RankineCycle wrote:Ocean thermal electricity generation is in most cases a losing battle, the temperature difference is so low that the work required to pump the water is greater than the work that can be extracted from the differences in temperature of said water.
I've done an idealized analysis on this. It will only work at all where the cold and warm water bodies are close to one another. The turbine efficiency is still horribly low (less than 5%) and must use a low-boiling point working fluid like R-134a.
Using ocean water as a source or sink for a heat pump, however, is a more reasonable proposition.
You are making machinery do a lot of un-necessary work the way you figured out this problem.
Think of it this way, you put a large diameter pipe from 3000 feet depth up to your shore lagoon facility. Have the pipe attach to the low end of the artificial lagoon you are using with a sealed structure. Enclose the lagoon from the open ocean with a dike. Pump the water in the lagoon over the dike into the surface waters. As you reduce the water level in the impounded lagoon the pressure goes down and creates a differential at the mouth of the pipe, the cold bottom water then flows up the pipe to replace the volume you are pumping out into the surface waters. Because of natural forces you only have to use enough energy to pump water as high as the top of the dike, nature abhors a vacuum and will replace the volume you remove through the only available pathway, up the deep pipe to the lagoon at the surface.
If that seems too complex for you, you can also run the system in reverse. Pump warm surface waters over the dike into the lagoon. What happens? The water level in the lagoon will go up until the pressure exceeds the density differential between the warm surface water and the cold bottom water. That is not actually very much, a few inches of elevation will do it, after which time the warm surface waters will be forced down the pipe to the cold bottom level. Put your thermocouples around the bottom of the pipe where the warm water exits and all you need are surface pumps pulling water into the impounded lagoon. For max efficiency use insulated pipe so that the waters maintain temperature all the way to the other end.
I prefer pulling the bottom water up because it is nutrient rich and when you dump it into the surface waters you will draw in all sorts of plankton, which in turn will bring in fish and other life. Either way let nature do the heavy lifting of filling the partial vacuum you create, don't do all the work mechanically.