In about 1859 William Rankine described a Rankine cycle in which water is vaporized to steam, work is produced by the steam and the steam is condensed. In about 1909 Georges Claude described the use of a Rankine cycle to produce work with the temperatures and pressures available in tropical seawater. One of the problems in producing work from tropical seawater is that dissolved gases are included in the water. Expanding and compressing the gases requires and wastes energy. Moreover the gases interfere with the heat flow characteristics, especially in the condenser. The present invention is directed to solving problems associated with the dissolved gases in seawater. The invention has special application to the removal of gases from water in open-cycle ocean thermal energy conversion (OC-OTEC) plants. The invention has application in situations in which it is desirable to deaerate or to remove gases from seawater. The invention also has application in the separation of gases from liquids.
Seawater contains dissolved gases, primarily nitrogen and oxygen. Dissolved gas concentrations in the cold water layer of the ocean near the Hawaiian islands indicate that oxygen is significantly below saturation and nitrogen is at or slightly below saturation with respect to air at atmospheric pressure. Carbon dioxide is significantly supersaturated in this cold water layer. The warm surface water is generally slightly supersaturated with both nitrogen and oxygen while carbon dioxide is below saturation.
In an open-cycle ocean thermal energy conversion plant, dissolved gases will be released in the evaporator and direct contact condenser (if used). By accumulating near the condensing surface, the inert gases lower the partial pressure of the steam, hence lowering the saturation temperature of the steam. Reduction of the temperature differential between the saturation temperature and the temperature of the condensing agent lowers the amount of heat flow in the condenser. Simultaneously, these gases raise the condenser pressure. In order to maintain the low pressure required present open-cycle thermal energy conversion design configurations remove these gases continuously by means of a vacuum pump attached to the condenser, i.e., at the lowest pressure in the system.
The usual plan for the removal of the non-condensible gases is to compress them to atmospheric pressures and release them to the atmosphere. This imposes a significant burden on the overall system efficiency with parasitic losses in the range of 10 to 15% of the gross power depending on the fraction of non-condensibles which evolve.
The other method for handling the non-condensibles is pre-deaeration. Pre-deaeration occurs upstream of the evaporator or the direct contact condenser (if used). It requires additional system components. The advantage of this deaeration strategy is that the evolved gases are removed at a higher pressure than the boiling point. In addition, accumulation of non-condensibles near the condensing surfaces does not occur and the related inefficiency can be avoided.
As noted, post-deaeration method gases are removed from the condenser at the lowest pressure in the system. Removal at this point requires greater compression power than for pre-deaeration. In post-deaeration, however, no additional components are required and additional hydraulic head losses, which are associated with pre-deaeration devices, are avoided.
The deaeration technique presented herein is a pre-deaeration process.