Energy Recovery Turbine Devices
Several devices have been designed and tested to recover the energy from the brine flow within SWRO. The first ERD used was a turbine coupled to the centrifugal HPP (High-Pressure Pump) shaft. Before the 1980s, Francis turbine were applied but they were replaced by the Pelton wheel. The latter technology operates at higher efficiency in high-head applications such as large SWRO plants. ERT’s have been widely accepted in large SWRO plants because of their reliability and proven efficiency, typically up to 88%.
The whole component efficiencies of a Pelton turbine. ERD range from 70% to 90%. The energy transfer efficiency of this kid of ERD is the product of the efficiencies of the nozzles (99%), the turbine (85%-90%) and the centrifugal HPP (75%-85%). In conclusion, the peak efficiency of a Pelton turbine ERD could be estimated around 85% – 90%.
Another type of centrifugal ERD is the hydraulic turbocharger, which was used for small and medium capacity SWRO plants at the beginning of the 1990’s. They are similar in operational concept to the Pelton turbine. The turbocharger and the HPP are not directly connected, providing a degree of flexibility in the operation of these devices. Also, turbochargers have a relatively small footprint and are easy to install but the overall efficiency of this ERD is typically 70-80%.
Energy Recovery Devices (Pressure Exchanger)
From the 1990s onward, several alternatives to centrifugal ERD have been designed and tested. These kinds of devices use the principles of isobaric chambers for SWRO plants and are known as isobaric pressure exchangers.
The HPP size and its motor is substantially smaller than ERT configuration. It should be more exactly to say that this kind of device saves energy instead of recovering energy as ERT.
There are several manufacturers of pressure exchanger devices nowadays. ERI (PX), Calder (DWEER), RO Kinetic and KSB (SalTec DT), Danfoss (iSave) are the main manufacturers of this kind of equipment and are now competing for designing the best system. Some optimized devices allow a real efficiency of energy from the brine stream to the feed flow of up to 97%. Besides, the innovative ERDs for SWRO plants imply an enormous advance in the SEC reduction in this type of facilities. These devices obtain excellent SEC from 1.80 to 2.20 kWh/m in new medium capacity SWRO plants with piston HPP installed.
The Pressure Exchanger device transfer pressure from a high pressure stream to a low pressure stream in a ceramic rotor. The rotor is fit into a sleeve between two end covers with precise clearances that, when filled with high pressure water, create an almost frictionless hydrodynamic bearing.