PhD project title: Enhancing the local water cycle via evaporation for a sustainable water supply (# of pos: 14)

Topic background
In many arid regions, water resources are scarce due to the low rainfall. Scarcity of water leads to lowered agricultural productivity and impairs the health and wellbeing of the population. This situation is expected to deteriorate due to climate change, growing population, and agricultural activities. Desalination techniques are available to produce drinking water, however they do not solve the root cause of the problem i.e. a lack of sufficient precipitation. To make more freshwater available, we propose a novel approach that aims at restoring the local water cycle sustainably. Counter-intuitively, restoring the water cycle requires additional evaporation of water, to lower the condensation level in the atmospheric boundary layer, which turn is expected to give enhanced water precipitation in the watershed making more water available. A restored water cycle can sustain itself as the evaporated water returns as precipitation within the watershed. Enhanced recycling can mitigate water scarcity issues as more useable water that is fully cleaned becomes available.

Research challenges
Crucial for restoring a well-functioning local water cycle, is the availability of water. Here we will look into technology that produce water vapour from brackish or seawater, via direct evaporation, driven by the abundant solar energy. The basic principle is that solar radiation is converted into heat that is used to increase the water temperature. This increased temperature leads to an increased vapour pressure, which in turn drives the transport of the water vapour to the atmosphere through a suitable membrane. This membrane is the interface between the seawater and the atmosphere and is required to prevent leaking of the brine to the environment. We will investigate the suitability of different processes namely membrane evaporation vs. pervaporation. The performance of this process will depend on the atmospheric conditions, like solar irradiation, turbulence in the lower boundary layer, temperature, wind speed and humidity profiles. These conditions are strongly variable in time and space. It is therefore of paramount importance to connect the mass transport inside the membrane with that of the atmospheric boundary layer. For this, a micrometeorological numerical model (such as PALM: Parallelized Large-Eddy Simulation Model) is employed to quantify the local scale environmental/hydrometeorological impacts of the membrane-based evaporation system.

Objectives and methodology
The aim of the project is to understand the performance of direct evaporation technology under realistic atmospheric conditions. As most membrane systems are used in engineered relatively constant environments, little is known on this interaction. As this direct evaporation technology is quite young, a prototype oriented approach will be used, i.e. prototype will be developed both to assess the performance and study underlying fundamental thermodynamic and transfer processes. The micro data obtained from the prototype are upscaled and validated using a process-based micrometeorological model. The model will assess the overall performance and impacts of the established membrane system on the localized water cycle for the target region. Therefore, the focus of the project lies in understanding the relation between performance of the membrane system and the surrounding lower boundary-layer atmosphere.