Modeling Of A Solar Thermal Energy Input For An Optimized Convection-Enhanced Evaporation System

Abstract

Desalination is the process of removing salts from a saline water source to obtainfresh water. All desalination processes produce a salt-rich brine; if not disposed of properly, brine can lead to negative environmental consequences including alteration of marine ecosystems or pollution of underground water reservoirs. Solar evaporation ponds facilitate its adequate disposal; however, this method requires a large footprint. Other brine management methods such as multi-effect evaporation, are only cost effect for large installations of >1000 m³/day. As an alternative for small-scale desalination plants, the Wright Lab at the Universityof Minnesota, in collaboration with QuadSun Solar Solutions, has developed a convection-enhanced evaporation (CEE) system which can be imagined as a “folded and stacked” version of an evaporation pond. A CEE unit consists of several packed horizontal or vertical surfaces, distributed by uniform spacing. Brine is injected along the width of the evaporation surfaces, forming a liquid thin film. A fan drives airflow parallel to the surfaces in a co-flow configuration, driving the evaporation of the liquid by a difference in vapor pressure between the air and the surface of the liquid. As ambient temperature and relative humidity vary on an hourly and daily basis, the Wright Group has previously developed an optimization model and controller that can determine the lowest energy operating parameters for the CEE system. Given a target evaporation rate and system size, the controller selects the optimal brine injection rate, brine temperature, and air speed. This set of optimized operating conditions includes a thermal energy load requiredto preheat the brine before it enters the evaporation system. This heat load oscillates in magnitude, as optimal brine inlet temperature varies from less than a degree Celsius up to 90 degrees Celsius, depending on time of the day and year. The goal of this project was to create a model that could 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). The model was developed considering the different system configurations present inthe solar desalination literature. The final design includes a two-axis solar concentrator; the thermal energy is supplied and stored using a fully mixed storage tank with two immersed-coil heat exchangers (IHX). A series of control-oriented parameters were included in the computational model to ensure precise compliance with CEE system operating conditions. The model was then used to perform a series of parametric analyses that covered differentoperating configurations of the CEE system, including different scheduled running times (24 vs. 12 hours), number of CEE surfaces (100, 200, and 500), and alternative all-electric operating conditions for hours when solar heating was not available or sufficient. 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 of forced air convection through the CEE system without preheating the brine. The specific energy and evaporation performance of the CSC-CEE system are similar to larger, commercially available systems. Further work is needed to understand the tradeoff in footprint required for the CSC system, the CEE system, and any non-evaporated brine storage. In the future, the computational model can be improved by including thermal lossesassociated with having the CSCs in series and any piping losses. Modeling the thermal stratification in the storage tank could lead to higher thermal performance, although the computational cost of performing this type of analysis will need to be evaluated. In addition, seasonal operation schedules should be evaluated, to take advantage of the most favorable ambient conditions of the year and further decrease the specific energy required to run the CEE system.