NCEDA analysis of IDA World Congress technical sessions
The energy footprint is one of the major challenges for seawater desalination projects. The energy recovery achieved from the SWRO brine stream has been one of the most sensational advances achieved in seawater desalination in the last decade. This innovation greatly contributed to improved efficiency and management costs for the facilities with benefits in the sustainability of water solutions, decreased energy and carbon footprint and decreased water cost. This segment of desalination technology remains in constant development and is a key to high efficiency, reliable performance and successful operation of SWRO processes. This session offered an overview of the current position and some insight to future directions
Solar thermal energy seawater desalination
Authors: Mr Toru Kannari, Mr Yoshiaki Miho, Mr Yuji Saito, Dr Rencai Chu, Prof Yoshiharu Horita
Mr Kannari, who delivered the presentation on behalf of his colleagues, began by backgrounding the difficulties associated with delivering desalinated seawater using solar energy despite its attractions. These range from problems with constructing and maintaining solar facilities to those associated with fine-tuning operations and building stability into the system.
Having identified current limitations, Mr Kannari then moved to describe a promising new solution to raise the operating efficiency of solar desalination systems by way of the addition of a thermoelectric generator (TEG) module to the system. In this configuration, solar heat is collected and concentrated in the solar thermal collector and used to generate steam, which is then employed as the heat source for the thermal desalination unit.
To optimize the system, TEG modules are attached across almost the entire surface of the heat exchange wall of the steam generator. A TEG module is composed of multiple elements each of which consist of two different semiconducting materials connected together as a “thermocouple” in order to convert heat directly into electricity.
The vaporization system used in this configuration can concurrently generate steam and electricity simultaneously without the need for a steam turbine generator. Part of the electricity generated by the TEG module is used for operating the heat desalination unit and the surplus electricity generated can be supplied directly to the grid or to the R.O. desalination plant for further production of water.
Since the system has no moving parts, is easy to operate and is highly reliable, it is expected to be invaluable for satellite systems of 5,000 or 20,000 m3/d capacity. Furthermore, the combination of this TEG technology with a trihybrid NF/RO/MED is expected to be more efficient in the utilization of heat and electric power and is considered to be one of the most suitable systems for next generation seawater desalination plants driven by solar thermal energy.
Desalination in remote areas: A prediction-based approach
Authors: Prof. Fernando Tadeo, Mr Luis G. Palacin, Prof. Cesar de Prada, Ms Johanna Salazar
The focus of this presentation, delivered by Luis Palacin, was on energy optimisation. He pointed out that without small plants, communities in North Africa in particular and parts of Spain would be without drinking water. A complication is that these communities are also unable to secure and keep skilled staff who can maintain and operate these facilities. Further, for communities in very hot, arid climates, storage of water as would occur when production was not closely linked to anticipated demand, carries a high risk of microbial and/or algal growth with the associated need for additional disinfection and health risk.
The current study commenced by first examining the efficiency gains achieved by large, as distinct from small desalination plants, in an effort to identify any factors in their use of energy that could be tweaked to build into the operational systems for the smaller plants.
A pilot plant designed to produce 30 m3/d treated water from a brackish source was set up for a small town in Tunisia. Power to the plant was provided via a three phase system comprising solar panels, wind turbines with battery storage to drive the RO membranes. Data from prediction of water demand for the coming 24 hours and data predicting the renewable energies that would be available from both solar and wind units for the same coming 24 hours was fed into a complex series of equations together with variables to ‘smooth’ the data – the ‘automated control’ computer system that serves to run the system.
The authors have concentrated on identifying the pluses from their work – reduced use of batteries and smaller sized storage tanks for treated water so as to reduce costs. Yet further big gains are yet to be achieved and will depend on refining system inputs and reliability especially in the incoming data. However the ideas in themselves provide a good starting point to resolve some important difficulties in supplying remote communities.
