Robin van Spaandonk wrote: >>Yes, but RO is more efficient. That is, it takes far less energy per >>gram of freshwater. Overall, it takes 4 to 30 times less than the >>best mult-stage flash (MSF) distillation process. > >My point is that using the waste heat is always going to be more efficient >overall than using valuable electricity, even if the process itself is less >efficient.
That is not true. If the RO process is 40 times more efficient than MSF (as it was in some situations, in some locations), then even though 60% of the heat used to generate the electricity is wasted, the overall efficiency is still far ahead of MSF. This is similar to the use of heat pumps instead of gas fired furnaces for space heating. > You need to look at the big picture, and not stare yourself blind on >a detail. It is not a detail. 40 X 30% = 12 times more efficient than MSF -- and I think the factor of 40 was actually based on overall fuel consumption. I believe that is with brackish water, in California. >>The large-scale Saudi plants are all hybrids, that produce both >>electricity and water, very efficiently. > >This is exactly what I mean. See >http://www.water-technology.net/projects/shuaiba/ . Yes. There have been recent improvements with MSF, as described here, especially with hybrid generator models. On cruise ships, all of the water is desalinated ocean water, from the engine cooling system. It is a freebee: they get enough cooling down the diesel-electric engines to supply all those swimming pools, tubs and showers for thousands of people. The early oceangoing steamships used to run short of freshwater for the boiler, until someone figured out they were sitting in a gigantic pool of cooling water which could be used to condense the steam. Marine engines have always been notably efficient, to conserve fuel, but they never had to worry about cooling. - Jed
