A novel design for solar-powered thermal desalination

Date of Award

2016

Degree Name

Ph.D. in Mechanical Engineering

Department

Department of Mechanical and Aerospace Engineering

Advisor/Chair

Advisor: Jun-Ki Choi

Abstract

One of the most promising applications of solar energy is water desalination, especially in regions where fresh water is scarce and sunlight is abundant. Desalination is a growing and necessary source of fresh water, but it is highly energy-intensive. Conventional desalination is currently supported with fossil fuels, and it is critical to explore renewable options to reduce pollution. Solar-powered desalination is currently being studied, with various small to medium scale plants at several locations around the world. Some methods use heat exchangers and medium fluids to transfer heat to the brine while others directly circulate the brine through solar array. Some systems produce distilled water during daylight hours only, while others employ control systems and thermal storage to achieve continuous production of distilled water. This work proposes two designs for solar powered thermal desalination. Both of the designs use a novel thermal storage system for efficiently managing and delivering solar energy to a desalination unit. The system consists of a pair of tanks that alternate roles each day between receiving energy from a solar array and delivering energy to the desalination unit. The first design couples the dual thermal storage tanks to an array of concentrating solar collectors and a multi-stage flash (MSF) desalination unit. MSF operates by partially flashing a stream of hot brine within a series of vacuum chambers, and condensing the vapors onto the outside of heat-exchange tubes that carry incoming feedwater. This process creates a flow of pre-heated brine which enters one of the storage tanks and is further heated by circulation through the solar array. The brine is directly heated in the array, such that no medium fluid or heat-exchangers are necessary. The other tank, which was charged in this same manner the previous day, provides hot brine to supply the MSF. The storage tanks alternate roles at approximately sundown each day. This design responds to different levels of daily solar energy by modifying the mass flow of incoming feedwater, such that different volumes of feedwater can be raised to nearly the same temperature each day. The dual storage tanks allow the brine heating and discharging operations to be decoupled, allowing for predictable operating conditions for the MSF. The second solar desalination design utilizes multi-effect desalination (MED). Each effect of an MED unit is a chamber in which cool feedwater brine is sprayed onto heat-exchangers, partially evaporating the brine. The vapors and heated brine serve to pass thermal energy to subsequent effects. The heat exchanger in the first effect of the system receives a flow of hot medium fluid, such that sensible heat is passed to the incoming feedwater. One of the dual storage tanks in this case provides this hot medium fluid, which flows through the first effect, losing heat, and then into the second tank. While in the second tank, the medium fluid is re-heated by circulation through the solar array. As with the MSF design, the tanks reverse roles at approximately sundown each day. Unlike the MSF design, this approach passes a fixed volume of medium fluid through the MED unit each day, and the variations in daily solar energy lead to daily variations in the stored medium fluid temperature. MED technology can operate at a range of temperatures, and the feedwater flow rate is adjusted accordingly. Both solar designs are mathematically modeled in order to verify performances. TMY3 data provides average hourly solar and ambient temperature activity, allowing for estimated system performance at a given location. The modeling uses the daily solar activity to predict distillate output each day of the year, and it allows for predicting the collector area necessary for reaching a desired production level. An economic analysis is conducted for the proposed MSF design. A detailed cost model is developed for a complete plant, and the economic performance is summarized using net present value (NPV) and Internal Rate of Return (IRR). Exploring these as functions of the plantäs scale and other parameters is useful for making design choices and for comparison with conventional desalination. A full blown Environmental Life Cycle Analysis (LCA) is performed to quantify the lifecycle environmental impact of the proposed design. LCA considers inputs/outputs of materials, energy, and wastes from the raw material extraction to the end-of-life of the proposed desalination plant. The results provide insights about various environmental hotspots of the proposed design and can be utilized for economic and environmental trade-off decision analysis of the desalination plant.

Keywords

Solar saline water conversion plants Design and construction, Solar saline water conversion plants Environmental aspects, Solar saline water conversion plants Cost of operation, Mechanical Engineering, Solar Multistage Flash Desalination, Solar Multi Effect Distillation, Economic Analysis of Desalination, Life Cycle Analysis of Desalination

Rights Statement

Copyright © 2016, author

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