Abstract:
In a Rankine cycle system wherein a vapor generator receives heat from exhaust gases, provision is made to avoid overheating of the refrigerant during ORC system shut down while at the same time preventing condensation of those gases within the vapor generator when its temperature drops below a threshold temperature by diverting the flow of hot gases to ambient and to thereby draw ambient air through the vapor generator in the process. In one embodiment, a bistable ejector is adjustable between one position, in which the hot gases flow through the vapor generator, to another position wherein the gases are diverted away from the vapor generator. Another embodiment provides for a fixed valve ejector with a bias towards discharging to ambient, but with a fan on the downstream side of said vapor generator for overcoming this bias.

Description:
FEDERALLY SPONSORED RESEARCH 
   The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the term of Contract Nos.: FC02-00CH11060 and FC36-00CH11060 awarded by the Department of Energy. 

   BACKGROUND OF THE INVENTION 
   This invention relates generally to Rankine cycle systems and, more particularly, to a method and apparatus for controlling of the flow of a fluid through a vapor generator thereof. 
   Power generation systems that provide low cost energy with minimum environmental impact, and that can be readily integrated into the existing power grids or rapidly sited as stand alone units, can help solve critical power needs in many areas of the U.S. Gas turbine engines and reciprocating engines are examples of such systems. Reciprocating engines are the most common and most technically mature of these distributing energy sources in the 0.5 to 5 MWe range. These engines can generate electricity at low cost with the efficiencies of 25-40% using commonly available fuels such as gasoline, natural gas, and diesel fuel. However, atmospheric emissions, such as nitrogen oxides (NOx) and particulates can be an issue with reciprocating engines. One way to improve the efficiency of combustion engines without increasing the output of emissions is to apply a bottoming cycle. Bottoming cycles use waste heat from such an engine and convert the thermal energy into electricity. One way to accomplish this is by way of organic Rankine cycle (ORC) power generators, which produce shaft power from lower temperature waste heat sources by using an organic working fluid with a boiling temperature suited to the heat source. 
   A concern with such use of an ORC is that, if the ORC cycle is interrupted, such as would occur with a failure of a pump, for example, then the refrigerant flow would discontinue and the temperature of the refrigerant within the system would eventually rise to the level of the heat source temperature, which could be well exceed the safe limit of around 350° F. for the refrigerant and cause the refrigerant and/or the lubricant therein to decompose. 
   Another concern in the design of organic Rankine cycles which use waste heat, is that of corrosion in the boiler. Hot gases from the combustion of natural gases or diesel fuel can be very corrosive if allowed to condense on the heat transfer surfaces of the boiler tubes. Normal practice is to design the boiler such that hot gas exits at 250-350° F., thereby preventing condensation of corrosive exhaust constituents such as sulfuric acid. However, there are times during start up or maintenance when this constraint is not met and condensation and corrosion can occur. Isolation of the boiler from the hot gas stream during these times could prevent condensation, but it is difficult and expensive to produce a high-temperature, low-leakage seal. 
   In addition to the above needs, there are some circumstances where it is beneficial to be able to divert or reduce the hot gas flow through the boiler. That is, if the exhaust gases being provided to the boiler are substantially in excess of 700° F., which can occur with gas turbine engines, then the refrigerant in the ORC will likely exceed a safe temperature threshold so as to cause decomposition of lubricant in the refrigerant, thereby forming coke which deteriorate boiler performance through excessive boiling and leads to oil loss of the system. Also, the refrigerant itself might decompose when it sees temperatures of excess of 350° F. 
   It is therefore an object of the present invention to provide an improved boiler heating arrangement for an organic Rankine cycle system. 
   Another object of the present invention is the provision in an ORC system for preventing excessive refrigerant temperatures in the event of a failure within the ORC system. 
   Another object of the present invention is the provision in an organic Rankine cycle system for preventing corrosion in a vapor generator/boiler thereof. 
   Yet another object of the present invention is the provision in the heating portion of an organic Rankine cycle system, for the control of the temperature thereof. 
   Still another object of the present invention is the provision in an organic Rankine cycle system which is economical to manufacture and effective and efficient in use. 
   These objects and other features and advantages become readily apparent upon reference to the following description when taken in conjunction with the appended drawings. 
   SUMMARY OF THE INVENTION 
   Briefly in accordance with one aspect of the invention in the event of a failure of the ORC refrigerant circulation system the heat source is diverted from the ORC boiler to prevent excessive temperatures. 
   In accordance with another aspect of the invention, at times when the vapor generator is allowed to cool down to the point where condensation will occur, provision is made for the reverse flow of air therethrough, to ambient, to thereby flush any harmful condensible gases that may be in the vapor generator. 
   In accordance with another aspect of the invention, a diverter/ejector is placed between the heat source and the ORC, and the ejector is operated such that, during normal operation, the gases flow through the ejector and to the ORC, while at times when the ORC vapor generator temperature will fall below a certain level, the ejector is adjusted such that the exhaust gases flow from the heat source through the ejector and to ambient, while at the same time drawing ambient air through the ORC vapor generator, through the ejector and to ambient to thereby flush out the gases that would otherwise condense in the vapor generator. 
   By yet another aspect of the invention, the ejector may be adjusted such that during normal operation, when the exhaust gases are flowing through the ejector and through the ORC, ambient air will be drawn in through the ejector and through the ORC, to thereby reduce the temperature of the gases to an acceptable level. 
   In there drawings as hereinafter described, a preferred embodiment is depicted; however, other various modifications and alternate constructions can be made thereto without departing from the true spirit and scope of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a further understanding of these and other objects of the invention, reference will be made to the following detailed description of the invention which is to be read in connection with the accompanying drawings, wherein: 
       FIG. 1  is a schematic illustration of a Rankine cycle system in accordance with the prior art. 
       FIG. 2  is a perspective view of the ejector portion of the invention. 
       FIG. 3  is a schematic illustration of the ejector as positioned to direct flow during normal operation. 
       FIG. 4  is a schematic illustration of the ejector as positioned to direct flow when the ORC is at a lower temperature. 
       FIGS. 5 and 6  show alternate embodiments of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring now to  FIG. 1 , a typical Rankine cycle system is shown to include an evaporator/boiler/vapor generator  11  which receives heat from a heat source  12  to generate high temperature vapor and provide motive power to a turbine  13  which in turn drives a generator  14  to produce power. Upon leaving the turbine  13 , the relatively low pressure vapor passes to the condenser  16  where it is condensed by way of heat exchange relationship with a cooling medium. The condensed liquid is then circulated to the evaporator by a pump  17  as shown to complete the cycle. The motive fluid in such Rankine cycle system is commonly water but may also be a refrigerant, in which case it is referred to as anorganic Rankine cycle (ORC). 
   Such an organic Rankine cycle system is susceptible to three possible problems. Firstly, if the pump  17  fails, then the temperature of the refrigerant can rise to excessive levels. Secondly, if the gases from the heat source  12  are at too high a temperature, the refrigerant in the vapor generator  11  will be heated to such a degree (e.g., 440° F.), that the lubricant within the refrigerant decomposes. The decomposed lubricant will be changed to coke, which causes a deterioration of the boiler performance as described above. Thirdly, if the vapor generator  11  is caused to have its temperature substantially lowered from its operating temperature, such as when it is shut down for maintenance and the like, any hot gases that are retained or which flow into the vapor generator would tend to condense and form acids that will be detrimental to the structure of the vapor generator  11 . All of these problems are addressed by the use of diverter/ejector device as shown in  FIGS. 2-4 . 
   One embodiment of the diverter/ejector  18  is shown in FIG.  2 . It comprises a box like structure having bottom and top walls  19  and  21 , and four side walls, three of which are shown at  22 ,  23  and  24 . Within those walls, there are provided a number of openings including bottom wall opening  26 , top wall opening  27 , and side wall opening  28 . These openings allow for the fluid flow into and out of the diverter  18  as will be described hereinafter. 
   Within the ejector  18  is a pair of stationary structures. An arcuate wall  29  interconnects the edge of opening  26  with an edge of the opening  28  and defines one side of a flow channel  31  between opening  26  and  28 . A flow divider island  32  is mounted adjacent the top wall  21  and side wall  24  and is cantilevered downwardly to a relatively sharp edge  33 . This member defines the other side of the flow channel  31  between opening  26  and  28 , and also defines, along with wall  22 , a flow channel  34  between openings  26  and  27 . 
   Also included within the diverter/ejector  18  is a modulating plate  36  which is rotatably mounted at its top edge  37 , near the sharp edge  33 . A space  38  is provided between the sharp edge  33  and the top edge  37  for the flow of fluid as will be described hereinafter. The modulating plate  36  is selectively rotated about its upper edge  37  to control the fluid flow within the ejector  18 . For example, in  FIG. 2 , it is moved to a position that shuts off the flow of air from the opening  26  to the flow channel  31 . In  FIG. 3 , it is moved to a vertically aligned position which allows the fluid flow coming into opening  26  to pass on each side of the modulating plate  36  so as to flow into both flow channels  31  and  34 . 
   Considering now the operation of the ejector  18  during normal operation as shown in  FIG. 3 , hot combustion products (e.g., from a gas turbine exhaust), pass into the opening  26  and, as mentioned above, when the modulating plate  36  is in the vertical position, the gases can flow to both openings  27  and  28 . When the modulating plate  36  is moved to the right as indicated by the dotted line, then all of the gases coming into the opening  26  will flow through the flow channel  31  and out the opening  28  to the vapor generator  11 . As this occurs, a low pressure area is created in the flow channel  31  such that ambient air is caused to flow into the opening  27 , through the flow channel  34 , and through the space  38  to enter the flow channel  31 . The introduction of this relatively cool air with that of the hot gases coming into the opening  26  causes a reduction in temperature of the gases that flow to the vapor generator  11 . In this way, the exhaust gas temperatures which may otherwise be excessive to create problems for the vapor generator as discussed hereinabove, can be avoided. Ideally, temperatures T 1  of the gases flowing into the vapor generator  11  are around 700° F., and those leaving the vapor generator  11  are around 200° F. If they are significantly higher, the refrigerant being circulated through the vapor generator will be heated to an excessive temperature that will be harmful to both the refrigerant and the lubricant within. If the temperature T 2  is substantially below 200° F., then condensation will tend to occur within the vapor generator  11  to thereby cause corrosive effects. The modulating plate  36  is therefore selectively adjusted in an effort to maintain the ideal temperature relationship. 
   It should be noted that the structure as shown provide for a fixed distance between the sharp edge  33  and the top edge  37  such that the space  38  remains constant. This distance can be established to meet the design requirements for the particular installation. However, the structure may, as well, be so constructed as to allow for the selective variation of that distance so as to thereby selectively vary the amount of ambient air that flows into the system during normal operation. 
   Considering now the situation where an ORC system failure occurs, such as a failure of the pump  17 , the reduced flow is sensed by a flow sensor  40  and, in response the modulating plate  36  is then moved to the closed position as shown in  FIG. 2 , such that all of the hot gases are diverted to flow upwardly to ambient air. This prevents the refrigerant in the ORC from being heated to excessive temperatures. Instead of a flow sensor  40 , a temperature sensor (not shown) can be installed in the vapor generator to sense temperatures that exceed a predetermined threshold level to activate the diverter. 
   Considering now the operational condition wherein the vapor generator  11  will be under temperature conditions which would cause condensation of gases therein, care must be taken to prevent such condensation. This would occur, for example, during periods of maintenance and start up. As shown in  FIG. 4 , during these operating conditions, the modulating plate  36  is moved to the far left position as shown to block off all flow of exhaust gases to the flow channel  31 . The exhaust gases will instead flow into the opening  26 , through the flow channel  34  and out the top wall opening  27  to ambient. Because of the low pressure condition that is created within the flow channel  34 , some of the fluid from flow channel  31  will be drawn in through the space  38  and into the flow channel  34 . In doing so, ambient air will be drawn in from the downstream side of the vapor generator  11  to thereby flush out any harmful gases that would otherwise remain in the vapor generator and which could condense to cause harm thereto. 
   Another embodiment of the present invention is shown in  FIGS. 5 and 6  wherein a fixed flap  39  is shown between the openings  26 ,  27  and  28 . There, rather than having a modulatable flap, a fan  41  is provided at the downstream side of the vapor generator  11  as shown. In  FIG. 5 , the system is shown in the condition wherein the vapor generator  11  is in a cooled condition, such that hot gases need to be flushed from the vapor generator  11 . Because the fixed flap  39  is in a biased position which causes the hot gases flowing into the opening  26  to pass out the opening  27  to ambient, the low pressure condition caused by that flow will cause air to be drawn to the left of the opening  28  such that a combustion gases in the vapor generator  11  are drawn out to the opening  27 . Thus, the fan  41  is in the off position and air will be drawn to the left as shown by the arrow. 
   In the full operating condition as shown in  FIG. 6 , because of the bias of the fixed flap  39  as mentioned above, it is necessary to create a low pressure condition on the downstream side of the vapor generator  11  in order to pull the hot gases away from the ambient opening  27  such that they will flow through the opening  28  to the vapor generator  11 . The fan  41  is therefore made to operate as shown so as to pull the flow of combustion gases to flow through the vapor generator  11 . 
   While the present invention has been particularly shown and described with reference to the preferred mode as illustrated in the drawings, it will be understood by one skilled in the art that various changes in detail may be effected therein without departing from the spirit and scope of the invention as defined by the claims.