Abstract:
The invention is an ocean thermal energy conversion method and a system in which a motive fluid having predetermined characteristics is circulated in a closed loop between a cold source in cold deep ocean water and heat sources in warm surface water. The motive fluid is compressed between the cold source and a first primary warm water heat source resulting in the motive fluid being substantially totally vaporized at an outlet of the warm water heat source. The motive fluid is heated downstream from the primary heat source by a secondary heat source. The thermal energy of the heated motive fluid is recovered from a turbine and the motive fluid is condensed in the cold source.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    Reference is made to French Application Serial No. 11/03.076, filed Oct. 7, 2011, which application is incorporated herein by reference in its entirety. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to Ocean Thermal Energy Conversion (OTEC) involving energy obtained using the temperature difference existing in tropical and subtropical regions between surface ocean waters and deep ocean waters, in particular at depths of the order of 1000 m. Surface waters are used for the heat source and deep waters for the cold source of a motive thermodynamic cycle. Given that the temperature difference between the heat source and the cold source is relatively low, the expected energy yields are also low. 
         [0004]    2. Description of the Invention 
         [0005]    Conventional OTEC plants generally work on a Rankine cycle. A variant of this cycle with superheating is known as the Hirn cycle. The Him cycle heats the motive fluid to a temperature sufficient for it to be still gaseous after expansion. However, these plants do not involve maximum optimization in the field of the invention. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention is an ocean thermal energy conversion method comprising: 
         [0007]    circulating a motive fluid having predetermined characteristics in a closed loop between a cold source and a heat source and the sources are respectively a heat exchanger disposed between a motive fluid and cold deep ocean water, and a heat exchanger disposed between the motive fluid and the warm surface ocean water; 
         [0008]    compressing the motive fluid between the cold source and the heat source so that in consideration of the thermodynamic conditions of the sources and the nature of the motive fluid, the motive fluid is a substantially totally vaporized motive fluid at the heat source outlet; 
         [0009]    heating the vaporized motive fluid is downstream from the heat source; 
         [0010]    recovering thermal energy of the vaporized heated motive fluid from a turbine; and 
         [0011]    condensing the motive fluid in the cold source. 
         [0012]    The invention increases the heat source temperature at a lower cost which considerably improves (and even doubles) the efficiency of a conventional OTEC plant, and thus the profitability thereof. It also allows the area of implementation of a plant of the present invention to be extended, which increases the potential market. 
         [0013]    The invention optimizes an ocean energy recovery system by basing the design of the plant on prevailing temperatures for cold deep ocean water to provide a cold ocean water source and warm surface ocean water to provide a warm water source providing primary heating from the warm surface ocean water which functions as a heat source causing substantial to total vaporization of the motive fluid and a secondary source providing complementary heating of the substantially to totally vaporized motive fluid. 
         [0014]    The thermal energy of the vaporized and heated motive fluid can be recovered through two turbine stages, and the motive fluid can be heated between the two turbine stages by heat exchange between the motive fluid and the warm surface ocean water. The motive fluid can be heated between the two turbine stages by heat exchange between the motive fluid and a motive fluid portion withdrawn downstream from the second turbine with the portion being recombined, after expansion, in the main motive fluid circuit. 
         [0015]    The vaporized motive fluid can be heated downstream from the heat source by a heat pump whose heat source is from heat exchange with the warm surface ocean water. 
         [0016]    The cold source can be a closed loop of a refrigerant fluid that exchanges thermally with the cold deep ocean water. 
         [0017]    The invention also relates to an ocean thermal energy conversion system comprising: 
         [0018]    means for providing closed-loop circulation of a motive fluid having predetermined characteristics between a cold source and a heat source with the sources being respectively a heat exchanger between the motive fluid and a fluid at the temperature of the cold deep ocean water and a heat exchanger between the motive fluid and a fluid at the temperature of the warm surface ocean water; 
         [0019]    means for compressing the motive fluid between the cold source and the heat source so that, (considering the thermodynamic conditions of the sources and the nature of the motive fluid,) the motive fluid is substantially totally vaporized at an outlet of the heat source; 
         [0020]    means for providing secondary heating of the vaporized motive fluid, downstream from the heat source; 
         [0021]    means for recovering the thermal energy of the vaporized heated motive fluid; 
         [0022]    means for condensing the motive fluid in the cold source. 
         [0023]    The means for recovering the thermal energy of the vaporized and heated motive fluid can comprise two turbine stages and a heat exchanger between the motive fluid and a fluid at the temperature of the warm surface ocean water, wherein the heat exchanger is disposed between the two turbine stages. The heat exchanger disposed between the two turbine stages can be a heat exchanger between the motive fluid and a portion of motive fluid withdrawn downstream from the second turbine by a flow splitter, compressed in a compressor and then, after expansion in a valve the portion is recombined in the main motive fluid circuit by a mixer. 
         [0024]    The means for providing secondary heating can comprise a heat pump having a heat source which is a heat exchanger exchanging heat with the warm surface ocean water. 
         [0025]    The cold source can comprise a closed loop of a refrigerant fluid that provides thermal exchange with cold deep ocean water. 
         [0026]    The plant design is based on the means used to obtain a specific heating method during the engine cycle, in particular in such a way that the heating energy from the surface water vaporizes the motive fluid and that the secondary heat source specific to the vaporized motive fluid provides an additional energy level that allows the cycle efficiency to be increased. 
         [0027]    The present invention is not limited to Rankine cycles. All of the thermodynamic cycles used by OTEC plants can be advantageously improved by the invention. 
         [0028]    The invention&#39;s “distributing the motive fluid heating work” among the warm ocean water (inexpensive) and an additional heat source allows good adjustability of the system according to the invention to local situations, with a higher profitability. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]    Other features and advantages of the present invention will be clear from reading the description hereafter of embodiments given by way of non limitative example, with reference to the accompanying figures wherein: 
           [0030]      FIG. 1  is a representation of the thermodynamic cycle of an OTEC plant whose heat source is improved in an enthalpy diagram; 
           [0031]      FIG. 2  diagrammatically shows a system for implementing the invention; 
           [0032]      FIG. 3  is an alternative embodiment which is a representation of the thermodynamic cycle of an OTEC plant having a heat source with an improved enthalpy diagram, 
           [0033]      FIG. 4  diagrammatically shows an alternative embodiment of the system according to the invention; 
           [0034]      FIG. 5  illustrates a heat pump; and 
           [0035]      FIG. 6  illustrates an alternative embodiment of a heat exchange between two turbine stages. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0036]    The invention is thus based on a distribution of the work among the warm water and an additional heat source. The principle of operation is as follows: the inexpensive warm ocean water provides vaporization of the working fluid, then another heat source increases the temperature thereof. 
         [0037]    The thermodynamic cycle is obtained in accordance with the following reasoning. The pressure of the cold source, which is deep ocean water, is provided by the temperature thereof. Indeed, the pressure must be sufficient for the cold source to be able to condense the working fluid simply by lowering the temperature thereof. 
         [0038]    Once pressure P cold  of the cold source is determined, the point of intersection between the isentrope passing through T cold  and P cold  (gaseous) and the horizontal line whose temperature is T warm water  in the bell of the enthalpy diagram is determined. The pressure of the heat source is thus obtained and the point of intersection gives temperature T warm  that optimizes the system. 
         [0039]    According to this reasoning and by taking 303 K and 278 K as the warm and cold water temperatures respectively, the thermodynamic cycle of  FIG. 1  is obtained from ammonia being the working fluid. 
         [0040]      FIG. 2  shows the block diagram of an OTEC plant working on a Rankine cycle and comprising a heat source exchanger  1  and a secondary heat source  2 . The secondary heat source can be provided by any source providing a temperature higher than the surface ocean water temperature (reference  3 ) and sufficient to improve the thermodynamic cycle. 
         [0041]    The system comprises a cold source exchanger  4  from cold ocean water (reference  5 ) taken from deep waters. Similarly, without departing from the present invention, a refrigerant fluid carrying the deep ocean water energy to the cold source exchanger can be used. The circuit of the motive fluid, which is ammonia in the present example, comprises a compressor  6  and a turbine  7  driving a generator  8 . 
         [0042]    The pressure (P in bar) and temperature (T in Kelvin) values depend on the cold water and warm water temperatures, and on the nature of the motive fluid that is practically totally vaporized at the heat source exchanger outlet. 
         [0043]    An alternative of the method similarly provides “distributing the work” among the warm water and an additional heat source, but with the invention allowing the energy supplied by the secondary heat source to be decreased. 
         [0044]    The thermodynamic cycle, for ammonia, is obtained with the following process: 
         [0045]    As above, the pressure of the cold source is provided by the temperature thereof. Indeed, the pressure must be sufficient for the cold source to be able to condense the working fluid simply by lowering the temperature thereof. 
         [0046]    Once pressure P cold  of the cold source is determined, the point of intersection between the isentrope passing through T cold  and P cold  (gaseous) and the isotherm at the warm water temperature is determined in order to obtain the second stage of the cycle. The intersection allows the intermediate pressure to be determined. From this last point, the isobar is followed up to the phase change start curve. The outlet pressure and temperature of the first stage are thus obtained. 
         [0047]    The intersection between the isentrope passing through the last point and the horizontal line whose temperature is T warm water  is then determined in the bell of the enthalpy diagram. The first stage of the cycle is thus obtained. This intersection allows the high pressure to be determined. 
         [0048]    Considering the warm and cold water temperatures of 303 K and 278 K respectively, the new thermodynamic cycle is obtained with ammonia as the working fluid as shown in  FIG. 3 . 
         [0049]    This alternative embodiment can be considered to recover power over several expansion stages. The goal is to decrease the energy supplied by secondary heat source  2 ′.  FIG. 4  shows a system with two energy recovery stages. In the case of such a plant, the circuit of the motive fluid which is ammonia here is the same temperature as the inlet of cold source exchanger  4  and the outlet of heat source exchanger  1   a . Downstream from this exchanger, the circuit comprises a secondary heat source  2 ′ that, in this alternative, provides less energy. In this example, it provides only half the energy in relation to the system of  FIG. 2 . After this secondary heat source  2 ′, the fluid is expanded in a first turbine  7   a . Since the temperature of the motive fluid is lower than in the single-stage case, the expansion rate is lower. The power recovered is also lower. In this example, half the power recovered is from the system of  FIG. 2 . At the outlet of this first turbine  7   a  (HP), the motive fluid is heated in a heat source exchanger  1   b  of the 2 nd  stage. It is then expanded in a second turbine  7   b  (LP). In the example, the second turbine allows recovery of the same amount of power as the first turbine  7   a . Each turbine can drive an electric generator, or both turbines can be arranged on a single shaft of the same generator. Thus, this alternative embodiment illustrates a method eventually providing recovery of the same amount of energy as the system of  FIG. 2 , and advantageously requires less power to be required from the secondary heat source. 
       Examples 
       [0050]    Table 1 gives the ammonia, warm surface ocean water and cold deep ocean water flow rates for a 10-MW OTEC plant without secondary heat source. 
         [0000]    
       
         
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Ammonia flow rate 
                 Warm water flow rate 
                 Cold water flow rate 
               
               
                   
               
             
             
               
                 0.25 t/s 
                 27 t/s 
                 38 t/s 
               
               
                   
               
             
          
         
       
     
         [0051]    Table 2 gives the powers involved in the various circuit elements of the system of a conventional plant. The compressor and turbine efficiencies were set at 0.9. 
         [0000]    
       
         
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                   
                   
                 Overall 
               
               
                 Compressor 
                 Turbine 
                 Cold source 
                 Heat source 
                 efficiency 
               
               
                   
               
             
             
               
                 0.093 MW 
                 10.045 MW 
                 312 MW 
                 322 MW 
                 3.1% 
               
               
                   
               
             
          
         
       
     
         [0052]    Table 3 shows the powers involved and the overall efficiency of the cycle of a plant with secondary heat source allowing verification of the temperature and pressure conditions given in  FIG. 2 . The water and ammonia flow rates are the same as for the conventional plant of  FIG. 1 . 
         [0000]    
       
         
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                   
                   
                 Cold 
                 Heat 
                 Heat 
                   
               
               
                 Compressor 
                 Turbine 
                 source 
                 source 
                 source 2 
                 Efficiency 
               
               
                   
               
             
             
               
                 0.22 MW 
                 20.4 MW 
                 312 MW 
                 322 MW 
                 10.6 MW 
                 6.1% 
               
               
                   
               
             
          
         
       
     
         [0053]    It can be seen that supplying approximately 10 MW heat (corresponding to 3% of the heat source) allows doubling of the efficiency, and especially doubling of the power recovered by the turbine. Practically all of the power delivered by the second heat source is converted to work in the turbine. 
         [0000]    
       
         
           
             
               
                 
                   
                     Note 
                      
                     
                       : 
                     
                      
                     
                         
                     
                      
                     efficiency 
                      
                     
                         
                     
                      
                     η 
                      
                     
                         
                     
                      
                     is 
                      
                     
                         
                     
                      
                     defined 
                      
                     
                         
                     
                      
                     as 
                      
                     
                         
                     
                      
                     η 
                   
                   = 
                     
                    
                   
                     
                       W 
                       Turbine 
                     
                     
                       Q 
                       Warm 
                     
                   
                 
               
             
             
               
                 
                   = 
                     
                    
                   
                     
                       
                         W 
                         Turbine 
                       
                       
                         
                           Q 
                           
                             
                               Warm 
                                
                               _ 
                             
                              
                             1 
                           
                         
                         + 
                         
                           Q 
                           
                             
                               Warm 
                                
                               _ 
                             
                              
                             2 
                           
                         
                       
                     
                     . 
                   
                 
               
             
           
         
       
     
       Two-Stage Alternative Embodiment 
       [0054]    Table 4 gives the powers involved and the overall efficiency of the cycle of a two-stage plant with secondary heat source, allowing verification of the temperature and the pressure conditions of  FIG. 4 . The water and ammonia flow rates are the same as for the conventional plant. The references correspond to those of  FIG. 4 . 
         [0000]    
       
         
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 4 
               
               
                   
               
               
                   
                   
                   
                   
                   
                   
                   
                 Effi- 
               
               
                 Com- 
                 HP 
                 LP 
                 Cold 
                 Heat 
                 Heat 
                 Heat 
                 cien- 
               
               
                 pressor 
                 turbine 
                 turbine 
                 source 
                 source 
                 source 
                 source 
                 cy 
               
               
                 (6) 
                 (7a) 
                 (7b) 
                 (4) 
                 (1a) 
                 (2′) 
                 (1b) 
                 η 
               
               
                   
               
             
             
               
                 0.22 
                 10.1 
                 10.0 
                 312 
                 322 
                 4.8 
                 5.4 
                 6.1% 
               
               
                 MW 
                 MW 
                 MW 
                 MW 
                 MW 
                 MW 
                 MW 
               
               
                   
               
             
          
         
       
     
         [0055]    It can be seen that the energy provided by the secondary heat source is divided in half, whereas the overall efficiency is the same. 
         [0000]    
       
         
           
             
               
                 
                   
                     Efficiency 
                      
                     
                         
                     
                      
                     η 
                      
                     
                         
                     
                      
                     is 
                      
                     
                         
                     
                      
                     defined 
                      
                     
                         
                     
                      
                     as 
                      
                     
                         
                     
                      
                     η 
                   
                   = 
                     
                    
                   
                     
                       W 
                       Turbine 
                     
                     
                       Q 
                       Warm 
                     
                   
                 
               
             
             
               
                 
                   = 
                     
                    
                   
                     
                       
                         W 
                         Turbine 
                       
                       
                         
                           Q 
                           
                             
                               Warm 
                                
                               _ 
                             
                              
                             1 
                           
                         
                         + 
                         
                           Q 
                           
                             
                               Warm 
                                
                               _ 
                             
                              
                             2 
                           
                         
                         + 
                         
                           Q 
                           
                             
                               Warm 
                                
                               _ 
                             
                              
                             3 
                           
                         
                       
                     
                     . 
                   
                 
               
             
           
         
       
     
         [0056]    Secondary Heat Sources 
         [0057]    Although the secondary heat source can be provided by many known means (from fossil fuels or renewable energy sources), using a heat pump is particularly advantageous in the field of OTEC where cold and heat sources are available. 
         [0058]    A heat pump (PAC) is a thermodynamic device allowing heat to be transferred from the colder medium (which therefore becomes even colder) to the warmer medium (which is thus heated), whereas heat naturally diffuses from the warmer to the colder medium until equality of temperature is reached. The efficiency η pac  of heat pumps is defined as follows: 
         [0000]    
       
         
           
             
               η 
               pac 
             
             = 
             
               
                 Q 
                 Warm 
               
               
                 W 
                 pac 
               
             
           
         
       
     
         [0059]    where W pac  is the power supplied at the heat pump. The efficiency can reach values of 7 in the best case. 
         [0060]    In the present case, heat pump PAC can have the warm ocean water as the cold source and thus can supply heat 24 hours a day. However, the heat pump has its own energy consumption to operate its compressor. This inevitably decreases the efficiency of the device. 
         [0061]      FIG. 5  shows the diagram of a heat pump PAC whose working fluid is ammonia which provides 10 MWt at 320 K, depending on the secondary heating element  2  integrated in the circuit of  FIG. 2 . Reference  10  corresponds to the circuit of the motive fluid of the main circuit that is heated by this heat source  2  (PAC) in a “cold source” exchanger  11 . “Heat source” exchanger  12  collects the thermal energy from the warm surface ocean water  13 . Heat pump PAC comprises a compressor  14  and an expansion valve  15 . 
         [0062]    Table 5 gives the calculation results for the powers of the elements, the flow rates and the efficiency of the heat pump, according to the conditions implemented in the diagram of the heat pump circuit of  FIG. 5 . 
         [0000]    
       
         
               
               
               
               
               
             
           
               
                 TABLE 5 
               
               
                   
               
               
                 Compressor 
                 Cold source 
                 Heat source 
                 Flow rate 
                 Efficiency 
               
               
                   
               
             
             
               
                 1.1 MW 
                 10.4 MW 
                 9.33 MW 
                 9 kg/s 
                 8.5 
               
               
                   
               
             
          
         
       
     
         [0063]    To achieve a secondary heat source according to the circuit of  FIG. 4 , an identical heat pump working at half the flow rate is sufficient. 
         [0064]      FIG. 6  illustrates another alternative embodiment where the secondary heat source is a portion of the motive fluid. The gaseous NH3 motive fluid at the outlet of second turbine  7   b  (LP) is separated into two portions in a flow splitter  17 , with about 4% secondary flux circulating in line A and 96% in line B. 
         [0065]    Line A leads to a compressor  9  and the secondary flux exchanges heat in exchanger  1   b  upstream from second turbine  7   b  and downstream from first turbine  7   a . The secondary flux is then expanded in a valve  16  prior to being recombined with the main flux in a mixer  18 . 
         [0066]      FIG. 6  gives by way of example the pressure and temperature values at various points of the secondary heat source circuit according to this variant.