can be compressed hydrostatically . As noted previously, non condensable gases, which include any residual water vapor, dissolved gases that have come out of solution, and air that may have leaked into the system, are removed by the vacuum compressor.
Open cycle OTEC eliminates expensive heat exchangers at the cost of low system pressures. Partial vacuum operation has the disadvantage of making the system vulnerable to air in-leakage and promotes the evolution of non condensable gases dissolved in sea water. Power must ultimately be expended to pressurize and remove these gases. Furthermore, as a consequence of the low steam density, volumetric Sow rates are very high per unit of electricity generated. Large components are needed to accommodate these Sow rates. In particular, only the largest conventional steam turbine stages have the potential for integration into open cycle OTEC systems of a few megawatts gross generating capacity. It is generally acknowledged that higher capacity plants will require a major turbine development effort.
The mist lift and foam lift OTEC systems are variants of the OTEC open cycle. Both employ the sea water directly to produce power. Unlike Claude’s open cycle, lift cycles generate electricity with a hydraulic turbine. The energy expended by the liquid to drive the turbine is recovered from the warm sea water. In the lift process, warm seawater is Sash evaporated to produce a two-phase, liquid}vapor mixture } either a mist consisting of liquid droplets suspended in a vapor, or a foam, where vapor bubbles are contained in a continuous liquid phase. The mixture rises, doing work against gravity. Here, the thermal energy of the vapor is expended to increase the potential energy of the Suid. The vapor is then condensed with cold sea water and discharged back into the ocean. Flow of the liquid through the hydraulic turbine may occur before or after the lift process. Advocates of the mist and foam lift cycles contend that they are cheaper to implement than closed cycle OTEC because they require no expensive heat exchangers, and are superior to the Claude cycle because they utilize a hydraulic turbine rather than a low pressure steam turbine. These claims await veriRcation .
Hybrid Cycle OTEC Some marketing studies have suggested that OTEC systems that can provide both electricity and water may be able to penetrate the marketplace more readily than plants dedicated solely to power generation. Hybrid cycle OTEC was conceived as a response to these studies. Hybrid cycles combine the potable water production capabilities of open cycle OTEC with the potential for large electricity generation capacities offered by the closed cycle.
Several hybrid cycle variants have been proposed. Typically, as in the Claude cycle, warm surface seawater is Sash evaporated in a partial vacuum. This low pressure steam Sows into a heat exchanger where it is employed to vaporize a pressurized, low-boiling-point Suid such as ammonia. During this process, most of the steam condenses, yielding desalinated potable water. The ammonia vapor Sows through a simple closed-cycle power loop and is condensed using cold sea water. The uncondensed steam and other gases exiting the ammonia evaporator may be further cooled by heat transfer to either the liquid ammonia leaving the ammonia condenser or cold sea water. The noncondensables are then compressed and discharged to the atmosphere.
Steam is used as an intermediary heat transfer medium between the warm sea water and the ammonia; consequently, the potential for bio fouling in the ammonia evaporator is reduced signiRcantly. Another advantage of the hybrid cycle related to freshwater production is that condensation occurs at signiRcantly higher pressures than in an open cycle OTEC condenser, due to the elimination of the turbine from the steam Sow path. This may, in turn, yield some savings in the amount of power consumed to compress and discharge the non condensable gases from the system. These savings (relative to a simple Claude cycle producing electricity and water), however, are offset by the additional backwork of the closed-cycle ammonia pump.
One drawback of the hybrid cycle is that water production and power generation are closely coupled. Changes or problems in either the water or power subsystem will compromise performance of the other. Furthermore, there is a risk that the potable water may be contaminated by an ammonia leak. In response to these concerns, an alternative hybrid cycle has been proposed, comprising decoupled power and water production components. The basis for this concept lies in the fact that warm sea water leaving a closed cycle evaporator is still sufRciently warm, and cold seawater exiting the condenser is sufRciently cold, to sustain an independent freshwater production process.
can be compressed hydrostatically . As noted previously, non condensable gases, which include any residual water vapor, dissolved gases that have come out of solution, and air that may have leaked into the system, are removed by the vacuum compressor. Open cycle OTEC eliminates expensive heat exchangers at the cost of low system pressures. Partial vacuum operation has the disadvantage of making the system vulnerable to air in-leakage and promotes the evolution of non condensable gases dissolved in sea water. Power must ultimately be expended to pressurize and remove these gases. Furthermore, as a consequence of the low steam density, volumetric Sow rates are very high per unit of electricity generated. Large components are needed to accommodate these Sow rates. In particular, only the largest conventional steam turbine stages have the potential for integration into open cycle OTEC systems of a few megawatts gross generating capacity. It is generally acknowledged that higher capacity plants will require a major turbine development effort. The mist lift and foam lift OTEC systems are variants of the OTEC open cycle. Both employ the sea water directly to produce power. Unlike Claude’s open cycle, lift cycles generate electricity with a hydraulic turbine. The energy expended by the liquid to drive the turbine is recovered from the warm sea water. In the lift process, warm seawater is Sash evaporated to produce a two-phase, liquid}vapor mixture } either a mist consisting of liquid droplets suspended in a vapor, or a foam, where vapor bubbles are contained in a continuous liquid phase. The mixture rises, doing work against gravity. Here, the thermal energy of the vapor is expended to increase the potential energy of the Suid. The vapor is then condensed with cold sea water and discharged back into the ocean. Flow of the liquid through the hydraulic turbine may occur before or after the lift process. Advocates of the mist and foam lift cycles contend that they are cheaper to implement than closed cycle OTEC because they require no expensive heat exchangers, and are superior to the Claude cycle because they utilize a hydraulic turbine rather than a low pressure steam turbine. These claims await veriRcation . Hybrid Cycle OTEC Some marketing studies have suggested that OTEC systems that can provide both electricity and water may be able to penetrate the marketplace more readily than plants dedicated solely to power generation. Hybrid cycle OTEC was conceived as a response to these studies. Hybrid cycles combine the potable water production capabilities of open cycle OTEC with the potential for large electricity generation capacities offered by the closed cycle.
Several hybrid cycle variants have been proposed. Typically, as in the Claude cycle, warm surface seawater is Sash evaporated in a partial vacuum. This low pressure steam Sows into a heat exchanger where it is employed to vaporize a pressurized, low-boiling-point Suid such as ammonia. During this process, most of the steam condenses, yielding desalinated potable water. The ammonia vapor Sows through a simple closed-cycle power loop and is condensed using cold sea water. The uncondensed steam and other gases exiting the ammonia evaporator may be further cooled by heat transfer to either the liquid ammonia leaving the ammonia condenser or cold sea water. The noncondensables are then compressed and discharged to the atmosphere.
Steam is used as an intermediary heat transfer medium between the warm sea water and the ammonia; consequently, the potential for bio fouling in the ammonia evaporator is reduced signiRcantly. Another advantage of the hybrid cycle related to freshwater production is that condensation occurs at signiRcantly higher pressures than in an open cycle OTEC condenser, due to the elimination of the turbine from the steam Sow path. This may, in turn, yield some savings in the amount of power consumed to compress and discharge the non condensable gases from the system. These savings (relative to a simple Claude cycle producing electricity and water), however, are offset by the additional backwork of the closed-cycle ammonia pump.
One drawback of the hybrid cycle is that water production and power generation are closely coupled. Changes or problems in either the water or power subsystem will compromise performance of the other. Furthermore, there is a risk that the potable water may be contaminated by an ammonia leak. In response to these concerns, an alternative hybrid cycle has been proposed, comprising decoupled power and water production components. The basis for this concept lies in the fact that warm sea water leaving a closed cycle evaporator is still sufRciently warm, and cold seawater exiting the condenser is sufRciently cold, to sustain an independent freshwater production process.
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