Abstract:
The present invention provides a method for operating a plurality of independent, closed cycle power plant modules each having a vaporizer comprising the steps of: serially supplying a medium or low temperature source fluid to each corresponding vaporizer of one or more first plant modules, respectively, to a secondary preheater of a first module, and to a vaporizer of a terminal module, whereby to produce heat depleted source fluid; providing a primary preheater for each vaporizer; and supplying said heat depleted source fluid to all of said primary preheaters in parallel.

Description:
FIELD 
     The present invention relates to the field of power plants. More particularly, the invention relates to a cascaded closed Rankine cycle power plant. 
     BACKGROUND 
     Low and medium temperature source fluids, hereinafter termed source fluids of the type described, are those fluids with a temperature less than about 177° C. (350° F.), such as geothermal fluids obtained from many production wells, and industrial liquids produced by various industrial processes. The East Mesa Development Project located in the Imperial Valley of Southern California near Holtville has been producing about 4 million pounds per hour of geothermal fluid at about 162° C. (324° F.). Such geothermal fluid is an example of source fluid of the type described. 
     Electricity is generally produced from source fluids of the type described using a closed Rankine cycle heat engine whose operating fluid is an organic fluid (e.g., Freon), such system being termed a power plant of the type described. A source fluid of the type described is applied to a vaporizer of a power plant of the type described containing liquid organic fluid whereby the latter is converted into a vapor. The vapor is expanded in a turbogenerator that converts some of the heat in the vapor to work and produces heat depleted organic vapor that is condensed in a condenser. The condensed organic fluid is returned to the vaporizer, and the cycle is repeated. 
     The condenser rejects the remaining heat in the heat depleted vapor into ambient air, if an air-cooled condenser is involved, or into cooling water, if a water-cooled condenser is used. Typically, the vaporizer is operated at a pressure that produces saturated or only slightly superheated vapor because the pressures involved are relatively low and the design of the heat exchanger that constitutes the vaporizer, the piping for conveying the vapor, and the turbine, are simplified. In order to maximize power output of a power plant of the type described, the temperature drop of the source fluid across the entire heat exchanger system of the power plant, and the evaporization temperature in the vaporizer must be optimized. 
     Prior art cascaded power plants utilizes a plurality of closed Rankine cycle power plant modules each having an associated heat exchanger, the source fluid being serially applied to the heat exchangers of each module. Whatever system is used, maximizing the net power produced by the system is of paramount importance. One technique for increasing the power is to extract more heat from the source fluid by increasing its temperature drop. With either a single stage or cascaded system, however, increasing the amount of heat extracted from the source fluid by increasing the temperature drop of the source fluid across the heat exchanger system has the effect of decreasing efficiency of the power plant because the mean temperature of the source fluid is reduced. This results in a reduction of the evaporization temperature of the operating fluid in the heat exchanger, thus reducing the Carnot efficiency of the power plant. 
     In another method for increasing the efficiency level of a power plant, a prior art power plant is operated by serially applying the source fluid to the vaporizers of the modules for producing heat depleted source fluid. A preheater is provided for each vaporizer, and the heat depleted source fluid is applied to all of the pre heaters in parallel. 
     In an effort to increase the efficiency of a power plant of the type described, and to extract more power from the source fluid, it has been proposed to operate at super critical temperatures and pressures. In such case, the temperature of the vaporized organic fluid produced by the heat exchanger system is higher than in the above-described typical Rankine cycle power plant. While this approach is effective to increase the efficiency of the power plant and to increase its work output, the gains are offset by the higher cycle pump power consumption, as well as increased cost and complexity of the power plant whose pressure vessels must be designed to operate at pressures in the range of 500-600 psia. 
     Consequently, the present invention provides a cascaded closed Rankine cycle power plant using low and medium temperature source fluid which advantageously can produce an increased power level relative to that produced by prior art power plants. 
     Other advantages of the invention will become apparent as the description proceeds. 
     SUMMARY 
     The present invention provides a method for operating a plurality of independent, closed cycle power plant modules each having a vaporizer comprising the steps of: 
     (a) serially supplying a medium or low temperature source fluid to each corresponding vaporizer of one or more first plant modules, respectively, to a secondary preheater of a first module, and to a vaporizer of a terminal module, whereby to produce heat depleted source fluid; 
     (b) providing a primary preheater for each vaporizer; and 
     (c) supplying said heat depleted source fluid to all of said primary preheaters in parallel. 
     In one aspect, the source fluid is geothermal fluid. 
     In one aspect, the source fluid is fluid generated from an industrial process. 
     In one aspect, each of the power plant modules is operated at different temperatures. 
     In one aspect, each of the power plant modules is operated at different pressures. 
     In one aspect, the motive fluid for the power plant modules is organic fluid. 
     In one aspect, the same type of motive fluid is used in each module. 
     In one aspect, each module is based on a Rankine cycle. 
     The present invention is also directed to a power plant of the type having a plurality of independent, closed cycle power plant modules each of which comprising a vaporizer to which a medium or low temperature source fluid is serially applied for producing heat depleted fluid, and a primary preheater for each of said vaporizers, each of said primary preheaters adapted to preheat motive fluid condensate by means of said heat depleted fluid which is supplied to all of said preheaters in parallel, the improvement comprising a secondary preheater to which said source fluid is serially applied from a first vaporizer and from which said source fluid is supplied to a terminal vaporizer, said secondary preheater adapted to preheat motive fluid condensate exiting from a first primary preheater before being introduced to a corresponding first vaporizer. 
     Each module advantageously comprises a vaporizer responsive to the source fluid for converting the motive fluid condensate to vapor; a turbogenerator responsive to motive fluid vapor produced by said vaporizer for generating power and producing expanded motive fluid vapor; and a condenser for condensing said expanded motive fluid and producing liquid motive fluid condensate that is supplied to the primary preheater associated with said vaporizer. 
     The condenser may be water cooled or air cooled. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings: 
         FIG. 1  is a block diagram of a cascaded power plant, according to one embodiment of the present invention; and 
         FIG. 2  shows an example of temperature-heat diagrams for the power plant modules. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention is an improved cascaded power plant using low and medium temperature source fluid (hereinafter the “source fluid”). One prior art power plant is operated by serially applying the source fluid to the vaporizers of the modules for producing heat depleted source fluid. A preheater is provided for each vaporizer, and the heat depleted source fluid is applied to all of the preheaters in parallel. Such power plant systems are described in U.S. Pat. No. 4,578,953, the disclosure of which is incorporated by reference. Furthermore, U.S. Pat. No. 4,700,543 discloses a similar cascaded power plant having a plurality of modules each of which being arranged in a plurality of levels. The disclosure of U.S. Pat. No. 4,700,543 is also incorporated by reference. In the present invention, an additional preheater is applied to one of the vaporizers. The temperature of the corresponding vaporized organic motive fluid is therefore increased, enabling more vapors to be extracted and to increase the power output of the power plant by the order of about 1-2%. 
       FIG. 1  illustrates a cascaded power plant generally designated as  10 , according to one embodiment of the present invention. Power plant  10  comprises a plurality of independent closed, Rankine cycle organic fluid power plant modules e.g. module  5 A, module  5 B and module  5 C. Three such power plant modules are shown; but the invention is applicable to two or more independent power plant modules. Each of these modules is similar and as a consequence, only module  5 C is described in detail. 
     Module  5 C has a piping system  3 C indicated by a thick line, through which the organic fluid circulates. Heated organic liquid is delivered to vaporizer  13 C and is vaporized by means of heat from the source fluid introduced from inlet I and flowing through source fluid piping system  11 . The organic liquid contained within vaporizer  13 C is vaporized producing essentially saturated or slightly superheated vapor which is applied to turbine  16 C of turbogenerator  15 C. The vapor expands in turbine  16 C, and work is produced so that electric generator  17 C driven by turbine  16 C produces electric power. The vapor exhausted from turbine  16 C is applied to condenser  18 C wherein the vapor is condensed into liquid by the application to the condenser of cooling water that flows through line  9 C. Alternatively, an air cooled condenser can be used. 
     By means of a pump (not shown), condensate produced by condenser  18 C is supplied via line  3 C into preheater  19 C that may be a physical part of or separate from vaporizer  13 C. Heat depleted source fluid, obtained from the outlet from vaporizer  13 C, is applied to preheater  19 C, to heat the organic fluid condensate. If the source fluid is geothermal, the cooled source fluid that exits preheater  19 C may be supplied to a rejection well; or, if the source fluid is an industrial chemical, the cooled fluid may be transferred back to the process. The organic fluid that is heated in pre-heater  19 C by the heat depleted source fluid is delivered to vaporizer  13 C. 
     After being injected into piping system  11  at inlet I, the source fluid is first delivered to vaporizer  13 A of module  5 A. The source fluid that exits vaporizer  13 A is delivered to vaporizer  13 B of module  5 B, and the source fluid that exits from vaporizer  13 B is applied to intermediate preheater  19 A 1  of module  5 A. Advantageously, preheater  19 A 1  can be portion of vaporizer  13 A where it can operate as a preheater zone. Thereafter, the source fluid that exits intermediate preheater  19 A 1  is delivered to vaporizer  13 C of module  5 C. The source fluid that exits from vaporizer  13 C is termed heat depleted source fluid because of the heat extracted from each of vaporizers  13 A,  13 B and  13 C as well as preheater  19 A 1 . This heat depleted fluid is applied to each of the preheaters  19 A 2 ,  19 B and  19 C, in parallel. That is to say, the present invention provides for serially applying a source fluid from inlet I to vaporizer  13 A, vaporizer  13 B, intermediate preheater  19 A 1 , and vaporizer  13 C and for applying heat depleted source fluid to each preheater  19 A 2 ,  19 B, and  19 C in parallel. The source fluid that exits from each of the preheaters  19 A 2 ,  19 B, and  19 C can be conveyed, as shown, to a rejection well if the source fluid is geothermal. With respect to module  5 A, the motive fluid condensate produced by condenser  18 A is delivered to first stage preheater  19 A 2  via line  3 A, additionally heated by intermediate preheater  19 A 1  and then vaporized by vaporizer  13 A and the motive fluid vapor produced is supplied to vapor turbine  16 A for producing power using electric generator  17 A run by vapor turbine  16 A. Alternatively, a recuperator can be used for utilizing heat present in the organic vapor exiting vapor turbine  16 A to heat motive fluid condensate produced by condenser  18 A before it is delivered to first stage preheater  19 A 2 . In addition, alternatively, an electric generator can be used for producing electric power from vapor turbines  16 A and  16 B. Furthermore, a recuperator can also be used in power plant module  5 B so that organic vapor exiting vapor turbine  16 B heats motive fluid condensate produced by condenser  18 B before it is delivered to preheater  19 B. In such a case, less heat can be extracted from the heat depleted heat source fluid. This can be advantageous particularly which geothermal fluid such as liquid or brine is used as the heat source fluid since, under such a situation, a further power plant module can be used to utilize heat still present therein. 
       FIGS. 2A ,  2 B and  2 C illustrate an example of a typical temperature-heat diagram for the three power plant modules  5 A-C shown in  FIG. 1 . From these Figures it can be seen that due to the additional pre-heating stage or pre-heater used in power plant module  5 A, a higher boiling or vaporizing temperature can be achieved. As a consequence, a higher overall power plant efficiency level can be achieved in power plant module  5 A. 
     While some embodiments of the invention have been described by way of illustration, it will be apparent that the invention can be carried out with many modifications, variations and adaptations, and with the use of numerous equivalents or alternative solutions that are within the scope of persons skilled in the art, without departing from the spirit of the invention or exceeding the scope of the claims.