Abstract:
A method and apparatus for controlling heat recovery coils in an exhaust stack. A set of heat recovery coils at least partially filled with a heat conducting fluid is positioned in a hot zone. The recovery coils are biased in a direction out of the hot zone to prevent accidental overheating in the event of a control or power failure. A heat transduction system is connected in fluid communication with the heat recovery coils. Heat energy is transferred from the hot zone into the heat conducting fluid, and the heated heat conducting fluid is then flowed into the heat transduction system where heat is removed from the heat conducting fluid. The extracted heat is then transduced into useful energy.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application Ser. No. 60/126,670 filed Mar. 29, 1999. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The present invention generally relates to heat recovery devices and, more particularly, to a control system for heat recovery coils. 
     BACKGROUND OF THE INVENTION 
     Although electric power is utilized in diverse ways in the economy and demand remains high at all times, the demand for electric power nevertheless fluctuates markedly during the course of a day. Business demand is high throughout daylight hours in the operation of stores and offices, but diminishes significantly thereafter. Residential demand is highest in the evening hours. Industrial demand is relatively steady and high at all times. Other demands, such as for urban transportation, peak at differing times. Additionally, demand can vary greatly seasonally and with short-term changes in the weather. For example, electricity usage soars on abnormally hot days due to widespread use of air conditioning equipment. 
     In an optimized power utilization system, all such demands would be complementary and thus provide a substantially constant power requirement which could be served readily by the various sources of electric power in a readily predictable manner. In reality, however, electric power demand is nowhere near constant. 
     The uneven demand for electric power requires that power generation capacity be sufficiently great to accommodate the maximum instantaneous demand. This, in turn, leads to uneconomic operation of generally over-sized electric power generation facilities. One approach to this problem has been the encouragement of off-peak usage of electric power in an effort to restructure the demand pattern. Another approach has been the installation of additional generating facilities intended for use during the periods of peak power demand. For example, an electric utility may lease one or more gas turbine electric generators in order to bring on-line more power generation capacity during warmer months of the year. 
     One such prior art gas turbine electric generator is illustrated in FIG.  1  and indicated generally at  10 . The turbine  10  is housed within a structure  12  having an air inlet  14  and an exhaust stack  16 . The gases exiting the top of the exhaust stack  16  are extremely hot, typically in the neighborhood of 900° F. 
     This exhausted heat is energy that is not being utilized by the system, thus drastically lowering the efficiency of the turbine  10 . This heat represents energy that is consumed by the turbine  10  but not turned into useful generated electricity. 
     Obviously, it would be desirable to recover the energy being lost as heat from the turbine  10  (or any other system that produces wasted heat exhaust) and convert this heat to a useful form. The present invention is directed toward this goal. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a method and apparatus for selectively introducing one or more sets of heat transfer coils into the path of heated gasses to facilitate reclamation of at least some of the heat for transduction into useful energy. One form of the present invention is a set of coils adapted to circulate a heat-conducting fluid under pressure. The coils are in fluidic communication with a fluid chilling assembly. The coils are further adapted to be partially or completely introduced into an environment containing hot gasses (a hot zone), wherein heat is transferred from the hot gasses to the fluid circulating in the coils. The heated fluid is circulated into the chillers, where the heat is removed and transduced into a conveniently useful form of energy, such as electricity. The coils may b e only partially introduced into the hot gasses so as to optimize t he heat transfer to the coils and to prevent overheating of the heat conducting fluid and damage to the coils. The extent to which the coils are introduced into the hot gasses is variable and is a function of the temperature of the gasses and the fluid in the coils. In t he event of a power or control failure, the coils may be provided with a failsafe configuration to automatically remove them from t he hot gas environment. 
     One object of the present invention is to provide an improved heat energy reclamation system with an automatic failsafe to guard against accidental overheating. 
     Related objects and advantages of the present invention will be apparent from the following description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a prior art gas turbine electric generator. 
     FIG. 2 is a schematic diagram of a heat recovery system of the present invention. 
     FIG. 3 is a plan view of a pair of heat recovery coil units of the present invention. 
     FIGS. 4A-B are schematic side elevational views of one of the heat recovery coil units of FIG.  3 . 
     FIG. 5 is a schematic drawing illustrating the relationship of the control system to the heat recovery coils of the present invention. 
     FIG. 6A is a schematic side elevational view of a first fail-safe configuration of the present invention. 
     FIG. 6B is a schematic side elevational view of a second fail-safe configuration of the present invention. 
     FIG. 7 is a schematic diagram of a heat recovery system according to a second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiment illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, and alterations and modifications in the illustrated device, and further applications of the principles of the invention as illustrated therein are herein contemplated as would normally occur to one skilled in the art to which the invention relates. 
     The use of a heat recovery system of the present invention with a pair of gas turbine electric generators  10  is illustrated schematically in FIGS. 2 and 7, and indicated generally at  20 . The heat recovery system  20  is illustrated in use with two turbines  10 , however it will be understood that the present invention may be used with any number of turbines  10 . In fact, the heat recovery system of the present invention may be used with any heat-producing device, and may be configured to work with any number of sources of such heat. 
     FIG. 2 is a schematic top plan view, such that the tops of the heat exhaust stacks  16  are visible. In order to capture the heat emitted from the exhaust stacks  16 , a system of heat recovery coils  22  are positioned above the stack  16  on a superstructure supported by posts  24 . This allows the heat recovery coils  22  to be supported above the exhaust stacks  16  upon their own superstructure, thereby allowing the heat recovery system  20  to be installed without modification to the turbine  10 . The present invention also comprehends an embodiment in which the heat recovery coils  22  are attached to the top of the exhaust stack  16  or otherwise physically integrated with the turbine  10 . 
     As is known in the art, the heat recovery coils work on a heat exchange principle, in which a fluid heat conducting medium, such as ammonia or water, is flowed through a series of coils positioned in the path of the exhaust emitted by the exhaust stack  16 , such that the fluid within the coils is heated by the exhaust. If the fluid within the coils is caused to continuously flow, the heat captured from the exhaust is moved away from the exhaust stack  16  to a place where it can be recovered (transduced) into useful energy. The use of ammonia in the heat recovery coils  20  is a preferred embodiment of the present invention; however, any heat conducting material may be used. For example, it is known in the art to use various oils for heat exchange (such as DOWTHERM manufactured by The Dow Corporation), in order to increase the temperature at which the heat recovery coils  22  may operate. It is also known in the art to pressurize the heat recovery medium, in order to allow it to absorb more heat. For example, a heat conducting liquid may be pressurized so that it may be heated to significantly higher temperatures before transitioning to a gaseous phase than would be the case if the liquid were at normal atmospheric pressure. The present invention comprehends the use of any material for the heat exchange medium. 
     In the preferred embodiment, the heat exchange fluid is pumped to the heat recovery coils  22  by means of a 16″ pipe  26  and is recovered from the heat recovery coils  22  by means of a 16″ return pipe  28 , having been heated by the placement of the heat recovery coils  22  in the path of the heated exhaust gasses (or hot zone  17 , as is illustrated in FIGS. 5,  6 A and  6 B). In a preferred embodiment, the fluid entering the heat recovery coils  22  is at approximately 230° F., while the fluid exiting the heat recovery coils  22  is at approximately 270° F. This 40° F. increase in the temperature of the fluid represents energy that has been recovered from the exhaust of the stacks  16 . This heated fluid is pumped into one or more pump/chiller combinations  30  (heat transduction systems) fluidically connected to the heat recovery coils  22 , which maintain the flow of fluid through the system and which also include chillers  30  for extracting the heat energy in the fluid, as is commonly known in the art. The number of pump/chiller units  30  required for the application depends upon the quantity of heat being recovered from the turbines  10 . In a preferred embodiment, the pump/chiller units  30  are contained within trailers in order to easily allow greater capacity to be added, or capacity to be taken away. 
     As is known in the art, the chillers  30  extract heat energy from the fluid flowing through the heat recovery coils  22  and transduce the extracted heat energy into useful energy for any desirable purpose. For example, this energy may be placed onto the electric grid that is being fed by the turbine generators  10 . As a further example, this energy may be used to power air conditioning coils  32  that are added to the air inlet  14  of each turbine  10 . The coils  32  cool the inlet air to the turbine  10 , thereby increasing the efficiency of the turbine  10 . 
     One concern with the use of the heat recovery coils  22  in the path of exhaust gases as hot as those exiting the stack  16 , is that if the fluid within the coils  22  is allowed to heat to too high a temperature, catastrophic failure of the system is possible. For example, if water is flowing through the heat recovery coils  22 , and the temperature of the water is elevated above the boiling point of the water (at the pressure at which it is maintained), then the water will turn to steam, greatly expanding its volume and causing catastrophic failure of the system through rupture of the coils. If the temperature rise is rapid enough, steam generation may occur so quickly that the failure mechanism may even be an explosion. Such a scenario may occur if the pumping units  30  fail and the water within the heat recovery coils  22  is not flowed at a high enough rate. 
     In order to guard against this problem, the present invention provides for heat recovery coils  22  as configured in FIG.  3 . Visible in the view of FIG. 3 is the superstructure  34  which rests upon the posts  24  and which holds the components of the heat recovery coils  22 . The superstructure  34  includes a central crossbeam  36  which crosses substantially over the centerline of the exhaust stack  16 . 
     The heat recovery coil  22  comprises two separate coil units  38  which are independently plumbed to the inlet fluid pipes  26  and the outlet fluid pipes  28 . In turn, each of the coil units  38  comprises three individual coils in the preferred embodiment. The number of coils or coil units is not critical to the present invention, and is considered to be a matter of design choice. 
     Each of the coil units  38  ride s upon wheels or other structures which allow it to be slid upon the side rails of the superstructure  34 . In this way the coil unit  38  may be moved into or out of the path of the exhaust flow exiting the stack  16 . Furthermore, the coil unit  38  may be moved partially into the exhaust flow, moved entirely into the exhaust flow, or moved completely out of the exhaust flow. Each of the two coil units  38  may be moved independently. In the view of FIG. 3, the upper coil unit  38  is shown positioned completely within the exhaust flow, while the lower coil unit  38  is shown positioned completely out of the exhaust flow. It can be seen with reference to FIG. 3 that when both coil units  38  are positioned completely within the exhaust flow, all of the exhaust produced by the stack  16  is forced to flow around the coils of the coil units  38 . In a preferred embodiment, the coil units  38  are moved by means of an electric motor  40  which drives a rack and pinion system attached to the superstructure  34 ; however, the present invention comprehends the use of any motor means  40  for moving the coil units  38 , the particular choice of coil motive means  40  not being critical to the present invention. 
     Because the fluid inlet pipes  26  and outlet pipes  28  are fixed and because the coil units  38  are moveable, some means must be provided for connecting these structures for fluid flow therebetween. In a preferred embodiment to the present invention, these connections are made by lengths of 5″ braided stainless steel flexible hose that connect both to the inlet pipes  26 /outlet pipes  28  and to the individual coils of the coil unit  38 . For each coil, one flexible hose  42  is provided for the inlet and a second flexible hose  42  is provided for the outlet. Therefore, for the coil units  38  illustrated in FIG. 3, three pairs of flexible hose  42  are required for each coil unit  38  (as illustrated in relation to the lower coil unit  38 ); only one pair of the hoses  42  is illustrated in relation to the upper coil unit  38 . As an alternative, each of the coils within the coil unit  38  may be chained together in a series, so that only one inlet hose  42  and one outlet hose  42  is required to service the entire coil unit  38 . 
     The hoses  42  are provided in a length sufficient to reach between the pipes  26 ,  28  and the coil unit  38  when the coil unit  38  is moved to a position representing its maximum distance from the pipes  26 ,  28 . In the embodiment shown in FIG. 3, this position is the position illustrated by the lower coil unit  38 . Conversely, when the coil unit  38  is moved to be completely within the exhaust path of the stack  16 , the hose  42  connections to the coil unit  38  will be very near the hose  42  connections to the pipes  26 ,  28 . Therefore, the hoses  42  will assume a generally U-shaped configuration therebetween. The hoses  42  are supported by a series of trays  44  no matter what position the hoses  44  are placed in. This is illustrated schematically in FIGS. 4A-B. In the view of FIG. 4A, the coil unit  38  is positioned entirely over the stack  16 , and the hose  42  assumes its shortest overall dimension. In the view of FIG. 4B, the coil unit  38  has been moved completely away from the stack  16 , extending the hose  42  to its longest dimension. In either position, the tray  44  supports a portion of the hose  42 , and the U-shaped configuration of the hose  42  allows it to transition between these two extreme positions without kinking. 
     With the configuration of the heat recovery coil  22  illustrated in FIG. 3, it is possible to actively control the position of the coil units  38  in relation to the temperature of the coil units  38 . FIG. 5 illustrates a control system  45  integrated with the heat recovery coil  22  in order to measure the temperature of the coil units  38  and actuate positioning of the heat recovery coils  22 . Accordingly, the control system  45  typically includes an electronic controller  46  and a temperature sensor  48  operationally coupled thereto. The sensor  48  may measure the temperature of the heat recovery coils  22  themselves, or the temperature of the heat exchange fluid flowing through the coils  22 . The temperature sensor  48  is adapted to send a signal proportional to the temperature of the heat recovery coils  22  (or the fluid therein) to the electronic controller  46 . Based upon this signal, the control system  45  may determine whether the coil units  38  should be moved farther into the stack  16  exhaust or farther away therefrom. The control system  45  may activate the motor  40  in order to achieve such movement. For example, upon receipt of a signal exceeding a first predetermined value, the controller  46  may actuate the motor  40  to completely remove the heat recovery coils  22  from the hot zone  17 . Alternately, upon receipt of a signal having a second predetermined value, the controller may position the heat recovery coils  22  partially within the hot zone  17 . Such control of the position of the heat recovery coil units  38  would not only prevent catastrophic failure of the system in the case of extremely elevated temperatures, but would also allow the temperature of the coil units  38  to be maintained at the optimum temperature for heat recovery. The position of the coil units  38  could therefore be continuously controlled by the control system  45  in order to achieve this optimum temperature. The implementation of such a control system  45  may utilize any appropriate hardware known in the art, and preferably utilizes a PLC control system commercially available from the Allen-Bradley Company. 
     As a fail-safe safety measure, the heat recovery coil  22  is preferably designed such that failure of the control system  45  will result in the coil units  38  automatically moving out of the exhaust path of the stack  16 . It is therefore necessary for the control system  45  to actively command the coil units  38  to be in the path of the exhaust of the stack  16  at all times. Failure of the control system  45  to send such control signals (for example, if there is a loss of power to the control system  45 ) will result in the coil units  38  automatically retracting away from the exhaust stack  16 . If such a fail-safe were not provided, failure of the control system  45  would result in the coil units  38  remaining in the path of the exhaust indefinitely, and could result in a dangerous elevation of temperature. 
     Several methods for implementing such fail-safe measures may be used. For example, as illustrated in FIG. 6A, the rails of the superstructure  34  upon which the coil unit  38  rolls may be angled away from the stack  16  such that work must be done to keep the coil unit  38  positioned in the hot gas stream. Therefore, upon loss of a control system signal activating the motor  40 , gravitational action upon the coil unit  38  will cause it to roll down this inclined ramp, away from the stack  16  and out of the hot gas stream. In this embodiment, the coil units  38  are moved by means of a hydraulic motor  40  which drives a piston attached to the superstructure  34 ; however, any convenient motor means  40  for moving the coil units  38  may be chosen. This configuration also offers the advantage of presenting an increased coil unit  38  area into the stack  16 , such that the energy transfer between the hot gasses in the stack and the heat transfer fluid in the coil unit  38  is increased. 
     In an alternative embodiment, illustrated in FIG. 6B, a cable  50  may be attached to the side of the coil unit  38  which is opposite to the stack  16 . This cable  50  may be routed through a pulley  52  suspended from the superstructure  34  and a large weight  54  attached to the other end of the cable  50 . Upon a loss of a command signal from the control system  45  activating the motor  40 , there would be nothing counteracting the gravitational pull on the weight  54 , and the weight  54  would act to pull the coil unit  38  away from the stack  16 . The size of the weight  54  is chosen to provide adequate force to move the coil unit  38 . Other methods for automatically moving the coil units  38  away from the stack  16  upon a loss of control signal to the motor  40  will be apparent to those having ordinary skill in the art, and are comprehended by the present invention. 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiment has been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.