PORTLAND, Ore.Energy Recovery Inc. has re-engineered its pressure exchangers, which enable fresh water to be derived from salt water worldwide in desalination plants, to run in reverse systems that generated power from mixing fresh- with salt-water. The first pilot plant, constructed by Norwegian-government owned renewable-energy company Statkraft is up and running. If successful, Statkraft plans to replicate its plants across the globe potentially generating enough energy to satisfy up to 50 percent of Europe's needs.
"Statkraft has selected our pressure exchangers to help make osmotic power production an economic reality," said Rick Stover, chief technology officer of Energy Recovery (San Leandro, Calif.). "Without our pressure exchangers, the forward osmosis process would be too inefficient to ensure the production of net-positive electricity."
For producing purified water, either from tap water or from salt water in desalination plants, a reverse osmosis process is used, whereby pressure is used to force salt water through a membrane that retains the particulate on one side and allows pure water to pass out the other side.
For producing electricity, a forward osmosis process is used, whereby water from the fresh water side of the osmotic membrane is drawn to the salty side by virtue of osmosis, thereby creating pressure in the salty outflow that can be used to drive an electricity turbine.
"The salt wants to go from the concentrated side of the membrane to the fresh water side, but is prevented by the membrane," said Stover. "Instead, the fresh water moves through the membrane, drawn to the salty side by osmosis."
The key component is Energy Recovery's pressure exchanger, which reduces the energy lost in the system by 60 percent, making electricity generation economical. The pressure exchanger has four connectionstwo allow the high pressure salt-water stream to enter and exit, while the other two allow the low-pressure fresh water stream to enter and exit. Inside the pressure exchanger, the only moving part is a ceramic rotor that transfers the pressure to the fresh water side from the salty water so it can drive a turbine that generates energy.
Statkraft's pilot plant showcases how the energy created through osmosis can be harnessed to generate a continuous source of renewable electricity, unlike solar, wind and wave which whose energy generation capabilities are dependent environmental conditions. If successful, Statkraft (Oslo, Norway) plans to build a full-scale osmosis energy production plant by 2015.
Energy Recovery has more than 7,000 pressure exchanger devices in the field at today, and claims to have patent protection that has already afforded it a 70 percent market share in desalinization plants. The company also claims to have the only pressure exchangers specifically designed to run in forward osmotic systems for power generation.