Ocean thermal energy can be described as the thermal potential energy produced by the temperature difference between the warm surface and the cold deep seawaters. Reference to the large ocean area, ocean thermal energy reserves are huge. Ocean thermal energy is considered green in the sense that its production is without pollution. Nevertheless, this renewable energy sources suffers from low temperature difference between the deep seawater and the surface, which is generally in the range of 15-25K. The source also suffers the weakness of low specific heat that is approximately 4J/(gK), while seawater heat of vaporization is approximately 2400J/g.
To enhance the efficiency of the ocean thermal energy, it has been found profound to use the ocean thermal energy for seawater desalination directly. Using it directly can help skip the numerous conversion steps for converting ocean thermal energy to electricity and then converting the resulting electrical energy to chemical energy. Considering the scarcity of fresh water, it becomes paramount to produce fresh water using ocean thermal energy. However, in a previous system, energy consumption on seawater transportation was observed to be very high and this led to poor economic tradeoff of the systems.
In addition, the effect of a number of parameters on the performance of this system was not assessed. Above all, placing the evaporator and the condenser into a single unit caused the system to be very large, and the system’s exergy efficiency should be determined. Therefore, Zhejiang University researchers Zhi-jiang Jin, Hao Ye, Jin-yuan Qian and in collaboration with Hao Wang at Air Liquide Hangzhou Co., Ltd. And Hao Li at Nuclear Power Institute of China explained the working principle of siphon flash evaporation desalination system and analyzed the exergy efficiency of the entire system. They created a simulation model in ASPEN PLUS and analyzed the effects of a number of factors on the functioning of the system through the model. Their work is published in Energy Conversion and Management.
The vapor produced in the flash evaporator is normally absorbed into the condenser chamber reference to the pressure difference between the condenser and the evaporator. The vapor is condensed into freshwater by the cold deep ocean water. Owing to a particular degree of vacuum difference between the evaporator and the condenser, then the vapor can be absorbed into the condenser continuously.
However, the initial vacuum degree of the condenser shell side must be the same as the evaporator. There are two main functions of the ocean thermal energy; one is to generate and maintain the vacuum difference between the flash evaporator and the condenser. This will ensure that the surface water is vaporized continuously and absorbed into the condenser without extra energy consumption. The second function is that the cold deep seawater is used as a condensing agent for condensing the vaporized water into fresh water.
Under design conditions, the authors realized that the exergy efficiency of the entire system turned out very well at 7.81%. This value was higher than the typical utilization of the ocean thermal energy. The exergy efficiency of the flash evaporator was observed to reduce with a rise in the surface seawater temperature, but the condenser efficiency remained unchanged.
The flow rate of the deep seawater decreased with a rise of temperature change of the deep seawater. However, the flow rate of the surface water decreased with the increase in change in temperature of the surface water. The surface water flow rate also influenced the pressure difference between the condenser and the evaporator. Non-condensable gases in the water might have caused this. Therefore, taking into account the influence of non-condensable gases in the actual production is paramount.
Zhi-jiang Jin, Hao Ye, Hao Wang, Hao Li, Jin-yuan Qian. Thermodynamic analysis of siphon flash evaporation desalination system using ocean thermal energy. Energy Conversion and Management, volume 136 (2017), pages 66–77.Go To Energy Conversion and Management