Water Desalination Using Low-Grade Heat

As a part of a $2M award from US Department of Energy, Solar Energy Technologies Office, a novel humidification-dehumidification (HDH) desalination technology protected under US Patent Application US62882953 is being developed.

First, saline water is atomized by thermally actuated air jets generated by using low grade thermal energy obtained from solar collectors. Then, the spray is evaporated using recouped heat in a first stage heat recovery process. Next, the solid salt particles are extracted in a cyclone separator. The heat is thereafter recouped from the pure humid air in condenser and then the liquid water is separated from the air in a two-phase separator. Finally, the warm freshwater is used to preheat the incoming saline water and then get discharged while the warm air is recirculated in the cycle. The figure shows the process cycle in one module. Multiple modules can be mounted depending on the product freshwater demand. Most of the processes were modeled analytically and numerically as well as tested experimentally through in-house built set-ups.

Desalination Diagram

Perforated Plate Atomizer

Atomization of saline using conventional nozzles faces serious technical problem of scale formation/fouling ultimately clogging the atomizer. In this regard, we propose a novel perforated plate atomizer which significantly reduces the scaling of the nozzle tip. In contrast to the conventional atomizer, perforated plate atomizer avoids saline water passing through any small orifice. A thin film of saline water is maintained on the top of a perforated plate with the help of primary and secondary manifolds, while air jet flows perpendicular to the saline thin film and atomizes it. An in-house experimental facility of perforated plate atomizer was developed and tested for different geometries and operating conditions including air and water temperatures, water salinity, air-to-water ratio and mass flow rates. Maximum salinity of 10% was tested during the experiments and no fouling was observed for three hours of operation. High speed imaging (5000 fps) was performed to characterize the atomized droplets in terms of droplet size, distribution, and uniformity. A shadow box fosters the atomizer assembly, which minimizes the interference of the ambient light and allowed to capture high quality high speed videos. Analysis of the high speed videos reveled droplets as small as 100 ┬Ám was achieved within the tested conditions. Future work focuses on developing design guides to obtain specific spray characteristics to optimize desalination.

Plate Atomizer

Spray Evaporation or Air Humidification

In addition to the investigation of droplet sizes and velocity distribution, evaporation of these atomized saline water droplets is an important parameter of interest for the desalination cycle. Therefore, a new spray evaporation measurement technique and analysis are being developed to account for the evaporation rate along with the distance from the nozzle tip. Analytical model is developed and validated against experimental data to pinpoint the most accurate thermodynamic state and fully characterize spray evaporation along the flow path. The study covers experimental work correlated to entropy analysis and fundamental analysis to obtain the evaporation rate at different conditions.

Spray Evaporation

Cyclone Separator

Analytical and numerical models were built to test and design a cyclone separator capable to meet the designed cycle specifications and achieve the maximum separation efficiencies for the smallest salt cut-diameters. The models were used to select a design of the cyclone to be 3D printed in-house. An in-house experimental setup was developed to validate the simulation results and a detailed parametric study is performed on cyclone to calculate separation efficiency. Some experiments showed a separation efficiencies approaching 99.6%. Further, tests are performed to test for salt separation in particle-laden streams for both dry and humid air.