Abstract:
In a waste heat recovery system wherein an organic rankine cycle system uses waste heat from the fluids of a reciprocating engine, provision is made to continue operation of the engine even during periods when the organic rankine cycle system is inoperative, by providing an auxiliary pump and a bypass for the refrigerant flow around the turbine. Provision is also made to divert the engine exhaust gases from the evaporator during such periods of operation. In one embodiment, the auxiliary pump is made to operate simultaneously with the primary pump during normal operations, thereby allowing the primary pump to operate at lower speeds with less likelihood of cavitation.

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 terms of DE-FC02-00CH11060 awarded by the Department of Energy. 

   BACKGROUND OF THE INVENTION 
   This invention relates generally to waste heat recovery systems and, more particularly, to a organic rankine cycle system for extracting heat from a reciprocating engine. 
   Power generation systems that provide low cost energy with minimum environmental impact, and which can be readily integrated into the existing power grids or which can be quickly established as stand alone units, can be very useful in solving critical power needs. Reciprocating engines arc the most common and most technically mature of these distributed energy resources in the 0.5 to 5 MWe range. These engines can generate electricity at low cost with efficiencies of 25% to 40% using commonly available fuels such as gasoline, natural gas or diesel fuel. However, atmospheric emissions such as nitrous 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 (i.e. an organic rankine cycle or ORC). Bottoming cycles use waste heat from such an engine and convert that thermal energy into electricity. 
   Most bottoming cycles applied to reciprocating engines extract only the waste heat released through the reciprocating engine exhaust. However, commercial engines reject a large percentage of their waste heat through intake after coolers, coolant jacket radiators, and oil coolers. Accordingly, it is desirable to apply an organic rankine bottoming cycle which is configured to efficiently recover the waste heat from several sources in the reciprocating engine system. 
   One problem that the applicants have recognized in such a system is that, if the organic rankine cycle (ORC) is disabled by component failure or for planned maintenance, the ORC working fluid will no longer be circulated through the reciprocating engine and the temperature of the ORC working fluid inside the engine as well as the critical engine components being cooled by this fluid will quickly exceed the safe level point of about 200° F., and it becomes then necessary to shut down the engine and cease operation. 
   A general concern with bottoming cycles is that of cavitation in the pump that circulates the working fluid. Such a system requires a pump with a relatively small flow rate (e.g. 18 lbm/s) and a large pressure rise (e.g. 250 psi). Optimum pump performance dictates a certain relationship between pump head (pressure differential), pump flow rate, and pump speed. For maximum efficiency, a small, high speed, radial pump is desirable. However, such a pump is subject to cavitation especially since it is downstream of the condenser where the liquid from the condenser is only slightly subcooled. Cavitation occurs when the liquid entering the pump starts to locally vaporize due to the initial flow acceleration. That is, since the higher local velocity results in a lower local pressure, vapor bubbles will be created if the local pressure is below the saturation pressure. 
   One approach to solving the cavitation problem is to use a less efficient regenerative pump, but this results in 35-45% efficiency rather than the 60-80% efficiency that is obtainable with radial pumps, which are more prone to cavitation. 
   It is therefore an object of the present invention to provide an improved ORC waste heat recovery system. 
   Another object of the present invention is the provision in an ORC system used to extract heat from a reciprocating engine, to allow continued operation of the engine when the ORC system is inactive. 
   Another object of the present invention is the provision in an ORC system for preventing cavitation of the pump. 
   Yet another object of the present invention is the provision in an ORC for prevention of pump cavitation while at the same time maintaining pump efficiency. 
   These objects and other features and advantages become more 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, an auxiliary pump is provided in the refrigerant flow circuit of an ORC, with the pump being driven by a dedicated shaft or by electrical power from a generator. Thus, when the primary pump is inoperative, the dedicated auxiliary pump can be activated to circulate the cooling fluid through the reciprocating engine and allow its continued operation. 
   In accordance with another aspect of the invention, a bypass arrangement is provided to bypass the ORC turbo generator such that the flow of coolant passes directly from the evaporator/boiler to the condenser, and also to divert the reciprocating engine hot exhaust gases from the evaporator. This reduces the amount of heat that is transferred to the refrigerant and allows for a smaller pump to be used as the auxiliary pump. 
   By yet another aspect of the invention, provision is made for simultaneous operation of two pumps in series, a primary and an auxiliary pump during normal operation such that the speed of both pumps can be reduced to thereby reduce the risk of cavitation. 
   In the drawings as hereinafter described, a preferred embodiment is depicted; however, various other modifications and alternate constructions can be made thereto without departing from the true spirit and scope of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic illustration of an organic rankine cycle system as incorporated with a reciprocating engine. 
       FIG. 2  is a schematic illustration of an organic rankine cycle system as modified in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to  FIG. 1 , there is shown a reciprocating engine  11  of the type which is typically used to drive a generator (not shown) for purposes of providing electrical power for consumer use. The engine  11  has an air intake section  12  for taking in air for combustion purposes and an exhaust  13  which may be discharged to the environment, but is preferably applied to convert a portion of the energy therein to useful purposes. 
   The engine  11  also has a plurality of heat exchangers with appropriate fluid for maintaining the engine  11  at acceptable operating temperatures. A radiator  14  is provided to take heat away from a liquid coolant that is circulated in heat exchange relationship with the portion of the engine where combustion occurs, while an oil cooler  16  is provided to remove heat from a lubricant that is circulated within the moving parts of the engine  11 . 
   The engine  11  may be provided with a turbo charger  17  which receives high temperature, high pressure exhaust gases from the exhaust section  13  to compress the engine inlet air entering the turbo charger  17 . The resulting compressed air, which is heated in the process, then passes to a charge cooler  18  and is cooled in a manner to be described hereinafter, prior to passing into the intake  12  of the engine to be mixed with fuel for combustion. The exhaust gases, after passing through the turbo charger  17 , pass through an evaporator  19 , which is a part of an organic rankine cycle (ORC) system that is shown on the left side of FIG.  1  and which is adapted to use the exhaust waste heat from the engine  11  while at the same time cooling the various components thereof and maintaining it at an acceptable operating temperature. 
   In addition to the evaporator  19 , the ORC includes a turbine  21 , a condenser  22  and a pump  23 . The turbine  21  receives hot refrigerant gas along line  24  from the evaporator  19  and responsively drives a generator  26 . The resulting low energy vapor then passes along line  27  to the condenser  22  to be condensed to a liquid form by the cooling effect of fans  28  passing ambient air thereover. The resulting liquid refrigerant then passes along line  29  to the pump  23  which causes the liquid refrigerant to circulate through the engine  11  to thereby generate high pressure vapor for driving the turbine  21 , while at the same time cooling the engine  11 . Both the fans  28  and the pump  23  are driven by electrical power from the grid  31 . 
   As will be seen in  FIG. 1 , relatively cool liquid refrigerant from the pump  23  passes sequentially through ever increasing temperature components of the engine  11  for providing a cooling function thereto. That is, it passes first through the charge cooler  18 , where the temperature of the liquid refrigerant is raised from about 100° to 130°, after which it passes to the radiator  14 , where the refrigerant temperature is raised from 130° to 150°, after which is passes to an oil cooler  16  where the refrigerant temperature is raised from 150° to 170°. Finally, it passes through the evaporator  19  where the liquid is further preheated before being evaporated and superheated prior to passing on to the turbine  21 . 
   In this system as described, it will be recognized that if the ORC system is not operating properly, such as, for example, if the pump  23  fails, the cooling effect of the refrigerant passing through the various heat exchangers will be lost and, if the engine  11  would continue to operate, it will heat up to unacceptable temperatures, requiring its shut down. 
   Also peculiar to the system as shown in  FIG. 1 , the pump  23  may be a small high speed radial pump that typically is high in efficiency but subject to the occurrence of cavitation. Alternatively, a regenerative pump which is generally not subject to cavitation but operates at much lower efficiencies, may be used. 
   Referring now to  FIG. 2 , there is shown the same system with certain additions being made for purposes of providing a means of cooling the engine  11  during periods in which the ORC is not operating. 
   Here a dedicated auxiliary pump  32  is provided in the line  29  for either boosting the pumping capacity when the pump  23  is on line or for replacing the pumping capacity of the pump  23  when the pump  23  is not on line. The various possible combinations will be described hereinafter. 
   Also provided are a number of valves that may be selectively operated to facilitate the continued operation of the engine  11  during periods in which the ORC system is inoperative. A pair of passively sprung vapor valves  33  and  34  are provided to bypass the turbo generator  21  during such periods. That is, to continue operation of the engine  11  when the ORC is inoperative, the valve  33  is closed and the valve  34  is opened such that the hot refrigerant gas from the evaporator  19  passes directly to the condenser  22 , with the resulting liquid refrigerant then being circulated by the auxiliary pump  32  through the various heat exchangers  18 ,  14 ,  16  and  19  to complete the circuit. 
   Recognizing that when the turbine  21  is not operating, the energy that is normally removed from the system by operation of the turbine  21  will be excessive, and the engine  11  will not be properly cooled if further changes are not made. Accordingly, provision is made to further remove heat from the system such that the auxiliary path as just described will be capable of maintaining acceptable temperature levels in the engine  1  when it continues to operate. 
   Recognizing that the majority of the heat passing to the ORC system in the conventional manner as described in respect to  FIG. 1 , comes from the engine exhaust  13 , exhaust diverter valve  36  is provided to selectively divert the exhaust gases from the evaporator  19  and pass them directly to the atmosphere as shown. This reduces the energy that is added to the refrigerant to that from the charge cooler  18 , the radiator  14 , and the oil cooler  16  such that the energy can be dissipated by the condenser  22  without operation of the turbine  21 . The pump  32  is properly sized such that the temperature of the refrigerant leaving the evaporator  19  is in the range of 170° F. 
   Considering now the possible operating modes of the two pumps  23  and  32 , one possibility is that of operating only the main pump  23  during normal operation and only the auxiliary pump  32  during periods in which the ORC is not operating. In such case, the main pump  23  must necessarily be of a relatively large head since it must bear the entire load. With the potential problem of cavitation in mind, a suggested pump for this use is a regenerative pump (such as the Roth 5258 pump). A suggested pump that could be used as the auxiliary pump  32  is the Sundyne P2000 pump. 
   In operation, the above described pump combination will be controlled as follows. During normal operation, when the valve  33  is open, the valve  34  is closed, and the valve  36  is set to allow exhaust gases to flow to the evaporator  19 , the main pump  23  is operating at all times and the auxiliary pump  32  is turned off at all times. During periods in which the ORC is inoperative, the valve  33  is closed, the valve  34  is opened, and the valve  36  is placed in a position so as to divert the exhaust flow from the evaporator  19 . In such case, the main pump  23  is turned off at all times and the auxiliary pump  32  is turned on at all times. 
   Considering now that the auxiliary pump  32  can be used during normal operation in order to deliver part of the head of the main pump  23 , it has been recognized that, for the main pump  23 , a lower speed pump, and thus one less likely to have cavitation problems, can be used. For example, rather than one having a head of 300 psi and a pump speed of 7000 rpm as described hereinabove, the pump head can be reduced to 150 psi with a pump speed of 5000 rpm. A suggested pump for this purpose would be the Sundyne P2000. 
   With such a pump combination as described hereinabove, during normal operation both pumps will be on at all times, and during periods of which the ORC is not operative, only the auxiliary pump will be on. 
   In the embodiment as described with respect to  FIG. 2 , the auxiliary pump  32  is placed upstream of the main pump  23 , but this order could just as well be reversed. Further, it is possible to have the two pumps in parallel relationship rather than in series, but this would not offer the advantages of head reduction, cavitation prevention and effective engine cooling during ORC shutdown and would appear to introduce certain disadvantages. 
   While the invention has been shown and described with respect to a preferred embodiment thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions and additions in the form of a detail thereof made be made without departing from the true sprit and scope of the invention as set forth in the following claims.