Energy performance enhancements of a 950 m³/d seawater reverse osmosis unit in Grand Cayman

This paper describes the upgrade of a seawater reverse osmosis (SWRO) unit by replacing the hydraulic turbocharger energy-recovery system with a Dual Work-Exchanger Energy-Recovery (DWEER) system. Before the upgrade, the unit operated at a capacity of 1,071 m³/d, and a specific electricity of 3.00 kWh/m³. Tests conducted on the unit after the upgrade, when operated at essentially the same membrane conditions, showed a 59% capacity increase to 1,699 m³/d, and a 26% reduction in specific electricity to 2.22 kWh/m³. A further test was then conducted on the unit, by temporarily modifying it such that it operated at essentially the same conditions as before the upgrade. The test resulted in a specific electricity of 2.32 kWh/m³, a reduction in specific electricity of 23% compared to the original performance. This confirmed that the reduction in specific electricity was due to the DWEER system and not economies of scale from the increased capacity. A detailed efficiency analysis showed that the upgraded unit had an overall desalination energy efficiency of 45%, which was broken down into 85% for pumping, 92% for energy recovery; and 57% for the membrane array. This indicates that further improvements in efficiency are most likely to be achieved in the membrane array. The operating experience of the unit for the first 6 months is presented.

This paper demonstrates that significant energy savings and expansion by utilizing existing equipment can be achieved by retrofitting the DWEER system to an existing train. Additional improvements in the DWEER efficiency can only reduce the specific energy by a maximum of 8%, so there is little room for further improvement in energy recovery technology beyond what has already been achieved. Significant additional savings will only be achieved through improvements in the membrane array.