Single-Stage, Venturi-Driven Desalination System (2020)

Water demand is increasing at a rapid pace due to population increase, industrial expansion, and agricultural development. The use of desalination technology to meet the high water demands has increased global online desalination capacity from 47 million m^3/d in 2007 to 92.5 million m^3/d as of June 2017 [49]. Membrane and thermal processes are the two mainstream desalination categories used worldwide for desalination plants. Reverse Osmosis (RO) is the most widely used membrane process and it has become the dominant technology for building desalination plants over recent decades. Thermal distillation, however, has become less and less competitive due to its high production costs, mainly due to a reliance on increasing fuel prices and large thermal energy requirements. Although heat recuperation is commonly used, it adds investment cost and increases complexity of the system. The concept of Single-Stage Venturi-driven (SSV) Desalination, a single-stage, thermal desalination system, using a Multifunctional Venturi Water Ejector (Venturi system), is proposed, analyzed, and demonstrated. The system requires only low-grade solar heat (< 60 °C) mainly to supplement the heat loss during operation. As compared to the conventional methods of solar desalination, the proposed system has the following intellectual novelties: First, the novel multifunctional water ejector integrates a vacuum pump for steam production, a compressor for condensation, and a starter for heat recuperation. Second, only residential-grade solar water heating is needed for the heat demand which greatly reduces the production cost of solar desalination, as compared to those systems using concentrated solar power (CSP). Third, the proposed system is operated standalone based solely on solar energy. The main objective of this research is to accurately analyze and model the SSV system, and achieve an estimated levelized cost of water (LCOW) close to the DOE target of $0.50/m3 (DE-FOA-0001778) [55]. Additionally, prototypes, operating at about 0.1 bar, were built to prove the concept that very low-grade heat sources can be utilized with the system. While similar to other thermal methods, such as MSF (multi-stage flash desalination), MED (multi-effect desalination), and VC (vapor compression desalination), the SSV system utilizes a unique water ejector to reduce vapor pressure in a “boiler” and operate at lower temperatures, thereby increasing the heat regeneration efficiency and decreasing the heat input temperature requirements. The concept, as well as the scalability, of the system is proven in the results. The performance of the Venturi System was simulated using Comsol Multiphysics. The simulation results were compared to both the theoretical and experimental results. The lowest experimental vacuum pressure achieved during operation was 0.07 bar, equating to a boiling point of 40 ℃. High-performance, customized Venturi water ejector designs are projected to further lower vacuum pressures. In this study, a thermo-economic analysis was performed as a theoretical baseline for the performance of the novel technology. In the future, the baseline results should be compared to experimental results of a pilot or operational SSV desalination plant. The resulting energy requirements of the system are calculated as 40.6 kWh/m3 for thermal and 0.23 kWh/m3 for electrical energy requirements. The performance ratio and exergy efficiency are calculated as 15.4 and 39%, respectively. Using all three modes of analysis, theoretical, experimental, and computer simulation, the system makes a strong case as a cost competitive desalination solution. Ultimately, the Thermo-Economic model estimated the LCOW at $0.67/m3, achieving a lower price point than most commercialized solar desalination technologies.