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Solar Collectors and Solar Thermal Energy Systems
Published in Radian Belu, Fundamentals and Source Characteristics of Renewable Energy Systems, 2019
A solar pond is a reservoir or pool of saltwater which collects and stores or transfer solar energy as heat. In a clear natural pond about 30% of the solar radiation reaches a depth of 2 m or so. This solar radiation is absorbed at the bottom of the pond. The hotter water at the bottom becomes lighter and hence rises to the surface, where the heat is lost to the ambient air and, hence, a natural pond does not attain temperatures much above the ambient. If some mechanism can be devised to prevent the mixing between the upper and lower layers of a pond, then the temperatures of the lower layers will be higher than of the upper layers. The simplest method is to make the lower layer denser than the upper layer by adding salt in the lower layers. The salt used is generally sodium chloride or magnesium chloride because of their low cost. Ponds using salts to stabilize the lower layers are called salinity gradient ponds. There are other ways to prevent mixing between the upper and lower layers. One of them is the use of a transparent honeycomb structure which traps stagnant air and hence provides good transparency to solar radiation while cutting down heat loss from the pond. The honeycomb structure is made of transparent plastic material. The saltwater naturally forms a vertical salinity gradient also known as a halocline, in which low-salinity water floats on top of high-salinity water. The layers of salt solutions increase in concentration (and therefore density) with depth. Below a certain depth, the solution has a uniformly high salt concentration. When the Sun’ rays contact the bottom of a shallow pool, they heat the water adjacent to the bottom. When water at the bottom of the pool is heated, it becomes less dense than the cooler water above it, and convection begins. In the case of a solar salt pond, a shallow pool with depth of 2–3 m with salt forms the solar collector for the energy systems, as shown in Figure 3.14.
Two-dimensional modeling of the vertical circulation of salt intrusion in the Sebou estuary under different hydrological conditions
Published in ISH Journal of Hydraulic Engineering, 2019
Soufiane Haddout, Abdellatif Maslouhi, Imad Baimik, Mohammed Igouzal, Hamid Marah
Model results of salinity during 4 and 22 February 2016, are shown in Figures 6a and 6b for comparison with the vertical profiles of measured values at Kenitra station. All together, eight vertical profiles of salinity at Kenitra station are presented in the comparison. A comparison of the model results and data indicates that the vertical salinity evolution at Sebou estuary is reproduced, under different river discharge (14 and 49 m3/s) and tidal conditions (at high and low tide). When the bottom water from the estuary encounters the saltier water from the oceanic region, there is an advection movement, causing the entrainment, which is responsible for transferring saltier waters from the bottom layers to the surface. The frontal pressure gradient caused by the encounter of waters with different densities causes a combination between friction force and entrainment, which results in a vertical circulation that characterizes active estuarine fronts (Garvine, 1977; de Barros et al., 2014). The entrainment and the turbulent diffusion are the main responsible for causing vertical mixing in the water column. Marine waters have a high density compared to fresh waters and tend to dive to the bottom of the river. The halocline is defined as a zone of rapid salinity increase with depth. The upper part of the halocline has very sharp gradient, and this is often visible to divers because of the neutrally buoyant organic matter that rests there (Dyer, 1991).