Our solution
The only way to avoid a suction limit is to push the water instead of aspirating it.
The first obvious solution is to put an immersed pump on the lower part of the cold pipe. However, this is not viable for three reasons:
- Pumps resisting to a 80 bar pressure are difficult to find and are very expensive.
- The electrical wire necessary for the electrical alimentation of the pump will need to be so thick that it would cost twice the price of the pipe.
- Maintenance operations 800 meters under the water surface will be too complex.
In order to avoid suction limit, while having the pumps above the surface, we have patented a new process:
In our scheme, we have a closed loop of fresh water that gets cooled by a heat exchanger located on the lower part of the loop.
Three parameters will therefore change from the initial scheme:
- The pipe is 50% longer to finish the loop, so the head loss will increase.
- Efficiency of the heat exchanger will increase the heat of the cold water by 1°C.
- We will use fresh water instead of salted water as coolant liquid, which has a better specific heat capacity (≈4200 J kg−1 K−1 for fresh water, ≈4000 J kg−1 K−1 for sea water), but smaller density [1].
Moreover, even if we are not subject to a suction limit, the pressure in the pipe will have to be limited to 16 bar, which is the mechanical resistance of the pipe.
With these new parameters, we are able to compare the energy balance of our system to the points we have formerly studied.
Comparaison between closed and open loops give the domain of each technology.
The closed loop system is therefore the only known system that allows making SWAC systems with thin pipes, giving the following advantages on middle sized remote locations:
- Pipes can be delivered in 100 m rolls and therefore are easier to deploy
- They can be handled by workers, and do not need expensive machinery
- The maritime deployment needs boats that can be found on every island (barges, fishing boats), when thick pipes need large specialized materials which are not usually available in remote locations.
Moreover, the loop system by itself gives some more advantages:
- Control on the quality of the circulating water
- No need for an end-pipe filter that requires regular maintenance
- No risk of aspirating sea water material or animals that could damage the system
The economic study is, in Tahiti, the return on investment has been calculated to be between 5 to 10 years, depending of the geography of the site.
