ABSTRACT:
A new and innovative concept was used to design and study an inverse-U shaped (∩) desalination unit, the system utilizes the low pressure (near vacuum) generated naturally by pushing water more than 10.33 meters using the atmospheric pressure as the main pumping force. By introducing low pressure on top of the water to be distilled, evaporation of water can be carried at lower energy levels compared to conventional types of water desalination units. The unit is a ∩ shaped pipe that consists of saline water chamber at one end and a fresh water chamber at the other end of the pipe, the saline water chamber is under solar irradiance (which is the main heat source) and the fresh water chamber is enclosed by a cylindrical heat exchanger that passes water to keep the chamber cold. The saline and fresh water chambers are at more than 10 meters height and connected to saline water inlet and fresh water outlet pipes, additionally, a pipe is connected to the saline water chamber to release the brine. Saline water evaporates and the vapor travels to the fresh water chamber to be condensed.
A theoretical study was carried out on the natural vacuum desalination unit under the operating conditions of Bahrain. A mathematical model was developed consisting of the suitable thermo-dynamical and heat transfer relations that closely describes the process of desalination. The natural vacuum desalination model was solved numerically using a customized built MATLAB code coupled with number of readymade functions. Two main cases were studied; hot and cold weather operating conditions, additionally, sensitivity analysis was carried out on a group of specific parameters.
The original configuration or design of the system (with 0.05 m^2 surface area of evaporation) was able to produce up to 0.54 kg/day of fresh for hot weather operating conditions and 0.149 kg/day of fresh water for cold weather operating conditions. The thermal efficiency and exergy efficiency were 44.58 and 5.44% for hot conditions and 9.98 and 0.375 % for cold conditions, respectively. For an evaporation area of 1 m^2, the output of the system was 3.60 kg/day, generally increasing the area of evaporation increased the system’s fresh water production, but stopped changing after 2 m^2.
The theoretical results of this study configuration have been compared to previous studies. For 1 m^2 area of evaporation surface, the NVD system achieved lower water production affected by the low efficiency of the current design.
Decreasing the depth of the saline water showed an increase in thermal efficiency (from 44.58 to 79.85%), but the fresh water production decreased considerably (from 0.54 to 0.313 kg). Additionally, the salts concentration increased when the depth was decreased (from 3.521 to 3.724%). The temperature of the cooling water was studied; decreasing the temperature 4°C(from 30 to 26 °C) increased the output of the system from 0.54 to 0.710 kg. Various withdrawal rates were studied, it was found that using withdrawal rate beyond 0.001 m3/hr stabilizes the salts concentration, but significantly decreases the fresh water production (from 0.331 to 0.228 kg for 0.1 m depth of saline water). Finally, the economic aspect was taken into consideration, the minimum cost of 1 kg of fresh water produced was found to be 0.166BD for a unit with 1 m^2 of evaporation surface.
