Patent Publication Number: US-7594399-B2

Title: System and method for power generation in Rankine cycle

Description:
BACKGROUND 
     This invention relates generally to power generation systems using a Rankine cycle. More particularly this invention relates to power generation systems using a Rankine cycle with a mixture of at least two liquids as the working fluid. 
     Rankine Cycles use a working fluid in a closed cycle to gather heat from a heating source or a hot reservoir by generating a hot gaseous stream that expands through a turbine to generate power. The expanded stream is condensed in a condenser by rejecting the heat to a cold reservoir. The working fluid in a Rankine cycle follows a closed loop and is re-used constantly. The efficiency of Rankine Cycles such as Organic Rankine Cycles (ORCs) in a low-temperature heat recovery application is very sensitive to the temperatures of the hot and cold reservoirs between which they operate. In many cases, these temperatures change significantly during the lifetime of the plant. Geothermal plants, for example, may be designed for a particular temperature of geothermal heating fluid from the earth, but lose efficiency as the ground fluid cools over time, thereby shifting the plant operating temperature away from its design point. Air-cooled ORC plants that use an exhaust at a constant-temperature from a larger plant as their heating fluid will still deviate from their design operating conditions as the outside air temperature changes with the seasons or even between morning and evening. 
     Therefore there is a need for a power generation system using a Rankine Cycle that can deal with fluctuations in the temperature of the hot and cold reservoir or heat sources without adversely affecting the efficiency or the stability of the power generation system. 
     BRIEF DESCRIPTION 
     In one aspect, a system for power generation includes a boiler configured to receive heat from an external source and a liquid stream and to generate a vapor stream. The liquid stream comprises a mixture of at least two liquids. The system also includes an expander configured to receive the vapor stream and to generate power and an expanded stream. A condenser is configured to receive the expanded stream and to generate the liquid stream. The system further includes a supply system coupled to the boiler or the condenser and configured to control relative concentration of the two liquids in the liquid stream. 
     In another aspect, a system for power generation includes a boiler configured to receive heat from an external source and a liquid stream and to generate a vapor stream, wherein said liquid stream comprises a mixture of at least two liquids. The system also includes an expander configured to receive the vapor stream and to generate power and an expanded stream and a condenser configured to receive the expanded stream and generate the liquid stream. A supply system is coupled to one of the boiler or condenser and is configured to control relative concentration of the two liquids in the liquid stream. The supply system includes a first tank to hold a liquid rich in said higher boiling point liquid and a second tank to hold a liquid rich in lower boiling point liquid. 
     In yet another aspect, a method of controlling a power generation system includes a boiler configured to receive a liquid stream and to generate a vapor stream, an expander configured to receive the vapor stream and to generate an expanded stream, and a condenser configured to receive the expanded stream and to generate the liquid stream. The method includes controlling relative concentration of at least two liquids in the liquid stream using a supply system coupled to the boiler or the condenser to supply a stream rich in one of the two liquids. 
    
    
     
       DRAWINGS 
       These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
         FIG. 1  illustrates an exemplary power generation system using a Rankine Cycle; 
         FIG. 2  illustrates the normal operation of the boiler of the exemplary power generation system of  FIG. 1 ; 
         FIG. 3  illustrates the operation of the boiler of the exemplary power generation system of  FIG. 1  when the temperature of the external heat source is low; 
         FIG. 4  illustrates the operation of the boiler of the exemplary power generation system of  FIG. 1  when the temperature of the external heat source is high; 
         FIG. 5  illustrates another exemplary power generation system using a Rankine cycle; and 
         FIG. 6  illustrates yet another exemplary power generation system using a Rankine cycle. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  represents an exemplary system  10  for power generation using a Rankine Cycle. The system includes a boiler  12  configured to receive heat from an external source  13  and a liquid stream  14  and to generate a vapor stream  16 . The power generation system  10  also includes an expander  18  configured to receive the vapor stream  16  and to generate power  25  by rotating the mechanical shaft (not shown) of the expander  18  and an expanded stream  20 . A condenser  22  is configured to receive the expanded stream  20  and to generate the liquid stream  14 . A supply system is coupled to the boiler  12  or the condenser  22  (with the “or” as used herein meaning either or both) and is configured to control relative concentration of the two liquids in the liquid stream  14  and the vapor stream  16 . The liquid stream  14  and the vapor stream  16  along with the vapor and liquid phase within the boiler  12  and condenser  22  form the working fluid of the Rankine cycle shown in  FIG. 1 . 
     The power generation system using a Rankine Cycle plant shown in  FIG. 1  uses a working fluid comprising a mixture of two or more component fluids, in place of a single pure substance. By the adjustment of the relative quantities of each component of the fluid, the properties of working fluid as a whole may be varied to accommodate changes in the external temperature conditions, as described below. In a Rankine cycle, the working fluid is pumped (ideally isentropically) from a low pressure to a high pressure by a pump  27  as shown in  FIG. 1 . Pumping the working fluid from a low pressure to a high pressure requires a power input (for example mechanical or electrical). The high-pressure liquid stream  14  enters the boiler  12  where it is heated at constant pressure by an external heat source  13  to become a saturated vapor stream  16 . Common heat sources for organic Rankine cycles are exhaust gases from combustion systems (power plants or industrial processes), hot liquid or gaseous streams from industrial processes or renewable thermal sources such as geothermal or solar thermal. The superheated or saturated vapor stream  16  expands through the expander  18  to generate power output (as shown by the arrow  25 ). In one embodiment, this expansion is isentropic. The expansion decreases the temperature and pressure of the vapor stream  16 . The vapor stream  16  then enters the condenser  22  where it is cooled to generate the saturated liquid stream  28 . This saturated liquid stream  28  re-enters the pump  27  to generate the liquid stream  14  and the cycle repeats. 
     As described above, the power generation system  10  represents a Rankine cycle where the heat input is obtained through the boiler  12  and the heat output is taken from the condenser  22 . In operation, the boiler  12  is connected to an inlet  30  and outlet  32 . The arrow  34  indicates the heat input into the boiler from the external heat source  13  and the arrow  46  indicates the heat output from the condenser  22  to the cold reservoir. In some embodiments, the cold reservoir is the ambient air and the condenser is an air-cooled condenser. In some embodiments, the liquid stream  14  comprises two liquids namely a higher boiling point liquid and a lower boiling point liquid. Embodiments of the boiler  12  and the condenser  22  can include an array of tubular, plate or spiral heat exchangers with the hot and cold fluid separated by metal walls. 
     To control the boiling and condensing characteristics of a mixture of two fluids in a thermodynamic cycle, the supply systems described herein actively manipulate the ratio of fluid concentrations. The method described herein uses the boiling and/or condensing stages that belong to any Rankine cycle as a means of changing the relative concentrations of the two fluids. After the point in the Rankine cycle where boiling or condensation has begun, but before the point where it completes (producing a vapor and liquid, respectively), two phases exist simultaneously in the boiler/condenser. The liquid phase, when compared with the homogeneous single-phase mixture, necessarily contains a higher concentration of the mixture species with the higher boiling point. The system and the methods described herein propose to change the overall concentrations of the working fluid by removing some of this liquid from the section of the boiler  12  or the condenser  22 , where the two phases coexist. 
     As shown in  FIG. 1 , the first supply system  24  includes a first tank  36  configured to hold and supply a first fluid  38  rich in higher boiling point liquid. The first supply system  24  may further include a second tank  40  coupled to the inlet line  44  to the boiler  12  configured to hold and supply a second fluid  42  rich in lower boiling point liquid. The first tank  36  is fluidically coupled to the boiler  12  through a valve  50  and the second tank  40  is fluidically coupled to the inlet line  44  of the boiler  12  through a valve  52 . The condenser  22  may be coupled to a second supply system  26 . The second supply system  26  includes a first tank  54  fluidically coupled to the condenser  22  through a valve  58  and a second tank  56 , fluidically coupled to the outlet  62  of the condenser  22  through a valve  60 . Although the embodiment shown in  FIG. 1  includes two supply systems  24  and  26  coupled to the boiler  12  and the condenser  22  respectively, alternate embodiments may include a single supply system coupled to either the boiler  12  or the condenser  22 . 
       FIG. 2  illustrates the normal operation of the boiler  12  along with the first supply system  24  coupled to the boiler  12 . When the temperature of the heat source  13  remains stable during operation, the valves  50  and  52  connected to the first tank  36  and the second tank  40  respectively remain closed and the first supply system  24  is not fluidically coupled to the boiler.  FIG. 3  illustrates the operation of the boiler  12  when the temperature of the external source  13  is lower than that during normal operation as shown in  FIG. 2 . When the temperature of the external source is low, the valve  52  attached to the second tank  40  opens to allow the fluid  42  rich in the lower boiling point liquid to be supplied into the inlet line  44  of the boiler  12 . Simultaneously, to keep the entire volume of working fluid inside the cycle constant, the valve  50  opens and pulls back equivalent amount of the liquid  38  rich in higher boiling point liquid from the boiler  12 . Since the liquid  21  inside the boiler  12  gets richer in the lower boiling point liquid, heat is removed more effectively from the heat source  13  at lower temperature. This boosts the power output of the cycle, hence regaining a portion of the power output lost compared to the design point. 
       FIG. 4  illustrates the operation of the boiler  12  when the temperature of the external source  13  is too high. In order to maximize the power generation level, the supply system  24  operates in such a way that heat is removed more effectively from the heat source. In order to achieve that, the mixture rich in lower boiling point liquid  42  is pulled back into the first tank  40  and the same volume of liquid rich in higher boiling point liquid  38  is pushed into the boiler  12 . Therefore the liquid mixture  21  in the boiler  12  is richer in the higher boiler point liquid and keeps the temperature and the amount of vapor generated in the boiler  12  optimal in spite of an increase in the temperature of the external source. As shown in  FIG. 1 , a controller  15  is electrically coupled to the boiler  12  and the supply system  24  configured to provide the signals for the opening and closing of the valves  50  and  52 . The working fluid may be pulled into the cycle and out of it by plungers  37  and  41  of the first supply system  24  and plungers  55  and  57  of the second supply system  26 . The plunger operations are governed by electric motors (not shown). 
     Although the working fluid is described herein as a mixture of a higher boiling point liquid and a lower boiling point liquid, the working fluid may also include more than two components. In some embodiments, the working fluid is a mixture of water and an alcohol. In one embodiment, the mixture comprises water and ethanol. In some other embodiments, the working fluid may include more than one hydrocarbon. In one embodiment, the working fluid comprises at least two of alkanes such as pentane, propane, cyclohexane, cyclopentane and butane. In some embodiments, the working fluid may also include fluorohydrocarbons, ketones and aromatics. 
       FIG. 5  illustrates another exemplary power generation system  100 , wherein the supply system  102  comprises a single chamber  104  and a movable barrier  110  situated in the chamber  104 . The movable barrier  110  is configured to separate two liquids: one rich in lower boiling point liquid  112  and another rich in higher boiling point liquid  114 . As shown in  FIG. 5 , the operation of the boiler  12  is illustrated using such a single chamber  104 . The two outlets  116  and  118  of the chamber  104  are attached to valves  106  and  108 . The liquid rich in higher boiling point liquid  114  is directly coupled to the boiler  12  through an inlet  120 . The liquid rich in lower boiling point liquid  112  is coupled to the inlet line  44  to the boiler  12 . In operation, when the temperature of the external source  13  is low, the movable barrier  110  is configured to move towards the valve  108  to push more liquid rich in lower boiling point  112  to maximize the amount of heat recovered. Simultaneously, the liquid rich in higher boiling point  114  is pulled back into the single chamber  104  through the opening of the valve  106  to keep the volume of the working fluid in the system constant. Alternatively, when the temperature of the external source  13  is too high, the movable barrier  110  is configured to move towards the valve  106  to push more liquid rich in higher boiling point  114  maximize the amount of heat recovered. Simultaneously, the liquid rich in lower boiling point  112  is pulled back into the single chamber  104  through the opening of the valve  108  to keep the volume of the working fluid in the system constant. As shown in  FIG. 5 , a controller  122  is electrically coupled to the boiler  12 , the heat source  13 , and the supply system  102  configured to provide the signals for the opening and closing of the valves  106 ,  108  and the movement of the movable barrier  110 . 
       FIG. 6  illustrates the operation of the condenser  22  connected to a supply system  202 , wherein the supply system  202  includes a single chamber  204  and a movable barrier  210  situated in the chamber  204 . As described earlier, the movable barrier  210  is configured to separate two liquids one rich in lower boiling point liquid and another rich in higher boiling point liquid. As shown in  FIG. 6 , the operation of the condenser  22  is illustrated using such a single chamber  204 . The two outlets  216  and  218  of the chamber  204  are attached to valves  206  and  208  respectively. The liquid rich in higher boiling point liquid  214  is directly coupled to the condenser  22  through an inlet  220 . The liquid rich in lower boiling point liquid  212  is coupled to the outlet  222  to the condenser  22 . In operation, when the temperature of the external cold reservoir is lower than normal conditions, the liquid generated in the condenser  22  is rich in the higher boiling point liquid and hence, maximize the amount of heat rejected, the movable barrier  210  is configured to move towards the valve  208  to push more liquid rich in lower boiling point. Similarly, the liquid rich in higher boiling point is pulled back into the single chamber  204  through the opening of the valve  206  to keep the volume of the working fluid in the system constant. Similarly, when the temperature of the external cold reservoir is higher than normal conditions, the liquid generated in the condenser  22  is rich in the lower boiling point liquid and hence, to keep the amount of the working fluid constant, the movable barrier  210  is configured to move towards the valve  206  to push more liquid rich in higher boiling point. Similarly, the liquid rich in lower boiling point is pulled back into the single chamber  204  through the opening of the valve  208  to maximize the amount of heat rejected As shown in  FIG. 6 , a controller  224  is electrically coupled to the condenser  22  and the supply system  202  configured to provide the signals for the opening and closing of the valves  206 ,  208  and the movement of the movable barrier  210 . 
     The systems and the methods described in the preceding sections can control the relative concentration of the higher and the lower boiling point liquids in the working fluid in a Rankine cycle. This allows the power generation systems to be operated at the optimum power output for a range of ambient temperature and heat source conditions. In some locations, the performance of the condenser in a Rankine cycle, such as an air-cooled condenser can be affected significantly by the temperature change between summer and winter. In desert climates, similar variations are observed between day and night. At many plants, the temperature of the external heat source may constantly vary due to a number of causes, including but not limiting to the change from full-load to part-load operation at power stations where waste-heat cycles are heated by turbine exhaust. By controlling the relative concentrations of the higher and the lower boiling point liquids in the working fluid, the instability of the power generation system is mitigated as the tendency of temperature variations to drive the plant&#39;s performance away from its design point is avoided. 
     While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.