A Feasibility Study - Aaltodoc - Aalto-yliopisto

8.3.4 Energy Recovery Device
In standard Seawater Reverse Osmosis (SWRO) plants the cost of energy is the single
largest operational cost. This is due to the required pressurization of feed water. In order
to decrease costs, various methods to improve the energy efficiency of the entire process
have been developed. In particular, the use of Energy Recovery Devices (ERDs) has
resulted in significant reductions in energy usage during the past decades. The ERD
recovers the energy from the high-pressure brine before it is discharged back to the sea,
and it may decrease the energy consumption up to 40% (Kim et al. 2009).
There are two categories of ERDs: centrifugal (e.g the Pelton wheel turbine, Francis
Turbine and the hydraulic turbocharger) and positive displacement devices (e.g. the
pressure exchanger). Centrifugal devices produce rotational energy (which can be used
to produce electricity) from the brine through various means while positive
displacement devices transfer the energy from the brine directly to the feed water. In
total, numerous different ERDs exist and the research in this field is on-going (Wetterau
2011). The properties of the most common ERDs have been summarized in Table 5.
Table 5. Properties of various ERDs (Jatzlau 2011).
APX = Axial Piston-Pressure Exchanger, DWEER = Dual Work Exchange Energy Recovery
The ERD for AaltoRO should be able to accomplish the following targets: It should be
able to maintain efficiency at variable flows. It should have a high efficiency and its
capital and maintenance costs should be low. Positive displacement devices fulfill the
criteria of having high efficiencies (over 95 %) and maintaining efficiency at variable
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