Following the recent posts, we are going to follow our research with the OTEC (Ocean Thermal Energy Conversion) devices.
Last post, Mikel wrote about the closed cycle OTEC. Well, the open cycle process relies on the same thermodynamic theory: A fluid that evaporates makes a turbine move generating electricity, using later cold water to condense the vapor. But unlike in the closed cycle, the substance that is vaporized in the open cycle is seawater, to be exact, surface seawater.
But of course, if pure H20 vaporizes at 101,3 kPa at a temperature of 100ºC, you might be wondering: how can the surface seawater, being at a temperature of around 20-25ºC vaporize? Well, the answer can be found in the Pressure-Temperature diagram of the water.
The fact is, that the chamber where the water is vaporized is a low pressure chamber. It has a vacuum pump that regulates the pressure so that the phase change can take place at those temperatures.
We can observe in the diagram, that if the pressure is lowered, the boiling point of water is consequently also lowered. Therefore, as the chamber is a low-pressurized, we can make the warm seawater change phase turning in to vapor.
This vapor is then used to make the turbine turn. But, what happens with it later? Well, this is where the second part of the process turns in to action! The cold water that is brought up from deep down under acts as a refrigerant , and turns the vapor back into liquid again. But of course, this is basically pure water, because as it vaporized, it left all the salt and mineral behind. This way, we obtain desalinized water! This water can be useful as drinking water or desalinized water.
Information was mostly gathered from wikipedia.