Abstract:
A heat recovery method and system that extracts heat from the exhaust of a gas turbine unit in a waste heat, heat exchanger and transfers the heat to an intermediate fluid, which can be pressurized water. The intermediate fluid in-turn transfers the heat to an organic working fluid resulting in the vaporization thereof. The vaporized organic working fluid drives a series of turbines which in turn drive a generator that generates electricity.

Description:
TECHNICAL FIELD 
     This invention relates to a waste heat recovery system and to a method of using the same. In particular it relates to a waste heat recovery system for a gas turbine system and to a method for using a heat recovery cycle with the exhaust gases produced by a gas turbine system. 
     BACKGROUND OF THE INVENTION 
     Throughout the world, gas turbines burning a combustable fuel are used to generate power. This power can be used for example to drive fluid pumps, to operate gas compressors, to operate other equipment and to generate electricity. Often these turbines are located in remote places around the globe where there are extreme weather conditions including freezing temperatures. When operating, the gas turbines produce exhaust gases that are usually extremely hot, and just as often, these hot gases are merely exhausted into the atmosphere instead of being utilized to generate additional power. 
     For example, high pressure natural gas transmission pipelines are conventionally used for transporting gas from production fields to customers remotely located from the fields. Gas compressors feeding such pipelines usually are powered by a gas turbine, and optionally, a heat recovery cycle can be employed to reduce the net power requirements by converting waste heat in the hot exhaust gases from the turbine into electricity. An installation of this type is illustrated in the U.S. Fisher et al. U.S. Pat. No. 5,632,143 issued May 27, 1997 which is incorporated herein by reference. In summary, this patent discloses a combined cycle power plant having a gas turbine system. In one embodiment a bottoming steam turbine power plant utilizes heat contained in the exhaust gases exiting the gas turbine system while in another embodiment a bottoming organic Rankine cycle power plant utilizes heat contained in the exhaust gases of the gas turbine system. Typically, the temperature of the exhaust gases is about 450° C. In accordance with this patent the temperature of the gases from which heat is transferred to the bottoming power plant is controlled using ambient air added to the exhaust gases of the gas turbine system. During cold weather, ambient temperatures may drop below freezing causing the steam condensate to freeze thus adversely affecting the operation of the heat recovery system. 
     On the other hand, organic fluids operating as the working fluid in such systems having relatively high temperatures may not be stable. 
     There is therefore a need for an improved heat recovery cycle for a gas turbine system which can be utilized in extreme temperature climates, on the one hand, and yet has an improved heat recovery cycle. 
     SUMMARY OF THE INVENTION 
     The present invention provides a heat recovery system for heat produced by a heat source, such as a gas turbine system. The heat recovery system uses an organic fluid as the working fluid so that the heat recovery system can be used in extreme temperature climates in which temperatures drop below the freezing point for water. In addition, the present invention provides an increased safety factor by utilizing an intermediate fluid to transfer the heat from the hot exhaust gases to the organic working fluid. 
     In a preferred embodiment of the present invention, four major systems are interconnected. The first system is a gas turbine system in which the gas turbine is a primary motive force for some particular application, such as driving a gas compressor remotely located geographically and used in a natural gas line. The gas turbine system generates large amounts of heat that usually is lost to the atmosphere by way of the gas exhaust stacks. The second system is a waste heat recovery system that takes turbine exhaust gas and diverts it from the exhaust stacks to extract heat contained therein and thus extract energy from that which was previously wasted. The third system is an intermediate fluid system which in a preferred embodiment is a pressurized water system and to which the waste heat removed from the turbine exhaust gas is transferred. The fourth system is an organic working fluid system to which the heat from the intermediate fluid is transferred to generate an organic fluid vapor that is used to drive an organic fluid turbine for producing power preferably by using an electric generator connected to the organic fluid turbine. 
     The present invention thus comprises a waste heat recovery system that transfers heat from a primary heat source, such as heat from the exhaust of a gas turbine, to an intermediate fluid, an intermediate fluid system that transfers the heat to an organic working fluid to generate a vapor, and an organic working fluid system whose vapor operates an organic turbine to generate further power from the waste heat. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     Embodiments of the present invention are described by way of example with reference to the accompanying drawing wherein; 
     FIG. 1 is a schematic block diagram of a waste heat recovery system having an organic energy converter using an intermediate liquid cycle. 
    
    
     DETAILED DESCRIPTION 
     Referring now to FIG. 1, reference numeral  10  designates a gas turbine system in accordance with the present invention. The gas turbine unit drives a power device or mechanical power device such as electric generator  14  for producing electric power or a gas compressor. Exhaust gases, exiting gas turbine  12  are supplied to waste heat recovery system  20 . 
     Waste heat recovery system  20  comprises heating coils  36  and  40  housed in housing  24  of heat exchanger  22  for transferring heat contained in the exhaust gases to intermediate fluid system  60 . When heat is transferred to intermediate fluid system  60 , gas turbine exhaust gases in line  18  enter waste heat recovery system  20  at inlet  26  and flow to coils  36  and  40  by opening valve  32  and closing valve  30 . Thereafter, heat depleted exhaust gases exit heat exchanger  22  via outlet  52  and flow to the atmosphere via stack  56 . If preferred, the route of the exhaust gases can be changed in accordance with the specific site. If, for some reason, heat exchanger  22  is to be bypassed, exhaust gases are supplied to the atmosphere by closing valve  32  and opening valve  30  for supplying the exhaust gases to the atmosphere. 
     Heat transfer fluid, preferably water, flowing in intermediate fluid system  60 , which is a closed, pressurized liquid water flow system, receives heat from exhaust gases flowing in heat exchanger  22 . The heat transfer fluid flowing in intermediate fluid system  60  exits heat exchanger  22  at  48  and transfers heat to organic fluid present in organic Rankine cycle working fluid system designated by numeral  90  through use of vaporizer  62 . Portion of heat depleted heat transfer fluid exiting vaporizer  62  is supplied by pump  64  to heat exchanger  22  at  44  while a further portion of heat depleted heat transfer fluid is supplied to pre-heater  68  for pre-heating the organic working fluid in organic e cycle working fluid system  90 . In a preferred configuration, pump  64  is actually two centrifugal pumps connected in parallel with each pump capable of supplying 100% of the pumping requirements, which at steady state full operation is about 130 kilograms per second (kg/s). The ratio of the amount of flow of heat transfer fluid returned to heat exchanger  22  at  44  to the amount of heat transfer fluid supplied to pre-heater  68  is determined by valve  66 . Usually the ratio is 70% flowing into heat exchanger  22  at  44  to 30% flowing to pre-heater  68  and preferably 72.5% to 27.5%. Further heat depleted heat transfer fluid exiting pre-heater  68  is supplied to heat exchanger  22  at inlet  42  for receiving more heat from exhaust gases in coil  36 . In a preferred embodiment, heat exchanger  22  has a capacity of transferring (i.e. recovering) about 33,000 kilowatts (kW) of energy. 
     When using water, the pressure of the water or heat transfer fluid flowing in intermediate fluid system  60  is maintained by pressurizer  76 . The lower or liquid side of pressurizer  76  is connected to line  70  in intermediate fluid system  60  via line  78  and pump or pumps  80  together with valve  82 . Valve  82  senses the pressure of the heat transfer fluid flowing in line  63  for maintaining the desired pressure. Typically, the pressure is maintained at about 3500 kPa with the range of 3000 to 4000 kPa in order to ensure that the water does not boil. Storage tank  72  is also connected to conduit  70  for accumulating excess pressurized heat transfer fluid and from which makeup fluid is supplied when required. Makeup heat transfer fluid is transferred to intermediate fluid system  60  according to the level of liquid in pressurizer  76  determined by level sensor  84 . Sensor  84  is also connected to level controller  86  for controlling the operation of pump  74 . If required, heat transfer liquid present in intermediate fluid system  60  can be emptied into storage tank  72 . Such operation can reduce the risk of the heat transfer fluid from freezing. 
     Organic Rankine cycle working fluid system  90  comprises vaporizer  62  for producing organic working fluid vapor which is supplied to organic vapor turbine  92 . Pentane is the preferred organic working fluid. Organic vapor turbine preferably comprises high pressure turbine module  94  which receives organic working fluid vaporizer produced by vaporizer  62  and low pressure organic vapor turbine module  96  which receives expanded organic working fluid vapor exiting high pressure turbine module  96 . Both high pressure turbine module  94  and low pressure turbine module  96  produce power and preferably drive electric generator  98  which can be interposed between these turbine modules. Further expanded organic vapor exiting low pressure turbine module  96  is supplied to condenser  102  via recuperator  100  where liquid organic working fluid exiting condenser  102  cool the further expanded organic vapor. Each turbine  92  and  94  can be a 3.75 Mw turbine rotating at 1800 RPM. 
     Heated liquid organic working fluid exiting recuperator  100  is preferably supplied to pre-heater  68  for receiving heat transferred from heat transfer fluid flowing in intermediate fluid system  60 . Further heated liquid organic working fluid exiting pre-heater  68  is supplied to vaporizer  62  thus completing the organic working fluid cycle. 
     In the above described waste heat recovery system, sufficient heat is removed from the gas turbine exhaust gases to lower the temperature of the gas from a temperature of about 463° C. to about 92° C. This removed waste heat results in the generation by generator  98  of a net electric power of about 5.8 MW and a gross power of about 6.5 MW, the difference in power between the two power figures being needed to operate the components of the system. 
     In the above describe embodiment, the heat recovery cycle is used to produce electricity. However the shaft power produced by the organic gas turbines  94  and  96  can alternatively be used for directly driving equipment, such as gas compressors or running such machinery without converting the shaft power into electricity. 
     Furthermore, while the above description specifies a gas turbine, other heat sources can also be used such as industrial heat, internal combustion engines such as diesel engines, gas reciprocating engines, etc. In addition, while the above description discloses a single organic working fluid heat recovery cycle, the present invention includes the use of cascaded, or parallel, operating units in a heat recovery cycle. If cascaded units are used, the higher pressure turbine or turbines may use water as a working fluid in closed cycles. 
     Moreover, while the above description discloses a power plant utilizing a simple closed cycle organic Rankine cycle or cycles having an air cooled condenser, air can be added to the exhaust gases of the gas turbine for controlling the temperature of the gases from which heat is extracted in the heat recovery cycle. By using a closed, organic Rankine cycle power plant for the heat recovery rather than a steam turbine, the construction, operation, and maintenance of the overall system is simplified permitting reliable and unattended systems to operate for long periods of time at remote locations. 
     The advantages and improved results furnished by the method and apparatus of the present invention are apparent from the foregoing description of the preferred embodiment of the invention. Various changes and modifications may be made without departing from the spirit and scope of the invention as described in the appended claims.