Abstract:
A system is described for allowing short term power augmentation of a gas turbine or a gas turbine combined cycle power plant. A reservoir of pressurized and/or liquefied gas is installed to store a temporary supply of gas turbine working fluid. This supply of working fluid is available for near instantaneous admission to the gas turbine at the compressor discharge or combustion system to boost output, as may be desirable to assist in supporting electrical grid frequency during a transient disturbance (frequency reduction).

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to a system for allowing short term power augmentation of a gas turbine or gas turbine combined cycle power plant.  
           [0002]    Gas turbine and gas turbine combined cycle power output capability typically drops in direct proportion to a grid frequency (shaft speed) reduction, whereas the desired output response is inversely proportional to the grid frequency (shaft speed) such that the target grid frequency (shaft speed) tends toward its target value. One contributor to this behavior is the natural reduction in compressor inlet air flow with reduced shaft speed. The control system may additionally limit compressor air flow to maintain adequate margin against compressor surge. Fuel flow is also limited by combustion stability and parts life considerations.  
           [0003]    Several means of gas turbine and combined cycle power augmentation have been proposed and implemented to increase the steady state power capability of gas turbine and gas turbine combined cycles as listed below:  
           [0004]    1. Evaporative inlet cooler  
           [0005]    2. Inlet fogger (possibly with over spray for wet compression)  
           [0006]    3. Inlet chilling  
           [0007]    4. Supplementary firing of the HRSG  
           [0008]    5. Water injection to the gas turbine  
           [0009]    6. Steam injection to the gas turbine  
           [0010]    Each of these alternatives have drawbacks with respect to providing a near instantaneous and predictable temporary output increase in the case of an electric grid frequency reduction. Alternatives 1-6 may already be operating when the frequency reduction event occurs, such that no further output increase is possible. The output boost potential of alternatives 1-3 is also constrained by ambient conditions (e.g., high ambient humidity, or low ambient temperature) and so can not be depended upon to meet the total output boost requirement at all ambient conditions. Alternative 4 will provide the slowest response due to the large thermal heat capacity of the HRSG and steam working fluid. Water injection according to alternative 5 has been associated with increased combustion dynamic pressures and combustion system modification. Steam injection according to alternative 6 provides only a weak boost unless combined with supplementary firing of the HRSG, which is slow response.  
           [0011]    Traditional control response to a grid frequency reduction is to increase air flow and/or firing temperature during the under frequency transient. Air flow can only be increased if the gas turbine is operating at less than base load when the event occurs, or the compressor capability was intentionally oversized to provide margin during these under frequency events, which is expensive. Over firing is the fastest response method of boosting gas turbine and gas turbine combined cycle power but is limited in amplitude by the strong relationship of firing temperature to gas turbine hot gas path parts life and maintenance costs.  
           [0012]    A few power plants have been built to store energy during low load hours, typically overnight, for later use during peak hours. In the case of gas turbine and gas turbine combined cycle plants, this involves extraction of compressor discharge air to a storage vessel, typically an underground cavern, during low load hours, with subsequent retrieval of this stored air to supply the turbine working fluid flow, and hence output, during peak load hours. Application of this arrangement typically requires suitable geologic circumstances (the cavern) and specialized turbo-machinery and controls.  
         BRIEF SUMMARY OF THE INVENTION  
         [0013]    The invention proposes an adaptation of the gas storage concept to the particular needs of a gas turbine or gas turbine combined cycle power plant faced with grid frequency support duties, i.e. intermittent short term power boost.  
           [0014]    The invention provides a simple means for temporarily boosting the shaft output of a gas turbine or gas turbine combined cycle. As such, the system provides low cost, fast response, minimal plant cost and layout impact and simple operation. Three exemplary embodiments of the invention are described hereinbelow, each demonstrating a varying balance of cost and performance.  
           [0015]    The invention is thus embodied in gas turbine/combined cycle system wherein a working fluid source is operatively coupled to the gas turbine system for selectively adding a working fluid to the gas turbine system downstream of the compressor and upstream of the gas turbine, to support a temporary plant power boost via gaseous working fluid injection. The working fluid source is preferably at least one vessel containing pressurized and/or liquefied gas that is coupled via a flow control valve to the gas turbine system. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    These, as well as other objects and advantages of this invention, will be more completely understood and appreciated by careful study of the following more detailed description of the presently preferred exemplary embodiments of the invention taken in conjunction with the accompanying drawings, in which:  
         [0017]    [0017]FIG. 1 illustrates the basic structure of a compressed gas storage system for a gas turbine and gas turbine combined cycle power boost application as an embodiment of the invention;  
         [0018]    [0018]FIG. 2 illustrates an alternate embodiment of the invention that employs liquefied gas storage;  
         [0019]    [0019]FIG. 3 illustrates a further embodiment of the invention that effectively combines the compressed gas storage system of FIG. 1 with the liquefied gas storage system of FIG. 2; and  
         [0020]    [0020]FIG. 4 shows another alternate embodiment of a hybrid compressed gas storage/liquefied gas storage system as an embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]    [0021]FIG. 1 shows the basic structure of a compressed gas storage system for gas turbine and gas turbine combined cycle power boost application. More specifically, the embodiment illustrated in FIG. 1 schematically illustrates a combined cycle system having power augmentation with working fluid injection and compressed gas storage. The combined cycle system of the illustrated embodiment is a multi-pressure reheat combined cycle, but the invention is equally applicable to any other bottoming cycle configuration.  
         [0022]    In this schematic illustration, steam flow is indicated by a solid line, water and liquid flow is indicated by a dashed line, air and gas flow are indicated by a long and short dash line, and fuel flow is indicated by a dash and two dot line.  
         [0023]    This example includes a gas turbine system  10  comprising a compressor  12 , a combustion system  14  and a gas turbine expander  16 , and a steam turbine system  18  including a high pressure section  20 , an intermediate pressure section  22 , and one or more low pressure sections  24  with multiple steam admission points at different pressures. The low pressure section  24  exhausts into a condenser  26 . The gas turbine system  10  and steam turbine system  18  drive the generator  28  (or other load). The gas turbine system  10 , steam turbine system  18 , and generator  28  may be arranged in tandem, on a single shaft  30  as shown in FIG. 1, or in a multi-shaft configuration wherein the gas turbine and steam turbine drive separate loads.  
         [0024]    The steam turbine system  18  is associated with a multi-pressure HRSG  32  which includes low pressure (LP), intermediate pressure (IP) and high pressure (HP) economizers  34 ,  36 ,  38 , respectively, an LP evaporator  40 , further HP and IP economizers  42 ,  44 , an IP evaporator  46 , an LP superheater  48 , a final HP economizer  50 , an IP superheater  52 , an HP evaporator  54 , an HP superheater section  56 , a reheater  58 , and a final HP superheater section  60 .  
         [0025]    Condensate is fed from condenser  26  to the HRSG  32  via conduit  62  with the aid of condensate pump  64 . The condensate subsequently passes through the low pressure (LP) economizer  34  and into the LP evaporator  40 . Steam from the LP evaporator  40  is fed via conduit  66  to the LP superheater  48  and then returned to the low pressure section  24  of the steam turbine  18  via conduit  68  and appropriate LP admissions stop/control valve(s) (not shown).  
         [0026]    Feedwater with the aid of feedwater pump(s)  70  passes (1) through the IP economizers  36 ,  44  via conduit  72  and to the IP evaporator  46 , and (2) through the HP economizers  38 ,  42  via conduit  74  and then on to the final HP economizer  50  via conduit  76 . At the same time, steam from the IP evaporator  46  passes via conduit  78  to the IP superheater  52  and thereafter flows via conduit  80 , is combined with the cold reheat steam  82  from the HP section  20  of the steam turbine  18  and sent through one pass  84  of the reheater  58  and through an attemperator  86 . After flowing through a second pass  88  of the reheater  58 , the reheated steam is returned to the IP section  22  of the steam turbine  18  via conduit  90  (and appropriate stop/control valves not shown).  
         [0027]    Meanwhile, condensate in the final HP economizer  50  is passed to the HP evaporator  54 . Steam exiting the HP evaporator  54  passes through the HP superheater sections  56  and  60  and is returned to the HP section  20  of the steam turbine  18  by way of conduit  92  and appropriate stop/control valves (if required, not shown).  
         [0028]    Heat is provided to the HRSG  32  by the exhaust gases from gas turbine  10  introduced into the HRSG via conduit  94  and which exit the HRSG to a stack (not shown) via conduit  96 . More specifically, exhaust from the gas turbine  16  enters the HRSG  32  where it encounters high temperature superheater  60  and  56  and reheater  58  sections  88 ,  84  disposed upstream of the HP evaporator  54  with respect to the direction of gas flow. As mentioned above, the IP superheater  52  discharge is combined with the cold reheat steam  82  from the HP section  20  of the steam turbine  18  and sent through the reheater  58 .  
         [0029]    The further discussion of the inventive system will be generally limited to those components provided or added as an embodiment of the inventive system. The reference numbers shown in FIGS. 2, 3 and  4  but not discussed hereinbelow are substantially identical to the corresponding components of the FIG. 1 system and are labeled to provide a frame of reference.  
         [0030]    A compressed gas, typically air or nitrogen, is stored in vessel(s)  100  until a requirement for power output boost is detected by the control system. When the event is sensed the control system will open the boost gas flow control valve  102  to supplement the air from compressor  12  with additional working fluid into the gas turbine between the compressor discharge and turbine inlet, with the particular location depending on hardware specifics. The additional mass flow allows more fuel to be burned in the gas turbine combustor  14  such that the turbine inlet temperature remains within its allowable limits. The heated working fluid then expands through the turbine expander  16  to make additional power, as compared to the un-boosted machine, as well as additional exhaust energy available to the bottoming cycle, if present. This system is very simple and would respond quickly, governed by the control system and the boost gas control valve  102  response rate. The duration of the power boost would be limited by the storage capacity of the storage vessel(s)  100 .  
         [0031]    [0031]FIG. 2 shows a variation which employs liquefied gas storage, typically nitrogen, to reduce the volume of storage required and/or extend the available boost within the same space constraints. The liquefied gas would be stored in a vessel(s)  110  at approximately atmospheric pressure, and thus would need to be pumped with pump  112  up to gas turbine admission pressure before vaporization in exchanger  116  and admission to the machine. Valve  114  controls the liquid pressure entering exchanger  116  and valve  118  is the boost gas flow control to the gas turbine. The preferred embodiment for gas turbine combined cycle applications uses the condenser circulating water supply  120  to vaporize the cold liquefied gas in exchanger  116 . In addition to vaporizing the gas, this cools the cooling water and further boosts plant output by reducing the steam turbine exhaust pressure in the steam condenser  26 . If the plant is a simple cycle gas turbine with no steam bottoming cycle, the liquefied gas could most readily be vaporized with air. Once vaporized, the control system will control the boost gas control valve  118  to supplement the air from compressor  12  with additional working fluid into the gas turbine between the compressor discharge and turbine inlet. The additional mass flow allows more fuel to be burned in the gas turbine combustor  14  such that the turbine inlet temperature remains within its allowable limits. The heated working fluid then expands through the turbine expander  16  to make additional power, as compared to the un-boosted machine, as well as additional exhaust energy available to the bottoming cycle, if present. This system requires rapid vaporization of large quantities of liquefied gas and is thus somewhat more complex than a compressed gas storage system of FIG. 1, but could offer a longer duration boost per unit of working fluid storage volume, again ultimately limited by the storage capacity of the storage vessel(s)  110 .  
         [0032]    [0032]FIG. 3 combines the compressed gas storage system of FIG. 1 (for fast transient response) with the liquefied gas storage system of FIG. 2 to extend the available boost duration within the system space constraints. This reduces the thermal gradients during startup of the liquefied gas vaporization subsystem by allowing it to be brought on-line more slowly. Initial response would draw the supplemental working fluid from compressed gas storage vessel(s)  122  via boost gas control valve  124  until the vaporized gas from vessel(s)  110  was available to supplant it. Otherwise, this embodiment corresponds to the embodiment of FIG. 2.  
         [0033]    [0033]FIG. 4 shows another embodiment of a hybrid compressed gas storage/liquefied gas storage system exhibiting lower cost but a higher space requirement. In this system, the entire transient need is met with the compressed gas storage vessel  130  (as in the system of FIG. 1), but the liquefied gas allows on site storage  110  of a second charge. When the control system senses the need for working fluid the immediate need is supplied from the compressed gas storage vessel(s)  130  via flow control valve  132 . At the same time, the liquid vaporization system is activated to begin replenishment of the compressed gas vessel(s)  130 . Liquid is pumped with pump  112  via pressure control valve  114  from the liquid storage tank or vessel(s)  110  to a high pressure suitable for admission to the gas vessel(s)  130 , and vaporized at exchanger  126  via heat exchange to air  128 , as shown in FIG. 4, or water  120  as in FIG. 3. The liquid flow rate in this system is approximately one to two orders of magnitude lower in relation to the compressed gas flow rate during the transient event, such that replenishment of the compressed gas vessel(s) takes a few hours.  
         [0034]    Although four embodiments of the present invention are presented, it is to be understood that numerous other variations may be developed within the constraints of a particular installation which nevertheless rely upon working fluid storage to support a temporary plant power boost via gaseous working fluid injection to the gas turbine downstream of the compressor and upstream of the turbine. These systems are distinct from prior art compressed air storage systems in that the storage is either liquid or highly compressed gas to minimize space requirements.  
         [0035]    The concept described hereinabove has been studied as an aid to address stringent grid regulations governing power plant response during a frequency reduction event. Nitrogen injection to the combustor was found to meet the entire power augmentation needs of the plant at 95% speed without causing the compressor to exceed its operating limit for surge. The severe under frequency events addressed by this invention are generally temporary in nature. Indefinitely sustained plant output at reduced frequency operation may require a combination of systems (such as the one proposed herein with supplementary firing and steam injection). This invention is applicable to all gas turbine and gas turbine combined cycle power plant configurations.  
         [0036]    While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.