Pressure exchangers recycle the water pressure.
The requirements for membranes for PRO are quite different from the requirement for membranes for reverse osmosis (RO). The difference basically relates to the structure of the membrane support. The efficiency of the PRO membrane is measured in watt/m2. In order to produce electricity at competitive costs the membrane efficiency should as a minimum have a value of 3 – 5 watt/m2. Such values can now be obtained in lab scale testing and the task is to achieve similar results in testing larger quantities of membranes in prototype and pilot systems.
In addition the pre-treatment technology is crucial for the entire operation and is the major factor determining the long term operation of the plant. The large volumes of seawater and fresh water are filtered through special pre-treatment systems in order to protect the membranes and minimize fouling of the membrane surfaces.
Most of the other components such as pressure exchangers, pressure vessels, pumps, valves & fittings are basically off the shelf components, which in some cases may need limited modification to cope with the special conditions in a PRO plant.
With a power efficiency of the membranes of 5 watt/m2 a 25 MW production of electricity would require 5,000,000 m2 of membrane area.