MVC desalination unit; Numerical modeling; Dynamic performance; Renewable source energy; Evaporation and condensation.
This paper presents a numerical model to analyze the thermal and fluid dynamic behavior of a mechanical vapour compression MVC desalination system. The MVC desalination method is an evaporation and condensation process that occurs at low pressure, which requires a compression work to increase the saturation temperature of the vapor. The compressed vapor is condensed and its latent heat is transferred to the feed seawater. The MVC desalination is used at low and medium scale in comparison with other techniques such as: multistage flash desalination (MSF) or reverse osmosis (RO). The MVC desalination unit studied in this paper uses renewable energy to supply the electricity required by the different devices that make up the unit. The reason to use renewable energy is that the MVC desalination system has been though to work in remote places, where an electric grid is not available. However, using renewable energy means variability in the supply power. This variability should be well defined to avoid damage and establish secure partial working operation of the MVC desalination system. The transient and steady-state behaviors of the desalination system are evaluated taking into account the variability of the renewable energy sources (solar and wind energy). The MVC desalination system has been divided in three different subsystems, following the strategy proposed by Mazini (2014). The first subsystem is the evaporator and condenser, in which the evaporation and distillation processes are performed. The second is the vacuum and deareation subsystem, where the low pressure is achieved and non-condensed gas (Oxygen) is stripped. The last subsystem is the mechanical compressor, which is modeled to know its energetic requirement in function of the desalination performance and the climatic conditions. The mathematical formulation of the evaporator/condenser and vacuum subsystem is based on mass, energy and salt balance conservation equations. Whilst the compressor model is based on the root blower laws, in which the volumetric flow, velocity, power and the displacement by revolution values are related. The group of equations is solved by means of the in-house object-oriented tool called NEST, which is capable to link and solve different elements that making up a system (Damle, et. al., 2011). The MVC desalination system that is presented in this paper has different components: an evaporator/condenser, a compressor, a deareator, two heat exchangers and a group of pumps. Although in this numerical platform each component is an object, the whole system resolution is carried out iteratively for solving all its components and transferring the appropriated information between them. A scalability study has been carried out to find the relation between the variability of the renewable energy sources and the capacity of the desalination system (distilled water production). The influence of the feed seawater conditions is also analyzed on the system performance.