Fresh water aquifer management by means of osmotic energy

Production cost per unit nominal power is not the only measure of the value of the energy produced by renewable sources. Other two key factors are the local environmental impact of the production device and, even more, the availability of the electric network nearby the production plant, where the energy is required.

In many countries there are large coastal areas where the main economic activity is agriculture, and the only source of fresh water is the underlying aquifer. The key idea is to exploit the energy potential of salinity gradients existing at the fresh-water/ saline-water interface along coastal aquifers for energy-free and self-sustained aquifer management, mainly in terms of extraction of fresh water for domestic and irrigation purpose.

The potential of the salinity gradient can be measured by the osmotic pressure existing between the sea water and the freshwater. Osmosis is the natural flux of freshwater occurring through a semipermeable membrane dividing two liquids with different saline concentrations and osmotic pressure is the pressure that must be applied to the high concentration liquid to balance the osmotic flux (see fig. 1).

 Fig.1: osmotic flux between fresh water and salt water divided by a semipermeable membrane

In the PRO technology (Pressure Retarded Osmosis), low salinity water passes through a membrane within a high salinity pressurized solution. With reference to the diagram of Fig 2, a fresh water flow rate Q is pumped in a confined tank named “Fresh water”, separated by a semipermeable membrane from a second confined tank named “Salt water”. An equal flow rate Q of salt water is pumped through the pressure exchanger in the “Salt water” tank, where the pressure is approximately half the osmotic pressure. The flow rate 2Q of “diluted seawater” obtained by the mixing goes to a turbine. Part of the electrical energy so produced is used for the pump and the “pressure exchanger”, the rest is the net production.

Fig. 2 Scheme of the PRO technique for osmotic energy conversion

Various configurations could be envisioned for the freshwater/saline water extraction and the allocation of the brackish water left after their mixing. The most compact and simple one seems the technology concept shown in Fig. 3. A well is drilled through the unconfined aquifer up to the saline water layer. In the same well two different pipes carry freshwater and saline water. The freshwater flow rate q₁₂ is pumped from a filter located above the transition zone, the saline water flow rate q₃ from a filter located below the same zone. The flow rate q₁₂ is split at the ground level in two parts, q₁ and q₂. q₁ is used for irrigation, q₂ is mixed with q₃ and is used to produce energy in the osmotic plant located on the head of the well. The resulting brackish water, with a flow rate given by the sum of q₂ and q₃, is then released in a second well of the same aquifer, located between the first one and the coast at the depth of the transition zone after recovering part of the energy through a turbine or a PAT (Pump As Turbine).

The  limit of osmotic plants is that filtration velocity, through the semipermeable membranes, is very low, of the order of 10^-6 m/s, and  production of  a large amount of energy requires the use of a very large membrane surface. For this reason osmotic energy is not, at present time, competitive with other forms of renewable energies with respect to the production cost per unit power. On the other hand  the osmotic pressure between freshwater and sea water is about 2600 KPa in the Mediterranean sea, and even small flow rates are sufficient in arid and semi-arid regions for irrigation purpose. The energy harvested from salinity gradients by means of the osmotic plant should be large enough for the extraction and the management of both the required freshwater and saline water, without the need of additional external energy sources.

The proposed plant has also two important advantages: 1) it provides an almost constant power along 24 h/day, b) the device machinery can be put in an underground chamber, completely hidden. Osmotic plants are the only plants for renewable energies that have an almost zero environmental impact, because they do not change anyway the surrounding landscape and the noise of the machinery is almost negligible. 

University of Palermo is at present time coordinating a research project on the potential use of osmotic energy and its environmental impact, funded by the PRIN 2010/11 MIUR program and involving  other six Italian universities.