Patent Publication Number: US-2009229265-A1

Title: Method and Device for Converting Thermal Energy Into Mechanical Work

Description:
The present invention relates to a method and to an apparatus for converting thermal energy into mechanical work. 
     Many kinds of cyclic processes and apparatus for converting thermal energy into mechanical work and, where required, from there to electric power are known. These processes are for example steam power processes, Sterling processes or the like. One possibility of utilizing such methods is to increase the efficiency of internal combustion engines by making use of the waste heat. The problem here however is that the available temperature levels are quite disadvantageous since the cooling circuit of internal combustion engines usually operates at temperatures of about 100° C. A similar problem arises when heat from solar power plants is to be converted into mechanical work. 
     A special solution for such a thermal power process is shown in the document WO 03/081011 A. In this document, there is described a method by which a hydraulic fluid is pressurized by heating a working fluid in a plurality of bladder accumulator means, said hydraulic fluid being worked off in a working machine. Although such a method is working in principle, it has been found that its efficiency is moderate and that, compared to the amount of energy that can be generated, equipment expense is quite high. 
     A discontinuously operated method capable of generating work through heat conversion at moderate efficiency is further known from U.S. Pat. No. 3,803,847 A. 
     It is the object of the present invention to configure a method of the type mentioned herein above in such a manner that high efficiency is achievable even under thermally disadvantageous conditions, with the equipment expense being as low as possible. 
     In accordance with the invention, such a method consists of the following steps, which are performed as a cyclic process:
         supplying a liquid working fluid from a storage reservoir to a work tank;   heating the working fluid in the work tank via a first heat exchanger;   allowing a fraction of the working fluid from the work tank to overflow into a pneumatic-hydraulic converter, this causing a hydraulic fluid to be urged from the pneumatic-hydraulic converter into a working machine for conversion of the hydraulic work of the hydraulic fluid into mechanical work;   returning the working fluid from the pneumatic-hydraulic converter into the storage reservoir by recirculating hydraulic fluid into the pneumatic-hydraulic converter       

     In the first step, a working fluid having an appropriate vapor pressure curve such as for example R 134   a,  that is 1,1,1,2-tetrafluoroethane, is drawn from a storage reservoir. The working fluid in this storage reservoir is in an equilibrium state between a liquid phase and a gaseous phase. The pressure is hereby chosen such that this equilibrium is maintained. In the case of R 134   a  and of an ambient temperature of about 20° C., this first-pressure will be about 6 bar. The working fluid is transferred to a work tank in which it is preferred that a second, higher pressure prevails. The second pressure is for example 40 bar. The energy expense for the transfer can be minimized if, in a preferred manner, only liquid working fluid is transferred to the work tank by pumping. In the second step, the working fluid is heated in the work tank. Heating causes the pressure to increase even more and the working fluid evaporates partially. Heating preferably occurs through waste heat, for example from an internal combustion engine. If the working fluid is heated to a temperature of 100° C., the waste heat can be optimally utilized. 
     In the third step, the working fluid is allowed to overflow into a pneumatic-hydraulic converter. This can occur after the second step, i.e., the heat is completely supplied first and the connection between the work tank and the pneumatic-hydraulic converter is established thereafter. These steps may however also be performed in part or in whole simultaneously, i.e., the fluid in the work tank is heated while it is flowing into the pneumatic-hydraulic converter. In this way, the efficiency can be optimized since the cooling effected by the expansion of the working fluid is immediately accommodated. Moreover, the cycle time is shortened. In the pneumatic-hydraulic converter, which is for example implemented as a bladder accumulator, the inflowing working fluid displaces a hydraulic fluid that is present in the hydraulic chamber and is being worked off in a suited working machine, for example a hydraulic motor, in order to produce mechanical work that may in turn be used to produce electrical energy. 
     In the fourth step, the pneumatic-hydraulic converter is re-filled with hydraulic fluid through a small pump, with the working fluid being displaced and recirculated into the storage reservoir. Where appropriate, the working fluid is thereby directed through a second heat exchanger, this making it possible to adapt the temperature to the ambient temperature. 
     After this fourth step, the cyclic process is continued with the first step. 
     The efficiency and the performance of the system can be optimized if the possible phase transitions are made use of accordingly. More specifically, in the first step, the working fluid should be moved in the liquid state only, whereas in the third step, only the gaseous phase will be transferred to the pneumatic-hydraulic converter. 
     Preferably, there is provided that during recirculation of the working fluid from the pneumatic-hydraulic converter into the storage reservoir the connection between the work tank and the pneumatic-hydraulic converter is interrupted. This permits to minimize overflow losses. 
     The efficiency may be optimized if the working fluid is cooled while being supplied from the storage reservoir to the work tank. Cooling can occur through an ambient heat exchanger, meaning through a current cooler, but it is also possible to use cold produced by the second heat exchanger provided it is not needed for some other purpose, for example for an air conditioning system or a cooling aggregate. 
     A particular effect of benefit is achieved if the hydraulic fluid is kept at a temperature that corresponds to the mean temperature of the working fluid in the pneumatic-hydraulic converter. This way, undesirable temperature compensating effects can be avoided. 
     As already explained, it is possible that the working fluid be directed from the pneumatic-hydraulic converter through a second heat exchanger. Depending on the way of conducting the method, low temperatures occasioned by the expansion of the working fluid may be generated in the second heat exchanger. These low temperatures can be used for cooling in order to economize the energy needed there. 
     Another improvement of the production of low temperatures can be achieved by causing the working fluid from the pneumatic-hydraulic converter to expand to an expansion pressure that is lower than the first pressure in the storage reservoir and is next compressed to the first pressure. 
     The invention further relates to an apparatus for converting thermal energy to mechanical work, said apparatus having a storage reservoir, a work tank and a working machine for converting hydraulic work into mechanical work. 
     In accordance with the invention, there is provided that the work tank is connected to a first heat exchanger for heating the working fluid, that the work tank is further connected to a pneumatic-hydraulic converter that transfers the pressure of the working fluid to a hydraulic fluid and that there is provided a recirculation line for recirculating the working fluid from the pneumatic-hydraulic converter into the storage reservoir. 
     In a particularly preferred implementation variant, there is provided that a plurality of work tanks and pneumatic-hydraulic converters are connected in parallel. 
     In practical implementation, five of the apparatus illustrated in  FIG. 1  are for example arranged parallel to each other in a side-by-side relationship and operated in a time-staggered fashion as this is for example the case in a five-cylinder internal combustion engine. This permits to achieve continuous operation without noteworthy cyclic fluctuations. 
    
    
     The method of the invention and the apparatus of the invention will be discussed in greater detail herein after with reference to the circuit diagram of  FIG. 1 , which illustrates the major component parts of the system.  FIG. 2  shows a typical vapor pressure curve of a working fluid. 
     A storage reservoir  1  holds a working fluid; a coolant such as R 134   a  can be utilized for example. The working fluid in the storage reservoir  1  is in phase equilibrium at ambient temperature and at a pressure of about 6 bar. The storage reservoir  1  is connected to a work tank  3  through a feed pump  2 , this connection being switchable through a valve  4 . In the work tank  3  there is disposed a first heat exchanger  5  that serves to heat the working fluid in the work tank  3 . Heat exchanger  5  is supplied with waste heat from an internal combustion engine that has not been illustrated herein via a booster pump  6 , with water at 100° C. being directed through the first heat exchanger  5  for example. Through an overflow line  7 , the work tank  5  communicates with a first working chamber  8   a  of a pneumatic-hydraulic converter  8  that is configured to be a bladder accumulator. The first working chamber  8   a  is separated from a second working chamber  8   b  by a flexible membrane  8   c  that separates the two working chambers  8   a,    8   b  while allowing for pressure compensation. The second working chamber  8   b  of the pneumatic-hydraulic converter  8  communicates with a hydraulic circuit consisting of a working machine  9  having a generator  10  flanged thereon, an oil tank  20 , a recirculating pump  17  and a third heat exchanger  11 . The third heat exchanger  11  is supplied from a pump  12 . Another work line  19  connects the first working chamber  8   a  of the pneumatic-hydraulic converter  8  to a second heat exchanger  16  that communicates through a booster pump  14  with the storage reservoir  1 . For the rest, the lines  7 ,  19  may be closed selectively by valves  7   a,    19   a.    
     The mode of operation of the apparatus of the invention will be explained in closer detail herein after: 
     In a first step, liquid working fluid is transferred from the storage reservoir  1  into the work tank  3  via the feed pump  2 , with the pressure being increased from 6 bar to 40 bar. 
     After the work tank  3  is completely filled with liquid working fluid, the valve  4  is closed and heating through the first heat exchanger  5  occurs. This heating constitutes the second step. Waste heat from another process can be used therefor. 
     By heating the working fluid to 100° C., part of said fluid evaporates in the work tank  3  and this vapor is transferred in a third step, through the line  7  with the valve  7   a  being open, into the first working chamber  8   a  of the pneumatic-hydraulic converter  8 . The pressure drop is compensated by further heating through the first heat exchanger  5 . Simultaneously, the membrane  8   c  of the pneumatic-hydraulic converter  8  is displaced toward the second working chamber  8   b  so that hydraulic fluid is urged through the working machine  9  driving the generator  10 . The third step ends as soon as the second working chamber  8   b  of the pneumatic-hydraulic converter  8  has largely emptied. 
     In a fourth step, hydraulic fluid is recirculated via the pump  17  from the tank  20  into the second working chamber  8   b  of the pneumatic-hydraulic converter  8  and the working fluid is directed from the first working chamber  8   a,  through the valve  19   a  in the line  19 , which has opened in the meantime, through the second heat exchanger  16  and is expanded. A booster pump  14  recirculates the working fluid back into the storage reservoir  1 . As denoted by the arrow  21 , the heat absorbed by the working fluid in the second heat exchanger  16  can be evacuated as cooling capacity for operating a cooling system or an air conditioning system. A partial flow through a heat exchanger  15  may also be used for cooling the working fluid during compression, though. 
       FIG. 2  illustrates a typical vapor pressure curve of a working fluid adapted for use in the cyclic process described herein above. Said working fluid is R 134   a,  which is known to be a coolant, meaning 1,1,1,2-tetrafluoroethane. As can be seen, at ambient temperature and at a pressure of about 6 bar, the liquid phase is in equilibrium with the gaseous phase. At a temperature of 100° C., this equilibrium pressure is about 40 bar. 
     With simple equipment structure the present invention allows for optimal use of waste heat from other processes, like for example from the operation of an internal combustion engine.