Rotem Industries: a new concept of Solar-Powered Desalination
The Israel Export & International Cooperation Institute
Most studies dealing with the potential of Renewable Energy Sources (RES) for desalination choose to use RES to generate electricity first, and then use this electricity to power desalination. Rotem Industries concept eliminates the phase of electricity generation by using solar thermal energy directly for distillation by evaporation. Saving the thermal to mechanical conversion losses allows the proposed Multi Effect Distillation (MED) process to compete economically with Reverse Osmosis (RO) process of significantly lower energy consumption.
The new opportunity to revive direct thermal evaporation arises from a new collector technology developed by Solel Solar Systems that is coupled to the familiar MED process modified by I.A.E. Technologies to match the solar steam characteristics.
Solel has applied its unique technique of selective coating to demonstrate a very efficient solar radiation collection, achieving high temperatures with relatively low installation costs. While the generated steam is not sufficient for efficient generation of electrical power to be used for RO - its quality far exceeds the minimum necessary for the existing methods of steam powered desalination by evaporation. We suggest a combination of a large number of effects of evaporation, together with high pressure saturated steam available for recycling. This configuration yields a dramatic improvement in the production rate of water desalination, accompanied by relatively modest increase of the plant installation cost. This implies that the distillate production costs are significantly lower with the new proposed combination. The installation cost increase is mainly due to the higher costs for the expansion of the desalination system, with a quite low additional cost for producing the high temperature steam due to the solar system nature.
A basic assumption, drawn from economic considerations and technological constraints, is that the desalination system should operate continuously, while the solar system, which is limited to daytime operation, would feed a steam generator combined with a storage system. Consequently either large and expensive heat storage capacity or fossil fuel backup are required. The relative storage to fuel cost dictates the optimal storage capacity. Advanced PCM storage increases the annual water production by almost 20% as compared to conservative sensible heat storage.
Several advanced configurations were analyzed. An attractive option, where electrical power is available and cheap during nighttime - is to run the MED system overnight by vapor compression recycling.
The option of co-generation power/desalination plant is also economically very attractive, where power and fresh water are both needed. Both configurations of extracted steam or back-pressure steam turbine are optional depending on the power demand.
The plant desalination capacity could vary from a small 1000 m3/day plant, typical for plants serving small settlements or industries at rural locations, isolated from fresh water and grid power sources, and up to full scale 100,000 m3/day plant connected to the national grid.
The new technology of affordable power and fresh water generation is clearly applicable along the Red Sea, North Africa and similar sites.
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