willdo said:
… Is there something obvious i am missing? Assuming that the ground is conveniently shaped and some thermally apropriate tubing, couldn't you allow this vapor to rise to a significant altitude?
The problem is that water vapour is buoyant, so it has greater potential energy at the bottom.
@willdo. How high exactly were you considering raising the air saturated with water vapour. What benefits do you expect to gain by doing that?
If we assume you want primarily to generate fresh water from saline, then you will need a “hothouse” and a source of energy to evaporate water from the saline. You then allow the warm saturated air to flow from the top of the hothouse.
To condense water from that air you must cool the air and then remove the latent heat energy originally invested to vaporise the water. Relatively warm wet air rises, so you may benefit if you can condense the water at a greater altitude than the hothouse. If you arrange the saturated air to rise diagonally to a pipe that runs buried? up a cool hillside, then as the airflow cools and water condenses the liquid will trickle back down that pipe. Water would be extracted from a collection sump at the bottom of the pipe. That density / thermal system would continue to run so long as the temperature of the saturated air flow reaching the top of the chimney was higher than the local air temperature. That requirement is a limitation on the length of the pipe.
The top of the cooling pipe would be an open chimney. If that chimney was heated, then the pipe might draw more efficiently and so the system would yield more water, but once heated, no additional water could be condensed from the air in the chimney as it is no longer saturated.
Unfortunately, the exhaust air from the system after condensation will still contain water vapour. The efficiency of the system can be improved by releasing air at the lowest possible temperature and pressure. You do not benefit by adding heat as the air rises, that just requires you go higher before condensation. I expect you are better taking the fresh water wherever you can get it.
Putting this all together, you get saline water flowing slowly through a hothouse, with a diagonal riser to take the warm air to a hill. A buried pipe then runs up the hill cooling and condensing water from the airflow, to be vented from a solar heated chimney. Liquid water trickles back down the same pipe for collection at the bottom.
There would be insufficient flow to recover energy from the system because all available energy is used to drive the flow through the system. There is no benefit from this system other than the production of valuable drinking water.
