Patent Publication Number: US-2011056227-A1

Title: Heat recovery system of plant using heat pump

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a heat recovery system of a plant using a heat pump. More particularly, the present invention relates to a heat recovery system of a plant using a heat pump that is capable of absorbing some heat energy wasted from a condenser of the plant, thereby improving power generation efficiency of the plant. 
     2. Description of the Related Art 
     As is already well known, a heat pump is a kind of technology that is widely applicable to production equipment of an industrial field, including refrigerators and freezers. The heat pump performs effective heat exchange with a high coefficient of performance (COP). Therefore, the heat pump is capable of effectively absorbing heat energy from the atmosphere, rivers, or industrial equipment that generates waste heat. In the most cases, the heat pump is used for cooling purposes although the heat pump is used for heating purposes in some cases. With the recent increase of interest in energy efficiency, technology for improving heat efficiency simultaneously using cooling and heating functions is gradually increasing. 
     The present invention relates to a heat recovery system that is capable of partially absorbing a large amount of heat energy wasted from a condenser of a plant using such a heat pump, thereby improving efficiency of the plant. 
     Meanwhile, a heat efficiency improvement apparatus of a plant using a thermoelectric element is disclosed in Korean Patent Application No. 2009-58580, which has been filed in the name of the applicant of the present application. The disclosed apparatus has an advantage in that the thermoelectric element is used, and therefore, no mechanical drive parts are provided, thereby achieving easy and simple application and noise prevention. On the other hand, the disclosed apparatus has a disadvantage in that the COP of the apparatus is excessively sensitive to temperature difference and current ratio between a high temperature part and a low temperature part, and therefore, the increase of the COP is limited, whereby it is difficult to apply the apparatus to large-scale equipment. 
     SUMMARY OF THE INVENTION 
     Therefore, the present invention has been made in view of the above problems of the related art, and it is an object of the present invention to provide a heat recovery system of a plant using a heat pump that is applicable to a large-scale plant and, in addition, is capable of recovering some heat discharged from a condenser of the plant due to a coolant, thereby improving efficiency of the plant. 
     In accordance with the present invention, the above and other objects can be accomplished by the provision of a heat recovery system of a plant using a heat pump constructed such that water condensed by a condenser in which a coolant flows from a coolant inlet port to a coolant outlet port to condense steam into water is supplied to a service water heater via a plurality of condensate pumps, wherein a heat pump is installed between the condensate pumps and the service water heater for receiving the water condensed by the condenser and the coolant discharged from the condenser, some of the condensed water and the coolant, having passed through the heat pump, are supplied to the coolant inlet port of the condenser, while the remaining portions of the condensed water and the coolant are supplied to the service water heater, the condensed water supplied to the service water heater is heated by a three way valve through which service water heating steam is supplied, and some of the supplied steam is directly recovered to the condenser via a combustion air heater. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a construction view illustrating a heat recovery system of a plant using a heat pump according to a first embodiment of the present invention; 
         FIG. 2  is a view illustrating the flow of heat in the heat recovery system according to the first embodiment of the present invention shown in  FIG. 1 ; 
         FIG. 3  is a construction view illustrating a heat recovery system of a plant using a heat pump according to a second embodiment of the present invention; 
         FIG. 4  is a construction view illustrating a heat recovery system of a plant using a heat pump according to a third embodiment of the present invention; and 
         FIG. 5(A)  is a schematic view illustrating input and output of energy into and from respective components of a plant before a heat pump is applied to the plant, and  FIG. 5(B)  is schematic view illustrating input and output of energy into and from respective components of a plant after a heat pump of the present invention is applied to the plant. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Now, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
       FIG. 1  is a construction view illustrating a heat recovery system of a plant using a heat pump according to a first embodiment of the present invention, and  FIG. 2  is a view illustrating the flow of heat in the heat recovery system according to the first embodiment of the present invention shown in  FIG. 1 . 
     The heat recovery system of the plant is constructed such that water condensed by a condenser  1  in which a coolant flows from a coolant inlet port  1   a  to a coolant outlet port  1   b  to condense steam into water is supplied to a service water heater  3  via a plurality of condensate pumps  2 . In this embodiment, a heat pump  4  is installed between the condensate pumps  2  and the service water heater  3  for receiving the water condensed by the condenser  1  and the coolant discharged from the condenser  1 . The coolant, having passed through the heat pump  4 , is supplied to the coolant inlet port  1   a  of the condenser  1 . The condensed water is supplied to the service water heater  3 . The condensed water, supplied to the service water heater  3 , is heated by a three way valve  5  through which service water heating steam is supplied. Some of the service water heating steam is condensed by a combustion air heater  6 , and is then directly recovered to the condenser  1 . 
     When the steam is condensed into water by the condenser  1 , a large amount of heat energy is wasted. In this embodiment, some of the wasted heat energy is absorbed by the heat pump  4 , and service water is heated by the absorbed heat energy. At this time, the amount of heat to heat the service water may be changed according to a coefficient of performance (COP) of the heat pump  4 . 
     Recently commercially available heat pumps have COPs of about 2 to 10. When a heat source is sufficient, and a heating temperature is about 15° C., the heat pumps may have a COP of 9 (Products of McQuay). 
     When a heat pump having a COP of 9 is used, therefore, it is possible to heat service water with the amount of heat equivalent to nine times the amount of electric power supplied to the heat pump. 
     As the service water is previously heated, service water heating steam may be left over. The steam has a relatively high temperature (about 60° C. or higher). After being used to heat combustion air, the steam is recovered to the condenser  1 . As a result, the amount of heat introduced into a boiler is increased without the additional increase of fuel, thereby improving the efficiency of the plant. 
     Meanwhile,  FIG. 3  is a construction view illustrating a heat recovery system of a plant using a heat pump according to a second embodiment of the present invention. The heat recovery system according to the second embodiment is identical to the heat recovery system according to the first embodiment except that the heat pumps  4  use steam to be supplied to the condenser  1  as a direct heat source. 
     In a heat recovery system of a plant using a heat pump according to a third embodiment of the present invention as shown in  FIG. 4 , service water is not heated by heat recovered to the condenser  1  using the heat pump  4  but combustion air is directly heated by the recovered heat such that the heated combustion air is supplied to the boiler. At this time, it is necessary to increase temperature difference at the same amount of heat since air has a lower specific heat than water. When the temperature difference is too large, however, the COP of the heat pump is lowered. For this reason, it is expected that this embodiment exhibits a lower increase in efficiency of the plant than the previous embodiments in which the service water is heated. 
       FIGS. 5(A) and 5(B)  are schematic views illustrating input and output of energy into and from respective components of a plant before and after a heat pump is applied to the plant, respectively. 
     For example, when the present invention is applied to a plant having a boiler efficiency of 90%, a turbine efficiency of 46%, and a generator efficiency of 94%, the efficiency improvement of the plant may be estimated as follows. Before the heat pump is applied to the plant as shown in  FIG. 5(A) , the total efficiency of the plant=boiler efficiency×turbine efficiency×generator efficiency=output/input=498.923/1282.051=38.916%. After the heat pump of the present invention is applied to the plant as shown in  FIG. 5(B) , on the other hand, the total efficiency of the plant=output/input=504.248/1282.051=39.331%. That is, an efficiency improvement of about 0.415% is achieved. Such efficiency improvement may save a considerably large amount of energy in a large-scale plant. 
     This is possible when the entirety of the amount of recovered heat is supplied to the boiler. For a plant in which an air preheater is already installed, loss of exhaust gas is increased due to the increase in temperature of the exhaust gas during heating of the combustion air, with the result that efficiency increase effect may be insignificant. In this case, it is necessary to directly recover heat from the exhaust gas, the temperature of which has been increased from about 117° C. to about 143° C., and to use the recovered heat in a predetermined process. The recovered heat may be used in the predetermined process as follows. In a district heating system, the recovered heat may be used to heat service water for district heating. In a heavy oil or light oil plant, the recovered heat may be used to preheat fuel, thereby improving efficiency of the plant. 
     As is apparent from the above description, in the heat recovery system of the plant using the heat pump according to the present invention, some of the heat discharged from the condenser due to the coolant is recovered to the heat pump, and the recovered heat is reused, with the result that efficiency of the plant is improved, thereby saving fuel and reducing the discharge of carbon dioxide and thermal waste water. 
     Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims. 
     DRAWINGS 
     
       FIG. 1 
         
         
           
             STEAM 
             TO SECOND SERVICE WATER HEATER 
             SERVICE WATER HEATING STEAM 
           
         
       
    
     
       FIG. 2 
         
         
           
             COOLANT 
             STEAM 
             HEATED COOLANT 
             SEAWATER 
             DISCHARGE OF SEAWATER 
             RECOVERY 
             CONDENSATE PUMPS 
             HEAT PUMP 
             SERVICE WATER HEATING STEAM 
             SERVICE WATER HEATER 
             SERVICE WATER 
             COMBUSTION AIR 
             COMBUSTION AIR HEATER 
             INTO BOILER FURNACE 
           
         
       
    
     
       FIG. 3 
         
         
           
             STEAM 
             SEAWATER 
             DISCHARGE OF SEAWATER 
             RECOVERY 
             RECOVERY 
             CONDENSATE PUMPS 
             HEAT PUMP 
             SERVICE WATER HEATING STEAM 
             SERVICE WATER HEATER 
             SERVICE WATER 
             COMBUSTION AIR 
             COMBUSTION AIR HEATER 
             INTO BOILER FURNACE 
           
         
       
    
     
       FIG. 4 
         
         
           
             COOLANT INLET PORT 
             STEAM 
             COOLANT OUTLET PORT 
             COMBUSTION AIR 
             TO BOILER 
             HEAT PUMP 
             CONDENSATE PUMPS 
             CONDENSED WATER 
             FIRST SERVICE WATER HEATER 
             ADDITION 
             TO SECOND SERVICE WATER HEATER 
           
         
       
    
     
       FIG. 5 
         
         
           
             (A) BEFORE APPLICATION OF HEAT PUMP 
             BOILER 
             TURBINE 
             GENERATOR 
             CONDENSER 
             (B) AFTER APPLICATION OF HEAT PUMP 
             BOILER 
             TURBINE 
             GENERATOR 
             POWER TRANSMISSION END 
             CONDENSER 
             HEAT PUMP