Ocean Thermal Energy Conversion (OTEC) systems have been studied and small scale units have been tried since the late 1920's and early 1930'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
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=t2−t1 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'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.
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'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.
In one embodiment, the solar collectors 32 can take the form of a flexible floating solar collector shown in
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 P1 of approximately 8.4 psi. This reduced pressure steam then flow through the turbine 26 into the condenser 20. The pressure P2 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
Another embodiment, shown in
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
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'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.
This application is related to and claims all benefit of U.S. Provisional Applications Ser. No. 61/743,236 filed Aug. 29, 2012 and Ser. No. 61/849,927 filed Feb. 5, 2013.
Filing Document | Filing Date | Country | Kind |
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PCT/US2013/054455 | 8/12/2013 | WO | 00 |
Number | Date | Country | |
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61849927 | Feb 2013 | US | |
61743236 | Aug 2012 | US |