duration of a year, a case study of the proposed hybrid powered RO
desalination is presented.
2. Stand-alone seawater RO desalination plant powered by
hybrid system of PV and PRO
A proposed stand-alone solar-salinity power driven RO desalination
system is illustrated in Fig. 1 and it shows the three main
sub-systems in this hybrid plant. The hybrid system consists of RO
desalination and renewable power generation including solar and
salinity power; the two parts are closely interacted. The renewable
energy generation supplies the power to the desalination plant and
the brine from the RO desalination is the source of the chemical
potential for salinity power generation. In operations of SSRO (solar-
salinity powered RO) during daytime, both the PV array and the
PRO plant areworking to generate electricity. In contrast, at night in
operations of SRO (salinity powered RO), only the PRO plant is
working. In Fig. 1(b), the detail diagram of the hybrid plant is
plotted. SW (seawater) is pressurised by a HP (high-pressure pump)
and a hydraulic ERD before it flows into an RO membrane module.
The implementation of ERD can significantly reduce the exergy
destruction of the RO plant [12]. The HP is driven by the induction
motor. The freshwater is produced from the SW in the RO plant.
Accordingly, two streams flow out from the RO module: the PW
(product water) and the CW (concentrated brine water). The CW is
further used to pressurise the SW in the ERD before it flows into the
PRO plant, and the PW is the product of the hybrid system. In
another sub-system, the solar power is harvested by solar PV
technology and the salinity power is generated by the PRO plant.
The low concentration streams (secondary wastewater and BW or
their mixture) are the potential candidates for the feed solution of
the osmotic membrane process [58]. In this study, IW (impaired
water) is selected as the feed solution for the early-stage investigation.
In order to overcome the pressure loss along the flow
channel, the IW is pressurised by a BP (boost pump) which is also
driven by the induction motor. Finally, the draw solution including
the permeated water from the PRO feed solution is expanded in a
HT (hydro-turbine) to generate electricity. Both renewable energy
generators, the PV array and the PRO plant, are interconnected to an
AC bus through DC/DC/AC and AC/DC/AC converters. For simplicity,
the efficiencies of all the converters and motors are assumed to be
100%.
2.1. Thermodynamic analysis of the stand-alone salinity-solar
power driven seawater RO plant
Before further analysis of the hybrid system, some key states of
the saline streams are presented in the pressureeflow rate (PeQ)
diagram as illustrated in Fig. 2. As can be seen from the diagram, the
pressure loss in the membrane and flow channels is negligible
compared with the hydraulic pressure applied on the saline
streams. So the applied pressure is considered as constant. Also, it is
assumed that no fouling or membrane deformation is occurred.
And because a very small amount of energy is used by the BP
compared to the energy consumed by the HP, in this study, the
energy consumption is only considered as the work of the HP in the
RO sub-system.
In the PeQ diagram, the energy consumption by the RO plant
and the energy generation by the PRO plant can be represented by
the areas illustrated in Fig. 2, i.e. the energy consumed can be
represented by the area O-0-1-C, the energy recovered by ERD can
be represented by the area 3-2-C-B and the energy generated by the