Evaluating the production and exergetic performance of point-of-use reverse osmosis devices for brackish water desalination

Abstract

An exergy analysis was conducted to investigate the high specific energy consumption (SEC) of point-of-use reverse osmosis (POU RO) devices. The RO module from one such device was experimentally characterized for desalination of 650, 1,000 and 1,800 mg/L sodium chloride solutions at 70–630 kPa feed pressures. The minimum SEC was 1.54 ± 0.04 kWh/m3, while the maximum second law efficiency and recovery ratio were 1.80% ± 0.05% and 24.6% ± 0.8%, respectively. Losses at the motor, pump, RO element, and flow restrictor respectively accounted for 36%, 25%, 8%, and 29% of the SEC at the intermediate concentration. By highlighting these inefficiencies, we also identified potential avenues for improving the system performance. Recovering brine pressure can decrease SEC significantly. Elevated feed pressures could also decrease SEC and raise recovery ratio but permeate flux would exceed recommended design limits (< 30 L/m2·h), thus increasing fouling risk. The same could be achieved by increasing membrane area provided that the resulting increase in cost and size of the system are acceptable. This work will help guide new developments to decrease the energy consumption of POU RO desalination.