Abstract:
Renewable energy is promising to become a major resource for future communities,
while water scarcity is becoming a major threat. The use of solar photovoltaic (PV) to power
reverse osmosis (RO) plants and produce water will enhance the sustainability of water supplies
in several dry coastal areas. Varying the operating power level of the RO plant has been
proposed in the literature as a solution to accommodate intermittent PV power sources. Such
variable operation is intended to match the RO load to the available PV power. Nevertheless,
such operation has not been used outside research laboratories and small pilot plants. In this
work, different case studies to evaluate the benefit of using variable operation and its effects
on system design, system operation and levelized cost of water (LCOW) were investigated. A
simulation model for the optimal operation of the system is developed using three-dimensional
dynamic programming (DP) to determine the power levels of the PV generators, battery, diesel
generator, and RO plant while optimal sizing of these plants and associated water tanks was
solved using an ordinal optimization (OO) approach. Simulations are performed under a
MATLAB environment. The use of OO permitted the examination of a large design search
space quickly but exhaustively using a simple model. The different designs were then ranked
in increasing cost order and assessed a reduced number of these using an accurate model to
simulate the system operation on an hourly basis for all the days of the year. This approach
relies on the fundamental tenet of OO: “order is robust to the noise introduced by the simple
model”. Different power modulation strategies are investigated, and their implications on the
hydraulic operating parameters are presented. The operation of the RO system at varying power
levels and different sizes of backup systems (battery and diesel generator) was investigated.
This ability to vary the RO operating level helped in a better matching of the system load to
the available, yet variable, PV power, even when the backup and storage systems were at a
minimum. Operating an RO plant with PV and backup systems is found to be far more cost
effective than operating it without backup systems, reducing costs by 37-57% for the case
studies considered.