Objective 1: Review the state-of-the-art in solar powered desalination technologies,
to critically assess and evaluate current popular solar RO systems and compare their
performances (Chapter 2).
With the help of the advanced technologies of both RO desalination and solar cells, medium and small scale solar PV-RO systems have been implemented widely in arid or semi-arid areas. They exhibit good performance in terms of their specific energy consumption (SECs) which can be as low as 5 kWh/m3 (depending on system size). However, comparing their SECs with the ideal theoretical limit of energy usage (at 70%
recovery ratio) which is 0.15 kWh/m3 for BWRO and 0.94 kWh/m3 for SWRO (Section 2.4.1), it is clear that their performances leave much scope for improvement. In other words, there is still huge room for them to be improved by typically 40 times for SW PV-RO systems and 200 times for BW PV-RO systems (Table 2.2) according to the theoretical solar energy conversion limit.
An alternative to the PV-RO system is the solar RC-RO system. Since they are still at the early stage of development, with very few plants having been built, only a few modelling and design studies have been carried out based on different solar collector types and working fluids (Table 2.3). In principle, the parabolic trough collector behaves better than other collectors. However, the choice of working fluid remains controversial. Further analysis on choosing a cost effective solar collector, a suitable working fluid and a practical high efficiency expanding machine is required. Owing to its promising features, such as the efficiency, a lower cost than PV and applicable to a co-generation system, the RC-RO system was selected in this work for detailed investigation.
Objective 2: Understand the loss mechanisms in solar powered RO desalination
system (Chapter 2).
For the large SWRO system adopted with high efficiency ERD, the energy losses are mainly dependant on membrane resistances which are heavily related to membrane types and conditions. Another energy loss is caused by longitudinal excess operation pressure. The energy losses can be much larger for current BWRO systems. Due to their low operational pressure and high recovery, these systems rarely include ERDs, resulting in further losses. Additionally, the smaller inverter and pumps which are adopted in the small or medium brackish water system have low efficiencies.
With regard to the solar PV-RO systems, usually mono- or multi-Si solar panels are adopted, not least because of their relatively low costs and commercial availabilities. Despite the fact that the RO desalination market is dominated by PV-RO, significant energy
inefficiencies of such systems still prevail. For the PV subsystem, successive losses (Figure 2.10) that happen during energy conversion mean the Si solar cells have less than 15%
efficiency, which is much lower than the theoretical solar energy conversion limit―86.8%. In respect to the solar RC-RO systems, the preliminary studies reveal that the energy conversion efficiency of the RC subsystem is comparable with the solar PV cells, which is around 20% (Table 2.3). However, this number will be reduced when applying solar RC technology to small scale BWRO systems due to the low efficiencies of small size components, such as turbines, pumps and converters. Energy losses are unavoidable during energy conversion, i.e., solar (PV) or mechanical (RC) energy to electrical energy. This leads to an idea of saving energy losses by cutting energy conversion steps in solar RO desalination, therefore increasing system efficiency eventually.
Objective 3: Compare different configurations of RO system with regard to efficiencies
and recovery, including multistage systems, system with and without energy recovery and
batch or closed circuit systems (Chapter 3).
For the RO subsystem, beside the concentrate energy loss which can be reduced or eliminated by recovering the high pressure concentrate, another major loss is caused by longitudinal excess operation pressure. To reduce this type of energy loss, multi-stage operation is an option (Section 3.2.1). Theoretically, it can effectively save energy loss (by 30% at 70% recovery ratio), depending on the stage numbers and recovery ratio (Figures 3.10 and 3.11). However, adding more stages of RO module and booster pumps can
significantly increase the capital and O&M costs; thus, an alternative means i.e. batch-RO system has been designed and developed to achieve an efficiency improvement. The main working principles of batch-RO system are (i) recycle high pressure concentrate back to feed water; and (ii) increase feed water pressure according to the rise of the feed water concentration. Theoretical analysis and comparison of batch-RO and other suggested RO systems with different configurations, i.e. multi-stage and CCD-RO systems have shown significant improvement of batch-RO systems (60% at 70% recovery ratio) in terms of SEC
(Figure 3.11). This improvement is reliant on assumptions regarding the robustness of the membrane module under variable-pressure (cycle of load and unload) conditions in batch operation.
At the stage of choosing a power system for the batch-RO, the solar PV and RC electrical technologies were discarded for system integration, because of additional inefficiencies in energy conversions. With regard to reducing the number of energy conversion steps, a mechanical means of using linkage subsystems was instead favoured; that is, the batch-RO is designed to be powered by mechanical energy (steam expansion) directly (Section 3.5).