In reply to Jed Rothwell's message of Fri, 5 Jan 2007 20:28:03 -0500 (GMT-05:00): Hi, [snip] >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.
No it isn't. The energy cost for MSF can be essentially zero, when attached to a thermal power plant, because that heat is normally thrown away anyway. > > >> 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. Yes, you may get up to 12 times more water for the same amount of fuel, but you are neglecting the fact that in thermal power plants the fuel is normally burnt anyway, and NO water is produced. If the electric output from the plant is used to produce water, using RO, then another plant has to be built to provide electricity for the usual customers. When MSF is combined with the thermal plant, no additional plant need be built, and the water (though less than from RO), is an added bonus. IOW with MSF, you water *in addition* to power, whereas with RO you get water *instead of* power. Regards, Robin van Spaandonk http://users.bigpond.net.au/rvanspaa/ Competition provides the motivation, Cooperation provides the means.
