Pressure Exchange Based Thermal Desalination Open Access
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The use of ejectors in distillation type water desalination systems is well known. However the energy efficiency of such systems is limited by the mechanical energy dissipation in the ejector. Conventional ejectors have been limited in their performance due to the dissipative mechanism of turbulent mixing upon which they rely.Recent advances in direct fluid-fluid flow induction provide potential for major improvement in the performance of ejectors based on the pressure exchange phenomenon compared to the conventional turbulent mixing controlled ejectors. Pressure exchange devices utilize the work of non-steady pressure forces acting across moving interfaces. The limits in performance of such devices can be determined through the use of the ideal turbomachinery analog. The analog is configured as a turbine-compressor unit, where the high energy primary fluid expands through the turbine that drives a compressor which compresses the low energy secondary fluid and the two then discharge in a common mixing chamber at a common intermediate pressure. The system implementation of the turbomachinery analog is similar to the conventional ejector. Thus the analog provides the highest possible performance that an ejector can achieve ideally. The analog defines the ejector efficiency as the product of adiabatic turbine and compressor efficiencies. An analytical single effect thermal vapor compression desalination model for 5m3/day capacity is developed. The turbomachinery analog which is the conceptually simplest kind of pressure exchange device, replaces the conventional ejector. The objective of the research is to investigate the improvement in performance of the system by employing pressure-exchange technology. Critical parameters that directly affect the cost of the distillate produced will be analyzed. The indicators will be compression ratio, top brine temperature, primary pressure, primary temperature and ejector efficiency. The system performance is expressed in the form of thermal performance ratio (TPR), energy performance ratio (EPR), entrainment ratio (Er) and specific mass flow rate of cooling seawater (sMcw). Results show that the use of pressure exchange technology results in three fold rise in energy efficiency.