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Salt-Gradient Solar Ponds for Renewable Energy, Desalination and Reclamation of Terminal Lakes
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Terminal lakes are water bodies that are located in closed watersheds with the only output of water occurring through evaporation or infiltration. The majority of these lakes, which are commonly located in the desert and influenced by human activities, are increasing in salinity. Treatment options are limited, due to energy costs, and many of these lakes provide an excellent opportunity to test solar-powered desalination systems. This dissertation investigates utilization of direct contact membrane distillation (DCMD) coupled to a salt-gradient solar pond for sustainable freshwater production at terminal lakes. The major advantages of this system are that renewable thermal energy is used so that little electrical energy is required, the coupled system requires low maintenance, and the terminal lake provides a source of salts to create the stratification in the solar pond. A fully coupled two-dimensional, numerical model that evaluates the effects of double-diffusive convection in the thermal performance and stability of salt-gradient solar ponds was developed. The model was successfully used to predict the behavior of a laboratory-scale solar pond. The solar pond was instrumented with a vertical highresolution distributed temperature sensing system. This instrument, which achieved temperature resolutions as small as 0.035 °C when 5-min integration intervals were used, allowed monitoring the vertical temperature profile every 1.1 cm. The temporal evolution of the temperature profile was used to estimate the heat extracted from the pond. This heat was used to power a DCMD unit. Freshwater fluxes in the order of 1.0 L hr-1 per m2 of membrane were obtained when the solar pond operated with 29% of efficiency, equivalent to a freshwater production of 0.12×10-3 m3 d-1 per m2 of solar pond. A heat and mass transfer model of this coupled system was developed and resulted in fairy well agreement with the experimental results. This model was used to evaluate the energy required to distillate the water that passes through the membrane. It was found that approximately 35% of the energy extracted was used in the distillation process, 35% was lost in the membrane module due to conductive heat losses, and the remainder was lost in the plumbing of the coupled system due to conduction. Furthermore, the heat and mass transfer model was utilized to evaluate the feasibility of freshwater production at a terminal lake (Walker Lake, NV). As results showed that freshwater flows are on the same order of magnitude as evaporation, the coupled system will only be successful if the solar pond is constructed inside the terminal lake so that there is little or no net increase in surface area. At Walker Lake, a potential water production on the order of 1.6×10-3 m3 d-1 per m2 of solar pond is possible in the DCMD module. When heat is extracted from the solar pond, the evaporation rate from the solar pond is smaller than that of the Lake. Thus, an additional 1.1×10-3 m3 d-1 per m2 of solar pond is obtained. A preliminary assessment of Walker Lake showed that the coupled system has the potential to contribute in the reclamation of this terminal lake. Using the previous water production values, and if 13% of the lake’s area is used to construct the solar pond, more than 100 years are needed to reduce the Lake total dissolved solids concentration below 12,000 mg L-1. If more research is carried out to improve the performance of this coupled system, shorter reclamation times would be needed. This also strongly points out that this system needs to be used in conjunction with other approaches, e.g., water acquisition, to expedite Walker Lake reclamation.