RO desalination is mostly used for large
application due to its low specific energy consumption compared to other
processes; especially when it is coupled with an energy recovery device 7. This is why it
is mostly adopted when considering RED 8. The energy
required for a RO plant is electrical energy to drive feed pumps that delivers
saline water to the RO membranes at a pressure exceeding the osmotic pressure.
The operating pressure of RO plants can range from 50 to 80 bars for SWRO and
10 to 25 bars for BWRO 7. The electrical energy required to drive the
RO process constitutes 44% of the plant cost structure 9. Electrical energy can be produced directly from
renewable energy by using solar photovoltaic systems or wind turbines. An
indirect approach to deliver electrical energy would be coupling a thermal
process to produce mechanical energy to drive an electric generator 10.
Intensive research was done during the last two
decades to improve the efficiency and sustainability of RO desalination
processes. Consequently, a noticeable improvement in membrane technology and
energy recovery devices was achieved. The specific energy
consumption of SWRO plants ranged from 5 to 10 kWh/m3 in the 1990s,
however, after the introduction of energy recovery devices the specific energy
consumption dropped to the current range of 3 to 4 kWh/m3 11. This dramatic improvement has defined RO
desalination as an affordable and reliable solution for sustainable water
production. Consequently, during the period from June 2015 and July 2016, a
total of 512 new plants were built, of which 52% were considered large and
medium sized plants, to deliver a production capacity of 3.7 million m3/day. Moreover, as of July 2016, the total number
of desalination plants constructed globally were 18,983, these plant provide
96.6 million m3/day of fresh water 12. The permeate recovery rate from RO
can reach from 35 to 50% for SWRO and nearly 90% for BWRO 7. Additionally, RO
systems are relatively simple in operation and are characterised by modular
design and fast start-up, which are important for variable operation.
1.1 Renewable energy source
There has been
major advancement in the field of renewable energy during the last decade. It
is now thought to be an alternative to using fossil fuel due to their abundant
availability for both developed and developing countries. During the period of
2014 to 2015, the installed capacity of wind power plants increased from 370 to
433 GW, additionally, during the same period, installed capacity of solar-PV
increased from 177 to 227 GW 13. Implementation of renewable energy is thought to
eliminate the link between energy prices and fossil fuel fluctuating price
Based on the nature of desalination processes as an
energy intensive process and their role in assuring water security, major
consideration was given to implement renewable energy in their operation.
Numerous researchers adopted this aim by characterizing the suitable RES for
each desalination process, which is presented in Fig. 1 14. Additionally, authors described the economic
benefit of implementing RES to drive desalination processes 7, 15.
The most widely used desalination processes are
currently based on membrane filtration or phase change (evaporation –
condensation). The main required energy for these systems is either electrical
or thermal energy depending on the process. Fig. 1 presents the applicability
of different RES to desalination processes 14. Desalination process such as RO, MVC and ED
require electrical energy to either drive pumps, compressors or provide an
electric potential. Electrical energy can be supplied by wind turbines that
convert wind energy into electric energy through a generator, solar energy that
is converted into electric energy through a PV cells. Moreover, wave energy can
be used to supply electric energy by using wave energy converters 16. Additionally, solar-thermal energy and geothermal
energy can be used to generate electric energy if they are coupled to a Rankine
cycle. On the other hand, other processes such as MSF, MED and TVC require
thermal energy to drive the desalination process. This thermal energy can be
provided by solar-thermal and geothermal energy.
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