These are systems that mix brine with river water to produce a brackish effluent and electricity. This type of power plant would be installed alongside the estuaries of large rivers. Alternatively they could extract power from the concentrated brine produced by desalination plants, salt mines or industrial plants wherever there is a suitable supply of minimally salt water for use as the dilute input stream.
Depending on who you talk to, these systems are said to be capable of supplying up to 7% of the world's energy needs as at March 2009. That is equivalent to 40% of the world's electricty requirement, estimated as 20827 Twh (Terawatt hours). This estimate assumes that these generating plants produce 2 MW of electricity for each cubic metre of river water input to the plant and that the entire flow of the source rivers is used. That would interfere with other uses of these rivers as well as having a large ecological impact, so restricting the systems to processing no more than 15% of the available river flow may be more realistic. This reduces the likely output to 1% of the world's energy needs (5.6% of the electricity requirement, 1250 Twh).
So far only laboratory scale systems have been demonstrated but small scale pilot plants are under construction. The cost of generating power this way is comparable to using wind turbines. Two processes are being prototyped:
The principle behind this is that, if fresh and sea water chambers are separated by a semi-permeable membrane, water will diffuse into the salty seawater chamber, raising its pressure. This increased pressure can be used to drive generators. The membrane allows water to pass through it but prevents sodium and chloride ions from diffusing through it. The diluted seawater is discharged as an outflow and replaced by pumping in more river and sea water. It is estimated that the pumps will absorb 20% of the generator's output.
Current laboratory scale systems generate 3 watts per sq. metre of membrane, though it seems that 5 watts/sq.metre can be achieved. This means that the generators will never be small: the pilot plant at the Södra Cell paper pulp factory in Tofte, Norway, which came on stream in November, 2009, has 2000 sq.metres of membrane and a design output of just 6 Kw. However it is actually producing 4Kw so, at 2 watts/sq.metre, the system is 33% less efficient than expected. Since the system depends on a mass flow of water through the membrane, another major issue is that of preventing the pores in the membranes from being clogged with silt and algae from the river water. PRO plants will work at pressures of around 20-25 bar (280-350 psi) across the membrane, so the membrane's mechanical strength is a major design constraint. It is thought that this technology will work better if the input brine is more concentrated than sea water.
This is almost the inverse of PRO, where water crosses the membrane and the ions stay put. By contrast, in a RED system the ions cross the membrane leaving the water behind. Two semi-permeable membrane types are used, one permeable to sodium ions and the other to chloride ions. Both types are impermeable to water. Several chambers are stacked with river water and seawater chambers alternating. There is an anode on one end of the stack and a cathode on the other end. The membranes are arranged so that positive sodium ions diffuse to the cathode and negative chloride ions diffuse to to anode. This results in a voltage difference across the cell. The number of chambers in a stack and the number of stacks connected in series are adjusted to generate the required voltage. The cross section of the stacks determines the membrane area, which in turn sets the available current.
RED systems seem to be much smaller than PRO systems. The pilot plant being built at the Harlingen salt mine in the northern Nederlands to use the brine stream pumped out of the mine and water from a local river occupies less than three cubic metres and should have a similar output to the PRO pilot plant.
The mass flow across RED membranes is about 1% of that required by PRO, so membrane silting problems should be much less and the pressure differences across the membranes is small too, being mainly due to water flows between the fairly closely spaced membranes.