Abstract
Desalination is the process of removing salts from a saline water source to obtain fresh water. All desalination processes produce a salt-rich brine. If not disposed of properly, brine can lead to negative environmental consequences. Solar evaporation ponds facilitate its adequate disposal; however, this method requires a large footprint. As an alternative for small-scale desalination plants, a convection-enhanced evaporation (CEE) system has been proposed.
A CEE unit consists of several packed surfaces, distributed by uniform spacing. Brine is injected along the width of the surfaces, forming a liquid thin film. As ambient temperature and relative humidity vary on an hourly and daily basis, a controller is necessary to select the optimal brine injection rate, brine temperature, and air speed that maintains a constant evaporation rate. The optimized operating conditions results in a thermal energy load required to preheat the brine before it enters the evaporation system. This thermal load oscillates in magnitude, as optimal brine inlet temperature varies from less than 1° C up to 90° C, depending on time of the day and year.
In this paper, a model is developed to evaluate the use of a concentrated solar collector (CSC) system for the supply of the CEE system’s thermal energy requirements. The model predicts the performance of the coupled CSC-CEE system in terms of specific energy consumption, solar fraction, and evaporation fraction (relative to the target).
Findings establish that due to the high thermal energy demand of the system, the solar fraction can be relatively low, with values of less than 20% for small collector-tank configurations. However, it was also observed that low solar fractions do not necessarily translate to low evaporation fractions, which can be attributed to the high variability in the magnitude of the hourly heat load, and the fact that a high percentage of the evaporating hours depend exclusively on forced air convection through the CEE system without preheating the brine. Further work is needed to understand the tradeoff in footprint required for the CSC system, CEE system, and any non-evaporated brine storage.
