After having explained two of the procedures to get electric power from ocean´s thermal gradient, you may be wondering… where is the need of a third one?
Well, the hybrid cycle combines the best characteristics of the previously shown processes (open cycle & closed cycle), so that this third way is expected to be easily penetrable on the marketplace. Actually, it was conceived as a response of some marketing studies. However, it is to be yet tested and works just theoretically.
Hybrid cycles combine the drinkable water generation capabilities of open cycles with the potential for large electricity production capabilities offered by the closed cycles. Let´s explain how it works:
Warm sea water (which comes from the surface of the ocean) enters a vacuum chamber where it is flash-evaporated. After that, this steam arrives to a heat exchanger where it will play the role of a warm fluid and will be used to warm the working fluid, which works in a closed loop. Usually, ammonia is used as this working fluid (it has good transport properties, it´s easily available and, in addition, it is a low-cost fluid. The negative point is that it is toxic and flammable). Other options are fluorinated carbons, such as CFCs and HCFCs or hydrocarbons.
Observation: An interesting part of the process is that as water evaporates, it leaves all the impurities and salt, so the consequent steam is drinkable.
Once heated and evaporated, the working fluid vapor flows through a closed-cycle power loop. At this point, the ammonia is used to turn a turbine which connected to an electricity generator, supplies the customers with electric power. After that, the ammonia is condensed using cold sea water for it (from the deepest ocean waters). The non-condensables are then compressed and discharged to the atmosphere.
After this brief explanation (click here for further data ) we conclude that the role of seawater is double: on the one hand, it works as an intermediary heat transfer medium; on the other, we get potable water after the condensation process.
– Drinkable water production and power generation are closely coupled. Changes or problems in either the water or power subsystem will compromise performance of the other.
– Possible ammonia leaks could contaminate potable water obtained after condensation.