Salt-Gradient Solar Ponds for Renewable Energy, Desalination and Reclamation of Terminal Lakes
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Authors
Suarez, Francisco
Issue Date
2010
Type
Dissertation
Language
Keywords
desalination , distributed temperature sensing , double-diffusive convection , membrane distillation , solar pond , terminal lakes
Alternative Title
Abstract
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.
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In Copyright(All Rights Reserved)