Energy Recovery Design

The simplicity of a pressure exchanger's mechanical design is playing a key role in the increased efficiency of seawater reverse osmosis (SWRO) systems. The result is very high transfer efficiencies between the concentrate and seawater feed streams, and reduced energy usage at lower recovery rates.

"By using energy recovery technology, applications can recover 55 to 60 percent of the energy required for the reverse osmosis process," says Jeremy Martin, director of engineering for Energy Recovery Inc. "The PX^(TM) pressure exchanger device itself has an efficiency of up to 98 percent and recovers virtually all of the pressure energy in the reject flow stream of the RO process."

ERI's PX devices have only one moving part, a high purity aluminum oxide rotor that turns at up to 1,200 rpm using an almost frictionless hydrodynamic bearing. Inside the rotor are channels in which the concentrate from the membranes and fresh salt water come into direct, momentary contact. The momentum of the water turns the rotor at a speed that adjusts to flow variations and nearly constant high efficiency over a wide operating range. The unit's design, which uses ceramic material, makes it unaffected by chemicals or aqueous corrosion, and three times harder than steel. It also provides unmatched durability in the PX device application.

"The heart of our innovation is using the engineering ceramic rotor and machining it to tight tolerances, so the spinning rotor can be powered by the flow of water going into the device," Martin says. "By maintaining very tight tolerances, there is very little leakage of water from the high- and low-pressure sides, which creates a very effective high-pressure seal."

The PX energy recovery device uses positive displacement and isobaric chambers to achieve extremely efficient transfer of energy from a high-pressure waste stream, such as the brine stream from a reverse osmosis desalination unit, to a low-pressure incoming feed stream. The key is that virtually no energy is lost in the transfer.

Energy Recovery is continuing to explore ways to further increase efficiency and build larger-capacity units. The SWRO industry operates at relatively high pressures (around 800 psi) but there is also a larger market for brackish reverse osmosis systems that operate at lower pressures.

Another potential application is osmotic power, an energy-generation process that uses the osmotic potential difference between seawater and fresh water to generate electricity. The plant is driven by osmosis that naturally draws fresh water across a membrane and toward the seawater side. This creates higher pressure on the sea water side, driving a turbine and producing electricity.

Statkraft, a large European producer of renewable energy with experience in hydropower that provides nearly all of Norway's electricity, aims to begin building commercial osmotic power plants by 2015. The main issue is to improve the efficiency of the membrane from around one watt per square meter now to about five watts, which would make osmotic power costs comparable to those from other renewable sources.