Why Use ERDs?
MAKING DESALINATION AFFORDABLE
ERI’s Pressure Exchanger technology was designed in the late 1990s to address the need for affordable desalination of water. ERI’s (PX™) energy recovery devices reduce the amount of energy required in membrane desalination by up to 60%, resulting in more economical production of drinking water and a reduced carbon footprint. Over 7,200 PX devices have been sold worldwide to support the production of drinking water for about 25 million people. These devices are estimated to save more than 750 megawatts of electrical power, while reducing carbon dioxide emissions by morethan 4.3 million tons per year.
ENERGY IMPROVEMENT
A variety of devices have been developed to recover energy from membrane reject stream and return it to the reverse osmosis process. Turbine-based, centrifugal energy recovery devices (ERDs), such as the Francis turbine and Pelton Wheel, have been employed since the 1980s. While these devices are still employed to in many older desalination plants, they are less efficient than isobaric chamber devices.
Impact of energy recovery devices on SWRO energy consumption
The impact of improved energy recovery technology on the energy required to producing drinking water with SWRO is illustrated above. The Jeddah 1 plant in Saudi Arabia, built in 1980, had no energy recovery devices and consumed over 8 KWh/m3. The Las Palmas, Canary Islands desalination plant, built in 1995, implemented Francis turbines and lowered energy consumption to 5 KWh/m3. Pelton turbines were used in a water filtration plant in Trinidad and lowered energy consumption to just under 4 KWh/m3. It should be noted that the Pelton turbines in the Trinidad plant are very large and considered state-of-the-art. Nevertheless, ERI’s PX technology and technological advancements ahve cut SWRO plant energy consumption from approximately 3.8 kilowatt hours per cubic meter of permeate produced (KWh/m3) to 3.2 or about 16%.
EQUIPMENT DESCRIPTION
The core of PX technology is an isobaric energy recovery device, made of uniquely engineered ceramics which captures the hydraulic energy from the high pressure reject stream of seawater reverse osmosis (SWRO) processes. It transfers this energy to low pressure feed water with an efficiency of nearly 98%. Pressurized seawater from the PX device is sent to the membrane feed, merging with the pressurized water from a high pressure pump. This significantly reduces the amount of water pressurized by the high pressure pump. Because the PX device itself consumes no electrical power, the overall energy consumption of an SWRO process is drastically reduced.
ERI’s PX technology reduces the energy required for membrane desalination
OPERATING DESCRIPTION
The heart of the PX device is a ceramic rotor floating in 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 rotor is turned by the momentum of the water at a speed that adjusts to flow variations to provide nearly-constant high efficiency over a wide operating range. There are no pistons between the concentrate and the fresh salt water, but the rotor and associated components keep high and low pressure separate, thereby sealing the high pressure portion of the SWRO process.