Patent Publication Number: US-2011056219-A1

Title: Utilization of Exhaust of Low Pressure Condensing Steam Turbine as Heat Input to Silica Gel-Water Working Pair Adsorption Chiller

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to steam turbines. A steam turbine is a mechanical device that converts thermal energy in pressurized steam into useful mechanical work. The steam turbine derives much of its thermodynamic efficiency through the use of multiple stages in the expansion of the steam. 
     Specifically, this invention relates to the utilization of low quality heat from the exhaust of a low pressure condensing steam turbine as the heat input to a silica gel-water working pair adsorption chiller to further improve the efficiency of the heat utilization of simple steam turbines and/or a combined cycle power plant. 
     This invention relates to a means of using a portion or all of the exhausted saturated steam that is currently expelled as waste from a steam turbine as the heat input to an adsorption chiller, preferably a silica gel-water working pair adsorption chiller. The product of the adsorption chiller (cold water) can be used for a variety of useful purposes such as industrial process cooling, air conditioning or gas turbine inlet cooling in a combined cycle plant. 
     The source of the steam for the steam turbine may come from a combined cycle plant consisting of one or more gas turbines matched to a steam turbine or from another other boiler process fired by fuel choices such as coal, oil, natural gas or nuclear power. 
     BRIEF SUMMARY OF THE INVENTION 
     The exhaust of a low pressure condensing steam turbine is a mixture of water and steam. It is at a temperature and pressure that are considered too low to provide any further value as steam for the turbine. The normal design of current turbine systems dumps this exhaust to a water-cooled surface condenser as waste heat. The condenser converts the exhaust back to water and, in doing so, the condensation of the steam and the change of state of the steam from a vapor to water creates a partial vacuum that helps pull the exhaust through the last stages of the low pressure turbine. The temperature of the exhaust as it exits the turbine is normally in the range of about 111°-125° C. (about 231°-257° F.). At that temperature in a partial vacuum, the exhaust is difficult to use effectively because it is a mixture of saturated steam and water at very low enthalpy. The resulting condensate in the “hot well” of the condenser is collected as condensate water for recirculation. The temperature of the condensate water has been reduced to about 40.5° C. (about 105° F.), a little above ambient temperature. During condensation, the heat is captured by the water tubes in the condenser and is transferred to a heat rejection device, often an evaporative (wet) cooling tower or simple circulation through river water. A cooling tower will transfer the heat a second time from the water to the surrounding atmosphere in the form of higher air temperature and higher humidity. Alternately, circulating the heated water from the condenser through lines through river water will transfer the heat a second time from the water in the water tubes to the surrounding river water in the form of higher water temperature. 
     The present invention provides a method and device to make use of the exhaust steam previously considered waste. The temperature of the exhaust steam is just above an ideal starting temperature for an adsorption chiller. This disclosure describes a method of extracting the heat in this exhaust steam and using it as one of the drivers for an adsorption chiller. 
     Adsorption chillers are ideally matched to a hot water source that is just below boiling temperature, at approximately 194° F./90° C. Hence the exhaust from a low-pressure turbine has the potential to provide for one more purpose, driving an adsorption chiller. 
     It is therefore an object of the present invention to improve the efficiency of conventional steam turbine plants by making more efficient use of the heat of the exhaust steam. 
     It is another object of the present invention to utilize one or more of the condenser&#39;s waterboxes to provide hot water for circulation through an adsorption chiller. 
     It is another object of the present invention to provide a heat exchanger intermediate to the low-pressure steam turbine and the condenser to capture heat from the exhaust steam to provide hot water to be circulated through an adsorption chiller. 
     It is another object of the present invention to provide a steam turbine having an adsorption chiller system intermediate to the low pressure turbine and the condenser. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The particular features and advantages of the invention as well as other objects will become apparent from the following description taken in connection with the accompanying drawings in which: 
         FIG. 1  is a block diagram of a condensing steam turbine including the adsorption chiller system of the present invention. 
         FIG. 2  is a schematic of an adsorption chiller system suitable for use in the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a block diagram of a conventional condensing steam turbine  10  which includes an adsorption chiller system  12  for usefully capturing the waste heat of the exhaust steam. Condensing steam turbines  10  are most commonly found in electrical power plants. A steam turbine plant consists of one or more steam turbines  10  operating in a Rankine cycle. The exhaust steam exiting such turbines  10  is in a partially condensed state, typically of a quality near 95%, at a pressure well below atmospheric. Steam quality is the proportion of saturated steam in a saturated water/steam mixture. 
     With the exception of the adsorption chiller system  12 , steam turbines  10  are known in the art and include, in serial flow relationship, a source of high pressure steam  14 , one or more steam turbines, such as high-pressure turbine  17 , intermediate-pressure turbine  20 , and low-pressure turbine  23 . Low-pressure turbine  23  is typically double ended as illustrated in  FIG. 1 , with steam flow going in both axial directions. High-pressure turbine  17  and intermediate-pressure turbine  20  are non-condensing while low-pressure turbine  23  is condensing. 
     The source  14  of the steam may come from a combined cycle plant consisting of one or more gas turbines matched to a steam turbine  10  or from another boiler process fired by fuel choices such as coal, oil, natural gas or nuclear. 
     High-pressure turbine  17 , intermediate-pressure turbine  20  and low-pressure turbine  23  are operatively connected to a rotor shaft  31  to turn a generator  28 . All turbines  17 ,  20 ,  23  of a steam turbine  10  are typically coupled in series to jointly drive rotor shaft  31  to drive the generator  28 , though alternate configurations are within the contemplation of this invention. High-pressure turbine  17  includes an inlet  18  and an outlet  19 . Intermediate-pressure turbine  20  includes an inlet  21  and an outlet  22 . Low-pressure turbine  23  includes an inlet  24  and one or more outlets  25 ,  26 . 
     High-pressure steam enters the inlet  18  of high-pressure turbine  17 . Higher pressure in front of the turbine and lower pressure behind the turbine creates a pressure gradient, which the steam follows through the impellers of the turbine, delivering kinetic energy to the impellers to turn the turbine and cooling down in the process. Intermediate pressure steam exits through outlet  19  and is channeled to the inlet  21  of intermediate-pressure turbine  20 . Again, a pressure gradient is formed across the impellers of the turbine  20 , and the impellers are turned as the steam travels through, exiting the intermediate-pressure turbine at outlet  22  as lower pressure steam and being channeled to the inlet  24  of low-pressure turbine  23 . The low pressure steam travels through the low-pressure turbine or turbines  23  and exits as exhaust steam through outlets  25 ,  26 . As it travels through the condensing steam turbine  10 , the steam crosses the saturation line and becomes saturated steam with ever decreasing quality as the steam flows. 
     In the present invention, all or a portion of the exhaust steam is directed from the outlets  25 ,  26  of the low-pressure turbine  23  to a chiller heat exchanger  36  for providing or producing a heated fluid, such as water, to adsorption chiller  40 . Chiller heat exchanger  36  may be any type of conventional heat exchanger, such a shell and tube heat exchanger, a plate heat exchanger, or any other suitable means for extracting heat from steam and outputting a flow of hot fluid that can be directed to an adsorption chiller  40 . 
     In a preferred embodiment of the present invention, the chiller heat exchanger  36  comprises a water-cooled surface condenser such as those typically used as the condenser  15  of the steam turbine  10  in present steam turbine plants. The present invention may be practiced by adding one or more additional water-cooled surface condensers connected via an isolated or isolatable inlet  37  and outlet  38  to the adsorption chiller  40  intermediate or between the low-pressure turbine  23  and the condenser  15  of the steam turbine  10 , or, alternatively, the heated fluid from one or more existing portions of the existing steam turbine condenser  15  may be redirected to provide hot water to the adsorption chiller  40 . 
     As will be explained in greater detail below, the adsorption chiller system  12  utilizes the hot water or other heated fluid generated by the chiller heat exchanger  36  to drive an adsorption chiller  40 , preferably a silica gel-water working pair adsorption chiller  40 . Adsorption chillers may also utilize other substances as the working pair, such as water and zeolite, ammonia and water, hydrogen and certain metal hydrides, activated carbon and a number of fluids, are a few of the available working pairs. While each type of working pair could theoretically be used, the silica gel-water working pair has been found to be preferable for the present invention based upon its range of working temperatures and the simplicity of its chemistry. 
     The adsorption chiller  40  produces as its output  63  chilled or cold water what can be put to beneficial use, such as industrial process cooling, air conditioning or gas turbine inlet cooling in a combined-cycle plant. The adsorption chiller system  12  utilizes the exhaust steam to extract additional work from the latent waste heat before the exhaust steam is finally directed into the condenser  15  of the steam turbine  10 . The exhaust steam itself cannot be channeled directly into the adsorption chiller system  12  as it is not a liquid and too hot. The temperatures inside the adsorption section of the chiller must be limited to lower ranges than the temperatures reached by the exhaust steam. Additionally, it would not be desirable to have to design the adsorption chiller to accommodate the high pressures reached by the exhaust steam. 
     Condensation from the chiller heat exchanger  36  is passed or channeled to the hotwell  51  of the condenser  15  for collection, but instead of merely rejecting all of the heat captured from the condensing steam to a cooling tower  29  or other heat sink as waste, the chiller heat exchanger  36  utilizes some of the heat captured from the steam as the heat source for driving the adsorption chiller  40 . 
     Exhaust steam not condensed at the chiller heat exchanger  36  is directed to the connected steam turbine condenser  15  which condenses and reclaims the water from the exhaust steam while rejecting waste heat from the exhaust steam and transferring it to the cooling tower  29 . 
     The exhaust of a low pressure condensing steam turbine  23  is a mixture of water and steam. The temperature of the exhaust as it exits the turbine is normally in the range of about 111°-125° C. (about 231°-257° F.). Adsorption chillers are ideally matched to a heated fluid or hot water source that is just below boiling temperature, having a temperature of about 85° C. to about 92° C., preferably at approximately 90° C. (194° F.) but may also make use of a hot water source having a temperature as low as about 50° C. (122° F.). At temperatures below about 50° C., the desorption phase become too long to be useful. Chiller heat exchanger  36  will produce hot water in the desired temperature range in the presence of exhaust steam in the range of about 90° C. 
       FIG. 2  is a schematic of an adsorption chiller system  12  suitable for use in the present invention. Numerous known designs of adsorption chillers are also within the contemplation of the present invention. An adsorption chiller system  12  uses an adsorbent  41  which can be regenerated. The presently preferred adsorbent is a silica gel. 
     An adsorption chiller  40  comprises in principle a pressure vessel  44  divided into a plurality of chambers, at least two or more adsorbent heat exchanger chambers  45 ,  46  located between a lower evaporator chamber  47  (also referred to as simply an evaporator) and an upper condenser chamber  48  (also referred to as simply a condenser). The adsorbent heat exchanger chambers  45 ,  46  are each connected to the condenser  48  and evaporator  47  by one or more valves  49   a ,  49   b  and  50   a ,  50   b.    
     Each adsorbent heat exchanger chamber  45 ,  46  contains a portion  53   a ,  53   b  of adsorption heat exchanger circuit  53  comprising tubing to carry fluid. Adsorption heat exchanger circuit  53  is a reversible circuit operatively connected to the chiller heat exchanger  36  (shown in  FIG. 1 ) and a pumping means for moving the fluid through the circuit. The portions  53   a ,  53   b  of adsorption heat exchanger circuit  53  within adsorbent heat exchanger chambers  45 ,  46  are packed with an adsorbent  41 , preferably silica gel. 
     A chilling-water circuit  60 , comprising tubing to carry fluid, passes through the evaporator  47 . A cooling-water circuit  61 , comprising tubing to carry fluid, passes through the condenser  48 . Many alternative plumbing layouts with various configurations of tubing and valves are well known in the adsorption chiller art, and their use is within the contemplation of the present invention. All that is necessary is for hot water to be directed through one of the adsorption heat exchanger chambers  45  or  46  while running cooling water through the other adsorption heat exchanger chamber  45  or  46  and the condenser chamber  48 . The condenser  48  and evaporator  47  are connected by a return  57  for returning condensed water from the condenser  48  to the evaporator  47 . 
     When the adsorption chiller system  12  is first started, the pressure vessel  44  is evacuated using an evacuation pump  56 . Once started, an adsorption chiller  40  operates automatically on a four step cycle. First, hot water is introduced into one adsorption heat exchanger chamber  45  through the heat exchanger circuit portion  53   a . This de-adsorbs (or desorbs or dries) the silica gel  41  in this chamber  45 . This forces the water vapor to open valve  49   a  and enter the condenser chamber  48 . Water vapor in the condenser contacts the cooling water circuit  61  where it condenses back into water. The water is then returned to the evaporator by way of the return  57 . When the drying of the adsorption heat exchanger chamber  45  is complete, the water flowing through the adsorption heat exchanger circuit  53   a  is switched from hot water to cooling water by means of external piping not described here. Cool, dry silica gel  41  has a large affinity to capture water vapor and will capture all of the available water vapor from the adsorption heat exchanger chamber  45 , reducing the pressure in this chamber, closing valve  49   a  and allowing the valve  49   b  to open. 
     Water evaporates in a vacuum at room temperature and thereby extracts heat from its surroundings. The evaporation of water introduced into in the evaporator chamber  47  chills the water flowing through the tubes in the chilling circuit  60 . The output of this useful cold water in the chilling circuit  60  is the product of the adsorption chiller system  12  and may be put to use for many desirable purposes. 
     The evaporated water passes through an opened valve  49   b  into adsorbent heat exchanger chamber  45  and is adsorbed into the adsorbent silica gel  41 . Cool water is circulated in this chamber through the adsorption heat exchanger circuit  53   a  to remove the heat deposited in this chamber  45  by the adsorption process. The adsorption process creates a slight decrease in pressure, creating a small vacuum differential between the evaporator chamber  47  and adsorbent heat exchanger chamber  45  that pulls evaporated water from the evaporator  47  through valve  49   b  and into the adsorbent heat exchanger chamber  45 . 
     When adsorbent heat exchanger chamber  45  is in the adsorption cycle, adsorbent heat exchanger chamber  46  is in the de-adsorption cycle; water in chamber  46  that has been adsorbed into the adsorbent  41  is driven from the adsorbent  41  by the circulation of hot water through the portion  53   b  of adsorbent heat exchanger circuit  53  running through chamber  46 . The de-adsorbed water vapor rises and exits adsorbent heat exchanger chamber  46  through opened valve  50   a , entering the condenser  48  where it is condensed by the cool water circulating through the cooling water circuit  61 . The cool water in the cooling water circuit  61  gains heat through the condensing process, which excess heat is transferred to the heat sink  29  (shown in  FIG. 1 ) of the plant, thereby adding to the total heat load to be dissipated through the heat sink  29 . The condensed water collects in the condenser  48  and is recycled through return  57  to the evaporator  47  where it is available for reuse in the adsorption process. 
     When an adsorbent heat exchanger chamber  45  is in the adsorption cycle, the pressure in that chamber  45  is slightly lower than in the evaporator chamber  47 , accordingly, a portion of the refrigerant, water, evaporates and is pulled into the adsorbent heat exchanger chamber  45 . At the same time, the pressure in the other adsorbent heat exchanger chamber  46  in the de-adsorption cycle, is slightly elevated as the water vapor is driven from the silica gel. That de-adsorbed water vapor in pulled into the condenser chamber  48  which has a lower pressure. 
     When the silica gel in the adsorption cycle chamber  45  is saturated with water and the silica gel in the de-adsorption cycle chamber  46  is dry, the adsorption chiller  40  automatically switches the adsorbent heat exchanger chambers  45  and  46  between adsorbing and desorbing by exchanging the flow of hot and cool water. The flow of hot water through the adsorption heat exchanger circuit  53  is switched from flowing through the portion  53   b  of the adsorption heat exchanger chamber  46  to flowing through the portion  53   a  to begin the de-adsorption process in adsorption heat exchanger chamber  45 . Cool water that was running through the portion  53   a  of the adsorption heat exchanger chamber  45  is switched to flow through portion  53   b  of adsorption heat exchanger  46  to begin the adsorption process. Valves  49   a ,  49   b ,  50   a  and  50   b  are preferably one-way valves which are actuated to their opposite condition (i.e., opened or closed) based upon changes in the air pressure differentials in the chambers on opposing sides of the valves. For example, when heat exchanger chamber  46  is in the de-adsorption cycle, valve  50   a  is pushed open by the increase in air pressure within heat exchanger chamber  46  due to the de-adsorption of water from the gel  41 , but valve  50   b  is held closed by this same pressure. As an adsorption cycle begins in heat exchanger  46 , the adsorption of water from the vapor creates a partial vacuum which pulls closed the valve  50   a  between the adsorption cycle chamber and the condenser chamber  48  but pulls open the valve  50   b  between the evaporator chamber  47  and heat exchanger chamber  46 , thereby allowing for the proper flow of vapor through the adsorption chiller  12 . Thus it can be seen that the adsorption chiller  12  requires only the switching of the flow of hot and cool water to function, but does not otherwise require the application of any power to properly function. 
     An adsorption chiller  40  is capable of operating with a wide range of temperatures for the hot, the cool and the cold water. Cycles are generally run for pre-determined amounts of time, depending on the conditions presented. In a presently preferred embodiment, peak performance is obtained when the hot water is about 90° C. (194 F), the cool water about 29.4° C. (85° F.) and the output cold water at about 3.3°-4.4° C. (about 38°-40° F.). 
     The utilization of some or all of the heat from the top of the condenser  15  of the steam turbine  10  to drive the adsorption chiller  40  changes neither the eventual temperature of the output of the condenser  15 , nor the condensate water at the bottom. The condensate water will still be slightly above ambient air temperature. The exhaust steam will still be fully condensed, so the partial vacuum created in the condenser  15  will remain the same. The use of some of the heat for the adsorption chiller  40  will neither impact the performance of the condenser  15 , nor the low pressure turbine  23 , nor the steam turbine as a whole. 
     When the adsorption chiller system  12  of the present invention is initially engaged, the thermal load on the cooling tower  29  is initially reduced because of the heat load transferred to the adsorption chiller  40  through the chiller heat exchanger  36 . However, all of the heat extracted for the adsorption chiller  40 , plus the heat extracted from the chilled water generated by the adsorption chiller  40  is returned back to the cooling tower  29  from the adsorption chiller  40 . 
     The cold chilled water output from the adsorption chiller  40  may be used in a variety of applications inside and outside the power plant. If the power plant is a combined cycle power plant, then it includes one or more gas turbines that are paired with the steam turbine. In a combined cycle plant, the exhaust from one or more gas turbines is captured in a heat recovery steam generator (HRSG) and is used to provide the heat into the Rankine cycle of a steam generator. The heat is transferred to the steam turbines as high pressure, high temperature steam. The steam produced in the HRSG is sent directly to the high pressure non-condensing steam turbine  17 . By utilizing the present invention in a combined cycle plant, the chilled water output from the adsorption chiller system  12  can be used to beneficially reduce the temperature of the inlet air entering the gas turbine. It is well known that chilling the air at the inlet of a gas turbine increases its density and hence the mass of air entering into the turbine compressor. This increased air mass allows more fuel to be burned in the gas turbine combustion chamber, increasing the output of the gas turbine and hence the connected generator. The increased heat flow from the turbine power section also increases the amount of heat available for the HRSG. This reduces the need for supplemental heating (duct firing or duct burning or bulk burning) within the HRSG to produce the desired steam flow rates for the steam turbine. The increased air mass has been demonstrated to add 15-30% to the output and has proven also to increase the heat rate and the cycle efficiency of the gas turbine. Accomplishing this air inlet chilling without having to resort to externally driven refrigerants will greatly increase the overall efficiency of the plant. 
     Another alternative in-process application of the chilled water is its use to provide intercooling of the air as it passes through the compressor stages of the gas turbine. Intercooling is extracting a portion of the air flow from near the center of the compressor, cooling it and reinjecting it into the air flow. This cooled air increases the density in a fashion similar to cooling the inlet air. 
     The chilled water may also be used for other out-of-process or auxiliary applications within the power plant. The generators for the steam turbine and the gas turbine need cooling for their windings and bearings. Using the chilled water for this purpose may further enhance the efficiency of the combined plant. 
     Additionally the chilled water can be used for cooling the components of the generator assembly cabinet from such heat loads as from the SCR&#39;s (silicon controlled rectifiers) and the generator structural legs. 
     The chilled water may also be used to cool the glands and sample valves in the steam and gas turbines. Keeping these elements cool prevents expensive and dangerous steam leaks in the steam turbines and similar gas leaks in the gas turbines. 
     The chilled water may also be used to cool the bearings and windings of the feedwater pumps. The main feedwater pump(s) in particular requires extensive cooling to control the buildup of heat in the pump as it pressurizes the water and moves it back into the HRSG from the condenser. The temperature rise of these critical pumps may limit the output of the combined plant if the temperatures are not controlled effectively. 
     Beyond the power plant itself, the chilled water can be used for an endless number of other uses including air conditioning, process cooling for industrial or food preparation processes or machine cooling. Examples of out-of-process auxiliary heat loads in a combined cycle or steam cycle plant that may be cooled using the chilled water include, but are not limited to, generator windings, bearings, supports and control cabinet loads, feedwater pump winding and bearing loads, and turbine bearings, supports, glands, packings and sample valves. 
     Although this invention has been disclosed and described in its preferred forms with a certain degree of particularity, it is understood that the present disclosure of the preferred forms is only by way of example and that numerous changes in the details of operation and in the combination and arrangement of parts may be resorted to without departing from the spirit and scope of the invention as hereinafter claimed.