Hybrid systems in seawater desalination-practical design aspects, status and development perspectives

Abstract

Ever since seawater desalination has been applied on an industrial scale, and particular in the countries of the Arabian Gulf, the application of desalination processes in dual-purpose facilities—water and power — as a hybrid configuration has been discussed in many feasibility investigations and even planning concepts. It is above all the combination of reverse osmosis with thermal processes that has found increasing interest with the aim of ensuring, as economically as possible, uniform water supplies under the specific, greatly varying load conditions in the Gulf countries, with large fluctuations of electricity consumption between summer and winter. Such design concepts for hybrid configurations encompass straightforward structures with a low degree of coupling between membrane and thermal desalination processes, but range up to very complex configurations with strong interconnections on both the water side and thermally, as well as with several desalination processes connected in series or in parallel. Simple, classical hybrid concepts in which the permeate from an RO desalination component is mixed with distillate from thermal desalination have already been implemented in Saudi Arabian dual-purpose plants, like Jeddah and Yanbu-Medina. Although hybrid systems of greater complexity have been addressed in many design studies and publications, up to now none has been brought to fruition. Coming into consideration as the design basis for determining the capacity shares of the various desalination processes operated in a hybrid configuration are: •arrangement of thermal cycle of the power plant component •water/power ratio of the dual-purpose seawater desalination and power plant •provision of undiminished water production of the desalination plant as electricity generation varies •provision of a specified drinking water quality with regard to composition and salt content •combination of all these aspects. Also gaining in importance are concerns of environmental pollution and sustainable development when selecting seawater desalination and power plant configurations, as well as their optimization when considering desalination and electricity generation as a whole. The hybrid configuration of reverse osmosis and thermal processes when combined with highly efficient and low-polluting power plants makes possible significant enhancements of environmental quality and abatement of carbon dioxide emissions for the seawater desalination process. In the practical design of hybrid membrane and thermal systems, aspects come to light, though, that restrict linking of the two systems andjoint utilization of facilities, as conceived in studies and conceptual design investigations. This applies both for common utilization of intakes and the use of heated up cooling water from thermal processes as a feed stream for the RO part of the desalination process. Additionally, requirements of drinking water composition, particularly chloride content, TDS and compliance with a specified residual content of boron, influence specifically the design of the membrane process part and its share in the total desalination capacity. Such practical aspects have greatly influenced the design and configuration of the Fujairah hybrid plant for which, from a total desalination capacity of 100 MIGD (454,600 m'/d), the share of 37.5 MILD (170,500 m3/d) makes its seawater RO plant the biggest currently being constructed anywhere in the world. From the findings of the engineering of this plant and the idea that, by increasing interconnection between the two processes on the water side, it is possible to advance a hybrid configuration of this type with regard to cost optimization in the membrane installation, but also by joint utilization of the intake equipment, perspectives result for applied research efforts over the near and long terms, for example: •long-term behavior of membranes at elevated temperatures •tendency for biofouling in membrane process with common utilization of cooling water and brine •influences of such interconnections on the overall availability of the facility. But also for the operation and maintenance organization of such large facilities, consequences can be foreseen for the future development of hybrid plants, particularly in the interplay of pre-treatment, monitoring of membrane replacement and cleaning, as well as controlling water quality. Also worthy of consideration is, for example, central hybridization vs. a decentralized arrangement of membrane processes in a power supply environment, with distributed membrane desalination plants supplied with electricity from dual-purpose seawater desalination and power plants that themselves employ solely thermal desalination processes.