Summary about energy recovery devices for seawater desalination plants

Introduction
The desalination market has grown dramatically in recent years. However, SWRO production cost still high resulting to high water production cost. The cost of energy in SWRO process is usually about 30 to 50% of the total production cost of water and can be as much as 75 % of the operating cost, depending on the cost of electricity. Therefore, the energy recovery devices (ERD) are utilized to recover wasted energy in the brine reject water which possible to reduce the water production cost by reducing energy cost.
 
Fig 1 SWRO with/without ERD

SWRO processes with ERD consume less energy about 20% than the conventional SWRO process. Energy recovery technology is driving down the energy cost and also it provides an indirect benefit by reducing carbon footprint of seawater desalination.
 
Fig 2 Energy consumption of SWRO which applied the different type of ERD

ERD categories 
ERD can be categorized into two types based on the working mechanisms: centrifugal and isobaric. Centifugal ERD recovery the energy from brine stream by capturing brine pressure energy with a turbine and transfer it to an impeller(s) spinning in the seawater (e.g., Francis turbines, Pelton wheels, and turbochargers). On the other hand, isobaric ERD accomplish a direct transfer of pressure from the brine to the seawater in constant pressure chambers, the famous model of isobaric ERD is the ERI™ PX Pressure Exchanger™ device. Over all product of ERD is provided in Table 1.

Table 1 Over all product of ERD 
    
ERD performance
ERD performance is very much dependant on the operating parameters. The main parameters which play an important role are feed flow and pressure. Therefore the clearest way to measure the effectiveness of ERD is by quantifying the high-pressure pump energy required.
                                                               
Pressure exchanger mechanism (PX) 
The PX device transfers 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 when filled with high-pressure water, the rotor create an almost frictionless hydrodynamic bearing. Pressure energy from the high-pressure brine reject of the SWRO membranes is transferred to filtered seawater with an efficiency that can exceed 98%. ERD capable to reduce energy consumption up to 60%. Pressurized seawater is conveyed to the SWRO membranes with a booster pump. Payback period is less than 1 ½ years.
PX configuration & advantages
The configuration of PX is showed in Fig 3. Moreover the summary of its advantages are provide below.
 
Fig.3 Configuration of PX

The summary of PX advantages
  Modular ? like membranes
     ▶ lightweight  
     ▶ Redundancy
  Ease of Installation
     ▶ small footprint
     ▶ no extra civil works
     ▶ no heavy supports
     ▶ no electrical connections
  Ease of Operation
     ▶flow controls and pressure limits only
     ▶no pistons or valves 
     ▶no regularly scheduled maintenance

Conclusions 
Large Scale SWRO with isobaric energy recovery devices is flexible and affordable. ERD can reach up to around 97% of energy recovery efficient in all Configurations. At least 20% better energy consumption as compared with legacy centrifugal ERDs. The efficiency of a PX is approximately 95%, efficiency loss in the PX is 5%. About 1% efficiency is lost to the compression of the seawater, 2% is lost to viscous friction through the PX, and another 2% goes to lubrication flow through the hydrodynamic bearing.

** Reviewer : Monruedee (moon@gist.ac.kr)