Wind-powered membrane desalination of brackish water
Park, Gavin Lawrence
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This thesis presents a detailed investigation of the technical feasibility, challenges and performance issues associated with the direct-connection of a wind turbine to a membrane (wind-membrane) system for treating brackish water in remote communities. The direct-connection of these two technologies negates the reliance on energy storage in batteries, which are traditionally used, but result in reduced system efficiency and increased life-cycle costs. Furthermore, the lack of knowledge of the safe operating window in which transient operation of membrane systems is beneficial or tolerable can be addressed. The impact of wind speed fluctuations on the performance of the wind-membrane system (using a BW30-4040 membrane and feed waters of 2750 and 5500 mg/L NaCl) showed that the performance deteriorated most under fluctuations at low average wind speeds with high turbulence intensity and long periods of oscillation. Therefore, the main challenge of operating with renewable energy is not the size of the fluctuations, but the effect of the power switching off. Further examination of the impact of wind intermittency (over one hour intervals with intermittent periods from 0.5 – 3 min) showed that the increase in permeate concentration was highest at off-times < 60 s, highlighting the potential for improved performance using short-term energy buffering. The safe operating window and the key constraints to safe operation were determined for several membranes and feed water concentrations to establish the optimum operating strategy for the wind-membrane system. Supercapacitors were used to expand the safe operating window by providing energy during periods of intermittency and enhancing the power quality delivered to the membrane system by absorbing wind fluctuations. When tested over 24 hours using real wind speed data (average 6 m/s), the wind-membrane system produced 0.78 m3 of water with an average permeate concentration of 240 mg/L NaCl and average specific energy consumption (SEC) of 5.2 kWh/m3. With the addition of supercapacitor storage, the system performance improved significantly with 0.93 m3 of water produced with an average permeate concentration of 170 mg/L NaCl and SEC of 3.2 kWh/m3.