tensive description of various designs of solar stills, from the single-box still to the sophisticated mutiple-effect distiller and greenhouse-inclusive types are described. These authors mentioned that direct solar desalination systems had low operating and maintenance costs, but required large installation areas and high initial investment. However, this is an appropriate solution for remote areas and small communities in arid and semi-arid regions lacking water. These authors [1] pointed out that the first conventional solar still plant was built in 1872 by Charles Wilson in the mining community of Las Salinas in Northern Chile. This still was a large basin-type still used to supply fresh water from brackish feed water to the community, with a total capacity of about 23 m 3 /day and lasted 40 years until the mines were exhausted. A notable historical account of this project can be found in [2], which offers a social and historical background about Wilson, the solar distillation plant and other authors who wrote about it.Few other historic reviews with different emphases have been reported [3] for designs and experiences during the 50 s to 70 s; and more recently, an extensive account of solar stills has been published [4].Solar distillation is one of the oldest methods used to produce fresh water for different basic human needs. According to [5] and [6], one of the first treatises about solar distillation is De distillationelibri IX, written by G.B. della Porta in 1589. This work describes a solar distillation with glass hemispheres directly exposed to the sun. In [7] mentioned Lavoisier and its method of solar concentrators for heating alembics in 1770. It is also described in [7], manufacturing and operation advances of conventional basin and portable solar stills during WWII. This author analyzes some isolated elements of the solar still independently, showing different experimental results, such as the efficiency and fresh water production as a f...
ResumenEn este trabajo se presenta un análisis teórico de exergía de un destilador solar simple en estado permanente. Mediante un balance de energía sobre los tres componentes principales de un destilador solar -colector, salmuera y cubierta de vidrio-para un conjunto de valores de parámetros -irradiancia solar, temperatura ambiente y espesor de aislamiento-, se obtienen las temperaturas de los componentes para distintas condiciones. Con estas temperaturas se pueden encontrar los flujos de exergía y eficiencias de segunda ley en un destilador solar. Los resultados muestran que la irradiancia solar es el parámetro más influyente en los procesos de transporte dentro del destilador, seguido por el espesor del aislamiento térmico. Para una irradiancia solar de 1,000 W/m², el colector cede 13% del total de exergía que llega al agua salada. El agua salada utiliza más del 6% de este total para la evaporación. Las irreversibilidades alcanzan 86% de la exergía total. La relación energía/exergía muestra que el componente más eficiente en el destilador solar es la masa de agua salada con más de 90% para cualquier valor de los parámetros estudiados mientras que el colector alcanza el 23% para un valor de 1,000 W/m² de irradiancia solar. El destilador solar es un dispositivo que aprovecha el calentamiento de un colector con alta eficiencia y la discusión sobre su uso, como todos los sistemas solares, no debe ser por sus limitaciones termodinámicas.
<p>En este trabajo se presenta una discusión sobre la metodología para el estudio teórico y experimental en la destilación solar, haciendo un breve recuento de los trabajos más significativos en este campo desde el punto de vista analítico y experimental. A partir de la discusión, se proponen los siguientes retos para promover el avance en el análisis de estos dispositivos: visualización de los patrones de flujo, medición de la velocidad y concentración con técnicas ópticas, métodos de optimización y teoría constructal, entre otros. El estudio refleja que la tendencia en el desarrollo de la destilación solar ha sido la metodología experimental de diversos diseños de destiladores para encontrar correlaciones y poder predecir coeficientes de transferencia de calor, tasa de producción de destilado y eficiencias; mientras que otros métodos analíticos y experimentales novedosos no se han utilizado hasta la fecha, dejando aspectos de la destilación solar pendientes de exploración.</p><p class="p1"> </p>
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