Abstract:
An Ocean Thermal Energy Conversion (OTEC) system having a turbine with an upstream side and a downstream side. Warm water under a partial vacuum is converted into a vapor, the vapor being supplied to the upstream side of the turbine at a pressure controlled by the temperature of the warm water. A condenser is situated on the downstream side of the turbine to cause the vapor, after passing through the turbine, to undergo a phase change back to a liquid, which can be used as potable water. The condenser is coupled to a source of a cooling liquid, and the pressure of the vapor on the downstream side of the turbine is determined by the temperature of the cooling liquid. A flexible floating solar collector supplies the warm liquid to the upstream side at a temperature higher than normal ambient temperature.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is related to and claims all benefit of U.S. Provisional Application Ser. No. 61/743,236 filed Aug. 29, 2012 and Ser. No. 61/849,927 filed Feb. 5, 2013. 
    
    
     BACKGROUND 
     Ocean Thermal Energy Conversion (OTEC) systems have been studied and small scale units have been tried since the late 1920&#39;s and early 1930&#39;s based on the principles of the Rankine Cycle developed in the mid-19th century. The systems involve a turbine having an upstream side and a downstream side. Warm liquid under a partial vacuum is converted into a vapor on the upstream side of the turbine with the vapor pressure being controlled by the temperature of the warm liquid. A condenser is situated on the downstream side of the turbine to cause the vapor, after passing through the turbine, to undergo a phase change back to a liquid. The condenser is coupled to a source of a cooling liquid, and the pressure of the vapor on the downstream side of the turbine is determined by the temperature of the cooling liquid. Small temperature differentials cause small pressure differentials which limit the effectiveness of the turbine. 
     To increase the temperature differential, OTEC systems are often located where deep water is available and temperature of the ocean surface is at its highest with the most hours of available sunlight. Typically, the most propitious sites are near the equator. To further increase the temperature differential, typical OTEC systems utilize water pulled up from extreme ocean depths of about 1000 meters as the cooling liquid. However, typical OTEC systems utilize about 80% of the energy that they can generate to pump warm surface water and to pump cooling water up from the extreme depths. 
     A typical prior-art land-based open loop OTEC system  10  is shown in  FIG. 1  adjacent to a body of water. The system  10  has a warm water inlet  14  close to the surface  40  of the body of water, where the water typically has a temperature t 2  of approximately 27° C. A cold water inlet  12  is provided at a depth of approximately 1000 meters, where the water typically has a temperature t 1  of approximately 5° C. Warm water is brought in through inlet  14  and pumped into a warm water degassing tank  16 . Air is pumped from the evaporation tank  18  by means of a vacuum pump  22  to achieve a partial vacuum. Degassed warm water is then pumped from the degassing tank  16  into the evaporation tank  18 , which results in a portion of the degassed water flashing into steam, thus increasing the pressure in evaporation tank  18  to an initial pressure P 1 . The steam is then fed into an upstream side of the turbine  26 . The steam then passes through the turbine  26  to the downstream side of the turbine and into condenser  20 . The condenser  20  is maintained at the temperature t 1  by virtue of being fed with the cold water being pumped by pump  24  up from cold water inlet  12 . Condensation of the steam in condenser  20  causes a drop in pressure in the condenser  20  and on the downstream side of turbine  26  to an exhaust pressure P 2 , which is lower than the initial pressure P 1 . The pressure difference ΔP=P 1 −P 2  causes the flow of steam through the turbine  26  causing the turbine  26  to spin. Turbine  26  then drives electricity generator  28 . Water in the form of the condensed steam can be withdrawn from condenser  20  and stored in a tank  30  for use as potable or distilled water. The electricity generator  28  can be connected to a suitable grid  42  for distribution and use. 
     Major difficulties have prohibited prior art OTEC systems from achieving commercial success. A first difficulty lies in deploying and maintaining a large pipe 10 meters in diameter and 1000 meters deep against the shedding currents typically found in the ocean. A second difficulty lies in the inherent inefficiencies of typical systems. An analysis of the Carnot efficiency with the temperature change Δt=t 2 −t 1  of 20° C. shows that the very best a system could achieve is around 7% efficiency. In practice, with low pumping pressure differentials ΔP between the turbine inlet and outlet (approximately 0.4 psi), pumping losses, and small Δt&#39;s, current OTEC systems run between about 1% and 2% efficiency. 
     Another problem for many of the systems attempted to date is that they are land-based systems where the deep ocean cold water had to be pumped not only up from the depths but it also had to be pumped a considerable lateral distance to reach the onshore plant which causes pressure losses and warming of the cooling water. Ocean front land can be prohibitively expensive and often are at risk due to hurricanes. Areas of highest temperature differential (equatorial Atlantic Ocean) are not close to areas requiring large quantities of energy. Bio-fouling of the entire system has also caused major failures in the past. 
     What is needed is alternative apparatus that can effectively and efficiently utilize the natural temperature differentials exhibited by selected areas of the ocean to generate a consistent level of usable power. 
     SUMMARY 
     OTEC systems of the present design can take the form of a system that can include a large flexible floating solar collector coupled to the warm water inlet. The solar collector can supply large volumes of very hot water (approximately 80°-85° C.) to the evaporator—steam generator, which reduces the amount of water needed to provide a desired quantity of steam. The increased inlet temperature improves the Carnot efficiency to about 21% with a Δt of 73° to 78° C. 
     OTEC systems of the present design can take the form of a system that can include a cooling water inlet that is located at or near to the surface of the ocean water rather than at a large depth. Using surface ocean water instead of the deep ocean water for cooling the condenser eliminates the need for the deep water pipe and only reduces the Carnot efficiency to approximately 17% with a Δt of 53° to 58° C. 
     Building the evaporator and condenser segments of the OTEC system of the present design, along with placing the generator turbine onboard a ship, and attaching the large flexible floating solar collectors to surround the ship, minimizes the pumping distance and therefore the efficiency losses due to pressure losses and thermal losses. A large insulated floating pool can be added in which hot water can be deposited from the solar collectors during the day, and then withdrawn during the evening and night time hours, to keep the generator ship functioning 24/7. 
     Additionally, OTEC systems of the present design no longer need to be located on the equator to get the maximum Δt&#39;s to improve the efficiencies. Now with a system improved with the solar collector water heaters, an OTEC system can be efficiently installed anywhere with water that does not freeze and with significant sunlight. If the system is onboard a ship it also can be moved to avoid inclement weather. 
     Other features and advantages of the present OTEC system and the corresponding advantages of those features will become apparent from the following discussion of preferred embodiments, which is illustrated in the accompanying drawings. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of operation. Moreover, in the figures to the extent possible, like referenced numerals designate corresponding parts throughout the different views. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a typical prior art land-based OTEC facility. 
         FIG. 2  is a schematic view of a land-based OTEC facility of the present design having a floating solar collector/heat exchanger. 
         FIG. 3  is a schematic view of a flexible floating solar collector/heat exchanger of the present design. 
         FIG. 4  is a schematic view of a land-based OTEC facility of the present design having a floating solar collector/heat exchanger and a near surface cooling water intake. 
         FIG. 5  is a schematic view of a ship-based OTEC facility of the present design having a floating solar collector/heat exchanger and a near surface cooling water intake. 
         FIG. 6  is a graph of a measured temperature rise in a demonstration solar collector similar to  FIG. 3 . 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 2  shows a first embodiment of an OTEC facility of the present design that can include one or more large floating solar collectors  32 . The solar collectors  32  are designed to heat the water introduced into the warm water inlet  14  to a temperature t 2  that exceeds the surface temperature of the adjacent body of water  40 . The floating solar collectors  32  can be constructed inexpensively, for example, by seaming at least two layers of a plastic material together to form a buoyant support for a pipe connected to the warm water inlet  14 . In the embodiment shown in  FIG. 2 , the condenser  20  is maintained at the temperature t 1  by virtue of being fed with the cold water being pumped by pump  24  up from cold water inlet  12  just as in the prior art shown in  FIG. 1 . This higher temperature t 2  for the water going into water inlet  14  translates into a greater Δt than that present in the prior art discussed in connection with  FIG. 1 . Additionally, as the water is introduced into the evaporation tank  18  to form the steam, the water that is not converted into steam, typically representing 90% or more of the introduced volume, can be recirculated to the solar collectors  32  through recirculation pipe  19 . The temperature of the recirculated water has been cooled by the steam evaporation process, but is still warmer than the surface sea water, thereby reducing the amount of solar energy required to achieve the desired temperature t 2 . This recirculation also reduces the energy used to degas the water incoming to the degassing tank  18  by as much as seven fold as at least part of the incoming water has been through the degassing process previously. 
     In one embodiment, the solar collectors  32  can take the form of a flexible floating solar collector shown in  FIG. 3  to be constructed of a plurality of reflective sheets  33  coupled adjacent to each other along a joining seam  31 . A transparent or translucent covering member  35  can also be connected to the edges of the reflective sheets  33  along seams  31 . The space between the reflective sheets  33  and the covering members  35  can be filled with a lighter than water material, such as air, to maintain the reflective surfaces in an approximately parabolic trough-like configuration. Additional intermediate webs  35 A can also be included to provide enhanced structural relations between the reflective sheets  33  and the covering members  35 . A pipe  34 , which is preferably black or other light absorbing color or texture, can be fixed to a middle portion of the covering member  35  so as to be positioned at an optimum position relative to the reflecting curved surfaces  33  of collector  32  to reflect sunlight to the pipe  34 . The pipe  34  can include curved intermediate portions  34 A so as to form a serpentine pattern mounted at the optimum point in the adjacent covering members  35 . Preferably, the reflective sheets  33  focus approximately 60% of the available sunlight falling on the collector  32  on pipe  34 . Multiple flexible floating collectors  32  can be arranged together such that if one solar collector  32  has maintenance problems of any kind it can be taken off line without affecting the flow of heated water from the remaining functional solar collectors  32 . 
     Pipe  34  can have an outlet end coupled to warm water inlet  14 . An inlet end opposite the outlet end of pipe  34  can simply be place into the surrounding body of water  40  adjacent to water surface, and can also be coupled to recirculation pipe  19 . The water not converted to steam in the evaporator  18  can be at an elevated temperature relative to the surrounding environment (typically, approximately 58° C.). Recycling this water through recirculation pipe  19  can provide additional efficiencies as it is already “preheated” and degassed, and the solar heating of this water to the desired output temperature in the range of 80° to 85° C. can occur more quickly. Water pumped through pipe  34  can be heated by the incident sunlight to temperatures in the range of 80° to 85° C. with simple horizontal solar tracking. The heated water can be delivered to the evaporator  18  where the heated water becomes steam at a reduced pressure P 1  of approximately 8.4 psi. This reduced pressure steam then flow through the turbine  26  into the condenser  20 . The pressure P 2  in the condenser  20  drops to about 0.5 psi as the steam condenses back to water. The difference in pressure ΔP between the evaporator  18  and the condenser  20  ensures the desired flow of steam through the turbine  26 . The comparatively higher pressure of steam on the inlet side of turbine  26  coupled with the lower pressure on the outlet side of turbine  26  causes turbine  26  to spin. This twenty-fold increase in pressure differential ΔP as compared to previously discussed prior art device can cause the turbine  26  to drive the electricity generator  28  at a much higher rate and with greater overall efficiency. 
     The embodiment shown in  FIG. 4  eliminates the deep water pipe and all of the deep water maintenance of that pipe, the location restrictions, the cost for pumping that much cold water from the depths, the thermal losses as the temperature of the cold water warms as it is brought to the surface. The input  12  for the water fed to the condensers  20  is, in this embodiment, situated adjacent to the surface  40  of the adjacent body of water, which is typically about 27° C. The warmer water obtained from this surface position typically only causes a 4% drop in Carnot efficiency from about 21% to about 17%. The embodiment shown in  FIG. 4  provides a Δt (t 1 −t 2 =58° C.) between the warm water temperature from a plurality of solar collectors  32  (t 1 =85° C.) and the cooling water temperature (t 2 =27° C.) for the condensing liquid from the surface of water body  40 . The Δt provided by  FIG. 4  is greater than the Δt provided by the prior art,  FIG. 1 , which is Δt=t 2 −t 1  of 22° C., where t 1  is the temperature of the warm water from the surface  40  of the adjacent body of water (t 1 =27° C.) and t 2  is the temperature of the condensing liquid drawn from the deep water draws at a temperature of about 5° C. 
     Another embodiment, shown in  FIG. 5 , can include a large floating hot water tank  36  in which to store hot water collected from the solar collectors  32  during the day light hours, and tapped during the evening and night time hours, thus allowing a 24/7 operating schedule. Additional solar collectors  32  can be added to fill hot water tank  36  while the remainder of the solar collectors  32  are supplying the hot water to the steam generator  18 . A 50 foot by 50 foot cubic water tank  36  would hold about one million gallons of hot water. In a 27° C. ocean, such a tank  36  full of water would cool from approximately 85° C. to about 80° C. in approximately 12 hours, thus potentially providing a source of solar heated water for round-the-clock operation. This embodiment also can comprise a floating transportable support  38 , such as a ship or barge, with an evaporator  18 , turbine  26 , generator  28  and condenser  20  onboard, and with a plurality of large flexible floating solar collectors  32  attached to and surrounding the floating transportable support  38 . Floating hot water storage tank  36  can be attached to, or an integral part of the floating transportable support  38  as well. The temperature of the water within the floating hot water storage tank  36  can be monitored relative to the outlet of the floating solar collector  32 . Water can be withdrawn from the floating hot water storage tank  36  at any time the temperature of the water in the storage tank  36  exceeds the temperature at the outlet of the solar collector  32   
     This embodiment also employs a means  42  to distribute the electricity generated to receiving stations on shore but is movable and thus could avoid threatening weather. For example, the means  42  can comprise one or more undersea power cables, each having a first end connected to a land-based power distribution network and a second end supported by a buoy at desired locations in the ocean. The system, supported by floating transportable support  38 , can be positioned to connect to one of the buoy-supported second ends of an undersea power cable so that the power produced by the system shown in  FIG. 5  can be utilized on shore. 
       FIG. 6  is a graph of a measured temperature rise in a demonstration solar collector similar to  FIG. 3 . The demonstration solar collector employed pipe  34  composed of black single-walled HDPE corrugated pipe of outside diameter of about 5 cm. The pipe  34  was centrally positioned above a forty centimeter wide reflector  32  having a total area of 1.98 m 2 . The ambient temperature of the air and water at the start of the test shown in  FIG. 6  was 27° C. After 49 minutes of exposure to sunlight, the temperature of the water within the pipe  34  had risen to 88° C. Scaling such a solar heat exchanger up, a solar collector of about one hectare would contain slightly over 27 kL of water. If the water in such a system circulated at a flow rate of slightly over 9 L/second, the round trip time for the water would be the 49 minutes used in the reported test, which would be sufficient time for the water to gain nearly 60° C. with each trip and delivering the gained heat to the steam generator  18 . 
     The descriptions in the above specification are not intended to limit this invention to the materials disclosed here. Rather, they are shown for illustration purposes only as one skilled in these arts could easily scale the invention&#39;s dimensions and materials to work with any size OTEC system from small to very large, from around 10 kilowatts to more than a gigawatt. Open loop systems produce steam for power generation and then condense the steam, producing large amounts of distilled water. Small versions of this system will be easily deployable to any coastal site, providing electric power and fresh water to any coastal site in emergency situations. Small systems could be used to supply water and power to remote locations, like oil rigs or small islands. No fossil fuels would need to be transported. A ten megawatt plant would produce more than 1 million gallons of distilled water per day, which could actually be worth more than the power generated in some areas of the world. 
     While these features have been disclosed in connection with the illustrated preferred embodiments, other embodiments of the invention will be apparent to those skilled in the art that come within the spirit of the invention as defined in the following claims.