Abstract:
A combination power station with power/heat cogeneration. The combination power station includes at least one gas turbine driven by combustion gases from a combustion chamber and at least one steam turbine working in a water/steam circuit. The water/steam circuit converts water into steam in a first apparatus by hot combustion gases emerging from the at least one gas turbine. The steam is supplied to the at least one steam turbine. A separate heat transfer medium is arranged in a heat exchange relationship with a heat consumer and is heated in a second apparatus by thermal energy extracted from the water/steam circuit at a plurality of extraction points each having a different temperature. The combination power station includes means for selectively connecting the second apparatus to the plurality of extraction points depending upon a thermal output and an inlet temperature required by the heat consumer.

Description:
This application claims priority under 35 U.S.C. §§119  and/or 365 to No. 197 20 881.9 filed in Germany on May 17, 1997; the entire content of which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of power station technology. It refers to a combination power station with power/heat cogeneration, having at least one gas turbine driven by the combustion gases from a combustion chamber and at least one steam turbine working in a water/steam circuit, within the water/steam circuit the water being converted into steam in a first apparatus by means of the hot combustion gases emerging from the at least one gas turbine and the generated steam being supplied to the at least one steam turbine, and a separate heat transfer medium, which is in exchange with a heat consumer, being charged in a second apparatus with thermal energy which is extracted from the water/steam circuit at a plurality of extraction points located at a different thermal level. 
     Such a combination power station is known, for example, from DE-C2-25 12 774. 
     2. Discussion of Background 
     In the case of combination power stations with power/heat cogeneration, the thermal energy always has to be provided in the form required at the respective point in time, that is to say as thermal output for current generation in the generator and as heat-charged heat transfer medium for the external heat consumer or heat consumers. Both the energy demand for current generation and the energy demand for the external heat consumer or heat consumers therefore fluctuate between various extreme values. In particular, according to FIG. 1 in the graph of the temperature (of the heat transfer medium) against the thermal output Pth, the thermal energy required by the external heat consumer fluctuates within a working range (represented by hatching and delimited by the corner points a-d) between a minimum inlet temperature VTmin and a maximum inlet temperature VTmax and between a minimum thermal output Pthmin and a maximum output Pthmax. 
     The thermal energy (heat) required for the heat consumer is transmitted from the water/steam circuit to the heat transfer medium (water or thermal oil) or to the heat consumer by means of direct or indirect heat transmission processes (by direct contact, for example by mixing, or indirectly, for example via heat exchangers). In order to obtain a working range lying between two extreme values, in this case the thermal energy is extracted from the circuit at a plurality of different thermal levels and is correspondingly transmitted to the heat transfer medium at different thermal levels by means of associated heat exhangers, usually condensers. A maximum number of extraction levels or extraction points results in a minimum loss of energy and maximum thermal efficiency. In the combination power station known from the publication initially mentioned, steam is extracted from the water/steam circuit at various points upstream and downstream of the steam turbine and is supplied in a plurality of individual heat exchangers (heating water heaters) which are connected in series with respect to the flow of the heat transfer medium and heat the heat transfer medium in succession. In this case, the last heat exchanger in the direction of flow is charged directly with fresh steam via a reducing station. 
     A disadvantage, here, is that, with an increasing number of extraction points and extraction levels, the number of corresponding plant components, such as heat exchangers, lines, control devices and the like, also increases and entails high capital costs as well as operating and maintenance costs. Furthermore, the extreme values of the power demand of the heat consumer, that is to say, for example, the maximum thermal output Pthmax at the maximum inlet temperature VTmax (point c in FIG.  1 ), do not occur very often or are usually only of short duration, so that the plant components necessary for this purpose are utilized to only a very low degree. 
     SUMMARY OF THE INVENTION 
     The object of the invention is, therefore, to provide a combination power station with power/heat cogeneration, in which heat transmission from the water/steam circuit to the heat transfer medium is drastically simplified by reducing the number of plant components, without any restriction in the number of extraction points and extraction levels. 
     In a combination power station of the type initially mentioned, the object is achieved in that the second apparatus can be connected selectively to the plurality of extraction points according to the thermal output and inlet temperature required by the heat consumer. The essence of the invention is not to assign each extraction point or each extraction level its own heat transmission arrangement, but to use a single heat transmission arrangement which is connected, as required, to the various extraction points or extraction levels in the water/steam circuit. 
     A first preferred embodiment of the combination power station according to the invention is distinguished in that individual lines equipped with controllable valves are provided for selectively connecting the second apparatus to the extraction points, said lines connecting the extraction points to the second apparatus, in that a valve control is provided for controlling the controllable valves, in that a temperature measuring point is arranged at the outlet of the second apparatus in order to determine the temperature of the heat transfer medium, and in that a first input of the valve control is connected to the outlet of the temperature measuring point. This design results in a very simple, reliable and efficient control and regulation of the thermal output which is transmitted from the water/steam circuit to the heat transfer medium. 
     A second preferred embodiment of the combination power station is defined in that the second apparatus comprises a condenser, in that steam is extracted from the water/steam circuit by means of the extraction points and in that condensate emerging from the condenser is recirculated into the water/steam circuit, in that a first extraction point for the extraction of high pressure fresh steam, a second extraction point for the extraction of high pressure steam and a third extraction point for low pressure steam are provided, and in that the valve assigned to the first extraction point is designed as a pressure reducing valve. A wide working range for the transfer of heat to the heat consumer can be implemented by the simplest means due to this selection of extraction points and extraction levels. 
     Further embodiments emerge from the dependent claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description, when considered in connection with the accompanying drawings, wherein: 
     FIG. 1 shows an illustrative working range for the thermal power consumption of a heat consumer in a graph of temperature against thermal output; 
     FIG. 2 shows, in a graph identical to that of FIG. 1, the transmission principle for the transmission of heat from the water/steam circuit to the heat transfer medium, as taken as a basis for the invention; 
     FIG. 3 shows the plan of a combination power station according to a preferred exemplary embodiment of the invention; 
     FIG. 4 shows a detail from the plan according to FIG. 3, with additional particulars of the control and regulation of heat transmission; and 
     FIG. 5 shows, in a graph identical to that of FIG. 2, the transmission of thermal energy by means of three extraction points or extraction levels, as is employed in the preferred exemplary embodiment according to FIGS.  3  and  4 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, FIG. 3 illustrates the plan of a combination power station according to a preferred exemplary embodiment of the invention. The combination power station  10  comprises a gas turbine  16  and a steam turbine  35  with two stages  36  and  37 . The gas turbine  16  drives a first generator  19  and the steam turbine  35  a second generator  40 . The gas turbine  16  works by means of the hot combustion gases from a combustion chamber  17 , to which, on the one hand, combustion air compressed by means of a compressor  18  arranged on the turbine axis is supplied via the line L 8  and, on the other hand, fuel is supplied via the line L 7  from an inlet E 4 . The fuel preferably flows through a preheater  20 , where it is preheated by heat exchange with heated water which flows through the line L 5 . The hot combustion gases from the combustion chamber  17 , after they have performed work in the gas turbine  16 , are conveyed via the line L 9  from the outlet of the gas turbine  16  through a waste heat steam generator  11 , where they heat the water of the water/steam circuit for the steam turbine  35  and convert it into highly pressurized steam. They then emerge at a flue gas outlet  15  and are discharged into the environment. 
     The waste heat steam generator  11  has an internal design, known per se, which comprises an economizer  12 , an. evaporator  13  and a superheater  14 . The feedwater delivered from a feedwater tank  30  via the line L 11  by a feedwater pump  31  is preheated in the economizer  12 . The preheated feedwater passes from the outlet of the economizer  12 , on the one hand, into a steam drum  27  and, on the other hand, via the line L 5  to the fuel preheater  20  and from there back into the feedwater tank  30 . The feedwater is pumped out of the steam drum  27  (which may even be omitted in other embodiments of the “once through” type), in which the liquid and vaporous phases coexist, by means of a circulating pump  26  through the evaporator  13 , where it evaporates. The generated steam flows via the steam drum  27  to the superheater  14 , where it is superheated and is conducted as fresh steam through the line L 16  and a fresh steam cooler  25  (inoperative when the plant is operating normally), which is supplied with feedwater via the line L 12 , to the steam turbine  35 . After the fresh steam has performed work in the two stages  36  and  37  of the steam turbine  35 , it flows through the line L 22  into a condenser  43  and is converted there into condensate which flows by means of a condensate pump  44  through the line L 15  via a feedwater preheater/deaerator  45  back into the feedwater tank  30 . The water/steam circuit belonging to the steam turbine  35  is thereby closed. 
     For better control of the water/steam circuit, a bypass and various taps are additionally provided in the circuit: a first tap is formed by the line L 10  which leads from the outlet of the economizer  12  via a (controllable) pressure reducing valve  28  to the feedwater tank  30 . A second tap is formed by the line L 13  which leads from the steam outlet of the steam drum  27  via a (controllable) pressure reducing valve  32  to the feedwater preheater/deaerator  45  and which serves for extracting steam from the steam drum  27 . A third tap is formed by the line L 14  having the (controllable) pressure reducing valve  33 , said line leading from the second stage  37  of the steam turbine  35  likewise to the feedwater preheater/deaerator  45  and serving for heating the feedwater preheater/deaerator  45 . The bypass is obtained by means of the line L 17  having the (controllable) pressure reducer  42  which simultaneously reduces the pressure and decreases the superheating of the steam. By means of this bypass, fresh steam can be conducted past the steam turbine  35  directly to the condenser  43  in the startup or shutdown phase of the plant. 
     The heat transfer medium (usually water) provided for the transfer of heat to a heat consumer  24  flows through the line L 3 . The heat transmission, necessary for heating the heat transfer medium, between the water/steam circuit of the steam turbine  35  and the heat transfer medium in the line L 3  takes place in two heat transmission arrangements  21  and  22  connected in series. The corresponding curves are reproduced in principle in the temperature/output graph of FIG.  2 . The heat transmission arrangement  22  is a condenser which at the same time eliminates superheating (“desuperheater”). Superheated steam with the outlet temperature T 3  and the thermal output P 3  is conducted through the condenser  22  and is extracted, as required, from the water/steam circuit of the steam turbine  35  at various extraction points or thermal extraction levels. The steam first cools from T 3  to T 2  in the condenser  22  according to curve C 1  from FIG. 2 (“desuperheating”), condenses at the constant temperature T 2  and at the same time discharges the heat to the heat transfer medium which is heated from the temperature T 6  to an inlet temperature VT somewhat above T 2  according to curve C 2  from FIG.  2 . The condensate has then discharged the thermal output P 3 -P 1  at the outlet of the condenser  22 . 
     The condensate from the condenser  22  is transported (FIG. 3) by means of a condensate pump  23  via the line L 1  to the heat transmission arrangement  21  which acts as a preheater for the heat transfer medium. At the inlet of the preheater  21 , the condensate from the line L 1  is mixed with colder feedwater which is delivered via the line L 2  from a tap at the economizer  12 . The temperature decreases from T 2  to T 1  (vertical segment of curve C 1  in FIG. 2) as a result of the mixing operation. The mixing water flows through the preheater  21 , cools from T 1  to T 5 , at the same time discharging heat to the heat transfer medium, and returns via the line L 4  and the pressure reducing valve  29  to the feedwater tank  30 . The heat transfer medium heats up in counteraction from the temperature T 4  to the temperature T 6  (curve C 2  in FIG.  2 ). The curves C 1  and C 2  in FIG. 2 stand for an individual point in the working range of the apparatus, at which the heat consumer requires only a specific thermal output and inlet temperature of the heat transfer medium. When other thermal outputs and inlet temperatures are required, other curves C 1  and C 2  are obtained correspondingly in the graph of FIG. 2 (see curves C 3 , . . . , C 7  and C 3 ′, . . . , C 7 ′ in FIG.  5 : in this case, the curves C 6 , C 7  and C 6 ′, C 7 ′ stand for the heat transfer medium to be heated and therefore correspond to the curve C 2  in FIG. 2; in this case, the curves C 3 , C 4  and C 3 ′, C 4 ′ and C 5 ′ stand for the medium to be cooled, namely the extracted steam, and therefore correspond to the curve C 1  in FIG.  2 . For heating along curve C 6 , cooling takes place along curve C 3 ; other pairs of associated curves are C 7 -C 4 , C 6 ′-C 3 ′ and C 7 ′-C 5 ′; a heating curve associated with the cooling curve C 4 ′ is not shown in FIG.  5 ). 
     A change of the operating point in the T, Pth graph is taken into account, according to the invention, in that the same condenser  22  extracts the steam or heat from another extraction point in the water/steam circuit and therefore at another thermal level. For this purpose, in the exemplary embodiment of FIG. 3, the steam-side inlet of the condenser  22  can be connected selectively, via various lines L 18 , L 19  and L 20  and corresponding controllable valves  34 ,  39 ,  38 , to an extraction point on the fresh-steam line L 16  (steam temperature T 8 ), an extraction point in the middle of the first stage  36  of the steam turbine  35  (steam temperature T 7 ) and to the outlet of the first stage  36  (steam temperature T 6 ). If the pressure reducing valve  34  is opened, fresh steam flows via the lines L 18  and L 21  to the condenser  22  and from there, as condensate, to the preheater  21 . At the maximum opening of the pressure reducing valve  34 , curve C 3 ′ in FIG. 5 corresponds to curve C 1  in FIG.  2 . The heat transfer medium can then be heated, according to curve C 6 ′, from the temperature T 4 , at which it returns from the heat consumer  24 , to the maximum inlet temperature VTmax and can transmit the maximum thermal output Pthmax, whilst the fresh steam cools from the temperature T 8  to the condensate temperature T 5 . If a lower thermal output is required in the case of a constant inlet temperature (VTmax), the pressure reducing valve  34  is throttled until (in the lower limit case) curves C 3  and C 6  are passed through, and, at the inlet temperature VTmax, the heat transfer medium transmits only the minimum thermal output Pthmin. For intermediate outputs, the pressure reducing valve  34  assumes an intermediate position. 
     If the heat transfer medium is to have an inlet temperature which is in the middle between the extremes VTmax and VTmin, the valve  39  is opened and high pressure steam is extracted from the first stage  36  of the steam turbine  35  via the lines L 19  and L 21  and conducted to the condenser  22 . Then, depending on the opening of the valve  39 , the steam cools from the temperature T 7  to the temperature T 5  along curves C 4  and C 4 ′ or intermediate similar curves, whilst the heat transfer medium heats up to an inlet temperature located between VTmin and VTmax and transports a thermal output located between Pthmin and Pthmax. 
     If the heat transfer medium is to be heated from T 4  merely to the minimum inlet temperature VTmin, this takes place along curve C 7 ′, in the case of maximum output Pthmax (valve  38  fully open) and along curve C 7 , in the case of minimum output Pthmin (valve  38  fully throttled). The low pressure steam necessary for this purpose is extracted at the outlet of the first stage  36  of the steam turbine  35  via the lines L 20  and L 21  by opening the valve  38  and is cooled from the temperature T 6  to the temperature T 4  in the condenser  22  and the following preheater according to curves C 5 ′ and C 5 . For outputs between the extremes, the valve  38  is in a corresponding intermediate position. 
     The curves C 6 , C 7  and C 6 ′, C 7 ′ in FIG. 5 mark the corner points (a-d) of the working range for heat transmission, as illustrated diagrammatically in FIG.  1 . Other values of the working range may be accessed very simply by opening one or more of the valves  34 ,  38 ,  39  to a greater or lesser extent. In particular, by a suitable choice of the valves and their position, maximum thermal efficiency can be achieved for heat transmission at any point in the working range at a minimum outlay in terms of apparatus and control. 
     A preferred exemplary embodiment of a control of the valves is illustrated in FIG. 4 which shows a detail from the plan according to FIG. 3 (like elements in the two figures are provided with the same reference symbols). The three extraction points or extraction levels of the exemplary embodiment are designated in FIG. 4 by the reference symbols I (fresh steam), II (high pressure steam) and III (low pressure steam). Each of the valves  34 ,  39  and  38  for controlling the inlet temperature and the thermal output is connected via corresponding control lines (A, B, C) to a temperature control  50  which is connected on the input side to a temperature measuring point  51  arranged at the outlet of the condenser and which receives an input value for the process via one of two signal lines  51 . The other signal line  51  controls a valve  52 , by means of which the stream of heat transfer medium to the heat consumer  24  can be set. In the temperature control  50 , the two input signals are compared and the valves  34 ,  39  and  38  are actuated via the control lines A-C, in such a way that the desired inlet temperature and thermal output are achieved optimally. 
     So that, when the valves  38  and  39  open simultaneously, high pressure steam cannot flow back from the extraction point II into the extraction point III for low pressure steam, it is expedient to provide a nonreturn flap  46  in the line L 20 . Furthermore, the temperature control can be even further improved and refined if the line L 21  to the condenser  22  has arranged in it a steam temperature control device  47  which is controlled via a valve  49  by a temperature measuring point  48 . In particular, fluctuations in the steam temperature can thereby be leveled out in a simple way. 
     For the sake of simplicity, the exemplary embodiment of FIGS. 3 and 4 shows a combination power station with only one gas turbine  16  and one waste heat steam generator  11 . However, within the scope of the invention, a plurality of gas turbines and a plurality of associated waste heat steam generators may, of course, act on a common steam turbine. In that case, the other waste heat steam generators are incorporated into the water/steam circuit of the steam turbine  35  via the inlets E 1 , E 2  and E 3  (together with the pressure reducer  41 ) and the outlet A 1 , which are shown in FIGS. 3 and 4. 
     Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.