Abstract

Powering desalination by waste heat is often proposed to mitigate energy consumption and environmental impact; however, thorough technology comparisons are lacking in the literature. This work numerically models the efficiency of six representative desalination technologies powered by waste heat at 50, 70, 90, and 120 °C, where applicable. Entropy generation and Second Law efficiency analysis are applied for the systems and their components. The technologies considered are thermal desalination by multistage flash (MSF), multiple effect distillation (MED), multistage vacuum membrane distillation (MSVMD), humidification-dehumidification (HDH), and organic Rankine cycles (ORCs) paired with mechanical technologies of reverse osmosis (RO) and mechanical vapor compression (MVC). The most efficient technology was RO, followed by MED. Performances among MSF, MSVMD, and MVC were similar but the relative performance varied with waste heat temperature or system size. Entropy generation in thermal technologies increases at lower waste heat temperatures largely in the feed or brine portions of the various heat exchangers used. This occurs largely because lower temperatures reduce recovery, increasing the relative flow rates of feed and brine. However, HDH (without extractions) had the reverse trend, only being competitive at lower temperatures. For the mechanical technologies, the energy efficiency only varies with temperature because of the significant losses from the ORC.

Highlights

  • Demand for water has been growing steadily due to growing population, industrialization, and consumer usage [1]

  • Chemical disequilibrium is negligible compared to other terms. Another major contribution is the top heater, especially for the 50 ◦ C case. This occurs because the energy regeneration is so low that significantly more energy has to be transferred to the feed stream from the top heater, generating large amount of entropy, and because the heat transfer occurs at lower temperature

  • The higher recovery ratio and overall good efficiency led to a relatively large role of entropy generation from chemical disequilibrium. As this is a simple one stage mechanical vapor compression (MVC) system, and since much of the entropy generation is in a mechanical component, the compressor, and in two phase heat transfer, significant gains in efficiency can be made with superior designs, such as multistage systems [43] and various technologies

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Summary

Introduction

Demand for water has been growing steadily due to growing population, industrialization, and consumer usage [1]. The irreversibilities analyzed included entropy generation in throttling to produce vapor (flashing), fluid expansion without phase change, pumping, compression, heat transfer, and mixing of streams at different temperatures (thermal disequilibrium) and different salinities (chemical disequilibrium) [17]. These results are used to compare the relative performance of technologies at different heat source temperatures, and to analyze which components are responsible for the major inefficiencies at different temperatures

Derivation of Performance Parameters for Desalination
Entropy Generation Mechanisms
Unused Temperature Reduction of Waste Heat Sources
Entropy Generation Analysis of Seawater Desalination Technologies
Modeling Approximations and Assumptions
Multistage Flash
Multiple Effect Distillation
12 Cond TD
Humidification-Dehumidification
Organic Rankine Cycle
Mechanical Vapor Compression
Reverse Osmosis
Applicability of Analysis to Systems of Different Costs and Sizes
Technology Comparison
Conclusions
Conflicts of Interest
Full Text
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