Ocean thermal power generation

Power generation principle

The ocean thermal power generation system uses the temperature difference between shallow and deep layers of seawater and different heat sources of temperature and cold to generate electricity through heat exchangers and turbines. In the existing Marine thermal power generation system, the source of heat energy is the warm sea water on the ocean surface, and there are basically two methods for power generation: one is to use warm sea water to evaporate the low boiling point working fluid in the closed circulation system; The other is that the warm water itself boils in a vacuum chamber. Both methods produce steam, which then drives a turbine to generate electricity. The steam after power generation can be cooled by cold sea water at a very low temperature and turned back into a fluid to form a cycle. Cold sea water is generally drawn from a depth of 600 to 1000m below sea level. Generally, the temperature difference between warm sea water and cold sea water is above 20 ° C, which can generate net electricity.

Cold seawater extracted from the deep sea, not only low temperature (generally 4, 5 ° C), sterile and nutrient rich, has a variety of uses, such as the production of fresh water, freezing, air conditioning, aquaculture, pharmaceutical, etc., can improve the economic value of ocean temperature difference power generation, this application is called deep sea water utilization (DOWA).

Systematic classification

closed

Closed circulation system uses low boiling point working fluid as working medium. Its main components include evaporators, condensers, turbines, working fluid pumps, and warm and cold sea pumps. Because the workflow system circulates in a closed system, it is called a closed cycle system. When the warm sea water pump pumps up the warm sea water and conducts its heat source to the working fluid in the evaporator, it evaporates. The evaporated working fluid expands adiabatically inside the turbine and pushes the turbine blades to generate electricity. The generated working fluid is transferred to the condenser and its heat is transferred to the cold seawater pumped from the deep layer, which is cooled and restored to a liquid, and then pumped through the circulating pump to the evaporator, forming a cycle. The working fluid can be recycled repeatedly, and its types are ammonia, butane, chlorofluorocarbons and other gas coolants with high density and high vapor pressure. Ammonia and HCFC-22 were the most likely working fluids. The energy conversion efficiency of closed cycle system is 3.3% ~ 3.5%. If the energy consumption of the pump is deducted, the net efficiency is between 2.1% and 2.3%.

open

The open circulation system does not use the working fluid as the working medium, but directly uses warm seawater. First of all, warm sea water is imported into a vacuum evaporator, so that it is partially evaporated, and its vapor pressure is about 3kPa (25 ° C), equivalent to 0.03 atmospheric pressure. The water vapor is adiabatically expanded in the low-pressure turbine, and after the work is done, it is introduced to the condenser, where it is cooled into a liquid by cold seawater. There are two methods of condensation: one is that water vapor is directly mixed into cold seawater, called direct contact condensation; The other is to use a surface condenser, where the water steam is not directly in contact with the cold seawater. The latter is the incidental method of preparing fresh water. Although the energy conversion efficiency of the open system is higher than that of the closed system, due to the uncertainty of the efficiency of the low pressure turbine, and the low density and pressure of the water vapor, the power generation device has a small capacity and is not suitable for large capacity power generation.

hybrid

The hybrid circulation system is somewhat similar to the closed circulation system, the only difference is the evaporator part. The warm water of the hybrid system is first passed through a flashevaporator (a device that rapidly compresses a fluid and then rapidly decompresses it to produce boiling evaporation), which converts some of the warm water into water vapor. The steam is then channelled into a second evaporator (a combination of evaporator and condenser). Here the water vapor is cooled and released to its potential; This potential in turn evaporates the working fluid at a low boiling point. The working fluid circulates through this to form a closed system. The purpose of the hybrid power generation system is to avoid the biological attachment of warm seawater to the heat exchanger. The system can also produce fresh water byproducts in a second evaporator. At the same time, the low capacity of the open power generation system can be improved.

There are not many countries in the world that develop Marine temperature difference technology, and Japan, France, Belgium and other countries have built a number of Marine temperature difference energy power stations, with power ranging from 100kW to 5MW. Japan has invested heavily in the research and development of Marine thermal energy, and is ahead of the United States in Marine thermal power generation systems and heat exchanger technology, and has built a total of three Marine thermal test power stations, all of which are shore-based. It is expected that by 2010, there will be 1,030 Marine thermal power stations in the world.