Cryogenic energy for indirect freeze desalination – numerical and experimental investigation

Due to the increasing demand for fresh water, desalination of sea water is viewed as a potential solution to overcome potable water shortages; therefore desalination technologies have been continuously developed. Renewed interest in freeze desalination has emerged due to its advantages over other desalination technologies. A major advantage of the freeze desalination technology over the evaporative methods is its lower energy consumption (latent heat of freezing is 333.5 kJ/kg and latent heat of evaporation is 2256.7 kJ/kg). Cryogenic fluids like LN2/LAir are emerging as an effective energy storage medium to maximise utilisation of intermittent renewable energy sources. The recovery of this stored cold energy has the potential to be used for freeze desalination. Computational Fluid Dynamics (CFD) is a powerful technique that allows investigating complex thermodynamic processes, including freeze desalination and evaporation of LN2. CFD modelling was developed to simulate the freeze desalination process in terms of the dynamics of ice layer growth and salt separation to investigate the amount of ice formed and salinity of the remaining brine at different operating conditions. The developed CFD model was validated by experiments conducted using a Peltier device and an ice maker machine showing good agreement with maximum deviation of less than 16.9%. Parametric analysis was then carried out using the validated CFD models, showing that as the freezing temperature decreased, the ice production increased due a faster rate of freezing of salt water. The initial salinity of salt water had a significant effect on the volume of ice produced and the output salinity. Therefore, different stages of freezing were needed to produce water with a salinity level below the required 0.1% recommended by the WHO as safe to drink. Parametric analysis was carried out on the geometry of the freezing tubes in the ice maker machine, where increasing the diameter and the length of the freezing tube, increased the volume of ice produced as this increased the total freezing surface area but this also increased the heat transfer rate and power consumption. It is seen that, the 20mm diameter with a 15mm length is the best geometry to use with a low heat transfer rate producing a higher volume of ice. Regarding the use of cryogenic fluids, the validated CFD model for freeze desalination was integrated with CFD modelling of liquid nitrogen evaporation, to investigate the feasibility of using cryogenic energy for freeze desalination. This integrated CFD model was validated using experimental heat exchanger test facility constructed, to evaporate liquid nitrogen to supply the cooling required for freezing. Tests were conducted without and with a copper mesh being inserted in the liquid nitrogen tube, where inserting the mesh improved the heat transfer rate to produce more desalinated water. The percentage of energy lost by water to form ice from liquid nitrogen increased significantly for the tests carried out with the mesh inserted as it was 70% and 63% compared to only 25% and 21% for the tests conducted without the mesh. The heat exchanger effectiveness improved considerably when the mesh was inserted as it was about 3.65 times more compared to the tests carried out without using a mesh. Parametric study on the LN2 flow rate to observe the volume of ice obtained was also examined using CFD, where increasing the velocity of LN2 by 6 times, increased the volume of ice obtained by 4.3times. A number of freezing stages were required in order to reduce the ice salinity from 1.5% down to 0.1% as regarded by the WHO as safe to drink. Hence, the overall efficiency was 0.12 for the ice maker machine to reduce the ice salinity from 1.5% to less than 0.1%. In the cryogenic desalination test rig, approximately 1.35 litres of liquid nitrogen was required to reduce the ice salinity from 1.5% to less than 0.1% with an overall efficiency of 0.23. Therefore, it is seen that the cryogenic desalination test rig had a better efficiency. Furthermore, the above results illustrate the potential of using the cold energy of cryogenic fluids such as LNG and LN2/LAir for freeze desalination applications as most cold energy during LNG regasification has been unexploited today