Renewable energy-powered membrane technology: Supercapacitors for buffering resource fluctuations in a wind-powered membrane system for brackish water desalination

The potential for supercapacitors to expand the safe operating window of a wind-powered reverse osmosis membrane (wind-membrane) system by buffering short term wind fluctuations and intermittency was investigated. Experiments were carried out using synthetic brackish water (5500 mg/L NaCl) with three sizes of supercapacitor bank to determine the effect of increasing the short term energy storage capacity. The wind speed ranged from 4 to 14 m/s for both intermittency and fluctuation experiments, with periods of no-power of 0.5–5 min and 15 s–20 min cycles, respectively. When the wind-membrane system was powered by the supercapacitors, wind speeds of >7 m/s were required for the supercapacitor bank state of charge (SOC) to increase, otherwise they discharged gradually to a threshold value dictated by the control electronics. While the SOC of the supercapacitors was above this threshold value, the operation of the wind-membrane system was as under steady-state conditions, thereby achieving independence of the wind speed fluctuations or intermittency. This resulted in an 85% increase in the average flux and 40% increase in permeate quality under fluctuations when compared to the system performance without supercapacitors. It is concluded that supercapacitors are an effective method of buffering short term wind speed fluctuations to provide steady-state performance and improve the productivity of renewable energy membrane systems.