SALINITY GRADIENTS

"Sucking" means creating a partial vacuum that will allow atmospheric pressure to push a liquid toward the one who may have thought he was pulling it, the sucker.

From which it follows that water cannot possibly be sucked to a height greater than a column whose weight (per area) balances the pressure of the atmosphere. That height is around 33 feet (the length of a barometer can be considerably shortened by using mercury rather than water for the liquid column).

So how do trees 100 ft tall get their water to the top branches?

By osmotic pressure. Osmosis is the diffusion of a solution with a lower concentration (i.e., a more dilute solution) through a semipermeable membrane into a solution with a higher concentration so as to equalize the concentrations. Osmotic pressure is what forces the liquid through the membrane, and this pressure (in the case of salty sea water on one side of the membrane and fresh water on the other) will hold up a column of not 33, but 775 feet!

Now a water level difference of several hundred feet is energy, for in principle, the water could flow from the higher to the lower level through a hydraulic turbine.

The amount of this energy is vast: A flow of only one cubic foot per second of fresh water into sea water (or sea water flowing into even saltier brine ponds) represents a whopping 850 kW. The salt domes in coastal regions contain 10 to 100 times more energy due to their salinity than due to the cherished oil they may contain.

But now we come to the salty bit, and that is the problem of tapping it.

Most versions of saline energy conversion use membranes, and thus introduce an element that is costly, subject to degradation, and in need of pretreatment of the solutions. One such device, in which (after many modifications and refinements) the sea water does directly drive a turbogenerator was found to result in a cost of about $10,000 per installed kilowatt.

That is 10 times more than for a nuclear plant even after paying tax, tribute, blackmail and ransom to the NRDC and other anti-nuclear obstructionists; nevertheless, it is still appreciably less than the cost of an installed kW in a solar plant (which can run as high as $30,000/kW when left to the DoE or the National Park Service).

There is, however, at least one method, called inverse vapor-compression, which does not have the drawback of a menbrane, though it may run into other problems. It utilizes the fact that fresh water evaporates more readily than salt water, and the difference in vapor pressures drives a steam turbine (see Olson et al below).

Like OTEC, but unlike windmills or ocean current turbines, salinity gradients have the potential of eventually offering electric power both economically and by thousands of megawatts from a single conversion facility. By this we mean that the possibility cannot be dismissed out of hand, though only intensive research will tell whether this goal can be achieved. At present neither OTEC (which is far further developed) nor salinity gradients are ready to produce power commercially.

The final of the conclusions reached by Isaacs and Schmitt in their survey (see below) is "Perhaps the most important relationship of the ocean to human power needs in the future will be the employment of seawater for heat rejection and of the deep region below the sea floor for the disposal of nuclear wastes.

[More: "Salt Power" by G.S. Wick, Oceanus, Winter 1979/80, pp.29-37; G.L. Wick, "Power from salinity gradients," Intrnl. J. of Energy, v.3, pp.95-100 (1978); M.G. Olson, G. Wick, J. Isaacs, "Salinity gradient energy: utilizing pressure gradient difference," Science, no.456, p.452 (1979); J.D. Isaacs and W.R. Schmitt, "Ocean Energy: Forms and Prospects," Science, 18 Jan 1980.]